Static Equipment

Refining-Petrochemicals-Chemicals-Engineering ———

CORROSION — Natural but controllable process —

I - DEFINITIONS AND MECHANISMS ..............................................................................................1 1 2 3 4 5 6 7 8

-

Introduction.........................................................................................................................................1 Economic aspect of corrosion ...........................................................................................................1 Review of chemistry and electrochemistry .......................................................................................2 Primary phenomena of wet corrosion .............................................................................................11 Secondary phenomena of wet corrosion ........................................................................................16 Effects of secondary reactions on the kinematics of wet corrosion ..............................................17 Dry corrosion ....................................................................................................................................29 Measurement and control - Corrosion tests ...................................................................................31

II - THE MANY FORMS OF CORROSION .......................................................................................35 1 2 3 4 5 6 7 8 9

-

Uniform attack ..................................................................................................................................35 Galvanic corrosion ...........................................................................................................................36 Crevice corrosion .............................................................................................................................38 Pitting ................................................................................................................................................40 Intergranular corrosion.....................................................................................................................42 Stress corrosion cracking ................................................................................................................45 Selective leaching ............................................................................................................................50 Fretting corrosion .............................................................................................................................51 Erosion corrosion .............................................................................................................................52

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III - CORROSION PREVENTION .......................................................................................................53 1 2 3 4 5 6 7 8

-

Materials selection ...........................................................................................................................53 Control of process variables ............................................................................................................68 Good engineering design.................................................................................................................70 Inhibitors ...........................................................................................................................................73 Anodic protection..............................................................................................................................75 Cathodic protection ..........................................................................................................................75 Metallic and inorganic coatings .......................................................................................................77 Paints - Organic coatings.................................................................................................................79

IV - HYDROGEN CORROSION..........................................................................................................82 1 2 3 4

-

Sources of nascent or atomic hydrogen .........................................................................................82 Hydrogen blistering ..........................................................................................................................83 Hydrogen embrittlement ..................................................................................................................83 Decarburization and hydrogen attack .............................................................................................85

V - CORROSION BY SULFUR DERIVATIVES IN THE ANHYDROUS PHASE.............................88 1 2 3 -

Corrosion by sulfur compounds.......................................................................................................88 Anhydrous phase H2S corrosion without hydrogen .......................................................................88 H2S corrosion with H2......................................................................................................................89

VI - CORROSION BY COLD HYDROGEN SULFIDE IN A HUMID ATMOSPHERE.......................99 1 2 3 -

Generalized corrosion......................................................................................................................99 Atomic hydrogen corrosion ..............................................................................................................99 Prevention.........................................................................................................................................99

VII - CORROSION BY COMBUSTION GASES IN FURNACES AND BOILERS............................100 1 2 3 -

Formation of SO3 ...........................................................................................................................100 Corrosion at high temperatures.....................................................................................................102 Low temperature corrosion............................................................................................................106

VIII - NAPHTHENIC ACIDS CORROSION.........................................................................................114 IX - CORROSION IN DISTILLATION UNITS ...................................................................................118 1 2 3 -

Corrosive agents in the crude .......................................................................................................118 Study of the reaction mechanisms related to corrosion of atmospheric distillation head equipment .......................................................................................................................................118 Corrosion combating methods ......................................................................................................119

X - CORROSION BY POLYTHIONIC ACIDS..................................................................................126 1 2 3 -

Introduction.....................................................................................................................................126 Initiating factors of PSCC...............................................................................................................126 Prevention of polythionic cracking.................................................................................................127

XI - CAUSTIC SODA CORROSION .................................................................................................128

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XII - BIOCHEMICAL CORROSION....................................................................................................130 1 2 -

Macro-organisms............................................................................................................................130 Micro-organisms.............................................................................................................................130

XIII - CORROSION IN AN AQUEOUS ENVIRONMENT ...................................................................135 1 2 3 4 5 6 7

-

Introduction.....................................................................................................................................135 Effect of oxygen concentration ......................................................................................................136 Effect of the solution pH.................................................................................................................137 Effect of strong and weak acids on the iron..................................................................................139 Effect of dissolved salts .................................................................................................................140 Effect of carbon dioxide .................................................................................................................142 Prevention of corrosion by water...................................................................................................144

XIV - ATMOSPHERIC CORROSION..................................................................................................145 1 2 3 -

Films formed by corrosion products..............................................................................................145 Atmospheric corrosion factors.......................................................................................................145 Remedies for atmospheric corrosion ............................................................................................146

XV - SEA WATER CORROSION .......................................................................................................146 1 2 3 4

-

Characteristics of sea water ..........................................................................................................146 Carbon steel corrosion...................................................................................................................147 Corrosion of stainless steels..........................................................................................................148 Corrosion of non-ferrous metals....................................................................................................148

XVI - DRY OXIDATION AT HIGH TEMPERATURE...........................................................................152 1 2 3 4

-

Mechanism .....................................................................................................................................152 Oxidation kinetics ...........................................................................................................................155 High temperature materials ...........................................................................................................155 Atmospheres encountered at high temperatures .........................................................................155

XVII - CORROSION BY LIQUID AMMONIUM.....................................................................................159 XIX - REINFORCED CONCRETE CORROSION...............................................................................160 1 2 3 4 -

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Corrosion of steel structures in a humid environment .................................................................160 Natural protection of steels by cement .........................................................................................160 Weakening of the natural protection of the embedded metal structure: alteration of the concrete................................................................................................................................160 Prevention.......................................................................................................................................166

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I-

DEFINITIONS AND MECHANISMS 1-

INTRODUCTION Corrosion is the most widespread degradation phenomenon in process units. It is the cause of most failures and malfunctions of pressure vessels. It is estimated that corrosion destroys a quarter of the world’s annual production in steel, which represents approximately 150 million tons per year, or 5 tons per second. In addition, corrosion is not limited to just steel, but also affects all metals, as well as polymers and ceramics. Here are a few examples of corrosion phenomena: -

transformation of steel into rust cracking of brass in the presence of ammonia oxidation of copper electric contacts hydrogen embrittlement of a high-resistant steel hot corrosion of a super-alloy in a gas turbine swelling of a polymer in contact with a solvent degradation of PVC by ultraviolet radiation attack of a nylon tube by a oxidizing acid attack of refractory bricks by slag attack of a mineral glass by an alkaline solution

Corrosion is an irreversible interfacial reaction of a material with its environment, which implicates a consumption of the material or a dissolution in the material of a component of the environment. This definition includes the positive effects of the corrosion, as well as the absorption of a component of the environment without consumption of the material. The absorption of hydrogen by steel is, for example, considered to be a corrosion reaction. For this reason, wet corrosion, or corrosion at room temperature, is distinguished from dry corrosion, that is, high-temperature corrosion.

2-

ECONOMIC ASPECT OF CORROSION The direct or indirect effects of corrosion are summarized below: -

cost of the parts to be replaced, the repairs to be made cost of maintenance and control (painting, cathodic protection) cost due to the use of more noble materials enhancement of safety factors contamination of the product by corrosion products production shutdown

According to the estimates noted in the literature, the yearly cost for corrosion amounts to 4% of the GDP (Gross Domestic Product), that is, several billion Euros annually for France. Results of a recent (2002) study show that the total annual estimated direct cost of corrosion in the U.S. is a staggering $ 276 billion - approximately 3.1% of the nation’s Gross Domestic Product (GDP). It reveals that, although corrosion management has improved over the past several decades, the U.S. must find more and better ways to encourage, support and implement optimal corrosion control practices.

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The total annual direct cost of corrosion for production and manufacturing is estimated to be $ 17.6 billion.

Oil and gas exploration and production 8% ($1.4 billion)

Mining 1% ($0.1 billion)

Petroleum refining 21% ($3.7 billion)

Home appliances 8% ($1.5 billion)

Chemical petrochemical pharmaceutical 10% ($1,7 billion)

Food processing 12% ($2.1 billion)

D MAC 1391 A

Agricultural 6% ($1.1 billion)

Pulp and paper 34% ($6 billion)

Annual cost of corrosion in the production and manufacturing category

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REVIEW OF CHEMISTRY AND ELECTROCHEMISTRY • Molecule’s structure The molecule is a structure composed of atoms, and the internal bonds are electrical in nature. The atoms of an element may be united to form a molecule by setting in common the 2 electrons (case of H2: H – H, 02: O = O).

+ Nucleus Electron

Repulsion force Attraction force Schematic representation of the Bohr model

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+

3

The atoms of different elements may be united under the same conditions (case of H2O, CO2). • Ions, anions, and cations An atom may lose one or more electrons from its peripheral layer. The result is one positive ion or cation. This positive ion may seize one electron and make an atom: +

M
M +e An atom can gain one or more electrons. The result is one negative ion or anion. As above, the anion may lose its additional electron and make an atom: A–

A + e • Ionization

Water is partially dissociated into ions according to the following equilibrium: H+ + OH–

H2O +

-

In pure water there are as many H ions as OH ions so that the charge is zero: [H+] = [OH–] = 10–7 moles per liter An environment is said to be acid when [H+] is greater than 10-7 in a solution. Otherwise, the environment is said to be basic. Definition of pH:

pH = Log 1/[H+] = - log [H+ ]

For neutral water, pH = - log [10-7] = 7 • Crystals and ionic solutions A salt crystal like sodium chloride is a structure resulting from the superposition of polyhedrons at the + atomic scale (cubes in the case of NaCl) at the summits of which are placed the Na and Cl ions in equal numbers and regularly distributed so that the crystal is electrically neutral. Cohesion is assured by electrostatic attraction forces.

0,558 nm Cl-

D MAC 2093 B

Na+

Diameter of Na+ = 0.196 nm Diameter of Cl- = 0.362 nm 02947_A_A

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If a sodium chloride crystal is put in water, it dissolves. In fact, the water molecule presents an electric dissymmetry (positive pole on the hydrogen atoms side and a negative pole on the oxygen atoms side).

O -

+

+

H

H

D MAC 2116 A

-

The Na+ ions of the crystal attract the water molecules by their negative side, while the Cl– ions attract the water molecules by their positive side. Thus, the water molecules reduce the electrostatic forces. The crystal is dislocated and the ions are free to move between the water molecule. A dissociation takes place: Na+ + Cl–

NaCl • Solid state metal structure

A metal is a compact structure of atoms composed of ions in an electron cloud. Some of the electrons are relatively free in mass and allow electric conductivity and heat conductivity.

FR

+

FA

+

FR

+

+

+

+

+

+

+

D MAC 2094 B

Electrons

Positive ions Schematic representation of the forces intervening in the metal bond: FR = repulsion force of the positive ions; FA = attraction force between the positive ions and the electron cloud. • Oxido-reduction phenomena Earlier, oxidation was defined as the fixation of oxygen on a chemical system and the reduction as an inverse phenomenon. Today this concept is generalized and oxidation corresponds to a loss of electrons, while reduction correspond to a gain of electrons. Fe

Oxidation

Fe++ + 2e

Reduction

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A system is said to be an oxidant when it may capture electrons. Example: oxygen in water: O2 + 2 H 2O + 4 e
4 OH

-

A system is said to be a reducer when it may release electrons. +

Example: sodium in water: Na  Na + e • Behavior of a metal plate in pure water (deaerated or without oxygen) At the plate’s surface the metal atoms lose electrons and transit from the metal phase to the liquid phase in the form of positively charged ions. The metal’s electric equilibrium is broken because the metal which contains an excess of electrons becomes negative, while a cloud of “positive ions” forms in the solution in the neighborhood of the metal surface. The electrostatic attraction forces between these charges of opposite signs hold the positive ions and the electrons formed in the immediate vicinity of the metal surface and the thus-created double layer generates an electric field of a constant value. The double electric layer formed at the metal’s surface corresponds to a potential difference between the metal and the solution (in the immediate vicinity of the metal or interface) and is denoted the
electrode potential”, which depends particularly on the concentration of metal ions in the solution. The “metal plate + water” system constitutes an electrode.

Layer of dipolar water molecules eeeeeeeeee-

+ + Solvated cations

+ + +

+

Helmholtz layer

+ + + + + +

Equivalent capacitor D MAC 2095 B

-

Schematic representation of the metal / pure water interface or electrode with equivalent electric capacitor 02947_A_A

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• Behavior of a metal plate in a solution of one of its salts (this is another electrode) If a metal place is placed in a solution of one of its salts (case of copper in a copper sulfate solution), the electrode potential depends on the concentration of metal ions in the solution.

Electrolyte H2O + CuSO4 Cu2+

2SO4

Cu2+

2SO4

Cu2+

Cu2+

Cu2+ Cu2+

Cu2+ Cu2+ Cu2+

Cu2+

Cu2+ Cu2+

Cu2+ Cu2+

e

Cu2+ Cu2+

e e

Cu2+ Cu2+

e e

Cu2+

δ

Cu2+

e e

e

e

e

D MAC 2096 D

e

Metal Cu

e

Schematic representation of a reversible copper electrode plunged in a copper sulfate solution; k = Helmholtz layer, compact and adsorbed on surface

-

In the general case where the concentration is [C] and n . e is the ion charge, Nernst has demonstrated that the electrode potential was equal to:

RT E = Eo + nF log [C]

R: T: n: C: F:

perfect gas constant absolute temperature number of electrons exchanged electrochemical reaction ion concentration in solution Faraday’s constant -3

in

the

This simplified formula is valid only for diluted solutions (concentration less than 10 moles per liter).

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In a normal solution (concentration equal to 1 mole of ions per liter), each metal assumes a particular potential, also called a standard or normal potential (E0).

Standard equilibrium potential E0 with respect to the hydrogen electrode at 25° C (V)

Reaction

Au 2 H2O Pt Ag 4 OH–

Au3+ + 3 e

+ 1.50

O2 + 4 H+ + 4 e

+ 1.23

Pt2+ + 2 e

+ 1.19

Ag+ + e

+ 0.80

O2 + 2 H2O + 4 e

+ 0.40

Cu

Cu2+ + 2 e

+ 0.34

H2

2 H+ + 2 e

0

Pb

Pb2 + 2 e

– 0.13

Sn

Sn2+ + 2 e

– 0.14

Ni

Ni2+ + 2 e

– 0.25

Cd

Cd2+ + 2 e

– 0.40

Fe

Fe2+ + 2 e

– 0.44

Cr

Cr3+ + 3 e

– 0.74

Zn

Zn2+ 2 e

– 0.76

Al

Al3+ + 3 e

– 1.67

Mg

Mg2+ + 2 e

– 2.37

Na+ + e

– 2.71

Na

• Electrode potential measurement A direct measurement is difficult because a potential tap is necessary in the liquid in the immediate vicinity of the metal. We prefer measuring the potential difference existing between the electrode studied in a laboratory and the so-called hydrogen electrode whose potential is equal to 0 under well defined temperature and pressure conditions. Practically speaking, reference electrodes are used associating under well defined conditions a metal and a salt at a fixed concentration (see figure on page 9).

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The table below gives the correspondences between the diverse reference electrodes.

Electrode

Electrolyte

Reactions

E/V

Calomel

KCl, saturated KCl, 1 M KCl, 0,1 M

Hg2Cl2 + 2e– = 2 Hg + 2 Cl–

Mercury sulfate

K2SO4 saturated

HgSO4 + 2e– = Hg + SO4

0.658

Silver chloride

KCl, saturated

AgCl + e– = Ag + Cl–

0.195

Copper sulfate

CuSO4, saturated

CuSO4 + 2e– = Cu + SO4

0.316

Hydrogen

H2SO4

2 H+ + 2e– = H2

0.000

2–

2–

0.241 0.280 0.333

In the case of the iron electrode, the metal in contact with the deaerated pure water containing the ferrous ions undergoes the following process: Fe

Fe++ + 2 e–

Nernst law applies and specifies the potential’s value: E = - 0.44 + 0.029 log [Fe++ ] which in an E, pH diagram does not depend on the pH, but on the concentration of Fe++ ions. The straight-line D corresponds to the solubility limit of the ferrous ions vs pH (see page 10).

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REFERENCE ELECTRODES Filling orifice

Saturated KCl solution

CALOMEL Hg 2 Cl 2 + 2e2Hg+ Cl-

Electrode body

Mercury (Hg) Calomel (Hg 2 Cl 2 ) Plug impregnated with saturated KCl SILVER CHLORIDE Ag Cl + e - Ag + Cl -

Excess KCl crystals Porous cap CU/COPPER SULFATE Cu 2 + + 2eCu

Araldite Screw

PVC

Rubber washer Plastic cap

Cable

Plastic pad PVC

Copper rod PVC Transparent plastic cartridge Excess copper sulfate crystals Silver wire AgCl gel Tender pine stopper

D MAC 1276 B

Fritted glass

Plug 02947_A_A

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Evolts 0.2

0

- 0.2 H2

D - 0.4 - 0.44

Fe

Fe++ + 2e

Fe++ = - 0.8

1 10-2 10-4 10-6

0

2

4

6

8

10

Iron electrode potential vs. pH with respect to the hydrogen electrode Fe/Fe

12

PH

D MAC 2097 B

- 0.8

2H + +2 e

++

In any electrolyte, any material in equilibrium assumes a potential called the dissolution potential. The table below gives the dissolution potential values for diverse materials in a chlorinated solution. Dissolution potential of diverse metals with respect to the saturated calomel electrode at 20° C in an aqueous NaCl 3% solution. Metals Platinum Gold Passive chromium Stainless steel (18 - 8) Mercury Silver Copper Hydrogen Nickel Tin Lead Active chrome Iron Al-Cu alloy Aluminum Cadmium Al-Mg alloy Zinc Magnesium

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Potential in Volt/ECS at 20°C + 0.30 + 0.22 + 0.20 to + 0.25 + 0.10 0 – 0.05 – 0.18 – 0.25 – 0.27 – 0.44 – 0.47 – 0.60 – 0.60 to – 0.70 – 0.60 to – 0.65 – 0.74 – 0.78 – 0.77 to – 0.80 – 1.06 – 1.63

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PRIMARY PHENOMENA OF WET CORROSION • Cathodic electrochemical reactions Let’s return to our example of the iron electrode in contact with a solution of ferrous salt having a ++ concentration [Fe ]. The following equilibrium results: Fe++ + 2 e–

Fe leading to a potential:

E = Eo + 0.029 log [Fe++] Let’s imagine now that the liquid’s potential is held fixed (constant concentration of ions in liquid) and by means of a suitable apparatus (DC generator connected to the metal and a reference electrode plunged in the liquid) the metal’s potential is artificially decreased (metal supplied with electrons). The system is going to alter itself to try to attain the value of the new lower potential. Since there exists an electric field between the solution and the metal, the positive ions Fe++ are attracted to the metal and retransit to the atom’s state according to the reaction: Fe++ + 2 e–

Fe

The Fe++ ions have been reduced (gain in electrons) and the metal is said to have been rendered cathodic.

R e e-

i Ea

Ec

e-

Fe++

EHN EHN Fe++ Electrolyte

D MAC 2098 E

Cathode Fe

Anode

Fe++

Principle of the apparatus allowing to measure the anodic and cathodic polarization curves (NHE: Normal Hydrogen Electrode) Therefore, on a cathode, the reaction which occurs is a reduction. Example of other cathodic reactions: -

+

-

in a deaerated environment: 2 e + 2 H
H2

-

in an aerated acid environment: 4 e + O2 + 4 H
2 H2O

-

in an aerated neutral and alkaline environment: 4 e + O2 + 2 H2O
4 OH

-

metal depositing: 2 e + Cu
Cu

-

+

-

-

-

2+

+

H (solvated proton) and O2 (dissolved oxygen) are the 2 main oxidants. They are reduced by gaining electrons. 02947_A_A

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• Anodic electrochemical reactions On the other hand, if the metal’s potential is increased (extraction of electrons from the metal) with respect to the solution, the created electric field goes from the metal to the solution and the already ++ existing Fe ions are repulsed and new iron atoms will be ionized and pass into solution according to the reaction: ++

Fe
Fe + 2 e

-

The metal passes into solution. The iron is said to undergo oxidation or corrosion (loss of electrons which remain in the metal). The metal is anodic.

e-

R i Ec

e-

Fe++

Corrosion

Anode Fe

Cathode EHN EHN Fe++ Electrolyte

Fe++

D MAC 2098 F

Ea

e-

The e– are “pumped” from the anode to the cathode
the Fe potential is increased and iron corrosion results

Therefore, on an anode, the reaction which occurs is oxidation. Examples of other anodic reactions:

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-

Zn

Zn2+ + 2 e

-

Al

Al3+ + 3 e

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• Wet corrosion principles An examination of the previous experiments allows the following principles to be disclosed: an electrochemical corrosion can only occur when the two electrode processes (partial reactions) are carried out at the same time, that is, one supplies the electrons (partial oxidation or anodic reaction), while the other consumes them (partial reduction or cathodic reaction).

Reduction of the oxidants Reduction Oxidation

cathodic reactions area

e-

anodic Oxidation = corrosion reactions area Uniform

D MAC 1196 B

-

Localized

Fe




Fe2+ + 2 e

anodic

2 H+ + 2 e




H2

cathodic

———————————————— Fe + 2 H+




Fe2+ + H2

global reaction

The partial anodic and cathodic reactions will explicitly make appear the electrons unchanged during the oxido-reduction reaction, contrary to the global reaction. Therefore, an electrochemical reaction is defined to be a chemical transformation which implicates a transfer of charges at the interface between an electronic conductor, called an electrode, and an ionic conductor, called an electrolyte. For any metal M with n valence electrons, the following anodic reaction may be written: M
Mn+ + ne For iron, Fe
Fe++ + 2 e (1) According to the environment in which the corrosion occurs, that is, according to the + electrolyte type, its concentration in H ions (or pH) and its content in dissolved oxygen, several cathodic reactions may take place, and here are the main ones.

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+

Deaerated acid environment (pH < 7): 2 H + 2 e
H2

Electrolyte HCl + H2O H+

H+

Cl-

Cl-

Cl-

Cl-

H+ H2 H+

Cl-

Fe2+ H+

H+ H+ Cathode

Anode D MAC 2096 E

e e

Méetal Fe

Dissolution of the iron in a deaerated solution of hydrochloric acid. The sites of the anodic and cathodic reactions continually change so that the iron dissolution is uniformly carried out. The chlorine ions Cl do not participate in the reaction. Acid environment (pH < 7) containing the dissolved oxygen: O2 + 4 H+ + 4 e
2 H2O Neutral or basic environment (pH > 7) containing the dissolved oxygen: O2 + 2 H2O + 4 e
4 OH Reduction of metal ions: Mn+ + n e
M

Electrolyte HCl + H2O + O2 H+

Cl-

ClO2

ClH2O Cathode

Cl-

H2O H+

O2

H+

H+ H+ H+ H+

Fe2+

Fe2+

Anode e

e e

D MAC 2096 G

e Metal Fe

Corrosion of iron in hydrochloric acid containing dissolved oxygen 02947_A_A

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In an aerated and neutral environment (for example, in salt water), the iron corrosion product is rust Fe (OH3). The anodic reaction corresponds to equation (1) and the cathodic reaction to equation (2). The products of both these reactions then react together to yield ferrous hydroxide, that is: 2 Fe2+ + 4 OH–
2 Fe (OH)2 The ferrous hydroxide precipitates, but since it is unstable, it oxidizes into ferric hydroxide, commonly called rust, and: 1 2 Fe (OH)2 + H21O + 2 O2
2 Fe (OH)3 -

other oxidants Other oxidants may also corrode the metals, such as: •

oxidizing metal cations: Cu2+, Fe3+, Sn4+

•

oxidizing anions: NO2–, NO3–, CrO4 , MnO4–, OCl–

•

dissolved oxidizing gases: O3, Cl2, SO3

2–

At high temperatures, some chemical substances normally inoffensive become corrosive. Among the oxidants responsible for dry corrosions here are a few: • • •

gaseous oxygen steam (water vapor) carbon anhydride, CO2

•

sulfur compounds: S2, SO2, SO4

2–

-

in the case of an electrochemical corrosion, there is always a dissolution of metal at the anode, while the cathodes are protected against any attack (deposit of metal or release of hydrogen, or formation of OH- ions in an aerated neutral environment);

-

in pure water free of oxygen or deaerated, the only metals liable to be destroyed by electrochemical corrosion are those whose dissolution potentials are less than the hydrogen electrode potential;

-

in aerated pure water, the metals whose dissolution potentials are less than the oxygen electrode potential are corroded (this is the case for copper, which does not corrode in a deaerated environment, but does corrode in an aerated environment).

• Center of cathodic reactions The diverse reduction reactions occur on cathodic areas which are composed of:

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any more noble metal, electrically coupled with that which is corroded, therefore, having a higher dissolution potential (which is what occurs in the case of galvanic corrosion)

-

any oxide located on a metal’s surface coming from metallurgical elaboration processes. The oxides generally have more noble potentials than those of the corresponding metal

-

in the case of cast irons, graphite has a more noble potential with respect to iron. Therefore, a dissolution of iron occurs and is known under the name graphitization of the cast irons

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SECONDARY PHENOMENA OF WET CORROSION • Description of these phenomena We have seen that corrosion starts by the transition of the metal into an ion state in the conductor environment in which it bathes. But then, the ions migrate in the solution, and react with the other ions already present in the environment. Thus, in water containing dissolved hydrogen sulfide, we have at average pHs the formation of an iron sulfur precipitate. In aerated water, we have the formation of hydrated ferric oxide depositing as a precipitate, etc. By considering the solubility products, these secondary reactions are of major importance because the salt deposit thus formed being more or less porous governs the intensity of ionic exchange phenomena. • Possibilities of these secondary reactions In the case of a fundamental aggressive environment, such as water, the possibilities of secondary reactions define in a pH potential diagram a certain number of areas. Under some conditions, corrosion phenomena may occur, while under other conditions, the primary corrosion reactions are more or less inhibited. Thus, for the iron-water system at 25°C, Pourbaix was able to define an equilibrium diagram in which the limits separate the regions where diverse reactions may occur. From a purely thermodynamic approach, these calculations could be worked out. They do not presume the reactional rate at which the diverse phenomena occur. -1

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16 2,2 2

2

1,8

1,8

Fe+++

1,6

02 + 4H+ + 4 e-

2 H2O

1,6

(b)

1,4

1,4 0

1,2

-2

-4

-6

1,2 1

1 O2

0,8 0,6

O2 + 2H 2 O + 4e -

H2O

0,4

0,8 0,6

4 OH -

0,4

CORROSION

0,2

(a)

0

2 H+

- 0,2

Fe + 2e -

0,2

++

PASSIVATION 0

H2

Fe(OH)3

H2 O

- 0,2

H2

- 0,4

- 0,4 Fe (OH)2

- 0,6

-6

0 -2 -4 -6

- 0,8

2 H2O + 2e-

-1

-4

- 0,6

HFeO2

- 0,8

H2 + 2 OH-

-1

Fe

- 1,2

- 1,2

IMMUNITY

- 1,4

- 1,4 - 1,6

- 1,6 - 1,8 -2

-1

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

- 1,8 16 pH

Electrochemical equilibrium diagram E, pH of the iron – water system at 25°C (Only Fe, Fe(OH)2 and Fe(OH)3 are considered to be solid bodies) 02947_A_A

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D MAC 2091 B

E(v) -2 2,2

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The equilibrium diagram thus constructed delimits the corrosion and immunity domains of the environment. The hydroxide precipitation domain is called passivation by assuming the precipitates are protectors, which is not always the case. Therefore, we see that a metal may be protected: -

by decreasing the metal’s potential below – 0.62 V; this is the cathodic protection by increasing the pH; this is the neutralization of the acid waters by adding oxidants at a rather high pH; this is the addition of inhibitors (oxidants)

In the practical usage of such a diagram, do not forget that the position of the equilibrium straight lines change at the same time as the concentration in Fe++ ions.

