HIGH-TEMPERATURE CORROSION OF ALLOYS AND COATINGS IN OIL-FIRED AND COAL-FIRED BOILERS

The high temperature corrosion processes which occur in oil- and coal-fired power utility boilers are summarized and plant experience is described. The dependence of corrosion rate on alloy composition is discussed and the frequent dichotomy between these requirements and those for mechanical strength and fabricability is emphasized. In aggressive corrosion conditions, it is therefore often necessary to separate the corrosion requirements from mechanical aspects by the use of 'coatings', where this means a surface layer of any thickness present for purely corrosion reasons. The lifetimes and reliabilities required in generating plant mean that coatings of several millimeters thickness are generally needed and this has led to the development and extensive use of coextruded tubes. Recent encouraging trials of plasma-sprayed tube are mentioned. Future requirements in optimizing coextruded tube compositions and fabrication routes are indicated.