Effect of Chloride Ions on Corrosion and Stress Corrosion Cracking of Duplex Stainless Steels in Hot Alkaline-Sulfide Solutions

The resistance of commercial duplex stainless steels (DSS) UNS S32205 and UNS S32101 to corrosion and stress corrosion cracking (SCC) has been studied using immersion tests and slow strain rate tests (SSRT) in hot alkaline-sulfide solution, with and without chloride ions, at 170°C. Weight-loss coupons of austenitic, superferritic, and DSS alloys were used to study the influence of alloy composition. SCC susceptibility of UNS S32205 and S32101 was evaluated in 150 g/L sodium hydroxide (NaOH, 3.75 M) and 50 g/L sodium sulfide (Na₂S, 0.64 M) with 0 to 100 g/L sodium chloride (NaCl, 0 to 1.7 M) at 170°C, using SSRT at an initial strain rate of 2 × 10⁻⁶ s⁻¹. The results indicate that there is a complex relationship among the chloride concentration, alkalinity, and percent sulfidity of the solution and behavior of the stainless steels. Corrosion resistance of the ferrite phase of DSS was superior to the austenite phase in chloride-containing solutions, which had an important role in SCC initiation. Crack initiation in the austenite phase and at austenite-ferrite phase boundaries of DSS was enhanced by the presence of chlorides. These findings demonstrate that chloride ions have an important role in the corrosion and SCC susceptibility of DSS in hot alkaline-sulfide solutions.