The influence of alternating current (AC) field on the pack boriding process for medium carbon steel was investigated through characterization of microstructure, phase composition, microhardness, and corrosion resistance of the boride layer and its mechanism was revealed. Results showed that the boride layer obtained by AC field boriding was composed of the outer FeB and the inner Fe₂B phase, which was similar to that of conventional boriding. Meanwhile, the effective thickness of the boride layer and proportion of Fe₂B increased gradually with increasing current during AC field boriding. The introduction of an AC field during the boriding process served dual purposes. First, it facilitated the decomposition of the boriding medium, leading to an elevation in the concentration of active boron atoms. Second, it reduced the activation energy required for atomic diffusion, thereby accelerating the diffusion of both boron and iron atoms. These combined effects significantly enhanced the hardness distribution and corrosion resistance of the steel. Further insights into the process were gained by fitting the parabolic kinetics curves, which confirmed that the boriding process in an AC field was exclusively controlled by diffusion. This study also clarified the growth mechanism of the boride layer within an AC field.

High-entropy alloy (HEA) coatings have attracted wide scientific attention in academic research and industrial innovation. In the present paper, the NiCoCrAlY HEA coatings are successfully synthesized on the surface of M50 steel to improve the corrosion resistance and tribocorrosion resistance of M50 steel in salt-contaminated lubricating oil. The corrosion and tribocorrosion behaviors of M50 steel and NiCoCrAlY coatings are studied systemically under the same conditions. The experimental results show that NiCoCrAlY coatings have good, densified microstructures and improve effectively the corrosion resistance and tribocorrosion resistance of M50 steel because the protective passivation films and oxide films are formed on the surface of NiCoCrAlY coatings. NiCoCrAlY coatings have high corrosion potential, a low corrosion current density, and a corrosion rate that is comparable with M50 steel. The corrosion potential of M50 steel decreases and the corrosion current density increases with the increase in load due to wear-induced corrosion. The corrosion and tribocorrosion mechanisms of M50 steel and coatings are discussed in light of the experimental results. The wear mechanism of M50 steel is abrasive wear. It is accompanied by corrosion wear for M50 steel and oxidative wear for NiCoCrAlY coatings.

Flow-passing components such as volutes and fixed guide vanes operating in high-velocity liquid flows with high sediment concentrations are subjected to long-term cavitation erosion and sand-particle abrasion, which cause substantial economic losses. To reduce the manufacturing and maintenance costs of mechanical equipment serving such environments, laser cladding was employed to fabricate Fe₄₅Cr₂₅Ni₂₀Ti₅Mo₅ multi-principal-element alloy coatings on Q235 steel substrate, and the effect of laser power on the microstructure and properties of the coatings was systematically investigated. On the basis of preliminary investigations, the present work elaborately analyzes the wear resistance and cavitation-erosion resistance of coatings fabricated under laser powers of 1000 W, 1200 W and 1400 W. The results reveal that the coating hardness gradually decreases from a maximum value of 485.67 HV_0.2 to 363.15 HV_0.2 with increasing laser power. When the laser power is 1200 W, the coating prepared under this laser power maintains a good balance between hardness and microstructural integrity. Under identical friction-and-wear test conditions, its wear rate reaches only 3.15 × 10⁻⁵ mm³/(N·m), which is reduced by 19.64%, 27.92% and 39.19% compared with the coatings produced at 1000 W, 1400 W and bare Q235 steel, respectively. After a 20h cavitation-erosion test, the mass loss of this coating is merely 2.56 mg, representing reductions of 72.88%, 81.5% and 96.91% relative to the 1000 W coating, 1400 W coating and Q235 steel substrate. The coating fabricated at 1200 W exhibits outstanding wear resistance and cavitation-erosion resistance. The results indicate that, under the experimental conditions of this study, the wear resistance and cavitation-erosion resistance of the coating can be effectively optimized by adjusting the laser power.