Estimation of the wear and corrosion synergism of borided Inconel 718 alloy immersed in a neutral aqueous solution

This work investigates the tribocorrosion properties of the nickel boride layer on an Inconel 718 superalloy immersed in a neutral solution of sodium sulfate and sodium chloride. The solution simulates the corrosive environment created by the combustion reaction of sodium, sulfur, and oxygen when Ni-based superalloys are used in turbine blades. A powder-pack boriding process was conducted at 950 °C for 6 h, obtaining a nickel boride layer thickness of around 50 μm on the surface of the superalloy; microstructural analysis revealed that the boride layer was composed of Ni₄B₃–Ni₂B–Ni₃B. Further, depth-sensing Vickers microindentation tests were performed to assess the ratios of H/E and H³/E² on the nickel boride layer. Additionally, tribocorrosion tests were conducted with a reciprocating tribometer and a typical three-electrode electrochemical cell. The counterpart was a 4.8 mm diameter alumina ball that applied 20 N for a sliding distance of 100 m. According to the ASTM G119-09 procedure, the total material loss rate due to tribocorrosion (T), which includes mass loss rate due to wear (W_c) and corrosion (C_w), was estimated. The results revealed a wear-dominant regime for the Reference material (Inconel 718 superalloy), attributed to passive film removal and high contact pressure at the tribopair, which increases the volume loss rate. On the other hand, the nickel boride layer on the Inconel 718 superalloy displayed a wear-corrosion regime due to boride layer debris that increased the corroded area. Under these experimental conditions, the presence of the nickel boride layer on the Inconel 718 superalloy improved tribocorrosion resistance by approximately three times.