C.T.J. Low

Plasma electrolytic oxidation (PEO) for production of anodised coatings on lightweight metal (Al, Mg, Ti) alloys

Abstract The introduction of plasma electrolytic oxidation (PEO) as a surface finishing technique has enabled a range of hard, dense oxide coatings to be produced on aluminium, magnesium, titanium and other lightweight alloy substrates. As with all surface coating technologies, successful development of PEO coatings requires adequate attention to substrate pretreatment together with careful control of electrolyte conditions and process variables. The principles and applications of the PEO coating process are considered, including the fundamentals of oxide deposition, the technology involved and the typical characteristics of the coatings. Industrial applications are considered together with their coating requirements. Plasma electrolytic oxidation coating is a specialised but well developed process. Suitable control of electrolyte and process conditions can realise a novel range of coatings having technologically attractive physical and chemical properties. The development of PEO technology over the last decade has provided coatings having controlled appearance, hardness, corrosion resistance and other tribological properties across an extending range of industrial sectors. Continuing developments are concisely reviewed and the PEO process is illustrated by the characterisation of anodised coatings on an AZ91 magnesium alloy surface.

Anodising of titanium in methanesulphonic acid to form titanium dioxide nanotube arrays

Abstract Titanium foil was anodised in an aqueous solution of methanesulphonic acid at a concentration up to 9·0 mol dm−3 to form titanium dioxide nanotube arrays at 295 K. The effect of ammonium fluoride concentration (0·01–1·0 wt‐%), applied cell voltage (1–30 V dc) and anodising time (1–24 h) on the formation of nanotubes was investigated. Surface morphology of the nanotubes was imaged by electron microscopy. Ordered and uniformly distributed nanotubes were readily formed on the surface of the titanium foil. The nanotubes had an inner diameter of 20–100 nm, a wall thickness of 10–20 nm, a tube length of 200–500 nm and a closed‐cup, rounded bottom. The as formed nanotubes were weakly crystalline and could be converted to an anatase phase after annealing in air at 400°C for 1 h.

Copper deposition at segmented, reticulated vitreous carbon cathode in Hull cell

Abstract The electrodeposition of copper from an acid sulphate solution has been studied in a Hull cell fitted with four types of cathode; a carbon plate or a reticulated vitreous carbon (RVC) sheet was used either in a continuous or segmented form. The rates of mass transport to the planar plate and RVC electrodes have been compared in static and stirred electrolytes containing 50, 75 and 100 mmol dm–3 CuSO4 in 0·5 mol dm–3 Na2SO4 at pH 2 and 298 K. The cathodes were divided into 10 equal sections and current vs. potential curves were obtained for each section at a constant current up to 140 mA. The current distribution over the cathodes followed a logarithmic decay with distance along the cathode; segments nearest to the anode experienced the highest rate of copper deposition.

Electrochemistry of tin deposition from mixed sulphate and methanesulphonate electrolyte

Abstract The electrodeposition of tin, at a copper surface, from a tin sulphate (0˙014 mol dm–3) electrolyte containing methanesulphonic acid (12 ˙5 vol.-%) at 295 K has been studied. Cyclic voltammetry, using potential sweep rates of 8–128 mV s–1, at a stationary copper electrode provided information on the potential ranges for tin deposition and stripping. Linear sweep voltammetry, at a copper rotating disc electrode was used to evaluate the mass transport characteristics of the system under controlled, laminar flow conditions. The changes in the limiting current density with a Sn2+ concentration of 0˙006–0˙078 mol dm–3 and an electrode rotation rate of 200–4800 rev min–1 were quantified. Randles-Sevčik and Levich equations were used to give an averaged diffusion coefficient for Sn2+ of 5˙4 × 10–6 cm2 s–1.

The formation of nanostructured surfaces by electrochemical techniques: a range of emerging surface finishes. Part 2: examples of nanostructured surfaces by plating and anodising with their applications

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.

