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Synthesis, environmental degradation and repairability of slurry aluminium coatings elaborated on steels employed in power plants

Abstract : Higher energy efficiencies can be achieved by the increase of temperatures and pressures in power plant, thus allowing the reduction of the polluting emissions. Therefore, more efficient materials, compared to those currently employed, have to be used in order to resist to harsh environmental conditions (high temperatures, steam, pressure…). Despite of the good mechanical properties of many steels, the elaboration of a protective coating able to form a dense, protective and stable oxide layer at high temperature, is necessary. Thus, in the framework of the “POEMA” European project, the mechanisms of the formation of aluminium diffusion coatings were studied on ferritic-martensitic and austenitic stainless steels that are currently employed in power plants. The sealing of cracks eventually created during the aluminisation of steels was also studied. The degradation mechanisms of the B2-(Fe (Ni))Al coatings were then studied in steam and in air at 650 and 700°C up to 10000 hours and under different pressures (1 and 300 bar). We have demonstrated that the pre-oxidation induced during the elaboration of coatings grew a thin protective oxide scale composed of α-Al2O3 which developed further upon oxidation. Nevertheless, the long exposures under high steam pressures promoted the cationic diffusion of Fe, thus allowing the formation of small surface oxide nodules. Finally, the dissolution mechanisms of coatings with a diluted acid have been studied in order to demonstrate the possibility of strip and realuminise the materials initially coated.
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Claire Boulesteix. Synthesis, environmental degradation and repairability of slurry aluminium coatings elaborated on steels employed in power plants. Materials. Université de La Rochelle, 2017. English. ⟨NNT : 2017LAROS022⟩. ⟨tel-01804936⟩

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