The impedance properties of the oxide film on the Ni-Cr-Mo Alloy-22 in neutral concentrated sodium chloride solution

The oxide film properties on Alloy-22 in the applied potential (E) range -600 mV to 600 mV (vs. saturated KCl, Ag/AgCl reference electrode) were characterized by Electrochemical Impedance Spectroscopy (EIS) in near neutral pH, 5 M NaCl solutions, at 30 deg. C. The impedance properties of the film were compared to the chromium content of the film determined by X-ray photoelectron spectroscopy (XPS). The oxide film properties on Alloy-22 may be divided into three applied potential (E) ranges: -600 mV ≤ E < -300 mV, -300 mV ≤ E ≤ 300 mV, and E > 300 mV. For the range -600 mV ≤ E < -300 mV the film resistance (Rfilm) increases with potential accompanied by an increase in Cr2O3 content; in the range -300 mV ≤ E ≤ 300 mV, Rfilm values and the Cr2O3 content of the oxide film achieve their maximum values; for E > 300 mV, a decrease in both Rfilm and Cr2O3 is observed accompanied by a significant increase in Cr(OH)3. Comparison of the impedance properties for Alloy-22 to those of Ni-Cr alloys indicate that the barrier layer oxide on Alloy-22 contains a lower number of less mobile defects, most likely Cr interstitials. Destruction of the barrier layer for E > 300 mV leads to the formation of a thicker, less protective bilayer, which is high in Mo content.