Near-atomic-scale study of corrosion oxide film on grain boundary of Al-Mg alloys at the initial stage of corrosion

摘要

Intergranular corrosion trigged by sensitization is the main reason for the failure of Al-Mg alloys with the Mg content > 3wt%[1 -3]. The early stage of corrosion occurred at GBs control the evolution of the Al -Mg alloy’s degradation process[4,5]. However, corrosion initiation and the oxide film formation on GBs before and after sensitization remain unsolved questions. Here, the early stage c orrosion of GBs of 5083 Al-Mg alloy before and after sensitization were investigated by using quasi in-situ transmission-electron microscopy (TEM) and focus ion beam (FIB) TEM. Sensitization caused the decrease of corrosion resistance due to the precipitat ion of Al-Mg phase on GBs. For the GBs of both unsensitized and sensitized alloys, the oxide film on GBs showed a deeper penetration into the alloy matrix compared with the neighboring grains. In addition, Mg segregation on the GB of the sensitized alloy led to the formation of a thicker and Mg -enriched oxide film on the GB surface. Density -functional theory (DFT) calculations suggest that the local electron accumulation at interface of the oxide and the sub-surface Al atoms provided possible sites for O ad sorption and oxide film formation. The GB showed a stronger tendency for oxidation than neighboring grains in terms of its structural and electronic properties, and Mg doping enhanced the tendency of O adsorption and electron localization on the GB, promoting the oxidation of Mg -segregated GBs. Our findings advance the mechanistic understanding of corrosion oxide film formation on GBs from near atomic -scale, and should help lower the susceptibility of Al alloys to IGC originating from the GB corrosion.