Erosion‐corrosion of laser and thermally deposited coatings exposed in fluidised bed combustion plants

Abstract

Coated low alloyed steel tubes were exposed in two combustion power plants of the type Circulating Fluidised Bed (CFB) and Pressurised Fluidised Bed Combustion (PFBC). The power plants were fired with wood chips and coal with a small addition of olive seeds, respectively. In addition to laser coating two thermal spray techniques were used; arc-spray (air as carrier gas) and High Velocity Oxy Fuel (HVOF). The sample locations in the PFBC plant were at the highest and lowest loops of a platen immersed in the fluidising bed. The material temperatures in each loop were 450 C and 400 C, respectively. The exposure lasted over two firing seasons for a total time of 8089 h. In the CFB plant a probe was located at the cyclone entrance where the material temperature was 630 C for an exposure time of about 2100 h before being reduced to 480 C for a further 920 h. The material wastage was determined from metallographic studies on cross-sections of rings cut from the exposed tubes. The nature and chemical composition of the corrosion products and deposits formed were determined by SEM/EDX, Auger spectroscopy, XPS and XRD. Cobalt based coatings show the best performance in both the PFBC plant and the CFB plant, while nickel based coatings are resistant to a corrosive atmosphere but very sensitive to erosion. The degree of corrosion is much larger in the CFB plant as is clearly seen from the chromium carbide containing coating, which totally degraded in this environment. Contrary the performance of the same coating was excellent in the fluidised bed due to its high erosion resistance. The material wastage of a coating with a specific composition is independent of the deposition method. No significant differences in spallation behaviour occurred among coatings deposited with the different techniques.

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