Paralinear Oxidation of Chromium in O2 + H2O Environment at 600–700 °C
The oxidation of chromium in dry O2 and in O2 + 10%H2O at 600 and 700 °C is studied. Scale morphology is investigated by several methods, including scanning electron microscopy (SEM) of cross sections prepared by focussed ion beam milling (FIB). In O2 + H2O at 600 and 700 °C, chromium forms a duplex scale consisting of an inner barrier oxide and a discontinuous outer oxide made up of blade-shaped crystals. Thermogravimetric (TG) measurements show that water vapour influences chromium oxidation by causing vaporization of the protective oxide, resulting in paralinear oxidation kinetics. An extension of the original treatment by Tedmon is deduced, which allows for the determination of the evaporation rate constant k s and the parabolic oxidation rate constant k d from TG data acquired during short exposures. The results show that k d is the same in dry O2 and in O2 + 10%H2O. Equivalently, the transport properties of chromia are the same in the two environments. The equilibrium constant of CrO2(OH)2 formation from chromia is reported. The activation enthalpy of the vaporization reaction is determined.
KeywordsOxidation of chromium Water vapour effect Chromia vaporization Paralinear oxidation
Swedish Foundation for Strategic Research (SSF) and High Temperature Corrosion Centre (HTC), Chalmers University of Technology, Göteborg, Sweden are acknowledged for financial support. A grant from the Knut and Alice Wallenberg Foundation for acquiring the FEG SEM instrument is gratefully acknowledged.
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