Corrosion behavior of two heat treatment Al-Zn-Mg-Cu alloys in different intergranular corrosion solution

The electrochemical behavior of two kinds of artificial aged Al-Zn-Mg-Cu alloys in two intergranular corrosion （IGC） solutions were studied using electrochemical impedance spectroscopy （EIS） and open circuit potential （OCP） at steady-state. EDAX result indicates that different artificial ageing methods change the composition and content of Cu and Zn in different zones. Zn/Cu depleted precipitation-free zone that plays a very important role in IGC is formed by heating the solubilized Al alloy for 135 ℃ at 16 h. All impedance spectra of the two alloys in two IGC solutions can be divided into three types. The two different states Al alloys takes on one time constant and two capacitive arcs at high-mediate frequency and low frequency in the NaCl+（NH4）2SO4 solution respectively; but in the NaCl+HCl solution, impedance displays one capacitive arc at the high-mediate frequency and an inductive loop at low frequency. OCP results show that more micro-galvanic cells in the NaCl+（NH4）2SO4 solution than that in the NaCl+HCl solution results in more potential fluctuation amplitude, and long-term drift of OCP is due to the long-term variation of the cathodic and anodic corrosion processes.