Chromate Coatings

Chromate conversion coatings are used on aluminum to retard and prevent corrosion, as a base for paint, and as a protective surface having lower electrical contact resistance than anodized coatings. They can be used to change the emissivity and absorptions properties of the metal and can be dyed for identification purposes.

These coatings are usually applied by immersion or spray, but application by brush or swab is also used, mainly for touch up. Roll coating is used for continuous strip processing.

The coatings generally provide protection in proportion to film thickness. They provide good protection against marine and humid environments. When coatings are dried at room temperature, hexavalent chromium compounds, loosely bonded in the film structure, are slowly soluble on exposure to aqueous media and will heal minor abrased or scratched areas.

The above self- healing characteristics are reduced upon heating above 71 degrees C., thus eliminating the self-healing and gradually reducing the protective value of the film as the temperature and time at temperature increase. Corrosion protection is reduced to a greater degree in heavy than in thin coatings. The paint-base properties of the coating are not adversely affected by the short elevated drying times normally used in production.

Class two coating is preferred. This is a general purpose coating process for corrosion resistance and as a paint base. Colors range from iridescent to yellow. Coating weight is from 110 to 380 mg/m2.
The performance of the coating depends on the environment of use~ in general it is in proportion to the thickness of the coating.