Near-atomic-scale study of the oxide films on the grain boundaries of Al-Mg alloys at the initial stage of corrosion: Experimental investigations and DFT calculations

Intergranular corrosion trigged by sensitization is the main reason for the failure of Al-Mg alloys with a Mg content > 3 wt%. The early stage of corrosion occurs at the grain boundaries (GBs) and controls the evolution of the Al-Mg alloy’s degradation. However, corrosion initiation and the oxide film formation on GBs before and after sensitization need to be further explored. Here, the early stages of GB corrosion of the 5083 Al-Mg alloy, before and after sensitization, are investigated using quasi in-situ and focused ion beam microscopy and transmission electron microscopy (FIB-TEM). Sensitization caused a decrease in the corrosion resistance of the Al-Mg alloy due to the precipitation of Al-Mg phase on GBs. On the GBs of both unsensitized and sensitized Al-Mg alloys, the oxide film penetrated deeply into the alloy matrix compared with the neighboring grains. Mg segregation on the GB of the sensitized Al-Mg alloy led to the formation of a thick, Mg-enriched, oxide film on the GB surface. Density-functional theory (DFT) calculations suggest that the local electron accumulation at the interface of the oxide and the sub-surface Al atoms provided possible sites for O adsorption. 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. This led to the formation of a less protective Mg-enriched oxide film on Mg-segregated GBs. Our findings advance the mechanistic understanding of near atomic-scale corrosion on the GBs of Al-Mg alloys.