Effect of Alkaline Etching on Formation Behavior and Corrosion Resistance of a Cr-Zr-Ti TCP Conversion Coating on AA2024 Aluminum Alloy

Trivalent chromium process conversion coating is considered to be the ideal alternative to traditional chromate conversion coating due to its high corrosion resistance and low toxicity. However, achieving a high corrosion resistance trivalent chromium conversion coating on 2xxx aluminum alloys has been a great challenge. Herein, based on a newly developed trivalent chromium conversion solution (CN patent, ZL 202210804263.2), the effect of alkaline etching on the formation behavior and corrosion resistance of the coating on AA2024-T351 aluminum alloy was investigated. It was found that under the same conditions, the mechanically polished sample and the sample alkaline etched for 30 s showed the highest corrosion resistance. When the alkaline etching time was too short or too long, the corrosion resistance of the coating deteriorated. This is ascribed to changes in the surface microstructure of the alloy during alkaline etching, i.e., reduced number of intermetallic particles and formation of a copper enrichment layer. Reducing the number of intermetallic particles on the alloy surface is beneficial for uniform growth of the coating. However, the copper enrichment layer participated in formation of the coating, resulting in doping of copper species in the coating, which is detrimental to the corrosion resistance of the coating.