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Traditionally, electroplating has involved the continuous deposition of a single layer of metal at constant current. However, electrodeposition of alternate layers can offer benefits such as reduced wear, improved corrosion resistance and higher tensile strength. The alternate layers can involve different morphology or thickness of metal, different metals or the alloy composition of layers with and without included particles. In the case of a single bath, electrocrystallisation is continuous but layers can be tailored to have different chemical composition, phase composition, morphology and microstructure. The composition of layers can also be systematically modified in a gradient fashion. The thickness of each metal layer can vary from >20 μm down to ≈1 nm; in the case of nanometre thick layers, up to 500 layers of 1 nm thick individual layers might be involved. Compact multilayer deposition from a single bath is often achieved by applying a potential waveform in the laboratory or pulsed current in industry. While multilayer electrodeposition is going through a phase of rediscovery, growth and diversification, the field can be traced back to a patent involving Cu–Ni multilayers, in 1905. Progress in multi-layered electrodeposition has made use of contemporary trends in electroplating research, including self-assembled layers, nanowire arrays and the use of deep eutectic solvents for electrolytes. The developing uses of multilayer deposits are seen to span industries as diverse as wear and corrosion resistant coatings, tool bits and heavy engineering. Speciality uses include electronic, optical and magnetic materials as well as catalytic electrode surfaces for electrochemical technology. Recommendations are made for topics which deserve further R & D. [ABSTRACT FROM AUTHOR]

The traditional importance of hard chromium electroplating in surface engineering is recognised and the key features of this well-established technology are summarised. Despite the high hardness, corrosion protection and wear resistance of chromium electrodeposits, a number of alternative coating compositions and application techniques have been developed for specific applications in tribology. Environmental challenges associated with hard chromium electroplating are highlighted and the need to develop and evaluate alternative coatings is stressed. Key examples of the alternative coatings are described, including their method of application, microstructure and tribological performance in controlled service environments. Research needs requiring rapid development are highlighted. A summary is given of the most competitive coatings and those having the potential to match the performance of hard chromium in selected applications are identified. [ABSTRACT FROM AUTHOR]

The versatility and convenience of 3D printing can be used to produce tailored metal mesh electrodes, which offer a high volumetric area and good gas release properties for applications in electrochemical technology. In this work, a titanium mesh with 20 ppi triangular pores was designed and then manufactured by 3D printing. A thin coating of platinum with strong adhesion was then applied by electrodeposition in a solution of hexachloroplatinic acid in HCl at 90°C. For this, a current of 0.1 A, corresponding to a current density of 0.92 mA cm−2 was applied for 60 min to the work piece, which was previously etched in 10% (w/v) oxalic acid at 80°C. Compact, adherent and silver-grey platinum deposits, 0.16 µm average thickness and 0.33 mg cm−2 Pt loading were obtained. The product can be used as an inert anode in nickel plating baths and as a porous electrode in organic electrosynthesis or cerium-based flow batteries. [ABSTRACT FROM AUTHOR]

Reticulated vitreous carbon (RVC) substrates were coated with a composite of PbO2 and titanate nanosheets (TiNS) by anodic electrophoretic depostion. The structure and morphological characteristics of the coating were evaluated by field emission scanning electron microscopy (FESEM) and Raman spectroscopy. The TiNS/PbO2/RVC coating contained the anatase phase and showed a well-defined, microporous morphology with hydrophilic character along the length and thickness of the RVC struts. Electrochemical and photocatalytic activity of the coatings facilitated RB-5 dye degradation as a model organic pollutant in wastewater. The electrochemical decolourisation involves the generation of hydroxyl free radicals over the TiNS/PbO2/RVC anode composite surface, whereas photocatalytic decolourisation was driven by the synergetic photocatalytic effect imparted by the photoinduced holes and free electron acceptors. The photocatalytic properties of the TiNS/PbO2 coating were achieved by calcination at 450 °C for 60 min in air which converted the titanate phase to anatase and modified its surface area. This enabled 98% electrochemical decolourisation of the RB-5 dye solution (measured by visible absorption at 597 nm) in a time of 60 min.ᅟ [ABSTRACT FROM AUTHOR]

Therapeutic drugs uploaded into conjugated conductive polymer matrices deposited on active magnesium alloys serve as controlled-dose, self-powered drug-delivery systems. Preferentially, drugs are added into polymer films in the largest amount possible, mostly to prevent long-term treatments. However, added drugs can interact with the polymer matrix affecting either the structure or the final mechanical properties of the polymer film. In this work, polypyrrole films (PPy) electrodeposited on an AZ31 Mg alloy in ibuprofen and salicylate-containing solutions are investigated in terms of their uploading capacity, surface morphology and mechanical properties. The techniques used to investigate the uploaded PPy films include cyclic voltammetry (CV), scanning electron microscopy (SEM), EDS, and depth-sensing indentation (DSI). A maximum ibuprofen concentration of 440 ± 40 μg cm was obtained in PPy films in the presence of sodium salicylate. The release fraction of ibuprofen as a function of time is fitted to Avrami's equation. The hardness and reduced modulus decreased by 54 and 40 %, respectively, when the PPy films are prepared in the presence of sodium ibuprofen compared with those prepared in sodium salicylate only, indicating a more plastic film with ibuprofen. [ABSTRACT FROM AUTHOR]

