Corrosion behaviour of AA7108 aluminum alloy

This study has investigated the influence of microstructure on the corrosion properties of AA7108 T6 aluminium alloy extrusions. Additionally, through the analysis of AA7108 alloy, the using of highresolution electron microscopy allowed the characterising of microstructure evolution during corrosion process from microscale to nanoscale. This provides previously unseen details of trenching corrosion, intergranular corrosion and crystallographic corrosion. Based on these details, the relevant corrosion mechanisms are proposed. Besides the general characterisation of intermetallic phases and grain structure of AA7108 alloy, the local microstructural variation related to Fe-containing constituent intermetallic particles is characterised. Non-uniform local plastic deformation and precipitate free zones (PFZs) are observed in the regions surrounding the particles; an aluminium oxide film and an Fe-rich layer beneath the oxide film are also observed on the Fe-containing particle surface. Further, discontinuity in the distribution of η'/η (MgZn₂) phase precipitates and variation in the width of PFZs are characterised at dislocations, sub-grain boundaries and grain boundaries. It is revealed that the size and the local discontinuity of η'/η (MgZn₂) phase, as well as the width of PFZs show the increase tendency with the degree of misorientation increasing. Corrosion test in neutral NaCl solution shows that trenching corrosion around Fe-containing particles is the primary corrosion phenomenon of AA7108 alloy. Up to even 5 hours exposure to the testing solution, intergranular corrosion or other types of severe localised corrosion are rarely observed. In contrast, corrosion test in NaCl solution acidified to pH 3.2 shows that intergranular corrosion and crystallographic corrosion are the main corrosion phenomena of AA7108 alloy. During trenching corrosion, Fe-containing particles as the net cathodes are not completely under cathodic protection. The corrosion of Fe-containing particles occurs as soon as the start of the immersion test, resulting in Fe-rich corrosion product on particle surface. As the net anode, alloy matrix dissolution leads to porous oxide film growth on the corroded Al matrix surface. The porous oxide film is perpendicular to the corroded matrix surface. The results suggest that these two phenomena are related to the local alkalisation due to the oxygen reduction reaction on particle surface. The corrosion morphology is consistent with the local alkalinity distribution. The mechanism of trenching corrosion induced by Fe-containing particles is proposed. Further, during the early stage, trenching corrosion might not be uniform, which is due to the non-uniformity of the local alkalinisation. The corrosion potential difference, the surface defects or the localised plastic deformation do not show significant influence on the non-uniformity of trenching corrosion. The test and associated characterisation also suggest that Clions do not play a significant role in trenching corrosion. In acidic NaCl solution, intergranular corrosion is initiated from the surface cavities introduced by the preferential dissolution of MgZn2 precipitate. Then, intergranular corrosion propagates via a combination of two factors: the driving force related to crystallographic dissolution and the driving force related to dissolution along the grain boundary interface. Crystallographic corrosion of grain interior propagates develops the typical half-cube corrosion morphology. The regions, with a high population of Al3Zr dispersoids, are resistant to crystallographic corrosion.