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EFFECTS OF SECONDARY REACTIONS ON THE KINEMATICS OF WET CORROSION • Coupling of two different metals Each metal isolated from the other and plunged in an electrolyte assumes a certain potential with respect to the solution.

−

ν

+

Cu D MAC 2098 C

Fe

R=∞ ν = ECu - EFe i = 0 When the samples are electrically connected, the one with the more negative charge will tend to become more positive (it is the anode), while the other receiving electrons will tend to become more negative (it is the cathode).

i

Α

e-

Fe

Cu

ANODE

CATHODE

R=0 i = (icorr.) Fe - Cu 02947_A_A

© 2009 - IFP Training

D MAC 2098 D

e-

18

The anode’s potential is therefore displaced toward more positive values and the cathode’s potential toward more negative values. The anodic reaction corresponds to a loss of metal schematized as follows: Mn+ + ne–

M

The electron flow corresponds to an anodic current ia. Like any electrochemical reaction, this reaction accepts an equilibrium potential Ea for which ia = 0. The cathodic reaction may be schematized as follows: oxidant + ne–

reducer

The arrival of electrons via the conductor on the cathodic surface corresponds to a current Ic. At equilibrium, the equilibrium potential Ec corresponds to ic = 0. The cathodic current is considered negative by definition. The curves E = f (ia) and E = f (ic) are called the anode and cathode polarization curves, respectively.
a corresponds to the over potential or anodic polarization.
c corresponds to the over potential or cathodic polarization.

E

Oxidant + ne-

reducer

Ec

Cathodic ηc polarization

M

Mn+ + neηa

Anodic polarization

D MAC 2117 B

EA

iC Reduction at the cathode

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Ι iA Oxidation at the anode

19

• Evans Diagram Evans proposed a graphic representation of the evolution of the anodic and cathodic potentials in which the cathodic curve is folded back on the straight section around the ordinate i = 0.

Eoc

ioc ηc

- Potential + +

Red

uctio

n

Ecorr. Eoa

ηa

ioa i

Log i

i corr.

D MAC 2099 C

ation

Oxid

Electrode polarization curves: anode and cathode polarization On the x-axis (abscissas), the current intensity is represented logarithmically

By means of this graphic construction, it is possible to determine by extending the anodic and cathodic curves the point of intersection which allows determining the dissolution potential or average corrosion Ecorr assumed by the two metals and corresponding to a corrosion current Icorr, such that: ioa = - ioc = icorr This corrosion current allows determining the corrosion rate of metal at the anode by the Faraday law:

M m =  F . ia . t

where

m: corroded metal’s weight in g M: metal’s atomic weight in g : metal’s valence ia: corrosion current in amperes T: time in seconds

Therefore, we see that the corrosion rate may be immediately deducted from the knowledge obtained from the individual polarization curves. Application If a metal is coupled to another less
noble
metal in a conductor environment or electrolyte, the second constitutes the anode of the battery and the first the cathode. This is the principle of the cathodic protection method by a reactive anode (iron protected by zinc or magnesium). • Case of a composite metal This is the general case which occurs when the same metal is placed in an electrolyte. It is not possible to plot the Evans diagram since the position of the anodes and cathodes is not known and thus the value of the current flowing between the electrodes cannot be measured. However, the graph given in the case of a coupling between two metals assumed to be ideally pure remains valid and only the measurement Ecc is possible. But another method must be used to determine the intensity of the corrosion current. 02947_A_A

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20

The most currently used technique consists of plotting the curve: E = f (i) or E = f (logi) where E is the measured potential and i the current supplied by an external source.

galvanometer

ν

DC current source ν = RI Ι

E

REF CE

D MAC 2100 B

WE

Measurement set-up of a galvanostatic polarization curve; WE: working electrode, REF: reference electrode; CE: counter-electrode This curve is called the
global polarization curve
.

E

X

Ec C

=

=

E"

A

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Ea

ic = icorr.

ia = icorr.

© 2009 - IFP Training

+i

Ι

D MAC 2101 A

-i

E'

Ecorr

21

Under these conditions, at any moment the global current I will be equal to the algebraic (signed) sum of the anodic current and the cathodic current: I = ia + ic In particular, when I =0, that is, when the global polarization curve intersects the potentials axis, then: ia = – ic = icorr Therefore, we obtain the corrosion conditions without any external current and, therefore, the corrosion potential Ecorr. Special case: When either/both of the reactions is/are governed by an activation polarization, the individual polarization curves will be easily determined. In this case, we obtain the straight-lines of the diagram E = f (log I) called TAFEL straight-lines.

E

X

Ec C

E"

A

02947_A_A

Y

E'

Ea

ic = icorr.

ia = icorr.

© 2009 - IFP Training

Log i

Ι

D MAC 2101 C

TAFEL straightline

Log - i

Ecorr

TAFEL straightline

22

io, M+/H2

eH+/H2

l Tafe

POTENTIAL

(Ec)

area

Ecorr.

(-)

eH/H+

Icorr.

io, M/M+

Tafe l

Applied current curves

area D MAC 2102 B

(+)

We immediately obtain the corrosion current icorr from the abscissa (x-axis value) from either of these straight-lines corresponding the corrosion potential. Practically speaking, we prefer to use the straightline of the cathodic reaction with the corrosion potential to determine the corrosion current icorr.

Anodic Cathodic

(Ea)

log iapp. Polarization curve of a metal in an acid environment

Application Cathodic protection by imposed current. As long as the potential imposed at the electrode is between Ecorr and Ea, the electrode, even though transported toward a more cathodic potential, is still the center of an anodic reaction. To protect the sample by an imposed current, it should be lowered to a more negative potential than the potential Ea. • Activation polarization The electrochemical reactions do not instantaneously occur; their kinetics depend, in fact, on certain intermediary reactions which may momentarily cause one of the products to pass, or one of the reactants via an activated state (a higher energy state). Let’s take, for example, the reduction reaction of hydrogen ions which are accompanied by a release of gaseous hydrogen: 2 H+ + 2 e

H2

and let’s examine all the phases which allow the H+ ions of the electrolyte to react in this way. In order to be absorbed at the cathode’s surface, the H+ ion must, first, be combined with an electron. Thus, H+ + e

Hads

This first phase is relatively rapid; however, for the reaction to be completed, it is necessary that two atoms adsorbed on the surface be sufficiently close to one another to form a hydrogen molecule, that is: 2 Hads

H2

Finally, several molecules must be grouped together to form a bubble with a sufficiently large size to allow it to escape from the cathode’s surface.

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23

In all these phases, this third reaction represents the slowest reaction and, as a result, governs the global kinetics of the reaction corresponding to the first equation. The reaction’s global kinetics is also a function of the electrode’s metal, the electrolyte and the temperature. The anodic reactions may also be the center of an activation polarization. By escaping from the metal’s crystalline structure, the metal ions transit through an activated state and this reaction may only occur at a finite rate. In any case (whether anodic or cathodic reactions), activation polarization is evidenced by an overvoltage, , determined by the Tafel law as previously seen and repeated below.

ioc

Eoc

Red

uctio

n

Ecorr.

ation

Eoa

ηa

Oxid

D MAC 2099 D

- Potential +

ηc

ioa i

Log i

i corr.

Electrode polarization curves: anode and cathode polarization On the x-axis (abscissas), the current intensity is represented logarithmically

For currents greater than the threshold values i0c at the cathode and i0a at the anode, the cathode’s potential decreases from its equilibrium value (open circuit), Eoc, while the anode’s potential increases from Eoc . Thus, for a current I, a cathodic overvoltage c is generated and an anodic overvoltage a. The variation in the anodic or cathodic overvoltages  is governed by the Tafel law for values of i greater than i0. Thus, i  =
log i 0 where
represents the slope of the polarization curves (generally expressed in volt per decade), E = f (i), when i is represented on a logarithmic scale. In anodic reactions (oxidation),
is positive, while in cathodic reactions (reduction),
is negative. The values of
depend on the electrolyte’s characteristics as well as the temperature. The current i0 is called the exchange current (or exchange curren density if reasoned in terms of surface unit). The importance of these curves is capital because the higher the absolute values of
are, the higher the corrosion current and, as a result, the lower the corrosion rates.

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24

• Diffusion polarization At the anode the release of positive ions leads to an increase in the concentration of these ions in the electrolyte; the rate at which they enter the solution is therefore limited by the diffusion rate of the ions outside of the diffusion layer. This diffusion rate determines the intensity of the limit current ilim. Diffusion polarization (also called concentration polarization) also occurs in some cathodic reactions (cathodic reduction of oxygen, for example). In this case, the limit current’s value is a function of the concentration in oxygen [O2] of the electrolyte; the reaction’s rate is limited by the diffusion of the oxygen molecules toward the cathode’s surface. When it is added to the activation overvoltage, the effect of the diffusion overvoltage is to
bend
the Tafel straightlines. These bent straightlines tend toward a limit current whose value will be that much higher as the concentration in O2 is higher in the electrolyte.

Tafel straightlines

4 OH-

[O2]1 < [O2]2

[O2]1

[O2]2

(ilim)1

(ilim)2

D MAC 2103 B

-

Potential +

O2 + H2O + 4e

log i Schematic representation of diffusion polarization for the reduction of oxygen for two oxygen concentrations [O2]; when [O2]1 < [O2]2 (ilim)1 < (ilim)2

• Passivation – Description of the phenomenon When they are subjected to corrosion, some metals and alloys adopt a behavior quite different from that described up to now; they become passive; their corrosion rate is thus generally less than 0.001 mm/a. A simple experiment performed by Faraday around the year 1835 focused on this phenomenon: an iron sample plunged in concentrated nitric acid is subjected to a dissolution accompanied by a release of hydrogen.

H 2O

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Fe

Fe

Fe

(a) HNO3 concentred

(b) HNO3 diluted

(c) HNO3 diluted

© 2009 - IFP Training

D MAC 2115 B

Sample scraped or immersed in diluted acid.

25

In (a) this reaction very rapidly stops and the corrosion rate becomes then almost zero. In (b) if the acid in which the iron sample is plunged is subsequently diluted with water, the iron remains inert. In (c), however, when its surface is scraped, a violent release of hydrogen occurs at this spot, which does not stop. In addition, a non-passivated iron sample immersed directly in diluted acid is attacked and the reaction does not stop. When in concentrated acid the corrosion current attains a high value, a passive layer 2+ forms on the metal’s surface, which literally inhibits the Fe ions from entering the solution. If the thus-passivated iron is then plunged in diluted acid, as long as this passivation layer remains intact, the iron will not be attacked; however, when this layer is destroyed, it may no longer be spontaneously reformed, contrary to like it did in the concentrated acid. Finally, a non-passivated iron sample plunged in diluted acid remains active and is rapidly corroded, and the reaction is accompanied by a violent release of hydrogen. In this example, the ratio between the corrosion currents in the active state and in the passive state is 4 on the order of 5 x 10 . Therefore, we see the importance which passivable metals and alloys represent for all usages where corrosion poses a problem. Iron is passivable only under very special conditions. The main passivable metals and alloys are stainless steels, nickel and chromium alloys (Inconel), cobalt and chromium alloys (Stellite), as well as titanium and its alloys, aluminum, tantalum, etc. • Nature of the passive layer The exact nature of the passive layers is still relatively not well known. Studies rendered possible, among others, thanks to Auger spectrometry (surface analysis of the first atomic layers) allow supposing that the passive layers contain oxygen in the form of oxide or hydroxide. The following reaction may, therefore, occur at the cathode: M + H2O

M (OH) + H+ + e -

According to this reaction, there will occur on the metal’s surface an adsorption of ions (OH ) and then the formation of an adsorption compound containing the metal ions. The passive layers are very thin (on the order of 2 to 3 nanometers), but they constitute an effective barrier which opposes the passage of metal ions from the metal to the electrolyte; thus, they considerably slow down the corrosion rate. The passivity of stainless steels, chromium alloys, nickel alloys and cobalt alloys is due mainly to the formation of an adsorption compound. On the other hand, the passivity of titanium, niobium and tantalum is attributable to the formation of an oxide layer much thicker than that of the adsorption compounds. • Corrosion of passivable alloys The figure below schematically illustrates the anodic bias curve of a passivable alloy. When the corrosion potential is greater than E 0a, but less than EF (the Flade potential), the current-potential relationship obeys the Tafel law and the corrosion current is controlled by anodic and cathodic overvoltages. The potential EF corresponds to the critical current icrit. For a potential greater than EF, the corrosion current amounts to ip (corrosion current at the passive state); from EF and Et this current value remains constant. When the corrosion potential exceeds Et, the alloy is in the transpassive state and the corrosion current increases again: as a result, the passive film breaks. 02947_A_A

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26

Transpassive state

Potential

+

Et

-

Passive state f EF

Eoa

Active state

D MAC 2104 B

+ + ne n M M

log i Anodic polarization curve of a passivable alloy

Let’s now examine how a passivable alloy (stainless steel, for example) behaves under diverse corrosion conditions. The figure below shows the polarization curves of a passivable alloy in three different oxidizing environments. The cathodic reactions are the same in the three cases; only the oxidizing power of each of the solutions varies. When the oxidizing power is high (cathodic bias curve C1), the corrosion potential is less than Et (point A); the passivity is stable. When the oxidizing power of the electrolyte is higher than that represented by the cathodic bias curve C1, the corrosion potential attains a value higher than Et. At the transpassive state, the passive film dissolves faster than it is formed; under these conditions, the corrosion is a pitting corrosion.

Potentiel +

ET

A C1 B C2

D

EF

C3 D MAC 2105 A

E F log i

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27

When the oxidation power is low (curve C 3), the corrosion potential attains a value represented by point F; the corrosion is then major but uniform. Under these conditions, it is obvious that passivation will not prevail and the anode will undergo corrosion. On the other hand, when the oxidizing power of the electrolyte is average (curve C2), the passivity is unstable: as long as the passive film is intact, the metal will remain in the passive state (point B) and the corrosion rate will be low. However, if the passive film is broken, it cannot be reformed; the corrosion potential which corresponds to this situation is represented by point E. When the corrosion potential is equal to EF (point D), the passivity is unstable.

Potential

(+)

Such a situation is extremely dangerous. In fact, accidentally depassivated areas will be corroded very rapidly, leading to pitting.

D MAC 2106 B

(-)

Increase T, (H+)

log i Effect of an increase in temperature and acid concentration on passivity

+ + ne Mn

(-)

Epass

Ecorr. 1 Ecorr. a

O 2 +2 H2 O + 4e 4OH -

Ipass

2H 2O + 2

e

H 2 + 2O H-

1 Icorr.a

2

ΙL 3

4

5

6 log i

Effect of deaeration, aeration, agitation on stainless steel corrosion (active or passive) in neutral sea water

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D MAC 2107 A

POTENTIAL

(+)

M

28

PotentialE(V) ECS

1.6

0.8

Fe Fe -10.5 Cr

0.4 - 0.0 - 0.4 - 0.6 10-1

1

10

102 103 104 105 Current density (μA/cm2)

ACTIF

D MAC 2108 B

Potential, E(V) ECS

1.2

0.8 0.6 (3)

0.4

(2) (1)

0.2 0 D MAC 2109 B

NOBLE

1.0

- 0.2 - 0.4 - 0.4

106

100

1 000

10 000

Current density (μA/cm2)

Effect of diverse castings and diverse samples of the same casting on the anodic polarization curve of a stainless steel 304 in H2SO4 1 N, deaerated, at 25° C

Anodic polarization of pure iron and an iron alloy with 10.5 % chromium

0.9 0.8

1 N H2SO4 1 N H2SO4 + IM NaCl

NOBLE 10

100

1 000

10 000 Current density (μA/cm2)

0.6 0.5 0.4 0.3

304 L stainless steel Hastelloy Alloy C

0.2 0.1

Pitting

0 - 0.1

D MAC 2111 B

Potential E(V) ECS

Hastelloy C-276 Hastelloy C Hastelloy B

Anodic polarization of nickel alloys in H2SO4 1 N at room temperature

02947_A_A

1.0

0.7

ACTIF

1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 - 0.1 - 0.2 - 0.3 1.0

D MAC 2110 B

ACTIF

Potential E(V) ECS

NOBLE

1.1

- 0.2 - 0.3 - 0.4 - 0.5

1

10

102

103

104

Current density (μA/cm2)

Effect of chlorides on the 304 L and a nickel alloy HASTELLOY ALLOY C in sulfuric acid

© 2009 - IFP Training

29

7-

DRY CORROSION • Dry corrosion (oxidation at high temperatures: T > 260°C) The dry corrosion of metals is a reaction between them and the gases which surround them. This reaction is oxidation. In fact, it occurs most often at the metal’s surface, a compound which leads to an effective loss and a degradation of the metal. This kind of corrosion leads particularly to in-service strength problems at temperatures clearly higher than room temperature. In most cases, the oxygen in the air combines with the metal’s atoms to form oxides. In sulfurous oxidizing atmospheres (SO2), on the other hand, the corrosion products are sulfurs. Finally, the attack may also be due to halogens (Cl, Br and I) and combustion environments (CO, CO2, H2O, etc.). • Oxide layer formation process The oxide layer formation process is a four-phase process schematically illustrated in the figure below. First, oxygen is chemically adsorbed on the metal’s surface. This adsorption leads to the formation of bonds between the oxygen and the metal ions which are generally ionic bonds (fig. a). This phase is favored due to the fact that on the surface the metal bonds of the atoms are not saturated; this is why these atoms exhibit a strong reactivity. After a certain incubation period, there is the germination of the oxide on the metal’s surface (fig. b), and then a lateral growth of the germs, which completely covers the metal’s surface with a film of oxide (fig. c). The uniform growth of the oxide then continues perpendicular to the metal’s surface (fig. d). This last phase is the phase which determines the oxidation kinetics since the oxide layer’s thickness increases to the detriment of the metal’s.

Chemical adsorption of oxygen

Oxide germination

(b)

Lateral growth of the germs and saturation of the surface

Uniform growth of the oxide

D MAC 2112 B

(a)

(d)

(c) Metal atom

Metal ion (cation)

Formation of an oxide layer on a metal’s surface

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Oxygen ion (anion)

30

The figure below shows what movements have to be made by the ions and electrons inside the oxide layer so that it may grow uniformly. For the oxide layer to grow, there must be a diffusion of the ions (cations or anions) and electrons across this layer. Therefore, the oxide must be an electric conductor (ionic conductor and electronic conductor). Since the oxides and the sulfides generally are ionic compounds, their electric conductivity remains always very low; they may, however, be assured by diverse types of faults, according to the compound’s stoichiometry. The stoichiometric ratio, R, of a compound of the general form MxOy is:
anions y = x R =
cations where x and y are the integers for a perfect stoichiometry.

O2

O2 O2

1 O + 2e 2 2

MO

O2

e

e O2

Oxide

M2+

Oxide

Metal

D MAC 2113 B

MO Metal

(Na ,M g, L i, N b) law ear

P

b a ra

oli c

la w

,C (F e

u at

hig

p h t em

e ra t u

re )

mperature r o om t e t a , u l, C ature F e, A te m p er la w ( h c g i i h i, a t it h m Cr, S gar

)

Lin Lo

D MAC 2114 B

Formed oxide masses

Oxide layer growth mechanism: a) Diffusion of metal ions (cations) and electrons to the oxide – gas interface; b) Diffusion of the oxygen ions (anions) to the metal – oxide interface

Time Metal oxidation kinematics

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31

8-

MEASUREMENT AND CONTROL - CORROSION TESTS The diversity of these tests does not allow developing them here. They are performed either in the laboratory or on the site or in a pilot factory. The corrosion rates are evaluated by the measurement of weight losses and by electric and electrochemical measurements (pitting tendency). Here are a few of the main tests: a - For stainless steels The sensitivity to intergranular corrosion may be evaluated by HUEY and MONYPENNY-STRAUSS tests. Huey Test: consists of exposing steel samples for 48 hours in an HNO3 solution (65%) at the boiling temperature. The sensitized sample becomes black and granular. The grains detach. The sample may be exposed in a liquid phase or in a vapor phase. Monypenny Test: a solution of SO4H2 – SO4Cu is used. b - Salty spray test The samples are subjected to the corrosive action of a sprayed solution. The vapors condense and the liquid drops drip down the samples. The environment may be aerated and the temperature is held constant. The pitting tendency in a chlorinated environment may be tested by this method. c - Stressed corrosion test The sample is generally turned on and insulated from the support by alumina joints which prevent galvanic corrosion. d - Corrosion tabs The reference boards are placed in a corrosive environment after polishing and weighing to within 1/10th of a mg. After an exposure of at least four weeks, the boards are removed for observation. The deposits are analyzed and then removed according to a cleaning method appropriate and carefully specified in the results. The weight after cleaning allows deducting the corrosion ratio. e - Measurement of average corrosion by electrical resistance A probe consisting of a composition filament close to the material subjected to corrosion is permanently plunged in the aggressive environment. During the read, an alternating current flows through the filament. The corrosion ratio is connected to the filament’s electrical resistance. f - Measurement of instantaneous corrosion by linear polarization (polarization resistance method) It involves measuring a current across two electrodes of the same composition as the facility’s metal. It was shown that a minor variation in the potential E on the order of 10 to 20 mV at the corrosion potential level on an electrode required the application of a current
I defined by the following equation:
l Icorrosion = K
E The electrodes must be carefully cleaned for each measurement so that no traces persist which could favor corrosion and falsify the results.

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g - Analysis of the corrosive environment For example, the quantity of iron is measured, but the initial contents have to be known before corrosion occurs.

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MEASUREMENT OF INSTANTANEOUS CORROSION BY POLARIZATION RESISTANCE CURVE DC current source

AUX

D MAC 1223 B

REF WE Potentiometer Reference electrode Auxiliary electrode

Working electrode

20

ΔEcorr (mv)

15

10

Slope =

ΔE

= R (ohms)

Iapplied

0 0

2

4

6

8

10

Applied current (ma)

Icorr. = K x

Iapplied K = R(ohms) E

Corr. rate (mpy) =

Icorr. A x K’


Area of corroding electrode (square inches)

02947_A_A

For iron:

0.46 mpy = 1
A/cm2

For Fe, Co, Ni alloys:

2000 corr. rate (mpy) = R(ohms) x A (square inches)

© 2009 - IFP Training

D MAC 1224 B

5

34

h - Measurement of hydrogen penetration in steels • Hydrogen created damages The hydrogen atom is one of the main products of corrosion reactions. Normally, these atoms H are transformed into molecules H2 at the steel’s surface, but it may happen, especially in the presence of 2-

3+

3+

catalysts such as S , As , or Sn that these H atoms penetrate into the steel. This causes serious problems in the presence of high stresses. The steel is embrittled and blistered by molecular hydrogen formed within the metal. Transfer environment Palladium foil (working electrode)

H

Interior side of canalization or device

Reaction to auxiliary electrode H

Auxiliary electrode H H

2H + 2e+

2H + 2e-

2H

H

H2

H

Reference electrode Electrolyte

H

Reaction to palladium foil sheet

D MAC 1273 B

Corrosion reaction Fe Fe2+ + 2eCathodic reaction + 2H + 2e2H

+

• Operating principle and description After the metal surface is cleaned, a product allowing the transfer of atoms (paraffin wax) and a palladium foil 0.25 mm thick are placed on the sheet metal. The electrochemical probe is, in turn, placed on the thin palladium foil. Tightness is assured by a pair of seals (Viton + Teflon). The palladium polarized by the instrument acts like a work electrode oxidizing the hydrogen which emerges from the metal wall: (2H
2H+ + 2 e) After a short build-up period, the electric current given by the device corresponds to the hydrogen’s penetration rate. The penetration rate may be characterized by a
diffusion pressure
of the gas across the material. The effect of an inhibitor is evidenced by an almost instantaneous drop in the diffusion pressure.

Viton seal

Teflon seal Collar Vent Connection of the palladium foil

Palladium foil Electric connector

Cell body 02947_A_A

© 2009 - IFP Training

D MAC 1274 B

Teflon seal Reference Auxiliary electrode electrode

35

II -

THE MANY FORMS OF CORROSION In most cases, the naked eye is sufficient to identify each form but sometimes magnification is helpful or required.

1-

UNIFORM ATTACK This is the most common form of corrosion, characterized by a reaction which proceeds uniformly over the entire exposed surface. The metal becomes thinner and eventually fails. Examples: -

piece of steel or zinc immersed in dilute H2SO4

-

rusting of iron in air -saturated water • •

-

initial corrosion rate: 100 mdd (mg/dm2 x day) steady-state corrosion: 10 to 25 mdd

steel in seawater: 25 mdd

a - Rate of uniform attack is reported in : inch per year : milligrams per square decimeter per day : mm per year

D MAC 2000 A

ipy mdd mm/year

No corrosion

Uniform

mg weight loss • 87,6 (area) • d • t mm/year

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mm2

© 2009 - IFP Training

g/cm3

h

D MAC 2173 A

Corrosion rate =

36

b - Metal classification -

< 0.002 ipy (< 0.05 mm/year)

Excellent

-

0.02 to 0.005 ipy (0.05 to 0.13 mm/year) Good (valve seats, pump shafts, impellers, springs, etc.) for critical parts.

-

0.005 to 0.05 ipy Satisfactory for tanks, piping, valve bodies, bolt heads, etc.

-

0.05 ipy (0.15 cm/year) Usually Unsatisfactory

c - Prevention

2-

-

Uniform corrosion is the easiest form to measure and unexpected failures can usually be avoided by regular inspection,

-

Use of proper materials, including coatings, inhibitors or cathodic protection.

GALVANIC CORROSION May occur when two different metals in contact are exposed to a conductive solution. The larger the potential difference between the two metals, the greater the probability of galvanic corrosion. The more "active" metal only corrodes. The less resistant metal becomes anodic (see the dry-cell). The relative areas of the two metals are also important. A such larger area of the noble, compared to the active metal, will accelerate the attack and vice versa. Galvanic corrosion can often be recognized by the increased amount of corrosion close to the junction of the two metals. More noble metal

Solution

D MAC 2001 A

Cu

Zn

a - Prevention select combinations of metals as close together as possible in the galvanic series avoid the unfavorable area effect of a small anode and large cathode insulate dissimilar metals wherever practicable (use of bakelite) apply coatings with caution, keep the coating in good repair

Porosity

Tin

Zinc

Steel -

02947_A_A

D MAC 2002 A

-

design for the use of readily replaceable anodic parts or make them thicker for longer life install a third metal which is anodic to both metals in the galvanic series add inhibitors

© 2009 - IFP Training

37

GALVANIC SERIES OF VARIOUS METALS AND ALLOYS IN FLOWING SEAWATER

(Active) - 1.6

- 1.4

- 1.2

- 1.0

- 0.8

- 0.6

- 0.4

- 0.2

0

(Noble) 0.2

Graphite Platinum Ni-Cr-Mo alloy C Titanium Ni-Cr-Mo-Cu-Si alloy G Nickel-ion-chromium alloy 825 Alloy 20 stainless steels, cast and wrought Stainless steel-types 316,317 Nickel-copper alloys 400, K-500 Stainless steel-types 302, 304, 321, 347 Silver Nickel 200 Silver-bronze alloys Nickel-chromium alloy 600 Nickel-aluminium bronze 70-30 copper nickel Lead Stainless steel-type 430 80-20 copper-nickel 90-10 copper-nickel Nickel silver Stainless steel-type 410, 416 Tin bronzes (G & M) Silicon bronze Manganese bronze Admiraity brass, aluminium brass 50Pb-50Sn solder Copper Tin Naval brass, yellow brass, red brass

Low-carbon steel, cast iron Cadmium Aluminium alloys Beryllium Zinc Magnesium

Galvanic series for seawater. Dark boxes indicate active behavior of active-passive alloys

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© 2009 - IFP Training

D MAC 028 B

Aluminium bronze Austenitic nickel cast iron Low-alloy steel

38

3-

CREVICE CORROSION This type of attack is usually associated with small volumes of stagnant solution caused by holes, gasket surfaces deposits, crevices under bolts. The deposits (sand, dirt, corrosion products) act as a shield and create a stagnant condition there under. a - Mechanism

Na + O 2– OH -

Cl -

Consider a piece of metal immersed in aerated water (pH = 7)

H+ + M D MAC 1225 A

e

Riveted plate Electrochemical reaction by differential aeration: M
M+ + e 02 + 2H2O + 4e
4 OH -

Initially occurs uniformly over the entire surface

After a short interval, the oxygen within the crevice is depleted because of restricted convection. The dissolution of metal M continues, because oxygen reduction continues with the external area. This tends to produce an excess of positive charge M+ which is necessarily balanced by the migration of chloride ions into the crevice (OH- also migrate but they are less mobile than Cl-). Secondary reactions M+ Cl- + H2O




M OH + H+ Cl-

The fluid within the crevice possesses a pH = 3 and the corrosion increases. b - Prevention

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-

Use welded "butt joints" instead of bolted joints. Sound welds and complete penetration are necessary to avoid porosity and crevices on the inside (if welded only from one side).

