Microscopic features in the transition from external to internal oxidation in an Fe-6 mol.% Si alloy annealed under various H2O-H2 atmospheres

The occurrence of external or internal oxidation in Fe-Si alloys is strongly affected by oxidation conditions. In the present study, X-ray diffraction, Auger electron spectroscopy, X-ray photoelectron spectroscopy, secondary-ion mass spectrometry, and glow-discharge optical-emission spectrometry were used for characterizing the microscopic features of oxide layers formed on the (011) surface of an Fe-6 mol.% Si alloy. The starting materials were annealed at 1473 K under dry hydrogen gas and were subsequently annealed at 1123 K under a 75% H-2-25% N-2 atmosphere with various partial pressures of water vapor. The results show that the microscopic morphology and elemental distribution in oxide layers strongly depend on oxidation conditions. The surface was found to become rough by annealing in higher partial pressures of water vapor. This phenomenon may be induced by internal oxidation. Corresponding to the morphological changes of the surface, changes in the distribution of alloying elements have systematically been characterized in surface layers. These experimental results are discussed in conjunction with thermodynamic data on oxidation of elements.