Abstract
Stellite-6 (Co-based) and NiAl coatings (Ni-based) were deposited via HVOF spraying onto 304 stainless steels and tested in a 20 kWth biomass fired bubbling fluidised bed (BFB) combustor for 20 h and an industrial scale anthracite fired CFB boiler for 1630 h. Stellite-6 showed excellent corrosion resistance in both fluidised bed combustion systems because of the formation of the outermost Cr2O3 layer and the spinel CoCr2O4 beneath, whereas NiAl coatings’ anti-corrosion performance was significantly depleted due to the chlorine attack, and the resultant formation of Al2O3 layer at the coating/substrate interface finally led to coating spallation in both systems.
Highlights
To prevent the severe impacts of climate change, the European Union (EU) has set ambitious targets for the energy sectors, including raising the renewable energy source consumption to 35% by 2030 [1]
Stellite-6 and NiAl coatings were high velocity oxy-fuel (HVOF) thermal sprayed onto 304 stainless steels and installed in a 20kWth biomass-fired bubbling fluidised bed (BFB) combustor, and an industrial anthracite fired circulating fluidised bed (CFB) boiler
The main reasons can be obtained by comparing the energy dispersive X-ray (EDX) spectrums across the corrosion products on the samples installed at BFB upper position (Fig. 6e) and CFB 8 m position (Fig. 6f): (1) as seen, sulphur content was detected only on the BFB sample and dissolved by the substrate forming nickel sulphides [34], acceler ating the corrosion rate; (2) Ca and Al were found on the CFB sample, covering the corrosion products in the form of calcium aluminosilicates
Summary
To prevent the severe impacts of climate change, the European Union (EU) has set ambitious targets for the energy sectors, including raising the renewable energy source consumption to 35% by 2030 [1]. The HVOF sprayed Stellite-6 coating (Co-Cr-W-C) has been demonstrated as one of the best corrosion pro tection methods in pulverised coal firing boilers at the superheaters under an average temperature of 900 ◦C given the formation of the spinel CoCr2O4 at the outermost layer [15] When it comes to biomass combustion, further concerns were raised against the protective Cr2O3 forming alloys as the alkali chlorides could react with Cr2O3 forming K2CrO4 which facilitated the chlorine penetration and led to continuous corrosion attack across the coating [16,17,18]. Stellite-6 and NiAl coatings were HVOF thermal sprayed onto 304 stainless steels and installed in a 20kWth biomass-fired bubbling fluidised bed (BFB) combustor, and an industrial anthracite fired CFB boiler Both systems were controlled at high-end operation bed temperatures (800 ◦C − 900 ◦C) which can emphasise the corrosion development on the 304 SS and highlight the anti-corrosion abilities of the coatings.
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