-

Design vessels for complete drainage (this facilitates washing and cleaning and tends to prevent solids from settling on the bottom of the vessel). Avoid sharp corners and stagnant areas.

-

Inspect equipment and remove deposits frequently.

-

Remove wet packing materials during long shutdowns.

-

Use nonabsorbent gaskets, such as Teflon, wherever possible.

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Good access of oxygen

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IRON

ANODIC ATTACK

2Fe

2Fe++ + 4e(+Fe(CN)6K3 —› blue)

Ferrous salts produced here

H2O + 3% NaCl

AIR

IMMUNE

Rust ring

Alkali produced 2H2O + O2 + 4e- —› 4OH(+phenolphalein —› pink)

SCHÉMATIC REPRESENTATION OF DIFFERENTIAL AERATION CELL (EVANS EFFECT)

39

D MAC 1363 A

40

4-

PITTING

NaCl O2

Cl

O2

++ + Fe+++ Cr++ Ni++++

Chlorides

D MAC 1364 A

++ +

Localized attack that results in holes in the metal. These holes may be small or large in diameter; sometimes isolated or so close that they look like a rough surface.

Stainless steel exposed to chloride solution

Pitting is one of the most destructive and insidious forms of corrosion. The shape of a pit is often responsible for its continued growth, for the same reasons mentioned under crevice corrosion. A pit can, in effect, be considered as a self-formed crevice.

a - Prevention -

the methods suggested for combating crevice corrosion generally apply also for pitting, particularly surface cleanliness.

-

Increasing velocity, often decreases pitting attack.

-

Surface finish improve pitting resistance.

-

Ordinary steel is more resistant to pitting than stainless steel alloys.

-

Addition of 2% M0 to "304" stainless steel to produce "316" steel results in a very large increase in resistance to pitting (more stable passive surfaces).

-

Materials that show pitting, or tendencies to pit, during corrosion tests should not be used to build the plant.

-

Adding inhibitors may be a dangerous procedure unless attack is stopped completely.

-

The following list of alloys may be used as a guide: • 304 • Hastelloy C • 316 • titanium • Hastelloy F

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21.0 15.5 16.3 22.0 22.0 21.0 21.5 25 20 29 21 20 17 27 25 26 25 25 17 20 22 21.5 17

F A/F cast A/F A/F A A

A/F

A/F A

Cr A A A A A A A A A F A A A A

Type

= % Cr + 3.3% Mo (Lorenz & Medavor 1969) = % Cr + 3.3% Mo + 16% N (Truman 1980) = % Cr + 3.3% Mo + 30% N (G. Herbsleb 1982) = 2.5% Cr + 7.6% Mo + 31.9% N - 41 (Renner & Al 1986) = 3.2% Cr + 7.6% Mo + 10.5% N - 81 (Renner & Al 1986) = % Cr + 1.6% Mo + 4.3%% Cb + 7.0% W (E.L. Hibner 1987)

Cabot Cabot Cabot Cabot Cabot Inco Inco Creusot-Marrel Avesta Allegheny VDM Allegheny VEW Sandvik VDM Uddeholm Langley Creusot-Marrel Creusot-Marrel Ugine-Guegnon Creusot-Marrel Sandvik Uddeholm Creusot-Marrel Sandvik VDM Creusot-Marrel —

Producer

6.2 12

6

25

4 6 6.2 6.7 16

58.0 59.0 68.0 44.0 49.0 61.0 42.0 25 18 — 25 25 16 31

Ni

2.5 2.5

3

4.5

4 3 3 3 5.5

13.5 16.0 15.3 6.5 7.0 9.0 3.0 5 6.1 4 5.9 6.6 6.3 3.5

Composition Mo

1.4 —

—

1.5

— 1.3-4 1.5 0.5 3

— — — 2.0 1.9 — 2.2 1.5 0.7 — — — 1.6 1.0

Cu

0.10 —

—

—

— 0.1 0.18 0.18 —

— — — — — — — 0.20 0.20 — 0.14 — 0.15 —

N

2.0 Cb, 2.5 Co, 1.0 W 1.0 W 3.6 Cb 0.9 Ti

2.5 Co, 3.0 W 4.0 W

Others

29.8 25.3

31.9

34.9

38.2 35.9 34.9 34.9 35.2

65.6 68.3 66.8 43.5 45.1 50.7 31.4 41.5 40.1 42.2 40.5 41.8 37.8 38.6

PRE

31.3 25.3

31.9

34.9

38.2 37.5 37.8 37.8 35.2

65.6 68.3 66.8 43.5 45.1 50.7 31.4 44.7 43.3 42.5 42.7 41.8 40.2 38.6

PREN1

32.75 25.3

31.9

34.9

38.2 38.9 40.3 40.3 35.2

65.6 68.3 66.8 43.5 45.1 50.7 31.4 47.5 46.13 42.2 44.7 41.8 42.3 38.6

PREN2

35.0 20.5

36.8

43.2

51.9 50.0 50.0 50.0 43.3

114.1 119.4 116.0 63.4 67.2 79.9 35.6 65.9 61.7 61.9 60.8 59.2 54.2 53.1

CPT °C

7.9 – 7.6

12.2

17.2

29.4 26.0 23.7 23.7 15.2

88.8 90.2 87.4 38.8 42.6 54.6 10.6 39.1 31.5 42.2 32.5 33.2 22.8 32.0

CCT1 °C

25.5 21.0

26.8

27.2

31.4 30.8 29.8 29.8 25.8

63.6 69.1 40.8 48.0 40.2 50.9 26.3 33.0 29.8 35.4 30.4 30.6 27.1 32.6

CCT2 °C

The critical pitting temperature CPT tests performed by Renner and Al and by E.L. Hibner lead to formulas for estimating the resistance to crevice corrosion by calculating the critical crevice temperature CCT. CPT°C = 2.5 (% Cr) + 7.6 (% Mo) + 10.5 (% N) – 41 CCT1°C = 3.2 (% Cr) + 7.6 (% Mo) + 10.5 (% N) – 81 CCT2°C = % Cr + 1.6 (% Mo) + 4.3 (% Cb) + 7.0 (% W)

PRE PREN1 PREN2 CPT CCT1 CCT2

Hastelloy C22 Hastelloy C276 Hastelloy C4 Hastelloy G Hastelloy G3 Inconel 625 Incoloy 825 Uranus SB8 254 SMO 29-4 C Cronifer 1925 HMO AL - 6x A 963 SANICRO 28 Nicrofer 3127 LC Monit Ferralium 255 Uranus 52 N Uranus 47 N NSCD Uranus B6 2 RK65 904 L Uranus 45 SAF 2205 Cronifer 2205 Uranus 50 316

Designation

CHEMICAL COMPOSITION, PITTING RESISTANCE AND CREVICE RESISTANCE OF STAINLESS STEELS AND NICKEL ALLOYS

41

42

5-

INTERGRANULAR CORROSION a - Metallurgical aspects B

C

D

E

F

DMAC 1365 A

A

Diagrammatic representation of various stages in the process of solidification of a molten metal.

The atoms of a metal are arranged in a regular repeating array. Ferritic steels have a body centered cubic structure; the austenitics are face-centered cubic. When a molten metal is cast, its solidification begins at many randomly distributed nuclei. Each of these grows in a regular atomic array to form a grain. However, because of random nucleation, the planes of atoms in neighboring grains do not match up. The area of mismatch between the grains is called a grain boundary. Grain boundaries are more active chemically and are usually attacked slightly more rapidly than grain faces when exposed to a corrosive environment. Intergranular corrosion can be caused by impurities at the grain boundaries, enrichment of one of the alloying elements, or depleting of one of these elements in the grain-boundary areas.

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43

b-

Austenitic stainless steels

Grain boundaries

Depletion of chromium in the grain boundary regions results in intergranular corrosion.

Chromium Carbide precipitate

D MAC 1366 A

Chromium depleted zone

The grain boundary phenomena that causes intergranular corrosion is sensitive to heat. Susceptibility to intergranular attack is usually a by product of a heat treatment (e.g. a welding or stress-relieving operation) and can be corrected by another heat treatment).

The sensitizing temperature range is 400 to 850°C.

°C 1000 900

1'

5'

1h

800

8h

100 h

Intergranular attack in boiling 16 % H2SO4 + 6 % CuSO4 solution D MAC 1367 A

700 600 500 10

102

103

104

105

106

Seconds

18 Cr 11Ni 0.05%C Effect of time and temperature

Spot welding, in which the metal is rapidly heated by a momentary electric current followed by a naturally rapid cooling, does not cause sensitization. Arc welding on the other hand, can cause damage, the effect being greater the longer the heating time. Sensitizing temperatures are reached some millimeters away from the weld metal itself. In the temperature range indicated, Cr23 C6 is insoluble and precipitates out of solid solution if carbon content is about 0.02% or higher. The chromium depleted zone near the grain boundary is corroded because it does not contain sufficient Cr to resist attack in many corrosive environments. In a severe case, entire grains are dislodged due to complete deterioration of their boundaries.

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44

When hit with a hammer, the steel gives a dull sound typical of a metal that has suffered intergranular corrosion, rather than a metallic ring, typical of sound metal.

Composite region

Unmixed zone Partially melted zone

Weld decay zone Unaffected base metal

True heat-affected zone

Schematic diagram of components of weldment in austenitic stainless steel

Prevention: -

heat-treatment: consists of heating to 1075°C followed by water-quenching. Cr carbide is dissolved at this temperature and a more homogeneous alloy is obtained.

-

adding elements (stabilizers) that are strong carbide formers (cb - titanium). These elements have a much greater affinity for carbon than does Cr.

-

lowering the carbon content to below 0.03% does not permit sufficient carbide to form to cause intergranular attack in most applications.

c - Ferritic stainless steels The sensitizing range for ferritic stainless lies above 925°C and immunity is restored by heating for a short time at 725°C (10 to 60'). In welded sections, damages occur to metal immediately adjacent to the weld and to the metal itself. Theory: the much more rapid diffusion rates of chromium and carbon in the body-centered cubic compared to the face-centered cubic lattice probably explain the shorter times at a given temperature required in the case of ferritic stainless steel to achieve carbide precipitation in the first place, and subsequently to reestablish a uniform chromium composition after carbides have precipitated.

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D MAC 033 C

Weld nugget

45

STRESS CORROSION CRACKING

D MAC 1368 A

6-

The simultaneous actions of a tensile stress and a corroding will in some instances result in the cracking of a metal alloy. The cracks can follow intergranular or transgranular paths, and there is often a tendency for crack branching. a - Stress effects Stresses that cause cracking arise from residual cold work, welding, thermal treatment, corrosion products which strain a wedging action, or may be externally applied during service.

The minimum stress required to prevent cracking depends on temperature, alloy composition and environment. In some cases, it has been observed to be as low as about 10% of the yield stress. b - Cracking time Initially, the rate of crack movement is more or less constant, but as cracking progresses the crosssectional area of the specimen decreases and the applied tensile stress increases. During later stages, the crack widens. c - Environments that may cause stress corrosion of metals

Aluminium alloys

NaCl solutions; sea water: air; water vapor; NaCl- H2O2 solution

Copper alloys

Ammonia vapors and solutions; amines; water and water vapor.

Inconel

Caustic soda solutions.

Monel

Fused caustic soda; hydrofluoric acid; hydrofluosilicic acid

Nickel

Fused caustic soda.

Ordinary steels

NaOH solutions; calcium, ammonium and nitrate soltuions; HCN solutions; mixed acids (SO4H2 -NO3H); H2S solutions; seawater.

Stainless steels

Seawater; H2S; MgCl2 solutions; NaOH-H2S solutions; NaOH-H2O2 solutions; condensing steam from chloride waters.

Titanium alloys

Seawater; methanol-HCl; red fuming nitric acid.

In NaCl and similar neutral solutions SCC is observed only if dissolved oxygen is present.

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46

d - Metallurgical factors

Time to failure (hr) 1000 The susceptibility to stress-corrosion cracking is affected by: chemical composition, preferential orientation of grains, composition of precipitates. The figure aside shows the effects of alloy composition in stainless steels.

Cracking Fissuration

ac k i n

g

100

ti m e

to c r

While generally the use of pure metals is often an available way for preventing cracking; it should be pursued only with caution.

im u m min

No cracking

Ni % 20

40

60

SCC of FeCrNi in boiling 42 % MgCl2

D MAC 2125 B

10

The resistance to SCC increases with the amount of ferrite in cast stainless steels. Pools of ferrite in the austenitic matrix tend to block the progress of cracks.

Solution at 154°C

SCC of iron Cr-Mi in boiling 42% Mg Cl2 e - mechanism It is not well understood. Some of the operating steps are:

02947_A_A

-

corrosion plays an important part in the initiation of cracks. A pit or other discontinuity on the surface of the metal acts as a stress raiser (stress concentration).

-

the simultaneous action of stress and corrosion is required (observations have been made when cathodic protection was applied then removed).

-

the rate of tensile stress has been shown to be important in rupturing protective films during both initiation and propagation of cracks.

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47

Alloy

Environment Hot acid chloride solutions Such as MgCl2 and BaCl2

Temperature 60°C 200°C

NaCl - H2O2 solutions Neutral halides: Br–, I–, F– Alkaline CaCl2 Austenitic Stainless Steels

Seawater Concentrated caustic solutions NaOH - H2S solutions Condensing steam from chloride waters For sensitized alloys: Polythionic acids (H2SnO6) Sulfurous acid Pressurized hot water containing 2 ppm dissolved oxygen

Ferritic Stainless Steels

H2S, NH4Cl, NH4NO3, hypochlorite. (resistant to most environments if free of nickel but may fail by other modes of corrosion in same media)

Duplex Stainless Steels

Susceptible to same environments as austenitic stainless steels but more resistant (immune to intergranular SCC in polythionic acid. Also greater resistance than ferritic stainless steels to other forms of corrosion)

Martensitic Stainless Steel

Caustic NaOH solutions (resistant to SCC in hot chlorides. Susceptible to hydrogen embrittlement) Caustic NaOH solutions NaOH - NaSiO2 solutions

Carbon Steels

> 120°C

RT RT 300°C

> 50°C > 255°C

Calcium, ammonium and sodium nitrate solutions Mixed acids (H2SO4 - HNO3)

Boiling RT

HCN solutions, acidified Acidic H2S solutions

Warm

Seawater Anhydrous liquid ammonia Carbonate/bicarbonate Amines CO/CO2 solutions

RT All

Environment-alloy combinations know to produce stress corrosion cracking

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48

Alloy Nickel Cr-Fe Alloys 600, 800 690

Environment High temperature chloride solutions aggravating factors: pH < 4, oxidizing species, such as dissolved oxygen, H2, free sulfur Polythionic acids and thiosulfate solutions, sensitized alloys with excess carbon Caustic alkaline solutions

Temperature > 205°C

RT 315°C

Ni-Cu

Acidic fluoride solutions

RT

Monel

Hydrofluoric acid

RT

Alloy

Hydrofluosilici acid

RT

400

Susceptible in cold-worked state. Resistant in stress relieved state H2S

Nickel Alloy 200, 201

Caustic alkaline solutions

290°C

Copper-

Ammonia vapors in water

RT

Zinc

Amines in water

RT

Alloys

Nitrites in water

RT

(brass)

Water, water vapor alone

RT

> 15% Zn

(45-50% Zn,
or
+  alloys) Nitrate solutions Some sulfate solutions

Aluminum Alloys

Titanium Alloys

Air with water vapor

RT

Potable waters

RT

Seawater

RT

NaCl solutions

RT

NaCl - H2O2 solutions

RT

Red fuming nitric acid

RT

Hot salts, molten salts

> 260°C

N 2O4

30 - 75°C

Methanol/halide

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RT

49

f - Prevention annealing in the case of residual stresses c.s 625°C (stress relief) s.s 900°C reducing the load thickening the section

• • • -

lowering the stress by: • • •

annealing in the case of residual stresses reducing the load thickening the section.

-

eliminating the critical species by degasification, demineralization, distillation.

-

changing the alloy: use Inconel (raising the Ni content) when 304 is not satisfactory. Although carbon steel is less resistant to general corrosion, it is more resistant to SCC. For example, heat exchangers used in contact with seawater are often constructed with ordinary steel.

-

applying cathodic protection (don't forget H2 evolution and embrittlement).

-

adding inhibitors: as in all inhibitor applications, sufficient inhibitors should be added to prevent the possibility of localized corrosion and pitting.

g - Corrosion fatigue Fatigue is defined as the tendency of a metal to fracture under repeated cyclic stressing. Usually, fatigue failures occur at stress levels below the yield point. During the propagation of a fatigue crack through a metal, the frequent cyclic stressing tends to hammer the fractured smooth surface, until the cross-sectional area of the metal is reduced to the point where the limit is exceeded and rapid brittle fracture occurs. In general, it is assumed that if a metal is stressed below its fatigue limit, it will endure an infinite number of cycles without fracture. Corrosion fatigue is defined as the reduction of fatigue resistance due to the presence of a corrosive medium. There is usually a large area, covered with corrosion products and a smaller roughened area resulting from the final brittle fracture. Corrosion fatigue is most pronounced at low stress frequencies; also influenced by oxygen content, temperature, pH. For example, steel and stainless steel possess good corrosion fatigue resistance in water, but in seawater they retain only 75% of their normal resistance. The failure is usually transgranular. It seems that pitting starts the failure. Prevention: the same methods as previously seen for SCC. Also, resistance can be improved by using coating such as electrodeposited zinc, chromium, nickel, copper coatings.

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50

7-

SELECTIVE LEACHING This is the removal of one element from an alloy. The most common example is the removal of zinc in brass alloys (dezincification). The corrosion is detrimental largely because it yields a porous metal with poor mechanical properties. Similar processes occur in other alloys in which al, Fe, Co, Cr and other elements are removed. a - Dezincification It is readily observed with the naked eye on yellow brass (30 Zn - 70 Cu). The alloy presents a red or copper color contrasting with the original yellow.

Potable water service

Dezincified portion

D MAC 1369 A

Unaffected portion

Corrosion is localized or uniform. The mechanism is not really understood. It seems the zinc is dissolved. Oxygen also enters into the cathodic reaction and hence increases the rate of attack. Prevention: Dezincification can be minimized by: -

reducing the aggression of the environment (i.e. oxygen removal),

-

cathodic protection,

-

addition of 1% tin to a 70-30 brass (admiralty). Further improvements were obtained by adding small amounts of arsenic, antimony, phosphate as "inhibitors". These elements are redeposited on the alloy as a film and thereby hinder dezincification.

b - Graphitization The cast iron appears to become "graphitized" in that the surface layer has the appearance of graphite. The graphite is cathodic to iron, and an excellent galvanic cell exists. The iron is dissolved leaving a porous mass consisting of graphite. The cast iron loses strength and its metallic properties. The surface shows rusting that appears superficial. Graphitization does not occur in nodular or malleable cast irons, because the graphite network is not present to hold together the residue.

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51

8-

FRETTING CORROSION Fretting describes corrosion occuring at contact areas between materials under load, subjected to vibration and slipping. Is appears as pits or grooves in the metal surrounded by corrosion products. It is also called "false brinelling" (the resulting pits are similar to the indentations made by a Brinell hard test). A common case occurs at the interface between a press-fitted ball-bearing race on a shaft.

Load Cyclic movement

D MAC 038 B

Fretting at tight fits subject to vibration

oxidized particules Before

After

D MAC 039 B

Contact point

Coldweld

A loss of tolerances and loosening appear. The relative motion necessary to produce fretting corrosion is extremely small - displacements as little as 1Å cause fretting damage. Mechanism: either the heat of friction oxidizes the metal and this oxide then wears away, or, the mechanical removal of protective oxides or corrosi10 - Erosion corrosion 53nued exposure of fresh surface that continues to actively corrode. Prevention:

02947_A_A

-

lubrication reduces friction between bearing surfaces and tends to exclude oxygen

-

increase the hardness of both materials

-

decrease the load at bearing surfaces (not always successful)

-

use gaskets to absorb vibration and to exclude oxygen (Teflon)

-

increase load to reduce slipping (effective if it is high enough to prevent slip)

-

design of contacting surfaces to avoid slipping completely (e.g. grit sandblasting or otherwise roughening the surface)

-

combination of a soft metal with a hard metal. A sufficiently high loads, soft metals serve to exclude air at the interface (Sn, Ag, Pb, Cd coated metals in contact with steel)

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52

9-

EROSION CORROSION

(a)

(b)

(c)

(d)

D MAC 2121 B

Erosion corrosion is the increase in rate of attack on a metal because of relative movement between a corrodent fluid and the metal surface. Erosion is usually attributed to the removal of protective surface films.

Turbulent eddy mechanism for downstream undercutting of erosion-corrosion pits The attack is characterized by grooves, waves, shallow pits and usually exhibits a directional pattern. Erosion corrosion thrives on high velocity conditions, turbulence impingement, galvanic effect, which remove the protective surface films. Fast-moving slurries that contain hard solid particles are also likely to cause problems. Erosion is frequently seen on pump impellers, piping particularly at elbows, inlet into exchanger tubes, etc. a - Prevention -

better material (Ni-Hard is a white cast iron containing about 4% Ni and 2% Cr). It is very hard (550 Brinell hardness 725) used for pump impellers in slurry services).

-

better design-involves: change in shape or geometry, increasing pipe diameter which decreases velocity, increasing the thickness of material strengthens vulnerable areas).

-

environment, filtration, reduce temperature, etc.

-

coatings that produce a resilient barrier, elastomer on impellers. Hard facings (stellite for valves). Repair attacked areas by welding.

b - Cavitation Cavitation is caused by the formation and collapse of vapor bubbles at the metal surface. Shock waves may increase up to 4000 bars. This forces can produce plastic deformation. The appearance of cavitation damage is similar to pitting, except that the pitted areas are closely spaced and the surface is usually roughened. Prevention: -

02947_A_A

the same techniques used in preventing erosion corrosion. smooth do not provide sites for bubble nucleation (smooth finishes on pump impellers). available NPSH > required NPSH.

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53

III -

CORROSION PREVENTION 1-

MATERIALS SELECTION One of the most important factors that must be considered is corrosion resistance. Materials selection charts presented here, indicate where materials are used for specific range of concentration and temperature in specific chemicals. Boundary lines for corrosion zones may shift with variations in aeration, velocity, impurities, etc. Plastics, as compared with metals and alloys, are much weaker, softer, more resistant to chloride ions and hydrochloric acid, less resistant to solvents, and have definitely lower temperature limitation. Fresh Portland cement contains lime (Ca (OH2)) so, it has an high alkalinity (pH = 12.6). Hence aluminum surfaces in contact with wet concrete may evolve hydrogen visibly. The corrosion rate is reduced when the cement sets, but continues if the concrete is kept moist or contains deliquescent salts (e.g. CaCl2).

