Micro-electrochemical insights into pit initiation site on aged UNS S32750 super duplex stainless steel

An aging treatment of UNS S32750 super duplex stainless steel at 1173 K for 1.0 ks produced sigma and secondary austenite (gamma(2)) phases in the alpha and gamma phase boundary regions. Small amount of Mn-Cr oxide and MnS complex inclusions were present in the steel. No pitting was observed during the potentiodynamic polarization of a small area (200 x 200 mu m) without inclusions in 1 M MgCl2 at 348 K. Because the electrode area included alpha, gamma, sigma, and gamma(2) phases, these phases and their boundaries alone could not act as initiation sites for pitting. The electrode area size was gradually reduced from 3 x 3 mm to 1 x 1 mm, and pitting corrosion was observed to occur at the Mn-Cr oxide and MnS complex inclusions in a region that appeared to be the gamma(2) phase near the sigma phase.