The corrosion of aluminium at room temperature has been investigated in nine decinormal solutions over the pH range 2.0–12.0. Corrosion was dependent more on the nature of the anion than on the pH of the solution.
The solubilities of hydrated and anhydrous aluminium oxide were examined and it was concluded that, except in the presence of chloride ions, corrosion was associated with the initial rate of solution of the anhydrous oxide.
Potential and anodic polarisation measurements indicated that whereas the normal anodic reaction is the formation of anhydrous oxide, in the presence of chloride, bromide and iodide ions soluble aluminium salts are formed.
It is suggested that when aluminium, carrying its air‐formed film, is immersed in a solution in the presence of oxygen, the film breaks down and a small corrosion current commences to flow. In the absence of chlorides, and similar ions, film repair can take place and the behaviour of the aluminium is controlled by the relative rates of film repair and film breakdown. In the presence of chlorides film repair does not occur and once corrosion has been initiated by film breakdown it proceeds apace. If initiation is suppressed, by saturating the solution with aluminium oxide, corrosion is inhibited.
Capacity measurements obtained with an a.c. bridge have been used to determine the thickness of non‐porous oxide films on iron and aluminium surfaces, and for showing the effect of both the solvent action of aqueous solutions and electrochemical processes on the thickness of these films.
These measurements could not be used, however, to detect the increase of permeability of the oxide film on aluminium placed in solutions containing chloride; electrode potential measurements were used in this case.
Measurements of capacity and of potential vs. time were used to show formation of pores through oxide films.
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