Selective oxidation behavior and anti-coking performance of 35Cr45NiNb alloy in H2-H2O atmosphere

This study investigates the selective oxidation behavior of 35Cr45NiNb alloy, a critical high-temperature material widely used in ethylene cracking furnace tubes and other petrochemical heating equipment due to its superior creep resistance and carburization resistance at temperatures above 900°C, in a H₂–H₂O atmosphere at 950°C. The morphological evolution and compositional changes of the oxide scale were systematically analyzed using multi-scale characterization techniques. The oxide scale exhibits a triple-layer structure from outer to inner: MnCr₂O₄, Cr₂O₃, and amorphous SiO₂. The distribution of amorphous SiO₂ plays a critical role in the growth of the oxide scale. During the initial oxidation stage (before 2 h), a locally continuous SiO₂ film forms on the austenite matrix surface, inhibiting further oxidation of the alloy substrate. However, the secondary M₇C₃ phase in the alloy contains no Si, preventing the formation of a continuous SiO₂ oxide film on its surface. Consequently, Cr and Mn elements continue to oxidize, forming oxide nodules. As the surface matrix elements deplete due to oxidation, the austenite matrix gradually transforms into ferrite. The critically annealed ferrite retained during subsequent cooling undergoes epitaxial growth. This epitaxial ferrite penetrates the amorphous SiO₂ oxide film, and upon exposure to the oxidizing atmosphere, forms new oxide nodules. During the simulated coking process, these mechanisms expose Fe and Ni elements in the alloy to the coking atmosphere, leading to catalytic coking. For sandblasted samples with increased specific surface area, the formation of a continuous amorphous SiO₂ film is delayed in the short term, providing additional pathways for the oxidation of Mn and Cr elements. The anti-coking performance of the oxide scales on both types of samples was compared. Experimental results confirm that the oxide scale on sandblasted samples more effectively blocks the catalytic coking reactions between transition metal elements (such as Fe and Ni) and the cracking atmosphere, providing valuable insights for optimizing surface pretreatment processes to extend the service life of cracking furnace tubes in industrial applications.