OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 3 2025 Ta b l e 3 Corrosion behavior parameters in 3.5 % NaCl Parameter Steel 0.4 C-Mn NiCrBSi (PR-NKh17SR4) NiCrBSi (PR-NKh17SR4)+10 % B4C NiCr/WC (VSNGN-85) Ecorr (vs. SCE) (mV) −690±15 -470±10 −260±10 −480±12 icorr (μA/cm²) 40.2±1.5 6.5±0.2 0.8±0.02 13.9±0.4 Rct (kΩ·cm²) 0.4±0.1 6.6±0.8 66.4±5.2 3.1±1.0 QCPE (µF·cm−2·s−1) 0.3 600 240 1100 n 1 0.63 0.66 0.6 The Tafel slopes for both anodic and cathodic reactions were determined from polarization curves plotted in semi-logarithmic coordinates (see Fig. 6), enabling calculation of corrosion current densities and potentials (Table 3). The results demonstrated that the boron carbide-modified coating exhibited the lowest corrosion current density (0.8 μA/cm2), which was one order of magnitude lower than both the base NiCrBSi (PR-NKh17SR4) composition (6.5 μA/cm2) and the NiCr/WC (VSNGN-85) coating (13.9 μA/cm2). Fig. 7. Randles-Ershler equivalent circuit Electrochemical impedance spectroscopy studies in 3.5 % NaCl aqueous solution revealed that the impedance data for all investigated coatings fitted well with the Randles-Ershler equivalent circuit model (Fig. 7). In this circuit, the constant phase element (CPE) describes the double layer capacitance, Rct represents the charge transfer resistance, and ZW corresponds to Warburg impedance associated with diffusion processes. The typical Nyquist plot featured a semicircular segment that rapidly transitioned into a sloping straight line (diffusion impedance) in the high-frequency region. However, the NiCr/WC (VSNGN-85) coating exhibited several inflection points, attributed to the presence of large heterogeneous phases (tungsten carbide and binder). The electrochemical impedance spectroscopy data of the coatings and the substrate with superimposed approximation lines using the Randles-Erschler equivalent circuit are presented in Fig. 8 as Nyquist plots. The approximation parameters (see Table 3) confirmed the formation of a dense protective film on the surface of the NiCrBSi (PR-NKh17SR4)+10 wt.% B4C coating, as evidenced by high values of polarization resistance and low values of the phase element constant. The low degree index of the studied coatings (0.6–0.7) indicates the heterogeneity and porosity of the coatings, which is consistent with the microstructure (see Fig. 3). At the same time, for the NiCrBSi (PR-NKh17SR4)+10 wt.% B4C coating, this parameter is higher for the others, that is, it indicates higher homogeneity and lower porosity. After corrosion tests, the surface of the coatings was examined using electron microscopy methods (see Fig. 9). On the NiCrBSi (PR-NKh17SR4)+10 wt.% B4C coating, shallow, predominantly surface corrosion was observed, while on the reference NiCr/WC (VSNGN-85) coating, numerous deep corrosion lesions of the metal bond were noted along the boundaries of tungsten carbide particles. Conclusion 1. The conducted study allowed us to comprehensively evaluate the corrosion properties of self-fluxing powder coatings obtained by detonation spraying. The results showed that the introduction of 10 % boron
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