Oxidation of ferritic–martensitic alloys T91, HCM12A and HT-9 in supercritical water Pantip Ampornrat * , Gary S. Was Nuclear Engineering and Radiological Sciences, University of Michigan, 2355 Bonisteel Blvd, Ann Arbor, MI 48109, USA Abstract The oxidation behavior of ferritic–martensitic (F–M) alloys in supercritical water (SCW) was studied in order to eval- uate the suitability of these alloys for use in the Gen IV supercritical water reactor (SCWR) concept. A series of exposure tests in SCW were performed with three F–M alloys: T91, HCM12A, and HT-9. The effect of temperature was evaluated over the range of 400–600 °C and the dissolved oxygen concentration was controlled at <10 ppb (deaerated condition), 100 and 300 ppb. The oxidation behavior was determined from weight gain measurements along with oxide structure analysis. The results indicated that the oxidation rate was strongly dependent on temperature and followed an Arrhenius behavior. Activation energies for oxidation were 172, 177, and 189 kJ/mol for HT-9, HCM12A, and T91, respectively. The time dependence of the oxidation rate followed an exponential law with time exponents 0.3–0.42. Reduction in oxidation rate was observed at intermediate values (100–300 ppb) of dissolved oxygen concentration. The oxide formed on the alloy sur- face consisted of an outer layer of porous magnetite (Fe 3 O 4 ) and an inner layer of iron chromium oxide, (Fe, Cr) 3 O 4 with spinel structure. A transition region lies beneath the inner oxide in which the metal content increases to bulk values and the oxygen content decreases to nearly zero. Iron chromium oxide, (Fe, Cr)O, with the wustite structure was observed in the transition layer at 600 °C. The relatively good agreement between the activation energies for oxidation and that for grain boundary diffusion of oxygen support an oxidation mechanism based on short circuit oxygen diffusion to the oxide–metal interface. Ó 2007 Elsevier B.V. All rights reserved. 1. Introduction The supercritical water reactor (SCWR) will operate above the critical point of water, in which 375 °C and 22.1 MPa [1,2]. Under these conditions, water is fluid with properties intermediate between those of a liquid and a gas. The properties affected include the density, hydrogen bonding, ionization product, dielectric constant, heat capacity, and the transport properties of water. These properties can have a very large impact on the corrosion properties of alloys. The density of SCW varies from around 0.1–0.8 g/cm 3 compares to the density of water as a liquid (1 g/cm 3 ) and a gas (0.001 g/cm 3 ) [3]. Since SCW can be considered as a dense gas, the density varies significantly with changes in temperature and pressure. Both the dielectric constant and the ionization product decrease dramatically when water passes into the critical region, implying that the oxidation rate in the sub-critical region should 0022-3115/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2007.05.023 * Corresponding author. Fax: +1 734 763 4540. E-mail address: pantipam@umich.edu (P. Ampornrat). Journal of Nuclear Materials 371 (2007) 1–17 www.elsevier.com/locate/jnucmat