The Effects of Bicarbonate on the Corrosion and Passivation of Iron

Abstract

The corrosion of iron in bicarbonate solutions has been investigated potentiostatically and by Auger electron spectroscopy. The dissolution of iron in the active and prepassive regions is greatly accelerated by the presence of bicarbonate, owing to the formation of the stable soluble complex anion Fe(CO3)22-. A mechanism is proposed for the dissolution process. Pitting of the metal surface is observed under conditions where dissolution as the complex occurs. Raising the potential into the passive region where Fe(lll) forms in the film prevents complexing and so anodically protects the metal from pitting. Possible implications of complex formation in the stress corrosion cracking of mild steel in bicarbonate electrolytes are discussed.