Comprehensive model for the activation mechanism of Al-Zn alloys produced by indium

Exploratory work on the electrochemical behaviour of the Al-4%Zn alloy in the presence of In3+ in weakly acid chloride and acetic media is reported. It was found that in chloride solutions, active dissolution takes place at very negative potentials (−1.5 VSCE). This significative enhancement in the electrochemical activity of the alloy is described and discussed. The aim of this paper is to explain the relationship between the operating potential (−1.1 VSCE) given by the commercial alloy (basically an Al-Zn-In alloy), its high efficiency, its quasi-uniform attack observed and the enhanced activity found (−1.5 VSCE). An unified model that explains this findings and the activation process of pure Al by In3+ is proposed. It was found that two activation mechanism are operative on Al: (i) When sufficient amount of In at the interface in quasi-liquid state is present, an In-Al amalgam is responsible for the activation process (−1.5 VSCE). This process is favoured by Zn reach zones that favours In enrichment at the interface by displacement reaction. (ii) At higher anodic potentials (near −1.2 VSCE), the presence of In at the interface facilitates a chloride adsorption process which depolarised the anodic reaction.