Abstract
Use of high-chlorine and alkali containing fuels such as biomass and refuse is increasing in production of heat and electricity. Fluidized bed boilers experience severe and harsh conditions, in which high temperatures combined with inhomogenous fuel cause severe material wastage to metallic parts in boilers. Thermally sprayed coatings have been reported to provide protection to the boiler tubes. However, thermally sprayed coatings have encountered serious problems, when corrosion has proceeded to the substrate material through voids and oxides in lamella boundaries. Sealing of the coatings can solve these problems. Sealing of coatings with commercial sealants and laser fusion treatment was investigated. Thermally sprayed HVOF metallic coatings were sealed with different commercial sealing agents and diode laser. The coatings were tested in molten salt, simulating condition of fluidized bed boiler superheaters. Optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive X-ray analysator (EDX) were used for examination of the tested and non-tested specimens. Some of the tested sealants protected the coatings adequately in the short time alkali chloride - alkali sulphate exposure test. Best sealant contained aluminium oxide and aluminium phosphate. Laser treated coatings had good corrosion resistance in a short-term test.
