ABSTRACT

Alloy 22 (N06022) is highly resistant to localized corrosion. Alloy 22 may be susceptible to crevice corrosion in pure chloride (Cl-) solutions under aggressive environmental conditions. The effect of the fluoride (F-) over the crevice corrosion induced by chloride ions is still not well established. The objective of the present work was to explore the crevice corrosion resistance of this alloy to different mixtures of fluorides and chlorides.

Cyclic potentiodynamic polarization (CPP) tests were conducted in deaerated aqueous solutions of pure halide ions and also in different mixtures of chloride and fluoride at 90°C and pH 6. The range of chloride concentration [Cl-] was 0.001 M = [Cl-] = 1 M and the range of molar fluoride to chloride ratio [F-]/[Cl-] was 0.1 = [F-]/[Cl-] = 10.

Results showed that Alloy 22 was susceptible to crevice corrosion in all the pure chloride solutions but not in the pure fluoride solutions. Fluoride ions showed an inhibitor behavior only in mixtures with a molar ratio [F-]/[Cl-] > 2. For mixtures with a molar ratio [F-]/[Cl-] of 7 and 10 the inhibition of crevice corrosion was complete.

INTRODUCTION

Alloy 22 (N06022) is nickel (Ni) based and contains nominally 22% Chromium (Cr), 13% Molybdenum (Mo) and 3% tungsten (W). Alloy 22 is one of the most versatile alloys of the Ni-Cr-Mo family and was designed to withstand the most aggressive industrial applications, including reducing acids such as hydrochloric and oxidizing acids such as nitric. The base element (nickel) is very resistant to hot alkalies, and the alloying elements chromium and molybdenum enhance its protection against oxidizing and reducing conditions respectively. Alloy 22 has showed excellent resistance to pitting, crevice corrosion and environmentally assisted cracking in hot concentrated chloride solutions. Applications of Alloy 22 include a variety of chemical processing, pickling and metal finishing, pollution control, nuclear waste treatment and pulp and paper industry, but the widest applications is in flue-gas desulfurization (FGD) plants. Due to its excellent performance in a wide variety of environments Alloy 22 has been selected for the fabrication of the corrosion-resistant outer shell of the high-level nuclear waste container for the proposed Yucca Mountain repository.

Alloy 22 can be considered not susceptible to pitting corrosion in practical applications in chloride containing environments. However, Alloy 22 might suffer crevice corrosion under certain aggressive conditions. The factors influencing crevice corrosion susceptibility of Alloy 22 can be classified into environmental (external) and metallurgical (internal). External factors include chloride concentration, temperature, applied potential, presence of inhibitors or deleterious species, pH, microbial activity, volume of electrolyte, crevice former geometry, crevicing material, etc. Internal factors include the metallurgical condition of the alloy (microstructure), presence of a weld seam, type of annealing, oxide film formed, surface finishing, etc. A more detailed discussion regarding this topic can be found elsewhere. Many of the factors listed above, such as chloride concentration, temperature and inhibitors (namely nitrate and sulfate) have been studied in some detail. The influences of other factors still need to be investigated. In particular the role of halides other than chloride is still not well established and will be briefly discussed in this paper.

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