Unraveling the precipitate-induced discontinuity of the surface oxide film on Al alloy
By Zhou, Y. T.; Jia, X. L.; Li, J. B.; Guan, G. G.; Ma, X. L.; Zhang, J. S.
Published in Applied Surface Science
2022
Abstract
Precipitate-induced discontinuity of the surface oxide film is a significant reason for the increasing pitting susceptibility of age-hardening aluminum alloys but lacking of direct characterization. In the present work, the oxide film heterogeneity induced by Mg(Zn,Cu)2 particles with different sizes in an Al-Zn-Mg-Cu alloy and the consequent corrosion behaviors were studied. Quasi in-situ transmission electron microscopy (TEM) observations indicated that the η'-Mg(Zn,Cu)2 precipitates with sizes less than 10 nm were electrochemically inert, while the grown η particles underwent de-alloying and then promoted pitting events of the nearby alloy matrix. By means of cross-sectioning, it was revealed that the surface film on the solid solution and peak-aged alloys were structurally uniform although oxidation of the η' resulted in local Mg enrichment in the surface film. For the grown η particle, the Mg-rich zone was remarkable and covered not only the particle but also the surrounding alloy matrix. The flawed structure in the Mg-rich area should be responsible for the immediately de-alloying of the precipitate and then the corrosion attack of the peripheric alloy.