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02947_A_A

16.0-18.0 17.0-19.0 16.0-18.0 17.0-19.0 17.0-19.0

17.0-19.0

17.0-19.0

18.0-20.0 18.0-20.0 17.0-19.0

19.0-21.0 22.0-24.0 22.0-24.0 24.0-26.0 24.0-26.0 23.0-26.0 16.0-18.0 16.0-18.0 18.0-20.0 17.0-19.0 17.0-19.0 17.0-19.0

303

303 Se

304 304 L 305

308 309 309 S 310 310 S 314 316 316 L 317 321 347 348

Cr

201 202 301 302 302 B

AISI Type N°

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10.0-12.0 12.0-15.0 12.0-15.0 19.0-22.0 19.0-22.0 19.0-22.0 10.0-14.0 10.0-14.0 11.0-15.0 90-12.0 9.0-12.0 9.0-13.0

8.0-10.5 8.0-12.0 10.5-13.0

8.0-10.0

8.0-10.0

3.5-5.5 4.0-6.0 6.0-8.0 8.0-10.0 8.0-10.0

Ni

Mn (max)

P (max)

S (max)

Si (max)

0.08 max 0.20 max 0.08 max 0.25 max 0.08 max 0.25 max 0.08 max 0.03 max 0.08 max 0.8 max 0.08 max 0.8 max

0.08 max 0.03 max 0.12 max

0.15 max

0.15 max

0.15 max 0.15 max 0.15 max 0.15 max 0.15 max

2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0

2.0 2.0 2.0

2.0

2.0

5.5-7.5 7.5-10 2.0 2.0 2.0

0.045 0.045 0.045 0.045 0.045 0.045 0.045 0.045 0.045 0.045 0.045 0.045

0.045 0.045 0.045

0.20

0.20

0.06 0.06 0.045 0.045 0.045

0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03

0.03 0.03 0.03

0.06

0.15 min

0.03 0.03 0.03 0.03 0.03

1.0 1.0 1.0 1.5 1.5 1.5-3.0 1.0 1.0 1.0 1.0 1.0 1.0

1.0 1.0 1.0

1.0

1.0

1.0 1.0 1.0 1.0 2.0-3.0

Other elements

2.0-3.0 Mo 2.0-3.0 Mo 3.0-4.0 Mo Ti 5 X C (min) Cb-Ta: 10 X C (min) Cb-Ta: 10 X C (min)

0.15 Se (min)

0.6 Mo (max)

0.25 N (max) 0.25 N (max)

Class: Austenicit (face-centered cubic, nonmagnetic, not heat treatable)

C

%

TYPES AND COMPOSITIONS OF WROUGHT STAINLESS STEELS — American Iron and Steel Institute —

Stablized grade Stabilized grade Stabilized grade 0.1 Ta (max) when radiation conditions require low Ta

Extra low carbon

Extra low carbon Lower rate of work hardening than 302 or 304

Easy machining, nonseizing

Easy machining, nonseizing

Resistant to high-temp. oxidation

Remarks

54

02947_A_A

11.5-13.0 11.5-13.5 11.5-13.5 12.0-14.0

12.0-14.0

12.0-14.0 15.0-17.0 16.0-18.0 16.0-18.0 16.0-18.0

11.5-14.5 16.0-18.0 16.0-18.0 16.0-18.0 23.0-27.0

416 Se

420 431 440 A 440 B 440 C

405 430 430 F 430 F, Se 446

Cr

403 410 414 416

AISI Type N°

© 2009 - IFP Training

— — — — —

— 1.25-2.5 — — —

—

— — 1.25-2.5 —

Ni

Mn (max)

P (max)

S (max)

Si (max)

1.0 1.0 1.0 1.0 1.0

1.25

1.0 1.0 1.0 1.25

0.04 0.04 0.04 0.04 0.04

0.06

0.04 0.04 0.04 0.06

0.03 0.03 0.03 0.03 0.03

0.06

0.03 0.03 0.03 0.15 min

1.0 1.0 1.0 1.0 1.0

1.0

0.5 1.0 1.0 1.0

0.08 max 0.12 max 0.12 max 0.12 max 0.20 max 1.0 1.0 1.25 1.25 1.5

0.04 0.04 0.06 0.06 0.04

0.03 0.03 0.15 min 0.06 0.03

1.0 1.0 1.0 1.0 1.0

Other elements

0.75 Mo (max) 0.75 Mo (max) 0.75 Mo (max)

0.15 Se (min)

0.6 Mo (max)

0.6 Mo (max) 0.15 Se (min) 0.25 N (max)

0.1-0.3 Al

Class: Ferritic (body centered cubic, magnetic, not heat treatable)

Over 0.15 0.20 max 0.6-0.75 0.75-0.95 0.95-1.2

0.15 max

0.15 max 0.15 max 0.15 max 0.15 max

Class: Austenicit (face-centered cubic, nonmagnetic, not heat treatable)

C

%

TYPES AND COMPOSITIONS OF WROUGHT STAINLESS STEELS — American Iron and Steel Institute —

Easy machining, nonseizing Easy machining, nonseizing Resistant to high temperature oxidation

Highest attainable hardness

Easy machining, nonseizing

Easy machining, nonseizing

Turbine quality

Remarks

55

56

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MATERIALS SELECTION CHART FOR PLANT PHOSPHORIC ACID Temperature (°F) 320

Atmospheric boiling point curve

280

4 3

240

2

200

5

7

160 120

1

6

40 0

10

20

1 Ceramics Hostelloy B*, C Durimet 20 R-55 Type 316* Lead (to 205 F.) Copper (air free) Monel (air free) Duriron* Impervious graphite Haveg 41 Rubber (to 150 F.) PVC (to 150 F.) Polyesters (65%) Epoxys Polyethylene (to 140 F.) Glass Cl. polyether Saran (to 75 F.) Ni-O-nel ally 825

30

40

50

60

70

80

Plant phosphoric acid (wt.%) Haveg 41 Epoxys Glass Cl. polyether Impervious graphite

3 Ceramics Hostelloys B*,C Durimet 20 R-55 Impervious graphite Glass Duriron* Ni-O-nel alloy 825 4 Ceramics Duriron* Glass 5

2 Ceramics Hostelloys B*,C Durimet 20 R-55 Type 316* Duriron* Ni-O-nel alloy 825 Lead (to 205 F.)

Ceramics Hastelloys B*,C Durimet 20 R-55 Type 316* Duriron* Polyesters Glass Ni-O-nel alloy 825

* Subject to attack by fluorine compounds 02947_A_A

90

© 2009 - IFP Training

100

110

120

D MAC 1371 A

80

6 Ceramics Hastelloys B*,C Durimet 20 R-55 Type 316* Copper (air free) Monel (air free) Duriron* Rubber (to 85 F.) PVC (to 85 F.) Glass Ni-O-nel alloy 825 7 Ceramics Hastelloys B* (to 400 F.) Durimet 20 R-55 Type 316* Monel (air free) Glass

Notes Ta, Mo & W 0.85% to 375 F. Zr 0-50% to 375 F.

58

MATERIALS SELECTION CHART FOR SODIUM HYDROXIDE Temperature (°F)

600

500 Stress cracking boundary 400 Atmospheric boiling point curve

300

4

5

30 mpy

3

200 2

0

1

0

20

Melting point curve

40

60

80

D MAC 1372 A

100

1 mpy

100

Sodium hydroxide (wt.%) 1 Steel and cast iron Most plastics Stainless steels Nickel Copper alloys Monel 400 Saran Cl.polyether (to 30%) Rubber Epoxies Haveg 60,61 Polyethylene PVC

02947_A_A

2 Nickel Monel 400 Hastelloys B, C Inconel Stainless steels Zirconium Ni-o-nel alloy 825 Haveg 60, 61 Neoprene

4 Nickel Silver Cast iron

3 Nickel Monel

© 2009 - IFP Training

Inconel Hastelloys Ni-o-nel

5 Steel Nickel Stainless steels 20 alloys

59

CORROSION RESISTANCE OF MATERIALS TO HYDROCHLORIC ACID

240

Boiling point

200 5

Temperature (°F)

4

160

3

120 2

1

D MAC 1373 A

80

10

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20 Concentration HCI %

© 2009 - IFP Training

30

40

60

CODE FOR HYDROCHLORIC ACID CHART

Zone 3 Zone 1 Chlorimet 2 Glass Silver Platinum Tantalum Hastelloy B Durichlor (Fe Cl3 free) Haveg Saran Rubber Silicon Bronze (Air Free) Copper (Air Free) Nickel (Air Free) Monel (Air Free) Zirconium Tungsten Titanium - up to 10% HCl at room température Worthite -up to 2% HCl at room température

Zone 2 Chlorimet 2 Glass Silver Platinum Tantalum Hastelloy B Durichlor (FeCl3 Free) Haveg Saran Rubber Silicon Bronze (Air Free) Zirconium Molybdenum Impervious Graphite

Chlorimet 2 Glass Silver Platinum Tantalum Hastelloy B (Chlorine Free) Durichlor (FeCl3 Free) Haveg Saran Rubber Molybdenum Zirconium Impervious Graphite Zone 4 Chlorimet 2 Glass Silver Platinum Tantalum Hastelloy B (Chlorine Free) Durichlor (FeCl3 Free) Monel (Air Free) up to 0.5% HCl) Zirconium Impervious Graphite Tungsten Zone 5 Chlorimet 2 Glass Silver Platinum Tantalum Hastelloy B (Chlorine Free) Zirconium Impervious Graphite

Materials in shaded zones having reported corrosion rate less than 0.020" per year

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CORROSION RESISTANCE OF MATERIALS TO SULFURIC ACID

600

500

10

Temperature (°F)

400 7 9

Avoid hastelloy B and D in the range at boiling point 300 6

Boiling point 200

1 3 2

5

8

4

20

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60 40 Concentration H2SO4 (%)

© 2009 - IFP Training

80

100 103,5

D MAC 1374 A

100

62

CODE FOR SULFURIC ACID CHART Zone 1 10% Aluminum Bronze (Air Free) Illium G Glass Hastelloy B and D Durimet 20 Worthite Lead Copper (Air Free) Monel (Air Free) Haveg 43 Rubber (up to 170°D) Impervious Graphite Tantalum Gold Platinum Silver Zirconium Nionel Tungsten Molybdenum Type 316 Stainless (up to 10% Aerated) Zone 2 Glass Silicon Iron Hastelloy B and D Durimet 20 (up to 150°F) Worthite (up to 150°F) Lead Copper (Air Free) Monel (Air Free) Haveg 43 Rubber (up to 170°F) 10% Aluminum Bronze (Air Free) Ni Resist (up to 20% at 75°F) Impervious Graphite Tantalum Gold Platinum Silver Zirconium Nionel Tungsten Molybdenum Type 316 Stainless (up to 25% at 75°F) Aerated

Zone 3 Glass Silicon Iron Hastelloy B and D Durimet 20 (up to 150°F) Worthite (up to 150°F) Lead Monel (Air Free) Impervious Graphite Tantalum Gold Platinum Zirconium Molybdenum

Zone 6 Glass Silicon Iron Hastelloy B and D (0.020" to 0.050") Tantalum Gold Platinum Zone 7 Glass Silicon Iron Tantalum Gold Platinum Zone 8

Zone 4 Steel Glass Silicon Iron Hastelloy B and D Lead (up to 96% H2SO4) Durimet 20 Worthite Ni Resist Type 316 Stainless (above 80%) Impervious Graphite (up to 96% H2SO4 ) Tantalum Gold Platinum Zirconium Zone 5

Glass Steel 18 Cr-8 Ni Durimet 20 Worthite Hastelloy C Gold Platinum Zone 9 Glass 18 Cr-8 Ni Durimet 20 Worthite Gold Platinum

Glass Silicon Iron Hastelloy B and D Glass Durimet 20 (up to 150°F) Gold Worthite (up to 150°F) Platinum Lead (up to 175°F and 96% H2SO4 ) Impervious Graphite (up to 175°F and 96% H2SO4) Tantalum Gold Platinum

Materials in shaded zones having reported corrosion rate less than 0.020" per year

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Zone 10

63

CORROSION RESISTANCE OF MATERIALS TO HYDROFLUORIC ACID

250 Boiling point

225

Temperature (°F)

200

175 6 4

150

5 3

125

100

7

75 1

D MAC 1375 A

2

10

20

30

40

Concentration (HF %) 02947_A_A

© 2009 - IFP Training

50

60

70

80

64

CODE FOR HYDROFLUORIC ACID CHART

Zone 1 Monel (Air Free) Copper (Air Free) 70 CU-30 Ni (Air Free) Lead (Air Free) Nickel (Air Free) Alloy 20 Ni Resist Hastelloy C Platinum Silver Gold Impervious Graphite Haveg 43 Rubber 25 Cr-20 Ni Steel

Zone 4 MONEL (Air Free) 70 CU-30 Ni (Air Free) Copper (Air Free) Lead (Air Free) Hastelloy C Platinum Silver Gold Impervious Graphite Haveg 43 Zone 5

Zone 2 Monel (Air Free) 70 Cu-30 Ni (Air Free) Copper (Air Free) Lead (Air Free) Nickel (Air Free) Alloy 20 Ni Resist Hastelloy C Platinum Silver Gold Impervious Graphite Rubber Haveg 43

Monel (Air Free) 70 Cu-30 Ni (Air Free) Lead (Air Free) Hastelloy C Platinum Silver Gold Impervious Graphite Haveg 43 Zone 6 Monel (Air Free) Hastelloy C Plainum Silver Gold Haveg 43 Zone 7

Zone 3 Monel (Air Free) 70 Cu-30 Ni (Air Free) Copper (Air Free) Lead (Air Free) Alloy 20 Hastelloy C Platinum Silver Gold Impervious Graphite Haveg 43 Rubber

Carbon Steel Monel (Air Free) Hastelloy C Platinum Silver Gold Haveg 43

Materials in shaded zones having reported corrosion rate less than 0.020" per year 02947_A_A

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65

CORROSION RESISTANCE OF MATERIALS TO MIXED ACIDS AT ROOM TEMPERATURE

100 % H2O

3

2 1

100 % H2SO4

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5

D MAC 1376 A

4

100 % HNO3

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66

CODE FOR MIXED ACIDS CHART

Zone 1 Steel Durimet 20 Worthite Glass Silicon Iron Tantalum Platinum Gold Lead

Zone 4 18 Cr-8 Ni Durimet 20 Worthite Glass Silicon Iron Tantalum Platinum Gold Zone 5

Zone 2 18 Cr-8 Ni Durimet 20 Worthite Glass Silicon Iron Tantalum Platinum Gold Aluminum

Cast Iron Steel 18 Cr-8 Ni Durimet 20 Worthite Glass Silicon Iron Tantalum Platinum Gold Lead Zone 3 Durimet 20 Worthite Glass Silicon Iron Tantalum Platinum Gold

Materials in shaded zones having reported corrosion rate less than 0.020" per year

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67

Remarks concerning some materials used in the charts Karbate: graphilor: graphite with resin impregnation Haveg: phenolic resin Saran: copolymer of vinylchloride and vinylidene chloride Lead (ch): chemical lead (0,06% Cu) Lead (Sb): hard lead (3 at 18% Sb) Duriron: ferrous alloy 82% Fe, 15% Si, 0.95% C Alloys 20: 20% Cr, 29% Ni, 3.25% Cu, 2.25% Mo, < 0.07% C Durimet 20 - cast carpenter 20 - wrought worthite: 20 Cr, 24 Ni, 3 Mo, 3 Si, 2 Cu Ni-resist: cast-iron
15% Ni +
3% Cr (the hardest grey cast iron) Illium - Ni alloy (21% Cr, 5% Mo, 6% Fe, 4% Cu, 1% Mn) Uranus 65: %C < 0.03, 25,5% Cr, 20% Ni, 0.25% Nb

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2-

CONTROL OF PROCESS VARIABLES

D MAC 1377 A

Corrosion rate

a - Concentration of major constituents

80 %

The rate of corrosion of a given material usually increases as the concentration of the corrodent increases (example: iron in hydrochloric acid). Corrosion does not always increases with concentration (iron in sulfuric acid and in nitric acid. Here, hydrogen ions concentration increases but at the higher levels, it decreases again.

Acid % weight Ambient temperature b - Effect of impurities Beneficial effects are dependent upon the same rules as the inhibitors. Chloride ion is deleterious because it interferes with the anodic reaction. Chlorides can be particularly effective at destroying passivity in material such as the stainless steels (Pitting). Impurities form complex ions and cause rapid corrosion (conversion of copper into copper-ammonia complexes when ammonia in the presence of oxygen, is present as an impurity). Ferric ions Fe3+ are deleterious to iron, but can be beneficial to stainless steel. There may be interference between impurities to reduce corrosion. The first step to mastering impurities is knowing what they are, how much is present, and where they come from. c - Temperature An increase in temperature will usually increase the rate of corrosion. There are a few exceptions due to a decrease in oxygen content of the solution (particularly near the boiling point). Oxygen is needed to depolarize the cathodic reaction. Also, corrosion problems appear at the dew point temperature. d - pH effect Acidity of a solution is designated by pH . The Pourbaix diagram relates the possibility of corrosion.

2

2

2

0

0

0

2 7 Tantalium

14

Passivation 02947_A_A

2 7 Titanium

14 Corrosion

© 2009 - IFP Training

7 Aluminium

14 Immunity

D MAC 1378 A

2

69

e - Influence of aeration Metals that do not show passivity and that are corroding will generally do so at a faster rate if the oxygen content of the solution is increased. Corrosion rate of materials that show passivity, depends of the state in which they are (active, passive). f - Velocity Has been seen before. Velocity acts on the following ways : -

remove deposits, erosion, concentration polarization.

g - Startup and shutdown procedures Startup problems are related to too high operating temperature, corrodent concentration, incomplete oxygen removal. Downtime problems are related to inadequate cleaning procedures for removing process residues. This can be deleterious in promoting localized corrosion (pitting of dirty stainless steel tubes left to drain slowly without cleaning). Improper cleaning can cause problems if the wrong reagents or inadequate rinsing procedures are used (cleaning stainless steel with HCl acid is a particularly dangerous practice).

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3-

GOOD ENGINEERING DESIGN

GOOD INGINEERING DESIGN BAD

GOOD

WELDS Crevice

Drainage deposits

Acid

Corrodent concentration

Acid

Hot gas Condensation

Hot gas

Métal A Weld Tube métal B

Dilution of metalB by A at weld Pad of metal B

Turbulent flow

Inlet Impingement

Tube métal A

métal B

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Galvanic corrosion

© 2009 - IFP Training

Tube

D MAC 001 B

Inlet

71

Faulty designs and good design t 6.5 mm Concrete t

Beam Steel rods and 25 x 3 mm stirrups

Inadequate access for maintenace painting

Faulty design in renforced concrete. The concrete cover is too thin to exclude water, which permeater through it and causes corrosion of the steel

Tubeplate

Seal-weld

Erosion damage

Flow direction

Shellside Erosion/corrosion downstream of a butt weld with too much root penetration

Tube/tube plate crevice (exaggerated in scale)

Tube

Water and dirt

Channels and angles

Poor

Drainage hole

Better

Poor

Better Contour used in construction

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© 2009 - IFP Training

D MAC 003 B

Even coating

Thin protection

72

Pipewall

Crevices

bad Seal weld

Crevices

Crevices at screwed joint

Bad

bad

Seal weld

Crevices Mauvais

Seal weld Good Vessel wall Seal weld Good Pipe butt welded to flange

Branch on vessel Crevice situation

Good

Bad

D MAC 002 B

Thermocouple sheath

Crevices. Formed (a) by the incomplete penetration of a butt weld, (b) by the use of a screwed flange and (c) by the use of a socketwelding flange. (d) Crevice-free slip-on-welding flange type, (e) crevice-free welding-neck flange type, (f) crevices created by choice of wrong size or wrong standard gasket for duty, (g) correct configuration if crevice corrosion is thought likely, (h) crevice situation created by too small a clearance between thermosheath and the containing branch, (i) crevice formed at the back of a tubeplate and (j) and (k) variants of sealing the tube/tubeplate crevice. 02947_A_A

© 2009 - IFP Training

73

4-

INHIBITORS An inhibitor is a substance which, when added in small concentrations to an environment, decreases the corrosion rate. It is possible to classify inhibitors according to their mechanism and composition. a - Adsorption, inhibitors In general, these are organic compounds which adsorb on the metal surface and suppress metal dissolution and reduction areas. They adsorb on the cathode and anode areas and stifle the reaction. They are commonly used in acid solutions (pickling operations, storage, etc.). b - Anodic inhibitors They retard the anodic reaction: -

film formers: that precipitate insoluble films on the anode area (alkalis, phosphates, silicates, benzoates),

-

oxidizing: (chromates, nitrates, ferric salts) that promote passivity.

Anodic inhibitors are often dangerous. If a small anode area is not protected, it will corrode very rapidly (pitting).

E-v

No in

hibito

E-v 2 H+ + 2

r

i

e –› H

2

E-v

i

D MAC 1379 A

i

c - Cathodic inhibitors They retard the cathodic process -

oxygen scavengers: they remove oxygen from the solution, and so prevent it from depolarizing the cathode. Examples are: • •

sodium sulfide: 2 Na2 SO3 + O2
2 Na2 SO4 hydrazine: 2 N2 H2 + O2
2 N2 + 2 H2O

They are used in closed systems like boiler waters. They can be deleterious with stainlesssteel operations where the conditions are borderline between active and passive corrosion.

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74

-

hydrogen evolution poisons: ions Arsenic, Antimony, Bismuth retard hydrogen depolarization 2H+ + 2 e
H2 They are useful with acid solutions, but ineffective when the major cathode reaction is oxygen reduction. In some cases, they cause hydrogen embrittlement (H into the metal instead of H2 evolution).

-

Insoluble films: they form in the cathode and reduce its effective area. Example 1: Addition of calcium bicarbonate to iron in neutral water converted in insoluble calcium carbonate. Example 2: Tension-active [R – N3]+ is a polar group. They form an adsorbed monolayer on the metal surface. An other layer may appear on the preceding one (constituted for instance by hydrocarbons).

e

+

Fe

-

+

H

Hydrocarbon

N-R 3

-

e

-

-

e e e

-

D MAC 1380 A

+

+ ++

d - Vapor phase inhibitors They have high vapor pressures that enable them to spread into the atmosphere and then adsorb into the metal surfaces. Used for metal parts in storage or during shipment. Care should be taken: -

to use enough inhibitor, they may contaminate environment.

When two or more inhibitors are added, the inhibiting effect is sometimes greater than that which would be achieved by the two substances alone. This is called a synergistic effect.

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5-

ANODIC PROTECTION Pontentiostat ΔE

+

Steel tank

H2SO4

Auxilary cathode

D MAC 1381 A

-

Insolated plug

Reference electrode Storage tank containing sulfuric acid Anodic protection is based on the formation of a protective film on metal by externally applied anodic current. This appears for metals with active-passive transitions such as Ni, Fe, Cr, Ti and their alloys. a potentiostat is an electronic device which maintains a metal at a constant potential with respect to reference electrode (CALOMEL).

Ev Applied current

E i

D MAC 008 B

i

This system is used in sulfuric producing plant.

6-

CATHODIC PROTECTION The purpose of a cathodic protection system is to apply a direct current, through an electrolyte such as earth or water, uniformly to the surface of a buried or immersed metal structure in order to stop corrosion. In cathodic protection, we make the metal behave entirely as a cathode. It has no anode areas and so does not corrode. Current used in excess of that required does no good, however, and may do harm to amphoteric metals or to coatings.

Ev Ec

Ea i

D MAC 010 B

Ι applied

The horizontal line at 0.62 V (S.H.E.), of the Pourbaix diagram means that iron will not corrode below this value to form a solution > 10-6 M Fe++ (M: molality).

10-6 M Fe++ means 0.056 mg of Fe++ per liter. In fact there is no corrosion at 10-3 M Fe++ = 10-3 ion gram per liter. This means – 0.85 volts versus copper/copper sulfate. The potential of a structure is determined with a high-resistance voltmeter.

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76

a - Impressed current

_ +

e-

I a.c line

1,5m

Transformer

C A Steel pipe

+

Carbonaceous backfill Impressed current

-

D MAC 012 C

Scrap iron anode

Rectifier

The direct electrical current is applied by connecting a d.c source to the corroding metal and an anode (scrap iron, carbon, etc.). The anode is buried in backfill such as coke to improve electrical contact. Example:

Pipe - 300 kms = 300 mm 6 rectifiers (4 to 13 A) metal potential – 1.5 to – 2 v total consumption = 0.85 kwh It was a new pipe very well insulated.

b - Sacrificial anode The basis is the inherent difference in the tendency to ionize of different metals. This is reflected by the relative position of the different metals in the galvanic series. Materials for sacrificial anodes are limited to those metals which are anodic to the metal of the structure (Mg, Zn, Al). EXAMPLES: pipe 85 kms - 300 mm (GDF) (1/3 in marshy ground electrolyte-resistivity = 4
m) 20 anodes Mg (350 kg) - (18 to 880 mA) metal potential – 0.85 to – 1 v Cost: 1/800 of investment.

Ground level

H Mg anode

02947_A_A

D MAC 012 B

Coated copper wire

The principal difference in the two systems lies in the fact that, with impressed current system, adverse environmental conditions can more easily be overcome through the selection of the output voltage of the rectifier and the current output per anode can be pushed to higher levels.

© 2009 - IFP Training

77

The most dramatic problems are the possible effects of stray currents on the corrosion of adjacent metal structures. A remedy is to connect electrically the structures.

+

Steel tank

Remedy = insulated connection

7-

Anode

D MAC 011 B

-

METALLIC AND INORGANIC COATINGS Hundreds of different coatings are available. This can make coating selection confusing. So, it is useful to separate coatings into a number of groups. Relatively, these coatings can provide a satisfactory barrier between metal and its environment (from 1μ to 0.5 mm depending on the method used). a - Electrode position (or electroplating) Consists of immersing an article to be coated in a solution of the metal to be plated and passing a direct current between the part and another electrode. It is common to plate several different metals in sequence to obtain optimum coatings (example: copper for adhesion, Ni for corrosion and Cr primarily for appearance). Zn, Ni, Sn and Cd are very often plated by electrode position. With a noble coating (Ni, Ag, Cu, Cr, Pb more noble than the metal being coated) extra care must be taken to avoid porosities or scratches (in service). b - Metallizing or metal spraying This process involves the application of a spray of molten-metal droplets. Thicknesses are usually in the range of 0.05 mm to 2.5 mm. In the wire and powder techniques, the coating metal is fed into the spray gun, where it is melted. Compressed air then atomizes and sprays to molten particles into the prepared surfaces to be coated. c - Cladding The bonding of two materials is most commonly done by hot rolling or welding, cladding of steel with stainless steel, copper, Ni alloys, titanium is the most popular. Problems of diffusion are encountered when austenitic stainless steel are clad to carbon steel. Carbon that diffuses from the steel can cause intergranular corrosion (due to chromium carbide formation) in the stainless steel, particularly when a vessel has to be stress-relieved at about 600°C (development of very low carbon stainless steels). d - Hot dipping

02947_A_A

Immersion of the part to be coated into a molten bath of the coating, which by necessity has a lower melting point (chiefly zing, tin, lead and Al). Galvanized steel is a popular example. Thickness of the coating is much greater than electroplates. © 2009 - IFP Training

78

e - Diffusion (or surface alloying) Involves heat treatment to cause alloy formation by diffusion of one element into the metal. Parts to be coated are packed in solid materials or exposed to gaseous environments which contain the element which diffuses. Sherardizing Zn – Fe chromizing Calorizing cementation Nitruratrion

Cr – Fe Al2 O3 C N


 
 

used for resistance to high temperature oxidation Surface HARDENING

f - Chemical conversion Anodizing Conversion of Al to Al2O3 by making it the anode in an electrolytic cell (sulfuric acid bath). Coatings are somewhat porous, so it is customary to seal them in boiling sodium dichromate solution, in order to improve their corrosion resistance. They are used for resistance to corrosion and abrasion, or as a paint base (good adherence). Phosphate coatings Dipping or spraying is used to apply to coating (approximate thickness 0.01 mm), following the sequence below: -

pickle to remove scale, preclean in alkaline and rinse, treat with conversion mixture and rinse with water.

Fe +

3 Zn (H2 PO4)2




Zinc phosphate

Zn3 (PO4)2 + Fe H PO4

+ 3 H3 PO4 + H2

phosphate coating insoluble in neutral or alkaline solution

They are almost always used as a base for paint or to absorb corrosion-preventive oils or waxes. g - Porcelain enamels and glass coatings They must have a suitable coefficient of expansion fused on metals. The coating is applied by spreading powdered glass on a sand-blasted or pickled surface. Then, heating the article to a high temperature (850°C approximately) softens the glass so that it flows and bonds with the metal. Borosilicate (Ca, Na, K, Co, Ni, etc.) is easier to bond to steel, whereas high silica glass has better acid resistance. This coatings can resist a wide variety of chemicals, including strong acids. The glass must completely isolate the metal (no sacrificial protection or inhibition). The main limitations of these linings are brittleness, and susceptibility to cracking due to thermal shock. h - Portland cement coatings Low cost and a coefficient of expansion (1 10-5/°C) approximating that of steel. Used for protecting steel from water and soil, also for fireproofing. Like glass, cements suffer from brittleness. 02947_A_A

© 2009 - IFP Training

79

i - Flushing compounds They are a kind of inhibitors used to protect steel surfaces temporarily from rusting during shipment or storage. They consist of oils, greases or waxes that contain small amounts of organic additives. The was usually provides a longer time of protection, while oil is easier to wipe or dissolve in solvents.

8-

PAINTS - ORGANIC COATINGS Paints are pigment containing compositions that form a liquid film on a surface and subsequently harden to a solid coating. Most paints are "barrier" coatings. That is to be effective, they must completely isolate the metal from the corrodent. Zinc-rich paints are an exception to this. a - Type and composition Paints usually consist of: Pigments -

some for opacity and color (TiO2, FeO, Lead chromate, etc.),

-

some for inhibition against corrosion: (red led Pb3O4, zinc chromate ZnCrO4, etc.), They must be soluble enough to supply the minimum ions necessary to reduce the corrosion rate. They are incorporated into the prime coat.

-

some for sealing: pigments having the shape of flakes oriented parallel (by brushing) to the metal surface (micaceous hematite).

Drier May accelerate the oxidation and/or polymerization stages of drying. Solvent or "Thinner" It is a volatile liquid that enables adequate coverage and ease of application of the paint, and then quickly evaporates. Base or "vehicle": it may be: -

natural oil: such a linseed, that when exposed to air oxidizes and polymerizes to a solid (they are permeable to moisture and their resistance to acid fumes is not very good).

