Breakdown of Protective Cr-Rich Oxide Scale Formed on Heat-Resistant Steels for Superheater Tubes in a Waste Power Generation Boiler

The high-temperature corrosion behavior of type 310 heat-resistant steel used for superheater tubes in waste-to-energy boilers was investigated to determine the breakdown behavior of the protective Cr-rich oxide scale in the initial stages of corrosion by conducting ash-embedded corrosion tests using combustion ash collected from an actual boiler. The corrosion tests were performed in air at a specimen temperature of 460 °C and an atmospheric temperature of 685 °C. After 24 h, the specimen surfaces were entirely covered with brown corrosion products, but there was almost no mass loss. However, the corrosion mass loss increased after 60 h. After 1–12 h, the specimen surfaces were partially covered with brown corrosion products, but in other areas, the interference color or a yellow hue remained, where no breakdown had occurred. Examination of the cross-sectional microstructures of these breakdown-free areas revealed that a protective Cr-rich oxide scale was formed on the surface after 1 h of corrosion, and sodium ferrite was distributed on the surface of the Cr-rich oxide scale. Although the breakdown of the Cr-rich oxide scale was not complete, iron chloride formation occurred at the substrate subsurface under the Cr-rich oxide scale, and the latter was partially exfoliated from the substrate in SUS310S. Cl₂ gas was also generated during the formation of sodium chromate. Therefore, despite the protective nature of the Cr-rich oxide scale, partial formation of chromate may cause chloride formation on the substrate followed by exfoliation of the scale. After scale breakdown, the Cl₂ gas caused intergranular or localized internal corrosion, and when the internal corrosion had spread to some extent, the Cr-rich oxide scale was formed again. This cycle led to the corrosion of the heat-resistant steel.