In order to clarify the relationship between crevice corrosion behavior of SUS304 and the pH in crevice（pH_c）, simultaneous measurement of pH in crevice by semiconductor chemical sensor at potentiostatic polarization.

The pH measured by the sensor is not influenced by the metal ion and shows the value reflecting［ H^＋］ and γ_H^＋.

When crevice corrosion is always progressing by potentiostatic polarization, among the metal ions dissolved in the crevice, mainly in the hydrolysis reaction of Cr^3＋ or formation of chloride complex ions, the entire crevice part becomes extremely low pH environment with pH_c＜0.0.

On the other hand, when polarization by two-step potentiostatic polarization, the amount of metal dissolved in the crevice decreases depending on the second potential（E₂）, and finally, eventually leading to re-passivation.

In the case of this study, the re-passivation pH after crevice corrosion progression was 2.5 to 3, and the repassivation potential was about 80 mV, which gave high engineering significance.

When crevice corrosion of stainless steel occurs and progresses, the proton and the chloride ion concentration is high and there is no dissolved oxygen in the crevice. On the other hand, when crevice corrosion of stainless steel stops, the proton and the chloride ion diffuses out of the crevice and dissolved oxygen penetrates the inside of the crevice. In this case, the state of the stainless steel changes from active state to passive state.

Since it is important to know the passivation behavior of the stainless steel, we invest the effect of chloride ion and dissolved oxygen concentration on passivation pH of SUS304 stainless steel.

The passivation pH of SUS304 steel increased with increasing chloride ion concentration and decreasing dissolved oxygen. When cathodic current density exceeds I_CRIT, passivation occurs.

Therefore, It is assumed that the rise of I_CRIT and the decrease of cathodic current density of the proton reduction with the rise of chloride ion concentration and the decrease of cathodic current density of the oxygen reduction with the decrease of dissolved oxygen concentration contributes to increasing passivation pH.