Corrosion behavior and mechanism of Al–Zn–Mg–Cu alloy based on the characterization of the secondary phases

Abstract

The corrosion behavior of Al–Zn–Mg–Cu alloys with different components in NaCl solution were studied experimentally in this work and the composition changes of micron and nano secondary phases during corrosion were semi-quantitatively characterized. Meanwhile, the effect of the type and quantity of the secondary phases caused by different compositions on the corrosion resistance and its mechanism were discussed. The results show that localised self-corrosion was noted in the S-phase for AA7050 and the corrosion rate of S-phase around Cu-rich corrosion products was more than one order of magnitude higher than that in the area where no corrosion products were formed. With the progress of corrosion, the Cu content in the area forming corrosion products first decreased and then increased, while the O content first increased and then decreased. In addition to self-corrosion of the σ-phase, severe corrosion occurred in the interface between matrix and σ-phase for AA7136, and the corrosion rate of interface was more than one order of magnitude higher than that of σ-phase. Due to the absence of the secondary phases (the volume fraction is only 0.02%) above micron-scale (S- and σ-phase), the pitting resistance of E2 alloy designed by authors was significantly better than AA7050 and AA7136. Zn and Mg elements were enriched around the nano dispersoids formed by Cr and Mn in E2 alloy, which reduced the content of Zn and Mg elements near the grain boundary, resulting in the potential difference between intergranular and intragranular were 10 mV ~ 40 mV lower than AA7050 and AA7136. Meanwhile, the dispersoids hindered the intergranular corrosion and improved the intergranular corrosion resistance of E2 alloy. • The corrosion process of S- and σ-phases in NaCl solution are characterized semi-quantitatively by quasi-in-situ observation. • The absence of the secondary phases above micron-scale can significantly improve the pitting resistance. • Dispersoids formed by Cr and Mn elements improve the intergranular corrosion resistance by enriching the content of Zn and Mg elements near the grain boundary.