Localized Corrosion Resistance of UNS S32750 and UNS S33207 Duplex Umbilical Tubes in Synthetic Seawater

The resistance to localized corrosion of girth-welded UNS S32750 and UNS S33207 duplex stainless steel umbilical tubes in synthetic seawater at different temperatures was investigated using electrochemical methods, such as cyclic potentiodynamic polarization (CPP) and potentiostatic polarization, and long-term immersion tests. Repassivation potentials as determined from the CPP tests were compared with the open-circuit potential (OCP) obtained from the long-term immersion tests to address the long-term localized corrosion resistance of umbilical tubes. Electrochemical results indicate that the critical pitting temperature (CPT) for the welded S32750 and S33207 tubes were 53°C and 68°C, respectively. The critical crevice temperature (CCT) for both welded tubes was similar to the CPT values. Interestingly, despite the critical temperatures mentioned above, the 45-day immersion tests show no pitting or crevice corrosion for the welded S32750 tubes at temperatures of 50~70°C and the welded S33207 tubes at temperatures of 60~80°C in aerated synthetic seawater.