Kinetics Ferritic steel Isothermal and non-isothermal oxidation Diffusion mechanism
Fecha de publicación:
Sociedad Española de Cerámica y Vidrio
Aríztegui A.,Gomez-Acebo T., Castro-Fernandez F. Steam oxidation of ferritic steels: kinetics and microstructure. Bol. Soc. Esp. Cerám. Vidrio 2000;39(3):305-311.
The ferritic 2.25Cr–1Mo steel has been subjected to isothermal and non-isothermal oxidation treatments in water steam at
several temperatures ranging from 550 to 700 °C for up to 56 days. Under isothermal conditions this steel follows a parabolic
oxidation kinetics, with an activation energy of 324 kJ mol
. This value corresponds to an apparent activation energy for the
global process, which includes both outward diffusion of Fe cations and inward diffusion of oxygen.
The oxidation products present in the oxide scales, which were characterised by X-ray diffraction and SEM, are in total agre
ement with the Fe-O phase diagram. Thus, magnetite is the most stable oxide at low temperatures and wustite starts to form
above 570 °C. Further studies of the effect of cooling rate have shown that wustite formed at 700 °C transforms into magne
tite during a slow cooling, whereas a rapid cooling inhibits this transformation to a certain extent.
For non-isothermal oxidation treatments consisting of a holding period at 550 °C followed by a single or double 4 hours
exposure at 700 °C, the oxidation process seems to occur in sequence, thus presenting an additive effect of the oxidation
treatments carried out at each temperature. This effect was observed both, for the type of oxide grown, and for the kinetics
of the process. Microscopic observations of the oxide scales formed after the various oxidation treatments revealed that the
oxide scales are constituted by sublayers of distinct microstructure and chemical composition changing from their surface to
the substrate interface.