Effect of Pre-Oxidation on High Temperature Oxidation and Corrosion Behavior of Co-Al-W-Based Superalloy

In 2006, stable gamma' phase was found in Co-Al-W alloy, which provides a new way for developing Co-based superalloys. In order to meet the requirements for applications, the Co-Al-W-based superalloys need to have good oxidation and corrosion resistance. But the oxidation and corrosion resistance of the Co-Al-W-based superalloys is relatively low. In this work, pre-oxidation treatments were to improve the oxidation and corrosion resistance. Three types of pre-oxidation treatments were carried out at 900 degrees C (in air), 950 degrees C (in air) and 1000 degrees C (in 1%O-2 +99%Ar), which were marked as 900-PreO, 950-PreO and 1000-LPreO, respectively. High temperature oxidation tests of both the pre-oxidation treated and untreated superalloys were carried out at 1000 degrees C. And hot corrosion behaviors were also investigated at both 800 and 850 degrees C. XRD, SEM and EDS were used to examine the characteristics of the oxidation and corrosion behaviors. The results show that the pre-oxide layers of the superalloys after different pre-oxidation treatments are compact. During oxidation at 1000 degrees C, the Cr2O3 scale in 900-PreO treated superalloy is oxidized to form volatile products, and the diffusion resistance of both oxygen and the metal elements through the oxide layer is poor. In 1000-LPreO treated superalloy, serious spallation of the oxides occurs and leads to poor oxidation resistance. In 950-PreO treated superalloy, since the continuous CoCr2O4 and Al2O3 scales help to reduce the diffusion of oxygen and the metal elements, the mass gain of 950-PreO treated superalloy is consequently decreased. During the hot corrosion process, the preoxide layer of 950-PreO treated superalloy can hinder the diffusion of corrosive medium. Therefore, the corrosion resistance can be significantly improved by 950-PreO treatment, and the mass gain is decreased by over 80%.