Chromate Conversion Coatings
March 17, 2011
Hexavalent and Trivalent Options
Chromate conversion coatings based on hexavalent chromium solutions have been used since the 1940s to provide corrosion protection for electrodeposited zinc and cadmium, zinc die castings and aluminum. Hexavalent chromium films provide corrosion protection by forming an impervious layer that acts as a barrier, short-circuiting the flow of electrons from cathodic to anodic sites therefore providing electrochemical protection. Activators, modifiers, accelerators and buffering ingredients are also incorporated in the chromate formulas to enhance protection, appearance and adhesion properties.
Due to the toxic and carcinogenic nature of hexavalent chromium, regulations have focused on eliminating its presence both in finishing shops and on the final products. “End of Life” limits for total chrome have become standards for automakers in the United States, Japan and Europe.
Active research and development has produced replacements, which are equal to or better than hexavalent chromium in corrosion resistance.
Chromate conversion coatings for electroplated zinc are generally based on the much less toxic trivalent form of chromium. The products currently on the market can meet salt spray specifications and even outperform the old technology when used in conjunction with appropriate seals.
New products for aluminum based on zirconium or titanium chemistry can likewise equal or exceed the old chromate formulations.
The operating parameters for this newer class of chromate conversion coatings are similar, although higher bath concentrations and operating temperatures may be required.
Chromate conversion coating are economical, easy to use and offer a variety of valuable properties to extend the service life and improve the performance characteristics of the parts being treated.
Click here for information on Heatbath’s Alchrome® Tri Tec – a trivalent chromate conversion coating for aluminum.
Click here for information on Heatbath’s Triumph products – trivalent chromate conversion coatings for zinc electroplate.Return to the List of Articles.