Inhibition Effect of Phosphate Ions on Crevice Corrosion of Alloy 22

Alloy 22, a nickel based alloy that belongs to the Ni-Cr-Mo-W family of alloys, shows an outstanding corrosion resistance in most environments. Studies carried out in chloride solutions under aggressive environmental conditions, reported crevice corrosion behavior of this alloy and also inhibition effect of several oxyanions, such as phosphate, nitrate, sulfate, carbonate and bicarbonate. Alloy 22 was a candidate for the nuclear waste container in the Yucca Mountain Project, and it might be considered as a candidate regarding the Argentine Repository System.

 Electrochemical tests were perfomed to study localized corrosion of Alloy 22 in deaerated aqueous solution of 1M NaCl and 1M NaCl with different phosphate additions at 90^oC and pH near neutral. Results of the tests indicated that the passivity range and the repassivation corrosion potential value, when localized corrosion was developed, depend on phosphate presence. Electrochemical tests with PCA specimens showed a strong  inhibitory effect of phosphate ions on chloride induced crevice corrosion of Alloy C22. R crit (molar ratio of phosphate/chloride) of 0.3 was determined based on Potentiodynamic-Potentiostatic-Potentiodynamic (PD-PS-PD) and Potentiodynamic-Galvanostatic-Potentiodynamic (PD-GS-PD) tests.

Keywords: Localized corrosion, N06022, repassivation potential, inhibition, phosphates.