The characteristic of oxide scales on T91 superheater tube after long-term service in an ultra-supercritical coal power plant was investigated using light optical microscopy, stereomicroscope, X-ray diffraction, electron probe micro-analyzer, scanning electron microscopy and energy-dispersive X-ray spectroscopy. It was found that the oxide scale formed on the inner wall of the tube was a multi-layer structure. The outer layer consisted of Fe3O4 and Fe2O3 with some pores and tended to exfoliate. The inner layer consisted of Cr-rich spinel. The Cr-rich oxide was observed as striations parallel to the oxide/metal interface. The possible mechanisms of oxidation and exfoliation are also discussed. (c) 2012 Elsevier B.V. All rights reserved.