Oxide Growth Characterization During Short-Time Oxidation of a Commercially Available Chromia-Forming Alloy (HR-120) in Air at 1,050 degrees C
Résumé
This study focuses on the oxidation behavior of commercially available HR120 in air at 1,050 A degrees C from 30 min to 100 h. The oxidation kinetics were first studied by thermogravimetry and isothermal exposure. The oxidation products were fully characterized using ex and in situ X-ray diffraction (XRD) and FEG-SEM observations. HR120 experienced at 1,050 A degrees C a non protective transient stage and formed a multilayered oxide scale (SiO2-Cr2O3-XCr2O4 with X = Mn and/or Fe, Ni). A series of complementary characterization methods (gold and isotopic marker experiments, photoelectrochemistry (PEC)) were implemented to elucidate the oxidation mechanism. The study identified a n-type semi-conductivity accompanied by an inward growth of the scale. Thus, assuming that diffusion in the oxide scale controlled chromia-scale growth, the oxygen vacancy was the major point defect governing the solid state transport. This result was attributed to the presence of a MnCr2O4 spinel layer at the top of chromia that strongly decreased the oxygen pressure at the interface spinel/chromia.