Super-austenitic stainless steel grades have been developed with high chromium (20 to 27%), high molybdenum (4 to 6%) contents and typically between 0.1 and 0.4% of nitrogen. These materials are significantly more resistant to corrosion than conventional austenitic grades such as 316L and can be considered as cost-efficient alternatives to nickel-based alloys. They are today widely used in various industrial sectors such as the chemical industry, water treatment and pulp & paper equipment. Some of them (UNS N08904 and S31254) are also commonly used for Oil & Gas applications.
NACE MR 0175/ISO 15156 standard is a reference document for material selection for the Oil & Gas production field. This standard limits the use of super-austenitic grades to 5,000 ppm chlorides in H2S environments. Several papers suggest that they could be resistant far above this limit.
Industeel has conducted an extensive test program to evaluate the corrosion and cracking resistance of various super-austenitic stainless steels: UNS S31254 (20%Cr, 18%Ni, 6%Mo), N08926 (20%Cr, 25%Ni, 6%Mo), S34565 (24%Cr, 17%Ni, 4.5%Mo), N08028 (27%Cr, 31%Ni, 3.5%Mo), N08031 (26%Cr, 31%Ni, 6.3%Mo) and finally S31266 (24%Cr, 22%Ni, 6%Mo). Various H2S partial pressures were investigated with chloride contents above the NACE limits. The paper will provide a ranking of cracking and localized corrosion resistance of these materials. The results are compared to what was obtained on the nickel-based alloys commonly used in the Oil & Gas industry UNS N06625 and N08825.