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ATOMIC ENERGY » AECL-8702 ATOMIC ENERGY »"5Ä L'ENERGIE ATOMIQUE OFCANADA LIMITED TiBV DU CANADA LIMITÉE MECHANISMS OF HYDROGEN ABSORPTION BY ZIRCONIUM ALLOYS Mécanismes d'adsorption de l'hydrogène par les alliages de zirconium B. COX Presented at 1984 Fall Meeting of the Materials Research Society, Boston, November 26-30. 1984 Chalk River Nuclear Laboratories Laboratoires nucléaires de Chalk River Chalk River, Ontario January 1985 Janvier ATOMIC ENERGY OF CANADA LIMITED MECHANISMS OF HYDROGEN ABSORPTION BY ZIRCONIUM ALLOYS B. Cox (Presented at 1984 Fall Meeting of the Materials Research Society, Boston, November 26-30, 1984) AECL-8702 Materials Science Branch Chalk River Nuclear Laboratories Chalk River, Ontario KOJ 1J0 Canada 1985 January L'ENERGIE ATOMIQUE DU CANADA, LIMITEE Mécanismes d'adsorption de l'hydrogène par les alliages de zirconium par B. Cox (Rapport présenté à la réunion d'automne 1984 de Materials Research Society tenue à Boston du 26 au 30 novembre 1984) Résumé Ce rapport résume la façon dont nous comprenons actuellement les trois mécanismes primaires par lesquels les isotopes d'hydrogène peuvent pénétrer dans les alliages de zirconium en service. Voici ces mécanismes: (i) Réaction avec l'hydrogène et adsorption de cet élément gazeux provenant d'atmosphères n'ayant pas suffisamment d'oxydant pour que soit maintenue la pellicule d'oxyde protectrice. (ii) Diffusion dans le métal d'une fraction de l'hydrogène dégagé dans la cellule partielle cathodique en cours d'oxydation dans l'eau ou la vapeur. (iii) Diffusion des isotopes d'hydrogène au travers d'une liaison métallurgique comprenant des métaux dissemblables formant un orifice traversant la pellicule d'oxyde normalement protectrice. Bien que ces trois mécanismes puissent être spécifiés qualitative- ment avec une certaine certitude et bien que des preuves circonstanciées soient souvent disponibles, on indique dans le rapport que les constantes de vitesse des diverses étapes des réactions sont essentiellement incon- nues. Département de science des matériaux Laboratoires nucléaires de Chalk River Chalk River, Ontario, Canada KOJ 1J0 Janvier 1985 AECL-8702 ATOMIC ENERGY OF CANADA LIMITED MECHANISMS OF HYDROGEN ABSORPTION BY ZIRCONIUM ALLOYS B. Cox (Presented at 1984 Fall Meeting of the Materials Research Society, Boston, November 26-30, 1984) ABSTRACT This paper summarizes our present understanding of the three primary mechanisms by which hydrogen isotopes can enter zirconium alloys in service. These are: (i) Reaction with and adsorption of hydrogen gas from atmospheres containing insufficient oxidant to maintain the protective oxide film. (ii) Diffusion into the metal of a fraction o.l: the hydrogen released in the cathodic partial cell during oxidation in water or steam. (iii) Diffusion of hydrogen isotopes through a metallurgical bond with a dissi- milar metal which provides a window through the normally protective oxide film. It is shown that while all three mechanisms can be specified qualita- tively with some certainty, and while supporting circumstantial evidence is often available, the basic rate constants for the various steps in the reactions are largely unknown. AECL-8702 Materials Science Branch Chalk River Nuclear Laboratories Chalk River, Ontario KOJ 1J0 Canada 1985 January -1- 1 . INTRODUCTION Delayed hydride cracking (DHC) of Zr-2.5 wt% Nb alloy pressure tubes was first observed in CANDU reactors in 1974 (Pickering-3) and 1975 (Pickering-4), and again in 1982 (Bruce-2) [1]. In the first two of these incidents, hydrogen absorption - as opposed to hydrogen in the tubes as installed - played no part in the process. Nevertheless, some unexpected hydrogen ingress into the tube ends was observed. The sudden failure of a Zircaloy-2 pressure tube in Pickering-2 in August 1983 [2], although crack propagation again was essentially by delayed hydride cracking until the critical crack length was obtained, was significantly influenced by hydrogen absorption after the installation of the tube. Thus, our earlier interests in hydrogen absorption mechanisms, which had been dormant for many years, were revived, not least by the large differences in hydrogen uptake behaviour shown by different zirconium alloys [3]. Zirconium alloys are normally protected against hydrogen ingress by the surface oxide film, which presents a good barrier both to ingress, and to the egress of hydrogen already in the metal. However, laboratory work has shown that, under conditions of straining at a notch, this oxide film offers little protection, and zirconium alloys are susceptible to rapid cracking (up to 10"^ m/s) in hydrogen gas [4,5]. These experiments revealed only small differences in the crack velocities of the same alloys which show surprisingly large differences in crack