The erosion-corrosion resistance of uncoated and aluminized 12% chromium ferritic steels under fluidized-bed conditions at elevated temperature

In this paper, some results from a study of the erosion-corrosion resistance of uncoated and aluminized 12% chromium steel in a fluidized-bed rig are reported. The aims of the research are to establish and compare the erosion-corrosion resistance of these materials for possible applications as heat exchangers in future power plants, and to obtain an increased understanding on their behaviour and mutual superiority in a range of conditions. Damage to the uncoated 12% chromium steel occurs by an oxidation-affected erosion process under all the studied conditions, with spallation of scale being the primary mechanism of material wastage. At a temperature of 550 degrees C, the uncoated steel follows the typical angle-dependence of a brittle material, while, at temperatures above 550 degrees C, it follows an angle-dependence that is more typical of a ductile material. This change in the angle-dependence with temperature is related to characteristics. i.e. uniformity, adhesion and density, of the formed oxide scales. The rate of material wastage increases with increase in speed and temperature, due to the development of thicker, more uniform and more dense oxide scales, that promote more severe scale spallation. The erosion-corrosion behaviour of the aluminized 12% chromium steel changes in the temperature range from 600 degrees C to 650 degrees C. This is due to a shift from a brittle-like to a ductile-like angle-dependence and to a more rapid oxide scale build-up at temperatures above 600 degrees C. At an impact angle of 30 degrees and at 550 degrees C and 600 degrees C, the prevailing erosion-corrosion process for the aluminized steel is oxidation-affected erosion. At 650 degrees C and 700 degrees C for an impact angle of 90 degrees, the primary erosion-corrosion mode is essentially erosion-enhanced oxidation. The results of the study have also demonstrated that the Al5Fe2 coating deposited by pack aluminization offers enhanced protection against erosion-corrosion at shallow impact angles at 550 degrees C and 600 degrees C and at steeper impact angles at 700 degrees C.