Effects of aging at 475(circle)C on corrosion properties of tungsten-containing duplex stainless steels

Abstract

Effects of aging at 475 degreesC on the corrosion and mechanical properties of Fe-25Cr-7Ni-0.25N-xMo-yW (x = 0-3, y = 0-6) duplex stainless steels were investigated by an anodic polarization test in HCl solution, a modified double-loop electrochemical potentiodynamic reactivation (DL-EPR) test, and an impact test. Corrosion resistance of the alloys was degraded with aging at 475 degreesC due to the depletion of Cr around alpha' precipitates where numerous micropits were formed during the anodic polarization test. Especially for over-aged alloys, a second anodic current loop appeared in the passive region during the anodic polarization in 1 M HCl solution. The peak value of the second anodic current loop as well as the ratio of the maximum current in reactivation loop to that in anodic loop (i(r)/i(a)) determined from the modified DL-EPR test were found to be an effective measure of the precipitation of alpha'-phase during the aging. However, the degradation in corrosion resistance and impact toughness of the alloys during the aging was retarded with an increase in the W content of the designed DSS, suggesting that W in duplex stainless steels delays the precipitation rate of a'-phase due to a slower diffusion rate of W compared with that of Mo in ferrite. Influences of aging on the galvanic corrosion behaviors between austenite and ferrite phases were discussed by atomic force microscopy observation. (C) 2002 Elsevier Science Ltd. All rights reserved.