The effects of Pb on the passive film of Ni-base alloy in high temperature water

Abstract

The effects of Pb on the passive film of Alloy 600 were investigated using polarization and immersion tests. The anodic dissolution behavior was observed using anodic polarization tests in water with up to 500 ppm of PbO at pH 4 and pH 10 at 90°C. The immersion tests were conducted at 250°C with lead concentrations of 0, 25, and 250 ppm. The chemical composition of the surface films formed after 5–10 days of immersion was analyzed by AES (Auger electron spectroscopy) and XPS (X-ray photoellectron spectroscopy). The metal ions dissolved from Alloy 600 during the immersion tests were analyzed by ICPS (ion coupled plasma spectroscopy). The results indicate that, as Pb concentration increased, the critical current densities of Alloy 600 increased at both pH 4 and pH 10. At pH 10, the outermost surface films were enriched in Cr and depleted in Ni with increased levels of Pb, but the inner layer showed the opposite trend. In solution analysis, higher concentrations of Ni were observed with increased levels of Pb, which indicates that Pb facilitates Ni dissolution. This is consistent with the analysis results of outer films by XPS. These results show that a Ni-depleted/Cr-enriched outer film and a Cr-depleted/Ni-enriched inner layer may be formed on Alloy 600 in mildly caustic water containing Pb.