Hydrogen Trapping Inside Metals and Metal Oxides
The anomalous behavior of hydrogen in terms of its solubility and diffusivity in metals and oxides has been the subject of repeated investigations [1–6]. The diffusion coefficients of hydrogen in metals reported in the literature have usually been determi
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Hydrogen Trapping Inside Metals and Metal Oxides
The anomalous behavior of hydrogen in terms of its solubility and diffusivity in metals and oxides has been the subject of repeated investigations [1–6]. The diffusion coefficients of hydrogen in metals reported in the literature have usually been determined under the assumption that the hydrogen concentration is governed by Fick’s law. Figure 5.1 summarizes some of the experimental data on the diffusivity of hydrogen reported in the literature [1]. It should be noted that small values of the diffusion coefficient were obtained for work-hardened samples (designated as curves 6). Figure 5.1 indicates that the diffusion coefficient is a function of other variables besides the temperature and that these neglected variables are in some way related to the work hardening experienced by the specimen. There are, therefore, some doubts about the validity of Fick’s law and the simple physical model of random motion through the electrode. Real metals are known to contain structural defects, e.g., dislocations, grain boundaries, microcracks, and internal interfaces, which can act as trap sites for hydrogen atoms. These trap sites deepen the potential well, and thus, a trapped hydrogen atom must acquire an activation energy larger than the energy for normal lattice diffusion to escape a trap site. The mean residence time of a diffusing hydrogen atom is significantly longer in a trap site than in a normal lattice site, resulting in the substantial decrease of the apparent diffusivity [7]. Darken and Smith [8] first suggested that hydrogen diffusion is impeded by lattice imperfections in cold-worked steel. They treated the diffusion process of hydrogen with trapping in a way analogous to the problem of oxygen diffusion into copper containing small amounts of silicon with which it could be trapped in the form of SiO2. McNabb and Foster [1] developed a more general modelistic formulation for the phenomenon of hydrogen diffusion with accompanying trapping at one kind of trap. Oriani [9] later reformulated the work of McNabb and Foster using the assumption of a local equilibrium between the hydrogen atoms in the normal lattice sites and trap sites. He applied this concept to obtain the trap density in steels from hydrogen charging or
S.-I. Pyun et al., Electrochemistry of Insertion Materials for Hydrogen and Lithium, Monographs in Electrochemistry, DOI 10.1007/978-3-642-29464-8_5, # Springer-Verlag Berlin Heidelberg 2012
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5 Hydrogen Trapping Inside Metals and Metal Oxides
Fig. 5.1 Literature data for the diffusion coefficient of hydrogen in iron and steels (Reprinted from McNabb and Foster [1], Copyright #1963 with permission from AIME)
t (°C)
APPARENT DIFFUSION CONSTANT D cm2/min
106
1000
500
3 3
2
10–2
200
100
25
4 5 10–3
6 7 6
10– 4 7
10– 5 8 10– 6
0.5
1.0
1.5
2.0
2.5
3.0
103/T (°K)
permeation measurements. Iino [10, 11] considered irreversible trapping when analyzing the hydrogen permeation data, and then, Leblond and Dubois [12, 13] studied the comb
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