The effect of σ-phase precipitation at 800°C on the corrosion resistance in sea-water of a high alloyed duplex stainless steel

Super-duplex stainless steels are recently developed high alloyed stainless steels that combine good mechanical properties with excellent corrosion resistance. Because of a high content of chromium and molybdenum, these alloys are susceptible to σ-phase precipitation during short exposure to temperatures between 650 and 950°C. The effect of 800°C aging on σ-phase formation and on the mechanical properties of a super-duplex stainless steel have been reported previously by the authors.1 This investigation concerns the effect of σ-phase precipitation at 800°C on the corrosion behaviour in seawater and comprises anodic polarization scans and potentiostatic crevice corrosion tests. A serious deterioration of the corrosion resistance is found after aging times longer than 7 min, resulting in a drop of both the critical crevice corrosion temperature (CCT) and the breakdown potential (Ebd). The initiation of localized corrosion takes place next to the σ-phase, in the newly formed secondary austenite (γ2).