Abstract
In relation to the crevice corrosion mode of type 304 stainless steel in high temperature chloride solutions, which changed from general dissolution to pitting at around 423K, breakdown behavior of passive film has been examined in deaerated acidic chloride solutions up to 423K. Only general dissolution was observed at 373K in solutions of any chloride concentration with pH below 1.8. At 398K, pitting was found in the 2mol dm-3 Cl- solution at pH 2.0, in addition to the usual depassivation in the solution of lower chloride concentrations at pH 1.8. At 423K, pitting became dominant mode of breakdown of passivity; the higher the chloride concentration, the higher the pH below which pits start. Since the corrosion potential of stainless steel was shown to follow the hydrogen electrode potential, the critical condition for pitting was derived by equating the hydrogen electrode potential to critical pitting potential determined separately in neutral solutions. Good coincidence of derived function with experimental data insured that pitting occurred with hydrogen evolution as a catholic reaction. The critical pH for pitting is called pitting pH (pHpit). Thus, pHpit is shown to become higher than depassivation pH (pHD) at 423K in relatively higher chloride solutions, which well explains the change in crevice corrosion mode at 423K.
