The role of fluoride compounds in the formation of chromate conversion coatings on aluminum alloys

Chromate conversion coatings are routinely formed on aluminum based surfaces for providing corrosion protection. Most widely used conversion coating treatments consist of hexafluorozirconate as major component besides chromate, ferricyanide and fluoride. The function of hexafluorozirconate has not been documented and is the subject of this study. This study reports results from contact angle measurements, surface morphology and chemistry investigated using scanning electron microscopy and energy dispersive spectroscopy, respectively. Extensive use of X-ray photoelectron spectroscopy has been made to determine the chemistry of the surface resulting from the hexafluorozirconate pretreatment. Our results indicate that hexafluorozirconate does not attack the oxide film and is involved in the formation of an Al-Zr-O-F based hydrated layer that increases the hydrophilicity of the surface, activates the surface and lowers the corrosion resistance. These factors combine to enhance the interaction of chromate with the alloy surface to result in the formation of a thicker conversion coating. Based on these results, a new model has been proposed for the formation of chromate conversion coatings.