Mechanistic Studies of the Localized Corrosion of Alloy 22 in Chloride-Nitrate Solutions

Alloy 22 is susceptibility to localized corrosion in concentrated divalent-cation chloride solutions at elevated temperature.  A number of oxyanions, most notably nitrate, sulfate, and carbonate, inhibit the aggressiveness of the chloride ion.  The ratio of aggressive to inhibitive anions is a key parameter in predicting the possibility of localized corrosion of waste packages in the Yucca Mountain repository.
The results of a preliminary experimental program to study the effects of chloride and nitrate ions on the propagation and stifling of the crevice corrosion of Alloy 22 will be discussed.  A coupled-electrode technique is being used, in which a large planar cathode is electrochemically coupled to a smaller creviced anode electrode.  Among the factors being investigated is whether nitrate ions preferentially migrate into the crevice and consequently stifle the propagation.  Analysis of the fluctuations in the coupled current and of the potential of the creviced electrode, in combination with examination of the creviced region following the test, are used to yield information on the crevice corrosion mechanism.  Simple solution analyses and analysis of the N:C ratio on the creviced surface are being used to determine whether the nitrate:chloride ratio in the crevice is different from that in the bulk solution.