Sealing of thermally sprayed coatings

Use of coatings is increasing in fluidized bed boilers due to harsh conditions, which modern boilers meet. High temperatures combined with inhomogenous, high-chlorine and alkali containing fuels like biomass, 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, such as corrosion proceeding on the substrate material through voids and oxides in lamella boundaries. Sealing of the coatings can solve these problems. This paper investigates the sealing of coatings. Thermally sprayed metallic coatings were sealed with different commercial sealing agents and laser treatment. The coatings were tested after sealing in the conditions simulating fluidized bed boiler superheater conditions. Tested and non-tested specimens were examined by optical (OM) and scanning electron microscope (SEM) and analysed by energy dispersive X-ray analysator (EDX). 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.