High-temperature corrosion properties of chromia- and alumina-forming alloys
Doctoral thesis, 2022

Electricity production, transportation, and manufacturing industry are some of the largest sources of greenhouse gas emissions. In many cases, these processes are carried out at high temperature and energy efficiency is limited by material degradation, so-called ‘high-temperature corrosion’. Understanding material degradation at high temperature is therefore crucial for making these processes more energy-efficient, thereby reducing greenhouse gas emissions. For an alloy to resist high-temperature corrosion, it must form a protective, slow-growing and adherent oxide scale on the metal surface. The type of oxide scale formed and how it evolves depend on the composition of the alloy and the operating conditions. Two common oxide scales formed on high-temperature alloys are chromia (Cr2O3) and alumina (Al2O3) scales. In this thesis, the formation and behaviour of these scales were studied. Focus was on two corrosion mechanisms: (i) how different scale microstructures are able to withstand the formation of volatile chromium-oxy-hydroxide; and (ii) how permeable the scales are to nitrogen. The study involved exposures of a wide variety of high-temperature alloys in environments with either high oxygen and water concentrations or high nitrogen concentration and low oxygen activity.

In environments with high concentrations of oxygen and water, chromia-forming Ni-base alloys suffered extensive volatilization of chromium-oxy-hydroxide. The resulting chromium depletion of the alloy triggered nickel oxidation which, in turn, caused a NiO layer to form on top of the chromia scale. The NiO ‘cap-layer’ reduced chromium evaporation rate, resulting in a secondary chromia scale being established at the oxide/metal interface. Cr-containing alloys forming alumina scales, showed a very limited evaporation rate of chromium-oxy-hydroxide. In the nitriding environment, the ability of the scale to prevent nitridation was studied. Gravimetric and GD-OES analyses showed that the presence of a chromia scale decreased the nitridation by 50-95%. A beneficial effect was observed for a two-layered scale that contained both chromia and silica, as this gave a nitridation reduction at the higher limit of the interval, i.e., 83-95%. Furthermore, the study showed that alumina scales without macro defects completely block the ingress of nitrogen.

High-temperature corrosion

Alumina scale

Chromia scale




KC, Kemigården 4
Opponent: Mathias Galetz, Dechema, Germany


Tommy Sand

Chalmers, Chemistry and Chemical Engineering, Energy and Material

T. Sand, A. Rajagopal, M. Sattari, S. Bigdeli, M. Hättestrand, J.E. Svensson, M. Halvarsson, L.G. Johansson. Nitridation of austenitic alloys at 1100 °C

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.

Subject Categories

Metallurgy and Metallic Materials

Corrosion Engineering

Areas of Advance

Materials Science



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5128



KC, Kemigården 4

Opponent: Mathias Galetz, Dechema, Germany

More information

Latest update