Effects of aging at 475 °C on corrosion properties of tungsten-containing duplex stainless steels

Abstract

Effects of aging at 475 °C 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 °C due to the depletion of Cr around α ′ 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 α ′-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 α ′-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.