COATING

A variety of coatings are applied to the metal surfaces to improve appearance, appeal and acceptability by the customer and to provide protection against likely contamination by human contact or atmosphere. Coatings also improve functional abilities such as insulation, corrosion, resistance, fire protection, etc. Coatings as available in the market have variations in the length of their life, ease in application, effectiveness of protection and the cost. Usually, there interim coatings, (b) exists a direct relationship between the applied cost of a coating and the degree of duration it will provide. From this consideration, coatings can be divided as durable coatings and (c) permanent coatings.

(a) Interim coatings: 

These provide rust prevention on the components which are in the process of manufacturing. Lubrication of surfaces is sometimes provided to help the further forming operations, particularly in deep forming and wire drawing. The best known and widely used interim coating process is that of chemical conversion (or conversion coatings). These are further divided into phosphate coatings, chromate coatings and anodic coatings.

(b) Durable coatings: 

Durable coatings last for longer period. These are further classified as (i) organic coatings derived from resins such as alkyds, vinyls and epoxies, and (ii) inorganic coatings derived from porcelain enamels and various plasma-sprayed materials.

(c) Permanent coatings or metallic coatings: 

Metal or inorganic coatings are considered permanent coatings. The principal coating materials include: zine, tin, lead, copper, cadmium, chromium, brass, aluminium and stainless steel. These are applied on job surfaces by various methods such a hot dipping, plating, metal spraying, vacuum metallizing and sputtering. Different types of important coatings used in industry are described in the following under subheads given below: (a) Conversion coatings

(b) Organic coatings
(c) Inorganic coatings 
(d) Metallic coatings

CONVERSION COATINGS

Certain metals are capable of reacting with various chemical agents in such a way that the resulting solid chemical compounds are precipitated on the surface. These coatings are produced in the form of a film of corrosion resistant (precipitated compounds) deposited on the surface of the base metal as a result of controlled chemical attack by the chemical agents. The most widely used conversion coatings are:

(a) Phosphate coatings

(b) Chromate coatings.

(c) Anodic coatings

Phosphate Coatings

Phosphate coatings are provided on metal parts through a chemical reaction where the part to be coated is treated with a hot solution of phosphoric acid or phosphate salt. The chemical reaction that takes place on metal surface results in the formation of a film of non-metallic coating. Phosphate coatings are principally used as a base for the application of paint or enamel as the coating itself does not become an effective rust-proof treatment on the base metals (usually iron and steel). Various phosphate coatings are: iron phosphate coating, zinc phosphate coating, lead phosphate coating and magnesium phosphate coating.

Chromate Coatings

Chromate coatings are used on non-ferrous materials for added corrosion resistance and a base for paint. The coating media are: chromate, dichromate and chromic acid. The coatings are provided by spraying or dipping in a chromate bath, which is an acidic solution containing hexavalent chromium compounds.

Anodic Coatings

The anodizing gets its name from the fact that the base metal becomes the anode (instead of cathode as in electroplating process). The anodizing is mostly used for aluminium parts. An electrolyte capable of yielding oxygen on electrolysis is used. When current is passed, oxygen is liberated at the job surface, forming an oxide film. After the anodic treatment, the oxide. film is sealed with boiling water. It is possible to impart excellent coloured coatings by immersing the parts in warm dye solutions, followed by sealing the dye in porous oxide coating by dipping in dilute nickel acetate. For anodizing aluminium and its alloys, only acid solutions are used as electrolytes and these are sulphuric acid, chromic acid and oxalic acid. Sulphuric acid gives a thin transparent film exhibiting natural colour of aluminium. Chromic acid gives an oxide film which is milky white in colour, which may be changed to grey colour by varying bath temperature. Oxalic acid gives an opaque oxide film which can be changed. to light yellow or cream colour by varying voltage and bath temperature.

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