The Al-5%Zn-1.7%Mg-0.23%Cu alloy, when submitted to suitable heat treatments, is a weldable, high-strength and SCC resistant material. A research plan is being developed in our laboratories to correlate the electrochemical polarization with the microstructure of such alloys. In this study, combined additions of Cr and Nb and different heat treatments were performed to obtain different microstructures and hardness, including a new age-hardening procedure based on the non-equilibrium segregation theories. In this review, the present state of the electrochemical corrosion study of these alloys, in chloride solutions, is reported. Open circuit potential measurements, cyclic polarization, potentiostatic polarization and EIS have been applied together with optical microscopy, SEM, TEM, STEM and EDX. It is shown that the electrochemical techniques are highly sensitive to the alloy microstructure. The open circuit, breakdown and repassivation potentials depended on the aluminium matrix solid solution composition. The microscopic study of the specimens with potential showed the grain regions sensitive to the localized corrosion, permitting to explain the SCC resistance of the present alloys when submitted to selected age-hardening procedures. Also, the electrochemical impedance spectroscopy was demonstrated to be an useful tool to characterize and to model the interphase suffering pitting corrosion.
Buy this Article