High temperature corrosion of low-alloyed and stainless steels: mechanistic study of chlorine-induced corrosion

Sammanfattning: The global demand on power generation is constantly increasing and so far, also its environmental impact. The environmental impact could primarily be directed to the power generation being based on fossil fuels, giving a net increase of CO2 to the atmosphere when combusted. The need of renewable fuels such as biomass and waste for power generation, leading to no net release of CO2, is therefore increasing. However, biomass and waste vary in composition and the considerably high amounts of alkali- and chlorine-containing species of these fuels result in a highly corrosive fireside environment for the metallic components of the boiler. Chlorine-induced corrosion is speculated to play an important role in the corrosion of these metallic components. However, the consequences of event leading to corrosion in the presence of chlorine is still not fully understood and the corrosion mechanism is under debate. Thus, this study aims at investigating mechanism of chlorine-induced corrosion. The study is divided into two parts; field exposures showing the extent and initiation of a chlorine-induced corrosion attack and laboratory exposures aiming at investigating the mechanism of the chlorine-induced corrosion attack. The field exposures were focused on the startup sequence of probe exposures. The results showed that the initiation of breakaway corrosion is very rapid in this environment. Thus, the primary protection, i.e. the Cr-rich oxide scale on stainless steels, was immediately destroyed and the oxides and metal chlorides formed set the boundary conditions for further corrosion, i.e. secondary protection. The results showed that the different startup sequences had only a minor effect on the initial corrosion attack. Based on the corrosion attack observed in the field-exposed samples, a set of laboratory exposures was designed. The objective was to investigate the mechanism behind chlorine diffusion through oxide scales at high temperatures. A series of pre-oxidations were performed in order to investigate the role of oxide composition, microstructure, and thickness on chlorine-induced corrosion. The investigation showed that the presence of either KCl(s) or HCl(g) accelerates the corrosion rate of all the investigated materials. Both thickness and microstructure of the Fe-rich oxide, i.e. secondary protection, influences the incubation time to breakaway corrosion. In addition, cracking and spallation of the Fe-rich oxide, as well as the presence of metal chlorides at the oxide/metal interface below a crack-free scale, were observed. Thus, the corrosion attack may be driven by both for crack formation and chlorine diffusion through the oxide scale. Mechanisms for both the influence of crack formation on the corrosion attack and alternative diffusion paths for chloride is proposed. DFT calculations showed that the diffusion of chloride ions through the oxide scale is energetically favoured to occur via oxygen vacancies.