EIS STUDY OF HEAT-TREATED AL-ZN-MG ALLOYS IN THE PASSIVE AND TRANSPASSIVE POTENTIAL REGIONS

The effect of the microstructure on the electrochemical passivation an d the corrosion of an Al-Zn-Mg alloy (5.00% Zn, 1.66% Mg, 0.23% Cu and 0.053% Nb) in 0.1 mol dm(-3) NaCl, has been studied using ocp measure ments, cv and EIS. A variety of microstructures, in which the basic di fferences were the aluminium matrix composition and the size of the Mg Zn2 precipitates, was obtained using well known heat treatments. The c orrosion potentials in oxygen-saturated (1 atm) 0.1 mol dm(-3) NaCl an d the repassivation potentials in 0.1 mol dm(-3) NaCl were determined, increasing in the order F < A approximate to ST < B approximate to C. The impedance measurements were performed at constant potential after achieving constant current conditions. Impedance spectra for all the specimens examined within the passive potential region were semicircul ar when data was presented in the Nyquist format. However, in the pitt ing potential region, inductive semicircles appeared. These were assig ned to the relaxation of adsorbed species. The depression of the capac itive semicircles was correlated with the size of the MgZn2 precipitat es. Equivalent circuits that simulate the experimental impedance diagr ams were developed on the basis of the microstructural observations.