PITTING CORROSION MECHANISMS OF LEAN DUPLEX, DUPLEX AND SUPER DUPLEX STAINLESS STEELS IN CHLORIDE SOLUTIONS

摘要

The pitting corrosion resistance of three types of lean duplex stainless steels (grades UNSS32003, UNS S32101 & UNS S32304) along with duplex stainless steel grades (UNS S31803& UNS S32550) and super duplex stainless steel (grades UNS S32750 & UNS S32760) wereinvestigated using the potentiodynamic polarisation technique. Results are discussed based ondata generated in an autoclave for wide range of chloride concentrations (15-22,000 ppm) andtemperatures (600C-1700C) in 8ppm dissolved oxygen. Super duplex stainless steel (SDSS)showed superior pitting resistance in all the conditions tested and demonstrated the highestpitting potentials (EP). But among all the lean duplex stainless steel (LDSS) grades tested,UNS S32003 with higher pitting resistance equivalent number (PREN) value has shown highresistance to pitting than corresponding LDSS (UNS S32304 & UNS S32101) for theconditions tested. A full scale evaluation of corrosion mechanisms was performed usingElectron Back Scattered Diffraction (EBSD) techniques to characterise the damage initiationprocesses and the behaviour of austenite and ferrite phases in corrosive conditions. It isobserved that the ferrite phase is preferentially attacked in LDSS. But in case of DSS theaustenite phase is selectively attacked, whereas in SDSS both austenite and ferrite phases areattacked.