Journal of Alloys and Compounds 383 (2004) 213–218 Effects of cathodic charging on hydrogen permeation in a Pd 80 Rh 20 alloy Dilson S. dos Santos a,∗ , S. Miraglia b , D. Fruchart b a PEMM-COPPE/UFRJ, CP 68505, 21945-970 Rio de Janeiro, Brazil b Laboratoire de Cristallographie—CNRS BP 166, Grenoble Cedex 09, France Abstract Samples of Pd 80 Rh 20 alloy were submitted to electrochemical hydrogen permeation tests at 40 ◦ C, using a 0.1N NaOH electrolyte, to study hydrogen permeation in a Pd 80 Rh 20 alloy for different cathodic hydrogen generating currents varying from 0.1 to 20 mA. It was found that the apparent hydrogen diffusivity and the hydrogen flux increased with increasing cathodic charging. For high levels of applied current, equal to or above 5 mA the hydrogen permeation curves present a double sigmoidal shape which corresponds to hydride formation during the test. The hydrides were characterized by X-ray diffraction after exposure of the sample to hydrogen. Phase separation was also observed in the alloy, having been provoked, for high applied currents, by the formation of a PdRhH hydride and a Rh-rich phase. © 2004 Elsevier B.V. All rights reserved. Keywords: Hydrogen diffusion; Palladium; Hydride; Phase separation 1. Introduction Palladium is a metal very frequently used as an academic tool, due to its special properties regarding hydrogen. But it is also of industrial interest when conveniently alloyed, such as applications in filters for hydrogen, separating hydrogen from other gases and the production of high purity hydro- gen, or as catalysts for hydrogenation, dehydrogenation and oxidation in the petrochemical industries, and also as au- toexhaust catalytic converters in the automobile industry [1]. The Pd–Rh system has been shown experimentally to have a miscibility gap [2,3]. The metastable disordered fcc Pd–Rh alloys are stable from ambient to relatively high tempera- tures and the phase separation did not occur either at high temperature and long time of heat treatment, for example, after 6 months at 873 K for Pd 0.74 Rh 0.26 [2]. However, in the presence of high hydrogen pressure the phase separation takes place in seconds as observed by Fukai and Okuma [4]. This can be attributed to the hydrogen-induced lattice mobility at high temperature [5], or a generation of super- abundant vacancies by high hydrogen pressure [4,6]. Flanagan et al. [7], using PCT experiments, have observed hydride formation in the Pd–Rh system and also that phase separation may occur in this alloy system. ∗ Corresponding author. E-mail address: dilson@metalmat.ufrj.br (D.S. dos Santos). Hydrogen permeation curves can be successfully applied to the study of hydrogen interaction kinetics in alloys and also the interaction of hydrogen with the microstructure and the possibility of hydride formation. The aim of this work is to study, by means of hydrogen permeation curves, hy- drogen diffusivity and the occurrence of hydride formation during permeation testing in Pd 80 Rh 20 , for different hydro- gen cathodic charging conditions. 2. Experimental techniques The Pd 80 Rh 20 alloy was melted in an arc furnace using a non-consumable electrode. The alloy was re-melted sev- eral times to homogenize the composition of the alloy. Af- ter melting, the alloy was cold-worked to obtain a thickness of 150 m. The samples were metallographically polished and then, submitted to electrochemical hydrogen permeation tests at 40 ◦ C, using a solution of 0.1N NaOH as electrolyte. These tests were undertaken using an electrochemical cell comprising two-compartments, separated by the sample un- der examination [8]. Nitrogen bubbling of the electrolyte was performed before, and maintained during, the test in both compartments to minimize the level of dissolved oxy- gen in the solution, thereby guaranteeing more precise mea- surements of current. In one of the compartments, the hydro- gen generation side, a constant cathodic current was applied 0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.04.063