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 KCI, Ag/AgCl reference electrode) were characterized by Electrochemical Impedance Spectroscopy (EIS) in near neutral pH, 5 M NaCl solutions, at 30 degrees 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 (R(film)) increases with potential accompanied by an increase in Cr(2)O(3) content; in the range -300 mV <= E <= 300 mV, R(film) values and the Cr(2)O(3) content of the oxide film achieve their maximum values; for E > 300 mV, a decrease in both Rfilm and Cr(2)O(3) 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. (C) 2010 Published by Elsevier Ltd.