Corrosion resistance of HVOC coatings in a biofuel boiler plant

Corrosion and erosion protection of superheaters in power plant boilers could be improved by using thermal spray coatings. By applying a dense coating layer over a conventionally used material, the properties of the component surface can be tailored more flexible based on the environmental demands, such as erosion and corrosion. In this paper corrosion and erosion resistance of thermally sprayed coatings in boiler superheater conditions is studied. HVOF coatings were tested on aircooled probe exposure in bubbling fluidized bed (BFB) boiler burning biofuels, recycled fuels and peat. Tested materials were NiCr and Cr3C2-based coatings and reference material was lowalloyed steel 10CrMo 9 10. Exposure times were several hundred hours. Specimens were analysed with SEM/EDX. Deposits on superheater tubes containing e.g. potassium, sulphur, calcium, sodium, and chlorine can cause severe corrosion problems for conventional metal alloys. Results of the exposure showed very good performance of coated specimens compared to the reference material 10CrMo 9 10. Critical factors of the coating are density and adherence to the substrate material. Some problems may rise from differences in thermal expansion coefficient, which can cause crack formation in the coating.