It was demonstrated theoretically and identified experimentally that the protection potential against pitting or crevice corrosion (Ep) was intrinsically the reversal potential of the current between the occluded cell and the external surface (ET). Below ET, the cathodic current flowed into the occluded cell, the pH value of occluded solution increased and Cl- migrated from the occluded cell to outside, resulting in an active-passive transition inside the pit or crevice. A simulated occluded cell was used to determine the ET at different propagating stages of the occluded cell for 1Cr13 and OCrl8Ni9 stainless steels. It was found that ET dropped suddenly as the pH value of occluded solution decreased below a critical value. ET was equal to the difference between the open-circuit potential of the occluded cell and the diffusion potential of the occluded solution into the bulk. The influencing factors of the Ep determintion and its poor reproducibility were also discussed.