Formation and Structure of Trivalent Chromium Process Coatings on Aluminum Alloys 6061 and 7075

Abstract

The formation mechanism and resulting structure of trivalent chromium process (TCP) conversion coatings on AA6061-T6 (UNS A96061) and AA7075-T6 (UNS A97075) were investigated. The formation of TCP on both alloys is driven by an increase in the interfacial pH caused by the dissolution of the passivating oxide layer, which leads to an elevated rate of proton-consuming cathodic reactions under open-circuit conditions. These reactions cause the interfacial pH to increase. This pH increase drives the hydrolysis of the fluorometalate precursors in the bath and results in the precipitation of a hydrated metal oxide coating on the surface. The coating appears to have a biphasic structure consisting of a hydrated zirconia (ZrO2·nH2O) and chromium hydroxide (Cr[OH]3) outer layer, and a fluoroaluminate (e.g., KxAlF3+x) interfacial layer. The coating thicknesses on both alloys are in the range of 80 nm to 100 nm. The TCP coating exhibited good stability on both alloys during full immersion testing in both naturally aerated sodium sulfate (Na2SO4) and sodium chloride (NaCl) electrolyte solutions. This was evidenced by no pits forming during immersion in chloride solution. The coating provided corrosion resistance to both alloys as polarization resistance (Rp) increased by about 100X in both Na2SO4 and the Na2SO4 + NaCl. Transient formation of Cr(VI) was detected in the coating on both alloys using Raman spectroscopy after immersion in air-saturated solutions.