Study On The Pitting And Intergranular Corrosion Behavior Of Super-Austenitic Stainless Steel 254SMO

254SMO is a super-austenitic stainless steel (SASS) with high levels of chromium, molybdenum, nitrogen and low level of carbon, which was first produced in Sweden in the 1970’s.254SMO offers chloride resistance superior to that of common austenitic stainless steels, such as Type 304,316 and 316L. It is often used as a technically adequate and much less costly substitute for nickel-base alloys and titanium in many cases and has been widely applied in pulp mill bleach system, seawater handling and desalination equipment, chemical processing equipment, flue gas desulfurization scrubbers and some other high-chloride environments. However, the high chromium and molybdenum contents of 254SMO will accelerate the precipitation of intermetallic phase (such as σ) and carbides during hot-working process and welding operation, which have a deleterious effect on resistance to pitting and intergranular corrosion and these failures have been identified as the main cause of premature failure of the structure. Therefore, it is significant to study the microstructure evolution, pitting and intergranular corrosion behavior of 254SMO after thermal treatment and welding. Based on the above background, in this paper, the resistance to intergranular and pitting corrosion of UNS S31254 was evaluated by means of chemical immersion and electrochemical methods. The mainly contents can be concluded as follows:(1) Intergranular corrosion property of UNS S31254 aged at 650℃ and 900℃ for different durations was evaluated by sulfuric acid-ferric sulfate immersion and boiling nitric acid immersion respectively, and the microstructure evolution was evaluated by microscopy techniques. The result showed that the precipitation of σ phase and carbides of specimens aged at 900℃ was much faster than that of specimens aged at 650℃, which can be explained by that high alloys have a higher nose temperature and 900℃ is closer to the nose temperature. Intergranularly precipitated σ phase and carbides can produce intergranular corrosion as a result of chromium and molybdenum depletion adjacent to the precipitates. As the aging time prolonged, the diffusion of chromium and molybdenum from the base metal healed the chromium depletion area, which decreased the susceptibility to intergranular corrosion.(2) Pitting corrosion property of UNS S31254 aged at 650℃ and 900℃ for different durations was evaluated by means of critical pitting temperature (CPT) technique. The result showed that the isothermal treatment at 650℃ hardly had effect on the resistance to pitting corrosion of the specimens, while the CPT value decreased significantly with the aging time aged at 900℃. The decrease of CPT value may be caused by chromium and molybdenum depletion area adjacent to the precipitates. The pitting corrosion property of UNS S31254 aged at temperatures ranging from 500℃ to 1150℃ for 10 min was also investigated by CPT technique and the result indicated that the specimens aged at 900～1100℃ had a lower resistance to pitting corrosion.(3) The influence of cooling rate and heat input on microstructure evolution and pitting corrosion behavior of UNS S31254 was investigated by means of CPT technique and image analysis. As cooling rate decreased and heat input increased, resistance to pitting corrosion decreased slightly. It is because that higher heat input and lower cooling rate means longer time at intermediate temperature from 900℃ to 1100℃, resulting in more precipitates, which have a deleterious effect on resistance to pitting corrosion.