The features of microstructure and micro-elements in oxide film appeared on inner surface of T91 steel superheater tubes after operation of 85 000 h have been analysed

and the formation mechanism of oxide film being discussed.Analysis shows that a transition zone exists between the oxide film and the metal matrix

in the said zone

Cr is oxidized

and Fe diffuses to the outer layer

leading to reduction of Fe concentration

and relatively increasing the Cr concentration.The oxide film has a structure of two layers

whose boundary surface is the original metal surface.The outer oxide layer is Fe-rich

and Cr isn't contained

its main composition being Fe3O4

while the inner oxide layer is composed of(Fe

Cr)3O4.