Corrosion is an important factor in the formation of deposits on the gas-swept heating surface of steam generating plant. The paper compares the behaviour of cast iron and steel under the corrosive attack of the sulphur acids occurring in boiler gases. Preliminary laboratory tests show a pronounced difference in behaviour of the two metals. Further investigation shows that cast iron has inherent properties which increase its resistance to corrosion over a wide range of conditions. These properties are shown to be linked with the silicon content of cast iron. Consideration is given to the influence of corrosion products, and of sulphurous acid. An explanation is offered of a number of observed phenomena in practice, including the puzzling “band corrosion” in air-heaters. This is shown to be a temperature and acid-concentration phenomenon. The paper ends with some comments on the modern practice of cleaning economizers and air-heaters by washing, and makes suggestions for the future guidance of designers.

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