Degradation mechanisms of aluminium diffusion coatings on 12% chromium steels under elevated temperature erosion – oxidation conditions

Abstract

Aluminium diffusion coatings offer one possible solution for improving the erosion–corrosion resistance of candidate steels for future power plants. However, their successful utilisation at temperatures higher than currently and under erosive load of solid particles requires an understanding of possible degradation phenomena that take place in both the short and the long term. This paper reports on the degradation resistance of aluminium diffusion coatings in the temperature range 550 – 700°C under impacts by silica sand particles at 30° and 90° for 200 h.For 90° particle impacts at 550°C and 600°C and 30° particle impacts at 650°C and 700°C, degradation of the coatings occurs rapidly by erosion, with oxidation or other diffusional processes in a minor role. For 30° particle impacts at 550°C and 600°C, the rate of erosion exceeds the rate of oxidation, with oxidation-affected erosion being the prevailing erosion – corrosion mode. Chipping and cutting wear pose the greatest challenge for long-term performance of the coatings. The coatings subjected to 90° particle impacts at 650°C and 700°C undergo more rapid oxidation and experience less erosive wear, with erosion-enhanced oxidation being the primary erosion – corrosion mechanism. Here, oxidation leads to depletion of the near-surface layer in aluminium and void formation near the coating surface, with consumption of the aluminium reservoir and an impairment of the coating integrity as primary consequences. Phase transformations and a thermally activated columnar growth were also observed to occur in the coatings, causing, for example, formation of new phases with decreased Al content and variations in the coating thickness.