Oxidation Behavior of Aluminide Coatings on Cobalt-Based Superalloys by a Vapor Phase Aluminizing Process

In this work, the oxidation behavior of an aluminide coating at 900, 1000, and 1100 °C was investigated. The aluminide coating was prepared on a cobalt-based superalloy using a vapor phase aluminizing process, which is composed of a β-(Co,Ni)Al phase outer layer and a Cr-rich phase diffusion layer. The experimental results showed that the oxidation of the coating at 900-1100 °C all obey the parabolic law. The oxidation rate constants of the coating were between 2.19 × 10^-7 and 47.56 × 10^-7 mg²·cm^-4·s^-1. The coating produced metastable θ-Al₂O₃ at 900 °C and stable α-Al₂O₃ at 1000 and 1100 °C. As the oxidation temperature increases, the formation of Al₂O₃ is promoted, consuming large amount of Al in the coating, resulting in the transformation from β-(Co,Ni)Al phase to α-(Co,Ni,Cr) phase. And the decrease in the β phase in the coating led to the dissolution of the diffusion layer.