Protective coatings for high-temperature steam oxidation in coal-fired power plants

Abstract Corrosion resistance of protective coatings on stainless steels against high-temperature steam oxidation was studied. The coatings made through the thermal diffusion process consist of aluminides, and they have a 3-layer structure or even more complex architectures when the aluminide layers are covered by thin oxide-based (e.g. Al2O3, SnO2, ZrO2) or BN layers. Oxidation behaviour of these coatings compared to bare materials (e.g. 310SS and 800H) was studied at 800 °C for 3000 h. In this work, along with determination of mass change of samples vs. time (kinetics of oxidation), structural analysis and hardness determination for the coatings and substrates were carried out. The developed multi-layered coating architectures demonstrated promising high-temperature steam oxidation performance with no breakaway degradation issues. Their performance is defined by the inertness of the aluminium intermetallides with a total thickness of protective thermal diffusion coatings of >100 μm, with strong diffusion-related bonding between the layers and with the substrate, and by the formation of the stable aluminium oxide films occurred due to surface oxidation. The additional layer from the selected oxides or BN applied on the top of the aluminized structure can provide additional corrosion protection; however, it is important to maintain a high level of bonding between this layer and the intermetallide structure. The thermal diffusion technology and obtained coating systems are applicable for protection of products with different sizes and shapes, including long tubing required in industry.