Determination of pitting initiation of duplex stainless steel using potentiostatic pulse technique

Duplex stainless steel simultaneously comprises various phases, inclusions, precipitates and grain boundaries. It is full of academic and valuable significance for materials designing and processing optimization to ascertain the corrosion resistance of corresponding phases and various microstructures. However, up to date, efficient evaluation technique is still scare mainly due to difficulty in controlling of micropits. In this work, for the first time, optimized potentiostatic pulse technique (PPT) was established and applied to study the rule of initiation of pits of solution annealed and aged DSS in 1.0 mol/L NaCl solution. The results showed that the size of pits can be well-controlled by adjusting the condition of potential pulse. Combined with SEM/EDS system observation, it was found that for solution-annealed specimens, sulphide and oxide mixed inclusions were the preferential pit initiation sites, while for aged specimens, majority pits occurred at inclusions in area impoverished in Cr/Mo, which is vicinal to σ phase. The phenomena that growing pits is retarded by precipitation rich in Cr/Mo, i.e. σ phase, was testified and explained.