In-Service Corrosion Performance of Automotive AA7075 Sheet Alloy

Aluminum alloys offer higher specific strength than advanced high strength steels, making them preferred material choice for automotive light weighting. Among them, AA7075 sheet alloy offers significantly higher strength than 5xxx and 6xxx alloys and has been developed to provide high in-service strength for automotive structural applications. However, there is a need to understand the stress corrosion cracking (SCC) susceptibility of AA7075 under “in-service” conditions for the automotive market. Currently there are no commonly accepted corrosion testing specifications or evaluation criteria for AA7075 sheets among material suppliers and automotive original equipment manufacturers. Furthermore, there is a lack of in-service data to validate the accelerated laboratory exposure testing. In this study, in-service SCC performance of the AA7075 test coupons was evaluated by road exposure for a two-year period under harsh Canadian winters. The SCC specimens were loaded using four-point bending frames, and were investigated in various tempers and surface conditions (bare vs. e-coated). Overall, the road exposure results were consistent with the lab accelerated testing such as slow strain rate tensile testing in specific environments. Coating provided sufficient corrosion protection such that the stressed coupons survived after 2-year road exposure without reduction in strength. For bare metals, overaged T73 and in-service (T6+paint bake) tempers outperformed T6 temper. Localized corrosion of AA7075 sheets with various tempers was also studied using potentiodynamic polarization technique. Acknowledgements: Dr. Danick Gallant, Aluminum Technology Center at National Research Council Canada; Kennesaw lab support in Novelis Global Research and Technology Center.