Influence of Sulfur on Passivation of Iron in Alkali Solutions

The presence of elemental sulfur caused pronounced changes in the passivation of iron. In higher alkali solutions, soluble divalent iron sulfur complexes are formed involving S22– ions. The trivalent oxides dissolve forming trivalent iron sulfur complexes, which cause complete passivation on further polarization of the surface by forming the polysulfide film. The reduction of the oxide film is found to involve sulfide ions. FeOOH reduction to soluble divalent iron is enhanced by the presence of sulfur.