The corrosion of a Fe-15 wt.% Ce alloy in coal gasification type atmospheres at 600 to 800 °C

The corrosion of a Fe-15 wt.% Ce alloy and of the two pure metals have been studied at 600 to 800 degreesC in H-2-H2S-CO2 mixtures providing high-sulfur and low-oxygen activities that are typical of coal gasification atmospheres. The alloy corrodes more slowly at 600 to 700 degreesC than pure Fe, more rapidly than pure Ce, while at 800 degreesC it corrodes at about the same rate as pure Fe. The scaling kinetics of Fe-15Ce are irregular and generally intermediate between linear and parabolic. The scale formed on Fe-15Ce shows a multilayered structure, including an outermost layer of base metal sulfide (FeS), an intermediate complex layer composed of a mixture of compounds of the two metals, and finally an innermost region of internal attack of Ce by both oxygen and sulfur. Ce is not able to diffuse outward in the metal substrate and remains in the alloy consumption region. In the intermediate region, FeS forms a continuous network that allows the growth of an external iron sulfide layer. A Ce content of 15 wt.% is insufficient to prevent the sulfidation of the base metal. These results as well as the scale microstructure are interpreted by taking into account the limited solubility of Ce in Fe and the presence of Ce-rich intermetallic compounds in the alloy examined.