Stainless steels are widely used in industries because of their high anti-corrosive performance due to their passivation ability. However, crevice corrosion sometimes occurs in chloride environments. It is known that crevice corrosion is initiated when electrode potential inside crevice changes from the passive to active state. Therefore, the potential is the important factor to understand the mechanism of crevice corrosion. In this research, visualization of the potential inside the crevice for type 430 and 304 stainless steels was performed in 0.01 M NaCl at pH 3.0.

An indium-tin oxide (ITO) glass with a sensing layer was applied for visualizing the potential. In the sensing layer, a redox indicator was entrapped. Figure 1 shows the schematic illustration of apparatus for the calibration procedure for color change of the plate. The ITO glass with the sensing layer (sensing plate) was used as a working electrode. In an electrolyte, a mirror polished Pt plate was put below the sensing plate to reflect the image of the sensing plate, and the optical microscope images were taken through the sensing plate by a CCD camera system. Electrode potential of the sensing plate was changed from -0.52 to 1.08 V in 0.01 M NaCl (pH 3.0 adjusted with HCl). All potentials reported in this work refer to Ag/AgCl (3.33 M KCl).

Figure 2 shows the optical micrographs of the sensing plate placed above the Pt. It is clear that the color was changed from light green to dark blue with increasing electrode potential. The sensing plate was applied to crevice corrosion tests in 0.01 M NaCl (pH 3.0 adjusted with HCl). In the crevice corrosion tests, the sensing plate was used as a crevice former. The distributions of electrode potentials inside the crevice of the stainless steels were monitored from the color of the sensing plate.