Mechanisms of Inhibition of Crevice Corrosion in Alloy 22
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0985-NN08-04
Mechanisms of Inhibition of Crevice Corrosion in Alloy 22 Raul B. Rebak Chemistry and Materials Science, Lawrence Livermore National Laboratory, 7000 East Ave, L- 631, Livermore, CA, 94550 ABSTRACT Alloy 22 may be susceptible to crevice corrosion in chloride-containing environments, especially at temperatures above ambient. The presence of oxyanions, especially nitrate, minimizes or eliminates the susceptibility of Alloy 22 to crevice corrosion. Other anions such as sulfate, carbonate and fluoride were also reported as inhibitors of crevice corrosion in Alloy 22. It is argued that the occurrence of crevice corrosion is due to the formation of hydrochloric acid solution in the creviced region. Inhibitors act by eliminating the occurrence of hydrochloric acid or by hampering its action.
INTRODUCTION Alloy 22 (N06022) belongs to the corrosion resistant family of Nickel-ChromiumMolybdenum (Ni-Cr-Mo) alloys. The required composition and mechanical properties of this family is described in ASTM B575 [1]. Alloy 22 has all the required alloying elements for protection in a variety of environments. Ni offers protection against hot chloride cracking and against hot caustic conditions, Cr offers protection against oxidizing acids, and Mo offers protection against reducing acids [2]. However, Alloy 22 may be susceptible to crevice corrosion in chloride-containing solutions. The susceptibility to this type of localized attack increases as the chloride concentration, the temperature, and the potential increase [3-10]. The presence of nitrate (NO3-) and other oxyanions in the solution minimizes or eliminates the susceptibility of Alloy 22 to crevice corrosion [8-24]. The value of the ratio R = [NO3-]/[Cl-] has a strong effect on the susceptibility of Alloy 22 to crevice corrosion [16-24]. The higher the nitrate to chloride ratio, the stronger is the inhibition by nitrate. It has also been reported that other ions in solution will inhibit the crevice corrosion by chloride ions. These ions include sulfate (SO42-) [3,20], carbonate (CO32-) [14-15], and fluoride (F-) [21]. The objective of this work is to discuss the viable mechanisms by which the presence of other anions in solution may inhibit the crevice corrosion of Alloy 22 promoted by chloride. CREVICE CORROSION INITIATION The crevice corrosion initiation mechanism for Alloy 22 in pure chloride (Cl-) solutions is the same as for other Cr-containing alloys such as austenitic stainless steels [22-23]. There is an initial separation between the anodic and cathodic areas of corrosion. The cathodic reactions, supporting the anodic dissolution of the metal, occur on the surface exposed to the bold solution. The anodic dissolution of the metal M takes place preferentially under the shielded portion of the material (M M+ + e-). This effect produces an excess of positively charged metal ions in the occluded area, forcing the migration of Cl- from the bulk of the solution to counter balance the electric charge in the occluded area, and forming salts (metal chlorides). These
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