The stress corrosion cracking of C-Mn Steel in CO2-HCO3--CO32- solutions .2. Electrochemical and other data

The polarization behaviour of a mild steel has been studied on static and rotating electrodes in solutions containing various concentrations of CO2, HCO3- and CO32-, from which it is shown that the lower bound of the intergranular stress corrosion cracking domain occurs at potentials a little above those at which a first oxidation peak is observed. The potential of the first oxidation peak decreased markedly with increase in pH value, also showing some dependence on HCO3- concentration; increases in the latter also causing an increase in the magnitude of the current density associated with the peak. It is shown that selective dissolution at grain boundaries in unstressed specimens of the steel begins at potentials approximating the lower bound of the cracking domain, lower potentials promoting non-selective dissolution. It is suggested that the upper bound of the intergranular cracking domain corresponds to the reaction whereby the stable phase changes from FeCO3 to gamma-Fe2O3, with the pH limits of the intergranular cracking domain at 6.7 and 11.0 resulting from the transformation from FeCO3 to Fe2+ and Fe(OH)(2), respectively, as the stable phases. The lower bound of the intergranular cracking domain is proposed as resulting from the beginnings of the formation of Fe2O3 in place of Fe3O4, the start of the formation of Fe2O3 inducing sufficient passivity to allow selective dissolution at grain boundaries. The quasi-cleavage that occurs in slow strain rate tests involving CO2-saturated dilute solutions at markedly different potentials from those that engender intergranular cracking correlates reasonably with those potentials at which hydrogen discharge is expected. Copyright (C) 1996 Elsevier Science Ltd