Metallic Coating

Protective coatings are used to protect the metals from corrosion. The main types of protective coatings are classified as follows.

Protective Coating

Metallic Coating

Hot dipping Galvanising Tinning Metal spraying Cladding Cementation Sherardizing Chromising Calorizing Electroplating or Electrodeposition

Non-metallic Coating

Surface or chemical conversion coating Chrome plating Phosphate coating Oxide coating Anodizing

Organic Coating

Paints Varnishes Lacquers Enamels

The protective coatings must be chemically inert to the environment and also sufficiently thick. Besides protection from corrosive conditions, such coatings can also give decorative appearance to the base metals. To be more effective, these coatings should adhere well to the surface. PRETREATMENT OF THE SURFACE OR PREPARATION OF MATERIALS FOR COATING The outermost surface of the base metal (which is to be protected) usually contains impurities like rust, scale, oil and grease: These substances, if present at the time of coating, will give porous and discontinuous coatings. In order to get a uniform, smooth and coherent protective coating, these substances are removed by the following methods. Degreasing Oil and grease may be removed by cleaning with organic solvents such as chloroform, toluene and acetone. Immersion in hot alkaline solutions is the most commonly used cleaning technique. For example, sodium carbonate and sodium hydroxide are used for this purpose. Removal of Oxide Scales or Descaling

Removal of the oxide scales and corrosion products (rust) from the surface is called descaling. In this process, the base metal is dipped inside the acid solution at higher temperatures. The acid penetrates through cracks and pores of the scales and then their dissolution takes place. Acids like sulphuric acid, hydrochloric acid and nitric acid are used under dilute conditions. Mechanical Cleaning Oxide scales, rust and corrosion products are also removed by abrasion such as grinding, wire brushing and polishing. Electrochemical Method The electrochemical method is used to remove oxide scales which are not removed by other methods. The base metal is made either anode or cathode with an electrolyte (acid or base). At the anode the oxide scale is dissolved in the electrolyte and leaves the base metal, whereas at the cathode the metal oxides are reduced to metal. METALLIC COATINGS Surface coatings made up of metals are known as metallic coatings. These coatings separate the base metal from the corrosive environment and also function as an effective barrier for the protection of base metals. Metallic coatings are mostly applied on iron and steel because they are cheap and commonly used construction materials. Metallic coatings are usually imparted by the following methods. Hot Dipping It is used for producing a coating of low melting metal such as Zn, Sn, Ph, Al etc on relatively higher melting metals such as iron, steel, copper etc. This is done by immersing the base metal covered by a layer of molten flux. The flux is used to keep the base metal surface clean and also to prevent oxidation of the molten metal. In the process of hot dipping, the metal to be coated is dipped in the molten bath of the coating metal and the thickness of the coating is adjusted by squeezing out the excess of the coating metal with rollers. Such hot dip coatings are generally non-uniform. The common examples of hot dip coatings are galvanizing and tinning. Galvanizing The process of coating a layer of zinc on steel is called galvanizing. The steel article is first pickled with dilute sulphuric acid to remove traces of rust, dust, etc, at 60-90°C for about 15-20 minutes. Then this metal is dipped in a molten zinc bath maintained at 430°C. The surface of the bath is covered with ammonium chloride flux to prevent oxide formation on the surface of molten zinc. The coated base metal is then passed through rollers to correct the thickness of the film. This method is widely used for protection of Fe from atmospheric corrosion in the form of articles like roofing sheets, wires, pipes, nails, screws, tubes etc. It is to be noted that galvanized utensils should not come in contact with acids.

