Abstract
The anodic passivation of iron was examined with respect to the degree of proton hydration as well as the acid structure. The investigation was based on the electrochemical measurements, XPS study and sulphur dioxide analysis. When acid concentration is low, the passivity of iron is a result of the precipitation of an oxide layer which is formed only in the presence of water. The role of sulphate ions in passivation grows with increasing acid concentration. For a concentration of about 5 M, the passivation is observed both in water and alcohol solutions. Above this concentration the passivation may occur as an effect of S 6+ ion reduction. The experiments revealed the sulphur dioxide evolution in the active and passive range of the potential. The increase of the volume of SO 2 with the acid concentration was also observed. An in-depth XPS analysis of the oxide layer for 18 M and 10 M solutions detected a gradual decrease in the oxidation number of iron as well as the reduction of sulphate ions to sulphides and the formation of iron sulphides in the bottom part of the oxide layer.
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