A filiform corrosion and potentiodynamic polarisation study of some aluminium alloys

The aim of the present investigation is a combined study of filiform corrosion of aluminium alloys by accelerated exposure tests and potentiodynamic polarisation measurements. The accelerated exposure tests are performed on binary Al-Cu, Al-Mg, Al-Si and Al-Zn model alloys, a ternary Al-MgSi alloy and on the two commercial alloys, AA2024-T351 and AA7075-T651, with variations of composition and surface treatments. The surface treatments cover simple degreasing, chromate and cerium based treatments. A trend of a higher filiform corrosion susceptibility with increasing alloying elements concentrations was observed for all model systems. Furthermore, the filiform corrosion susceptibility varies with the solute atom, in particular Cu was found to have a detrimental effect on the filiform corrosion properties. Both chromating and cerating improve the filiform corrosion resistance of the alloys significantly. To explain the trends observed in the exposure tests, polarisation measurements were performed on the untreated Al-Cu and Al-Zn alloys in bulk anolyte and catholyte solutions which are characteristic for the local anodic and cathodic sites in the filaments on the aluminium substrates. From these measurements a filiform corrosion current, defined as the intercept of the anodic and cathodic curves, can be determined. The present set of experiments shows a correlation between the filiform corrosion properties during accelerated exposure tests and the potentiodynamic polarisation measurements for the Al-Cu alloys. When comparing the results for the Al-Cu and Al-Zn binary alloys it can be concluded that the correlation factor differs significantly with the solute atom and the filiform corrosion current proves to be a non-uniquely discriminating parameter for the filiform corrosion susceptibility of the model alloys. The difference in correlation factor for the Al-Cu and Al-Zn alloys is attributed to differences in the electrochemical behaviour of these alloys with local variations in substrate composition. For the Al-Cu and Al-Zn model alloys the filiform corrosion initiation characteristics are related to the passive range and thus implicitly to the ease of pitting of the alloy. A smaller passive range corresponds to a higher filiform site density for both the Al-Cu and Al-Zn alloys.