<p>The effect of a NiCr₂O₄ spinel-containing oxide layer on the corrosion resistance of Fe–20Ni–25Cr–1Si (wt%) alloy was investigated in a synthetic CaO-based ash containing alkali metal chlorides at 520&#xa0;°C in air. To promote spinel formation, the alloy surface was first coated with Ni by electrodeposition and subsequently pre-oxidized in air at 900&#xa0;°C for 144&#xa0;h. This treatment produced a duplex oxide structure consisting of an outer Ni-rich oxide layer and an inner Cr₂O₃ scale with an intermediate NiCr₂O₄ spinel layer. Corrosion tests revealed that the Ni-coated alloy exhibited significantly lower mass gain and improved oxide-scale stability compared with the bare alloy, which formed a single Cr₂O₃ scale. Microstructural and phase analyses showed that the bare alloy underwent progressive chromia degradation through chromate formation, leading to scale fragmentation and accelerated corrosion. In contrast, the NiCr₂O₄-containing oxide scale suppressed chromia degradation and limited the penetration of ash constituents into the oxide scale. The improved corrosion resistance was attributed to the ability of the NiCr₂O₄ spinel layer to reduce direct interaction between Ca-containing species and Cr₂O₃ while restricting the transport of reactive chromium species toward the ash/oxide interface. These findings demonstrate that the controlled formation of a NiCr₂O₄ spinel layer is an effective strategy for enhancing the durability of chromia-forming alloys in Ca-rich ash environments relevant to waste-to-energy systems. However, the long-term durability of this protective effect remains to be established, as progressive accumulation of ash deposits and corrosive salts may eventually compromise the integrity of the spinel-containing oxide layer.</p>