High-temperature oxidation of precipitation hardening steel

The oxidation of precipitation hardening (PH) steels is a rather unexplored area. In the present work an attempt is made to estimate the oxidation mechanism and the kinetics for a PH steel. For this purpose specimens of the material under examination were isothermally heated at 850, 900 and 950 degrees C for 15 h. The as-treated samples were examined with SEM and XRD, while kinetics was based on TGA results. From this examination it was deduced that a thick scale is formed on the substrate surface, which, regardless the oxidation temperature, is composed by different oxides arranged in the form of successive layers. Of course the distribution in the different layers is not uniform but the oxides of the minor elements tend to be gathered at certain layers. The layer close to the substrate is compact while the Upper layer seems more porous. Regarding kinetics, the mathematical analysis of the collected data shows that the change of the mass of the substrate per unit area vs time is described by a parabolic law. Hence oxidation is impeded as the exposure time increases. (C) 2008 Elsevier B.V. All rights reserved