Effect of Chlorides on Stress Corrosion Cracking of Duplex Stainless Steels in Hot Alkaline Sulfide Solution

Duplex stainless steels (DSSs) are often chosen for equipment exposed to aggressive alkaline solutions. In many chemical process streams, chloride is always present in limited amounts. The current work systematically evaluated the synergistic role of chloride and sulfide ion attack on the stress corrosion cracking (SCC) behavior of UNS 31803 and UNS 32101 DSS. Slow strain rate testing (SSRT) was conducted in an autoclave in alkaline solutions at 170 ⁰C containing varying amounts of sodium hydroxide and sodium sulfide as well as 10 g/L sodium chloride. The percent reduction in area and time to failure were found for each test. The fracture morphology was characterized using scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS). Additionally, the surface cracks were cross-sectioned with a focused ion beam (FIB) and examined with the SEM backscatter electron detector (BSE). The fracture surfaces were mounted and observed under optical microscope, and the maximum crack velocity was measured for each specimen. The results show that chloride can alter the mechanism of SCC. Intergranular corrosion of DSSs can occur when chloride is added to sulfide-free alkaline solution. DSSs are susceptible to interphase attack (IPA) when chloride is added to sulfide-containing alkaline solution, whereby crack growth proceeds along phase boundaries. The nickel-rich, austenite phase undergoes more attack than the ferrite phase. Chloride addition is shown to enhance the SCC susceptibility of DSSs for the conditions in the current study.