Determining the probability of stable pit initiation on aluminium alloys using potentiostatic electrochemical measurements

A method is described that determines the probability of stable corrosion pits being formed on aluminium alloys. The method employs potentiostatic measurements of metastable pit equivalent volumes, in coulombs, at the open circuit potential. The metastable pit volumes are statistically analysed using an extreme value statistics procedure that returns an expected distribution of metastable pit volumes. This enables the probabilities of the occurrence of metastable pits of given volumes with time to be calculated. The size requirement for a metastable pit to transition into a stable pit was then determined using both optical microscopy and electrochemical tests. This size requirement when combined with the metastable pit size distributions was used to determine the probability of the formation of a stable pit. The method is demonstrated for aluminium alloy AA2024-T3 in chloride media with and without chromate additions and validated against optically measured pitting rates.