Inhibiting effects of nitrates on the passive film breakdown of alloy 22 in chloride environments

The influence of nitrates and chlorides on the passive film of alloy 22 (Ni-Cr22-Mo13-W3-Fe3) in chloride-containing solutions (1 and 4 m NaCl) from pH 5 to -1 at 60 and 90 degrees C is characterized. The combination of nitrates and chlorides in low pH solutions leads to a variety of different corrosion states, including: active, passive, active/passive, and transpassive. For each corrosion state, electrochemical impedance spectroscopy (EIS) is utilized to characterize the passive film properties. Active corrosion is a sensitive function of the nitrate-to-chloride ratio, rather than the total amount of anions in solution. At pH < 0, transpassive dissolution is initialized in all nitrate solutions independent of the tested temperatures and nitrate-to-chloride ratios. Raman spectroscopy analysis shows that Mo is a significant dissolution product during the initial stages of transpassive dissolution. The dissolution is attributed to a transformation from an insoluble Mo oxide to soluble Mo complexes at high open-circuit potentials and low pH. Finally, the results are correlated with critical nitrate/chloride ratios that lead to crevice corrosion in occluded volumes. (c) 2006 The Electrochemical Society.