Alkali Chloride-Induced High-Temperature Corrosion of Alloys - Utilising long-term corrosion mechanisms to predict boiler corrosion

Doktorsavhandling, 2025

Biomass and waste fuels show strong potentials as alternative renewable energy sources that can help meet the increasing global energy demand while reducing the net release of CO2 into the atmosphere. The combustion of biomass and waste is however associated with the release of flue gases that contain high levels of alkali salts (e.g., KCl) that cause breakaway oxidation, and this leads to accelerated corrosion of metallic boiler components. The rapid corrosion restricts boiler operating parameters, i.e., steam temperature and pressure, and thereby limits the electrical efficiency. One way to tackle this issue is to use more-corrosion-resistant materials for boiler applications, to enable operations at higher temperatures and pressures. To achieve this, it is crucial to gain an in-depth understanding of the corrosion mechanisms of boiler materials. However, there is lack of comprehensive long-term investigations that have employed well-controlled systems to elucidate the corrosion phenomenon as well as the long-term corrosion kinetics.

To date, most of the high-temperature corrosion studies related to biomass and waste combustion have involved short-term laboratory investigations with a focus on understanding the initial stages of corrosion. This thesis concerns long-term high-temperature corrosion research in the laboratory through the development of a state-of-the-art experimental set-up that mimics key corrosive species in the boiler environment at a relevant temperature. This approach offers the possibility to study several materials and coatings with the aim of evaluating the corrosion properties during long-term exposure to the corrosive environment. The corrosion products are characterised using x-ray diffraction (XRD) and advanced electron and ion microscopy, such as SEM/EDX, EBSD and TEM, on cross-sections prepared using broad ion beam (BIB) milling and focused ion beam (FIB) milling. Moreover, thermodynamic calculations and kinetics-based simulations are applied to elucidate the growth mechanisms of oxide scales.

The results reveal that all of the investigated alloys experience breakaway oxidation and form multi-layered oxide scales, referred to as secondary protection. Secondary protection could be divided into two categories in the presence of KCl: (i) fast-growing and less-protective iron-rich oxide scales, representing poor secondary protection; and (ii) slow-growing and more-protective chromium/aluminium-rich corundum-type oxide scales, representing good secondary protection. The long-term oxidation kinetics studies in the presence of KCl, together with the oxide microstructural evolution showed that the scale growth is diffusion-controlled, and that the properties of the secondary protection may be influenced by the alloying elements and the bulk microstructure. In addition, the findings showed that long-term laboratory oxidation kinetics can be utilised to understand the mechanism and predict the corrosion of metals in the complex boiler environment. The insights gained from this thesis will help to improve predictions of material corrosion and facilitate the design and development of high-temperature alloys.