Metals And Some Aspects Of Corrosion.pptx

Arjeane Doctor Roy Vincent Domalaon Reden Clerigo Vanessa Cometa

METALS

 The exception of hydrogen, all elements that form positive ions by losing electrons during chemical reactions called metals. Thus metals are electropositive elements with relatively low ionization energies.

Properties of Metals: • State: metals are solids at room temperature with the exception of mercury, which is liquid at room temperature. Luster: metals have the quality of reflecting light from their surface and can be polished.

• • Malleability:

metals have the ability to withstand hammering and can be made into thin sheets

known as foils. Ductility: metals can be drawn into wires.

• • Hardness:

all metals are hard except sodium and potassium, which are soft and can be cut with

a knife.

• Valency: metals typically have 1 to 3 electrons in the outermost shell of their atoms. • Conduction: metals are good conductors because they have free electrons. Silver

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and copper are the two best conductors of heat and electricity. Lead is the poorest conductor of heat. (bismuth, mercury, and iron are also poor conductors. Density: metals have high density and are very heavy. Iridium and osmium have the highest densities whereas lithium has the lowest density.

• Melting and Boiling Point:

metals have high melting and boiling points. Tungsten has the highest melting and boiling points whereas mercury has the lowest. Sodium and potassium also have low melting points.

Chemical Properties of Metals • Electropositive character: metals

tend to have low ionization energies and typically lose electrons(i.e. are oxidized) when they undergo chemical reactions they normally do not accept electrons.  Compounds of metals with non-metals to be ionic in nature. Most metal oxides are basic oxides and dissolve in water to form metal hydroxides.  Metals oxides exhibit their basic chemical nature by reacting with acids to form metal salts and water.

CORROSION  Corrosion is a chemical process that gradually breaks down materials.

 Electrons are exchange and new molecules form.

Corrosion Theory for Metals  Pure metals contain bound energy, representing a higher energy state than that found in nature as sulphides or oxides.

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Energy state of metal in various forms.

Anode and Cathode

 In analysing corrosion, the first thing that must be determined is whether a metal reacts with its environment. If so, the nature of the reaction must be understood. It is generally accepted that corrosion processes are caused by the formation of electrochemical cells. The electrochemical reactions in these cells can be divided into two reactions:

1) Anodic reactions 2) Cathodic reactions where each reaction is called a half-cell reaction. In the anodic reaction, metal goes into solution as an ion. The reaction is generally written as: M --> Mn+ + newhere M is a metallic element, e- is an electron and n is the valence of the metal as an ion. An example of this is Zinc where: Zn --> Zn2+ + 2e-

CORROSION CELLS

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 The current flows through the lamp from the copper plate to the steel plate.

The copper will be the positive electrode and the steel will be the negative electrode.

Anode: Electrode from which positive current flows into a electrode. Cathode: Electrode through which positive electric current leaves an electrolyte.

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The prerequisites for the formation of a bimetal cell are: 1. Electrolyte 2. Anode 3. Cathode 4. Oxidation medium, such as dissolved oxygen (O2) or hydrogen ions (H+)

When positive iron atoms go into solution from the steel plate, electrons remain in metal and are transported in the opposite direction, towards the positive current.

Corrosion in Micro-Cells

 The anode part of the surface corroded.

Parameters affecting the corrosion rate: •

Oxidizing agents

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The electric conductivity of the electrolyte

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Temperature

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Concentration

Types Of Corrosion 1) Dry or Chemical Corrosion •

by oxygen, other gases and liquid metal

 Chemical corrosion can be seen as oxidation and occurs by action of dry gases, often at high temperatures.

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Oxidation of metals in different temperatures.  Oxidation reaction: The anode reaction, by which electrons are given up to the electrochemical cell.

 Reduction reaction: The cathode reaction, by which electrons are accepted from the electrochemical cell.

2) Wet or Electro Chemical Corrosion •

by galvanic, pitting, crevice, waterline, stress and concentration cell or differential aeration.

 Electro chemical corrosion on the other hand takes place by electrode reaction, often in humid environments.

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An electrochemical cell is composed of pure lead electrodes in solutions of their respective divalent ions. For 0.6 M concentration of Cu2+, the lead electrode is oxidized yielding a cell potential of 0.507 V. calculate the concentration of Pb2+ ions if the temperature is 25℃.

3) General Corrosion •

General corrosion on a competitors cast iron pump.

 General corrosion is characterized by an overall attack on the surface. The corrosion takes place without distinguished and cathode areas.

5) Pitting Corrosion •

Pitting corrosion on a stainless steel stator housing operating in sae water.

 Typical examples of pitting corrosion can be seen on aluminium and stainless steels in liquids containing chloride. (e.g. Water)

4) Galvanic Corrosion •

Galvanic corrosion of eye bolt connected to an stainless.

 When two different metals are electrically connected and in contact with an electrolyte(liquid), they will form a galvanic cell where the more noble material is cathodic and the less noble anodic.

6) Crevice Corrosion •

Crevice corrosion on a stainless nut exposed to seawater.

 Crevice takes place in confined liquid filled slots and crevice where the liquid circulation is prevented.

8) Stress Corrosion •

Austenitic stainless steel subjects to stress corrosion cracking.

 Tensile stresses arise for example during cold work of steel sheet or as a result of directly applied load. Stress corrosion is generally connected with austenitic stainless steels in contact with liquids containing chlorides.