Effect of anions on the corrosion of aluminium in sodium hydroxide solutions Part I. The chromate ion

Abstract

The corrosion rates and potentials were measured at 30°C for aluminium in 0.1–0.5 M unstirred aerated sodium hydroxide solutions containing 10 −4 –1 M Na 2 CrO 4 . Galvanostatic anodic polarisation measurements were carried out within the current density range 10 −6 –10 −1 A cm −2 . At large dilutions the chromate ion markedly promoted the corrosion of aluminium; the promoting effect increases with concentration up to 10 −3 M . At higher concentrations the ion behaves as inhibitor, and the corrosion rate decreased sharply with concentration. The corrosion potentials and the polarisation curves gave indications that the chromate ion accelerates both the cathodic and anodic reactions. Suggestions concerning their acceleration mechanisms are given. The inhibiting action was attributed to the formation of a protective layer of chromium oxide. Similar to the corrosion rate, the passivation current increases with concentration up to 10 −3 M , and then decreases with further increase of concentration. The non-formation of the inhibiting film below 10 −3 M was explained on the basis that chromium oxide is formed through the disproportionation of an intermediate adsorbed species of valency less than six.