Inhibition of Cathodic Kinetics by Zn 2+ and Mg 2+ on AA7050-T7451

Abstract

© The Author(s) 2019. Published by ECS. The inhibitory effects of Mg2+ and Zn2+ on the cathodic kinetics of AA7050-T7451 were investigated by both electrochemical techniques and surface characterization tools. This study focused on the effects of Zn2+/Mg2+ concentration and immersion time. It has been found that in the cathodic potential regime where the oxygen reduction reaction (ORR) was dominant, Mg(OH)2 and NaZn4(SO4)(OH)6Cl6H2O were precipitated on Cu-bearing intermetallic compounds (IMCs) embedded in AA7050-T7451 in Mg2+-and Zn2+-containing solutions, respectively. Both formed a barrier to oxygen diffusion, thus lowering the cathodic limiting current density. The decrease in cathodic limiting current density pertinent to ORR was proportional to Mg2+/Zn2+ concentration. At short times, both ions decreased the open circuit corrosion rate, with the inhibition increasing with increased cation concentration. Better inhibition was provided by Zn2+ than by Mg2+ at the same ion concentration. The corrosion inhibitive effect of Mg2+ was inferior to Zn2+ during the first 120hrs, but was inferred to be superior to Zn2+ for an immersion exposure period longer than 120hrs.