The kinetics of oxidation of diffusion coatings in superheated steam

Abstract

The kinetics of oxidation of Cr-rich, Si–Cr-rich and Al-Cr-rich diffusion layers have been studied in superheated steam within the temperature range 650 and 750°C. The diffusion layers were generated in test-pieces of alloy type P-92 using the pack cementation approach. The thermogravimetric analytical technique was used to determine the kinetic in superheated steam over a period of 100 h. The results were benchmarked against the oxidation kinetic of the bare P92 alloy at 650°C. The results of these short-term experiments indicated a significant improvement in oxidation resistance where the diffusion layers did not experience any apparent attack. The oxidation kinetics of a test-piece of alloy type S304H were also determined at 750°C. The latter type of samples experienced oxidation with the formation of a 1 μm thick Fe–Cr spinel type oxide with discrete patches of break-away oxidation. By comparison the diffusion layers showed no apparent attack. The diffusion layer chemistry remained stable even at 750°C for all coated composition. The characteristics of the diffusion layers will be presented.

Keywords:

• kinetics of oxidation
• diffusion coatings
• superheated steam