Effect of Fluoride Ions on Crevice Corrosion and Passive Behavior of Alloy 22 in Hot Chloride Solutions

Abstract

Alloy 22 (UNS N06022) is a Ni-Cr-Mo-W alloy 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 fl uoride (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 fl uorides and chlorides. Cyclic potentiodynamic polarization (CPP) tests were conducted in deaerated aqueous solutions of pure halide ions and in different mixtures of chloride and fl uoride at 90°C and pH 6. The range of chloride concentration [pH, microbial activity, volume of electrolyte, crevice former geometry, crevicing material, etc. Internal factors include 7 the metallurgical condition of the alloy (microstructure), presence of a weld seam, type of annealing, oxide fi lm formed, surface fi nishing, etc. A more detailed discussion regarding this topic can be found elsewhere. 7 Many of the factors listed above, such as chloride concentration, temperature, and inhibitors (namely nitrate and sulfate), have been studied in some detail. 6,[8][9][10][11] The infl uences of other factors still need to be investigated. 7 In particular, the role of halides other than chloride is still not 7,12-17 and will be discussed briefl y in this paper.Meck, et al., did not fi nd crevice corrosion in prismatic Alloy 22 specimens (a variation of the ASTM G5 specimen, 18 which contained an artifi cial crevice formed by a polytetrafl uoroethylene [PTFE] compression gasket) tested in 1 M sodium chloride (NaCl), pH 6 and in 1 M sodium fl uoride (NaF), pH 9 solutions at

The crevice corrosion of Alloy 22 in the Yucca Mountain nuclear waste repository

“…Chloride concentration has a signifi cant effect on the resistance of Alloy 22 to crevice corrosion: as chloride concentration increases, the susceptibility to crevice corrosion increases. 13,16,17,19,22,[30][31][32][33][34] It has been shown for Alloy 22 that E R,CREV decreases linearly as the chloride concentration increases logarithmically ( Figure 3). 13,22,25,34 That is, the relationship between E R,CREV and chloride concentration has the form of Equation 1…”

“…The value of the ratio R = [ion]/[Cl -] has a strong effect on the susceptibility of Alloy 22 to crevice corrosion. 14,18,20,[23][24][25][26][27][28]30,35 The higher the R value, the stronger the inhibition. R. Rebak 29 considered that there could be four basic mechanisms of chloride-induced crevice corrosion inhibition in Alloy 22 by nitrates.…”

“…29 A signifi cant amount of experimental work shows that ions such as nitrate (NO 3 -), sulfate (SO 4 = ), carbonate (CO 3 = ), and fl uoride (F -) act as inhibitors for the localized corrosion of Alloy 22 in chloride-containing solutions. 14,[16][17][18][19][20][23][24][25][26][27][28]30,31,[35][36][37][38][39] That is, the alloy is less susceptible to crevice corrosion in a chloride solution containing these ions than in pure chloride solutions of the same concentration and pH. The value of the ratio R = [ion]/[Cl -] has a strong effect on the susceptibility of Alloy 22 to crevice corrosion.…”

Crevice corrosion of nickel-based alloys considered as engineering barriers of geological repositories

“…Even apparently detrimental species, such as fluoride and organic acids, behave as crevice corrosion inhibitors though their R CRIT are large. 36,53 R…”

“…E R,CREV was initially obtained by cyclic potentiodynamic polarisation (CPP) tests using artificially creviced specimens. [34][35][36] However, it was pointed out that CPP method put Ni-Cr-Mo alloys in the transpassive range of potentials where chromates and probably molybdates (both localised corrosion inhibitors) are released to the testing solution. 37 Consequently, other testing methods were applied in which transpassivity was avoided by limiting the maximum applied potential or current density.…”