EFTEM and EELS analysis of the oxide layer formed on HCM12A exposed to SCW

The inner-diffusion layer interface of an HCM12A sample oxidized in 600°C supercritical water (SCW) was analyzed using EFTEM and EELS. The EFTEM analysis showed the presence of chromium-rich zones linked with the porosity within the inner layer, as well as a nanometric iron–chromium separation, which may be linked with the presence of both Fe3O4 and FeCr2O4 in this layer. The diffusion layer was characterized by large chromium-rich oxides located at the tempered martensite lath boundaries, which suggested the preferential grain boundary diffusion of oxygen and the preferential oxidation of the chromium carbides present at these boundaries. The metal grains of the diffusion layer contained nanometric chromium-rich spinel oxides. The presence of large chromium-rich oxide precipitates in the diffusion layer appears to help improve the corrosion resistance of these alloys.