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 a t oxygen partial pressures ranging from 10(-8) mbar up to 10(-5) mbar. Expe riments were conducted at 600-650 degrees C for heating times 5-120 min. Th e 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 SiO2 formation. After the first monolayers of oxides have grown the process becomes simpler and different oxidation kinetics accounts for the further growth. The micr ostructure of steel P91, tempered martensite along with chromium carbide pr ecipitates leads to preferential chromium-rich oxide formation along marten site laths. The results are also discussed in view of diffusion data taking into account material transport by bulk and fast diffusion paths. (C) 2000 Elsevier Science B.V. All rights reserved.