Filiform corrosion of AA3005 aluminium analogue model alloys

The effect of the metal substrate microstructure on filiform corrosion (FFC) susceptibility was investigated for super purity based model alloys with compositions based on the specifications of AA3005. Variations in alloying levels of the elements iron, silicon and copper were investigated. Alloys with high silicon content were more susceptible to FFC than alloys with low silicon content. The iron content, at the levels investigated, did not strongly affect FFC properties. The apparent detrimental effect of a high silicon content is attributed to the influence of silicon on secondary intermetallic particle precipitation. Given the same thermo-mechanical treatment, alloys with high silicon content underwent more extensive secondary precipitation of manganese containing intermetallic particles than those alloys with a low silicon content. The resulting microstructure is characterised by a higher density of finely dispersed intermetallic particles and a lower content of manganese in the adjacent supersaturated solid solution. These conditions provide a large number of potential corrosion initiation sites on the surface and also enhance microgalvanic coupling between intermetallic particles and the surrounding aluminium rich matrix, thus promoting the propagation of filamental corrosion attacks. Additions of copper had a detrimental effect on the FFC resistance. The role of copper in promoting FFC is attributed to preferential dissolution phenomena during the corrosion process, whereby copper is locally enriched on the corroding surface. This copper enrichment provides additional area for cathodic reaction, thus enhancing the corrosion process.