Characterization of trivalent chromium process coating on AA2024-T3

Abstract

Trivalent Chromium Process (TCP) coatings, which have been used recently as an alternative to chromate conversion coatings (CCCs), have been characterized. The TCP coating was found to be a dense layer consisting of particles, hundreds of nm in size on the coating surface, which is similar to CCCs. The TCP coating has a two layered structure, with zirconium-chromium mixed oxide in the outer layer and aluminum oxide or oxyfluoride at the metal/coating interface. No Cr(VI) was found, which supports its use as an environmentally friendly replacement for CCC. The TCP thickness was in the range of 40–120nm, considerably thicker than the zirconium based coating without chromium species. The high vacuum condition in the SEM dehydrates the coating quickly resulting in the formation of mud-crack artifacts. The TCP coating provides corrosion protection to the AA2024-T3 through suppressing the oxygen reduction reaction on aluminum alloy surfaces by acting as a protective barrier layer. Two pretreatments were investigated but found to result in little difference in the resulting TCP formed on AA2024-T3.