Influence of surfactants on electrodeposition of a Ni-nanoparticulate SiC composite coating

Abstract Nickel coatings containing well-dispersed, submicron (150–500 nm) silicon carbide particles were electrodeposited on copper from an agitated, modified Watts nickel electrolyte at 60°C using current densities of 10–100 mA cm− 2. Coumarin additions (0–5 g dm− 3) to a bath containing 10 g dm− 3 SiC at 6 A dm− 2 resulted in lower microhardness but considerably reduced abrasive wear in the composite coatings. Over 20 other surfactants including anionic, cationic, and non-ionic types were evaluated for their influence on the surface and tribological properties of the nanocomposite coatings. The surfactant levels (typically 0–3 g dm− 3) were chosen to give good particle dispersions in the bath while avoiding any obvious deposit quality problems. Coating microhardness (via nanoindentation measurement), surface coefficient of friction (COF) and abrasive wear performance of the coatings under three-body, water-based conditions were investigated. The surfactants affected the degree of silicon carbide nanosized particles incorporated into the coating, from 5 to 54 vol.-%, and altered the surface microstructure from a matte to a bright surface finish and from porous to nodular, compact coatings. The nickel-nanosized silicon carbide composite coatings showed improved resistance to abrasive wear compared to a plain nickel (PN) deposit by a factor of 2–20.

Effects of additives on microstructure and properties of electrodeposited nanocrystalline Ni–Co alloy coatings of high cobalt content

Abstract Saccharin and 2-butin-1,4-diol (BD) were investigated as electrolyte additives to electrodeposit high quality nickel-cobalt alloys (78±2 at-%Co) as a protective surface coating on steel for tribological applications. The additives facilitated controlled electrodeposition of nanocrystalline coatings. The properties of the coatings investigated included surface morphology, grain size, crystalline texture and hardness. Tribological performance against a steel counterpart was studied via a reciprocating ball-on-disc apparatus. The coating microstrain could be manipulated from tensile to compressive and texture could be modified from (100) for hexagonal close packed (hcp) structure to (0002)hcp/(111) for face centred cubic (fcc) structure. The inhibition effect of absorbed species on electrodeposited nanocrystalline coatings is explained via analysis of grain size and texture. The coating from the bath with an optimised additive content had a high hardness (500 HV) due to its reduced grain size (11±1 nm) and improved tribological properties due to a high proportion of hcp structure.

The formation of nanostructured surfaces by electrochemical techniques: a range of emerging surface finishes – Part 1: achieving nanostructured surfaces by electrochemical techniques

Abstract 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.

Composite, multilayer and three-dimensional substrate supported tin-based electrodeposits from methanesulphonic acid

Tin and tin–alloy deposits enjoy many applications in the electronics, tribology and engineering industries with potential applications as electrodes for lithium batteries and as electrocatalyst coatings. Methanesulphonic acid (MSA) has become a favoured electrolyte due to its environmental benefits and ability to offer a vehicle for many metal alloy, conductive polymer and composite coatings. A number of emergent uses require less common compositions or structures of alloy, polymer or composite deposits. This paper concisely provides diverse examples of modern tin-containing deposits from aqueous MSA, including Sn–Cu alloys having a very wide composition together with a wide range of colours (golden-yellow–dark-brown) and surface finishes, a Sn–Cu composite deposit containing ceramic, protonated titanium oxide nanotubes for batteries, a tin–copper–bismuth ternary alloy and tin deposits supported on an inert reticulated vitreous carbon or carbon felt substrate to provide a porous, three-dimensional tin surface for electrocatalysis and batteries. The importance of controlled current distribution and electrode/electrolyte movement is illustrated by the use of the rotating disc electrode, rotating cylinder electrode and rotating cylinder Hull cell.

Copper deposition and dissolution in mixed chloride–sulphate acidic electrolytes: cyclic voltammetry at static disc electrode

Abstract The electrochemistry of copper deposition and dissolution in chloride–sulphate electrolytes has been investigated using cyclic voltammetry. A wide range of chloride ion concentration (0.02 to 2.0 mol dm−3 NaCl) in 0.05 mol dm−3 CuSO4 and 0.5 mol dm−3 Na2SO4 was used to examine the electrodeposition of copper onto platinum from chloride containing solution at 295 K. Cyclic voltammetry indicated that the stable dichlorocuprous anion was involved in the deposition and dissolution of copper at a platinum disc electrode. The transition from a single, two-electron transfer in a sulphate solution to two, single electron transfers in a chloride–sulphate solution was observed and the effect of chloride ion concentration on the deposition and the stripping charge density were investigated. Cu(II)/Cu(I) was found to be a quasi-reversible couple; the degree of reversibility increased at higher chloride ion concentrations. The formal potential for the reaction Cu(II)→Cu(I) steadily increased while its exchange current and Tafel slope gradually decreased at higher concentrations of chloride ion. The participation of an adsorbed cuprous chloride film in the oxidation and reduction reactions is discussed. These findings indicate that the presence of high chloride ion levels in a dilute copper solution inhibits both the deposition and the dissolution of copper.