Effect of microstructure on exfoliation corrosion resistance in an Al-Zn-Mg alloy

In this work, the effect of microstructure on exfoliation corrosion (EXCO) resistance in an Al-Zn-Mg alloy were investigated using standard EXCO test, together with detailed microstructural examinations mainly by scanning electron microscopy (SEM) and high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). An EXCO product model was built based on morphologies of EXCO product on longitudinal section and cross section for further measurement and analysis. A quantitative characterization of EXCO resistance by measuring average thickness of EXCO product was conducted, showing average results of 6.94 mu m, 10.21 mu m and 18.14 mu m, which are corresponding to EXCO ratings of P, EA and EB under ASTM G34 standard, respectively. It was concluded that recrystallization, high fraction of high-angle grain boundaries (HAGBs) and high coverage ratio of eta phases on HAGBs reduced EXCO resistance.