Filiform corrosion of binary aluminium model alloys

The effect of alloy composition and microstructure on filiform corrosion (FFC) susceptibility was investigated for super-purity based binary model alloys of the systems Al–Mg, Al–Mn, Al–Fe and Al–Cu. Corrosion testing indicated that the presence of electrochemically noble second phase particles is a necessity for FFC to occur. Single phase, solid solution alloys of the Al–Mg, Al–Mn or Al–Fe systems did not support FFC. Heat treatment caused precipitation of the electrochemically noble intermetallic phase FeAl ₃ in the Al–Fe alloy, resulting in extensive FFC. Precipitation of the phase MnAl ₆ , which has electrochemical properties similar to that of the aluminium rich matrix, by heat treatment did not impair the corrosion properties of the Al–Mn alloy. Significant surface oxidation and magnesium enrichment of the surface oxides by heat treatment did not affect the FFC properties of Al–Mg alloys. However, the solid solution binary Al–Cu alloys exhibited severe FFC. The detrimental effect of copper in solid solution is attributable to selective dissolution phenomena during the corrosion process, whereby copper was locally enriched on the surface as copper-rich particles providing efficient cathodic sites.