Over the last 40 years, electrically conductive polymers have become well established as important electrode materials. Polyanilines, polythiophenes and polypyrroles have received particular attention due to their ease of synthesis, chemical stability, mechanical robustness and the ability to tailor their properties. Electrochemical synthesis of these materials as films have proved to be a robust and simple way to realise surface layers with controlled thickness, electrical conductivity and ion transport. In the last decade, the biomedical compatibility of electrodeposited polymers has become recognised; in particular, polypyrroles have been studied extensively and can provide an effective route to pharmaceutical drug release. The factors controlling the electrodeposition of this polymer from practical electrolytes are considered in this review including electrolyte composition and operating conditions such as the temperature and electrode potential. Voltammetry and current-time behaviour are seen to be effective techniques for film characterisation during and after their formation. The degree of take-up and the rate of drug release depend greatly on the structure, composition and oxidation state of the polymer film. Specialised aspects are considered, including galvanic cells with a Mg anode, use of catalytic nanomotors or implantable biofuel cells for a self-powered drug delivery system and nanoporous surfaces and nanostructures. Following a survey of polymer and drug types, progress in this field is summarised and aspects requiring further research are highlighted. [ABSTRACT FROM AUTHOR]

The importance of Co-Ni-P alloy deposits is summarised and recent developments are highlighted. Electroplating and electroless deposition of nanocrystalline Co-Ni-P ternary coatings are considered. Nanostructure, physical and mechanical properties (including corrosion resistance) of various bath types and compositions (including pH and electrolyte additives) as well as plating conditions (including current density, temperature and agitation) are summarised. Applications range from wear and corrosion resistant coatings, particularly as a hard chromium replacement to speciality hydrogen evolution electrodes in water electrolysis. Following this concise review, future research needs are briefly listed. [ABSTRACT FROM AUTHOR]

In part 1 of this review, emerging practice to realise nanostructured metallic coatings by electrodeposition, anodising and electrophoresis has been considered. Conventional, aqueous electrolytes may be utilised in some cases if workpiece preparation and process conditions are well controlled. Such coatings can provide wear and corrosion resistance or a catalytic or high active area compared to more conventional coatings. An overview of the principles involved in deploying electrochemical techniques to produce nanostructured surfaces and factors influencing developments in this rapidly emerging field were considered. The strategies, which can be adopted to electrodeposit nanostructured metallic coatings, include grain refinement, application of a pulsed current, inclusion of nanoparticles into the coating and the use of nanoporous templates. In part 2, examples of nanostructured coatings and their properties are illustrated with research findings from the authors' laboratory and the literature. Nanostructured metallic coatings include nanocrystalline, functionally graded, nanocomposite and recently introduced hierarchical structures. The potential uses for these coatings in engineering industries (including tribology and energy conversion) are summarised. Finally, future developments necessary to realise and deploy the coatings in increasingly demanding environments are considered. [ABSTRACT FROM AUTHOR]

Emerging practice to realise nanostructured metallic coatings by electrodeposition, anodising and electrophoresis is considered. Conventional, aqueous electrolytes may be utilised in some cases if workpiece preparation and process conditions are well controlled. Such coatings can provide wear and corrosion resistance or a catalytic or high active area compared to more conventional coatings. An overview of the principles involved in deploying electrochemical techniques to produce nanostructured surfaces and factors influencing developments in this rapidly emerging field is given. The strategies, which can be adopted to electrodeposit nanostructured metallic coatings, include grain refinement, application of a pulsed current, inclusion of nanoparticles into the coating and the use of nanoporous templates. Part 2 will consider examples of nanostructured surfaces together with their potential applications. [ABSTRACT FROM AUTHOR]

Freely-standing and flexible films (0.075 to 9 μm in thickness) of electrically conducting polypyrrole were synthesised via anodic electrodeposition onto a stainless steel substrate from methanesulphonic acid under stirred conditions at 295 K. Cyclic voltammetry was used to study the effect of pyrrole monomer concentration (0.01 to 1.0 mol dm−3) and methanesulphonic acid level (1.0 to 6.0 mol dm−3) on the formation of polypyrrole films. The films were prepared for deposition times of 30-240 s at constant current densities of 1 to 15 mA cm−2. The ionic conductivity of freely-standing polypyrrole membranes in aqueous methanesulphonic acid was studied. Scanning electron microscopy was used to image the surface microstructure. The polypyrrole films, which were prepared in the oxidised (methanesulphonate doped), conductive state, showed an ionic area resistance as low as 10 ohm cm2. The films were readily doped with the methanesulphonate anion and the membrane ionic conductivity was dependent on the electrolyte composition used for their deposition. In the presence of anodic oxygen evolution, the films showed a 'template-free' porosity due to film growth around the bubbles. [ABSTRACT FROM AUTHOR]

