Corrosion protection by surface coatings

The final solution for corrosion protection is the coating of the surface with a film of an inert substance. This can be a layer of a metal with better protection properties than the bulk material. One example is the covering of iron or unalloyed steel by a zinc layer with a thickness of some micrometers. Zinc coating is possible by dipping into a hot zinc bath or by electroplating. The zinc layer must be further treated for sufficient corrosion protection. This can be chromatizing as described in the previous chapter or by a newly developed process free of chromate. 

Another example is chromium plating on steel parts. The typical process consists of a primary copper layer, a secondary nickel layer, and the final chromium plating. 

Nickel-plating itself is used for corrosion protection without additional chromium. Usually two nickel layers are plated from different plating baths. A nickel layer of dull finish covers a bright nickel layer. The nickel layers have slightly different potentials, which develop some cathodic protection. 

An inorganic coating with a non-metaUic material is an enamel layer. Enamel is a glasslike film. The metallic part is covered by the enamel mass, usually a liquid suspension, by dipping into the suspension or by spraying. After drying the part is heated to 800 or 900 °C. 

The third possibility for corrosion protection is a coating with an organic polymer. This can be a thick rubber-like material, a polymer, or just by painting. New materials are being 

---