Three common conversion coatings

There are three common types of conversion coating used to protect metal parts from corrosion. Not only do these coatings extend the lifespan of metal parts, they deliver no dimensional change and create a ready primed surface.

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Also referred to as chem film or chromate coating, these types of surface treatments have very clear advantages, whether an additional finish is to be applied or not. Economical and easy to use, conversion coatings fall into three main categories – oxide, phosphate and chromate coatings.

Oxide coatings

Oxide treatments are conducted by electrochemical reaction, heat or chemicals and produce an extremely thin coating that is resistant to corrosion.

– Gun-bluing involves heating the metal, usually steel, to 370°C in a steam atmosphere. This type of coating provides little resistance to wear or corrosion

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– Chemical baths produce a similar result using a chemical dip

– Black oxide treatments are applied as a paste to steel and copper and produce an aesthetic finish with no hydrogen embrittlement

– Anodising works through a process of electrochemical conversion and provide excellent levels of corrosion resistance. The surface won’t tarnish and retains a natural metallic sheen. This is a complex surface coating and is best created by a specialised company like

Phosphate coatings

This process of chemical conversion uses either manganese or zinc to coat metals like carbon steel and cast iron. These coatings are applied by immersion or spraying to a range of products including bearings and springs, transmission systems and military equipment, where a Chemical Agent Resistant Coating (CARC) is required to resist harsh environments.

Phosphating is one of the most useful non-metallic coatings and provides strong adhesion and corrosion resistance. These surface coatings can also improve friction for sliding parts like bearings.

Chromate coatings

Chromate coating is another chemical conversion process that creates an extremely thin film that is also highly corrosion resistant. Applied to aluminium, this type of coating ensures that the good conductive properties of that metal are retained.

Aluminium is immersed in a water solution of chromic salts or acid which produces a gelatinous film that should not be disturbed for 24 hours. A chromate coating makes an excellent primer with high adhesion for a paint finish. This type of coating is also widely used for household products including hardware items like screws and hinges which exhibit a distinctive yellow-brown lustre.

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