텅스텐(W)이 함유된 25 Cr이상(二相) 스테인리스강의 부식특성에 미치는 475℃ 시효처리의 영향

Abstract

Duplex austenitic/ferritic stainless steels are known to undergo `475℃ Embrittlement` due to spinodal decomposition in ferrite phase when exposed to temperatures of 300∼550℃. Effects of aging at 475℃ on the corrosion and mechanical properties of Fe-25Cr-7Ni-0.25N-xMo-yW (x = 0∼3, y = 0∼6) alloys were investigated by anodic polarization tests in HCl solution, a modified DL-EPR (double-loop electrochemical potentiodynamic reactivation) test and an impact test. Corrosion resistance of the alloys was degraded with aging at 475℃ owing to depletion of Cr around α` precipitates where numerous micro pits were formed during anodic polarization. Specially for the over-aged alloys, a second anodic current loop appeared in the passive region during anodic polarization in 1M HCl solution. The current value of the second anodic loop as well as the ratio of the maximum current in reactivation loop to that in anodic loop (i_r/i_a) were found to be an effective measure of the precipitation of α` phase during the aging. However, the degradation in corrosion resistance of the alloys during the aging was retarded with an increase in the ratio of wt% W/wt% Mo of the alloys, suggesting that the W in duplex stainless steels delays the precipitation rate of α` phase due to a slower diffusion rate of W compared with that of Mo. Galvanic corrosion behaviors between austenite and ferrite phases for the aged and the unaged alloys were discussed by AFM (atomic force microscopy) observation.