Propagation of Corrosion Pits

Like all passive metals, aluminium is prone to localised corrosion caused by a local rupture of the passive film [3]. This results in a corrosion pit that can propagate, provided that the 

The electrochemical mechanisms of pitting corrosion are very complex and not totally understood. The practical relevance of these theoretical models is still limited [4]. Pitting corrosion shows two distinct stages initiation and propagation. 

It has long been known that pitting corrosion develops in the presence of chloride. Chloride ions Cl are adsorbed on the natural oxide film [5], followed by the rupture of the film at weak points, with formation of microcracks that are a few nanometres wide. Many pits are initiated within a very short time, up to lO cm . Their density depends on the alloy from 10 cm on 1199 to 10 ° cm on an alloy containing 4% copper. 

However, most pits will stop after a few days. Polarisation studies have demonstrated that when pits stop growing, they will be repassivated. When the metal is polarised once again, these passivated pits will not be reinitiated again, but pitting will start on fresh sites. 

Oxygen will be reduced slowly in cathodic areas. These areas seem to be intermetallic phases underneath the oxide layer that more or less covers them. Where the film cracks, aluminium will oxidise rapidly, and a complex intermediate chloride AlCl will be formed. 

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