The contribution of hydrogen evolution processes during corrosion of aluminium and aluminium alloys investigated by potentiodynamic polarisation coupled with real-time hydrogen measurement

Water reduction, which leads to the evolution of hydrogen, is a key cathodic process for corrosion of many metals of technological interest such as magnesium, aluminium, and zinc; and its understanding is critical for design of new alloys or protective treatments. In this work, real-time hydrogen evolution measurement was coupled with potentiodynamic measurements on high-purity aluminium and AA2024-T3 aluminium alloy. The results show that both materials exhibit superfluous hydrogen evolution during anodic polarisation and that the presence of cathodically active alloying elements enhances hydrogen evolution. Furthermore, it was observed for the first time that superfluous hydrogen evolution also occurs during cathodic polarisation. Both the anodic and cathodic behaviours can be rationalised by a model assuming that superfluous hydrogen evolution occurs locally where the naturally formed oxide is disrupted. Specifically, during anodic polarisation, oxide disruption is due to the combined presence of chloride ions and acidification, whereas during cathodic polarisation, such disruption is due to alkalinisation. Furthermore, the presence of cathodically active alloying elements enhances superfluous hydrogen evolution in response to either anodic or cathodic polarisation, and results in 'cathodic activation' of the dissolved regions. Corrosion of aluminium: the contribution of hydrogen evolutionAqueous corrosion is an electrochemical reaction resulting in materials degradation, involving simultaneous oxidation of metal and reduction of species in a wet environment. The overall corrosion rate depends on how fast the two processes proceed. Although the reduction of oxygen is the most important cathodic reaction, for more reactive materials such as Al and Mg the reduction of hydrogen is thermodynamically allowed and could contribute to the overall rate. Now a team led by Michele Curioni at University of Manchester look at the hydrogen evolution behaviour of high purity aluminium and AA2024T3 alloy utilising coupled real-time hydrogen evolution and potentiodynamic measurements. Superfluous hydrogen evolution was observed during both anodic and cathodic polarisation and associated in both cases to local oxide disruption. The derived understanding of corrosion could enable us to develop new protection treatments.