Coatings for high temperature corrosion protection in advanced power plants

When aiming to decrease of greenhouse gas emissions and higher cost-efficiency, power plants would benefit from increased efficiency. Advanced power plants are aiming for up to 750°C steam temperatures. Higher steam temperatures will set higher requirements to tube materials, as especially difficult fuels can cause severe corrosion conditions. Biomass and waste include chlorides, sulfates and heavy metals, such as NaCl, KCl, K2SO4, Pb and Zn, that can substantially accelerate the corrosion even at lower temperatures. Corrosion protection properties of thermal spray coatings were validated in a co-fired boiler using biomass, coal and solid recovered fuel. Material testing was performed with a temperature controlled probe at 550 and 750°C for 3000 hours. Corrosion performance of six high velocity oxy-fuel (HVOF) coatings, FeCr, Ni46Cr, Ni26Cr, NiCrAlY and Al2O3, were evaluated, and a ferritic steel T92 and a nickel based alloy 740H were applied as reference materials. Maximum corrosion rates varied from 0.01 mm/y of Ni-based coatings to 0.4 mm/y of T92 at 550°C and from 0.1 mm/y of A740H to 0.5 mm/y of Fe-based coating at 750°C. The nickel-based coatings and the ceramic Al2O3 coating gave excellent protection to the ferritic steel at 550°C and sufficient protection at 750°C.