Identification of Initial Oxide Films on 18-8 Stainless Steel

Abstract

An attempt was made to identify an oxide film formed on the surface of 18Cr-8Ni steel, heated at 500°, 800° and 1050°C in air for less than a few minutes. The structures of the oxide film were determined with the aid of an electron microscope equipped with an X-ray microanalyzer, which enables us to obtain electron micrographs, diffraction patterns and characteristic X-ray spectra from the same selected area of the film. The oxide film was composed of three kinds of common oxides through all the experimental conditions; namely, (1) oxide with a corundum structure detected from the area of a continuous thin film, which is identified to be Cr₂O₃, (2) oxide with a spinel structure corresponding to aggregates of fine particles, Fe₃O₄ and (3) oxide of electron opaque clusters in small quantities, either MnO or Mn₃O₄. From all the oxide films concerned, no spectrum of Ni could be detected. The results differ considerably from those obtained after prolonged oxidation; in the latter, several complex oxides with some metallic components of the alloy are found instead of mono-metallic oxides.