Exploring protective properties of passive films formed upon CoCrFeNiMo0.4 high-entropy alloy in H2O2-containing electrolytes via graphical processing of electrochemical impedance data

The electrochemical behavior of CoCrFeNiMo_0.4 high-entropy alloy both in the passive state and at depassivation in 0.6 M NaCl + HCl (pH 1) and in the presence of H₂O₂ (10, 50, or 100 mM) was studied. The surface film properties were investigated by means of polarization measurements, Mott-Schottky analysis, and electrochemical impedance spectroscopy utilizing graphical methods for enhanced processing of the results. The quantitative parameters of the passive layer, namely its resistivity at the oxide/electrolyte interface and the charge carrier concentrations, were determined. Also, the film thickness was calculated using the complex capacitance representation. The alloy underwent spontaneous passivation regardless of the H₂O₂ concentration, but the anodic dissolution in the depassivated state intensified as the oxidizer content increased. The passive layer became thicker and less conductive with rising formation potential in the background solution, but became thinner and more defective as the H₂O₂ concentration increased. Nevertheless, the interfacial resistivity remained at the level of 10⁶ Ω∙cm, irrespective of the H₂O₂ content. Thus, the detrimental effect of the oxidizer on the protective ability of the passive layer was weak, indicating the high stability and effectiveness of the passive state. Deterioration in the protective ability occurred at the depassivation potential, as evidenced by the decrease in interfacial resistivity and thinning of the film. The decline was more pronounced in the presence of H₂O₂. The variations of the film properties were consistent with the polarization measurements. Additionally, the alloy exhibited better passive layer properties compared to the 316L steel, indicating its higher corrosion resistance.