Differences in reactivity of oxide growth during the oxidation of Zircaloy-4 in water vapour before and after the kinetic transition

Abstract

International audienceThe oxidation of Zircaloy-4 by water vapour has been studied between 500 and 550 °C, the water vapour partial pressure ranging in 13-80 hPa, using isothermal and isobaric thermogravimetry, and calorimetry. During gravimetry experiments, sudden changes in temperature or water vapour pressure have also been performed. It results that the approximations of steady state and rate-limiting step are only valid before the kinetic transition. In the post-transition region, a significant influence of water vapour and hydrogen partial pressures has been found, contrarily to the kinetic behaviour before the transition (which is in this last case, in good agreement with a rate-limiting step of diffusion of oxygen vacancies). It comes out that the post-transition kinetic behaviour is definitely not the same as before the transition

Oxidation kinetics of ZrNbO in steam: Differences between the pre- and post-transition stages

“…In a previous work dedicated to the oxidation of Zircaloy-4 by water vapour [7], we have shown that great differences exist between the pre and post-transition stages, from the kinetic point of view: before the transition, the oxidation rate is controlled by the diffusion of oxygen vacancies in the oxide layer, as usually suggested in the literature data, whereas in the post-transition stage, the assumption of the rate-limiting step is no more valid.…”

“…(1) can be applied. This method, that we have named the "ΦE test", has been successfully used in previous works [7,[17][18][19][20][21][22]. The results are indicated on Fig.…”

“…The variations of Φ with a gas partial pressure P can easily be obtained by performing sudden changes, from P 0 to P, at a given thickness X, the ratio of the rates being then equal to ( ) ( ) 0 P P Φ Φ [7,17,19,21]. The experiments were carried out at 500°C, with 10 hPa in hydrogen and a water vapour pressure varying from 13 hPa (P 0 ) to 80 hPa after the jump.…”

“…Finally, due to the lack of consistent data on the effect of water vapour on the oxidation rate before and after the transition, and in order to confirm (or not) these interpretations, we have studied the oxidation of a Nb-containing zirconium alloy, ZrNbO alloy, at 530°C, following the same methodology as in the case of Zircaloy-4 [7]. This approach is principally based on the verification of kinetic assumptions that are usually made a priori.…”

“…It is well known that after a pseudo-parabolic first period [1-3], a kinetic transition occurs when the thickness of the oxide layer exceeds a critical value, which corresponds to an increase in the oxidation rate, associated with the appearance of cracks and pores in the oxide layer [2,4,5]. In the post-transition stage, the oxidation curves are considered to be either linear [4] or to result from cyclic periods of increasing and decreasing rate [6].In a previous work dedicated to the oxidation of Zircaloy-4 by water vapour [7], we have shown that great differences exist between the pre and post-transition stages, from the kinetic point of view: before the transition, the oxidation rate is controlled by the diffusion of oxygen vacancies in the oxide layer, as usually suggested in the literature data, whereas in the post-transition stage, the assumption of the rate-limiting step is no more valid.As far as the oxidation of Nb-containing alloys is concerned, the shape of the kinetic curves (weight gain versus time) is qualitatively similar to that obtained with zircaloys. Nevertheless, Nb-containing alloys are oxidised faster than recrystallised Zircaloy in oxygen and in water vapour [4,[8][9][10][11][12], at the beginning of the oxidation, but the kinetic transition is not so sharp in Nb-containing alloys and it occurs after a longer time.…”