Effect of aging at 475℃ on the corrosion properties of 25Cr duplex (α+γ)stainless steels containing tungsten

摘要

Duplex austenitic/ferritic stainless steels are knownto undergo '475 deg C Embrittlement' due to spinodaldecomposition in ferrite phase when exposed to temperatures of300~550 deg C. Effects of aging at 475 deg C on the corrosionand mechanical properties of Fe-25Cr-7Ni-0.25N-xMo-yW (x =0~3, y = 0~6) alloys were investigated by anodic polarization testsin HCl solution, a modified DL-EPR (double-loopelectrochemical potentiodynamic reactivation) test and an impacttest. Corrosion resistance of the alloys was degraded with aging at475deg C owing to depletion of Cr around #alpha#' precipitateswhere numerous micro pits were formed during anodicpolarization. Specially for the over-aged alloys, a second anodiccurrent loop appeared in the passive region during anodicpolarization in 1M HCl solution. The current value of the secondanodic loop as well as the ratio of the maximum current inreactivation loop to that in anodic loop (i_r/i_a) were found to bean effective measure of the precipitation of #alpha#' phase duringthe aging. However, the degradation in corrosion resistance of thealloys during the aging was retarded with an increase in the ratio ofwt% W/ wt% Mo of the alloys, suggesting that the W in duplexstainless steels delays the precipitation rate of #alpha#' phase dueto a slower diffusion rate of W compared with that of Mo.Galvanic corrosion behaviors between austenite and ferrite phasesfor the aged and the unaged alloys were discussed by AFM (atomicforce microscopy) observation.