Oxides are chemically stable and wear resistant materials. Because of these properties, they are often applied as protective coatings in harsh environments. However, their chemical and mechanical stability at high temperature in chlorine containing environments is uncharted. These conditions are present in waste-to-energy and biomass boilers in which the currently available metallic and metal matrix composite coatings provide unsatisfactory protection. To be effective in these conditions the coatings should be chemically inert, erosion resistant and act as environmental barriers. For this purpose, this research studies the corrosion behavior and microstructural features of HVOF and APS-sprayed Al 2 O 3 -, Cr 2 O 3 -, TiO 2 -based coatings. Their chemical stability was evaluated by high temperature corrosion testing of self-standing coatings under KCl salt deposit at 550, 650 and 720 °C for the duration of 72 h.

Highly corrosion and wear resistant thermally sprayed chromium carbide (Cr 3 C 2 ) based cermets coatings are nowadays a potential highly durable solution to allow traditional fluidised bed combustors (FBC) to be operated with ecological waste and biomass fuels. However, the heat input of thermal spraying processes causes carbide dissolution in the metal binder. This alters the coating structure and forms carbon saturated amorphous and nanocrystalline metastable areas, which can affect the behaviour of the materials under the corrosive chlorides containing environment of the flue gases. This study analyses the effect of carbide dissolution in the metal matrix of MMC coatings and its effect on the onset of chlorine induced high temperature corrosion. Four Cr 3 C 2 -NiCrMoNb coatings were thermally sprayed with high-velocity air-fuel (HVAF) and high-velocity oxygen-fuel (HVOF) spray processes in order to obtain microstructures with increasing amount of carbide dissolution in the metal matrix. The specimens were heat treated in an inert argon atmosphere at 700°C for 5 hours to induce secondary carbide precipitation. As-sprayed and heat-treated self-standing coatings were covered with KCl and their corrosion resistance was investigated with thermogravimetric analysis (TGA) at 550°C for 4 hours. High carbon dissolution in the metal matrix appeared to be a detrimental factor in the initial stage of corrosion. The microstructural changes induced by the heat treatment hindered the corrosion onset in the coatings. Moreover, an optimal amount of oxides and melting degree seemed beneficial.

Hot corrosion tests have been conducted on Ni- and Cr-based laser coatings, a high-velocity oxy-fuel (HVOF) sprayed coating and various wrought alloys covered with a synthetic salt of Na 2 SO 4 -V 2 O 5 and exposed at 650°C for 1000 h in air. Coating microstructures and reaction product layers were analyzed with scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). The hot corrosion resistance of tested specimen was evaluated by measuring its mean thickness loss. Generally, wrought alloys, HVOF coating and Cr-based laser coatings suffered from selective corrosion beneath salt film, that is, distinct Cr-depleted layer was formed at alloy/salt interface. Cr-based laser coatings exhibited extended solid solubility and they transformed towards equilibrium condition. Cr-rich phases enriched further with Cr and they were prone to corrosion. Low diluted laser coatings and HVOF coating were more resistant to hot corrosion than commonly used industrial standard alloy, Nimonic 80A. Ni-based laser coating exhibited resistance equivalent to Cr-based coatings and superior to corresponding wrought alloy.

The significance of biofuels and the other chlorine-containing fuels in energy production is in strong increase. Serious erosion-corrosion problems in boilers combusting fuels with high chlorine-content have been detected frequently. A series of erosion-corrosion and corrosion tests were performed on thermal sprayed coatings and coating precursors in chlorine-containing environments in order to evaluate possibilities to utilize thermal sprayed coatings for erosion-corrosion protection in boilers combusting chlorine-containing fuels. A series of hot erosion and erosion-corrosion tests were performed on thermal sprayed coatings at elevated temperatures with and without chlorine. Carbide-containing HVOF coatings performed well in hot erosion tests, but they were completely destroyed in the presence of chlorine due to rapid oxidation of carbides. Metallic HVOF coatings with high chromium content performed well in both conditions. Iron-based arc-sprayed coatings with unhomogeneous microstructure suffered more hot erosion and erosion-corrosion damages than metallic HVOF coatings. The E-C (erosion-corrosion) resistance of carbide-containing coatings in the presence of chlorides was worse than expected. A series of oxidation tests were performed on various carbides in order to elucidate the effect of chlorine on high temperature oxidation behavior of carbides. TGA and isothermal oxidation tests proved that gaseous chlorine-containing species and also solid chlorides have a detrimental effect on oxidation resistance of tested carbides.