Metastable pitting characteristics of aluminium alloys measured using current transients during potentiostatic polarisation

In this study we report a comparison between the metastable pitting observed upon AA2024-T3, AA5005-H34, AA5083-H116, AA6022-O, AA7075-T651 and pure Al, over a range of potentials. Metastable pitting increased with microstructural (hence electrochemical) heterogeneity of the alloys studied, which roughly correlates with hardness. In alloys exhibiting a low pitting propensity, metastable pitting activity completely ceased as the underpotential from Epit was increased, whereas for corrosion prone alloys, metastable activity continued independently of underpotential. The selection of the test potential is also a key factor in using the method described herein. It is shown that the Epit value bears little correlation with the number of pits formed during long-term exposure as measured using optical profilometry, whereas the metastable pitting rate bears a correlation to the actual pit density.