It is used to protect roofing sheets, wires, pipes, tanks, nails, screws, etc. Tinning The coating of tin on iron is called tin plating or tinning. In tinning, the base metal is first pickled with dilute sulphuric acid to remove surface impurities. Then it is passed through molten tin covered with zinc chloride flux. The tin coated article is passed through a series of rollers immersed in a palm oil bath to remove the excess tin. Tin-coated utensils are used for storing foodstuffs, pickles, oils, etc. Galvanizing is preferred to tinning because tin is cathodic to iron, whereas zinc is anodic to iron. So, if the protective layer of the tin coating has any cracks, iron will corrode. If the protective layer of the zinc coating has any cracks, iron being cathodic does not get corroded. The corrosion products fill up the cracks, thus preventing corrosion. Tinning is widely used for coating steel, Cu and brass sheets which are used for making containers for storing food studs, oils, kerosene & packing food materials. Tinned Cu sheets are used for making cooking utensils & refrigeration equipments. Metal Cladding Cladding is a method by which the surface to be protected is sandwiched between two layers of the coat metal and pressed between rollers. The clad metals used are aluminium, chromium and nickel. In this process, a thick homogeneous layer of coating metal is bonded firmly & permanently to the base metal on one or both the sides. This method cnhanceds corrosion resistance. The choice of cladding material depends on the corrosion resistance required for any particular environment. Nearly all existing corrosion resisting metals like Ni, Cu, Al, Ag, Pt and alloys like stainless steel, Ni alloys, Cu alloys can be used as cladding materials. Cladding can be done by different means. a.

Fusing cladding material over the base metal

b.

Welding

c.

Rolling sheets of cladding material over base metal.

Generally cladding is done by arranging thin sheets of the coating metal and the base metal sheet in the form of a sandwich which are then passed through rollers under the action of heat & pressure. For aircraft applications, sheets of duralumin are sandwiched between 2 layers of pure Al.

Copper-clad steel is used in the electrical industry, because this kind of steel has good electrical conductivity and high strength. Cementation In cementation, the base metal is heated with the coating metal in the form of fine powder in order to promote the diffusion of the coating metal into the base metal. The coatings obtained are of uniform thickness. The base metal is generally steel and the coating metals used are zinc, chromium and aluminium. When the coating metal is zinc, the process is called sherardizing. When the coating metal is chromium, the process is called chromizing. When the coating metal is aluminium, the process is called calorizing. 1. Sherardizing: Cementation with zinc powder is called sherardizing in honour of Sherard Cowpercoles who developed the process in 1900. The base metal is heated with zinc dust in a metal drum maintained at a temperature of 350-370°C. The drum is closed tightly and rotated with constant heating for two to three hours. During this process zinc gets diffused into iron forming an alloy of Fe-Zn on the surface. Sherardized coatings are used for protecting small steel parts such as nuts and bolts against atmospheric corrosion. 2. Chromizing: The base metal is heated with a powdered mixture of 55 per cent chromium and 45 per cent alumina at a temperature of 1300-1400°C for about 3-4 hours in a closed drum. The purpose of using alumina is to prevent the coalescing of chromium particles. The outermost surface of the base metal is converted into a chrome alloy which protects the metal against corrosion. This method is used to protect gas turbine blades. 3. Calorizing: Here the base metal is heated with a powdered mixture of aluminium and alumina in a drum at a temperature of 840-930°C for 4-6 hours. Calorized steel is used for making furnace parts.

CHEMICAL CONVERSATION COATINGS OR SURFACE CONVERSATION COATINGS These coatings are produced on the surface of a metal or alloy by chemical or electrochemical reaction. The metal is immersed in a solution of suitable chemical which reacts with the metal surface producing and adherent coating. These coatings protect the base metal from corrosion. Moreover many of these coatings are particularly useful to serve as excellent bases for the application of paints, enamels and other protective

coatings. The most commonly used surface conversion coatings are chromate coatings, phosphate coatings and chemical oxide coatings.

1.

Chromate coatings

There are produced by the immersion of the article in a bath of acidic potassium chromate followed by immersion in a bath of neutral chromate solution. The surface film consisting of a mixture of trivals and hexavalent Cr is formed. Chromate coatings possess more corrosion resistance and can also be used as a base for paints. These are applied on Zu, Cd, Mg and Al

2.

