A model for H-Absorption by metals
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cold-worked (cw) and 68 pct cold-worked plus aged at 500 ~ for 100 h (cw + a). Charging was conducted galvanostatically (constant current density) to simulate the manner by which the alloys are H-charged in service 6 and for relatively short charging times of several hours to allow comparison with mechanical property studies 1,2,7,8of HE. Of primary importance in this study was the quantity of H absorbed, for a given amount of H + reduced, by the cw and cw + a materials during the first several hours of charging. The electrolytic permeability cell technique provides a simple and direct measure of that quantity and allows the H-concentration at the charging surface to be determined. 9,1~However, the technique was not applicable for this study. Alloy C-276 has a quite low H-diffusivity at room temperature. 11 Even with the use of very thin foils (as thin as 16/~m) to overcome that difficulty, times on the order of 15 to 20 h have been reported for steady-state H-diffusion to be reached) l As will be seen later in this paper, large changes in surface H-concentration are likely to occur during the first several hours of galvanostatic charging. Therefore, another method had to be developed to study H-charging in Alloy C-276. The method involved three distinct steps: Step 1. The surface H-concentration was related to the electrochemically generated H-fugacity, fr~, which in turn was related to a measurable quantity, the H-overpotentiak ~. Step 2. Precise measures of H-diffusivity 1~and solubility 12were used to mathematically model nonsteadystate H-diffusion during both the first several hours of ingress or charging and also following completion of charging. Step 3. Total H-concentrations in charged samples were measured as a function of egress or outgassing time. The results of the three steps were combined to obtain precise determination of the H-distribution in cw and cw + a Alloy C-276 as a function of ingress and subsequent egress. As stated previously, although this analytical method for studying H-absorption was developed for Alloy C-276, it is quite general and can be applied to the study of H-charging for a wide range of alloy systems and charging conditions. It is particularly useful for the study of galvanostatically charged low H-diffusivity alloys.
ISSN 0360-2133/81/0211-0183500.7510 METALLURGICALTRANSACTIONSA 9 1981 AMERICAN SOCIETY FOR METALS AND THE METALLURGICALSOCIETY OF AIME
VOLUME12A, FEBRUARY 1981--183
H-Diffusivity and Solubility Modeling of H-ingress and egress requires that Hdiffusivity (D) and solubility constant ( K s ) be known precisely for each thermomechanical condition of interest. Recently both D n and K s 12 have been determined between 20 and 100 ~ for Alloy C-276 in a number of conditions. The studies were made by the electrolytic permeability cell technique, 9,1~and results for c w and c w + a Alloy C-276 at 25 ~ are listed in Table I. The H-permeability study n also established two important characteristics of H-motion in Alloy C-276: 1) No evidence for H-trapping or short circuit
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