Pitting corrosion resistance of duplex stainless steels multipass welds

Duplex stainless steels might undergo phase precipitations during welding thermal cycles, without almost any change in volumetric fraction of ferrite. This is the case of thermal. cycles in temperature range from 950degreesC to 650degreesC. The microstructure in HAZ might have a small amount of intermetallic phases that impair mechanical and corrosion resistance properties. To analyze this phenomenon, three commercial steels (UNS S32304, UNS S32750 and UNS S32760) were submitted to GTAW multipass welding processes and thermal cycle sample simulation using three different heat inputs (0,64 kJ/mm, 0,8 kJ/mm and 1,0 kJ/mm). HAZ microstructures were characterized by optical, scanning, and transmission microscopy. Pitting corrosion resistances of simulated samples were determined by cyclic polarization test in artificial seawater. X-Ray diffraction analyses were utilized to identify precipitated phases. Results showed a total agreement between actual and simulated HAZ microstructures. It was observed no phase precipitation in HAZ of LNS S32304 steel in any of heat inputs studied. On the other hand, UNS S32750 and UNS S32760 steels presented intermetallic phases precipitation. Cyclic polarization test depicted low pitting (Epit) and protection (Epp) potentials for the UNS S32304 steel (Epit = 481 to 531mV and Epp = -164 to -150 mV). UNS S32750 and LNS S32760 presented almost same Epit (1070 mV) but different Epp, respectively -67 to -90mV and 250 to 307mV.