High temperature corrosion of cast heat resisting steels in CO + CO2 gas mixtures

Two commercial variants of the cast heat resistant grade HP40Nb (Fe-25Cr-35Ni, Nb modified) were exposed to CO/CO₂ gases at 982 and 1080 °C in order to simulate exposure to the carbon and oxygen potentials realised in steam reformers under normal and overheated conditions. Both alloys developed external chromium-rich oxide scales, intradendritic silica precipitates and interdendritic oxide protrusions where primary, interdendritic carbides were oxidised in situ. Surprisingly, the lower silicon content alloy developed a more continuous internal silica layer, thereby slowing external scaling. Intradendritic oxidation was fast in both alloys, and is attributed to interfacial oxygen diffusion. Both alloys underwent rapid internal carburisation, indicating that their oxide scales failed to prevent carbon access to the underlying alloys under these reaction conditions.