No abstract

Alloy 22 (UNS N60622) is a nickel-based alloy, which is extensively used in aggressive industrial applications, especially due to its resistance to localized corrosion and stress corrosion cracking in high chloride environments. The purpose of this work was to characterize the anodic behavior of Alloy 22 in concentrated calcium chloride (CaCl2) brines and to evaluate the inhibitive effect of nitrate, especially to localized corrosion. Standard electrochemical tests such as polarization resistance and cyclic polarization were used. Results show that the corrosion potential of Alloy 22 was approximately −360 mV in the silver-silver chloride (SSC) scale and independent of the tested temperature. Cyclic polarization tests showed that Alloy 22 was mainly susceptible to localized attack in 5 M CaCl2 at 75°C and higher temperatures. The addition of nitrate in a molar ratio of chloride to nitrate equal to 10 increased the onset of localized corrosion to approximately 105°C. The addition of nitrate to the solution also decreased the uniform corrosion rate and the passive current of the alloy.

Boron containing stainless steels are used in the nuclear industry for applications such as spent fuel storage, control rods and shielding. It was of interest to compare the corrosion resistance of three borated stainless steels with standard austenitic alloy materials such as type 304 and 316 stainless steels. Tests were conducted in three simulated concentrated ground waters at 90°C. Results show that the borated stainless were less resistant to corrosion than the witness austenitic materials. An acidic concentrated ground water was more aggressive than an alkaline concentrated ground water.

The approach of isolating high-level nuclear waste in the designated site of Yucca Mountain (Nevada) is to separate it from the environment using a series of engineering and natural barriers. The container for the waste will consist of two concentric metal cylinders. The outer cylinder is going to be fabricated of Alloy 22 (N06022). If water is present at the site, several corrosion processes may occur. These include passive or general corrosion, localized corrosion and environmentally assisted cracking. The occurrence of one (or more) mode of corrosion over another will be determined by the redox potential of the aqueous electrolyte that may enter in contact with the container. This redox potential will also control the corrosion potential (E,,,) of the container. This paper summarizes the findings of an extensive laboratory testing aimed at measuring E,,, of Alloy 22 in presence of a variety of electrolyte solutions. Some of these solutions are multi-ionic electrolytes that may simulate concentrated ground waters. Other environments are chemical solutions of pure salts, which are highly unlikely for an underground repository but that may establish an extreme bounding condition. Current results show that the highest measured potential for Alloy 22 was approximately +0.3 to 0.4 V in the saturated silver chloride [SSC] scale. Most of the E, , values are in the order of 0 V [SSC] or below.

Nickel based Alloy 22 (N06022) is extensively used in aggressive industrial applications, especially due to its resistance to localized corrosion and stress corrosion cracking in high chloride environments. The purpose of this work was to characterize the anodic behavior of Alloy 22 in oxalic acid solution and to compare its behavior to sodium chloride (NaC1) solutions. Standard electrochemical tests such as polarization resistance and cyclic polarization were used. Results show that the corrosion rate of Alloy 22 in oxalic acid solutions increased rapidly as the temperature and the acid concentration increased. Extrapolation studies show that even at a concentration of M oxalic acid, the corrosion rate of Alloy 22 would be higher in oxalic acid than in 1 M NaCl solution. Alloy 22 was not susceptible to localized corrosion in oxalic acid solutions. Cyclic polarization tests in 1 M NaCl showed that Alloy 22 was susceptible to crevice corrosion at 90°C but was not susceptible at 60°C.