01/01/2021
Orientation distribution map for Fe with 9% chromium before and after exposure to 0.5% SO2 at 650°C

Orientation distribution map for Fe with 9% chromium before and after exposure to 0.5% SO2 at 650°C. The right side shows the pole figures for ferrite (Fe) and hematite (Fe2O3) according to the defined crystal orientation relationship.

Source: BAM, division Materialography, Fractography and Ageing of Engineered Materials

When used at high temperatures, almost all alloys of technological interest oxidize and degrade. The nature of the oxide layer formed, its microstructural properties, and consequently its morphology, determines how protective it is to the material under continued hot gas exposure. Chromium is added to many alloys for their use in the medium to high temperature range, which promotes the formation of chromia (Cr2O3) layers. A dense and well adhering chromia layer provides an efficient protective layer for many service environments. It is well established that the formation of such a layer and its protective properties depend on the Cr content in the alloy, operating temperature and surrounding gas atmosphere. An aspect of the formation of a protective oxide layer that has received little attention to date is the crystallographically controlled growth as a result of the crystal orientations of the alloy occurring at the surface. In this article, the orientation relationships between two Fe-Cr alloys and the forming oxide layers are investigated by electron backscatter diffraction (EBSD) in the scanning electron microscope at identical positions before and after corrosion. In order to be able to find the measurement positions again after oxidation despite the microscopically rough surface, microhardness impressions were introduced before the experiments. Subsequently, exposure took place at 650°C and under the supply of highly corrosive gas containing SO2 for only a few minutes. This study shows the initial stages of corrosion and the growth process dominated by the structural relationships between the closed packed atomic layers. As a rule, despite the occurrence of orientation relationship along the interface, misfits occur, which, depending on their size, can lead to significant stresses. Therefore, in order to form a dense and protective chromium oxide layer, the reduction of interfacial tensions via the chromium content can be an essential factor.

Elucidation of orientation relations between Fe-Cr alloys and corrosionproducts after high temperature SO2 corrosion
Christiane Stephan-Scherb, Martina Menneken, Kathrin Weber, Leonardo Agudo, Gert Nolze
published in Corrosion Science, Vol. 174, page 108809
BAM, division Materialography, Fractography and Ageing of Engineered Materials