Protection mechanism of enamel-alumina composite coatings on a Cr-rich nickel-based superalloy against high-temperature oxidation

As the service environment has become increasingly harsh, high-temperature protective coatings were widely researched and applied in industrial fields. K444 is a typical Cr-rich Ni-based superalloy used in power system. Its protection at high temperatures relies on the formation of chromia which is not alone but accompanied by the growth of other oxides or compounds thereby decreasing the oxidation resistance of chromia. In this paper, an enamel-alumina composite coating was prepared on the K444 alloy. Its oxidation behavior at 900 °C was investigated and the oxidation mechanisms were carefully elucidated. Results indicate that oxidation resistance of the K444 superalloy has been increased for more than six times by the enamel-alumina composite coatings. As the traditional theory suggested, the enamel composite coatings retard oxygen diffusion to the superalloy surface, leading to the decrease of oxidation rate. However, it is found that the protection mechanism is more associated with the change of oxidation mode of the alloy substrate, i.e. preventing Cr oxidation while promoting Al and Ti oxidation by the enamel coatings. Furthermore, this protection depends strongly on the alumina content of the enamel composite coating. With higher alumina content, the enamel composite coating retards oxygen diffusion more effectively, and the oxidation mode with respect to the selective oxidation of Al keeps unchanged for longer times.