The dissolution mechanism of steel in inorganic acids

The dissolution behaviour of steel in 1 N solutions of HCl, H2SO4 and HClO4 acid containing 10 per cent (v/v) ethanol was investigated at 30-60degreesC using gas volume measurements, polarisation and electrochemical impedance spectroscopy techniques. The data revealed that the rate-of corrosion decreased in the order Cl- > SO42- > ClO4-. The corrosion mechanism of the acid dissolution for steel at low pH was found to be dependent on the counter ion present. The mechanism was discussed on the basis that the acid anion is first adsorbed on the steel surface, followed by the formation of a surface complex in the anodic process and finally the complex was desorbed from the surface. In general, if the adsorbed anion or the surface complex was stable, the corrosion of steel was suppressed. The calculated values of the activation energy for the steel in different acid indicated that the dissolution process proceeded via similar mechanisms.