During the oxidation, in laboratory air, of thin foils of Fe-20Cr-5Al based alloys, voids were formed in the substrate beneath the outer protective alumina scale after times varying from 50 h at 900 8C to 10 min at 1200 8C. Once the substrate aluminium level had dropped below a critical value ( 0.5 wt%), it no longer sustained the alumina scale formation and, as a consequence, continuing oxidation resulted in the initiation and development of a Cr-rich oxide sub-layer formation. At the lower temperatures, the voids filled with chromia leading to a scallop-shaped inner layer beneath the alumina scale. In contrast, at higher temperatures, the Cr-rich sub-scale layer was continuous. If the Fe-20Cr-5Al based alloys are deposited as coatings, for example as a compliant layer onto a stronger substrate, there is a risk that other elements (such as silicon) from the substrate may diffuse through the coating and influence the subsequent oxidation behaviour of the coating. In order to simulate this, sandwiches of Fe-Cr-Al and a silicon rich substrate were fabricated and tested over a range of oxidation temperatures. It was then found that the silicon did indeed diffuse through the Fe-Cr-Al layer and change the oxidation mechanism. The voids formed under the alumina were now found to contain silicon oxide rather than chromia, but the void filling mechanism also appeared to be different. With chromia filled voids the filling commenced from the alumina scale, with the oxide growing inwards, while the silica rich regions grew outwards into the voids from the substrate. Scanning electron microscopy and EDX analysis were used to follow these changes and those in other more complex situations. Detailed mechanisms for void and chromia sub-scale formation and development will be discussed in the paper.
The deposition of carbon from carbonaceous gas mixtures onto the surface of critical components is of particular concern to the petrochemical and chemical industry. In highly carbonaceous atmospheres, a type of corrosion occurs by which the metal (Fe, Ni or Co) disintegrates into a dust of fine metal-rich particles and graphite. This is called metal dusting.In this project, low alloy 1Cr 0.5Mo ferritic steels were exposed to a 50% CO and 50% H 2 (vol.%) gas mixture at temperatures between 500 C and 650 C for times up to 100 hours. Metal dusting occurred on all the test samples. Analysis of cross-sections through the samples in a scanning electron microscope (SEM) showed the formation of a cementite layer together with a layer of cokeygraphite deposited on the surface after short times. The amount deposited on the alloy's surface increased with both time and temperature over the temperature range studied.High levels of sample coking were observed after 100 hour tests. Deposited carbon from these tests was characterised by SEM, X-ray diffraction, and (scanning) transmission electron microscopy, S(TEM). TEM of the carbon depositycoke on selected samples showed a complex convoluted network of carbon filaments with small iron-rich particles at their tips. High resolution TEM was used to study these carbon filaments in greater detail and showed that there were two completely different orientation relationships between the catalyst particles and the graphite.
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