Galvanic Corrosion of Aluminum

Laboratory tests were carried out to compare the electrochemical characteristics of galvanic couples made of aluminum alloys (1050, 5086, 2024, and 7075) with other metals (plain steel, Type 316 stainless steel, titanium) in tap water and in artificial sea water. It was found that: One must first choose materials which individually resist corrosion. Only when this requirement is fulfilled can the risk of galvanic corrosion be evaluated. In aluminum-steel assemblies, taking into account only the risk of galvanic corrosion leads to the following bad choices: (1) the choice of copper containing aluminum alloys which are less anodic than copper free alloys (poor choice except when justified by mechanical factors), and (2) the choice of plain steel which is less noble than stainless steel. Although they generate weaker galvanic currents, field tests show that those assemblies made of corrosive materials have shorter life than stainless steel assembled with a copper free aluminum alloy. The galvanic current produced by the couple decreases rapidly with time, then stabilizes within a few days. The comparison of these stabilized currents seems to be the best way to compare the risks of galvanic corrosion. For a given couple of different metals, the life span of the assembly depends upon the conductivity and the aggressiveness of the environment; in other words the complexity and the cost of the protection to be used, for a given couple increase with the aggressiveness of the environment.