Phosphate coating

These are produced by the chemical reaction of base metal with aqueous solution of phosphoric acid and a phosphate of Fe, Mn or Zn. The reaction results in the formation of a surface film consisting of phosphate of a surface film consisting of phosphates of the metal. These coatings are usually applied by immersing or spraying or brushing. These coating do not give complete corrosion resistance but can serve as base for painting. These are applied on metals like Fe, Zn, Cd, Al and Sn.

3.

Chemical Oxide Coatings

These types of coatings are formed on the surface of metals like Fe, Al, Mg etc by treating the base metal with alkaline oxidizing agents like potassium permanganate. This treatment increases the thickness of the original oxide film on the metal, there by increasing the corrosion resistance. Oxide coatings form a good base for paints. These oxide coatings have got only poor corrosion resistance. However, for better protection the thickness of the oxide film can be increased 100 to 1000 times by electrolytic oxidation or anodisation.

Anodisation or Anodised Coatings Anodised coatings are generally produced on non – ferrous metals like Al, Zn, Mg and their alloys by anodic oxidation process. In this process, the base metal is made as anode and the cathode is an inert electrode like graphite. The electrolytic bath is usually of H2SO4, chromic acid, boric acid, phosphoric acid, oxalic acid etc

The base metal to be anodized is suspended from the anode. The process is carried out by passing a moderate direct current through the electrolytic bath. As the anodized coatings are somewhat thicker than the natural oxide film and they posses improved resistance to corrosion. Anodizing on Al has gained considerable commercial importance. Al coated surface require oxidation to convert the metal to its inert oxide. Anodising on Al is carried out by an electrolytic process. The Al article is made as the anode by suspending it from the anode material. The cathode can be made of any inert conducting material like graphite. The electrolyte is dil. H2SO4. Electrolysis is carried out at moderate temp (35 – 40 0C) & moderate current density. The O2 evolved at the anode oxides the outer layer of Al to the oxide film, Al2O3. the oxide film initially very thin, grows from the metal surface outwards and increases in thickness as oxidation continues at Al anode. The outer part of the oxide film formed is porous and to reduce porosity, the article after electrolysis is kept immersed in a boiling water bath. This treatment changes porous alumina into its monohydrate (Al 2O3.H2O) which occupies more, volume, thereby the pores are sealed. 4 Al

+ 3 O2

Al2O3 + H2O

Al2O3 Al2O3.H2O

Anodized coatings may be coloured with organic dyes and inorganic pigments to give decorative effects. Applications Aircraft parts, refrigerators, reflectors, machine parts etc are anodized by this method Al articles used as doors, windows, showcase panels & household utensils are anodized by this method.

Electroplating or Electrodeposition

Electroplating is a process in which metals are deposited or plated on base metals from solutions containing metallic ions by means of electrolysis. The objectives of electroplating are as follows: I.

To obtain improved resistance to corrosion and chemical attack

2.

To get better appearance

3.

To get increased hardness

4.

To change the surface properties of metals and non-metals

In the electroplating process, the freshly cleaned base metal which is to receive the coat is made the cathode in a suitable electrolyte bath containing (a)

a solution of the salt of the metal to be electrodeposited

(b)

buffer solution to control the pH

(c)

additional reagents to enhance conductivity and to aid the formation of smooth, dense and coherent coating.

The concentration of the salt solution is maintained by the addition of the metal salt at regular intervals or by the use of continuously dissolving anode of the metal. The plating is usually done at a high current density. The nature of the deposit depends upon the current density, pH and the concentration of the bath. A typical electroplating process is shown here.

