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Abstract
When iron is passivated in chromate solutions in the pH range 7·5–1·0, an oxide film forms on the surface and thickens logarithmically with time. At pH ≤ 1·6, dissolution of iron takes place at a measurable rate that is considerably higher than the rate of oxide film growth at the same pH. The amount of iron in solution varies in a parabolic manner with time and the spontaneous dissolution current is given by log idiss = –0·84 pH + constant. The dissolution current of active iron in sulphuric acid solutions is related to the pH of the solution by the expression log idiss = –0·36 pH + constant.
These relationships are interpreted in terms of electrochemical reactions occurring at the surface of the iron, when it is active in sulphuric acid solutions, or at the oxide/solution interface in chromic acid solutions, when the iron is passivated and so covered with a surface film of oxide.