Influence of Heat Treatment and Scandium Addition on the Electrochemical Polarization Behavior of Al-Zn-Mg-Cu-Zr Alloy

In this study, the electrochemical polarization behaviors of Al-Zn-Mg-Cu-Zr (7010) alloy in three different heat treatments, namely, underaged, peak-aged, and overaged, were examined in 3.5 wt pct NaCl solution. Experimental results show that the cathodic current increases marginally in the order of underaged < peak-aged < overaged alloys, which has been attributed to an increase in copper content of the precipitates in general and the grain boundary precipitates (GBPs) in particular. The change in the precipitate chemical composition has been found to affect the anodic polarization behavior even in a more significant way. Thus, both the anodic polarization curves of underaged and peak-aged alloys exhibit two distinct breakdown potentials and current reversal immediately below the second breakdown potential, whereas such a phenomenon is found to be absent in the overaged alloy. The overaged alloy exhibits only one breakdown potential without any current reversal. Detailed study of the polarization data and corroded surfaces of the alloy shows that the anodic current reversal is due to H₂ evolution on the alloy surface just after the occurrence of passive film breakdown along the grain boundary. Notably, it is only those heat treatments that are prone to intergranular corrosion (IGC) seems to exhibit the tendency to reduce H⁺ ions, when they are anodically polarized. The chemical composition of the precipitates that can be altered by heat treatments is responsible for this behavior. The addition of 0.25 wt pct scandium to type 7010 Al alloy did not show any improvement in the corrosion resistance of the alloy. The E _corr of scandium containing alloy shifted toward the active direction as compared to the base alloy. Noticeably, the peak-aged scandium containing alloy also exhibited two distinct breakdown potentials in the anodic polarization curve similar to the peak-aged base alloy, thus revealing its susceptibility to IGC and pitting corrosion.