Enhanced Molten-Salt Hot-Corrosion Resistance of an SLM Ni–Cr–Co–W Alloy Enabled by Protective Cr2O3 Spallation and a Stable Bilayer Scale

Abstract

This paper employs selective laser melting (SLM) to fabricate a Ni–18Cr–16Co–20W alloy and systematically investigates its hot-corrosion behaviour at 900 °C. Results indicate the alloy’s hot corrosion process undergoes three characteristic stages: rapid oxide scale formation, significant spallation of the Cr₂O₃ layer, and the establishment of a stable double-layer oxide scale. Owing to Cr’s strongest affinity for O, a predominantly Cr₂O₃ oxide scale rapidly forms on the alloy surface during the initial corrosion phase. However, influenced by the corrosive salt medium, this rapidly generated oxide scale exhibits numerous defects and is prone to spallation. The migration of Cr³⁺ ions creates Cr-poor regions within the matrix, thereby promoting extensive oxidation reactions of the relatively less reactive Ni and Co. Benefiting from the protective effect of the outer layer, the resulting NiO and CoO films exhibit a more compact structure. Following the spallation of Cr₂O₃, the concentration of surface corrosion salts decreases, and the hot corrosion process gradually shifts towards a high-temperature oxidation mechanism. This ultimately leads to the formation of a distinctive double-layer oxide structure composed of NiCr₂O₄/CoCr₂O₄ and Cr₂O₃. Based on this, the spallation behaviour of Cr₂O₃ can be considered protective.