Oxidation of steel T91 in flowing lead–bismuth eutectic (LBE) at 550°C

This paper reports results of exposure experiments with martensitic steel T91 for 1200, 2998 and 5016 h in the COR-RIDA loop. Owing to initial problems with the enrichment of oxygen in the circulating LBE, the conditions with respect to the oxygen activity (expressed as the activity of lead oxide, a PbO ) varied significantly. At 550 °C, oxidation is the only mode of material degradation for T91 in flowing LBE (2 m/s) when the oxygen activity corresponds to 10 -4 ≤ a PbO ≤ 10 -2 . The mean oxidation rate in the course of the first 1200 h approximates 0.13 mm/year. Lower oxygen activity in the LBE (10 -6 ≤ a Pbo ≤ 10 -4 ) may be beneficial with respect to the oxidation rate, but undershooting a certain value around the threshold for Fe 3 O 4 stability (a PbO = 10 -5.2 ) poses the risk of liquid-metal attack accompanied by high local material consumption. The composition and structure of the oxide scales on the surface of T91 are analysed and a mechanism for scale growth initiating from oxide nuclei of various compositions is proposed.