Corrosion Behavior of Model Zirconium Alloys in Deaerated Supercritical Water at 500°C

Abstract

Several zirconium alloys with differing weight percentages of Cr, Fe, Cu, and Mo were exposed to fl owing, pure supercritical water at 500°C for up to 150 days in an effort to determine their corrosion behavior for consideration in the supercritical water reactor. The weight gains of the alloys were measured, and oxides were characterized after various times. The test results showed a wide range of corrosion behavior depending on the alloy composition and process temperature. The alloys most resistant to corrosion were those containing Cr and Fe, three of which showed protective stable oxides, low corrosion rates, and no breakaway behavior. The ZrCr, ZrCu, ZrMo, and ZrCuMo alloys all exhibited high corrosion rates and nonprotective oxides. Analysis of the oxide layer showed that the oxide consisted mostly of monoclinic zirconia (ZrO 2). The structure of the oxide-metal interface in the fi ve protective alloys exhibited characteristics that were also seen in protective oxides formed at low temperature, especially the presence of a suboxide layer and an intense (002) T peak at the interface, indicating the presence of a highly oriented tetragonal phase associated with the protective oxide. The change in corrosion kinetics from cubic to linear was directly linked to the size and density of cracks in the oxides.

Zirconium alloys for supercritical water reactor applications: Challenges and possibilities

“…A similar diffusion layer has been observed in oxides formed in zirconium alloys at low temperature, in transmission electron microscopy both by imaging and by energy dispersive X-ray fluorescence [12]. The oxygen content was measured to be about 30%, or approximately equal to the solid solubility of oxygen into Zr [13]. The calculated weight gain of the samples submitted to SCW testing when translated into an oxide layer by assuming only ZrO 2 is formed (14.8 mg/dm 2 = 1 lm) does not correspond well either to the ZrO 2 layer or to the sum of the ZrO 2 layer and the inner layer.…”

Corrosion issues in supercritical water reactor (SCWR) systems

“…Model alloys including Zr-Nb, Zr-Fe-Cr, and Zr-Cu-Mo appeared to have better corrosion resistance [30,[41][42][43][44] but still exhibit higher oxidation rate than the austenitic stainless steels.…”

Determination of the initial oxidation behavior of Zircaloy-4 by in-situ TEM

“…These methods include microbeam synchrotron diffraction [23][24][25], transmission electron microscopy (TEM) [14,[26][27][28], scanning electron microscopy (SEM) [12,14,[23][24][25]29], optical microscopy [23,30], and x-ray diffraction [25,29,30]. These techniques have provided an extensive knowledge base for the morphology and structure of the oxides formed on these alloys, including cracking behavior [23,26], crystal structure [23,25,26,29], and the start of breakaway corrosion [9,31]. A variety of oxidation environments for zirconium-based alloys have been studied as well, including oxygen [32,33], steam [7,8,34,35], water [7,9,25], water vapor [32], air [36][37][38], with all studies showing similar oxide structures and appearance, with only changes in kinetics of oxidation being noted.…”

“…These techniques have provided an extensive knowledge base for the morphology and structure of the oxides formed on these alloys, including cracking behavior [23,26], crystal structure [23,25,26,29], and the start of breakaway corrosion [9,31]. A variety of oxidation environments for zirconium-based alloys have been studied as well, including oxygen [32,33], steam [7,8,34,35], water [7,9,25], water vapor [32], air [36][37][38], with all studies showing similar oxide structures and appearance, with only changes in kinetics of oxidation being noted. The majority of these experiments, however, have been done using autoclave or reactor corroded samples over long periods of time, which has not allowed for the study of the initial steps of corrosion.…”

“…As such, while the oxide structure on corroded zirconium-based alloys is known to be composed of both of tetragonal and monoclinic ZrO 2 , the structure of the initially formed oxide is not yet known. It is known that the oxide structure formed on zirconium-based alloys is highly textured, and that this may affect the corrosion rate of the base alloy [9,23,27,39]. It is thought that the texture in the monoclinc phase is the result of stress minimization, but the effect of this texture on oxidation rate is unknown [9,39].…”