THE EFFECT OF SIGMA-PHASE PRECIPITATION AT 800-DEGREES-C ON THE CORROSION-RESISTANCE IN SEA-WATER OF A HIGH ALLOYED DUPLEX STAINLESS-STEEL

Super-duplex stainless steels are recently developed high alloyed stai nless steels that combine good mechanical properties with excellent co rrosion resistance. Because of a high content of chromium and molybden um, these alloys are susceptible to sigma-phase precipitation during s hort exposure to temperatures between 650 and 950-degrees-C. The effec t of 800-degrees-C aging on sigma-phase formation and on the mechanica l properties of a super-duplex stainless steel have been reported prev iously by the authors.1 This investigation concerns the effect of sigm a-phase precipitation at 800-degrees-C on the corrosion behaviour in s eawater and comprises anodic polarization scans and potentiostatic cre vice corrosion tests. A serious deterioration of the corrosion resista nce is found after aging times longer than 7 min, resulting in a drop of both the critical crevice corrosion temperature (CCT) and the break down potential (E(bd)). The initiation of localized corrosion takes pl ace next to the sigma-phase, in the newly formed secondary austenite ( gamma2).