The various' types of electroplating are as follows. Chromium plating For protecting iron, chromium plating is widely adopted. A warm bath of chromic acid and a high current density (100-200 mA/cm2) are used. The temperature maintained is about 45-50 °C. The anode is lead and the cathode is the base metal to be plated. Chromium coats give an excellent shining appearance and are stable because of the formation of an invisible, strongly, adherent protective

layer of Cr2+ on exposure. However, chromium coats are porous and contain fine cracks. Hence the article to be chromium plated is given a copper coating to give protection from corrosion which is then nickel plated. Chromium is finally placed on the nickel coat. Nickel plating A bath of nickel sulphate and nickel chloride is used. A pure nickel sheet is used as anode and the base metal as the cathode. The operating conditions are as follows: • Current density : 20-50 mA/cm2 • Temperature : 45-55°C • pH : 4-5 Nickel coats are not quite stable, the highly polished surface deteriorates with age with the appearance of rust spots. Copper plating A bath of acidified copper sulphate solution and copper anode are used. The base metal is used as the cathode. The operating conditions are as follows: • Current density : 20-50 mA/cm2 • Temperature : 30-40oC Copper plating is used in printed circuit boards. It is coated at the bottom of the stainless steel cooking utensils to effect better heat transfer. Gold plating Double cyanide of gold and potassium is used as the electrolytic bath. A pure gold plate is used as the anode and the base metal as the cathode. The operating conditions are as follows: • Current density : 1-10 mA/cm2 • Temperature : 60°C • pH : 9-13 It is used for high quality decoration and high oxidation-resistant coatings. Factors affecting electroplating The following are the factors affecting electroplating: 1.

Cleaning of the article is essential for a strong adherent electroplating.

2.

Concentration of the electrolyte is a major factor in electroplating.

3.

Low concentration of metal ions will produce uniform coherent metal deposition.

4.

Thickness of the deposit should be minimized in order to get a strong adherent coating

5.

Additives such as glue and boric acid should be added to the electrolytic bath to get a strong adherent and smooth coating.

6.

The electrolyte selected should be highly soluble and should not undergo any chemical reaction.

7.

The pH of the electrolytic bath must be properly maintained to get the deposition effectively.

Applications of electroplating The following are the applications of electroplating: 1.

Plating for decorative purposes Gold, silver, copper, nickel, chromium, etc. are the most widely used metals for decorative plating. The base metals used are steel, zinc, lead, copper, aluminium alloys, etc.

2.

Plating for protection In order to protect steel from rusting and chemical attack, it is electroplated. Protective metals applied are zinc, cadmium, tin, etc. To provide both protection and good appearance, steel is given successive coating layers of copper, nickel and chromium. Copper and nickel provide protection against rusting and chemical attack. The chromium plate gives a clean metal appearance. In case of food containers, tin plating is preferred as it not only protects steel but also does not contaminate the food.

3.

Plating for special surface and engineering effects Electroplating occupies an important place in engineering. A composite part can be designed in which the base metal is an alloy. It possesses the required mechanical properties but has poor resistance to surface attack under severe conditions. The required surface stability can be provided by means of electroplating.

ELECTROLESS PLATING In electro less plating, metal ions in solution are reduced to metal atoms and are deposited on the substrate surface without using electrical energy. It is a technique of depositing a noble metal from its salt solution on a catalytically active surface of a less noble metal by employing a suitable reducing agent without using electrical energy. The reduction is accomplished by the presence of a reducing agent. The reducing agent supplies the electrons for the reduction of metallic ions to metal which gets deposited over the substrate surface giving a uniform thin coating. M2+ + 2e- (from the reducing agent) ~ M (Deposited over substrate surface)

