Chromating was developed to curb occurrence of white rust on electroplated zinc surface and also to enhance adhesion with painted layers. Following success of application to zinc, cadmium processing was developed for similar purpose and brought forth further application of similar conversion coating by using materials such as silver, copper, beryllium, tin, copper alloys, aluminium, magnesium, zinc die castings and chrome plating layers.
Chromating films are formed because pH rises in interface between metal surface and solution when small amount of metal ion is melted from metal surface. That is, chromating layers, which have gel shape made of chrome(No. 6 and 3) and processed metals, are formed on surface of processed metals. Gel normally formed on surface is soft and needs care for its handling. But after being dried, it holds abrasion-resistance relatively. If heat treatment is needed to prevent hydrogen brittleness, it should be done prior to chromate coating. It is because films are destroyed due to occurrence of dehydration on coated films if heat higher than 65℃ were to be put.
Chromate coating layers have good adhesion with metal surface as well high anti-corrosion adaptation to agricultural and industrial areas, inland and coastal areas. Its anti-corrosion function is provided by soluble chromates contained within film. The bigger width of film of processed materials and the bigger volume of chromates content, the stronger anti-corrosion works.
Compared to transparent film, rainbow-colored or olive film has more content of chromates and maximum corrosion resistance.
Chromate coating can be divided into two types.
Chromate solution polishes surface chemically and softens to improve surface illuminance and endow reflectivity and luster like a mirror. Also chromating eradicates dim film - drops surface illuminance - existing unevenly on the surface.