velocity under DHC conditions with only internal hydrogen [6]. As a result of this revived interest in hydrogen absorption, as a critical first step in any hydride cracking processes at elevated temperatures, earlier work on hydrogen absorption (some previously unpublished) has been reviewed, and new experimental programmes have been started. There are three fundamentally different processes by which hydrogen isotopes can enter a fabricated zirconium alloy component. The first, and most trivial mechanistic- ally, is by diffusion into the zirconium alloy via a direct metallurgical contact with another metal having a high diffusivlty for hydrogen, and a higher fugacity of hydrogen than the zirconium. The other two ingress routes operate through the surface oxide film. The first of these occurs when zirconium is exposed in a hydrogen atmosphere containing insufficient oxidizing species to maintain the protective nature of the oxide, while the second occurs as part of the normal oxidation process in aqueous media. These three processes will be dealt with individually. 2. ABSORPTION OF HYDROGEN GAS All zirconium components, unless held at high temperature in a good vacuum, carry a ZrÛ2 film on their surfaces. In air at room temperature, the thickness of this oxide is limited by electron tunneling to 3-5 nm. Thickening of this oxide can proceed by a variety of processes. By increasing the electric field across the oxide, further oxygen ion transport through the air-formed film can be initiated either at room temperature or above. This same process will proceed by thermal activation if the temperature is raised in the presence of an oxidant. Such an oxide film is normally a very good barrier against reaction with hydrogen gas and, provided sufficient oxidant is present in the environment, -2- may remain so indefinitely. "Sufficient" in this context was established by Shannon [7] to be that required to maintain the normal oxidation rate at the temperature concerned (Figure 1); the "normal" oxidation rate being that obtained at environmental pressures sufficiently high for the pressure dependence of the oxidation rate to approach zero [8]. These observations led to the postulate that, for any conditions of temperature, pressure and oxidant, there should be a critical hydrogen/oxidant ratio above which the oxide film would remain protective, and gross reaction with hydrogen would be prevented [9]. There could still be a slow absorption of hydrogen by reaction with the oxidant if this were a hydrogen containing molecule. Several investigators [9,10] have established these critical ratios, for a limited range of environments, and they are generally observed to be in the range ÎO^-IO^ (Table 1). Thus, very small concentrations of oxidizing species in hydrogen gas are sufficient to prevent direct surface reaction with hydrogen at reactor operating temperatures. This effect was borne out in experiments on cracking in hydrogen gas where small oxygen additions were suffi- cient to inhibit crack growth [5]. However, it is notable (Table 1) that more oxygen appeared to be necessary to ensure passivation of strained metal at the crack tip, than appears to be the case for reaction on a smooth surface. If insufficient oxidant is present to maintain the oxide film in good repair, then, after a very variable incubation time required for "breakdown" of the surface oxide film [10-13], a rapid direct reaction with hydrogen gas ensues (Figure 2). The progress of the oxide "breakdown" process can be monitored, during the incubation period, by measuring the electrical resistivity (Figure 3) of the oxide film [7]. The resistivity of the oxide declines rapidly with time, as a result of the increasing hypostoichiometry of the oxide, and hydriding commences once the resistivity declines below a critical level. The hypostoichi- ometry changes because the rate of dissolution of oxygen atoms from the oxide into the metal now exceeds the rate at which replacement oxygen atoms can be acquired from the environment. In addition to the increasing electronic conduc- tivity associated with this process, an increase in the number of anion vacancies will result. As a result of these observations, investigators studying the rate of reaction uf hydrogen with oxidized zirconium surfaces have been tempted to postulate that hydrogen is migrating via these anion vacancies as interstitial H2 [14,15]. From their experiments they have then calculated dif fusivitief; for hydrogen in the Zr(>2 lattice. However, there is no unambiguous evidence from these experiments to support an argument that hydrogen is migrating via the ZrÛ2 lattice at all. The diffusivities which they calculate are very high for such a lattice diffusion process, and lie in a range more typical of surface
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