Abstract
A combination of focussed ion beam milling and X-ray tomography has been used to evaluate the steam grown oxide on plant-exposed Grade 91 material, comparing the features observed with those on laboratory coupons, which have not been ground. A particular focus was the characterisation of any pre-existing oxides and features; and the effect these have on elemental segregation and the formation of porosity in the Fe–Cr spinel. In particular, X-ray computed tomography (XCT) has been used to characterise the defect structure within the magnetite layer of the scale. The results have shown that laboratory tests on as-received material replicate features seen in plant-exposed material, and that macroscopic crazing-like cracks extend from the surface of the oxide through the spinel layer, and that this provides a mechanism for oxide healing and densification around the cracks.
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