Investigation of the passive behavior of a Ni–Cr binary alloy using successive electrochemical impedance measurements.

The passive behavior of a Ni–20Cr binary alloy was studied using an original methodology, based on successive electrochemical impedance spectroscopy (SEIS), over a wide potential range. The proposed method was first validated by comparing the resulting potentiostatic polarization curves with more conventional potentiodynamic polarization curves and determining the effect of the dwell time (30, 120, and 600 s) and potential step size (25 and 50 mV). Using the optimized parameters from this preliminary work, the passive behavior of the Ni–Cr alloy was then studied in solutions of varying pH: 10 g/L Na 2 SO 4 with H 2 SO 4 (pH = 2), 0.01 M Na 2 SO 4 (pH = 5.8), and 0.05 M H 3 BO 3 + 0.075 M Na 2 B 4 O 7 10H 2 O (pH = 9.2). The obtained impedance diagrams were interpreted using enhanced graphical representation and fitted using the power-law model to determine the resistivity distribution throughout the passive film during polarizations. The results demonstrated that SEIS provides complementary information to conventional potentiodynamic curves. The evolution of the passive film properties was monitored in situ within the potential range corresponding to stable passivity. With the help of this original method, complementary to the evolution of the current density (measured at each potential step), the thickness, chemical composition, and reactivity changes were also highlighted. Image 1 • Successive electrochemical impedance technique was validated to study passivity. • Accuracy of enhanced graphical method and fit using power law model was evidenced. • Evolution of passive film's properties was assessed on a wide potential range. • This original methodology permits the in-situ monitoring of passive behavior. [ABSTRACT FROM AUTHOR]