A simulated occluded cell which can be loaded by lever system was designed to study the changes of potential and pH in occluded cell with polarization of bulk specimen. The system studied was Crl8Ni9Ti austenitic stainless steel-0.5M NaCl at 40℃. It was found that the critical crevice (or stress cor-rosion) potential of this system lies almost in the same range as the corrosion potentials of the occluded and bulk potential (0V to -0.2V SCE). The rule of changes of the potentials and pH values in the occluded cell could be classified into three categories as follows. 1. When the bulk specimen was polarized above the critical potential

the occluded potentials did not rise with bulk potential

but remained nearly constant within corrosion potential range (-0.10 to- 0.13V SCE). Meanwhile

the pH in the occluded cell decreased rapidly from 7 to 2-3.5. 2. When the bulk specimen was polarized below the critical potential

the occluded potential always kept a few mV more positive than the bulk potential

and the pH in the occluded cell increased higher than bulk pH (>7). 3. When the potential of bulk specimen remained in the range of the critical potential

no change of pH and potential in the occluded cell took place within a long period. The tensile load of occluded specimen caused both the potential and pH in the occluded cell lower than those without load under anodic polarization of the bulk specimen

but no effect under cathodic polarization. The important phenomenon of incubation period for crevice or stress corrosion could be interpreted properly by the present results. The crevice or stress corrosion starts only when the bulk potential rises above the critical po-tential so that a corrosion cell of sufficient voltage difference between bulk cathode and occluded anode could be established. A rapidly accelerating process will result from pH fall and Cl- migration in the occluded cell. While the bulk potential is lower than the critical potential (cathodically polarized)

the crevice or stress corrosion will not occur since no significant voltage difference exists between bulk and occluded cell. It is very interesting that the potential and pH in the occluded cell have a simple relation with the bulk potential. It could be easily to evaluate occluded potential or pH if one knows the bulk potential. By controlling the bulk potential one could make the initiation and propagation of crevice or stress corrosion prevented. Besides

one may determine whether there is danger of crevice or stress corrosion by measuring the bulk potential.