Electrochimica Acta 54 (2009) 6028–6035 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Electrodeposition of rhenium–nickel alloys from aqueous solutions A. Naor a , N. Eliaz a,∗ , E. Gileadi b,1 a Biomaterials & Corrosion Laboratory, School of Mechanical Engineering & The Materials Science and Engineering Program, Tel-Aviv University, Ramat-Aviv 69978, Israel b School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel-Aviv University, Ramat-Aviv 69978, Israel article info Article history: Received 2 December 2008 Received in revised form 1 March 2009 Accepted 2 March 2009 Available online 17 March 2009 Keywords: Rhenium–nickel alloy Electrodeposition Induced codeposition abstract Rhenium–nickel alloys were deposited on copper substrates in a small three-electrode cell, under galvano- static conditions. The bath solution consisted of ammonium perrhenate, citric acid and nickel sulfamate. The effects of bath composition and deposition time were studied. The Faradaic efficiency (FE) and partial deposition current densities were calculated based on mass gain and elemental analysis using energy dis- persive spectroscopy. The surface morphology was characterized by scanning electron microscopy. The thickness of the coating was measured on metallographic cross-sections. The results are discussed with emphasis on routes to increase the Faradaic efficiency and rhenium content in the coating. A plausible mechanism for the electrodeposition of rhenium–nickel alloys is presented. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction Rhenium (Re) differs from the other refractory metals (Nb, Ta, Mo and W) in that it has a hexagonal close-packed (hcp) structure, and does not form carbides. Its structure eliminates a ductile-to- brittle transition and allows Re to remain ductile and strong from subzero to high temperatures. It has the second highest melting point of all metals (after W), 3157–3181 ◦ C [1], the fourth high- est density (after Os, Ir and Pt), 21.00–21.02gcm -3 [1], and the third highest modulus of elasticity (after Ir and Os), 461–471GPa [1]. Rhenium is hard, 2.6–7.5GPa [1], and has a low coefficient of friction. It also has one of the highest strain hardening exponents of all elements, 0.353 [2], giving it a high shear modulus, 155GPa [2], and excellent wear properties. Compared with other refrac- tory metals, Re has superior tensile strength, 1000–2500 MPa [1], and creep-rupture strength, 10 MPa for 100 h at 2200 ◦ C [2], over a wide temperature range. The attribute ranges reflect different thermal conditions and suppliers of the commercially pure metal. At elevated temperatures, Re resists attack in hydrogen and inert atmospheres. It is also resistant to hydrochloric acid and seawa- ter corrosion. While pure Re is vulnerable to oxidation in moist air above 600 ◦ C due to formation of Re 2 O 7 and its penetration into the grain boundaries, improvements in this and other properties are being sought through the development of Re alloys and the application of an oxidation-resistant top coating (e.g., Ir, Pt or Rh) [3]. ∗ Corresponding author. Tel.: +972 3 640 7384; fax: +972 3 640 7617. E-mail address: neliaz@eng.tau.ac.il (N. Eliaz). 1 ISE Fellow. The unique combination of properties of Re is useful in different applications, including aircraft, aerospace, nuclear, electrical, catal- ysis and biomedical [3]. For example, it has been considered for use in divert and attitude propulsion subsystems, e.g., as a protective coating of carbon–carbon composites, in rocket engine exhaust noz- zles, etc. [4–9]. In the aircraft industry, Re is also used as an alloying element of single crystal Ni-based superalloys for turbine blades, as a coating of face seal rotors, in air turbine starter components for gas turbine engines, as a diffusion barrier (e.g., on top of graphite [10], Ni 3 Al-based superalloy for vanes [11] or Nb-based alloy in advanced jet engines [12]). Most of the published reports to-date deal with fabrication of Re-based items by chemical vapor deposition (CVD). However, CVD is an expensive, complex and energy intensive process that results in delamination-prone coated components. Electroplating at near- room temperature using non-toxic bath chemistries may become a successful alternative and also allow for uniform Re coatings on complex shapes. Electrodeposition of Re and its alloys has recently been reviewed by Eliaz and Gileadi [3]. Rhenium belongs to a group of metals that are difficult to produce by electrolysis of their aqueous solutions, mainly because of its very low overpotential for hydrogen evolution. Its most stable form in solution is ReO 4 - , which is iso-electronic with WO 4 2- . However, unlike W, it can be electrodeposited from aqueous solutions as a pure metal, similar to Os, Ir and Pt, but the Faradaic efficiency (FE) is low and the resulting coating is brittle, as a result of absorbed hydrogen. The Pourbaix diagram of Re [13] indicates that both potential and pH must be finely controlled to maximize deposition of metallic Re and minimize hydrogen evolu- tion. In addition to electrochemical control, improvements are also expected through the inclusion of alloying elements. 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.03.003