Nickel-based HVOF coatings promoting high temperature corrosion resistance of biomass-fired power plant boilers

• HVOF thermal spray coatings were exposed for two years in a biomass boiler.
• Nickel based HVOF coatings showed excellent corrosion resistance.
• A tube leakage had occurred in the circulating fluidized bed boiler.
• Economizer tubes of carbon steel St35.8 had corroded strongly in the conditions.
• Corrosion was caused by chlorine together with copper and zinc compounds.

There are over 1000 biomass boilers in Europe, and the number is increasing due to actions for reducing greenhouse gas emissions. Biomass boilers often experience strong corrosion due to harmful elements in fuels. In biomass burning, detrimental components include especially chlorine, potassium and heavy metals, which can cause chlorine-induced active oxidation or hot corrosion by molten phases even at fairly low temperatures. In order to increase the corrosion resistance of heat exchanger components, either more alloyed steels or protective coatings should be applied. High velocity oxy-fuel (HVOF) sprayed coatings may provide corrosion protection for low alloy tube materials. Three nickel based thermal spray coatings (Ni–24Cr–16.5Mo, Ni–22Cr<5Fe–9Mo–4Nb and Ni–22Cr–10Al–1Y) were tested for two years in a 40 MW circulating fluidized boiler (CFB), which had experienced severe corrosion and a tube failure. The coated tubes were installed to the cold and the hot economizer. After the exposure the coatings and the substrate materials were analyzed with SEM–EDX. The uncoated boiler tubes corroded strongly, whereas the thermal spray coatings exhibited excellent corrosion performance. This paper presents the tube failure at the cold economizer, exposure conditions, the analysis of the coated and uncoated samples, and the corrosion mechanisms of the steel tubes.