High Temperature Corrosion of FeCrAl Alloys in Biomass- and Waste-fired Boilers - The Influence of Alloying Elements in Prediction and Mitigation of Corrosion in Harsh Environments

Doktorsavhandling, 2020

Combustion of biomass and waste for heat and power production is an alternative to fossil fuels and can be an important step towards a more sustainable future. The electrical efficiency of the fuel-to-energy conversion process is largely dependent on the steam parameters (temperature and pressure) of the combined heat and power (CHP) plants. Meanwhile, the boiler environment when utilizing these fuels is complex and can be characterized by high levels of corrosive species, such as water vapor and alkali chlorides. These species contribute to highly corrosive conditions that results in rapid material degradation of boiler components and limits the operating temperature.

Stainless steels are commonly used to reduce material degradation in these types of corrosive environments because of their high temperature corrosion resistance. This is attributed to their ability to form a protective chromium-rich oxide scale. However, in the highly corrosive environment of biomass- and waste-fired boilers these scales have been found to rapidly break down and result in the formation of less protective iron-rich oxide scales. The present thesis elucidates the potential of improving the corrosion resistance by introducing alumina-forming alloys (FeCrAl alloys) and/or improving the properties of the fast-growing oxide scales formed after breakaway oxidation.

The results show that the oxidation process can be divided into a primary and a secondary corrosion regime, i.e. the corrosion behavior before and after breakaway oxidation. This concept was utilized when investigating the influence of alloying elements on the corrosion resistance of FeCr(Al) model and coatings in a broad range of corrosive environments. Cr, Al and Si was found to significantly influence the corrosion behavior of FeCr(Al) alloys in both the primary and secondary corrosion regime. However, the criteria for a high corrosion resistance differed for the two corrosion regimes. HVAF-sprayed and overlay-welded coatings demonstrated the potential of providing high corrosion resistance within the primary and secondary corrosion regime, respectively. The applicability of the findings was shown to extend from simplified laboratory environments to the complex conditions of a waste-fired boiler.

The novel insights, presented in this work, contribute to new perspectives on high temperature corrosion resistance, which are valuable in material development and corrosion prediction in harsh environments.