Passivation and its breakdown reactions have been studied on Mo-containing stainless steel specimens using different electrochemical techniques. Mo-containing stainless steel specimens were polarized in both naturally aerated NaCl and Na2SO4 solutions of different concentrations at 25 +/- 0.2degreesC between -1000 and 1500 mV versus saturated calomel electrode (SCE). The results of potentiodynamic polarization showed that i(corr) and i(c) increases with increasing either Cl- or SO42- concentration indicating the decrease in passivity of the formed film. EIS measurements under open circuit conditions confirmed that the passivity of the film decrease with increase in either Cl- or SO42- concentration. (C) 2004 Elsevier Ltd. All rights reserved.