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

Abstract An aging treatment of UNS S32750 super duplex stainless steel at 1173 K for 1.0 ks produced σ and secondary austenite (γ 2 ) phases in the α and γ 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 × 200 µm) without inclusions in 1 M MgCl 2 at 348 K. Because the electrode area included α, γ, σ, and γ 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 × 3 mm to 1 × 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 γ 2 phase near the σ phase.