In situ high temperature oxidation analysis of Zircaloy-4 using acoustic emission coupled with thermogravimetry

Abstract

h i g h l i g h t sThermogravimetry associated to acoustic emission (AE) improves knowledge on the corrosion of metals at high temperature. Kinetic transition is detected under air oxidation tests at 900°C of Zircaloy-4 by a change in the rate of mass gain and by the AE activity. AE analysis is complementary to characterizations of post mortem oxidized samples. AE allows us to distinguish the cracks which occur during the Zircaloy-4 oxidation from the cracks which arise during the cooling of the samples. a b s t r a c tZircaloy-4 oxidation behavior at high temperature (900°C), which can be reached in case of severe accidental situations in nuclear pressurised water reactor, was studied using acoustic emission analysis coupled with thermogravimetry. Two different atmospheres were used to study the oxidation of Zircaloy-4: (a) helium and pure oxygen, (b) helium and oxygen combined with slight addition of air. The experiments with 20% of oxygen confirm the dependence on oxygen anions diffusion in the oxide scale. Under a mixture of oxygen and air in helium, an acceleration of the corrosion was observed due to the detrimental effect of nitrogen. The kinetic rate increased significantly after a kinetic transition (breakaway). This acceleration was accompanied by an acoustic emission activity. Most of the acoustic emission bursts were recorded after the kinetic transition (post-transition) or during the cooling of the sample. The characteristic features of the acoustic emission signals appear to be correlated with the different populations of cracks and their occurrence in the ZrO 2 layer or in the a-Zr(O) layer. Acoustic events were recorded during the isothermal dwell time at high temperature under air. They were associated with large cracks in the zirconia porous layer. Acoustic events were also recorded during cooling after oxidation tests both under air or oxygen. For the latter, cracks were observed in the oxygen enriched zirconium metal phase and not in the dense zirconia layer after 5 h of oxidation.

In-situ digital image correlation for fracture analysis of oxides formed on zirconium alloys

“…Research has indicated this acceleration is associated with the formation of large cracks perpendicular to the metal-oxide interface, and the formation of a defective, heavily-fractured oxide layer [21,24,25]. Although in this study oxides are formed at a much lower temperature, the results could still be of value to research carried out at high temperatures.…”

“…In order to understand oxidation during loss of coolant accidents (LOCA) many authors have researched oxidation of zirconium alloys in the 600-1200°C temperature range [21][22][23][24][25].…”

Peridynamic simulations of the tetragonal to monoclinic phase transformation in zirconium dioxide

“…In oxides formed on zirconium alloys cracks are known to form parallel and close to the metal oxide interface (N. P Platt et al, 2015;P. Platt et al, 2015a), and perpendicular to the metal-oxide interface at higher temperatures (Omar et al, 2015;Parry et al, 2013). In both cases these cracks are linked with a significant reduction in the protective capacity of the oxide layer.…”

Acoustic emission monitoring and damage mode discrimination of APS thermal barrier coatings under high temperature CMAS corrosion

“…To avoid mechanical and electromagnetic noise, the low-and high-filter frequencies were set as 0.1 MHz and 1 MHz, respectively. As the conventional piezoelectric transducer can only be used at ambient temperature, a waveguide with the good transmission of acoustic signals (low attenuation of the signal energy) was designed using Nichrome wire to resist the chemical corrosion (oxidative or reductive atmospheres) [26]. This waveguide connects the specimen and transducer perfectly, consequently avoiding the transducer locating at high temperatures.…”