Phase-field investigation of intergranular corrosion mechanism and kinetics in aluminum alloys

The resistance to intergranular corrosion (IGC) is crucial for 7xxx series high-strength aluminum alloy. The grain boundary characteristics and nanoscale intergranular precipitates are the most significant factors affecting the IGC performance of alloys. Exploring the influences of the two factors on the mechanism and kinetics of corrosion is a fundamental issue for IGC processes. In this paper, the multi-phase field model for electrochemical corrosion is employed to study the IGC processes. The effects of grain size, GB overpotential, and the status of nano-precipitates on the mechanism and kinetics of IGC are systematically and quantitatively investigated. Polycrystalline alloys with the averaged grain sizes in micrometer and nanoscale show typical IGC (GB network propagation) regime and massive corrosion regime respectively. The IGC tendency of micrometer-sized grains increases with the average grain size, while that of nanoscale grains exhibits the opposite trend due to different corrosion mechanisms. For the IGC in the presence of nano-precipitates, our simulations quantify that the orientation, size, and distribution of precipitates could substantially affect the corrosion rate. Nanoscale precipitates exhibit completely different corrosion regimes in the process of IGC, with trenching and penetrating corrosion patterns depending on the overpotential. The multi-phase field model lays a foundation for revealing the corrosion mechanism as well as designing new corrosion resistant materials.