Electrochemical studies were conducted to determine the critical amount of oxygen necessary to prevent corrosion by maintaining the open-circuit passivity of Fe-18Cr samples initially passivated at 0.6 V normal hydrogen electrode (NHE) in 1n H2SO4 solutions. Samples passivated at 0.6 V (NHE) and then released to open circuit in O2-saturated (29.4 ppm O2 dissolved) solution maintained a state of passivity. Samples passivated and released to open circuit in N2-purged solution decayed to a state of active corrosion in 800 to 2,000 min. A passive state, however, could be maintained if O2 were added to the N2 flow so that a minimum of 1.7 ppm O2 was present in solution. Furthermore, this critical amount of O2 had to be added before open-circuit decay reached 0.45 V(NHE). Auger electron spectroscopy measurements indicated that following open-circuit stabilization of passivity of O2 the percentage chromium in the film increased with increasing open-circuit potential. X-ray photoelectron spectroscopy measurements indicated that the film thickness decreased with increasing open-circuit potential.