Effect of Mo Element and Heat Treatment on Corrosion Resistance of Ni2CrFeMox High-Entropy Alloy in NaCl Solution

As a new alloy design concept, the high-entropy alloy (HEA) and the formation of simple solid solution introduce excellent properties such as high hardness, high strength and corrosion resistance. Investigations have shown that the single solid solution CrCoFeNi alloy possesses good corrosion resistance. The addition of Mo is beneficial to the corrosion resistance of the HEAs for potential industrial applications in 3.5%NaCI (mass fraction) simulating seawater type environments. The major effect of Mo is to promote the pitting potential of the alloy and inhibit the dissolution of the passivation film by forming and retaining molybdenum oxyhydroxide or molybdates (MoO42-). Considering that the cost of pure Co is higher, Ni and Co elements have similar atomic size and valence electron concentration, and the corrosion resistance of pure Ni is higher than that of pure Co, Ni2CrFeMox HEA was designed by replacing Co element with Ni element in CoCrFeNiMox HEA. As the Mo content increases in the Ni2CrFeMox HEAs, the interdendrite is a Cr and Mo rich a phase, and the dendrite is a Cr and Mo depleted fcc phase. The potential difference between interdendrites and dendrites leads to galvanic corrosion, which accelerates the localized corrosion of alloys. Here, a solution heat treatment process is selected to reduce the precipitation phase and improve the corrosion resistance of the alloy. The effects of Mo element and heat treatment on the corrosion resistance of Ni2CrFeMox HEA in 3.5%NaCI solution were tested. The results show that the corrosion resistance of as-cast Ni2CrFeMox HEA is obviously higher than that of 316L stainless steel. The Ni2CrFeMo0.2 alloy has the best corrosion resistance because of its minimum dimensional passive current density and corrosion current density. However, the addition of excessive Mo leads to the precipitation of a phase and galvanic corrosion, which reduces the corrosion resistance of the alloy. After solution treatment, the uniformity of alloy structure and element distribution weakens galvanic corrosion, and the corrosion resistance is obviously improved.