Pitting of aluminium in synthetic waters

The corrosion characteristics of commercially pure aluminium in synthetic ‘tap’ waters, containing sodium chloride, calcium carbonate and dissolved copper, have been studied. In the presence of any two of the constituents, the loss in weight is very small, but nodular pitting is observed when the three are present. The maximum weight loss occurs when the Ca 2+ content is 50 p.p.m., irrespective of the chloride ion concentration, the copper content being constant. Specimens which have been anodised and subsequently sealed, show no pitting in two weeks; unsealed specimens are less effective in this respect. Localised corrosion is most severe after the oxide film has been removed in a phosphoric/chromic mixture. A theory is put forward to explain the initiation of pits and their subsequent development with time.