Effects of Si, Al and Ti on corrosion of Ni-20Cr and Ni-30Cr alloys in Ar-20CO2 at 700 °C

Highlights: • Binary Ni-20Cr and Ni-30Cr alloys had insufficient Cr content to form a uniform single Cr₂O₃ scale. • Silicon and aluminium significantly improved oxidation resistance of both Ni-20Cr and Ni-30Cr by forming additional layers of SiO₂ or Al₂O₃. • The addition of titanium resulted in titanium oxides, promoting Cr diffusion in Cr₂O₃ and growth of Cr₂O₃ scales. • All alloys underwent internal carburisation, forming intergranular carbide precipitates beneath their scales. - Abstract: Model alloys Ni-(20, 30)Cr (wt.%) with (0.1, 1)% Si, 1.5% Al or (1, 2)% Ti were exposed to Ar-20CO₂ at 700 °C. Samples were electropolished before reaction to avoid cold working effects on alloy diffusion. The Ni-20Cr alloy formed a thick NiO scale and an internal oxidation zone, whereas Ni-30Cr formed a duplex scale of NiO and Cr₂O₃ layers. The Si and Al additions increased the oxidation resistance by forming additional protective layers of SiO₂ or Al₂O₃. Titanium accelerated chromia growth, promoting alloy surface coverage with protective oxide. All alloys formed intergranular Cr-rich carbide precipitates beneath their scales.