Corrosion Mechanism of Al-zn-in Alloys in Chloride Solutions

The electrochemical behavior of Al-ZnIn alloys was investigated in chloride media by means of potentiodynamic techniques, complemented by Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Analysis (EDX) and Transmission Electron Microscopy (TEM). The influence of local alkalization produced by previous cathodization was analyzed at potentials more negative than the rest potential. The electrochemical response of alloys is given by the sum of those corresponding to the different present phases. Thus, the electrochemical behavior of each metal and bimetallic alloy (Al-In, In-Al and Zn-In) was analyzed in the same solution conditions at a potential region near the corrosion potential of the ternary alloy. The low solubility of In in Al favors its segregation at grain boundaries, where the formation of Zn-In alloys was detected. The initiation of the attack in the Al-Zn-In alloy is mainly localized in these zones. This is a consequence of the enhanced adsorption and surface accumulation of Cl promoted by In at potentials more positive than 1.1 VSCE. Close similarities of the anodic behaviors of the Al-In and ternary alloys lead us to propose a mechanism where the In distribution plays the major role. The In dissolution from homogeneous solid solution phases allows an enrichment of this element at the Al and Zn rich phases by displacement reactions. (Al,Zn)In + Cl → InCl + 3 e 2 InCl + 3 Zn → 2 In + 3 Zn + 2 Cl This, in turn, assures an accumulation of adsorbed Cl, which maintains an active state of the Al matrix. Thus, the rapid Zn dissolution reaction ensures a better redistribution of In, so bringing the system towards potentials near those for Cl adsorption on In. The attack morphology was also analyzed in order to explain the smooth wide cavities embracing many grains. KeywordsAluminium, Al-Zn-In alloy, corrosion of Aluminium, Chloride adsorption.