Chromate conversion coating
[1]: p.1265 [2] The coating serves as a corrosion inhibitor, as a primer to improve the adherence of paints and adhesives,[2] as a decorative finish, or to preserve electrical conductivity.
It also provides some resistance to abrasion and light chemical attack (such as dirty fingers) on soft metals.
The gel consists of a three-dimensional solid skeleton of oxides and hydroxides, with nanoscale elements and voids, enclosing a liquid phase.
The structure of the gel depends on metal ion concentration, pH, and other ingredients of the solution, such as chelating agents and counterions.
The trapped solution keeps reacting with any metal that gets exposed in the cracks, so that the final coating is continuous and covers the entire surface.
For example, in the coating formed on aluminum by a commercial bath, about 23% of the chromium atoms were found to be hexavalent Cr6+, except in a region close to the metal.
[8][9][10] The protective effect of chromate coatings on zinc is indicated by color, progressing from clear/blue to yellow, gold, olive drab and black.
ASTM B633 Type II and III specify zinc plating plus chromate conversion on iron and steel parts.
[5] Hexavalent chromium compounds have been the topic of intense workplace and public health concern for their carcinogenicity, and have become highly regulated.
[16] In particular, concerns about the exposure of workers to chromates and dichromates while handling the immersion bath and the wet parts, as well as the small residues of those anions that remain trapped in the coating, have motivated the development of alternative commercial bath formulations that do not contain hexavalent chromium;[17] for instance, by replacing the chromates by trivalent chromium salts, which are considerably less toxic and provide as good or better corrosion resistance than traditional hexavalent chromate conversion.
[18] In Europe, the RoHS and REACH Directives encourage elimination of hexavalent chromium in a broad range of industrial applications and products, including chromate conversion coating processes.
