ALUMINUM PITTING IN CHLORIDE SOLUTIONS - MORPHOLOGY AND PIT GROWTH-KINETICS

Results from morphological studies on aluminum pitting in chloride solutions are discussed on the basis of in situ optical microscopy observation of pit propagation. It is shown that corrosion tunnelling plays the prominent part, irrespective of chloride concentration, electrolyte temperature or the value of the applied electrode potential. The macroscopic result of pitting therefore depends on the distribution of tunnel initiation sites. On a microscopic scale, pit morphology is determined by the crystallographic form of tunnels and their propagation mode. Within this framework, the pit morphology at the pitting potential as well as the morphologic changes observed after tests at potential values far beyond the pitting potential can be explained easily. No transition to a new pit growth mechanism at high electrode potentials has to be postulated. © 1990.