Atmospheric air oxidation of 9Cr-1Mo steel: Depth profiling of oxide layers using glow discharge optical emission spectrometry

Modified 9Cr-1Mo steel is used as a steam generator tube material of fast breeder reactors. Although air oxidation behavior of modified 9Cr-1Mo steel is qualitatively studied, the exact nature of oxide layers formed across the thickness of the film from nanometer to several microns depth is not studied systematically. In this work, the oxide scales formed on modified 9Cr-1Mo steel on air oxidation at 650 degrees C up to 200 h duration using Glow Discharge Optical Emission Spectrometer (GDOES) with depth profiling, X-ray Diffraction (XRD), Laser Raman Spectroscopy (LRS) and Scanning Electron Microscopy coupled with Energy Dispersive Spectroscopy(SEM-EDS) were investigated. The weight changes increase and did not follow the conventional parabolic kinetics. The top layer of the oxide scales after 25 and 50 h air oxidized samples comprised of MnCr2O4, Cr2O3 and Fe, Cr spinel oxides; however, the oxide scales of 100 and 200 h oxidized samples mainly consist of hematite. GDOES analysis indicated the presence of Fe rich oxides on the top surface of the oxide films in the case of 100 and 200 h oxidized samples. For 25 and 50 h samples, the oxide layer consists of manganese oxides in addition to Cr and Fe spinel oxides. All the samples consist of Fe and Cr spinel oxides at the interface of the oxide and alloy surfaces. SEM analysis indicated the formation and growth of oxides from small crystal and flakey type morphology to fully grown bigger size particles as the oxidation progresses. GDOES technique provided useful insights into the nature of oxide films through the thickness of the film. The intensity depth profiles of the oxide layers obtained by GDOES-DiP (Differential Interferometric Profiling) is complemented by other techniques such as XRD, LRS and SEM-EDS.