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

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 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 in different mixtures of chloride and fluoride at 90°C and pH 6. The range of chloride concentration [Cl–] was 0.001 M ≤ [Cl–] ≤ 10 M and the range of fluoride to chloride molar concentration ratio [F–]/[Cl–] was 0.1 ≤ [F–]/[Cl–] ≤ 10. Results show that Alloy 22 was susceptible to crevice corrosion in all the pure chloride solutions but was 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.