Effect of pH and activity coefficient of proton in high concentration brines on stress corrosion cracking of stainless steels

The environmental factors which may have influence on the Stress Corrosion Cracking (SCC) susceptibility of stainless steels are temperature of solution, dissolved oxygen, chloride ion concentration, pH, cation species and so on. In this study, the effect of high concentration brines on the SCC susceptibility of stainless steels has been investigated. Especially, the variation of pH and activity in high concentration brines has been closely studied for the fundamental aspects to determine the effect of the cation species on the variation of pH. The pH imaging sensor (Scanning Chemical Microscope - SCHEM which can measure the two-dimensional pH distribution) was applied to examine the local pH distribution in high concentration brines. It is elucidated that the pH in CaCl₂ or MgCl₂ solution is more variable than in NaCl or KCl solution due to their higher activity coefficient of proton and lower buffer powers. The local pH measured by SCHEM in the lower buffer power solution (i.e. CaCl₂) is very variable with the addition of a small amount of proton. The SCC mechanism with the active path corrosion (APC) in high concentration brines can be proposed on the basis of the effect of the high activity coefficient of proton and low buffer power on the pH. It can be also proposed that the effect of the cation in brines due to its variability in pH on SCC sensitivity should be considered.