The super-austenitic stainless steel UNS S31266 is due to its optimum of alloying elements a suitable material for seawater applications as for example seawater cooled heat exchangers. Producing UNS S31266 as seamless tubes gives the possibility to build heat exchangers with both tubes and plates in UNS S31266. The super-austenitic stainless steel UNS S31254 also has excellent corrosion resistance in a variety of industrial environments.
In this study the critical pitting temperature (CPT) for UNS S31266 was compared to UNS S31254 with the newly developed electrochemical method, modified ASTM G150 developed by K. Lund et al., using 3M magnesium chloride (MgCl2) instead of 1M sodium chloride (NaCl). CPT-values measured by the modified ASTM G150-method were shown to be 10-20 °C lower than when using 1M NaCl. Also, the crevice corrosion resistance of UNS S31266 was compared to UNS S31254 measured in NaCl solution.
The average CPTmod in 3M MgCl2 for UNS S31266 was 86 °C and for UNS S31254 it was 66 °C. The CPTmod was thus 20 °C higher for the UNS S31266 seamless tubes compared to UNS S31254. The CCT in NaCl of UNS S31266 was approximately 85 °C and for UNS S31254 it was approximately 60 °C.
The development of super-austenitic steels has been driven by the increased demands on pitting and crevice corrosion resistance in seawater handling and chemical industries. Main alloying elements in super-austenitic steel UNS S31266 is Fe, Cr, Ni, Mo, Mn, W and N. Main design concepts in UNS S31266 are a reduced Mo content replaced with W and a high N content. The N level can be increased since Mo has partially been replaced with W. Due to the higher N content the mechanical properties are increased because of the high amount of interstitial N dissolved in the austenite. N further increases the suppressing of intermetallic phase precipitation. The high content of Ni, Cr and N additions in UNS S31266 gives a stable microstructure less prone to precipitate intermetallic phases.1,2 UNS S31266 is used as plate in combination with for examples hyper duplex UNS S32707 seamless tubes in seawater cooled heat exchangers. Tests performed by J. Eidhagen et al. showed that the UNS S31266 plate and UNS S32707 tubes in a seawater cooled scale model heat exchanger with 0.5 ppm residual chlorides during 18 months was corrosion resistant up to a tube skin temperature of 95 °C inside in the seawater.3