VOLTAMMETRIC STUDIES OF THE TRANSPASSIVE DISSOLUTION OF MILD STEEL IN CARBONATE/BICARBONATE SOLUTIONS

Abstract The transpassive dissolution of mild steel in aqueous equimolar carbonate/bicarbonate normally aerated solutions has been investigated using cyclic voltammetry. For the first time a transpassive anodic peak preceding oxygen evolution at E >1.100 V ( sce ) has been detected together with the corresponding reduction peak. The formation of Fe (VI) species is proposed. Sweep reversal and sweep rate studies revealed that at least three processes are involved in the kinetics of peak V: charge transfer under resistive control, a coupled chemical reaction and diffusion of electroactive species. An increase in the charge of the peak corresponding to the reduction of Fe(III) to FE(II)(−0.800 V ( sce )) is thought to be due to the chemical decomposition of Fe(VI) yielding Fe(III) species. An increase in the transpassive peak current with increasing temperature (12–73°C) was also indicative that a chemical step is involved. An apparent activation energy of ≈10 kJ mol −1 was obtained.