Void formation and filling under alumina scales formed on Fe-20Cr-5Al based alloys and coatings, oxidised at temperatures up to 1200℃

摘要

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℃ to 10 min at 1200℃. 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.