-

Synthetic resin: that dries by evaporation of the solvent in which it was dissolved or by polymerization (heat may be required). • • • • • •

02947_A_A

alkyd paints: not suitable for chemical services, polyvinyl acetates and acrylics: ease of application and clean up, urethane: good toughness and abrasion resistance, chlorinated-rubber: resistant to water, but not strong oxidizing media and < 65°C, neoprene: may be applied over chlorinated rubber to increase resistance to weathering, vinyl: resistant to a variety of aqueous acid and alkaline media, but not above 65°C,

© 2009 - IFP Training

80

•

epoxy: 3 paints Amine hardened epoxy consists of 2 components, the hardener and the resin, that are mixed just before using. These are the most resistant to chemicals. Polyamide hardened epoxy ; less resistant to acids but tougher and more moisture proof. COal-tar epoxy: can be used without a primer (good for water, soil, inorganic acids),

•

silicone paints: are used for high temperature service (up to 650°C),

•

coaltars: underground applications,

•

zinc paints: consist of metallic zinc dust in an organic or inorganic vehicle. They use galvanic protection by the zinc to prevent pitting at holes in the coatings.

b - Surface preparation Involves surfaces roughening to obtain mechanical bounding as well as removal of dirt, rust, mill scale, oil, grease and other impurities. -

blasting: it is the best method. Scale is removed by high velocity particules projected on to the surface by an air blast. Blast material usually consists of sand or sometimes steel grit,

-

others methods: pickling by SO4 H2 and inhibitor, wire brushing, cleaning by solvents or alkaline solutions.

c - Application of a paint Remember the cost for labor equals two to three times the cost of the paint. The final stages of surface preparation and the application of the primer should be performed on the same day. Weather conditions are also important. If possible, the coating should not be applied to a metal surface that is too hot (> 60°C) or too cold (< 5°C). The surface should be dry, relative humidity should be below 85% and metal temperature should be at least 3°C above the dew point. Fresh paint should be used and carefully mixed, just prior to application. The commonest methods of painting are: spraying, brushing, dipping. Spraying is the most popular. The paint is atomized and driven onto the surface by compressed air or hydraulically (air less). Paint thickness should be checked with special gages during application. This ensures adequate thickness and guards against too thick a coating. A paint system is usually constituted of 3 or more coats. -

primer coats: they contain pigments inhibitor (or zinc) and in addition act as barriers. Wettability is needed so that crevices and other defects will be filled rather than bridged, second coats: paint is applied for sealing, moister proof and adherence, top coats: multiple coats are needed so a pinhole in one coat is covered by a complete fim of another. Top coats are applied for: • • • • • • • • • •

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sealing appearance (color) resistance to infrared and ultraviolet radiations resistance to abrasion and mechanical shocks (hardness) anti-fouling (particularly under water) resistance to temperature resistance to acids, alkalines, and solvents ease to clean up and touch up resistance to cold adherence

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d - Inspection importance Inspection of coatings is particularly important because it will generally be possible to touch up the coating in a problem area, and delay the need for a complete repainting (the underlying metal will often have poor resistance to the corrodent). Possible causes of paint defects Blistering -

painted over wet surface undercoat not dry metal too hot when painted (more than about 65°C) metal too cold when painted (less than about 10°C) improper solvent used if sprayed-too much air pressure or water in line

Peeling, flaking -

dirty starting surface undercoat not dry metal too hot or too cold coating too thick

Cracking -

not enough thinner undercoat too thick surface unclean, too hot or too cold paint not completely mixed if sprayed - oil or water in air line

Wrinkling -

coating too thick drying in too hot or too moist an environment

Running -

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too much solvent solvent vaporizes too slowly coating too thick if sprayed - air pressure too low if dipped - withdrawn too quickly or jerkily

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IV - HYDROGEN CORROSION The presence of hydrogen in a metal network can cause considerable damage to the metal associated with a drastic drop in metal properties. The hydrogen atom (H) is the only species capable of diffusing through metal. The molecule form (H2) does not diffuse. Therefore only the atomic form of hydrogen can damage metal.

1-

SOURCES OF NASCENT OR ATOMIC HYDROGEN • Gas phase hydrogen Penetration occurs through adsorption of the molecules on the surface of the metal. Atoms are then formed by molecule dissociation through catalytic action of the metal, which the atomic hydrogen then penetrates. The higher the temperature, the greater the dissociation of H2 molecules into atoms. • Hydrogen in solution Penetration from a solution occurs when the metal surface is the seat of an electrochemical reaction (cathode). The reduction of the hydrogen ion (H+ ) first produces hydrogen atoms which then form H2 molecules. Certain substances such as sulfur ions, arsenic and phosphorus compounds and cyanides lower the reduction reaction of hydrogen ions: 2 2 H+ + 2 e –
2 H
H2 The second stage (formation of molecules) would seem to be that mainly concerned. In the presence of these substances there is greater concentration of hydrogen atoms on the surface of the metal. There are four types of damage by hydrogen:

1 - Hydrogen blistering 2 - Hydrogen embrittlement 3 - Decarburization

Occur in chemical processing units, welding and pickling operations or as a result of corrosion or cathodic protection. Occur only at high temperatures

4 - Hydrogen Attack

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HYDROGEN BLISTERING H+

Certain H atoms diffuse through the metal and are halted by voids or inclusions (e.g. manganese sulfides in rolled steels) where they combine in the form of hydrogen molecules. H/H2 equilibrium pressure is very high and capable of causing the rupture of any equipment through blister formation.

H H

H+

H

H2

H

e

H

e

H

H2

H

H H

Void H

H2

H

• Prevention -

use of killed steels (non- porous)

-

use of metallic, organic or inorganic cladding and lining • • •

3-

cladding with 304-316 type steels or Monel. Hydrogen does not diffuse in austenite or Monel (66 Ni - 30 Cu) when cold. porous guniting favoring the transformation of H into H2 by catalysis. rubber and plastic;

-

removal of poisons such as sulfides, arsenic compounds, cyanides and phosphorous ions which in the presence of acids, cause blistering (e.g. in refineries light fractions are washed with water to eliminate and dilute the H2S and cyanides);

-

use of inhibitors which reduce corrosion and the H+ ion rate of reduction, but can only be employed in closed and recycled systems.

HYDROGEN EMBRITTLEMENT This too is caused by the penetration of atomic hydrogen inside defects or dislocations near the surface of the metal. In plastic zones, dislocations build up at the particle responsible for the plastic deformation blocking their progress. The atomic H fixed onto this accumulation of dislocations acts as a wedge on the metal, which at the same time is subjected to tensile stress, and a fissure may then form. Once initiated, the fissure will grow, preceded by a plastic zone (which will also fix the H atoms) at its tip. The time taken by the hydrogen to initiate a fissure is called ´†incubation time†ª. Here it can be seen that this is a stress type corrosion, caused by the release of hydrogen (reduction of H+). Steels with high mechanical properties are more sensitive to hydrogen embrittlement, and the higher the stresses they undergo, the more sensitive they become.

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D MAC 042 C

2-

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Surface

Grain boundaries

Grain

Dislocations pile-ups

Voids betwen grains

Exemples of existing defects

Grain

Grain

Cracks (e.g corrosion pits) opened on the surface

Cracked or uncracked carbides

Oxide at the grain boundary

Elongated plastic strain zones

HH

Molecular hydrogen (pressure)

HH

HH H H

Absorbed hydrogen

Existing defect

H H H

HH

Effects of hydrogen

TCH = Cohesive force reduced in the prescence of absorbed or trapped hydogen

Crack

Hydrogen absorbed or trapped + aggregates and dislocation stacking

TTH = Sum of applied and residual stresses + hydrogen induced stresses (e.g pressure)

With hydrogen

TC° = Cohesive forces

TT° = Sum of applied and residual stresses

No hydrogen

D MAC 1382 A

Elongated plastic strain zone

MECHANISMS OF HYDROGEN EMBRITTLEMENT IN THE PRESENTS OF DEFECTS

84

85

A tendency to embrittlement is strengthened by the concentration of hydrogen on the metal, but the phenomenon is reversible. This means that it is almost possible to recover the initial mechanical properties by displacing the hydrogen. The figure, on the following page, shows that after a given, length of time, cracking occurs at successively higher stresses as hydrogen concentration in the metal decreases. 2100

300

+

Uncharged + +

+

+ 1850

250

Bake 24 h

200

1350 Bake 12 h 1100

150

Bake 7 h

850

Bake 3 h 600 350 0.1

1

10

Also, the alloys are most susceptible to cracking in their highest strength level.

100

Bake 0.5 h

0.01

Applied stress (ksl)

Bake 18 h

100

50 1000

D MAC 1370 A

Applied stress (MPa)

1600

Facture time (hr)

Fracture of AISI 4340 steel baked at 150°C after initial cathodic hydrogen charging • Prevention

4-

-

reduction of the corrosion rate. During pickling operations an inhibitor should be used to protect the parent metal from the large quantities of hydrogen released.

-

avoid hydrogen release during plating operations

-

baking at 150°C to displace the hydrogen

-

avoid steels having high mechanical strength. Rule of thumb recommends not exceeding 22 Rockwell C hardness (HRC < 22). Nickel and molybdenum reduce embrittlement sensitivity

-

welding should be done using low-hydrogen electrodes. It is also most important to weld in dry conditions (dried electrodes) as water and steam are sources of hydrogen (dissociation at high temperatures).

DECARBURIZATION AND HYDROGEN ATTACK In conversion units using hydrogen at high pressures and temperatures, the steel structures coming into contact with the gas phase can show signs of hot hydrogen embrittlement (>200°C). The main effect of hydrogen at high temperatures is decarburization of the alloy: Fe3C + 2 H2
3 Fe + CH4. a - Incubation phase The oxides segregated at the grain boundaries play an important role in the first, or incubation, period, which precedes actual embrittlement.

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The H2 reduces the oxides with the formation of water vapor which retards the methane embrittlement action. The following reactions are involved: Fe2O3 + 3 H2 Fe3O4 + 4 H2 H2O + CH4






3 H2O + 2 Fe 4 H2O + 3 Fe CO + 3 H2

The carbon monoxide formed during the incubation period, diffuses into the steel along the grain boundaries, avoiding phenomena of fragilization by overpressure. b - Attack When all the oxides have been reduced, the final phase of carbide attack (cementite) followed by embrittlement, begins. The methane cannot diffuse and generates intergranular cracking (methane will have accumulated preferentially at the grain boundaries). The zones surrounding the crack contain a partially decarbonized pearlite (the cementite plates vary in density and discontinuity). c - Prevention • Replacement of cementite with stable carbide Stable carbide steels are produced by adding stabilizing elements which, in increasing order of efficiency are: Mn, Mo, Cr, W, Va, Ti, Nb. Ni, Si, and Cu which do not produce carbides, have no effect. Ti steels are difficult to forge and Cr-Mo steels are preferred. Nelson curves will provide information on the limits of Cr-Mo steels. For very severe conditions of temperature and pressure 18/8, 18/8/3 stainless steel with very low carbon content or stabilized, will be used. But the danger of intergranular corrosion must be eliminated. • Use of homogeneous structure, fine grain steels Spheroid cementite is more stable than plate cementite. • Weld relaxation Welds are coarse grained, have a wide range of harnesses, high residual stresses, and are therefore prone to hydrogen embrittlement. • Avoid local or accidental overheating Even for fairly short periods (2 or 3 hours), the penetration of decarburization is considerably accelerated by an increase in temperature (this has been observed between 500°C and 650°C).

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V-

CORROSION BY SULFUR DERIVATIVES IN THE ANHYDROUS PHASE 1-

CORROSION BY SULFUR COMPOUNDS The sulfur content of crude oils varies between 1.8 and 2.6% and is tending to increase due to current market conditions. This sulfur is in the form of more or less complex organic sulfides with thiophene, mercaptans, disulfide etc. bases, and corrosivity varies according to composition. These compounds have been the subject of numerous studies by oil companies and each compound has been rated on a numbered index so that it can be placed on the steel-corrosion scale. It is thus possible, given the exact composition of the sulfide derivatives of a crude, to have an idea of its potential agressivity in relation to existing installations. • Corrosion mechanisms The major corrosive substance in the oil industry is nascent hydrogen sulfide coming from the crude but also and mainly from thermal decomposition of sulfide compounds. Thermal cracking occurs mainly in the pipes of tube-fired heaters in distillation units, and in zones where temperatures reach 380°C to 400°C can cause drastic corrosion of carbon steel, sometimes associated with steel carburization. This is why 5% Cr steels are used in straight distillation vacuum stills. In refineries there are two distinct types of corrosion:

2-

-

H2S corrosion without hydrogen, which is the case in straight distillation units, thermal and catalytic cracking and visbreakers

-

H2S corrosion with hydrogen, which is the case in desulfurization, reforming and catalytic hydrocracking units. In this case there is a synergetic effect between H2S and H2 which increases corrosion

ANHYDROUS PHASE H2S CORROSION WITHOUT HYDROGEN This corrosion is mainly governed by the H2S concentration and temperature; it is independent of pressure. To start with attack is very fast and for carbon steel it is high from 285°C. This corrosion then tends to lessen with time, the layer of iron sulfide formed, preventing penetration of the sulfur ions. This factor must be taken into consideration when interpreting corrosion results on removable probes or specimens. The specialist literature contains many curves giving the corrosion rate of carbon steels and alloys commonly used in refineries for hydrocarbon fractions and cuts corresponding to given crude. However, these are only mean results and care should be taken when extrapolating them to specific cases. The corrosion rate can be seen to increase rapidly with temperature between about 300°C and 400°C. At this temperature hydrocarbons start to generate some coke and also the ferrous sulfide becomes less porous and forms a protective film (< 0.5 mm/year) from 460°C. However, in units treating light hydrocarbons, the formation of coke is insufficient and corrosion continues to increase - beyond 400°C. Limits of use of alloys in hydrogen sulfide corrosion.

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Usually carbon can still be used up to 285°C with a corrosion allowance compatible with the useful life of the equipment. For higher temperatures Cr and Cr Mo low alloy steels will be used. The most commonly used for vacuum tube fired heaters is 5% Cr. For extreme cases of high turbulence, which tends to eliminate the protective FeS film by abrasion, 9% and 13% Cr should be considered. Above 460°C, although the FeS formed provides an effective barrier to penetration by S, C steel is not recommended due to graphitic corrosion phenomena (425°C). Regarding the use of 304-304L or 316 austenitic steels, which have a good specific resistance to H2S, secondary problems of corrosion by polythionic acids must be born in mind, as these may occur on opening the apparatus (presence of water and air). For this reason, if austenitic steels are required only 321 or 347 niobium stabilized grades should be used whenever it is impossible to neutralize prior to opening. Sulfur corrosion is associated with a problem of plugging by scales of ferrous sulfide removed from the points of corrosion. This problem is thought to appear when the rate of corrosion exceeds 0.3 mm/year. Plugging of tube bundles or certain filters can occur.

3-

H2S CORROSION WITH H2 This type of corrosion occurs most on desulfurization units where the sulfur is eliminated by hydrogenation as H2S, the effluent being a gas loaded with H 2S (high proportion of H2). The attack threshold temperature is much lower than without H2, and the corrosion rate increases as a linear function of the temperature without passing through a maximum (here the molecules are saturated and do not polymerize to produce coke). Due to the presence of hydrogen and considering its partial pressure, alloy steel would be recommended (see Nelson curves). The 5% Cr is corroded by H2S just like C steel. Therefore, from 315°C stabilized austenitic steel grades should be used if possible. Laboratory tests show that the partial H2S pressure has no impact on the corrosion rate of the steel up to 0.4 bar, above which the corrosion rate will depend solely on temperature. Apart from the austenitic steel grades, recent studies in the US have shown the excellent resistance of aluminized Cr Mo steels or 9% Cr 1% Mo spray coated with a 0.1mm layer of aluminum. Here too corrosion products can cause plugging. This is major problem as the catalyst beds may become plugged. The corrosion rate must be kept below 0.3 mm/year.

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RATE OF CORROSION OF SOME ALLOYS BY H2S WITHOUT HYDROGEN — Versus temperature, with a crude oil containing 1.5% —

Rate of corrosion mm/year 3.5

Ordinary steel

3.0

2.5

2.0

4-6 Cr 0.5 Mo steel 1.5

1.0

0.5

18 Cr-8 Ni steel Temperature (°C) 300

350

400

450

500

(T.S. CLAIREBORNE - A.W. COULTER)

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D MAC 1233 B

12% Cr steel

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150

0.25

0.51

0.76

1.02

1.27

1.52

1.78

2.03

2.28

Corrosion rate mm/ year

250

xxxxxxxxx 350

xxxx

xxxx

450

x x x x x x x xx 550

650

18 % Cr 8 % Ni steel

12%Cr steel

5 % Cr 0.5 % Mo steel

2 % Cr 0.5 % Mo steel

Ordinary steel

750

xxxx xxxx xxxxxx

D MAC 1234 B

temp. (°C)

91

92

RATE OF CORROSION OF SOME STEELS — Versus temperature, in a hydrogen sulfide environment without hydrogen, from products containing 0.6 sulfur —

Corrosion rate mm/year 2.5 l

tee

1.2

s ary

in

Ord

eel

st Cr

5%

0.25 9

r %C

el

ste

0.12

eel r st C % 12

0.025 18-8

l

stee

Temperature (°C) 250

300

350

400

(MC CONNOMY, PROCEEDINGS, API DIVISION OF REFINING VOL. 43 (III). 1963.)

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D MAC 1235 B

0.012

93

CORROSION RATE BY SULFUR WITH HYDROGEN

3.5 H2 S alone

Corrosion rate mm/year

H2 S + H2

3.0 5 Cr 0.5 Mo STEEL

2.5

ORDINARY STEEL 2.0

9 Cr 1 Mo STEEL

1.5

1.0 11 - 13 Cr STEEL

0.5

D MAC 1236 B

5 Cr 0.5 Mo STEEL 18 Cr 8 Ni STEEL

300

400

500 Temperature °C

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Weight loss (mg/cm 2)

EFFECT OF PRESSURE ON STEEL CORROSION 9 Cr 1Mo. by H2S + H2 (H2S 5% vol. 400°C).

300

200

100

Total pressure (bar) 35

Weight loss (mg/cm 2) 150

70

105

140

175

EFFECT OF H2S PARTIAL PRESSURE ON STEEL CORROSION 9 Cr 1 M. in a mixture H2S + H2 (36 bar. 400°C).

100

H2S partial pressure (bar) 0.15 0.28

0.56

2.1

2.8

3.5

4.2

4.9

5.6

6.3

From J.D. Mc Coy, Materials Performance - May 1974 p.21

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D MAC 1237 B

50

95

CORROSION RATE OF SOME STELLS in an H2 + H2S environment (H2S content > molecule) versus temperature

Corrosion rate (mm/year)

2.0

12% Cr

9% Cr

2.5 Carbone steel 5% Cr steel 1.5 1.0

18 - 8

0.5

300

350

400

450

500 Temperature (°C)

CORROSION RATE OF CHROMIUM STEELS AT 371°C Versus H2S content in an H2 + H2S environment

0 - 5% Cr

Corrosion rate (mm/year) 1.25

0 - 5% Cr 1.00 12% Cr 0.75 0.5

D MAC 1238 B

Temperature 371°C

1

2

3

4 Temperature (°C)

From J.D. Mc Coy, Proceeding, API Division of Refining, Vol. 43 (III). 1963 p.20

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% H2S molecules

96

CORROSION RESISTANCE OF SOME STEELS IN THE PRESENCE OF HYDROGEN SULFIDE AND HYDROGEN °C 600

STEEL WITH 0 À 5% Cr

Temperature (°C)

CORROSION RATE 500 > 1 mm/year 400 0.5 à 1 mm/year 300 < 0.5 mm/year

H2S partial pressure (bar) 0.02

0.04

0.06

0.08

0.1

0.2

0.4

°C

Temperature (°C)

600

12% Cr STEEL CORROSION RATE

500 > 0.25 mm/year 400 0.12 à 0.25 mm/year 300 < 0.12 mm/year H2S partial pressure (bar) 0.02

0.04

0.06

0.08

0.1

0.2

0.4

°C 600

STAINLESS STEEL 18%Cr 8%Ni -

500 > 0.12 mm/year 400

0.02 à 0.15 mm/year 300

< 0.02 mm/year H2S partial pressure (bar) 0.02

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0.06

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0.08

0.1

0.2

0.4

D MAC 1239 B

Temperature (°C)

CORROSION RATE

97

EFFECT OF TEMPERATURE ON CORROSION BY SOUR CRUDES

Corrosion rate (mm/year)

3.5

3

382°C 2.5 338°C 2

1.5

1 310°C 277°C D MAC 1240 B

0.5

0

2

4

6

(From G.R. PORT Petroleum Refiner,Vol.40, Mai 61, p.154)

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10 % Cr

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0 275 277

1

2

3

4

Corrosion rate (mm / year)

300

310 325

5%Cr

338

CARBONE STEEL

350

9%Cr

7%Cr

375

382

D MAC 1241B

400 Temperature (°C)

98

EFFECT OF CHROMIUM CONTENT OF CORROSION BY SOUR CRUDES

99

VI - CORROSION BY COLD HYDROGEN SULFIDE IN A HUMID ATMOSPHERE 1-

GENERALIZED CORROSION Hydrogen sulfide is a weak bi-acid. Ionization occurs in two stages : H2S
H+ + (HS)– (1)

(HS) –
H+ + S2– (2)

In a humid environment hydrogen sulfide generates considerable generalized corrosion for pH values of between 6 and 8 (see dissociation curve for H2S in water). Here, sulfide films provide little protection, they are dissolved by the pH variations and especially by erosion (high flow velocities of more than 1m 50 per second, considerably stimulate corrosion). Protection can be provided against this type of corrosion by SAKAPHEN type coatings, or if economically feasible, by using metals such as titanium.

2-

ATOMIC HYDROGEN CORROSION A reaction occurs: H2S + Fe
S Fe + 2 H+ This corrosion is similar to the phenomenon of hydrogen embrittlement and results in:

3-

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-

stress corrosion: this appears on steel or welds with a hardness greater than 22 Rockwell C, on carbon steel pipes or curved austenitic stainless steel which has not had any relaxation heat treatment, or if the heat treatment has been ineffective

-

blistering: this phenomenon has already been mentioned and appears in the formation of blisters and splitting. As we have seen, this attack is due to the diffusion of the atomic hydrogen and its transformation into molecular hydrogen which does not diffuse

PREVENTION -

the gas should be dried

-

coating to prevent the H2S from acting on the metal

-

inhibitors (filament amines or ammonium polysulfides which transform into sulfo-cyanides)

-

use of special quality steel plates (high inclusion related cleanliness, low sulfur and hydrogen contents) made to resist splitting.

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VII - CORROSION BY COMBUSTION GASES IN FURNACES AND BOILERS The clogging and corrosion of tube bundles disrupts operation of tube fired heaters and boilers and causes frequent stoppages. Also, the environment is degraded by corrosive emissions from chimneys. Installations are designed according to regulations established by the Authorities (fuel quality, chimney height etc.). The fuels used contain undesirable products such as sulfur, sodium, vanadium which play a major role in the formation of deposits and corrosion.

1-

FORMATION OF SO3 Operating difficulties due to clogging and corrosion at high and low temperatures are the source of So3 forming from the sulfur contained in the fuel. Heavy LSC fuels with less than 1% S, are becoming rarer, and more and more HSC are being used with 1 to 5% S. Combustion of S produces: S + O2
SO2 The presence of excess air then gives the equilibrium: 1 SO2 + 2 O2
SO3 In theory there is no SO3 in the flame over 1000°C. In fact there is at least 5%, and some say that there is as much as 50% SO3 for the following reasons: -

catalysis of the reaction by Fe2O3 (ferrous oxide) and V2O5 (vanadium pentoxide)

-

the atomic oxygen in the flame at high temperatures has a much greater chemical activity than molecular oxygen CO + O2 H2 + O2 SO2 + O

-

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CO2 + O H2O + O SO3

a substantial excess of air favors the chemical reaction.

© 2009 - IFP Training

101

SO3 FORMATION VERSUS TEMPERATURE

SO2 + 1 2

%

N° of SO3 molecules

O2

SO3

= probability

Total n°. of SO2 + SO3 molecules

100

90

At 100 ° almost all the SO2 has been transformated into SO3

80

70

60

At approx.520° there is 50% SO3 50

40

30

20

10

°C 0

100

200

300

400

500

600

700

800

900

1000

From EDF document

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1100

D MAC1242 B

At approx 1000° there is no SO3

102

2-

CORROSION AT HIGH TEMPERATURES a - Effect of metal temperature This corrosion appears above 540°C. Above this temperature corrosion doubles for an increase in metal temperature of 35°C, the temperature of the fumes remains constant. This corrosion in fact limits the yield of large boilers which have estimated yields of: 39.5% at 540°C 40% at 593°C 42% at 649°C b - Influence of the fumes temperature The temperature of the fumes affects the metal temperature and certain secondary chemical reactions, which impact the rate of corrosion. The rate of corrosion doubles when the fumes temperature increases by 150°C. c - Influence of excess air This influence is mainly characteristic of very hot fumes > 950°C: -

the higher the temperature, the greater the action of the oxygen

-

a high flame temperature gives highly reactive atomic oxygen

-

the presence of S and O2 produces SO3 as a function of temperature

-

the presence of oxygen increases the probability of transformation of vanadium oxide V2O3 into vanadium pentoxide V2O5 which is more oxidizing and acts as catalyst in the transformation of SO2 into SO3.

d - Influence of vanadium, sodium and sediments Sodium and vanadium compounds have different melting temperatures depending on the Na and V contents. At combustion chamber temperatures most of the substances are gas phase; they condense at 1500°C. Some have a very low melting point, about 500°C. Sediments such as nitrogen, Si, Ca, Hg, K and Al oxidize in the flame and produce other gas, liquid or solid phases on coming in contact with pipes. All these liquid compounds dissolve the ferrous oxides protecting the pipes. Disastrous oxidation can occur. Liquid phase oxidation is much more rapid than oxidation by oxygen in a gas phase. Moreover, all the pasty substances formed, facilitate metal deposit adhesion and thus increase clogging.

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e - Influence of sulfur The SO3 formed reacts with the parent metal, producing through oxidation-reduction (Redox): SO3 + 3 Fe
Fe2O3 + Fe S which explains the presence of ferrous sulfide at the metal-deposit interface. f - Combating high temperature corrosion -

elimination of impurities: • • •

possible for Na costs are very high for V and Ni and fuel viscosity increases distillation units all have a desalting installation which eliminates 90% of the salts obtained in the crude (except for vanadium and Ni).

-

limiting excess air

-

inhibition with additives

There are 3 types of action: -

raising the melting temperature of ashes by creating the appropriate eutectic mixtures reducing the SO3 (by neutralization or adsorption) transformation of the physical structures of the ashes to facilitate elimination

• Magnesium additives: (in the form of oxide) MgO + V2O5
MgOV2O5 melt above 1000°C The Na content should be as low as possible as first the Na2O, V2O4, 5 V2O5 complex forms which is pasty and favors the formation of deposits. The reaction with Mg only occurs afterwards. Some authors advise a Mg/V ratio of 2.5 for a temperature of 700°C. • Calcium additives This is a very fine grained CO3Ca powder. Decarbonization of the powder under the action of the flame forms highly reactive quick lime. The compounds obtained have high melting points and the ashes are fine, pulverulent and noncorrosive (they form a protective film). • Metallurgy Ferritic steels are more resistant than austenitic steels although the latter have better mechanical behavior at high temperatures which justifies their use. For fired heaters tube supports which reach temperatures of 1000°C, 25 Cr, 20 Ni alloys are used or, for more severe conditions, 50 Cr, 50 Ni or 60 Cr, 40 Ni.