The history of Ni-Co alloy deposits is summarised and recent developments are highlighted. Electrodeposition of nanocrystalline Ni-Co binary alloys and, briefly, Co-Ni-P ternary coatings is considered, including chemical composition, phase composition and crystalline nature. Nanostructure, morphology, physical and mechanical properties (including corrosion resistance) vs bath type and composition (including pH and electrolyte additives) and plating conditions, including current density, temperature and agitation are summarised. Applications range from wear and corrosion resistant coatings, particularly as a hard chromium replacement to speciality hydrogen evolution electrodes in water electrolysis. Following this extensive review, future research needs are briefly listed. [ABSTRACT FROM AUTHOR]

The importance of nanostructured deposits and methods for achieving them are considered. The scope for nickel and cobalt nanocrystalline single metal deposits is summarised and developments in the last decade are highlighted. In each case, chemical composition, phase composition and crystal structure are included. Nanostructure, physical and mechanical properties (including corrosion resistance) v. bath type and composition (including pH and electrolyte additives) together with plating conditions, including current density, temperature and agitation are summarised. While the majority of published studies have featured nickel, the more expensive cobalt has also been considered due to its better corrosion and high temperature oxidation resistance in some environments. An overall comparison between the two metals is made. The major applications of nanocrystalline nickel have been in wear and corrosion resistant coatings, particularly as a hard chromium replacement, but speciality emerging uses include catalytic electrode surfaces in water electrolysis. Future research needs are briefly listed. [ABSTRACT FROM AUTHOR]

Following a brief overview of their history, which dates back to the 1920s with marked developments during the 1960s and 1970s, the principles of composite coatings, achieved by including particles dispersed in a bath into a growing electrodeposited metal layer, are considered. The principles and role of electroplating compared to other techniques for realising such coatings, are considered. A good quality particle dispersion (often aided by a suitable type and concentration of surfactants) appropriate choice of work-piece shape/geometry and controlled agitation in the bath are seen to be prerequisites for achieving uniform coatings having a well-dispersed particle content by electroplating. Examples are provided to illustrate the influence of bath composition and plating conditions on deposit properties. Engineering applications of included particle composite layers are illustrated by examples of hard ceramic, soft ceramic and polymer inclusion composite coatings from the recent literature. Current trends in the development of composite plated coatings are summarised and their diverse applications are seen to include the use of finely structured (especially nanostructured) and functionally active particles together with hybrid and more complex, e.g. hierarchical, structures for applications ranging from tribology to speciality electronics, magnetic and electrochemical energy conversion materials. [ABSTRACT FROM AUTHOR]

Impedance spectroscopy recorded at the open circuit potential in 1M H2SO4 at 293 K is shown to provide a convenient test for detecting the degree of through porosity in electroplated nickel coatings, electrodeposited from a Watts bath, on mild steel. The impedance data can be interpreted in terms of a simple equivalent circuit, and the corrosion resistance provides a reliable guide to the quality of the deposit. The quantitative estimation of porosity from these data is considered. [ABSTRACT FROM AUTHOR]

Over the last century, electrochemical engineering has contributed significantly to societal progress by enabling development of industrial processes for manufacturing chemicals, such as chlorine and the Nylon precursor adiponitrile, as well as a wide range of metals including aluminium and zinc. In 2011, ca. 17 M tonne Cu p.a. was electro-refined to 99.99%+ purity required by electrical and electronic engineering applications, such as for electrodepositing with exquisite resolution multi-layer inter-connections in microprocessors. Surface engineering is widely practised industrially e.g. to protect steels against corrosion e.g. by electroplating nickel or using more recent novel self-healing coatings. Complex shapes of hard alloys that are difficult to machine can be fabricated by selective dissolution in electrochemical machining processes. Electric fields can be used to drive desalination of brackish water for urban supplies and irrigation by electrodialysis with ion-permeable membranes; such fields can also be used in electrokinetic soil remediation processes. Rising concerns about the consequences of CO2 emissions has led to the rapidly increasing development and deployment of renewable energy systems, the intermittency of which can be mitigated by energy storage in e.g. redox flow batteries for stationary storage and novel lithium batteries with increased specific energies for powering electric vehicles, or when economically viable, in electrolyser-fuel cells. The interface between electrochemical technology and biotechnology is also developing rapidly, with applications such as microbial fuel cells. Some of these applications are reviewed, the challenges assessed and current trends elucidated in the very active area of Chemical Engineering bordering with material science and electrochemistry. [ABSTRACT FROM AUTHOR]