Coatings of Metal: 12 Methods | Industries | Metallurgy

The following points highlight the twelve main methods adopted for coating of metals. The methods are: 1. Hot Dipping 2. Galvanising 3. Tin Plating 4. Lead Coating 5. Metal-Cladding 6. Diffusion 7. Sherardizing 8. Calorising 9. Chrome-Diffusion 10. Siliconising 11. Anodising 12. Parkerizing.

Method # 1. Hot Dipping:

Hot dipping coatings can be used for coating high melting point metals such as iron and steel, zinc, tin, lead and an alloy of lead and tin called terne.

The coating metal should be of such a composition that it forms an alloy at the interface between the base metal and the coating. If the coating does not form an alloy, pre-coating with a metal which alloys both with a base and the principal coating can be used.

Zinc forms a more protective coating on iron than tin and terne, but it is unsuitable for the preparation and storage of foods, where tin coatings are quite satisfactory. This is due to the frequent acid nature of foods which are strong enough to dissolve the zinc but do not attack tin. Terne coating is never used for food preservation due to danger of lead poisoning.

Method # 2. Galvanising:

It is the name given to the coating of zinc by hot dipping. The quality of the coating is directly related to its thickness which is measured in gms/m² of surface for all products except sheet where it is measured in gms/m² of sheet (2 sides).

Heavy coatings of zinc are most easily applied by hot dipping than electrolytically. The purity of zinc affects its bonding property and the appearance of the coating, and besides that it has no effect. So where bonding does not matter, low grades of zinc can be used.

It is mainly used in roofing, wire fencing for gardens, etc., buckets and water cans, steel pipes and for structure purposes exposed to atmosphere.

Method # 3. Tin Plating:

Tin plating has a pleasing appearance, good corrosion resistance and it solders well. Copper wire is very often tin-plated to increase its suitability for soldering. This coating is usually applied by hot dipping before the wire has been drawn down to final diameter and the subsequent operations draw down the copper and tin coating together.

The various applications of tin plating include tin cans for food, biscuit-tins, kitchen utensils, copper wire, copper tube used in refrigeration etc.

Method # 4. Lead Coating:

The lead coatings for hot dipping process contain 2½% tin and 2% or antimony, the presence of which strengthens the coatings considerably. The coatings used generally are thicker than terne but thinner than zinc.

To compensate for the drawback of poor adhesion of coatings lead has decided advantage because it does not alloy with steel. The steel is not embrittled and can be used for deep drawing etc. It takes paints and solders well. To obtain the best possible adhesion, the steel must be perfectly cleaned before coating.

Lead coating is a cheaper substitute for terne and it can be used for spinning and deep drawing operation.

Method # 5. Metal-Cladding:

Cladding is usually performed by attaching plates of relatively thick metals together such as by welding and then rolling them down together. The centre core and plating metal retain their proportion as the cross-section is reduced.

The use of “Aluminium Clad Sheet” has already been mentioned in connection with aluminium alloys where the superior corrosion resistance of aluminium is added to the strength of duralumin. It is much used in air-craft construction.

In a like manner steel may be clad with copper, cupro-nickel, and rolled gold is made by rolling a gold alloy into a brass or cupronickel-base. This method in general allows the protection of a material which combines the strength of a core with the corrosion-resistance of the plated material. Unlike electroplating, there is no danger of porosity.

Method # 6. Diffusion:

It resembles the process of cementation for making steel and case hardening. The article being treated is heated in contact with the metal being diffused which penetrates into the surface thereby producing an alloy.

The concentration of diffused metal in the article is highest at the surface and falls off rapidly within less than 1.25 mm. Steel is treated chiefly with four metals which form useful diffusion alloys at the surface, namely zinc, aluminium, chromium and silicon. Hardly any dimensional change occurs in the treated articles.

Method # 7. Sherardizing:

So called after the name of its originator is the name given to zinc diffusion process which has been used extensively. The parts to be treated are trembled in a barrel to 350°C with powdered zinc or a powder of zinc and zinc oxide known as blue powder.

The period of heating depends upon coating required varying from 3 to 12 hours for a coating of (0.025 to 0.075 mm) thickness. However, there is a drawback of brittleness in this process and so its use is restricted to treatment of small nuts and bolts exposed to atmosphere, and small castings etc.

Method # 8. Calorising:

It is a diffusion process of aluminium powder mixed with roughly equal proportion of aluminium chloride. This is carried out at a much higher temperature (about 900°C) i.e., well above the melting point of aluminium. The time of process is 4 to 6 hours, but this may be followed by retaining for such a longer period at 950°C out of contact with aluminium in order to homogenise the layer and to increase its depth.

Calorised parts are resistant to both corrosion and at elevated temperatures are protected by the impervious alumina films which is the characteristics of aluminium. It is used principally as a protection against corrosion and heat which occur in furnace and exhaust pipes.

Method # 9. Chrome-Diffusion:

It produces a surface layer similar to stainless steel. It is carried out by heating the steel articles in a mixture of 55% of chromium powder or powdered ferro-chrome and 45% of alumina in the presence of hydrogen atmosphere. The temperature used is above 1000°C at which one hour is sufficient to produce coating of about 0.1 mm thickness and containing 10—20% of chromium.

These coatings are used where the corrosion or wear is severe, but this application does not merit considerably because of its higher cost. Also it is not a good substitute as compared to stainless-steel because coating produced by diffusion tends to be brittle.

Method # 10. Siliconising:

It is carried out for two hours at above 1000°C in a mixture of silicon-carbide and ferro-silicon in an atmosphere of chlorine. The parts siliconised are very hard and process is carried out for wear corrosion or heat resistance. A typical application is in valves, valve seats, and guides of internal combustion engines.

Method # 11. Anodising:

It is an oxidising process used for aluminium and magnesium articles. The article to be anodised is made anode and sulphuric, oxalic and chromic acids are used as electrolyte. The coating is produced entirely by the oxidising process and not by plating. The coating so produced is hard but at the same time it is porous enough and hence advantageous from decorative point of view.

Such oxide coatings enable organic coating and dyes to be successfully used on aluminium article surfaces. Modern aluminium glasses and pitchers arc the examples of this class.

Method # 12. Parkerizing:

It is a process used for making thin phosphate coatings on steel to act as a base or primer for enamels and paints. In this process steel articles are dipped in a heated solution of magnesium dihydrogen phosphate at temperature of 88°C for about 45 minutes. During the dipping period, the phosphate from magnesium dihydrogen phosphate decomposes and phosphate separates out and forms a thin coating on the steel articles.