High-temperature corrosion properties of chromia- and alumina-forming alloys

Doctoral thesis, 2022

From the industrial revolution in the middle of the 18th century up to present day, the concentration of CO2 in the atmosphere has been increasing steadily and at an alarming rate. This increase is attributed to human activities and the use of fossil fuels, which have resulted in warming of the planet and increased the likelihood of extreme weather patterns, including droughts, floods and heat-waves. The largest sources of CO2 emissions are electricity and heat generation, transportation, and industrial and manufacturing processes. The emissions from these sectors need to be reduced significantly to slow climate change. This can be achieved by making industrial processes more energy-efficient and/or through the development of new technologies. Heat-resistant alloys will play a major role in processes mitigating climate change. In many cases, the limiting factor for increased efficiency and market adoption of new technologies is the harsh environment that causes degradation of the alloys used in the construction of critical components such as heat exchanger. At high temperatures, metals interact with the environment and form different compounds that can be harmful for the material. It is, therefore, important that the alloy can, through reactions with oxygen, form a barrier that separates it from the environment, a so-called oxide scale. How effective this oxide scale is at preventing further degradation depends on the composition of the alloy and the environment in which it forms. Two common oxide scales formed at high temperature on high-temperature resistant alloys are chromia- and alumina scales. In this study, two important degradation mechanisms for high-temperature alloys have been studied. The first mechanism relates to how different oxide scales are able to withstand the formation of volatile chromium-oxy-hydroxide in environments that contain oxygen and water vapour. The formation of this volatile specie from the scale is known to reduce the lifetime of the alloy and can also cause contamination of down-stream processes. The second mechanism concerns how effective different oxide scales are at preventing nitrogen from being transported from the environment, through the scale and into the metal, where it forms brittle phases with alloying elements. The gained understanding of these mechanisms will be valuable for material selection and for the design of new alloys for use in processes with reduced carbon footprints.