Erosion - Corrosion Resistance and Adhesion of Laser and Thermally Deposited Coatings in Fluidised Beds

Doktorsavhandling, 2004

High temperature erosion - corrosion is the major source for degradation of components such as heat exchangers, tubes, cyclones and water walls used in Fluidised Bed Combustion (FBC) plants. By coating the exposed components with a protective material, the degradation rate can be decreased. The aim of this work was to determine the erosion - corrosion resistance of coatings deposited with the arc-spray, High Velocity Oxy Fuel (HVOF) and laser techniques. The erosion - corrosion tests have been performed in three different power plants; one Circulating Fluidised Bed (CFB) plant fired with biomass, one Multi Bed Combustion (MBC) plant fired with a mixture of coal and biomass and one Pressurised Fluidised Bed Combustion (PFBC) plant fired with coal. The erosion - corrosion resistance of coatings has also been examined in one laboratory jet nozzle test rig and one fluidised bed test rig. Further, the adhesion strength of coatings deposited on substrates of a low-alloyed steel (1Cr0.5Mo) has been tested. A number of commercially available coating qualities have been used in this study. The reference material is the Fe-based Metcoloy 2 coating, which was deposited with all the three methods available. The other coatings can be divided into Ni-based, Co-based, carbide - containing and Fe-based. Under active - service conditions Co-based coatings show the best overall performance. It is suggested that the erosion resistance is partly due to the presence of carbide rich phases dispersed in the coating. The softer Ni based coatings are generally more subjected to erosion. The corrosion resistance of Fe-based coatings is poor at higher temperatures in more corrosive environments. Chromium carbide - containing coatings are excellent in the coal fired erosive environment but degrade severely in the biomass fired plants showing that stable hard phases, such as carbide, at the exposure temperature decrease the degradation rate. Tungsten and Ti-carbide containing coatings suffer from severe degradation in all environments. The coated samples were subjected to erosion- corrosion in a jet nozzle rig in air at 520ºC or 550ºC. Compared to a solid 304L steel (Fe19Cr10Ni), the erosion - corrosion resistance of the coatings tested (Fe-based, Ni-based and carbide - containing) is higher. The oxides formed on the eroded area of the Metcoloy 2 coating are 10 - 60 times thicker than those formed on the non - eroded area. Consequently, erosion has increased the oxide growth rate. The increased oxide thickness on the eroded areas is due to the formation of cracks and pores in the oxide scale. These will enhance the inward transport of molecular oxygen through the oxide layer to the oxide/metal interface where the metal phase is oxidised. Erosion - corrosion tests performed in the fluidised bed test rig at 550°C in air shows that the erosion resistance of thermally sprayed coatings is higher in comparison to laser deposited coatings. The use of acoustic emission for determining the adhesion strength of the coatings proved to be successful and more reliable than the traditional four point bending test with a subsequent metallographic examination. HVOF sprayed coatings start to delaminate at a strain of about 1%, arc sprayed at a strain of about 1.4 – 2% while laser deposited coatings did not delaminate for strains up to 15%. However, by increasing the splat size in the arc sprayed coatings the strain at which delamination occurred was increased to a bout 10%.