Effect of microstructural variables on the kinetic stability of aluminium and its alloys

Microstructure plays a major role in the corrosion of engineering metals and alloys, particularly for aluminium alloys. For any alloy system, microstructure is not well-accounted for when considering electrochemical stability, which is typically described in terms of equilibrium thermodynamics, i.e. Pourbaix diagrams. However, in an engineering context, a likelihood of corrosion as predicted by thermodynamics is not as useful as knowing the kinetic response of reactive metals such as aluminium (i.e. how fast a metal reacts in a certain environment). This work presents a first step towards developing a physical, mechanistic, understanding and representation of kinetic stability for aluminium-based metals, as an engineering alternative to Pourbaix diagrams. The rationale for this is to be able to subsequently compare the impact of ‘structural’ features, which are not presently considered in equilibrium thermodynamics – however such features are critical to engineering materials. Three key microstructural heterogeneities are studied as model structural variables. This includes: the presence of differing grain boundary density, different number density of precipitates, and varying degrees of crystallinity. To accompany the electrochemical studies, a range of careful specimen preparation and microstructural characterisation has been carried out.