Beyond Breakaway Corrosion: Secondary Corrosion Protection of Iron-based Alloys

Licentiate thesis, 2020

Metallic materials intended for high temperature applications must resist both mechanical and environmental degradation. The ability to withstand corrosion is an important aspect of high temperature materials and is of major concern in, for example, heat and power production. Nevertheless, corrosion is often a limiting factor in the lifetime of boiler components and it reduces the electrical efficiency and hinders the development of more economical and environmentally sustainable processes. The challenge of high temperature corrosion is often addressed by the use of high-alloyed steels, such as stainless steels and FeCrAl alloys. The corrosion resistance of stainless steels and FeCrAl alloys rely on the formation of a slow-growing, chromium- and/or aluminium-rich, corundum type oxide. However, in harsh corrosive environments these oxides are known to break down (i.e. 'breakaway corrosion') and a less protective, multi-layered, Fe-rich oxide is formed. One such example is in biomass- and waste-fired boilers, where the combustion process produces a corrosive environment, often resulting in breakaway corrosion in an early stage of operation. Thus, the corrosion propagation and lifetime of many key parts of the boilers, depend on the oxide scale formed after breakaway. This oxide scale is often considered non-protective and studies on the oxidation mechanisms controlling the corrosion propagation after breakaway are scarce.
 
In order to address, and systematically investigate the corrosion behaviour after breakaway, this thesis introduces the concept of primary and secondary corrosion protection for the oxide scales formed before and after breakaway, respectively. The concept is considered to be important for the development and selection of materials to be used in applications where the breakaway event cannot be prevented, e.g. in biomass- and waste-fired boilers, as well as for the development of lifetime predictive modelling tools for corrosion. A systematic study of the secondary corrosion regime is performed by well-controlled breakdown of the primary corrosion protection of Fe-based model alloys. The resulting oxide scales are subjected to detailed microstructural investigation to study the general aspects of the secondary corrosion protection and how its properties and microstructure changes e.g. by altered alloy composition. 
 
The results show that the oxide scales formed after breakaway exhibit similar microstructural features on all the exposed FeCr(Ni/Al) model alloys and that the growth of the secondary corrosion protection is mainly diffusion-controlled. Thus, lifetime predictive tools using diffusion-based simulations, such as DICTRA, could be developed to predict corrosion both before and after breakaway. However, it is also shown that corrosive species (e.g. KCl) may affect the mechanical integrity of the oxide scale, resulting in growth processes that requires other types of models. Furthermore, the results show that the growth rate in the secondary corrosion regime may be influenced by the alloy composition, for example by adding Ni or a combination of Al/Cr. This behaviour is not directly connected to how well the primary corrosion protection withstands the exposure environment (i.e. the incubation time to breakaway). Thus, these findings indicate that research on the secondary corrosion protection has a large potential to improve the selection and development of alloys for use in corrosive environments, such as biomass- and waste-fired boilers.