Pitting corrosion of aluminium in sodium chloride solutions

Abstract

The anodic corrosion process in an artificial pit in aluminium undergoing active corrosion in dilute sodium chloride has been examined by pH and potential measurements and by schlieren observations. Evidence is produced that corrosion continues and spreads mainly in a downwards direction due to the special ability of aluminium chloride solutions to activate passive aluminium surfaces, thus extending the area of active metal along the flow path of this solution.It is shown that the anodic solution can dissolve large amounts of aluminium hydroxide yet still exhibit this behaviour. Insoluble anodic products, postulated by previous workers, would seem to occur in a minor degree, if at all, inside of the pit itself; any anodic hydrogen evolution would probably produce a basic aluminium chloride solution, not a mixture of hydroxide and stoicheiometric chloride.