The trapping and transport phenomena of hydrogen in nickel
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I.
INTRODUCTION
T H E search for solutions to hydrogen embrittlement problems in crystalline materials needs a detailed analysis of behavior of these metals in hydrogen environment, especially the hydrogen solubility, diffusivity, and trapping phenomena. In general, the fairly low hydrogen embrittlement sensitivity of fcc structures under normal conditions is related to their low diffusivity and a higher hydrogen solubility compared to ferritic structures. At this point, knowledge of the factors determining the solubility and transport properties of hydrogen would seem to be important in developing an understanding of the mechanism of the embrittlement. Although there have been extensive studies 1-9 on the solubility and diffusivity of hydrogen in pure nickel by means of hydrogen permeation techniques, etc., considerable uncertainty remains. The present investigation has been conducted in attempt to gain a better understanding of the trapping and diffusion of hydrogen in nickel by using a hydrogen thermal analysis technique. This technique has been successfully applied to the studies of hydrogen trapping behavior in various matedais with bcc structure.l~
If.
T H E O R E T I C A L BACKGROUND
where x = (No - N)/No No = the amount of hydrogen in trapping site at t=O N - the amount of hydrogen in trapping site at tr A = reaction constant R = gas constant T = absolute temperature In Eq. [2], the term (1 - x) expresses the amount of hydrogen remained at trapping site and e x p ( - E , r / R T ) represents the probability of hydrogen escape from the trap site to a normal lattice site. Therein, it is assumed that the overall hydrogen evolution reaction is controlled by detrapping process from a trap site to a normal lattice site, i.e., the lattice diffusion of hydrogen is fast enough to ignore as in bcc iron. Thus, when the hydrogen charged specimen is heated with a uniform heating rate, the hydrogen evolution rate peak is formed at a certain temperature related to the trap activation energy of each trap site. On the other hand, when the activation energy for lattice diffusion (E~o) is very large relative to the trap activation energy (Ear), a diffusion controlled hydrogen evolution will be measured. Thus, the hydrogen evolution rate peak related to bulk diffusion would appear. Therefore, in hydrogen thermal analysis experiments, the character of the hydrogen evolution rate peak is determined depending on what the rate controlling step in overall evolution reaction is.
The hydrogen evolution reaction from trap sites can be described as Eq. Ill and the energy level of hydrogen around the trap site is assumed as in Figure 1. ['-H-'~trap ~
~-~trap + Hinlattice
[1]
The hydrogen evolution rate, as a thermally activated process, from trap sites is written as Eq. [2]
__dx dt
( eoq
A(I - x) exp -
RT/
[21
SUNG-MAN LEE, Graduate Student, and JA1-YOUNG LEE, Professor, arc with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, Korea
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