Primary stage of Fe-Cr-C alloy oxidation at 1100°C

Purpose: To study the primary oxidation stage of Fe-Cr-C steels in more detail, to correlate the oxidation kinetics with O2 absorption, with CO2 evolution and with the morphology of the scale layers formed during the course of oxidation. Design/methodology/approach: Analysis of oxidation kinetic of Fe-Cr-C alloys containing 13 % Cr and carbon ranging from 0.15 to 1.63% at 1100°C for 3hr was carried out using a SETA¬RAM-1000 thermobalance-chromatograph. Three specimen geometries were used: cylindrical specimens, bar-like samples, wire-like samples. As a results the weight gain relations, measured rates of carbon loss, rates of oxygen absorption and morphology of the oxide scales was analysed. Findings: The oxidation rate increased with increasing carbon content in the alloys. The measured time variation of CO2 evolution showed that, during the first period of oxidation (0-40 min), a compact surface layer of FeO formed, which prohibited the free transfer of CO3 into the streaming oxidation atmosphere. The measurable CO2 evolution started between 40 and 50 min, and the most rapid evolution occurred in the interval t = 60-90 min of oxidation. This effect corresponds well to scale-layer damage and to the maximum O2 absorption. Research limitations/implications: Results enable to interpret the primary stage of the oxidation of Fe-Cr-C alloys and to discuss the relations between the cation diffusivities in the individual oxide sublayers, i.e., in wustite, magnetite, and hematite. Originality/value: More detailed description of the primary oxidation stage of Fe-Cr-C steels.