The Change of pH in Crevice Due to Crevice Corrosion of Stainless Steels

Abstract

The change of pH in corroding crevices has been investigated. Crevice specimens were immersed in NaCl solutions, and the pH and the Cl- ion concentration in crevices were determined. The potential inside the crevice was also measured with a capillary inserted into the crevice.
The results show that the pH in crevice becomes lower with development of crevice corrosion. The stationary pH of the SUS 304 crevice is 2.5 to 4. The enrichment ratio of the Cl- ion in the SUS 304 crevice is increased, as the concentration of the Cl- ion in the test solution or the crevice volume is decreased. Judging from the pH in crevice and the potential inside crevice, it seems that active state is attained in the SUS 304 crevice. The steels which contain Mo more than 2% have a tendency to show lower pH in crevices in comparison with low Mo steels.