Alloy 22 is susceptibility to crevice corrosion in concentrated 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 the crevice 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 are described. A coupled-electrode technique was used to study the stifling of crevice corrosion in various solutions following artificial initiation achieved through galvanostatic polarization. The environments studied included CaCl2-NaNO3 mixtures with varying [NO3-]:[Cl-] ratio and total salinity and a solution of 5 mol·dm-3 NaCl with and without added NO3-. The test temperature in all cases was 120oC. Even with electrochemical polarization, it was found difficult to initiate crevice corrosion in CaCl2 solutions containing nitrate. Some superficial damage was observed in concentrated solutions. In contrast, initiation occurred readily in NaCl solutions, although the coupled current decreased quickly in nitrate-containing solutions, indicating stifling of the crevice.
Crevice corrosion of Alloy 22 is one of a number of potential corrosion processes of interest for the waste packages in the proposed Yucca Mountain repository.1-6 Localized corrosion is only possible during the intermediate stages of the thermal pulse, and then only if the drip shields have failed and seepage water can contact the waste package surface. Under these conditions, concentration of the seepage drips by evaporation on the hot waste package surface could lead to aggressive concentrated brines. The alternative scenario of the initiation of localized corrosion in small volumes of water formed by the deliquescence of salt assemblages in dust deposits on the waste package surface has been shown to be highly unlikely based on multiple lines of argument.1,7 The crevice corrosion of Alloy 22 has been studied in some detail, with particular attention paid to conditions leading to the initiation of localized attack.2,4-6 The proposed approach to predicting the susceptibility of the Alloy 22 outer corrosion barrier to crevice corrosion is to compare the corrosion potential ECORR to a critical potential for localized attack. For this purpose, the selected threshold criterion is the re-passivation potential (ER,CREV) measured on creviced samples, even though this potential corresponds to that at which a propagating pit ceases to grow. Less is known about the factors controlling the propagation of crevice corrosion. Conservatively, the DOE have chosen to assume that, once initiated, crevices will continue to propagate at a constant rate.1 In the absence of measured crevice propagation rates, a distribution of rates was defined based on the rate of general corrosion of Alloy 22 in concentrated acidic chloride solutions containing Fe(III) and other aggressive cations. The DOE have also considered an alternative crevice propagation model, in which the rate of crevice penetration decreases with time in a process referred to as “stifling.”
