Anodic Passivation of Iron in Carbonate Solutions

Abstract

The anodic passivation of iron in aqueous carbonate solutions, pH 11·3, at room temperature gives potentiostatic polarisation curves with two well defined current peaks. the potential of the first oxidation pea. −0·58 v., indicates that passivation is primarily due to the formation of an oxide film, probably mamly magnetite. the iron surface also becomes covered with a film containing ferrous carbonate. The second oxidation peak, at −0·22 v and less than half the size (coulombic equivalent) of the first peak, is due to the oxidation of the ferrous carbonate film to form hydrated γ-ferric oxide. The sizes of the two current peaks depend on the carbonate concentration and on pH. Dissolved oxygen in the solution has no direct effect on the passivation processes in 0·1 M sodium carbonate.