Chemical dissolution of iron in aqueous solutions

By anodic oxidation of hydrogen on the Pt ring of a metal-disk-Pt-ring rotating electrode in aqueous acid (pH 0.5-3) sulfate solutions, it is shown that during the polarization of the metal disk hydrogen co-evolves by two parallel reactions. One is the well known electrochemical charge transfer process, while the other one is a direct chemical reaction of the metal, i.e. iron, with water molecules from the electrolyte. This process was proposed a long time ago by Kolotyrkin and co-workers, but their experimental results were subjected to serious criticism which is taken care of in the present paper. The chemical reaction is potential- and pH-independent and, depending on the actual conditions during the corrosion processes (actual potential, pH, etc.), can be of greater or smaller importance in the overall process. The consequences of the existence of this direct chemical reaction of metal with water molecules (i.e. H2O-induced corrosion or chemical corrosion) on pitting and stress corrosion cracking of metals is discussed.