Critical Pitting Potentials for Type 304 Stainless Steel in High-Temperature Chloride Solutions

Abstract

The critical pitting potentials (Epit) of type 304 (UNS S30400) stainless steel (SS) were determined as a function of temperature (373 K to 523 K) and chloride (Cl−) concentration (1.00 x 10−2 mol/kg-H2O to 2.09 mol/kg-H2O [4.54 x 10−3 mol/lb-H2O to 9.48 x 10−1 mol/lb-H2O], m). Steady polarization tests were performed at intervals of 20 mV around Epit for not < 50 h. Epit was supposed to lie between the noblest potential, where no grown pit was found, and the least noble one, where pitting occurred. Results were expressed by Epit = a – b log[Cl−]. In regard to temperature dependency, a decreased with temperature, while b was almost constant up to 448 K. At 523 K, the decrease in a ceased, and b became lower, which reflected the change in pitting morphology at this temperature. Epit was compared with the pitting potential (E′pit) and the repassivation potential (ER), as determined using the stepwise cyclic polarization method. The practical importance of ER was emphasized, since ER was always less noble than Epit, although ER could not always be determined uniquely.