Simultaneous visualization of pH and Cl− distributions inside the crevice of stainless steel

A sensing plate for the simultaneous measurements of pH and Cl− concentration was fabricated. Terbium–dipicolinic acid complex (Tb–DPA) and quinine sulphate were used to measure the pH and Cl− concentration, respectively. In the incubation period of the crevice corrosion, the pH inside the crevice gradually decreased from 3.0 to ca. 2.0, and the Cl− concentration increases from 0.01 to ca. 0.18 M. The generation of the micro-pit led to a sharp decrease in pH to below 0.5 and an increase in the Cl− concentration to above 4 M. This situation allowed the crevice corrosion to proceed without spontaneously stopping.