Stress corrosion cracking of type 321 stainless steels in simulated petrochemical process environments containing hydrogen sulfide and chloride

The susceptibility to stress corrosion cracking (SCC) of type 321 stainless steel (type 321s) in a simulated petrochemical process environment containing hydrogen sulfide and chloride (20 wt.% NaCl + 0.01 M Na2S2O3, pH 2) was assessed using the slow strain rate tensile (SSRT) test and static load (U-bend) tests at the free corrosion potentials. In the SSRT, effects of environmental factors, such as chloride (Cl−) plus thiosulfate (S2O32−), Cl− concentration, solution pH, and temperature, on the susceptibility to SCC of type 321s were critically examined. In addition, factorial design experiments using Yates's algorithm quantitatively estimated the individual and interactive effects of temperature, Cl− concentration, and solution pH on the SCC susceptibility of type 321s. In the U-bend tests, specimens were immersed in an autoclave containing deaerated 20 wt.% NaCl + 0.01 M Na2S2O3 aqueous solution (pH 2) for 1400 h at either 80 or 300 °C. Results of the SSRT tests indicated that the effects of environmental factors on the SCC susceptibility of type 321s decreased in the following order: temperature effect ≫ solution pH effect > Cl− concentration effect. The mechanism of SCC induced by corrosion pits or TiC particles (5-10 μm) is discussed. In addition, results of the U-bend tests show that the susceptibility of type 321s to SCC decreases with increasing temperature, which is related to the more compact surface film containing chromic oxide (Cr2O3) and magnetite (Fe3O4) that forms at the higher temperatures.