Performance of hydrotalcite conversion treatments on AA2024-T3 when used in a coating system

Hydrotalcite (HT) conversion coatings are a possible alternative to chromate conversion coatings (CCC) for the pretreatment of aluminum aerospace alloys, but they must meet several criteria for acceptance. Relevant to acceptance are the abilities of these coatings to provide stand-alone corrosion protection, suitable adhesion with subsequent organic layers, and overall corrosion protection when used within a coating system. This study examines the salt spray performance of nine different HT treatments on AA2024 (UNS 92024) as stand-alone conversion coatings and as a substrate for an epoxy primer. In addition, the adhesion of epoxy to the HT and the corrosion characteristics of HT coatings as assessed by electrochemical impedance spectroscopy (EIS) were determined. Salt spray testing was performed under pertinent military specifications, and CCC was used as the benchmark. Two epoxy primers were evaluated: one with a chromate inhibitor (Class-C) and one with a nonchromate inhibitor (Class-N). The stand-alone HT treatments did not perform as well as the CCC but showed favorable performance relative to bare AA2024. In the primed samples, the CCC and bare AA2024 performed better than the HT-coated samples. Current HT coating chemistries have been optimized for stand-alone corrosion resistance. A correlation between stand-alone salt spray performance and EIS response was observed. The stand-alone corrosion resistance or epoxy adhesion could not explain the salt spray performance of the epoxy-primed samples, supporting the need for a total system testing approach in coatings development.