New approach to Metal Dusting studies
Metal dusting corrosion can occur when steels and Ni-alloys are exposed to a strongly carburizing gas, such as synthesis gas, at temperatures between 400 °C and 800 °C. The corrosion product may include carbides, oxides, and graphitic carbon. Coke deposition is a key component of the process and is encouraged by the presence of metallic nanoparticles in the corrosion product. Low chromium steels are understood to corrode by a process that involves the initial formation of a layer of the metastable carbide, Fe3C, at the alloy surface. Alloys with a higher Cr content are initially protected by a surface Cr2O3-rich oxide. However, defects in the oxide can allow ingress of carbon. This leads to carburization of the alloy under the corroding surface and localized pitting corrosion.
Important techniques for studying the MD phenomenon are to apply electron microscopy techniques like Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). TEM is a powerful tool, but sample preparation for the study of alloy surfaces is demanding. MD-corroded surfaces are inhomogeneous in terms of precipitation and morphology and the most important, near-surface, regions have proven extremely difficult to prepare during earlier TEM studies. The Focused Ion Beam (FIB) technique provides the possibility to prepare samples from specific positions in a material microstructure. A Focused Ion Beam (FIB) technique has been used to "cut" out samples for analysis by Transition Electron Microscopy (TEM) and together with elemental analysis we have been able to identify new Metal-Carbon phases. The goal of this project is to understand the corrosion mechanism so that measures can be taken to avoid this catalyst degradation.
The pictures are SEM images of a metallographically prepared cross section sample. The whole surface is corroded and pits are present underneath the coke deposit and corrosion product, as indicated by arrows. Around 300 μm of the original alloy thickness has been lost due to corrosion. After exposure, the thickness of the coke and corrosion product layer was ∼100 μm .
: "The evolution and oxidation of carbides in an Alloy 601 exposed to long term high temperature corrosion conditions" J.C. Walmsley, J.Z. Albertsen, J. Friis and R.H. Mathiesen. Corrosion Science 52 (2010) 4001–4010.