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D MAC 1383 A

Corrosion rate

CORROSION RATE AGAINST EXCESS AIR

0

1

2

3

4

5

10

Air excess %

EDF document

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MELTING POINT SODIUM-VANADIUM COMPOUNDS

1000

3Na2O, V2O5

900

850 800 Na2O, V2O5

Na2O, V2O4, 5V2O5 700

2Na2O, V2O5

5Na2O, V2O4, 11V2O5

659

632

600

632

577

500

400

300

200

Na2O V2O5 + NaO 0 0

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0.1

0.2

0.3

0.4

0.5

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0.6

0.7

0.8

D MAC 1384 A

100

0.9

106

3-

LOW TEMPERATURE CORROSION The improved yield of modern equipment has meant that the fumes temperature has been reduced, so that any item of equipment downstream the acid dew point becomes vulnerable to low temperature corrosion. The equipment in question may be water heaters, air heaters or ventilation and chimney ducts. The sulfuric acid is generated from SO3 and steam produced by the combustion of hydrogen in the fuel. There are 2 equilibrium:

1 SO2 + 2 O2
SO3 SO3 + H2O
SO4H2

The “water-acid†” mixture condenses on cold elements as soon as the temperature reaches the acid dew point between 100°C and 150°C. a - Mechanism -

as the sulfuric acid condenses it attacks the steel: H2SO4 + Fe
Fe SO4 + H2 (ferrous sulfide)

-

in the presence of oxygen another reaction prevails: 1 2 Fe SO4 + 2 O2 + H2SO4
(SO4)3 Fe2 + H2O

-

the ferric sulfate formed may react on the iron and return to a ferrous state: (SO4)3 Fe2 + Fe
3 SO4 Fe this is yet another reason for burning without excess air.

b- Influence of water content The sulfuric acid in vapor phase has a negligible activity. SO4H2 hydration is practically complete at dew point temperature (see curve attached). For a certain type of fuel and 10% excess air, the following stages can be identified:

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-

at 170°C pure acid, which is only slightly active, condenses.

-

From 140°C to 100°C water starts to condense and, by diluting it, causes the acid to ionize and become active

-

from 100°C to 60°C the corrosion stabilizes and a certain degree of passivation prevails

-

below 60°C corrosion becomes extensive, passivation is no longer maintained

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CHEMICAL REACTION

SO3 + H2O

H2SO4

%

[ H2SO4 ]

Moles percent

= probability

[ H2SO4 ] + [ SO3 ]

100

Bellow200° at the SO3 is transformed into H2SO4 (vapor phase)

80

60

Above 500° there is no H2SO4

20

Temperature (°C)

0

100

200

300

400

500

600 From EDF document

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D MAC 1243 B

40

108

Dew point (°C) DEW POINT OF FUMES VERSUS SO3 CONTENT

150

100

50

20

40

60

80

100 Teneur en SO3 ppm

Dew point (°C) DEW POINT OF FUMES VERSUS THE SULFUR CONTENT OF THE FUEL (% CO2 = 7,5 %) 150 140 130 120

D MAC 2122 B

110 100 1

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2

3

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Sulfur content (%)

109

SO3CONTENT IN THEV FUMES VERSUS EXCESS AIR (400 T/H ELECTRIC GENERATOR)

ppm vol. SO3 content 8

6

4

2 02 % in fumes 0.5

1

1.5

2

2.5

3

ACID DEW POINT VERSUS EXCESS AIR (400T/H ELECTRIC GENERATOR)

°C Acid dew point 120

100

D MAC 1244 B

80

60 02 % in fumes 0.5 02947_A_A

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1

110

0.5

0.4

2

ox yg en

O

1.5 %

%

1.5 t o

0.2

1 to

3% O

3

in f

um es

in fum es

0.3

in fum es

SO2 content in fumes % volume

SO3/H2SO4 VERSUS OXYGEN AND SULFUR CONTENTS

1 to 5 0.

0.1

Sulphur content in fuel (weight) 0

1%

2%

3%

4%

Dew point temperature 140 130

3.

120

ulphu 6%s

110

il r fuel-o

1 . 8 % su l p h ur

100

fuel-oil

90 80 70 60 50 40 30 O2 content in fumes

10 0 02947_A_A

D MAC 1385 A

20

0

0.5

1

1.5

2

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2.5

3

3.5

4

111

Film forming rate (μ a/min)

100

VERY HEAVY CORROSION

MODERATE CORROSION

SLIGHT CORROSION

HEAVY CORROSION

50

ACIDE PH

ABUNDANT WATER CONDENSATION

20

40

60

80

100

120 Temperature (°C)

Corrosion rate (g/m2.h) 16 14

ACID FORMATION AND STEEL CORROSION RATE

EXCESS 02

12

- Versus wall temperature and excess oxygen in the flame

10 % 10 8 6

5%

4 D MAC 1245 B

2%

2

<1% 50

100

150

Temperature (°C) (HOFFMAN)

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60°C

70°C

+ +

Corrosion of steel wall by sulphuric acid condensation

H2O

875°C

1150°C

=

Fissuring of refractory material through shrinkage O S H2SO4 3

Condensation temperature of water in fimes

Tube

0 0 0+ < 550°C Melting of protective oxide and reoxidation of tube metal

FeO Fe2O3 Fe3O4

350°C

10 m3 air

+

1kg FO

1.51

N2

NOX

Fuel-oil n° 2 PCI 9500 kcal/kg

Vaporization of light cuts

C - S - V - Na - Ni

SO2 - CO - Na2 V2O3 - V2O4 - V2O5 Cracking of heavy cuts

1st stage of oxydation

5V2O5 - V2O4 - Na2O

2nd stage of oxydation

Ash deposits

Soot deposits

H 2O

SO2 - SO3 - CO - CO2

281

Fumes 11m3

CORROSION BY FUEL COMBUSTION

2.1% 6.4% 20.6% 0.7% )0.0% 9.1% 3.2% 0.9% 3.5% 53.5% 100%

C = 85% H = 10.6% S = 3.8% N = 0,3%

Na = 38 ppm V = 113 ppm Ni = 35 ppm

COMPOSITION

[METHANE - BUTANE - PROPANE - …]

VANADIUM PENTOXIDE VANADIUM SESQUIOXIDE VANADIUM BIOXIDE ALKALINE SULFATES HYDROCARBONS + CARBON AND VARIOUS ELEMENTS

VANADATES AND EUTECTIC MIXTURES MELTING POINT < 550°C

Organic matter silicates (SIO2) + miscellaneous Sulfates (SO3) Lime (O) Magnesia (MgO Sodium (Na2O) Total iron (Fe2O3) Aluminium (Al2O3) Nikel (NIO) Vanadium (V2O5)

COMPOSITION

112

D MAC 045 B

113

c - Combating low temperature corrosion -

use desulfurized fuels (expensive solution)

-

reduce the percentage of O2 contained in the fumes by reducing the excess air which will in turn reduce: • • •

formation of SO2, SO3, Nox, V2O5 (all pollutants) high temperature V2O5 corrosion low temperature SO3 - SO4H2 corrosion

-

try to discharge fumes at 160°C in order to have a temperature of at least 140°C on the inside walls (with very good insulation). Yield will be reduced by substantial heat loss increase

-

install glass air heaters (Pyrex)

-

inject pulverulent additives: • •

MgO magnesia the double Ca - Mg carbonate: CaMg (CO3)2 (dolomite)

These provide protection against the following types of corrosion: -

through formation of salts, which only melt at high temperatures (sulfates) low temperature by neutralization of SO4H2 according to: CO3 Ca + H2SO4
Ca SO4 + CO2 + H2O Mg O + SO4H2
Mg SO4 + H2O However, these, like vanadium and sodium oxides and when coming in contact with hot refractory material, will contribute to the production of low melting point eutectic mixtures. The gangue thus formed and fluidized, can rapidly degrade refractory linings.

-

injection of ammonia (NH3) into the fumes: NH3 + H2SO4 2NH3 + H2SO4





NH4H SO4 (NH4)2 SO4

acid sulfate melting at 147°C solid ammonium neutral sulfate

It is also known that NH3 dissociates between 370 and 700°C 2 NH3
N2 + 3 H2. Injection of NH3 at a temperature greater than 300°C should be avoided. As can be seen, this process is not simple to implement.

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VIII - NAPHTHENIC ACIDS CORROSION This corrosion is caused by the organic acid present in certain crude. These acids have a variable molecular weight; they have a saturated cyclic type radical containing acid groups. For example:

CH2

CH2

CH2

CH

COOH

D CHI 138 8 A

CH2

Bubble point 60°C at 200 mm Hg Molecular weight = 114 (This is only an order of magnitude) Most naphthenic acids in crude are found in the gas oil and light oil fractions. Ordinary steel attack starts between 220 and 400°C, usually for an acid number greater than 0.5 mgKOH/g. In fact, acid numbers of up to 10 mgKOH/g may be encountered. The corrosion product, iron naphthenate is soluble in hydrocarbons, so that attack is not slowed down by the corrosion product as it is in the case of sulfur corrosion. There is therefore rapid corrosion which is accentuated in the zones of turbulence and high flow rates. Corrosion pits with sharp edges and perpendicular to the surface of the metal is characteristic of this type of attack. Corrosion can also take the form of oriented groove with smooth and deposit-free cavities. There is no corrosion of ordinary steel below 220 to 230°C; and corrosion only appears above this range. Corrosion rates will increase rapidly with the temperature until about 400°C, above which, the acids decompose. The following figure shows the corrosion rate of the various steels and alloys versus temperature. As can be seen, only 18-8-3 (AISI 316) stainless steel has an acceptable resistance. The evident difference in corrosion rates between 18-8 and 18-8-3 steels can serve to identify naphthenic corrosion if needs be. A specimen of each alloy is placed in a liquid phase medium; if corrosion is naphthenic corrosion, only the 18-8 steel will be attacked, but if corrosion is due to sulfur, neither of the specimens will be attacked. Note that, for a given temperature, the corrosion rate is directly proportional to the acid number: corrosion of ordinary steel and low alloy steels doubles when the acid number doubles, and it is only from type 18-8 steels that corrosion rate decreases. Laboratory tests have shown that corrosion by naphthenic acids occur mainly in liquid phase. The result is that their vaporization of these naphthenic acids, which for some starts at about 230°C, causes a drop in liquid phase concentration. This can offset the increase in the corrosion rate due to a rise in temperature. By contrast, local condensation, e.g., on certain trays, enriches the liquid phase in naphthenic acids and can cause local corrosion. Paradoxically, corrosion increases, sometimes very severely, in zones of high vaporization. This can only be explained by the effect of "liquid-vapor" mixture flow rate which is very high at this point and can affect the corrosion rate.

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The activity of acids also affects the corrosion rate. Activity is considered as increasing in the following cases: -

when the molecules become smaller when the acid bubble point is approached in the liquid phase rather than the vapor phase

It thus appears that naphthenic acid corrosion depends on: -

material quality acid concentration (acid number) temperature flow velocity the activity of the naphthenic acids.

• Equipment concerned and remedies The equipment vulnerable to naphthenic corrosion attack in a distillation unit are the atmospheric column in the zones having a temperature which reaches 220-230°C; vacuum tube fired heaters (tubes, transfer lines), the vacuum column and ancillary equipment: trays, pumps, valves and pipe work. Neutralization by soda or potash can be an acceptable solution in certain cases, for instance light fractions. Heavy fractions demand highly concentrated solutions (with considerable clogging of the installations), and it is often preferable to use 18-8-3 (AISI 316) steel which has a good resistance. It is used either solid, sometimes centrifuged for heater tubes, or in plating for the walls of vessels or certain transfer lines.

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ATMOSPHERIC RESIDUE

395

Vapor

GRILLS

PACKED BED

VACUUM DISTILLATION COLUMN

VACUUM

350

Gas oil circulating reflux

250

90 mbar

360

300

Internal reflux

DISTILLATE

Circulating reflux

Internal reflux

50 200

60 mbar

CORROSION IN THE VACUUM COLUMN — Reduced crude —

VACUUM RESIDUE

DISTILLAT E2

DISTILLATE 1

VACUUM GAS OIL

342

366

320

300

280

260

220

140

67

0

AVERAGE TEMPERATURE °C

up to 20

0.3

0.6

CORROSION RATE mm/yaear

3.0

3.1

4.4

4.2

4.7

2.8

ACID NUMBRES mg KOH /g

460

410

380

350

340

260

MOLECULAR WEIGHT

0.03

0.03

0.08

0.04

0.11

LIQ.

0.37

0.50

0.58

0.71

1.50

1.95

VAP.

FLOW RATE m/sec.

116

D PCD 1332 B

117

CORROSION RATE OF SOME STEELS — With a crude containing naphthenic acids and sulfur compounds versus temperature —

Corrosion rate (mm /year) Carbone steel 3.5 5-9 % Cr. steel

3.0

2.5 18 Cr-8 Ni steel

2.0

12 % Cr. steel 1.5 18 Cr-8 Ni -3 Mo steel at high velocities 1.0

18 Cr-8 Ni-3 Mo steel at low velocities 300

350

Temperature (°C) 400

450

(From G.A.Nelson).

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D MAC 1246 B

0.5

118

IX - CORROSION IN DISTILLATION UNITS The main cause of corrosion at the head of "crude" distillation units is the presence of chlorides which can hydrolyze when hot, and produce hydrogen chloride. The chlorides come from drilling water and pollution by sea water during tanker ballasting and deballasting operations (75% NaCl, 15% MgCl2 and 10% CaCl2). • Clogging The solid sediments contained in the crude oil are found partly in the preheat and in the heater tubes, where they accentuate a tendency to coke. In order to eliminate these deposits, a minimum of 400 hours storage hold-up time would have to be respected, which is rarely the case.

1-

CORROSIVE AGENTS IN THE CRUDE In a decreasing order of corrosive action: 1-

At the head, the chlorides Mg Cl2, CaCl2, NaCl, generate hydrochloric acid, through hydrolysis in the 120 to 180°C temperature range. Such temperatures are common during preheating before and after desalting (in fact, NaCl usually only hydrolyses at about 900°C).

2-

Sulfur compounds such as H2S, sulfides, mercaptans and by decomposition in the furnaces produce mainly H2S, which is also found at the head of the column and in the condenser.

3-

Oxygen, although only found in small quantities in crude (1 ppm), acts as a depolarisant of the cathodic zones and can come from electrolysis in the electrostatic desalter.

4-

Naphthenic acids Desaturated cyclic compounds have no direct action on corrosion of the head of the tower, but regarding crude, they favor hydrolysis of sodium chloride at temperatures where this would not otherwise occur. Considering that the ClNa represents 75% of the crude salt content, the quantity of HCl thus generated is not negligible.

2-

STUDY OF THE REACTION MECHANISMS RELATED TO CORROSION OF ATMOSPHERIC DISTILLATION HEAD EQUIPMENT a - Hydrochloric corrosion An average example of the distribution of chlorides in the water contained in the crude is 85% NaCl, 11% Ca Cl2 and 4% Mg Cl2. The latter (Mg Cl2) hydrolyses when hot according to the following reaction: MgCl2 + 2 H2O
2 HCl + Mg (OH)2 Dry HCl does not corrode carbon steel. However, in the presence of water, it forms a highly ionized acid, very aggressive for the steel, which it dissolves, producing FeCl2, FeCl3. This phenomenon is encountered in the column head equipment, where the dew point of the acid solution is reached. This acid can reach azeotropic concentration, i.e. 6.6 times the normal value (241 g/liter).

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b - H2S corrosion Hydrogen sulfide is produced by the cracking of sulfide molecules at the outlet of the distillation heater. As regards the temperature of head equipment (104 to 120°C), H2S alone will have a fairly slight action, the FeS produced, tending to form a protective film against S-- penetration. However, in the presence of free HCl, this FeS film is dissolved with the formation of FeCl2 and H2S, and as there is not more protection, corrosion increases with time, and eventually pierces the condenser tubes. This phenomenon can also occur on the first trays of the topper head if cold reflux is used. In this case, the trays should be designed, as should the top of the tower, to resist this aggressive environment (Hastelloy B - Monel - Nichromaz).

3-

CORROSION COMBATING METHODS There are three possible types of action. • Use of corrosion resistant alloys This is the method used in the years when the oil industry was expanding, but it is extremely expensive. The top of the tower was Monel clad. The first 3 trays of the head were in Nichromaz C, and the head condensers in Cupro Nickel 70 30 or even titanium. This method was used before modern and effective neutralization methods were developed. b - Crude desalting and use of a pre-flash drum The desalter consists, in most cases, of a large capacity horizontal drum fitted with a system of electrodes which accelerate the decantation of solid water drops suspended in the crude (electric field). The desalter can be considered as the element which regulates the chloride content at the unit inlet. It prevents massive influx of salt water, reduces clogging of the preheating train by trapping the muds and is an indispensable complement to neutralization, which is the next treatment stage. A recent study shows that the addition of a desalter can economize on average 1 condenser bundle per year. • Role of a pre-flash drum Among other advantages, the pre-flashing of crude eliminates the water from the crude salts which are in a state of anhydrous crystal suspension in a hydrocarbon phase. This results in a lower rate of hydrolysis and a decrease in the clogging of the preheating train. c - Neutralization and inhibition At present, the practice of simultaneous neutralization in the crude and at the tower head is the most effective means to combat corrosion in the head equipment.

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120

• Neutralization of the crude * Objective The purpose of neutralization is to transform Ca and Mg chlorides which have a high hydrolysis potential into ClNa which is much less likely to hydrolyze. This practice reduces the chloride content in condensed water at the head to below 20 ppm (almost 1 to 2 ppm - NaCl). Apart from the action of the chlorides, part of the H2S and naphthenic acids are neutralized. • Implementation of neutralization This operation is done by injecting simultaneously soda (< 60 ppm) and ammonia (< 1 ppm) into the crude after desalting. • Extra neutralization at the head Injection in the form of gaseous NH3 into the head vapor line has the purpose of maintaining the pH of condensed water at between 6 and 7, corresponding to minimum corrosion. This is a complement to injection of NH4OH in crude limited to 1 ppm NH3, which is not always enough to neutralize HCl in the head. H2S is less dissociated in SH- and S ions - about pH5 - but for HCl it is preferable to have pH 7; this is a compromise which has to be accepted. The use of neutralizing volatile bases at the distillation head can generate crystal deposits (NH4Cl) which can form before water condensation. These deposits will be partially entrained by the hydrocarbons which have already condensed and will cause fouling. Therefore means should be employed to ensure that the water dew point is higher than the NH4Cl point of crystallization (there is a connection between the chloride content at the head and the crystallization temperature). Action should be taken during desalting and on complementary neutralization mainly in order to avoid fouling. • Role of extra protection by corrosion inhibitors The role of corrosion inhibitors is to form a protective film between the surface of the equipment and the surrounding corrosive environment. They make it possible to eliminate corrosion peaks when there is a momentary failure of neutralization, but cannot replace this. Classic inhibitors are long chain film forming amines which have a dipole behavior and are adsorbed on the metal surfaces on which they lose their charge and have to be replaced by other molecules, which implies a continuous injection, even for homeopathic doses (2 to 5 ppm).

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• Criteria for injection of neutralizing agents * Injection of soda Soda is injected in a solution of 3 to 6° BeaumÈ in order to obtain a chloride content in the condensed water at the head of less than 20 ppm. However, this injection is limited to 60 ppm (NaOH) due to crude train plugging and caustic embrittlement phenomena of the heater tubes and transmission lines. Although the use of spent soda with a view to saving fresh soda and in order to eliminate residues is effective, it is not recommended from the point of view of corrosion and plugging of the preheating train. • Injection of ammonium hydroxide into the crude This injection of ammonium hydroxide is implemented in the form of a 10°B solution. The injection flow rate is adjusted in order to obtain a maximum content of 1 ppm NH3 relative to the crude. Initially, this injection adjusts the pH at the head of the tower in condensed water at a value of 6 to 7. • Monitoring neutralization efficiency The end objective of neutralization is to reduce the corrosion rate of head condensates to 0.2 mm/year on carbon steel. A daily check of the various parameters should be made in order to verify the efficiency of the various injections, with corrosion monitored by corrosion probes. • Analytical criteria Desalted water and sometimes water from the tank bottom and head drum condensers are analyzed daily to determine the pH, chloride content (NaCl ppm), H2S content and dissolved iron. • Corrosion monitoring Probes are placed respectively in the condensed water circuits at the outlet of the head drums. The corrosion rate curves can thus be plotted versus time. Simultaneous to these in situ controls, the injection parameters of the various products must be recorded.

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DISSOCIATION OF HYDROSULFURIC ACID H2S IN WATER VERSUS pH

% of H2S dissociated in sulfides HS- et S--

F.C.C. condensates

75

50

25

Condensates of the atmospheric distillation head drum 5

pH 6

7

8 From "Les méthodes de la chimie analytique" G. CHARLOT

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D MAC 1247 B

Compromise

123

HYDROLYSIS OF MAGNESIUM CHLORIDE AND CALCIUM CHLORIDE DURING DISTILLATION

100 Hydrolysis (%)

80 Mg Cl 2

60

40

20 D MAC 1275 B

CaCl 2 Temperature (°C) 100

200

300

400

(From LE PETRECO MANUAL "IMPURITIES IN PETROLEUM" fig.1 p.6).

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CRUDE

20 to 300 g/t 0.1 to 0.6% volume

© 2009 - IFP Training Mixing valve

DESALTER

Washes the crude

Dissolves the mineral salts

WATER + SALTS

neutralises HCl

BASE SUBSTANCE

Transforms part of the residual salts into NaCl

CAUSTIC SODA

PREHEAT FURNACE

ATMOSPHERIC COLUMN

100 to 150°C 3 to 8% volume/crude 20 to 30 minutes about 95%

Desalting conditions Temperature: Water flooding ratio: Residence time: Desalting efficiency:

DESALTER WATER

PREHEAT

Prevents stable emulsions stable due to asphaltenes

DEEMULSIFIER

Mineral salt content: Water content:

Inlet crude oil characteristics

ATMOSPHERIC DISTILLATION UNITS — Corrosion prevention —

Steam

Protects metal from attack by H2S and HCl

INHIBITOR

GAS OIL

ATMOSPHERIC RESIDUE

Steam

KEROSENE

Steam

WATER

GAS + GASOLINE

pH monitoring

124

D PCD 481 B

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CARBON STEEL

pH

H

basique

BLISTER

1

H 2S

H2

H

2

HS -

Iron sulfide

(Hydrogen sulfide)

3

HCN

3

CN -

H2 S

4

H

Droplet of condensed water

HS -

H

5

In FCC gas there is a gaseous mixture of steam, ammonia, hydrogen sulfide and hydrocyanic acid which will dissolve when steam condenses (alcaline condensate) The carbon steel is corroded by hydrogen sulfide. Iron sulfide is created and is poreous and does not cling to carbon steel. (Fe + 2HSFeS + S2- + 2 H+). The iron sulfide is itself corroded by HCN to create ferrocyanide (blue). (FeS + 6 CN- Fe (CN)4- + S2-)

FCC gas + steam

(Ammonia) NH 3

1

2

(Hydrocyanic acid)

CORROSION MECHANISM BY HYDROGEN SULFIDE IN A BASIC ENVIRONMENT AND HYDROGEN BLISTERING OF CARBON STEEL IN A FCC UNIT

125

D PCD 692 B

126

X-

CORROSION BY POLYTHIONIC ACIDS 1-

INTRODUCTION Polythionic acids are not naturally present in petroleum products. The general formula is H2S x O6,and they are produced by oxidation and hydrolysis of sulfides in the presence of water. This type of corrosion can be considered as a spin off of hydrosulfuric corrosion. It is an intergranular corrosion phenomenon occurring in austenitic steels under cold conditions. It is found in structures under load (stress/strain) covered in sulfide deposits when oxygen and water are present (e.g. on opening apparatus).

2-

INITIATING FACTORS OF PSCC (Polythionic Stress Cracking Corrosion) On all equipment which may carry H2S and having austenitic structure, hydrofiners, catalytic reforming, cat cracker, etc., sulfide deposits may form during normal operation. During stoppages, damp air may enter the equipment and the iron sulfide immediately hydrolyses in the presence of oxygen and water, which produces compounds known as polythionic acids according to the reaction: FeS + H2O + O2
H2Sx O6 (x may be equal to 2, 3 or 4), the pH drops between 3 and 5. Conditions in which intergranular corrosion by polythionic acid occurs are: -

sensitization of the steel structure by carbide precipitation at grain boundaries. This is related, as we know, to the steel composition (high carbon) and the temperature exposure time for carbide migration

-

presence of sulfides or polysulfides on the surface of the metal

-

presence of oxygen and water;

-

presence of residual stress, in particular in the HAZ of welded assemblies or rolled sheets which have not been quenched (case of certain linings).

Carbon steel is not sensitive to corrosion by polythionic acids. a - Sensitization of AISI 300 austenitic steels Prevention of the sensitization of austenitic steels is the most important factor when combating PSCC. Sensitization of austenitic is due to the precipitation of carbides which accumulate at the grain boundaries creating preferential passages for any intergranular type of attack in the vicinity of the chromium depletion zones. This phenomenon occurs very rapidly at high temperatures, and therefore around welds. This concerns the construction phase of the plant and during operation when temperatures exceed 400°C over longer periods. The sensitization temperature is higher than 450°C for so-called stabilized titanium or type 321 or 347 Nb steels having carbide migration and precipitation kinetics which are much slower. Among structures insensitive to this type of cracking, are dendritic structure austenitic weld overlay which can be for lining material in certain hydrofining reactors.

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b - Presence of sulfide deposits These deposits result from hydrosulfic corrosion during operation of stainless steel installations or C steel lines located upstream. If these deposits can be eliminated by oxidation during decoking operations or by sanding or alkaline chemical cleaning before opening, PSCC can be avoided. c - Presence of water and oxygen These two elements are always present when installations are opened. To avoid contact with the deposits, it is recommended, when stopping without opening, to pressure-up the apparatus with nitrogen in order to avoid any influx of air and keep the internal temperature above the water vapour dew point. d - Presence of stresses in the steel Generally, prevailing stresses are always above the threshold of the critical level, below which no cracking will occur in the presence of polythionic acids.

3-

PREVENTION OF POLYTHIONIC CRACKING Use Ti or Niobium stabilized steels, or at least 304 L and 316 L low carbon austenitic steels. Avoid any influx of air or water during stoppages (keep the apparatus which are not opened under nitrogen pressure). Before opening, neutralize existing sulfide deposits and recirculate a 2% sodium carbonate solution with 0.5% sodium nitrite and a wetting agent, for four hours, to penetrate the hydrofobic surface of the deposits.

