Dilatometric and electrochemical measurements for studying hydrogen transport through oxide layers Y. Oren a, * , A. Tamir b , Y. Lederman b , Z. Gavra a a Chemistry Dept., Nuclear Research Center, Negev, PO Box 9001, Beer-Sheva 84190, Israel b Chemical Engn. Dept., Ben-Gurion University, PO Box 653, Beer-Sheva 84105, Israel Received 18 October 2000; accepted 30 January 2001 Abstract Simultaneous dilatometric and electrochemical measurements are employed for studying the kinetic parameters and mechan- ism for hydrogen transport through metal oxides. As a model, CeO 2 layer on a palladium wire substrate is investigated. Hydrogen is dissolved in aqueous solutions where their acidity and the oxide layer thickness are the studied parameters. Based on both open circuit potential and length changes, it is deduced that molecular diffusion through pores and imperfections in the ceria lattice is the rate determining step and no chemical reactions are involved in the oxide bulk. The following diffusion coef®cients are calculated by using a diffusional transport model: D 1 N KOH)  1.69 £ 10 211 cm 2 s 21 D 1 N H 2 SO 4 )  1.72 £ 10 211 cm 2 s 21 . q 2001 Elsevier Science Ltd. All rights reserved. 1. Introduction The fact that, in most of the cases, absorption of hydrogen onto metals is impeded by a surface passivation layer has important implications on the rate of absorption and, there- fore, this phenomenon has received much attention. Usually, this layer is composed of an oxide or a mixture of oxides, depending on the type of the metal or the alloy used. However, in real systems it may contain also hydroxiles, oxycarbides, carbides, nitrides and metallic contaminants [1,2], depending on the temperature and the chemical envir- onment to which the system is exposed. Whether a surface layer is bene®cial or disadvantageous depends on the application of the metal±hydrogen system. In gettering and storage systems as well as with hydrogen compressors and metal±hydride batteries [3±5], where absorption and desorption of hydrogen should be as fast as possible, means are taken to eliminate surface impeding layers or reduce their thickness as much as possible. In cases where metals are used as construction materials e.g. in nuclear reactors, aviation industry etc.) which may fail due to exposure to hydrogen, they should be protected by preserving the naturally occurring surface layers or coat- ing the metals with layers composed of non-penetrable materials such as oxides, copper, or nickel [6±9], titanium carbide or diboride, silicon carbide, boron or graphite [10]. Transport of hydrogen through surface oxides was studied both theoretically and experimentally. Andritschky et al. [11] developed a diffusion limited transport model with linear boundary conditions assuming a two phase oxide layer, to calculate the time dependence of the permeation. They also found a good agreement with the results obtained with oxidized Hastalloy and Inconel 600 in which, the oxide layer mainly consists of Cr 2 O 3 . Cohen et al. [12] calculated the time required to build up a limiting hydrogen concentra- tion `nucleation induction time') on the oxide surface assuming the following steps: dissociative H 2 chemisorption on the surface, jumps of H atoms between adjacent oxide atomic layers and the transition of the atoms across the oxide±layer interface. Among the experimental techniques that have been employed in studying kinetic parameters of hydrogen trans- port through oxides, perhaps the electrochemical methods are the most popular. Thus, the dual-cell utilizing a bi-layer separating membrane was used by Pyun et al. [13] to study a TiO 2 /Pd system. The same concept was used by Yoon and Pyun [14] to investigate the NiOH) 2 /Pd couple and by Song and Pyun [15] to study the transport of hydrogen through an Journal of Physics and Chemistry of Solids 63 2002) 57±64 0022-3697/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0022-369701)00048-8 www.elsevier.com/locate/jpcs * Corresponding author. Tel.: 1972-7-6477167; fax: 1972-7- 6472960. E-mail address: yoramo@bgumail.bgu.ac.il Y. Oren).