Abstract
Generally, oxide scales formed on high Cr steels are multi-layered and the kinetics are strongly influenced by the alloy grain boundaries. In the present study, the oxidation behaviour of an austenite steel TP347 with different grain sizes was studied to identify the role of grain-boundaries in the oxidation process. Heat treatment in an inert gas atmosphere at 1050 °C was applied to modify the grain size of the steel TP347. The mass gain during subsequent oxidation was measured using a microbalance with a resolution of 10-5 g. The scale morphology was examined using SEM in combination with energy-dispersive X-ray spectroscopy (EDS). Oxidation of TP347 with a grain size of 4 µm at 750 °C in air follows a parabolic rate law. For a larger grain size (65 µm), complex kinetics is observed with a fast initial oxidation followed by several different parabolic oxidation stages. SEM examinations indicated that the scale formed on specimens with smaller grain size was predominantly Cr2O3, with some FeCr2O4 at localized sites. For specimens with larger grain size the main oxide is iron oxide. It can be concluded that protective Cr2O3 formation is promoted by a high density of fast grain-boundary diffusion paths which is the case for fine-grained materials.
Highlights
The formation of a protective Cr2O3 scale is required to avoid degradation by severe corrosion processes for alloys used in high temperature
The oxidation behaviour of the austenitic steel TP347, which is used in superheaters of power plants, with different grain sizes was studied to quantify the role of grain-boundaries on the oxide scale formation
On the coarse-grained specimen the oxide scale consists of an outer scale of iron oxide and an inner scale of mixed oxide phases containing Fe, Cr, Mn and Ni, similar to the oxide nodules formed on the fine-grained specimens
Summary
The formation of a protective Cr2O3 scale is required to avoid degradation by severe corrosion processes for alloys used in high temperature (up to 1000 °C). The influence of grain size of the oxide[6,7] as well as the effect of doping by rate earth elements (e.g. yttrium or cerium) on the growth kinetics of Cr2O3 have been carefully investigated and even the diffusion coefficients of chromium and oxygen in the bulk and along grain boundaries of Cr2O3 are available. It is well established[5,8,9] that Cr O scales grow by counter-current diffusion of Cr and O. The oxidation behaviour of the austenitic steel TP347, which is used in superheaters of power plants, with different grain sizes was studied to quantify the role of grain-boundaries on the oxide scale formation
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