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XI - CAUSTIC SODA CORROSION Soda only attacks iron above approximately 40% weight concentration and as a function of the temperature and oxygen content of the solution. This is an electrochemical corrosion which gives the following reactions: -

anodic: (formula) cathodic: (formula)

FeO2– combines with soda, and hydrogen is released. For weaker soda concentrations of between 15 and 40% and temperatures over 80°C, generalized corrosion gives way to intergranular stress cracking. This type of corrosion only occurs if the metal has previously strain-hardened. This cracking mechanism appears to be related to the very slow rate at which the passivation layer forms. Before the formation of this layer, the metallic ions enter solution. This is more evident along the grain boundaries. When the boundaries are too deeply attacked, the passivation layer cannot form at the base of the boundaries subjected to increasingly high stresses, and corrosion is all the more rapid as a potential differential prevails between the base of the grains and the surface (passivated). Glass, tantalum, aluminum and titanium are not particularly soda resistant. A soda resistant material will also resist potassium. Sensitivity of the boundaries to corrosion is enhanced by the presence of carbon and nitrogen accumulated along the boundaries. • Remedies — Titanium steel grades to avoid the presence of free nitrogen and carbon in the steel — Addition of nickel. use of Monel-Inconel-Uranus 50 — Avoid hot spots (tracers, etc.) — Avoid points where soda can accumulate (low points) — Relaxation of welds and curves — Do not exceed HRC 22 hardness — Use of organic coating (paints) — Some authors advise raising the potential which could cause cracking corrosion to disappear through passivation.

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SELECTION OF MATERIALS — to carry caustic soda —

Above 300 °C, use a 99 % nickel - low carbon (C ≤ 0.22 %) alloy 250 240 230

Limit of use for stainless steel (304) (316) due to cracking corrosion

220 210 200

Zone

D

190 Nickel 99% C≤ 0.15 %

180

Limit of use for stainless steel due to cracking corrosion

170 160

Température °C

150 140

Zone

130

C

Stainless steel Type 304

120

Zones

110 B

Zone

C

valve faces

In nickel alloy or stellite

100 90

B and

carbon steel Stress relief necessary

80 70 Zone

A

60 carbon steel Stress relieving unnecessary

50 40

Melting point curve 30 D MAC 021 C

20 10 0

10

20

30

40

50

60 70 80 Soda concentration in % weight

From Chenical Engineering-November 12-1962 "Gilby-Fodor" catalogue (Wiggin et Huntington nikels alloys) 02947_A_A

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XII - BIOCHEMICAL CORROSION This is not actually a type of corrosion, but rather a degradation resulting from the activity of living organisms. These organisms include micro-organisms such as bacteria and macro-organisms such as mussels, algae, etc. They can live and reproduce in environments where the pH varies between 0 and 11, temperatures from - 10°C to + 80°C and under pressures of several hundred bars. These living organisms contribute to the destruction of materials through: -

1-

depolarization of anodic and cathodic reactions production of corrosion deposits through differential aeration production of corrosive agents (SO4H2, for example).

MACRO-ORGANISMS These animal and plant species attach themselves to the walls. An accumulation of these organisms creates the conditions required for crevice corrosion and fouling (the latter effect will limit more particularly heat transfer in the exchangers). • Prevention

2-

-

fast fluid flow in order to hamper attachment of the organisms

-

rough or scaly surfaces hampering the attachment of species

-

use of anti-fouling paints (these paints contain toxic substances such as copper compounds)

-

regular cleaning

-

in closed systems (recirculated water), products such as chloride can be injected which destroy the living species

-

removal of shellfish.

MICRO-ORGANISMS It is mainly bacteria which are involved in metal corrosion. They are about 1
in size and can reproduce very fast by simple cellular division. The necessary energy to sustain bacteria is provided by enzymes and catalyzed metabolic reactions. Biological corrosion often appears in the form of closely packed tubercles which can generate crevice corrosion. This laminated tuberculiform accumulation is frequently hollow and sometimes contains a blackish fluid mass. The presence of iron sulfide can easily be detected by adding a few drops of hydrochloric acid to the rust (release of H2S). Here, we will only review the principal types.

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a - Sulfate-reducing bacteria (desulfovibrio, desulfuricans) These are sulfate-reducing anaerobic bacteria, which produce the following sulfites: 2–

SO4

+ 4 H2
S2– + 4 H2O (production of sulfides and hydrogen sulfide).

The hydrogen is supplied by the cathodic ranges or organic compounds in the environment. b - Sulfur oxidizing bacteria (thiobacillus and thiooxydans) These aerobic bacteria can oxidize the sulfur contained in the following sulfur compounds: 2 S + 3 O2 + 2 H2O
2 H2 SO4. They are found in oil reservoirs and in sewage systems where they cause rapid attack of the cement. Here, there is in fact a combined effect of the sulfate-reducing bacteria at the lower part (anaerobic fermentation of settled muds) and aerobic bacteria in the upper part. c - Iron bacteria (gallionella) bacterial filaments of ferrous oxide These live in an aerobic environment and acquire their synthesis energy by consuming ferrous ions and their oxidation from ferric ions (ferric oxides which form a sheath around the cell). 2 Fe2+ + 2 OH– + H2O + O2
Fe2 O3, 2 H2O + Q kcal This reaction consumes the ferrous ions and the hydroxyl ions and depolarizes both: -

Fe
Fe2+ + 2 e 1 cathodes 2 e + 2 O2 + H2O
2 OH– anodes

with a resulting increase in corrosion. The growth of ferrous bacteria also eliminates oxygen and covers the steel with tubercles, thus favoring crevice corrosion. d - Prevention — Coating the buried structures with coal tar, plastic tape, concrete, etc. However, there are certain moulds which attack plastic coating. — Cathodic protection; the metal is also protected by a coating. — Alteration of the environment (compounds containing sulfur can be displaced by airing the sewers). — Increasing the rate of flow in order to remove deposits.

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— Injection of bactericides (chlorine, chlorinated compounds, etc.). Care should be taken regarding the association of different bacteria: sulfate-reducing bacteria are not the only ones in the water, and other species can degrade the inhibitor (which may have been very effective in another environment). Before injection of a bactericide, a list of flora and all micro-organisms present should be made. Certain laboratories advise only changing the bactericide when the analysis of recirculated water shows the beginning of habituation (alternation of 2 bactericides should be prohibited). — In certain cases, the steel pipes can be replaced by asbestos or plastic piping.

Release H2S Deposit with thiobacilus bacteria

Aerobic medium

Muds with desulfovibrio bacteria

Anaerobic medium

Production of sulphides

Double bacterial attack of a metal pipe conveying waste water

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D MAC 2126 B

H2SO4 condensation

Contribution of oxygen in humid environment

133

AE TOZ O

MOULDS

PRO

FUN YEA GI ST

ALGAE

PLANTS

ANIMALS

PROTISTS

SUPER PROTISTS (unicellular) E ALGA n) r g ee (blue-

BACTE RIA

SUBPROTISTS (acellular)

BACTERIA

Streptococcus

COCCI

Staphylococcus

Fusobacterium Corynebacterium

Bacillus BACILLT Vibrio

Spirochaete

Treponema

D MAC 1386 A

SPIRIL

ACTINOMYCETES

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From J.D. Millez, 1971

4Fe (OH)2 + O2 + 2H2O

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3Fe2+

© 2009 - IFP Training 4Fe2+

4Fe2+

ANODE

3Fe (OH)2

4Fe (OH)3

FeS

(+ O + H2O)

Fe (OH)2

S2-

4H2O

8 e-

6OH-

S2(H+)2 4

M (OH)2

2OH-

8H

8H+

From GATTELIER - IFP

CATHODE

8H2O

134

D MAC 1248 B

135

XIII - CORROSION IN AN AQUEOUS ENVIRONMENT 1-

INTRODUCTION Here, two reactions prevail on the surface of the metal: -

anodic reaction: Fe  Fe++ + 2e–

(1)

This reaction is fast in most environments, but it is usually controlled by a slower cathodic reaction. -

cathodic reaction: 2 H+ + 2e  H2


•

in an acid deaerated environment

•

in an alkaline or neutral deaerated environment 2 H2O + 2e  2 OH– + H2


•

in an aerated acid environment

1 2 H+ + 2 O2 + 2e  H2O

•

in an alkaline or neutral aerated environment

1 H2O + 2 O2 + 2e  2 (OH)–

(2)

(3)

This process is called depolarization. The cathodic reaction is accelerated as long as the oxygen can reach the surface of the metal. The reaction balance (1) + (3) gives: 1 1 Fe + H2O + 2 H2O + 4 O2  Fe (OH)3 It is the hydrated ferrous oxide (FeO, nH2O) which forms a barrier against diffusion on the metal surface. The pure substance is white, but it is usually grey due to oxidation which initiates in the presence of air. On the outer surface of the oxide film, the dissolved oxygen transforms the ferrous hydroxide into ferric hydroxide. 1 1 Fe (OH)2 + 2 H2O + 4 O2  Fe (OH)3 The hydrated ferric oxide (brownish-red color) forms most of the "rust". Depending on the oxidation rate, ferrous oxides with various degrees of hydration and various crystallographical structures will be found on the surface. The ferrous salts which form initially are not very adhesive; the rust is easily remove, and metal attack develops. In brief, the role of the oxygen in the process of rust formation is dual: -

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depolarization of the microcathodes oxidation of the formed products.

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2-

EFFECT OF OXYGEN CONCENTRATION

Corrosion of mild steel in slightly stirred distilled water at 25°C (48 hour test).

Air satured

Critical value

60

40

20

6

10

20

25

Dissolved O2 concentration (cm3/L)

D MAC 1249 B

This critical value increases with the content of dissolved salts and temperature, and decreases with the pH and turbulence. The reduction of corrosion beyond the critical value is thought to be due to the transformation of the ferrous oxide film into another film providing greater protection.

Corrosion in mg/dm2 day

In the absence of dissolved oxygen, the corrosion rate of low alloy steel is negligible. However, as soon as the oxygen concentration increases, corrosion will increase by depolarization. Beyond a critical concentration, the corrosion rate drops to a very low value.

With high O2 concentrations, pitting may appear, especially at higher temperatures in the presence of halogen ions (Cl-, B- and l-). This behavior limits the practical use of high oxygen concentrations as a means to reduce steel corrosion. • Temperature effect When the temperature increases, the solubility of all the dissolved gases, in particular free CO2 and O2 is reduced (up to 110°C for the O2). Beyond this temperature, there is a recovery of this gas, and at 200°C, there may be as much dissolved oxygen as at 30°C. In particular, if a system is open, the oxygen released by a rise in temperature can escape; the corrosion rate increases with temperature up to 80°C, and then drops at about 100-110°C.

50

C 30° 25

10°C 2

4

6

8

0

10 PPM 0 2

Effect of oxygen concentration on corrosion at different temperatures (Betz, Handbook)

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0,75

em

yst

s sed

Clo

0,50 0,25

e Op 40

tem sys d ne

80

D MAC 1250 B

Corrosion rate mm/year

70

50° C

Corrosion rate mm/year

However, in a closed system, the oxygen cannot escape, and corrosion continues with the increase in temperature until all the oxygen has been consumed.

120 160 Temperature (°C)

Effect of temperature on corrosion of iron in water containing dissolved oxygen

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A rise in temperature also affects:

3-

-

the decomposition of carbonates and bicarbonates: formation of carbon dioxide (CO2) corrosion by carbonic acid (CO3 H2)

-

the solubility of alkaline-earth salts (calcium and magnesium salts). Solubility decreases with the rise in temperature when the various equilibrium established in water are thoroughly disturbed

EFFECT OF THE SOLUTION pH

1 From pH = 4 to 10, ferric oxide is soluble; it is constantly renewed by the corrosion rate which is stable. In fact, 0,75 the surface of the metal always remains in contact with an alkaline solution of hydrated ferrous oxide saturated at 0,50 pH = 9.5

Corrosion rate (mm/year)

0 14 13

DMAC 1251 B

Iron in soft aerated water at room 0,25 temperature.

10 9

4 3 2

pH

Corrosion will depend only on the diffusion rate of the oxygen. At about pH = 10, the iron becomes passive, and the corrosion rate decreases in the presence of oxygen and alkaline. If alkalinity increases sharply, corrosion will increase slightly with the formation of Na Fe O2 or, in the absence of dissolved oxygen, the formation of Na Fe O2 and of Na Fe O2 with release of H2. Between pH = 4 and pH = 10, and for as long as the oxygen controls the reaction, small variations in the composition of the water, the heat treatment and the carbon percentage have no impact on the corrosion rate. However, for pH < 4, oxygen diffusion no longer governs the reaction alone. The hydrogen generation rate is also involved. Impurities or phases present in the steel (e.g. carbon) have a lower hydrogen over voltage, and therefore a depolarizing role. A high carbon steel corrodes at a higher rate in acids than a low carbon steel. A pH increase in water containing a solution of calcium bicarbonate and magnesium sulfate will cause, precipitation of the calcium carbonate and the magnesium hydroxide, respectively, on the surface of the metal. Deposition of these salts can form an even, adhesive layer which is actually desirable in certain cases (cathodic protection of offshore platforms).

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CORROSION OF STEEL — Versus pH of boiler water —

+

8.5 pH 12.7 pH

1

2

3

4

5

6

7

8

pH

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9

10

11

12

13

14

D MAC 1252 B

Safety zone

139

4-

EFFECT OF STRONG AND WEAK ACIDS ON THE IRON As we have seen in strong acids (H2SO4, HCl), the oxide film (diffusion barrier) on the metal surface dissolves when pH < 4. For weaker acids (acetic or carbonic), the dissolution of the oxide film appears at a higher pH. Thus, the corrosion rate of iron will increase and be associated with the appearance of hydrogen at a pH of 5 to 6. This difference can be explained by the total affinity or the greater neutralizing capacity of a partially dissociated acid compared with a totally dissociated acid for a given pH. In other terms, for a given pH, there is more H+ available to react and dissolve the oxide film using a weak acid than a strong acid. An increase in the corrosion rate of iron when the pH decreases is not only due to an increase in hydrogen generation. In fact the greater accessibility of the oxygen to the metal surface caused by dissolution of the surface oxide, favors depolarization by oxygen, and this is often the chief reason. Oxidizing acids and non-oxidizing acids or the role of dissolved oxygen on the corrosion of mild steel in acid. Ratio = corrosion rate with O2/Corrosion rate without O2

PRESENCE OF OXYGEN

IN PRESENCE OF HYDROGEN

RATIO

6% acetic

0.55 ipy

0.006 ipy

87

6% H2SO4

0.36

0.03

12

4% HCl

0.48

0.031

16

0.04% HCl

0.39

0.0055

71

1.2% HNO3

1.82

1.57

1.2

ACIDS

As can be seen, the oxidizing acids act independently of the dissolved oxygen; they act as depolarizers. This is not so for non-oxidizing acids. In general, the ratios are higher in more dilute acids. In more concentrated acids, the rate of hydrogen generation is such, that the oxygen has difficulty in reaching the surface of the metal.

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5-

EFFECT OF DISSOLVED SALTS The effect of sodium chloride on corrosion of iron in aerated water at atmospheric temperature is indicated in the following diagram.

Corrosion rate

D MAC 1253 B

2 1

0

3

5 10 15 20 NaCl concentration (% weight)

25

Effect of NaCl concentration on iron in an aerated solution at room temperature

At the beginning, corrosion increases with the salt concentration and then decreases; the rate value drops below that found in distilled water when saturation point is reached (25° NaCl). When oxygen depolarization governs the rate from one end of the NaCl concentration range to the other, it is of interest to understand why the rate increases to begin with, reaches a maximum at approximately 3% NaCl (sea water concentration), and then decreases. The solubility of oxygen in water decreases steadily with the sodium chloride concentration, thus explaining a drop in the corrosion rate at high concentrations of sodium chloride. The initial increase seems to be connected to a change in the nature of the rust film forming a diffusion barrier which is produced by the iron corrosion. In distilled water, with low conductivity, the anodes and cathodes have to be fairly close together. Consequently, the OH- ions which form at the cathodes according to the following process: 1 – 2 O2 + H2O + 2e
2 OH are always near to the Fe++ ions which form at the anodes. The result is the formation of an Fe (OH2) film adhering to the metal surface. This produces a film which is an effective barrier against diffusion. By contrast in sodium chloride solutions, conductivity is greater. Thus the anodes and cathodes can operate at greater distances from each other. For such cathodes, the NaOH does not react immediately with the Fe Cl2 formed at the anodes. Instead, these substances diffuse in solution and react to form Fe (OH)2 far from the surface of the metal. Consequently, all the Fe (OH)2 thus formed does not generate a protective film on the surface of the metal. More dissolved oxygen can reach the cathodic zone in diluted sodium chloride solutions, so the iron will corrode faster. Above 3% NaCl, the drop in dissolved oxygen is much greater than the modification of the diffusion barrier layer, so corrosion decreases.

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The alkaline metal salts (KCl, Li Cl, Na2 SO4, Kl, Na Br, etc.) affect the corrosion rate of iron and steels in the same way as sodium chloride. Acid salts which hydrolyze and produce acid solutions, cause corrosion, with the appearance of hydrogen and depolarization by the oxygen. Examples of such salts are AlCl3, Ni SO4, MnCl2 and FeCl2. Ammonium salts (e.g.: NH4Cl, NH4NO3) are also acid and produce a higher corrosion rate. The increase in corrosion due to the ability of NH4+ ions to transform the iron into a state of Fe++ ions to form a complex mole. The ammonium nitrate in high concentrations is more corrosive than chlorides or sulfates, partly because the NO3- ion has a depolarizing capacity. The complex molecule in this case is (Fe(NH3)6) (NO3)2. Alkaline salts which hydrolyze to form pH > 10 solutions act as corrosion inhibitors. They passives the iron in the presence of dissolved oxygen in the same way as NaOH (see the curve representing the effect of pH on corrosion in aerated water). Examples of such salts: Na3 PO4 (sodium phosphate), Na2 B2 07 (sodium tetraborate), Na2SiO3 (sodium silicate), Na2CO3 (sodium carbonate), with added dissolved oxygen to enhance iron passivation, these salts form a layer (corrosion products) of ferrous phosphates or ferric phosphates in the case of Na3P04 or similar components in the case of Na2SiO3. These layers form more effective diffusion barriers than FeO. Oxidizing salts These salts are: either good depolarizers, and consequently corroding, e.g.: FeCl3, CuCl2, HgCl2 Fe+++ + e–
Fe++ (depolarizing action) or effective passivators and inhibitors, e.g.: Na2CrO4, Na2Cr2O7 NaNO2, KMn O4, K FeO4 2–

(passivation) Cr2O7 + 8 H+ + Fe (surface)
2 Cr+++ + 4 H2O + O2 + O (adsorbed on the iron). Salts contained in natural water Natural fresh water contains calcium and magnesium salts dissolved in various concentrations. If the concentration of such salts is high, the water is said to be hard. If the concentration is low, the water is said to be soft. Soft water is usually more corrosive than hard water; this will be developed in the following paragraph.

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6-

EFFECT OF CARBON DIOXIDE (CO2 - CARBONIC ACID H2C03) The presence of carbon dioxide in water in the absence of oxygen results in the formation of carbonic 2– – acid H2C03, which partially ionizes to produce the ion H+, HCO3 and CO 3 . 2–

–

2 H+ + CO3

H+ + HCO3

H2CO3

CO2 + H 2O

(1)

The appearance of H+ ions contributes to the lowering of the pH: the water becomes more acid. These H+ ions also act as an electron pump so that corrosion is either initiated or accelerated. In the –

2–

case of iron and steels, the Fe+2 ions formed during dissolution react with the HCO 3 and CO3

ions

to produce soluble ferrous bicarbonate and ferrous carbonate which, as soon as the solubility product is reached, precipitate on the surface of the steel. –

Fe+2 + 2 HCO3
Fe (HCO3)2 2–

Fe+2 + CO3

(2)


FeCO3

It has been demonstrated that attack of the iron by carbonic acid is negligible for concentrations equal to or less than 10-6 mole/l. This limit concentration is a function of the pH and total CO2: Total CO2

–

HCO3 + CO3– –

+

H2CO3

CO2 + H2O

CO2 bonded

excess CO2 For a given total CO2 concentration, the higher the carbonic acid H2CO3 concentration, the lower the pH will be. Water with a H2CO3 content greater than this limit value will attack iron and steel, and will be considered as steel corrosive. It is important to clearly distinguish between this aggressiveness of water towards steel and the aggressiveness of water towards marble. Water is aggressive towards marble when the equilibrium: CaCO3 + CO2 + H2O

1 2

Ca(HCO3)2

(4)

H2CO3 is displaced towards 1 (case where there is too little calcium carbonate to balance all the CO2). The carbonate then dissolves in the form of bicarbonate (limy aggressiveness). However, if reaction 4 evolves towards 2, calcium carbonate precipitation occurs, and the water becomes scale-forming.

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Equilibrium (4) like any other equilibrium depends on many parameters, and in particular on the total CO2, the pH and temperature. By combining the theories of limy aggressiveness and corrosivity GIRARD has defined characteristic zones regarding these two chief properties on a total CO2-pH diagram so that it can be determined whether the aggressiveness of a given water will be corrosive, aggressive, non-corrosive, calcifyingcorrosive, or calcifying-non-corrosive. The advantage of such a diagram is that it is possible from a qualitative point of view to forecast the evolution of a given water in the presence of iron. It should be noted that many authors have compiled indexes for forecasting from the pH, TH, TAC, etc. whether a water will tend to be aggressive or scaling, corrosive or non-corrosive, among others: Langelier and Rysnar. It is useful to use this type of index to determine whether a water is aggressive or scaling, but precautions should be taken when using them to predict corrosion (non-protective scale, under-scale corrosion, pitting).

(H2CO3)Ca

H2CO3 = 10-6 mol/l

CaCO3 + H2CO3

6

Total CO2 in (mol/l)

4

Corrosive water

2

Aggressive water

1 0.80 0.60

Calcifuing water

0.40

D MAC 2123 B

0.20

0.1 5

6

7

8

9

10

11

If the carbonic acid is consumed (corrosive action of the water with formation of iron carbonate which precipitates), equilibrium (4) evolves towards the formation of this acid (2), which generates the formation of sodium carbonate. The deposit that forms on the surface of the steel will thus be a mixture of calcium and iron carbonate. This mixed deposit can be adhesive and provide protection against corrosion; this is known as scale inhibition.

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• Appearance of carbonic acid corrosion in the presence of oxygen - role of oxygen Although no tubercles can be seen when oxygen is not involved in the corrosion process, the presence of dissolved oxygen causes local attack with the formation of pits below the tubercles (products of reaction between the oxygen and the iron carbonates or bicarbonates). The corrosiveness of the water is increased by the presence of oxygen. 1 2 Fe (HCO3)2 + 2 O2
Fe2O3 + 4 CO2 + 2 H2O 1 3 Fe (HCO3)2 + 2 O2
Fe3O4 + 6 CO2 + 3 H2O Likewise, the reactions: Fe ( HCO3)2
FeO + 2 CO2 + H2O (high temperature) and Fe (HCO3)2
Fe (CO3) + CO2 + H2O are also possible. Carbonic acid corrosion in the presence of oxygen gives corrosion products containing FeO, Fe2O3, Fe3O4, and Fe (CO3) in more or less hydrated forms.

7-

PREVENTION OF CORROSION BY WATER 1 - Choice of suitable materials or coating the material with paint, resins or galvanization. 2 - Reducing the temperature 3 - Treatment of water to remove corrosive species (02, CO2, etc.). Deactivation: consisting in passing the water slowly over steel bars in a closed vessel. The oxygen will corrode this steel and be consumed. In order to eliminate CO2, the water is neutralized by running it through a neutralizing product (marble). Dearation: this generally consists in thermal degassing by flowing the water counter-current the vapor: O2 and CO2 are entrained. 4 - Corrosion inhibitors. 5 - Cathodic protection. 6 - Ensure cleanliness and homogeneity of the surfaces.

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XIV - ATMOSPHERIC CORROSION Steel does not corrode without humidity. For corrosion to occur, an electrolyte must be present. Corrosion depends on humidity, dust and gaseous impurities which favor condensation of the humidity on the metal.

1-

FILMS FORMED BY CORROSION PRODUCTS

Weight loss in g/dm 2 6 Rust becomes a protection with time. The corrosion rate drops with time. The oxide 4 film of low alloy steels is compact and adhesive. 2

D MAC 1254 B

Low alloy steel

0

2-

2

3

4

Years

ATMOSPHERIC CORROSION FACTORS The rates of atmospheric corrosion of metals are generally lower than those encountered in water or soil. • Dust Industrial atmosphere contains particles of carbon, metallic oxides, H2SO4, (NH4)2, NaCl and other salts which, when combined with humidity, can form galvanic cells, or through differential aeration or due to their hygroscopic properties, form an electrolyte on the surface of the metal. • Gases CO2 does not accelerate corrosion, in fact it forms a protective film. The most corrosive component is SO2 (which converts into SO4H2). Stainless steel, aluminum and lead have a better resistance than zinc, iron, nickel or cadmium. Copper forms a protective film of base sulphate. A green patina of CuSO4, 3 CU (OH)2 forms. In a coastal environment, this products a base chloride. • Humidity In an uncontaminated atmosphere at constant temperature, no corrosion should be found for a humidity of less than 100%. In practice, however, due to temperature fluctuations and to the presence of hygroscopic impurities in the atmosphere or on the metal itself, the relative humidity will be above a critical value (approximately 60%) and will create water condensation on the metal surface so that corrosion starts rapidly.

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3-

REMEDIES FOR ATMOSPHERIC CORROSION Organic, inorganic or metal coating. Vapor phase inhibitors for storing manufactured products. Use of low alloy steels (addition of small quantities of Cu, P, Ni, Cr are most effective against atmospheric corrosion). Corten steel (0.09% C, 0.4% Cu, 0.8% Cr, 0.3% Ni, 0.09% P) produces a highly protective rust in an atmosphere which is alternately dry and humid (constant plunging in water, the rust film is not more protective than the rust on ordinary steel). Stainless steel and aluminum are also highly resistant. In a marine atmosphere, the following can be used: C Hastelloy (54% Ni, 17% Mo, 5% Fe, 15% Cr, 4% W) or low alloy steels: Corten and Mariner by U.S. Steel, IN 787 by Inco or Marine 50 by Nippon Kokan. In certain special cases, it is possible to reduce the relative humidity in the air to below 50% by heating.

XV - SEA WATER CORROSION 1-

CHARACTERISTICS OF SEA WATER • Salinity This is the total quantity (in grams) of solid matter contained in 1 kg of sea water. It is in the order of 35 g/litre. The principal ions present in sea water are in decreasing order of content: Cl-, Na+, SO4--, Ca++, K+ , HCO3-, Br-, Sr++ , F NaCl represents 77.8% weight of the dissolved salts. MgCl2 MgSO4 CaSO4 K2SO4

10.9% 4.7% 3.6% 2.5%

• Dissolved gas The gases contained in air dissolve in the surface layers of the sea water. Regarding oxygen, the O2 produced by photosynthesis must be included (algae and bacteria assimilate CO2 and sunlight to produce O2). At 20°C and for a normal salinity of 3.5%, we have: Oxygen Nitrogen Argon CO2

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5.25 ml/l 9.60 ml/l 0.25 m/ 0.23 ml/l

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• pH The pH is between 7.8 and 8.3. It can drop to 7 in the presence of H2S. • Resistivity In the range of 20 to 30
cm (perfect electrolyte). Sea water also contains dissolved organic compounds (< 2 ppm), suspended solids (5 mg/l), colloidal mineral and living organism compounds, etc. (their average size varies from 10 to 50
).

