The Electrochemical Behavior of Alloy in Extreme Chloride and Bitrate Environments

Abstract

Alloy 22 specimens were tested in high temperature (100 to 160°C), high concentration chloride and nitrate environments. Results of this study indicate that increasing nitrate to chloride ratio to 0.5 in these electrolytes increases resistance to localized breakdown and enhances repassivation. In these extreme environments, localized corrosion occurred by pitting even though specimens were tested using artificial crevice formers. Open circuit (E,,,), breakdown and repassivation potentials all increase, and pitting morphology changes as nitrate to chloride ratio increases from 0.05 and 0.15 to 0.5. Results also indicate that increasing the temperature from 100 to 160°C increases E,,, values, while breakdown potentials and repassivation potentials peak at 130°C for the 0.5 nitrate to chloride ratio electrolytes. Keywords:Alloy 22, localized corrosion, nitrate, chloride, welds, deliquescence INTRODUCTION ' Alloy 22 is a nickel-based alloy that is highly corrosion resistant. It falls into the general category of nickel-chromium-molybdenum alloys known for their high corrosion resistance. Like other alloys in this family, Alloy 22 exhibits both low general corrosion rates and high resistance to localized corrosion in a wide range of environment^.'-^ Alloy 22 is the corrosion barrier material for the waste packages intended for the underground disposal of radioactive waste at Yucca Mountain. One of the most probable exposure mechanisms of the waste packages to aqueous solutions will be through the process of deliquescence. Deliquescence is the process by which a crystalline solid absorbs moisture and subsequently dissolves. As the waste packages cool and the relative humidity rises, the likelihood of deliquescence occurring on the package surface increases. The initial solutions that form through this process are predicted to exist at high temperatures (probably above 100°C) and contain high salt concentration^.^' lo Solutions containing high amounts of chloride and nitrate have been identified as one type of deliquescence solution that can forrn,in the repository.10 Nitrate has been shown to have an inhibitive Knolls Atomic Power Laboratory, Schenectady, NY + Integrated Science Solutions Inc., Walnut Creek, CA effect on the localized corrosion behavior of Alloy 22 in chloride solutions at lower concentrations and temperatures than those used in this study.2$ 3' 79 I ' Dunn and Brossia found that for welded Alloy 22 specimens tested in 0.5 M NaCl solutions at 95OC, there was no crevice corrosion initiated in potentiodynamic tests for nitrate to chloride ratios of 0.2 or greater but crevice corrosion did occur at ratios of 0.1 and below.3 Evans et. al. found that in potentiodynamic testing of Alloy 22 in 5 M calcium chloride (CaC12) solutions, the onset of localized corrosion could be shifted from 7S°C to 105OC by adding 0.5 M calcium nitrate (~a(N03)2).~ AS the ratio of nitrate to chloride is increased, an increase in both breakdown potentials and repassivation potentials occurs, and the difference between the...