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Abstract: Pack-aluminized 9% chromium steel specimens were exposed to angular silica sand particles in a fluidized-bed erosion–oxidation rig for 200h. The exposures were conducted in air at temperatures of 550°C to 700°C for particle impact angles of 30° and 90°, at speeds of 7.0–9.2ms−1. Subsequently, the mean thickness changes of the specimens were determined and the specimens were examined and analyzed by scanning electron microscopy and X-ray diffraction. The specimens experience only slight thickness changes for 30° angle impacts but significant material loss for 90° angle impacts, typical of a brittle erosion process. Under 30° angle impacts, the coatings were mostly retained on the substrate surface and slightly deformed. Thin oxide scales were detected on the surface at all test temperatures. Under 90° angle impacts, thickness losses increased with increase in speed and temperature up to 650°C, resulting in complete loss of the coating in the test period. A porous, cracked, but continuous, oxide scale was observed on the surface of the exposed substrate. At 700°C, the coating was partially retained on the substrate, with the residual coating thickness decreasing with increase in speed. Explanations for these observations are presented, the interactions between the erosion and oxidation processes for the specimens are discussed and the degradation mechanisms for the coatings under the test conditions are described in this paper. [Copyright &y& Elsevier]

Abstract: Aluminium diffusion-coated 9% Cr steel specimens were exposed to particles of two grades of silica sand, with similar particle size distributions but dissimilar angularities, in a fluidized-bed rig for 200h. The exposures were conducted at 550 and 600°C for particle impact angles of 30° and 90°, at speeds in the range 7–9.2ms−1. The results show that the specimens experience only slight thickness changes for 30° angle impacts but significant material loss for 90° angle impacts, typical of a brittle erosion process, irrespective of the silica sand grade used. Under 30° angle impacts, the two silica sands introduce dissimilar stress distributions in the specimen surfaces; particles of the more rounded sand A introduce relatively uniform plastic deformation throughout the surface by a cutting type wear, while particles of the more angular sand B cause plastic deformation to be concentrated in certain areas of the surface. Here, for both sands, the prevailing erosion–oxidation mode is oxidation-affected erosion of the coating. At an angle of 90°, there is a threshold speed, below which some of the coating was retained throughout the substrate and above which only the residues were retained on some areas of the surface, resulting in exposure of the steel substrate to impacting particles at the locations of complete loss of coating. Overall, the coatings were significantly less damaged, i.e. less of the substrate was exposed, following impacts by sand A than by sand B. By calculating the threshold particle energy for coating loss, the erosivity of the sands can be evaluated: it is much higher for sand B than for sand A, mainly due to the greater angularity of the former. For 90° angle impacts, the prevailing mode of damage is erosion of the coating. Once the substrate surface is exposed to impacting particles after loss of the coating, damage continues by oxidation-affected erosion of the substrate. [Copyright &y& Elsevier]

Abstract: Protective coatings, deposited mainly by thermal spraying and diffusion techniques, are considered a solution to extend the lifetime of many components in the energy production sector, such as heat exchangers. In this paper, some results are presented for uncoated, aluminized and chromized-aluminized 9Cr–1Mo steel, subjected to air and to impacts by 200μm silica particles at angles of 30° and 90° and speeds of 7.0–9.2ms−1 at 550–700°C, in a laboratory fluidized-bed rig, to determine whether or not aluminized and chromized-aluminized diffusion coatings could protect the steel under such conditions. Erosion-oxidation damage was characterized by measurement of the mean thickness changes using a micrometer and examination of worn surfaces by scanning electron microscopy. Under most conditions, the coatings provided some protection to the substrate: under 30° impacts, up to 650°C, and under 90° impacts, at 700°C, both coatings were effective, whereas under 90° impacts, up to 650°C, only the chromized-aluminized coating gave significant protection. However, for 30° at 700°C, the oxide scale on the substrate was protective and the coatings were not needed. Explanations for these observations are presented in this paper, in terms of interactions between the erosion and oxidation processes for the materials. [Copyright &y& Elsevier]