2-

CARBON STEEL CORROSION This is an electrolytic type corrosion Fe
Fe2+ + 2e-

Anodic: Cathodic:

1 2 O2 + H2O + 2e
2 OH 2 H+ + 2 e-
H2 ou 2 H2O + 2 e -
H2 + 2 OH-

Secondary reactions persist and the ferrous hydroxide formed is oxidized by the dissolved oxygen into hydrated magnetite and ferric hydroxide. Cl- is also involved. This corrosion can be uniform, but heterogeneities on the surface of the metal or in the electrolytic layer tend to localize the anodes (differential aeration under deposit). The exact composition of the steel is of little importance; however, the condition of the surface (in particular calamine) has an important role. The layer of calamine, which is very protective, is unfortunately not unbroken and pits will form. Calamine has a higher potential than steel (0.3 V). The presence of galvanic couples is particularly harmful. The general expression of uniform corrosion rate is:

Corrosion rate =

O2 diffusion coeff. . O2 concentration . actual diffusion area thickness of the diffusion layer

• Temperature The corrosion rate doubles every 10°C. The use of steel is limited to 43°C. Moreover, for higher temperatures, the precipitation of: carbonate for t > 65°C sulfates for t > 143°C should be taken into consideration.

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• Circulating velocity The thickness of the diffusion layer will depend on the flow regime (laminar or turbulent). The corrosion rate increases with an increase in circulation velocity. The turbulence maintains high concentration of oxygen by stirring up the water. • Suspended solids These have an abrasive effect which lowers the protective capacity of corrosion products. • Fouling Fauna (barnacles, mussels, bacteria, etc.) and flora (algae) can accumulate on the surface of the steel and form cells. Flowing at rates greater than 1 m/s will usually remove them. Remedies -

3-

suppression of living organisms in the case of exchangers by intermittent chlorination medium and steady circulating velocity cathodic protection (soluble anodes for the exchangers) previous descaling (tank bottoms - exchangers) Brai-epoxy + anti-fouling type coating (for offshore structures)

CORROSION OF STAINLESS STEELS These are sensitive to: -

pitting corrosion (absorption of Cl- through the passive film) crevice corrosion (differential aeration + acidization) intergranular corrosion stress corrosion and fatigue corrosion.

• Austenitic steels Grade 316 L has a good resistance to pitting, but like all austenitics, it is sensitive to crevice corrosion. Impurities such as sulfur have a disastrous effect. Integration of at least 2.5% Mo is highly recommended. • Ferritic and austeno-ferritic steels Uranus 50 by Creusot Loire has a better resistance than 316, but has still not adequate resistance to crevice corrosion.

4-

CORROSION OF NON-FERROUS METALS • Nickel alloy Monel: overall corrosion is proportional to the O2 content and this alloy has no anti-fouling capacity, as the Cu content is not high enough (avoid aerated water). Ni - Cr - Mo alloys: Incoloy 825, Hastelloy C and Inconel 625, together with Titanium are almost exempt from sea water corrosion (the Inconel 625 also has good weldability and can be used for surfacing steels).

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• Aluminum alloys Good resistance of these alloys in oxygenated water (constant replacement of oxide film). • Copper alloys These are sensitive to erosion, and also, deposits on the surface can initiate differential aeration corrosion. Copper and its alloys do not owe their good resistance only to a passive film, but to an intrinsic resistance due to the nobility of this metal.

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0.06 0.06

< 0.03

0.02

< 0.03

0.02

0.02

0.02

316 317

Uranus 50

Uranus B6 904 L Nicrofer

NSCD

Sanicro 28

254 SMO

Hastelloy C

Alu 5454

Titane

< 0.3

15

20

27.5

17.5

20

22

17 17

Solde

18

31

16

25

8

12 12

66.5

16

6.1

3.5

>5

4.5

2.5

2.5 3.5

0.7

1

3

1.5

1.5

31.5

70

28.4

Monel 400

90

8.4

Cu

Cupro-Nickel

Mo

76

Ni

Aluminum brass

Cr 70

C

Naval brass

METAL

99.8

Ti

<1

?

?

1.2 ? ?

<2

<2 <2

1

Mn

22

29

Zn

Solde

2

Al 1

Sn

5

1.25

1.6

1.6

Fe 0.04

As

< 4.5

Mg

65

40

38

34

34

30.5

25.5 28

Eq. Cr

> 20

"

"

"

20-50

20-50

20-50

3

2.5

2

1.5

Vitesse M/sec <

Sweden

when

Seems valid (results not well known)

This is sensitive to under-scale corrosion from 140°C. But it can withstand high velocities and provides a weight gain

Avesta

Sandvick

Ugine and Vallourec

Creusot Loire

Austeno-ferritic recommended stress cracking is expected

This is the minimum which can be used for stainless steels

Good resistance in moving sea water

The cupro-nickels are not H2S resistant (< 0.1 ppm)

Here, there is a Fe2O3 gamma layer which provides good resistance to localized erosion corrosion

The NH4+ ions and Hg are harmful for brass

Addition of As inhibits zinc depletion. The Sn forms a protective compound

150

151

Brass (Cu - Zn) Brasses resist erosion better than copper alone. They are sensitive to dezincification and stress corrosion. Naval brass (60 Cu - 39 Zn - 1 Sn) and manganese bronze have good erosion resistance but are sensitive to dezincification. Aluminum brasses (76 Cu - 22 Zn - 2 Al - 0.04% As) are used for condenser tubes (they have a good resistance to erosion and are inhibited with arsenic to avoid) dezincification. The presence of magnesium neutralizes the action of arsenic. Cupro-Nickels: 70 Cu-30 Ni with added iron is used for the exchanger tubes. Cupro-aluminums: Inoxyda 90, misnamed aluminum bronze (10 Al - 5 Ni - 2 Fe - 85 Cu) is used for the tubular plates of exchangers (molded). Titanium is still very expensive, and there is a critical temperature (about 110°C), above which differential aeration corrosion may occur. Plastics The advantage of these materials is that they do not corrode and are light. However, there are limits related to risk of combustion, low mechanical properties and lower resistance to hydrocarbons and solvents. They are also sensitive to UV rays (although this problem seems, at the present time, to have been reduced by the use of effective pigments) which is a drawback when repairs are necessary. For the piping, either thermoplastics: polyethylene, polypropylene, PVC are used, or thermo-hardening plastics: epoxies, fiberglass reinforced polyesters. There are also plastic coatings: -

epoxy paints or epoxy pitch baked formophenolic products powdered epoxy and polyamide combination, which have the combined advantages of thermoplastics (flexibility and resilience) with those of powdered epoxy (high temperatures).

Coatings Galvanizing in a bath of melted metal or Schoopage. Concrete cladding.

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XVI - DRY OXIDATION AT HIGH TEMPERATURE A cross-section of an oxidation film formed on iron at 700°C shows a thick layer of ferrous protoxide FeO in contact with the iron, overlaid with a layer of magnetite Fe3O4 and a fine layer of sesquioxide Fe2O3. For temperatures below 570°C, FeO is unstable, and the film contains mainly Fe3O4 overlaid with Fe2O3. The FeO formed at high temperatures partially decomposes when cooled according to the equilibrium: Fe3O4 + Fe

4 FeO

1-

MECHANISM Pilling and Bedworth scale: where

Wd R = Dw

W = molecular weight of the oxide w = molecular weight of the metal D and d = respective densities of oxide and metal

R represents the volume of oxide formed for a unit of metal volume. For R < 1, the volume of oxide formed is insufficient to cover all the metal. For R > 1, compressional stresses enter the oxide, which tends to crack. A protective oxide should have the following properties: -

1 < R < 2 (R should be near to 1) expansion coefficient close to that of the metal good adhesive properties high melting point low vapour pressure low diffusion coefficient (low electrical conductivity) high temperature plasticity (to resist cracking) TABLE R for some oxides Protective Non-protective Cu Al Cr Ni Pb Si

1.68 1.28 1.99 1.52 1.4 2.27

Na Mo W Ti Li V Fe (FeO) Mg

0.57 3.4 3.4 3.4 0.57 3.18 2.06 0.84

• Electrochemical aspects of oxidation Metal oxides are semi-conductors. The ions can also move through the oxides via vacancies, but the electronic conductivity is much greater than the ionic conductivity, and it is this which governs oxidation. 02947_A_A

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There is oxidation of the metal Fe
Fe2+ + 2 e and 1 reduction of the oxygen 2 O2 + 2 e
O2-

Fe ++

1 Global reaction Fe + 2 O2
FeO.

O 2e-

D MAC 1255 A

Oxidation is mainly retarded by reduction of the diffusion of ions through the oxide film. This diffusion is reduced in certain cases by doping, i.e., the addition of elements which reduce the vacancies in the oxide itself. These vacancies are a result of the stoichiometric difference.

O2

In fact, the oxide is not stoichiometric; there is an excess of oxygen or a lack of iron due the ferrous ions becoming ferric.

O- -

Fe++

O- -

Fe++

Fe++

O- -

Fe++

O- -

O- -

Vacancy

O- -

Fe+++

Fe+++

O- -

Fe++

O- -

O- -

Fe++

O- -

Fe++

Fe++

O- -

Fe++

O- -

On the following figure, it can be seen that the presence of 2 Fe+++ ions compensates for the absence of Fe++ regarding the electrical charge. There are therefore vacancies in the oxide through which the ions migrate.

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• Prevention of iron oxidation Ferrous oxidation can only be limited by reducing the vacancies. This is difficult for FeO, as part of the Fe++ ferrous ions oxidise to Fe+++ and stoichiometry is not achieved. Nickel is not suitable either, as it also lacks cations, nor is the addition of chromium advised, as it transforms to Cr+++ (up to 5% Cr, this is observed with Fe-Ni alloys). In fact, a very adhesive and protective binary or ternary compound Ni Cr2O4 type is obtained, which prevents any diffusion. It has been noted that, for high pressure and high temperature services, it is the 18-20 Cr and 8-10 Ni alloy steels which resist oxidation best.

Fe2O3 Fe3O4

Fe

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D MAC 2172 A

FeO

155

2-

OXIDATION KINETICS For iron, the layers formed flake off, and oxidation occurs almost only by diffusion of Fe++ cations through the FeO oxide. The law of oxide growth is linear to begin with, before the film has formed, and then becomes parabolic.

Weight gain mg/cm2 Weight gain mg/cm2

t 2 =K x

1

D MAC 1256 B

x=

Kt

10

10

3-

100 Time (min)

Linear (catastrophic oxydation Na,K, Nb, Ta) Parabolic (Fe,Co,Cu) Logarithmic (observed in oxide films at low temperatures Al, Cu,Fe at ambient for films < 1000 Å) D MAC 1257 B

100

Time

HIGH TEMPERATURE MATERIALS Alloy steels and refractory alloys developed for use at high temperatures must have: -

4-

good creep resistance (deformation when hot under permanent load) good atmospheric resistance (mainly oxidizing environments) resistance to thermal shocks (fast cooling from 900°C to ambient temperature) For very high temperatures, Mo, W, Nb and Ta should be added to improve mechanical properties.

ATMOSPHERES ENCOUNTERED AT HIGH TEMPERATURES • Carburizing atmospheres and carburization Atmospheres with a high CO content (burning of fuel or hydrocarbons) generate chromium carbides which precipitate at the grain boundaries and cause chromium depletion of the matrix; this then oxidizes more easily. The increase in the Ni content slows carburization. In general, carburization alternates with oxidation, giving: -

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formation of carbide and Cr depletion in certain zones oxidization of depleted zones appearance of cracks in the oxidized zones under the action of stresses

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Temperature (°C)

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400 20

600

800

1000

1200

22

Feα + wustite

Feγ + wustite

24

Feα + magnetite

Wustite + (FeO)

26 Weight % oxygen

Wustite + magnetite

28

Magnetite (Fe3 O4)

Wustite + hematite

30

Hematite (Fe2 O3)

Hematite + oxygen

32

156

IRON/OXYGEN DIAGRAM D MAC 1258 B

157

Atmosphere

Oxidising

Maximum temperature for constant service use (°C)

Steels to use

Observations

650

5-6% Cr Steel

Delicate welding

850

17% Cr Ferrite

Delicate welding, low creep resistance

900

18-8 Austenite and derivatures

Avoid welding, low creep resistance

1100

28% Cr Ferrite 25-12 Austenite 25-20 Austenite 35% Ni Austenite and 20% Cr 60% Ni and 15-20% Cr Alloys

1150 80% Ni and 15-20% Cr Alloys

Carburization Reducing

900

25-12 Austenite 25-20 Austenite

1000

35% Ni and 20% Cr Austenite

1100

60% and 15-20% Cr Alloy 80% and 15-20% Cr Alloy

Sulphidation reducing

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700

18-8 Austenite

750

17% Cr Ferrite

900

25-12 Austenite 25-20 Austenite

1000

28% Cr Ferrite

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To avoid welding

To avoid welding, low creep resistance

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0

100

200

300

350

500

700

800

r 7C

900 °C

1 18-8 18-8 Nb 18-8 Mo 18-8 Ti

12 C 12 C r r-Al 5C r -0, 5M o 1,5 Cr

Comparaison of corrosion rates for ≠ steels at T° of 595 at 925°C for 1000 h

600

5

o; 2C r 0 , 5M C o; -r 0, 2 , 2 5 Cr5M 1 Mo 9 Cr - o 1 Mo

M l 0,5 e e t ne s Carbo

mg/cm2

0,5

1,00

1,5

2,00

530

mm/year

650 Oxydation rate versus T°

600

9 Cr-1 Mo °C 700

,25 Si

5 Cr-0,5 Mo

2 Cr-0,5 Mo

1 Cr-0,5 Mo

1 ,5 Mo3 Cr-0 5 Cr-0,5 Mo-1,25 Si

Carbone

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steel

OXYDATION OF STEEL VERSUS TEMPERATURE

0,5

1,0

550

3C r0 , 5M o 700

From document ELF

Oxydation at high temperature effect of silicium

650

i 1,25 S o M 5 3 Cr-0, 600

mm/year

D MAC 1259 C

°C

158

159

Carburization can also occur by diffusion of the carbon through the steel and from the circulating product. There is a risk of hardening (when cooling during a stoppage) with a sharp drop in ductility. • Reducing atmospheres The alternation between carburizing and oxidizing atmospheres is dangerous. The protective oxide film will be destroyed with gradual chromium depletion of the metal.

XVII - CORROSION BY LIQUID AMMONIUM The presence of oxygen (in the form of impurities) in a tank containing liquid ammonium favors stress corrosion cracking. The ammonium fixes the oxygen during transfer. A content of 2.5 ppm (weight) in the liquid phase is considered the upper safety threshold in the presence of 100 ppm water. Larger oxygen contents can be accepted for greater water contents, but it is always best to keep the oxygen content as low as possible. If an item of equipment is empty over a long period, it is essential to maintain a nitrogen atmosphere throughout the duration, or a small amount of ammonium. Regarding vessels which have not been relaxed or only partially, the following must be respected: -

the oxygen content should be kept as low as possible whenever possible, keep a water content > 0.2% weight otherwise, refer to the following figure and perform regular analysis of the oxygen content

Liquid water in ppm (weight) 10000

1000

A

Low risk of stress corrosion cracking

100

High risk of stress corrosion cracking

10 1

10 100 Liquid O2 in ppm (weight)

1000

D MAC 1260 B

B

Do not put the equipment on stream in conditions delimited by this zone. The oxygen must either be reduced or water added* for A or B services conditions. * If water is added intentionally, it should be distilled water or an equivalent quality condensate.

(Source: APAVE technical publication)

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XIX - REINFORCED CONCRETE CORROSION (Reference Document SIKA - Martine MASSON) 1-

CORROSION OF STEEL STRUCTURES IN A HUMID ENVIRONMENT The corrosion of steel embedded in reinforced concrete is one of the major causes of degradation. The formation of expansive rust (rust expansion: 800%) causes cracks to appear and then splitting and scaling, with the metal structure being laid bare. Corrosion of the embedded irons is an electrochemical phenomenon, the principle of which is the formation of microcells on the surface of the metal in an aqueous medium containing dissolved salts (chloride, carbonate, nitrates, etc.). Water, with a high salt content (marine environment, for example), can reach the steel structures via the pore network of the concrete. The metal which, in a natural state, is oxidized, tends to recover this stable form by emitting positive ions. The presence of microscopic heterogeneities (impurities, oxides, etc.) favors the formation of a microcell or electrolytic couple.

2-

NATURAL PROTECTION OF STEELS BY CEMENT The pH of water contained in the pores has an decisive impact on the corrosive potential of a metal. In PORTLAND cements, which are likely to be used in reinforced concrete, the alkalis (soda potassium) dissolve in the water at the time of hydration. Similarly, as the silicates hydrate, they produce free lime and the pH becomes clearly basic (11 to 12). The pH then remains at this value due to the steady generation of lime by the silicates. The lime causes the formation of a ferrite Fe2O3CaO which forms deposits on the surface of the metal: the steel structure is passivated by this protective film. As long as the metal is surrounded by this basic solution there is no corrosion risk. Also, in the case of aluminous cements, hydration of the monocalcic aluminates liberates alumina, which forms deposits on the steel structure and protects it.

3-

WEAKENING OF THE NATURAL PROTECTION OF THE EMBEDDED METAL STRUCTURE: ALTERATION OF THE CONCRETE Several agents can attack this natural protection and favor corrosion of the steel. They are: -

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carbon dioxide oxygen sea water aggressive agents pure water frost

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a - Action of carbon dioxide: carbonation Air contains 0.03% carbon dioxide; this content can vary with temperature, pressure, the environment (towns, industrial areas) and reach 0.10%. The lime liberated by hydration of the silicates can carbonize according to: Ca (OH)2 + CO2

CaCO3 + H2O

This reaction, catalyzed by the atmospheric humidity, progresses from the outside inwards and causes gradual neutralization of the cement alkalinity: the basic medium (pH = 11 to 12) loses its alkalinity and the pH drops to below 9. Natural protection of the embedded metal is no longer ensured. Moreover, water containing carbon dioxide generates weak acid (H2CO3: carbonic acid) and attacks the lime and lime carbonate according to the following reactions: Ca (OH)2 + CO2
Ca (HCO3)2 Ca CO3 + CO2 + H2O
Ca (HCO3)2 The lime bicarbonate thus formed is soluble in water and the concrete is destroyed by gradual watering down of the binder (photos 2 and 4) and sometimes stalactites form. Rate of carbonation: the layer of carbonate hampers the diffusion of carbon dioxide. However, for concrete in an ordinary atmosphere at 65% R.H., carbonation progresses as follows: 1 year .......... 4 years .......... 25 years ..........

5 mm 10 mm 25 mm

}

Dept at which natural protection is eliminated

This is why the metal structures are covered with 2 cm concrete. • Factors affecting carbonation: -

type of cement: the more lime contained in the cement, the slower carbonation

-

dosage of water and cement: when the cement content is increased, the carbonation rate will decrease fast as the quantity of lime for carbonation will be greater (figure 3).

If a large quantity of water is used for making up the cement, it can leave voids or porosity which favor penetration and diffusion of carbon dioxide: -

implementation: strong binding increases the density of the concrete and makes it more waterproof; this will slow down carbonation.

-

environment: in a water saturated atmosphere, the fixing of carbon dioxide is weak; water only dissolves its own volume of gas.

In a dry atmosphere, the carbon dioxide hardly reacts at all and the water acts as a catalyst. In practice, the carbonation rate is maximum for a mean relative humidity (approximately 50%) (figure 2).

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b - Action of the oxygen When the carbon dioxide has removed the passivating lime film, the oxygen can reach the metal structures: ferrous hydroxide, and then ferric hydroxide form. This chemical attack, unlike electrolytic corrosion, is found at all points of the metal structure and does not require an electrolytic environment. c - Action of sea water or gritting salt The mechanism of salt water action on cement is highly complex, and here we will only give an overview. Salt water contains mainly: -

sodium chloride magnesium chloride magnesium sulfate calcium sulfate potassium carbonate acid

NaCl MgCl2 MgSO4 CaSO4 KHCO3

Magnesium sulfate is the most harmful of these salts. It reacts with the hydrated lime to produce gypsum CaSO4, 2 H2O and brucite Mg (O)2. Also, its action on the aluminates in the cement result in the formation of ettringite (Ca3Al2O6, 3 CaSO4, 31 H2O). Expansion due to crystallization of the ettringite (approximately 300%) causes cracking and the sea water penetrates through to the metal. The ettringite or CANDLOT salt can also form when the water has a high concentration of selenite. Corrosion in the splash zone: In this zone, the concrete undergoes desiccation-rehydration cycles which have various effects: -

the aggressive environment allows anhydrous salts to crystallize. These are rehydrated when coming in contact with the water and cause swelling, which can eventually cause the concrete to burst (e.g., reaction): Na2SO4 + 10 H2O
Na2SO4 , 10 H2O (corresponds to a volume increase of 320%!).

-

in addition, there is an incremented effect of oxygen and carbon dioxide in these zones, thawing phenomena, effect of light and temperature.

d - Action of aggressive agents In an industrial atmosphere, the highly aggressive environment contains acids, alkalis or oxidizers which can seep into the concrete and attack the metal directly. Rainwater can entrain certain aggressive agents such as sulfur dioxide, SO2, coming from the burning of petroleum and coal wastes or nitrogen oxides from engine exhausts. The rain is acidized, and a pH in the range of 2.5 in certain large industrial centers (North of France, Ruhr, England, etc.) can be observed. This aggressive water can dissolve the salts in the cements and depassivate the metal structure. In some zones (marshes, forests), the humid acids can, by capillarity, rise in the concrete and considerably reduce the pH. In this type of environment, corrosion can also occur through the development of bacteria.

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e - Action of pure water The purity of rainwater or condensation water makes it a remarkable solvent which entrains the soluble salts and waters down the binder. Gradual deterioration of concrete exposed to rain •

Rainwater penetrating the concrete by capillarity reduces compactness.

•

Carbonation, accelerated by the fact that the structure is in a zone polluted by CO/CO2 (centre of Paris), will have penetrated in depth and reached the irons. Protection by the lime in the cement is thus eliminated.

•

Steel structures will oxidize and exert a pressure strain on the layer of concrete, the resistance of which (cohesion) will be weakened and will burst.

f - Frost effect When the water in the capillary network of the concrete freezes, the pressure of the ice causes microcracking (expansion rate of ice: 9%). Successive freezing-defreezing cycles will enlarge the cracks gradually until the concrete bursts.

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- Figure 1 Chloride ion

Sodium ion Na +

Cl -

Na Cl Sodium chloride

Hydrated ferrous ion

-

Ferrous ion

+ e-

e-

e-

Impurities

Fe 2+

Fe 2 O3 Redish-yellow rust (precipitate)

Fe (OH) 3

Cathode

Fe 2+

water

O2 + H2O + 2 ( e - )

Fe Cl2 Ferrous chloride

r wate

O 2 oxygène

Na

Fe Cl3 Ferrous chloride

water

Hydroxide ion

Na OH soda

water

water water

+

Electrical charges (electrons)

EMBEDDED METAL STRUCTURE (external cell circuit) 1,0 3

DEGREE OF CARBONATION

/m

g 0k

20

0,8

0,6

3

0,4

2

30

3

m

g/ 0k

4

3

kg/m 350 /m3 kg 400 /m3 g 500 k

1 0,2 Scale time 3 month 1 year 0

- Figure 2 -

20

40

60

80

100%

5 year

Influence on cement dosage on carbonation rate (CERILH document)

RELATIVE HUMIDITY Influence on relative humidity on carbonation (CERILH document)

- Figure 3 -

+2

pH easily monitored using phenol-phtaleine

Carbonation not yet initiated

III

+1

Passivation Fe2

0

+

Alkaline corrosion

Acid corrosion Immunity

-1 0

1

2

3

- Figure 4 -

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Fe3 O4

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IV 4

5

6

Fe 7

8

II

9 10 11 12 13 14 15 16

pH POURBAIX diagram for iron

Fe2

+

D MAC 1261 B

pH > 9 → concrete turns pinkish

POTENTIEL (VOLT)

pH < 9 → concrete does not change colors Carbonation occurs

I

165

PROGRESSIVE DEGRADATION OF CONCRETE

Atmosphéric pollution

Marine environment and highways

Rain Condensation

Mécanical action

Pure water

CO2

SO2,NO2...

carbonate water

Aggressive water

Sea water or defrosting

O2 (oxygen) Pore Network

Expansive salts (ettringite.…)

Cracks

Solution of salts

Depassivation of metal structures

Oxydation of metal structures

Opening of cracks Bursting metal structure Laid bare

Expansive rust

Watering down Efflorescence

Widening of cracks

1st step Fine crack Efflorescence Traces of rust

2nd step Widening of crack Grumbling of corners Swelling of concrete

3rd step Bursting between cracks Bursting along steels Deep corrosion of steel

Gravity of damages

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D MAC 2124 B

Frost

166

4-

PREVENTION All failures are due to compactness or density. The penetration of water into the pore network is the responsible vector. The water can rise through capillarity over a height of: 15 m in a 1
diameter tube 7.5 m in a 2
diameter tube 3.25 m in a 4
diameter tube. For outside structures, dry climates will therefore favor long life for the concrete more than climates which have frequent changes of rain and wind, causing an influx of water and rapid evaporation. It is necessary to take preventive measures against corrosion by installing barriers between the outside environment and the embedded metal. Care should be taken to choose concrete of the right quality and to ensure direct protection of the steel. a - Concrete quality The concrete provides the main protection of the metal and must prevent the water from filtering through to the metal. The two main factors are therefore high compactness and low cracking potential: -

high compactness: in order to reduce the porosity of the concrete, it is necessary to reduce the quantity of water used in making up the cement to a minimum. Plastifiers should be used, as these make it possible to reduce the water by 10 to 15%. Integrated water repellents can provide good protection, but by a different means, as they leave crystals in the pores which hamper the progress of the water.

-

low cracking potential: the metal structure must be sufficiently covered and the steel properly configured.

b - Protection of the concrete Penetration of water into the concrete can be avoided either by taking protective measures (projecting cornice, etc.) which prevent rainwater washing over the structure, and also by waterproofing the surface with paints. Structures can also be protected without changing the surface appearance, by using a silicone water repellent. Water repellent mortar cladding provides a very effective protection against water infiltration. c - Protection of the metal Apart from natural protection of the steel by cement, there are many possible solutions. A cement coating made up with a resin and applied by brush or dipping. Thanks to the sealing properties and alkalinity of the film, this method provides long lasting protection.

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Other processes used more rarely: • Metallization: it is possible to spray a layer of metal onto the iron: -

if the metal is more noble than iron, the protection will only be provided if the layer is smooth and thick

-

if the metal is less noble, this protection is ´†sacrificial†ª. This is the case of galvanization in which zinc acts as anode and iron acts as cathode. Protection is thus provided by the activity of the iron-zinc cell at the expense of the zinc. The activity of the protective cell ceases with the removal of the last traces of zinc.

There are other methods for protecting prestressed cables: protection using oil, plastic coating, paints, etc. d - Concrete of the future Fluidizers are integrated (liquefaction of concrete without water, which is added at the site) and silica smoke. (The diameter of the particles is 0.1 micron, whereas a grain of cement represents 10
.) The density obtained is much greater. Porosity is reduced, impermeability increases and so does resistance to freezing and degradation in general; bubbles are fewer and smaller. The cladding of the metal structure is improved. It is easy to achieve a resistance to compression of 800 bars which corresponds approximately to the resistance of the granular material. The best formula corresponds to 2.5% fluidizer and 6% silica smoke. Carbonation does not exceed one millimeter. Any crack which may appear would have catastrophic effects, as here, pH is low. Moreover, for a compressive strength of 1,000 bars, the concrete will be fragilized due to a sort of vitrification (which is why it is limited to 800 bars).

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