General Corrosion and Passive Film Stability

Abstract

We have studied Alloy 22 corrosion and passive film stability in nitrogen-purged Na-K-Cl-NO{sub 3} brines having NO{sub 3}:Cl ratios of 7.4 at 160 C and NO{sub 3}:Cl ratios of 0.5 and 7.4 at 220 C in autoclave experiments under a slight pressure. The experiments were done to show the effect of high nitrate brines on the durability of the Alloy 22 outer barrier of the waste canisters. Ratios of NO{sub 3}:Cl used in this study were lower than expected ratios for the repository environment at these temperatures and atmospheric pressures (NO{sub 3}:Cl > 25), however they were thought to be high enough to inhibit localized corrosion. Localized corrosion occurred on the liquid-immersed and vapor-exposed creviced specimens under all conditions studied. Crevice penetration depths were difficult to quantify due to the effects of deformation and surface deposits. Further characterization is needed to evaluate the extent of localized corrosion. The bulk of the surface precipitates were derived from the partial dissolution of ceramic crevice formers used in the study. At this time we do not know if the observed localized corrosion reflects the corrosiveness of Na-K-Cl-NO{sub 3} solutions at elevated temperature over nine months or if it was an artifact of the experimental protocol. Nor do we know if much more concentrated brines with higher NO{sub 3}:Cl ratios formed by dust deliquescence will initiate localized corrosion on Alloy 22 at 160 and 220 C. Our results are consistent with the conclusion that nitrate concentrations greater than 18.5 molal may be required to offset localized corrosion of Alloy 22 at 160 and 220 C. Stability of the passive film and general corrosion were evaluated on the liquid-immersed and vapor-exposed non-creviced specimens. Elemental depth profiles of the vapor-exposed specimens are consistent with the development of a protective Cr-rich oxide near the base metal. The combined passive film and alloy oxide of the immersed specimens was much thicker than for the vapor-exposed specimens. This may be attributed to the inability to transport reactants away from the surface with limited amount of fluid in the condensate compared to the large reservoir for the liquid-immersed specimens. Elemental depth profiles of the liquid-immersed specimens suggest that Cr(III) and Mo(II) in the passive film are oxidized to Cr(VI) and Mo(VI) and are dissolved in the high nitrate brines, because the alloy oxide layers were enriched with Ni relative to Cr and Mo in the base metal. An alumino-silicate-chloride precipitate was identified on specimens immersed in solutions with a NO{sub 3}:Cl ratio of 0.5 at 220 C. Further characterization is needed to identify all secondary phases. The inability to extract reliable rates from weight loss measurements suggests that other techniques are needed to evaluate long-term general corrosion of Alloy 22.