Microstructural modification arising from alkaline etching and its effect on anodizing behavior of Al-Li-Cu alloy

Surface and near-surface microstructures of alkaline etched AA2099-T8 aluminum alloy have been characterized. The effect of alkaline etching on the anodizing behavior of the alloy in both acidic and neutral electrolytes has been investigated by comparing the electrochemical responses and the resultant anodic films of mechanically polished and alkaline etched alloys. It was revealed that alkaline etching could locally remove intermetallics from the alloy surface, with development of a copper-enriched layer immediately beneath a residual alumina film of about 5 nm thickness. The 'footprints' of alkaline etching, i.e. the removal of intermetallics and the formation of copper-enriched layer during alkaline etching, could affect subsequent anodizing behavior of the alloy, particularly in the early stages. The removal of intermetallics from the alloy surface could not only improve anodizing efficiency but also contribute to reduced discontinuities in the resultant anodic films, potentially benefiting corrosion and fatigue resistances of the anodized profiles. The difference in anodizing behavior caused by the copper-enriched layer developed during alkaline etching was considered limited to the early stages of anodizing, since a similar copper-enriched layer could be formed quickly during subsequent anodizing process, regardless of the surface pre-treatment. © 2013 The Electrochemical Society.