ABSTRACT

The resistance of Alloy 22 (N06022) to localized corrosion, mainly crevice corrosion, has been extensively investigated in the last few years. However, the behavior of Alloy 22 in concentrated aqueous solutions that may simulate concentrated ground waters was not fully understood. Systematic electrochemical tests using cyclic potentiodynamic polarization as well as the Tsujikawa-Hisamatsu electrochemical method were performed to determine the crevice corrosion susceptibility of Alloy 22 in simulated concentrated water (SCW), simulated acidified water (SAW) and basic saturated water (BSW). Results show that Alloy 22 is immune to crevice corrosion in SCW and SAW but may suffer crevice corrosion initiation in BSW. Results also show that in a naturally aerated environment, the corrosion potential would never reach the critical potential for crevice corrosion initiation.

INTRODUCTION

Alloy 22 (N06022) is nickel (Ni) based and contains nominally 22% Chromium (Cr), 13% Molybdenum (Mo) and 3% tungsten (W). ¹ Alloy 22 belongs to the Ni-Cr-Mo family of nickel based alloys, which also include alloys such as C-4 (N06455), C-276 (N10276), C-2000 (N06200), 59 (N06059) and 686 (N06686). ¹ The Ni-Cr-Mo alloys were designed to withstand the most aggressive industrial applications, including reducing acids such as hydrochloric and oxidizing acids such as nitric. Chromium is the beneficial alloying element added for protection against oxidizing conditions and molybdenum is the beneficial alloying element to protect against reducing conditions. ^2-4 The base element (nickel) protects the alloy against caustic conditions. ^2-4 All three elements, Ni, Cr and Mo act synergistically to provide resistance to environmentally assisted cracking in hot concentrated chloride solutions. ^2-4 The alloying elements Cr and Mo also provide resistance to localized corrosion such as pitting and crevice corrosion in chloride containing solutions. Some of the Ni-Cr-Mo alloys also contain a small amount of tungsten (W), which may act in a similar way as Mo regarding protection against localized corrosion. ⁵ Ni-Cr-Mo alloys are practically immune to pitting corrosion but they may suffer crevice corrosion under aggressive environmental conditions under the effect of chloride. The presence of other anions in the electrolyte inhibits crevice corrosion. 6-16 These anions include mainly nitrate, sulfate and carbonate. ^6-16 A minimum ratio of inhibitor to chloride is needed for the inhibition to occur. ^6-16 For example, for nitrate, it is generally accepted that a ratio (R) of concentration nitrate over concentration of chloride of 0.5 may be sufficient to inhibit crevice corrosion initiation and propagation in Alloy 22. ^7,9,10,17

Due to its excellent resistance to all forms of corrosion, Alloy 22 (N06022) has been selected to fabricate the external shell of the Yucca Mountain high-level nuclear waste containers. ¹⁸ The environment at the repository site is mostly dry or unsaturated. If water enters in contact with the containers it would be in the form of two main modes: (1) Dripping from the drift crown and walls and (2) Deliquescence of salts or dust collected during the early dry period.