Hydrogen trapping and permeation in nickel thoria
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I.
INTRODUCTION
THE permeation of solids by hydrogen is an important technological phenomenon. Its presence sets the performance and safety limits on many materials and processes. Hydrogen distributes very rapidly in most metals and can have a deleterious effect on mechanical properties. Safety requirements for thermonuclear fusion reactors call for walls that allow minimal internal accumulation of the hydrogen isotope tritium. For fuel and storage cells, the need is for materials with a high storage capacity and a structure which allows hydrogen to move freely. A large body of experimental data has been accumulated for diffusion of hydrogen in metals and alloys,v,z,31Data on some metals, such as nickel, are consistent, while data on other materials, such as iron and its alloys, are dispersed. Surface reactions and internal trapping are likely to be responsible for such inconsistencies. Modulated permeation experiments have been proposed as a means of separating surface and bulk processes in a macroscopic determination of diffusivity and solubility. Apart from slow surface reaction rates, the presence of imperfections, dislocations, microvoids, and second-phase particles may all affect the transport, acting as trapping sites. A detailed theory for the analysis of diffusion of hydrogen in metals which contain uniformly dispersed traps was presented by McNabb and Foster in which, it is assumed that hydrogen is trapped as atoms. [4] Others, including Ellerbrock et aLt51 Kass,[ 6] and Allen-Booth and Hewitt[7] have modeled the accumulation of hydrogen gas in voids. Oriani, by assuming dynamic equilibrium between hydrogen in the bulk and hydrogen in traps, was able to estimate trapping densities and trapping energies for the data he collected from the literature,ts] Robertson performed permeation experiments (time lag) A.K. ALTUNOGLU, visiting research fellow at the Open University Oxford Research Unit, and N.St.J. BRAITHWAITE, Co-director of the Oxford Research Unit, are with the Oxford Research Unit, The Open University, Oxford, United Kingdom OX1 5HR. Manuscript submitted May 23, 1995. METALLURGICALAND MATERIALSTRANSACTIONSA
on nickel thoria (NITD) and estimated trapping parameters and concluded that NITD shows nonequilibrium trapping of hydrogen.t91 In this work, permeation experiments based on harmonically modulated gas flux are performed with particular attention to the effects of cold work on permeation. The materials investigated in a comparative study are nickel and NITD. A mathematical approach to such modulation experiments has been developed by Cummings and Blackburn.[111 Their model describes the propagation of flux and concentration modulations of a gaseous diffusant through a homogeneous foil under conditions of diffusion limited flow, surface limited flow (simplified two rate constant model), and internal trapping (linearized MacNabb and Foster model). Their approach was to link fluxes with concentration and pressures by a series of linear equations in which the diffusivity, solubility, surface reacti
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