Determination of the Acceptability Conditions for a Safe Use of Super Duplex Stainless Steel through SSRT Screening

Abstract The present work mainly focuses on determining the limits of use of a UNS S32750 super-duplex stainless steel tube product in NACE standard solution A under 0.5 bar H2S, at different temperatures between 23 and 80°C, and pH 2.8 to 6 through slow strain rate testing. The acceptability conditions for SCC/SSC resistance were compared to those obtained on plate products of UNS S32205 duplex and UNS S32707 hyper-duplex stainless steels using the same experimental technique. Two different plate products in UNS S32205 were tested, with two different thickness reductions upon manufacturing: a 17 mm thick plate and a 2 mm thick foil. The super duplex stainless steel tube material UNS S32750 showed higher SCC and SSC resistance compared to the plate duplex stainless steel UNS S32205. It has been observed that both the duplex and super-duplex stainless steels are suffering from selective dissolution in low pH at 80°C, however, in different phases. Selective dissolution of austenite is observed in alloy UNS S32750 at 80°C when pH ≤ 3.5, whereas only ferrite dissolution is observed in UNS S32205 (as plate) at 80°C and pH 4.5. No SSC has been observed in UNS S32750 at lower temperature down to 23°C and at pH 2.8. However, dissolution in both phases has been evidenced in UNS S32205 (as foil) at temperature as low as 23 and 50°C and pH between 2.8 and 4.5. EBSD investigation has shown that the microstructural difference between the thin and thick plate products of UNS S32205 is most likely process-related. In addition, the UNS S32707 plate showed the highest resistance to SCC and SSC. No degradation has been observed for UNS S32707 in any of the conditions tested in this study.