Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat A review of electrodeposited Ni-Co alloy and composite coatings: Microstructure, properties and applications Abdossalam Karimzadeh a , Mahmood Aliofkhazraei a, ⁎ , Frank C. Walsh b a Department of Materials Engineering, Tarbiat Modares University, P.O. Box 14115-143, Tehran, Iran b Electrochemical Engineering Laboratory, National Centre for Advanced Tribology, Faculty of Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, UK ARTICLE INFO Keywords: Cobalt Corrosion Electrodeposition Microstructure Nickel Tribology ABSTRACT NieCo alloy electrodeposits have been widely employed in industry due to their good corrosion and wear re- sistance, high mechanical strength, moderate thermal conductivity and outstanding electrocatalytic and mag- netic properties. This review aims to provide an insight into the mechanism of electrodeposition and efect of operational parameters and deposit microstructure, together with the mechanical, electrochemical and tribo- logical characteristics of NieCo alloys and included particle, composite deposits. Potential applications of the coatings have also been considered in applications as diverse as additive manufacturing, micro-tools, micro- sensors, electronic imaging and electrochemical energy conversion. 1. Introduction Most failures in metallic components originate from the interaction of their surface with the environment. Problems can include wear, abrasion, fatigue, fretting, corrosion, oxidation, stress corrosion cracking (SCC). The practical use of engineering components is severely limited by these failures. The surface of the component plays an es- sential role and can restrict its uses. Appropriate attention to surface engineering can help to transform the microstructure, chemical com- position and the resultant properties of materials. Coating is an efective method of improving the service life of the components [1]. Electro- deposited NieCo alloys and nanocomposite coatings provide a good example of such coatings. NieCo alloys possess great practical im- portance due to their protective and decorative properties. Among the production methods of NieCo coatings, electrodeposition is commonly used because of its moderate costs, fexibility (single layer or multilayer deposition as intermittent or gradient), high efciency and simple mass production procedure with little need for high temperatures and high pressures, compared to other production processes, such as chemical vapour deposition (CVD), sputtering and fame spraying [2–4]. While the electrodeposition of Co or NieCo alloys is more expensive than Ni, due to the higher price of Co and its salts, signifcant benefts can be achieved due to the improved engineering properties of the alloy deposit. The electrodeposition of Ni and Co single metals has been considered in classical texts and reviews. The benefts of electrodepositing nickel and cobalt single metals, their binary alloys and ternary composites in a nanostructured form was highlighted in a series of three reviews [4A–4C] [5–7]. The often superior corrosion and tribological characteristics of CoeNi compared to Ni are accompanied by higher costs; the specifc cost of cobalt metal is often more than twice that of nickel. Many coating techniques are available, such as chemical vapour deposition, CVD, high velocity oxygen fuel (HVOF) fame spraying, plasma spraying and laser cladding. The advantages of electroplating include versatility, an acceptable rate of deposition and moderate costs together with the ability to control the thickness and structure of the deposits, even on complex shapes. In the case of NieCo alloy electro- deposits, A review [6] considered that there was a need for continued developments in several areas: 1. A clarifcation of the role of triboflms and wear debris on the tri- bological behavior of NieCo deposits. 2. Developing techniques, such as compositionally graded coatings and application of pulsed current have limitations including complex equipment and high cost. 3. The deposit microhardness needs to be increased further and direct nanostructured alloy deposition needs to be achieved by optimizing bath additives and plating conditions. 4. Despite many studies on coarse-grained deposits, there have been relatively few investigations of nanocrystalline NieCo coatings. https://doi.org/10.1016/j.surfcoat.2019.04.079 Received 5 March 2019; Received in revised form 20 April 2019; Accepted 25 April 2019 ⁎ Corresponding author. E-mail addresses: khazraei@modares.ac.ir, maliofkh@gmail.com (M. Aliofkhazraei), f.c.walsh@soton.ac.uk (F.C. Walsh). Surface & Coatings Technology 372 (2019) 463–498 Available online 29 April 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved. T