ИСТИНА

The design of spontaneously self-passivating alloys is essential for safe operation of metal parts and structures. A lot of high-entropy alloys (HEAs) of 3d transition metals have demonstrated high corrosion resistance [1]. In this regard, the Co-Cr-Fe-Ni-Mo HEAs are of great interest. Still, the effectiveness of passivation can vary considerably with an increase in the oxidizing ability of a solution. The objective of this work was to determine the quantitative parameters of the surface film on the CoCrFeNiMo0.4 HEA in an acidic solution containing chloride ions and an oxidizer. The protective properties of the oxide layer formed upon the HEA 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) were investigated by means of polarization measurements, Mott-Schottky analysis, and electrochemical impedance spectroscopy utilizing graphical methods, including complex capacitance representation [2]. 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 resistivity of the surface film at the oxide/electrolyte interface remained at the level of 106 Ω∙cm, irrespective of the H2O2 content. Thus, the adverse effect of the oxidizer was weak. 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.