Abstract
The high-temperature corrosion of low-alloyed steels and stainless steels in the presence of KCl(s) has been studied extensively in the last decades by several authors. The effect of KCl(s) on the initial corrosion attack has retained extra focus. However, the mechanisms behind the long-term behavior, e.g., when an oxide scale has already formed, in the presence of KCl(s) are still unclear. The aim of this study was to investigate the effect of the microstructure of a pre-formed oxide scale on low-alloyed steel (Fe–2.25Cr–1Mo) when exposed to small amounts of KCl(s). The pre-oxidation exposures were performed at different temperatures and durations in order to create oxide scales with different microstructures but with similar thicknesses. After detailed characterization, the pre-oxidized samples were exposed to 5%O2 + 20%H2O + 75%N2 (+KCl(s)) at 400 °C for 24, 48, and 168 h and analyzed with scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and focused ion beam. The microstructural investigation indicated that Cl-induced corrosion is a combination of oxide thickness and microstructure, and the breakaway mechanism in the presence of KCl(s) is diffusion-controlled as porosity changes prior to breakaway oxidation were observed.
Highlights
Renewable fuels, such as biomass and waste, are attractive replacements for fossil fuels in heat and power production in order to achieve a sustainable society
The aim of the pre-oxidations was to map out the oxidation behavior of the lowalloyed steel Fe–2.25Cr–1Mo in the temperature range of 400–700 °C to be able to design exposures in order to generate oxide scales with different microstructures with a similar thickness
Exposures performed at different temperatures were expected to generate different microstructures, i.e., the grain size and distribution of phases
Summary
Renewable fuels, such as biomass and waste, are attractive replacements for fossil fuels in heat and power production in order to achieve a sustainable society. The superheater and waterwall areas are some, among others, of the affected parts where a corrosion attack may lead to failure and costly outages. For this reason, biomass and waste-fired boilers are often operated at a lower steam temperature, resulting in decreased electrical efficiency. In order for renewable fuels to replace fossil fuels, it is necessary to both lower the material costs and increase the efficiency of the production of power, which can be done by increasing steam temperatures and/or using less expensive materials, such as low-alloyed steels. It is of great interest to gain better understanding of the corrosion mechanisms of low-alloyed steels in biomass- and waste-fired boilers
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