THE PASSIVITY OF IRON AND CARBON-STEEL IN ANHYDROUS PROPYLENE CARBONATE SOLUTIONS

The passivation behavior of high purity iron and 1018 carbon steel in anhydrous propylene carbonate (PC) containing 0.5 M LiClO4 or 0.5 M LiAsF6 was studied. The air-formed film on Fe was stable and protective over a wide applied potential range in perchlorate solutions. Scratch tests showed that both bare Fe and steel passivate in the anhydrous PC solutions of either electrolyte by chemisorption of PC molecules below the oxidation potential of PC (PC(ox). At higher potentials, bare Fe passivates in PC/LiClO4 via the formation of a Fe(ClO4)2 salt film. The air-formed film on carbon steel was protective up to the PC(ox) potential in either electrolyte. Above PC(ox), deterioration of the air-formed film on steel was associated with sulfide surface inclusions. Pits initiated and grew at these inclusion sites. Scratch tests showed slower repassivation of the bare steel by the Fe(ClO4)2 salt film than on iron. Sulfides impeded this repassivation. Above PC(ox), neither the iron or the steel are in passive in PC/LiAsF6 solutions. The higher solubility of the Fe(AsF6)2 salt prevents precipitation of a protective salt film.