Analysis on the Structure and Mechanism of Cracking and Spalling for Super304H Steel Oxide Films in High-Temperature Steam

In this paper, by means of Scanning Electronic Microscope (SEM), X-ray diffraction analyzer (XRD) and Cross hatch scanning analysis, the Structure and mechanism of cracking and spalling for Super304H steel oxide films in high temperature steam were investigated. With the oxidation proceeding, the surface of Super304H steel specimens is covered by a large amount of tiny holes, which causes the occurrence of an outer oxide layer. The Super304H oxide film generally has a double-layer structure: the outer layer mainly for iron oxide Fe₃O₄, also being covered by a small amount of Fe₂O₃, and the inner layer with the FeCr₂O₄ spinel phase as primary component. Because of the inhomogeneity of oxide distribution, large amounts of smaller gaps appear in the Super304H steel oxide films. Due to the differences among the outer layer, the inner layer and steel matrix in thermal expansion coefficient, when the steam temperature around the specimen changes periodically, the stress variation arise in the Super304H steel and oxide films, which causes the cracking and spalling of oxide films.