Effect of variation of local PH on stress corrosion cracking of stainless steels in high concentration brines

Recently, high density clear brine has been utilized in the oil and gas exploration field as completion or packer fluid in the annulus between the production casing and tubing. CaCl₂ has been widely applied for the clear brine, however, some failure cases of duplex stainless steels or super martensitic stainless steels in high density CaCl₂ brine have been reported. The effects of oxygen and/or production gas (CO₂/H ₂S) contamination, or H₂S formation as decomposition products of NaSCN-type inhibitor have been discussed. In this study, pH in high concentration brines was evaluated to discuss SCC susceptibility of stainless steels. Especially, the stability of pH in high concentration brines has been closely studied for the fundamental aspect to determine the effect of the variation of local pH on SCC behavior. 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. SCC susceptibilities of martensitic stainless steels and duplex stainless steels were evaluated in the combination of various brine concentrations and temperature conditions. The addition of caustic soda to high concentration CaCl₂ brine made it stable at higher pH, and also reduced the SCC susceptibility. SCC susceptibility seems to depend on the stability of passive film. Therefore, the depassivation pH is considered to be the important factor affecting SCC in high concentration brines. The effect of the hydrolysis reaction of dissolved metallic ion makes the local pH easily dropped in high concentration brine condition because of its higher activity coefficient of proton and lower buffer power.