Initial stages of oxidation of a 9CrMoV-steel: role of segregation and martensite laths

The initial stages of oxidation of steel P91 were studied in a UHV system at oxygen partial pressures ranging from 10 −8 mbar up to 10 −5 mbar. Experiments were conducted at 600–650°C for heating times 5–120 min. The oxide scales were analyzed by means of Auger electron spectroscopy (AES), scanning Auger microscopy (SAM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The very initial stages up to growth of the first few oxide layers correspond to a complex surface situation and are influenced by Cr-nitride formation, P segregation and SiO 2 formation. After the first monolayers of oxides have grown the process becomes simpler and different oxidation kinetics accounts for the further growth. The microstructure of steel P91, tempered martensite along with chromium carbide precipitates leads to preferential chromium-rich oxide formation along martensite laths. The results are also discussed in view of diffusion data taking into account material transport by bulk and fast diffusion paths.