High Temperature Oxidation of Ni–20Cr–5Al Alloys with Small Additions of Cerium

Abstract

The oxidation behavior of Ni–20Cr–5Al alloys containing 0.01, 0.04 and 0.40 mass% cerium was studied in air at 1473 K for periods up to 720 ks by mass-change measurements, X-ray diffraction, scanning electron microscopy and electron probe microanalysis. The mass gain slightly decreased in a 0.01% Ce alloy, and increased in 0.04%Ce and 0.40%Ce alloys. The surface oxides on a Ce-free alloy and 0.01%Ce and 0.04%Ce alloys were predominantly α-Al₂O₃. The major surface oxides on a 0.40%Ce alloy were α-Al₂O₃ and NiAl₂O₄ with small quantities of rod-shaped CeO₂. The surface oxides on a Ce-free alloy and 0.01%Ce and 0.04%Ce alloys spalled during cooling. No spalling of the surface oxides occurred in the 0.40%Ce alloy in any of the oxidation conditions studied. The oxide/alloy interfaces of the Ce-free alloy and the 0.01%Ce and 0.04%Ce alloys were fairly smooth. On the other hand, α-Al₂O₃ and rod-shaped CeO₂ in the 0.40%Ce alloy protruded from the surface. The improvement of the oxide adherence of the alloy was attributed to the protrusion of the surface oxide into the alloy substrate.