CORROSION BEHAVIOUR OF AN Al-Zn-Mg-Cu ALLOY AFTER DIFFERENT HEAT TREATMENTS

Abstract

The variation of factors such as the amount of alloying elements in solid solution, thermal and thermo-mechanical treatments changes the corrosion tendency in the Al-Zn-Mg-Cu alloy. The measurements of electrical conductivity enabled the estimation of the structural state of the alloy, i.e. the type and the degree of precipitation.The corrosion behaviour was examined through three electrochemical methods(Ecorr., polarization measurements, EIS).The measurement of corrosion potentials (Ecorr.) gave information about the distribution of the alloying elements between the solid solution and the precipitation phases. The polarization measurements have shown a more positive value of pitting potential (Epit.) and a greater resistance to corrosionfor the two-step aged alloy. The electrochemical impedance spectroscopy (EIS) has also shown that the two-step aged alloy has better corrosion propertiescompared to the one-step aged alloy.The fracture mechanics (FM) method wasused for SCC testing and hasshown that the alloy after two-step precipitation hardening is significantly more resistant to SCC. The obtained results enable a greaterinsight into the effects of both heat treatment and alloying elements, primarily copper, on the corrosion resistance of the alloy.