High-temperature erosion–oxidation of uncoated and FB-CVD aluminized and aluminized–siliconized 9Cr–1Mo steel under fluidized-bed conditions

Fluidized-bed combustion is one of the methods to generate energy in a clean and efficient way from a variety of fuels. However, conditions in fluidized-bed boilers: high temperature, oxidizing atmosphere and impacts by fluidized sand particles, can cause significant degradation of some boiler components, such as heat exchangers, by a combination of oxidation attack and erosion wear. Protective coatings, deposited mainly by thermal spraying and diffusion techniques, are considered a solution to extend the lifetime of such components. This paper allows evaluation whether diffusion coatings, applied using a fluidized-bed chemical vapour deposition (FB-CVD) technique, could be used to provide protection for 9Cr–1Mo steel against high-temperature erosion–oxidation. In this paper, the results from laboratory studies of the erosion–oxidation behaviour of uncoated, aluminized and aluminized–siliconized 9Cr–1Mo steel, subjected to air, at temperatures of 550–700 °C and impacts by 200 μm silica sand particles, are presented. The tests were carried out in a fluidized-bed rig, using speeds of 7.0–9.2 m s −1 and angles of 30° and 90°. Erosion–oxidation damage was characterized by measurement of the mean thickness changes using a micrometer and examination of worn surfaces by scanning electron microscopy. The results show that the coatings, particularly the aluminized–siliconized coating, protect the 9Cr–1Mo steel for some period of the test under the given conditions, but, once the coatings are penetrated, aluminizing and aluminizing–siliconizing are no longer effective in preventing erosion–oxidation of the substrate. The interactions between erosion and oxidation processes are discussed and explanations for differences in behaviour of uncoated and coated specimens are presented. Finally, the challenges in developing thicker coatings to provide longer term protection of the steel against erosion–oxidation are considered.