Material Solutions for Mitigating High Temperature Corrosion in Biomass- and Waste-fired Boilers - Utilizing Novel FeCrAl Alloys and HVAF-sprayed Ni-based Coatings

Licentiate thesis, 2018

Combustion of biomass and waste for heat and electricity production is a promising alternative to fossil fuels and is thereby an important step towards a more sustainable future. However, due to substantial amounts of alkali- and chlorine-containing species as well as moisture in biomass and waste, the combustion of these results in a highly corrosive environment which accelerates the degradation of steel components in the boiler, drastically increasing the maintenance costs. One way to reduce the corrosivity in the boiler is to reduce the operating temperature but this simultaneously reduces the electrical efficiency of the boiler process. To retain the combination of optimized operating parameters of the boiler and low maintenance costs, new and improved material solutions has to be considered. In this study two different approaches has been investigated; improving corrosion properties of the material by altering the alloy composition and utilizing thermally sprayed high alloyed coatings.

The first approach, to improve corrosion resistance by altering the composition, has been focused on FeCrAl alloys. FeCrAl alloys has been shown to exhibit good corrosion properties at elevated temperatures due to the formation of α-alumina (above 900 °C) and transient forms of alumina (below 900 °C). However, in very harsh environments these protective oxides tend to break down, resulting in rapid material degradation. It is therefore of interest to further improve the corrosion resistance of FeCrAl alloys, one possible approach is to alter the alloy composition.  Minor additions of silicon has been shown to have great beneficial effects on the corrosion behaviour of stainless steels. It is however not known if the presence of silicon affects the corrosion behaviour of FeCrAl alloys in the same way. Thus, in this study, FeCrAl model alloys with varying silicon content were exposed in O2, O2+H2O and O2+H2O+KCl at 600 °C in order to investigate the possibility of improving the corrosion properties of FeCrAl alloys. The results showed minor additions of silicon drastically improved the corrosion resistance of FeCrAl alloys in the presence of water vapour and KCl. In the presence of water vapour, the silicon containing alloys retained a protective oxide while displaying breakaway oxidation in the absence of silicon. In the presence of KCl, all alloys formed iron oxide but addition of silicon drastically reduced the oxide growth rate.

Utilizing coatings to protect different components from degradation due to corrosion attack is used in many applications. However, in very harsh environments, the density of the coating becomes crucial for achieving a good corrosion protection. The more novel thermal spraying technique, High Velocity Air-Fuel (HVAF) has been shown to obtain coating with higher density and improved substrate adherence than other techniques. Therefore, in this study, the possibility of using coatings for corrosion protection in biomass and waste-fired boilers was investigated by exposing three Ni-based HVAF-sprayed coatings (NiCr, NiAl and NiCrAlY) in O2+H2O and O2+H2O+KCl at 600 °C. The results showed that the NiCr coating failed to prevent the diffusion of corrosive species such as chlorine to the substrate, leaving it vulnerable to severe corrosion attacks. However, both the NiAl and NiCrAlY coatings managed to prevent the diffusion of chlorine and only displayed minor oxide formation at the coating surface.