Maria Oksa

Sealing of Thermally Sprayed Coatings

Abstract The use of coatings in fluidised bed boilers is increasing owing to the harsh conditions that modern boilers meet. High temperatures, combined with inhomogeneous, high chlorine and alkali containing fuels such as biomass, cause severe material wastage to metallic parts in boilers. Thermally sprayed coatings have been reported to provide protection to boiler tubes. However, thermally sprayed coatings have encountered serious problems, such as corrosion on the substrate material owing to 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 by simulating fluidised bed boiler superheater conditions. Specimens were examined by optical microscopy and SEM and analysed by energy dispersive X-ray analysis. Some of the tested sealants protected the coatings adequately in a short term alkali chloride– alkali sulphate exposure test. The best sealant contained aluminium oxide and aluminium phosphate. Laser treated coatings had good corrosion resistance in a short term test.

Optimizing NiCr and FeCr HVOF Coating Structures for High Temperature Corrosion Protection Applications

In order to achieve a desired dense structure for coatings employed in high temperature corrosion conditions, thermal spray process optimization with diagnostic tools can be applied. In this study, NiCr (51Ni-46Cr-2Si-1Fe) and FeCr (Fe-19Cr-9W-7Nb-4Mo-5B-2C-2Si-1Mn) powders were sprayed with HVOFGF (gas-fueled) and HVOFLF (liquid-fueled) systems, and the spray processes were monitored with diagnostic tools, including SprayWatch for measuring the temperature and velocity of the spray stream, and in situ coating property (ICP measurement) for measuring the stress state. Various spray parameters were applied to attain the best coating characteristics for high temperature applications. Selected coatings were exposed to high temperature corrosion conditions both in laboratory and actual power plant. The coatings were analyzed by microscopic means and mechanical testing. The application of process-structure-properties-performance methodology with the process monitoring, analysis of the coating characteristics, and results of corrosion performance are presented in this paper.