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

The critical pitting potentials (E_pit) 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 E_pit for not < 50 h. E_pit 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 E_pit = 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. E_pit was compared with the pitting potential (E′_pit) and the repassivation potential (E_R), as determined using the stepwise cyclic polarization method. The practical importance of E_R was emphasized, since E_R was always less noble than E_pit, although E_R could not always be determined uniquely.