A number of metals like nickel, cobalt, copper and some precious metals can be deposited by electroless plating. The most popular metal deposited electrolessly is nickel. Various Steps Involved in Electroless Plating The various steps involved are as follows: 1. Preparation of active surface of the substrate: This is achieved by using anyone of the following methods. (a) Etching: The unwanted particles are removed from the surface by acid treatment (b) Electroplating: A thin layer of the metal or any other suitable metal is electroplated on the surface. (c) Treating with stannous chloride followed by dipping in palladium chloride: This treatment yields a thin layer of palladium on the surface. This method is followed in the case of plastics and printed circuit boards. 2. Preparation of plating bath: The constituents of the electroless solutions are mainly: (a) Metal salts to provide metal ions for deposition. (b) Reducing agents like formaldehyde and hypophosphite for reduction of metal ions into metal atoms. (c) Complexing agents like EDT A and trisodium citrate to complex metals ions to prevent bulk decomposition. (d) A stabilizer like thiourea to prevent the decomposition of plating bath solution. (e) Accelerators like succinates and fluorides to accelerate the reduction process. (f) Brighteners like cadmium ions and lead ions to improve the brightness of the deposit. Some Electroless Platings Electroless nickel plating Electroless nickel plating involves the following steps: 1. Pretreatment of the surface: The preparation of surfaces for electroless nickel plating depends on the metal on which the deposition is to be done. (a) Metals such as cobalt, steel and aluminium will be directly nickel plated without any pretreatment. (b) Metals like lead, cadmium and tin are to be given a first coating of electrodeposited nickel prior to electro less nickel coating. (c) Non-conductors like glass, plastics and ceramics are first activated in a solution of SnCl2 and HCI followed by dipping in a solution of PdCl2 and HCI. On drying, a thin layer of Pd is formed on their surface.

2. Bath composition: The following is the bath composition of electro less nickel plating: Nickel chloride (coating solution)

30 g/l

Sodium hypophosphite (reducing agent)

10 g/l

Sodium acetate (buffer)

50 g/l

Sodium succinate (complexing agent)

15 g/l

pH

4-6

Optimum temperature

85-95°C

3. Process: The pretreated surface is immersed in the plating bath. The reduction of nickel ions occurs and the nickel gets deposited over the surface.

Properties The following are the properties of electro less nickel platings: 1. They possess better corrosion resistance than electroplated nickel deposits. 2. They give rise to harder surface with better wear resistance. 3. The coating is solderable and weldable. Applications The applications of the electroless nickel platings are as follows: 1. They are used in the electronic industry for fabricating printed circuits, diodes, etc. 2. They are used to prevent galling of aluminium, titanium and stainless steels. Electroless copper plating The process involves the following steps: 1. Pretreatment of the surface: The preparation of surfaces for electroless copper plating depends on the metal on which the deposition is to be done. (a) Metals like Fe, Co and Ni can directly be copper plated without any pretreatment. (b) Non-conductors like glass, plastics and ceramics are first activated in a solution of SnCl2 and HCl followed by dipping in a solution of PdCl2 and HC!. On drying, a thin layer of Pd is formed on their surface.

2. Bath composition: The following is the bath composition of electroless copper plating: Copper sulphate (coating solution)

12 g/l

Fonnaldehyde (reducing agent)

8 g/l

EDTA (Complexing agent)

20 g/l

Buffer solution (i) NaOH

15 g/l

(ii) Rochelle salt

14 g/l

pH

11

Optimum temperature

25°C

3. Process: This process involves the reduction of cupric ions to their metallic state at a controlled rate.

Applications The preparation of a printed circuit board is done by electro less copper plating The process is known as substractive method. A thin layer of copper is first electroplated over the printed circuit board (PCB) made up of ~ass reinforced rubber. Then selected areas are protected by employing electroplated image. The remaining area is now etched away so as to obtain the required type of circuit pattern. In order to pack more number of components in a small space, usually double-sided tracks are made. Finally the connection between the two sides of PCB is made by drilling holes followed by electroless copper plating through holes. Electroless copper plating is used in double or multilayered boards in which plating through holes is required. However such holes cannot be copper electroplated.

Advantages of Electroless Plating over Electroplating The following are the advantages of electro less plating over electroplating: 1. Uniform deposits are produced. 2. Deposits may be directly produced on non-conductors or insulators (plastics, glass, etc, 3. Coatings produced have unique chemical and physical properties. 4. No electrical energy is required. 5. Deposits are usually less porous. Limitations of Electroless Plating Though electro less plating is versatile in its applications, the process has serious limitations like high chemical cost, slow coating rate and poor stability of the solution. ************