Oxidation Behaviors of Different Grades of Ferritic Heat Resistant Steels in High-Temperature Steam and Flue Gas Environments

For steam tubes used in thermal power plant, the inner and outer walls were operated in high-temperature steam and flue gas environments respectively. In this study, structure, microstructure and chemical composition of oxide films on inner and outer walls of ex-service low Cr ferritic steel G102 tube and ex-service high Cr ferritic steel T91 tube were analyzed. The oxide film was composed of outer oxide layer, inner oxide layer and internal oxidation zone. The outer oxide layer on the original surface of tube had a porous structure containing Fe oxides formed by diffusion and oxidation of Fe. More specially, the outer oxide layer formed in flue gas environment would mix with coal combustion products during the growth process. The inner oxide layer below the original surface of tube was made of Fe–Cr spinel. The internal oxidation zone was believed to be the precursor stage of inner oxide layer. The formation of internal oxidation zone was due to O diffusing along grain boundaries to form oxide. There were Fe–Cr–Si oxides discontinuously distributed along grain boundaries in the internal oxidation zone of G102, while there were Fe–Cr oxides continuously distributed along grain boundaries in that of T91.