Kinetics of hydrogen absorption in alpha titanium
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I.
TITANIUM'S hydrogen absorption and desorption kinetics will determine its suitability for several applications. For example, titanium is a leading choice for long term storage of tritium, t" Also, titanium sublimation pumps are used extensively in hydrogenic vacuum environments. Titanium absorption-desorption kinetics determine the rates there and for hydrogen and tritium removal for helium cooled nuclear reactors. [2] A general description of hydrogen dissolution kinetics in alpha titanium is thus desired for predicting and correlating experimental data, and for understanding hydridization phenomenon. To these ends, a thermodynamic surface energy model is developed for correlating hydrogen absorption into alpha titanium. Experimental absorption data by Gulbransen and Andrews m and by Laser t41 for cleaned, polished titanium, show linear weight gain with time, as opposed to a root time dependency (t'/z), suggesting surface limited kinetics rather than diffusion limited absorption.tSl Further, in Gulbransen and Andrew's study, [41 the hydrogen pressures could have produced titanium hydride, Irl but no surface hydride was observed until the total amount of absorbed hydrogen exceeded the alpha phase saturation limit. Again, this is contrary to any bulk-diffusion limiting model, but is in line with surface limited kinetics as can be checked by a finite element approximation of the diffusion equation for constant hydrogen flux at the surface and the known bulk dimensions.
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INTRODUCTION
iT H2(g) ~
CHARLES C. B R O W N is with Road, Rolling Meadows, IL 60008. Professor, Department of Chemical sity, East Lansing, MI 48824-1226. Manuscript submitted January 2,
I B M Research, Tower 1, 1701 G o l f R O B E R T E. B U X B A U M is Assistant Engineering, Michigan State Univer1987.
METALLURGICAL TRANSACTIONS A
~__[ "
T
Hab
Ec
i_
I
bulk Ti
Fig. 1 - - R e j e c t e d one-dimensional e n e r g y surface. Ec = enthalpy of c h e m i s o r p t i o n = 77.4 - 31.4 | k J / m o l . H,b = heat of absorption = 48.1 k J / m o l . Hag = activation e n e r g y of d i f f u s i o n = 61.5 k J / m o l . H,b = s u r f a c e - t o - b u l k activation. H,t, = Ec + H a i f f - nab = 9 0 . 8 -31.40 kJ/mol.
MetalSurface H2( g ) ~ Hb
SURFACE E N E R G Y MODELING
A surface energy model similar to that used by Pick tTl for hydrogen absorption by Nb and Ta is described below, and sketched in Figure 1. Modifying this model, Figure 2, to include a new activation peak will be found to match experimental data for titanium. Hydrogen chemisorption involves an energy well, the depth of which must equal the heat of chemisorption, Ec in
MetalSurface
Ec
] bulk Ti
Fig. 2 - - P r o p o s e d one-dimensional energy surface. E: = enthalpy of c h e m i s o r p t i o n = 77.4 - 3 1 . 4 0 k J / m o l . Hob = heat of absorption = 4 8 . 1 k J / m o l . Hb, = b u l k - t o - s u r f a c e a c t i v a t i o n = 134.7 k J / m o l . H,b = s u r f a c e - t o - b u l k activation. H,b = E, + Hb, -- Hao = 164.0 3 1 . 4 0 kJ/mol.
VOLUME 19A, JUN
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