The significance of the formation of thick oxide scales during exposure of Cr steels in steam-containing environments on the service life of components is discussed. The quantitative effects of the loss of load-bearing cross-section on the creep rupture life are presented. For tubes of 6 mm wall thickness, the reduction in rupture life due to material loss will be significant after comparatively short exposure times of 10 000 It at 600 degrees C. Potentially much more damaging is, however, the thermal insulation effect of the porous, thick oxide scales, which leads to overheating of heat transfer tubes. The higher metal temperatures will then lead to early failure by creep. The scale present after 10 000 h exposure in steam may be sufficient to cause a temperature increase of 50 K which will reduce the rupture time for a constant stress by a factor of 30. Oxide scale spalling can also lead to overheating due to the restriction of the flow inside tubes. The design should ensure that spalled oxide cannot accumulate inside the tubes. (c) 2006 Elsevier Ltd. All rights reserved.
