Both pH and pCl sensing plates were fabricated using ethyl violet (pH indicator) and quinine sulfate (fluorescent dye for pCl indicator). The sensing plates were also used for crevice formers. The crevice corrosion for 18Cr-10Ni-5.5Mn stainless steel was studied at 0.3 V (vs. Ag/AgCl, 3.33 M KCl) in 0.01 M NaCl solution (pH 3.0, 298 K). The pH inside the crevice gradually decreased with time, whereas chloride ion concentration did not change. An etch pit was initiated inside the crevice at the pH of 1.7 and the chloride ion concentration of 0.007 M. Just after the etch pit initiation, the pH of the surrounding area of the pit suddenly decreased to 0.4, the chloride ion concentration sharply increased to above 4 M. The dissolved metal ions from the etch pit led to increased acidification, and then the depassivation of the steel matrix occurred.