INHIBITION EFFECT OF PHOSPHATE IONS ON CREVICE CORROSION OF ALLOY 22

摘要

Alloy 22, a nickel based alloy that belongs to the Ni-Cr-Mo family, shows an outstanding corrosion resistance in most environments. Alloy 22 was extensively studied regarding its crevice corrosion behavior both in pure chloride solutions and in solutions containing other ions such as nitrate, sulfate, carbonate, bicarbonate and phosphate. Electrochemical tests were perfomed to study the localized corrosion of Alloy 22 in deaerated aqueous solution of 1M NaCI and 1M NaCI with different phosphate additions at 90°C and pH near neutral. Results of the tests indicated that the length of the passive potential range and the repassivation corrosion potential value, depend on the presence of phosphate ions. Electrochemical tests with PCA specimens showed a strong inhibitory effect of phosphate ions on chloride induced crevice corrosion of Alloy 22. A critical molar concentratinn ratio, Rcrit , of 0.3 was determined based on Potentiodynamic-Potentiostatic-Potentiodynamic (PD-PS-PD) and Potentiodynamic-Galvanostatic-Potentiodynamic (PD-GS-PD) tests.