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Chromic acid anodising has been used since 1923 as a means of affording a level of corrosion resistance to aluminium alloys; the original method of anodising was patented by Bengough and Stuart and is still used today although more commonly a modified version is used. Recently, environmental pressure has resulted in alternatives to chromic acid anodising being sought. A key to finding a suitable alternative is to fully understand why chromic acid anodising affords a higher level of corrosion resistance compared with other forms of protection. The current research focuses on the pre-treatment and anodising of two aluminium alloys, AA 1050A and AA 2024T351, the film development process being investigated with the high purity 1050 alloy and the corrosion resistant properties afforded by chromic acid anodising investigated with the 2024 alloy.

High strength aluminium alloy AA2024 is susceptible to localized corrosion in the form of pitting and intergranular corrosion (IGC). The corrosion behaviour of this alloy is strongly influenced by the presence of different intermetallic particles. In this study, the effect of applied stress and the role of intermetallic particle removal by surface treatment on the initiation and propagation of localized corrosion are investigated. It was found that applying a stress to as polished AA2024 leads to a drop in breakdown potential. The development of micro/nano crevices adjacent to the intermetallic particles is thought to act as initiation sites for localized corrosion, lowering the breakdown potential of stressed AA2024. The electrochemical behaviour of specific intermetallic particles and the particle free matrix of AA2024 have been studied as a function of stress state using the micro-capillary electrochemical cell in combination with an in situ stressing stage. These experiments emphasized the role of intermetallic particles as the possible key contributing factor in determining the corrosion properties of the alloy under stressed conditions. In situ X-ray synchrotron microtomography experiments were used to observe the evolution of corrosion attack as a function of continuous exposure time in both unstressed and stressed AA2024. Quantitative growth rate measurements indicates that application of stresses of 70% yield strength or above has a significant effect on the localized corrosion propagation.

Wrought Al-Mn alloys can develop a thin deformed layer on the surface as a result of hot and cold rolling. Subsequent heat-treatment precipitates fine secondary intermetallic particles which effect corrosion susceptibility. This work focuses on the effect of surface preparation and deformation on the electrochemical behaviour of Al-Mn alloys. The first part of the work investigated the effect of surface preparation such as mechanical grinding and polishing, alkaline etching and desmutting, and nitric acid treatment on electrochemical behaviour of an Al-1Mn-0.4Fe-0.3Si model alloy. Different surface preparations of this alloy show different electrochemical behaviour. In the second part of the work, the electrochemical reactivity of the surface layers of commercial rolled AA3005 sheet was investigated by profiling through the surface with GDOES (glow discharge optical emission spectroscopy). The microstructure and electrochemical reactivity was examined at different depths in order to compare the behaviour of the surface layers with that of the bulk alloy. In order to understand the role of deformation on corrosion behaviour of Al-Mn alloy, an Al-1Mn-0.4Fe-0.3Si model alloy was deformed by uniaxial compression and equal channel angular extrusion (ECAE) and followed by annealing. It was found that deformation is likely to have two effects on the surface of Al-Mn alloy. One effect is to cause the precipitation of particles that act as local cathodes and pit initiation sites. The other effect is that formation of precipitates will deplete the adjacent matrix in solute, making it more susceptible to dissolution.