Analysis of surface oxides on an Fe-12Cr alloy by reduction in a hydrogen flow

Abstract

I have previously used a temperature-programmed reduction in hydrogen to identify surface oxides on nickel base alloys [1]. The reduction onset temperature of oxides depends on the cation-oxygen bond strength [2-6]. Sufficiently large differences in the reduction onset temperatures make identification of oxides possible [7-9]. The amount of water vapour produced as a result of the reduction is directly proportional to the amount of the oxide reduced. The high sensitivity of the thermal conductivity detectors (TCDs) used in gas chromatography allows detection of the minute amounts of water vapour evolved during the reduction of surface oxide on samples with small surface area [1]. The quantity of the oxides is determined by measuring the area under the peak of the detector signal curve, which is directly proportional to the water vapour content. Thin films, consisting of iron oxide and chromium oxide mixtures, their solid solutions or iron-chromium spinel form during oxidation of Fe-Cr alloys below 500°C [10-12]. In this work surface oxides were identified on an Fe-12Cr alloy oxidized in air at 400 and 500 ° C, the composition of the oxide films were calculated and the oxidation rates of the alloy components were compared. Samples of a commercial (Middelburg Steel and Alloys, South Africa) 3CR12 alloy (11.5% Cr, 0.6% Ni, 0.25% Ti, 0.4% Si, 0.9% Mn, 0.02% C and 0.015% N) were oxidized at 400 and 500°C in air. The 10 mmx 10 mm x 7 mm samples were polished before oxidation with 1/~m diamond paste. In the experiments the following were used: 99.9% pure electrolytic iron (Metallurg Co., South Africa), 99.5% pure electrolytic 2 to 3 mm chromium particles (BDH Chemical Co., UK), 99% pure ferric oxide (Cerac Co., USA), 410L water-atomized stainless steel (0.071% C, 0.86% Si, 0.49% Mn, 0.024% S, 12.3% Cr and 0.012% O; Glidden Metals, USA), and 99.99% pure hydrogen, containing < lp.p.m, oxygen and having a dew point below - 72 ° C. The analysis technique was described in detail in [1]. A specimen was placed in a quartz reactor with a hydrogen stream of 4dm3h ~ The reactor temperature increased at a rate of 15°Cmin ~. A