High-temperature oxidation of duplex stainless steels in air and mixed gas of air and CH4

TGA, isothermal and thermal-cycle tests were carried out to investigate the oxidation behaviour of duplex stainless steels at 1220 degreesC in various atmospheres. The weight gain in air is initially in parabolic relation to time. This is followed by a linear relationship for super alloy and a dramatic growth one for standard alloy that is subject to breakaway oxidation by forming nodules. The thickness of gamma-layer increases with increasing annealing time and oxygen content in the atmosphere, and it is slightly higher in standard alloy than in super alloys. A single layer of (Fe,Cr)(3)O-4 spinel is noticed in the external oxide scale after annealing for 2 h, and two layers are found after 4 h with an inner layer of the spinel and an outer layer of iron oxides. Nodules grow on the delta/gamma packets where the oxidation of austenite is accelerated and that of ferrite is suppressed. Both chains and Si-enriched scales grow inside the gamma-layer along grain and phase boundaries. (C) 2004 Elsevier Ltd. All rights reserved.