Journal of Alloys and Compounds 479 (2009) 342–347 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Corrosion resistance and microstructure characterization of rare-earth-transition metal–aluminum–magnesium alloys E.P. Banczek, L.M.C. Zarpelon, R.N. Faria, I. Costa ∗ Centro de Ciência e Tecnologia de Materiais, Instituto de Pesquisas Energéticas e Nucleares, IPEN-CNEN/SP, Av. Prof.Lineu Prestes, 2242, Cidade Universitária, 05508-900 São Paulo –SP, Brazil article info Article history: Received 5 September 2008 Received in revised form 12 December 2008 Accepted 15 December 2008 Available online 25 December 2008 Keywords: Energy storage materials Rare earth alloys and compounds Corrosion Scanning electron microscopy, SEM Electrochemical impedance spectroscopy abstract This paper reports the results of investigation carried out to evaluate the corrosion resistance and microstructure of some cast alloys represented by the general formula: La 0.7-x Pr x Mg 0.3 Al 0.3 Mn 0.4 Co 0.5 Ni 3.8 (x =0, 0.1, 0.3, 0.5, and 0.7). Scanning electron microscopy (SEM) and electrochemical methods, specifi- cally, polarization curves and electrochemical impedance spectroscopy (EIS), have been employed in this study. The effects of Pr substitution on the composition of the various phases in the alloys and their cor- rosion resistance have been studied. The electrochemical results showed that the alloy without Pr and the one with total La substitution showed the highest corrosion resistance among the studied alloys. The corrosion resistance of the alloys decreased when Pr was present in the lowest concentrations (0.1 and 0.3), but for higher Pr concentrations (0.5 and 0.7), the corrosion resistance increased. Corrosion occurred preferentially in a Mg-rich phase. © 2009 Elsevier B.V. All rights reserved. 1. Introduction In recent years, nickel–metal hydride (Ni/MH) alloys for the negative electrode of a secondary battery have been incorporat- ing many elements in addition to the basic composition of LaNi 5 with the purpose of improving the electrode performance [1–6]. In some cases, high hydrogen storage capacity, improved kinet- ics of hydrogen absorption and desorption, long cycle life, and good corrosion resistance have been achieved. Systematic stud- ies to evaluate the cyclic stability of R–Mg–Ni type alloys (R: rare earth, Ca or Y) suggested a degradation mechanism com- posed of the following stages: pulverization of the alloy particles, oxidation/corrosion, and oxidation/passivation of the alloy active components [7,8]. The effect of the La–Mg–Ni–Co type alloys com- positions on their microstructure, electrochemical properties, and hydrogen storage capacity has been thoroughly investigated by Liu et al. [9–14] and by Pan et al. [15–20]. Praseodymium has been incor- porated into the alloys together with mish metal (MM) [21–30]. Pr content varying from 0 to 0.4 at.% in MMAl 0.3 Mn 0.3 Co 0.4 Ni 4.0 alloys (MM = LaNdPr) has also been reported [31]. It has been shown that the electrochemical properties were greatly improved in an electrode alloy containing about 20 at.% Pr in the MM. Bat- tery size cells in which the (LaNdPr)Al 0.3 Mn 0.4 Co 0.8 Ni 3.5 electrode alloy contained about 17at.% Pr in the MM showed a very long ∗ Corresponding author. Tel.: +55 11 31339226; fax: +55 11 31339276. E-mail address: icosta@ipen.br (I. Costa). cycle life (1400 cycles) with reasonable rate capacity (1100mAh when discharged at 5 C) [31]. In the annealed condition, the La 0.7 Mg 0.3 Al 0.2 Mn 0.1 Co 0.75 Ni 2.45 alloy showed a maximum dis- charge capacity of 370.0 mAh/g [32]. The aim of this work is to investigate the influence of the substitution of La with Pr on the corrosion resistance of original La 0.7-x Pr x Mg 0.3 Al 0.3 Mn 0.4 Co 0.5 Ni 3.8 hydrogen storage alloys (x =0, 0.1, 0.3, 0.5, and 0.7). The corrosion resistance of these alloys has been evaluated in the medium to which they are exposed inside the battery (6.0 M KOH solution) and in a more aggressive medium (0.6M NaCl solution). Attempts have been made to correlate the corrosion behavior to the alloy microstructures. 2. Experimental 2.1. Microstructure and composition characterization The alloys used in this investigation were prepared by Less Common Metals Ltd. (UK) in 5kg batches melted in an induction heating furnace and cast in a water- cooled copper mold. The microstructures of these alloys were examined using a scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis facilities. The chemical compositions of these alloys, a general view of their as-cast microstructures, the X-ray diffraction (XRD) patterns, and a detailed phase analyses have all been reported in a previous paper [33]. In the present study, the surface of the specimens was also evaluated by SEM (+EDX) after polarization to investigate the presence of corrosive attack. 2.2. Corrosion resistance characterization The corrosion resistance of the alloys was evaluated by electrochemical methods, specifically, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves (anodic and cathodic, separately). A three-electrode set-up cell 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.12.075