UNS S32654 Superaustenitic stainless steel for demanding offshore applications

molybdenum content together with increased levels of chromium (20%) and nitrogen (0.20%) gave this grade excellent corrosion resistance and improved mechanical properties. Because of its resistance to pitting and crevice corrosion UNS S31254 has been extensively used in the offshore industry for seawater handling. Other alloys with even higher alloying contents were later developed such as UNS S34565. Despite the lower level of molybdenum (4.5%) this grade exhibits increased corrosion resistance because of the higher levels of chromium (24%) and nitrogen (0.45%) (2). The high nitrogen solubility for UNS S34565 is achieved by alloying with manganese (5.5%). Even though the above mentioned grades show excellent resistance to pitting and crevice corrosion, there are limitations to the use of these alloys for seawater handling under harsher conditions such as elevated temperature or applications with severe crevice configurations (e.g. plate heat exchangers) (3). In such applications nickel-base alloys or titanium had to be used. Thus UNS S32654 (654 SMO®) (1), a second generation superaustenitic stainless steel was developed by further increasing the levels of chromium (24%) and molybdenum (7.3%) which made it possible to reach an even higher nitrogen content (0.5%) (4, 5). UNS S32654 is still one of the highest alloyed austenitic stainless steel grades produced to date and because of its extremely high corrosion resistance it can replace nickel-base alloys in certain applications. As UNS S32654 recently has been reintroduced to the market, this study presents results from laboratory standard tests regarding pitting and crevice corrosion, as well as stress corrosion cracking. Previous studies in natural and artificial seawater and laboratory data for sour service environments are also reviewed. The results are compared to those of superaustenitic stainless steels UNS S31254 and UNS S34565, and also nickel-base alloys UNS N06625, UNS N10276 and UNS N06022.