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This is a case study of tube wear in a nuclear power steam generator. The tubes are worn in regions of contact with the support plates. Wear studies were performed by SEM and AES, in addition to conventional metallographic techniques, and comparisons were made with a laboratory test. The real case tube is covered by a 0.1-2 microm oxide layer. A wavy topography is characteristic of the wear surface, in which no plastic deformation was detected. It is concluded that wear occurs by flow enhanced removal of material from the superficial layer of the oxide film. Contributing mechanisms involve liquid erosion, cavitation and chemical dissolution. This is contradictory to the results of the laboratory test and the general opinion that flow induced wear of steam generator tubes is caused by fretting.