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High temperature corrosion studies of HVOF sprayed coatings in molten sulphate salts environment
Lencová, K. ; Česánek, Z. ; Houdková, Š. ; Lukáč, František ; Mušálek, Radek
The generally applied requirements on lowering of emission production in the field of power industry have led, among others, into the application of low-emission combustion process of coal fired power plants. Besides the emissions reduction, this process has led to an increase of boilers parts corrosion degradation due to the reducing atmosphere in low-emission boilers, responsible for sulphide corrosion and molten salt attack. In such environment, the protection of the exposed surfaces gains importance. Among others, the thermal spraying technology can offer a suitable way of surface corrosion protection. In the present work, the high temperature corrosion behaviour of CoCrAlYTaCSi and Cr3C2-25%NiCr High Pressure/High Velocity Oxygen Fuel (HP/HVOF) sprayed coatings is evaluated and compared to the behaviour of bare structural steel (SPT360) substrate. The specimens were exposed to a molten salt environment, i. e., 12% Na2SO4 + 82% Fe2(SO4)3 at 690 °C under cyclic condition, simulating the working environment in low-emission boilers of coal-fired power plants. The thermogravimetric method was used to establish the kinetics of corrosion. X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) techniques were used to analyse the corrosion products and determine the corrosion mechanism. The thermogravimetric measurement shows high oxidation rate of SPT360 steel. Cr3C2-25%NiCr coating provides slightly better performance compared to CoCrAlYTaCSi coating. Based on the obtained results, both coatings were found to have higher corrosion resistance than the SPT360 steel substrate, offering a sufficient surface protection in a given environment.
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Characterization of flame sprayed abradable seal NiCrAl-bentonite coatings
Houdková, Š. ; Česánek, Z. ; Schubert, J. ; Lukáč, František ; Bystrianský, M.
The NiCrAl-21% Bentonite abradable coating, deposited by flame spraying, was exposed to 720°C / 1 hour air-annealing to evaluate its resistance against heat-induced changes. The SEM and XRD microstructure evaluation was done. After the air-annealing, the increase of HR15Y surface hardness was recorded and accompanied with decrease of abradability evaluated by Progressive Readability Hardness (PAH) test, although no demonstrable microstructural changes was recorded by SEM. XRD analyses evidenced the formation of nickel oxides and ordered Ni3Al phase during the air-annealing. Besides the abradable coating evaluation, this work brings the verification of the potential of PAH for scratch hardness testing. The measurement provides the results with low scatter, compared to usual surface hardness testing, and enables to distinguish between the depth of indentation in loaded and unloaded state, pointing out to the elastic/plastic deformation ratio.
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