Chapter C.2 - Pitting corrosion

Abstract

Like all metals coated with a passive oxide film, aluminium is sensitive to pitting corrosion, especially in environments with a pH close to neutral and therefore in natural environments: surface water, tap waters, seawater, humid air, etc. Experience shows that if there is going to be pitting corrosion, it always occurs within the first few weeks of commissioning. The initiation of pits is a very rapid phenomenon and, moreover, is of a random nature since it is not possible to predict the exact place where a pit can start. There is no incubation period for the initiation of pitting. This localized corrosion of aluminium is mainly due to galvanic coupling between microcathodes formed by intermetallic metals and microanodes distributed over the metal surface. The driving force is due to the galvanic coupling between the intermetallic particles and the matrix. It is now widely accepted that the rate of pitting corrosion of aluminium is not proportional to time and that phenomena of pit repassivation intervene to stop further progression. Experience shows that, in most cases, the rate of deepening of the pits formed in natural environments such as freshwater, seawater, and rainwater, decreases over time. This explains the very long service life (several decades) of applications in construction (roofing, etc.), shipbuilding, etc.