High-Temperature Oxidation Behavior of 214Cr-1Mo Steel in Air—Part 1: Gain of Mass Kinetics and Characterization of the Oxide Scale

Abstract

The oxidation behavior of 214Cr-1Mo steel was studied at 600, 700 and 800°C for a sequence of times up to 1000 h, using cylindrical specimens of the material, similar to testpieces used in high-temperature creep testing. The various samples exposed to different degrees of oxidation were investigated by optical and electron scanning microscopy and X-ray diffractometry. The oxidation rate expressed as gain of mass per area was observed to gradually slow down with the exposure time according to an approximated parabolic behavior, at all temperature levels. Multilayer oxide formation was observed to occur involving oxides with various compositions and crystallographic structures. Three main oxide layers were detected: (a) an internal one, of an FeO⋅Fe2−x.CrxO3 spinel form, containing: molybdenum; (b) an intermediate one, of an FeO⋅Fe2O3 magnetite type; (c) an external one consisting of α Fe2O3 hematite. In some instances, there was a fourth stratum formation in local spaces left by a separation between the hematite and magnetite interfaces. Each of these layers was observed to exhibit different morphological aspects, with needlelike crystals of hematite occurring at the free external surface of the material.