Corrosion properties of coatings produced from self-fluxing powders by the detonation spraying method

Introduction. This paper presents the results of a comprehensive study of the corrosion properties of innovative coatings based on self-fluxing NiCrBSi alloys (PR-NKh17SR4) modified with 10 wt.% boron carbide (B4C) nanoparticles, produced by detonation spraying. The relevance of the study stems from the critical need to develop new high-performance materials for protecting essential equipment operating under extreme conditions, including marine environments, chemically aggressive solutions, and elevated temperatures. Particular attention is paid to a detailed analysis of the influence of B4C on corrosion mechanisms, the formation of protective passivating layers, and the relationship between microstructure and functional properties of the coatings. Objective. A comprehensive evaluation of the effect of 10 wt.% B4C addition on the corrosion resistance, microstructure, and mechanical properties of coatings in comparison with the base alloy NiCrBSi alloy (PR-NKh17SR4) and the commercially available counterpart NiCr/WC alloy (VSNGN-85), widely used in industry. Methods. The coatings were applied to 0.40% C-Mn steel substrates using a multi-chamber cumulative detonation spraying unit (MKDU). Modern analytical methods were employed for thorough characterization: scanning electron microscopy (SEM, Mira 3) with energy-dispersive spectroscopy, X-ray diffraction (XRD, ARL X'TRA diffractometer) with quantitative phase composition assessment using the Rietveld method. Corrosion tests were conducted in a 3.5% NaCl solution simulating marine environments, using potentiostatic measurements and electrochemical impedance spectroscopy on a SmartStat PS-10-4 potentiostat-galvanostat. The depth of corrosion penetration was evaluated using confocal laser microscopy (Lext OLS5000) with a resolution of 10 nm. Results and discussion. It was established that the addition of 10 wt.% B4C leads to the formation of a unique multilayered coating structure with an amorphous phase content of up to 12.3% and promotes the formation of passivating chromium (Cr₂O₃) and boron (B₂O₃) oxides. Electrochemical measurements revealed an exceptionally low corrosion rate of 0.0014 mm/year, which is an order of magnitude lower than that of the base alloy (0.021 mm/year) and 30 times lower than that of the commercial counterpart NiCr/WC alloy (VSNGN-85) (0.041 mm/year). The modified coating exhibits remarkably high polarization resistance (215±25 kΩ•cm²) and minimal porosity (0.6±0.1%). The microhardness reached 680±40 HV, significantly exceeding that of the base alloy (520±30 HV), which is attributed to the formation of dispersed NiB₂ particles. XRD and EDS analyses confirmed the catalytic effect of B4C, facilitating a more complete transition of silicon into nickel silicide (NiSi). The developed coatings possess a unique combination of high corrosion resistance, wear resistance, and adhesive strength. The obtained results recommend this technology for creating protective coatings for critical components in the oil and gas industry, shipbuilding, and energy sectors. Future research prospects include optimizing powder compositions and spraying parameters for various operational conditions, including elevated temperatures and combined loads.