It is shown that the macroscopic dimensional effect of the composition and properties (microhardness and corrosion resistance) of coatings obtained by induced co-deposition of the iron group metals with tungsten (the effect of the surface area of electrodeposition on the composition and properties) due to the presence of oxide-hydroxide layers in the surface layer, as well as hydrogenation, is a special case of effects of this kind; this, in turn, requires a constant volumetric current density (mA/L) during electrodeposition. It has been established, using examples of electrodeposition of Fe-W and Co-W alloys from a citrate bath, that a change in the volumetric current density at a fixed electrodeposition current density leads to a change in the potential, the current output, and in the composition of the alloy in the coating. Anodic dissolution of the modified surface layer increases the microhardness, but does not eliminate the effect of dependence of the composition and properties of coatings on the electrodeposition surface area.

By the example of electrodeposition of Co–W alloys, this work shows that observed peculiarities of induced codeposition, including the macroscopic size effect in the composition and properties of deposited layers and their nanocrystallinity, are a consequence of the fact that the deposition-inducing species (a complex of the deposition-inducing metal) has the form of a high-molecular-weight polymer. Under the conditions of (relatively) high current loading on a plating electrolyte (high volume current density), this results in involvement of water molecules in the electrochemical process, formation of oxy-hydroxide layers, hydrogenation, an increase in the alloy tungsten content as a result of the side reaction of hydrogen evolution, alkalization of near-electrode region, and polymerization of the deposition-inducing metal species. Because of the presence of macroscopic size effect (i.e., the dependences of composition and properties of deposited coatings on the electrodeposition surface area), industrial scaling up of this electrodeposition technology will require maintaining the current loading on a plating bath at a constant level, along with other parameters traditional for electrochemical materials science.

Using the example of obtaining Co-W alloys it is shown that the observed features of the induced co-deposition, including the macroscopic dimensional effect of composition and properties, as well as nanocrystallinity, are a consequence of the fact that the inducing agent (a metal-precipitator complex) is a polymer complex with a high molecular weight. Under conditions with a high current load on the electrolyte, water molecules participate in the electrode process. This leads to the formation of the surface oxide-hydroxide layers and hydrogenation, as well as to an increase in the concentration of tungsten in the alloy due to a “side” reaction of hydrogen release, alkalinization and polymerization of the inducing agent. At a large-scale transfer from laboratory tests to industrial technology, along with maintaining the usual electrochemical parameters, the presence of a macroscopic dimensional effect (dependence of composition and properties on the surface area of electrodeposition) requires to maintain a constant current load on the electrolyte (volumetric current density).

This work demonstrates that the macroscopic size effect, which consists in the influence of electrodeposition surface area on the composition and properties (microhardness and corrosion resistance) of alloy coatings prepared by induced codeposition of iron group metals with tungsten, is a particular case of this type of effects and this requires maintaining the volume current density (VCD) constant during electrodeposition. The effect is associated with the formation of surface oxy-hydroxide and hydrogenated layers. For electrodeposition of Fe–W and Co–W alloys from a citrate bath, it is established that a change in VCD at a definite value for current density of electrodeposition results in changes in the electrode potential, current efficiency, and alloy composition. Anodic dissolution of the modified surface layer allows increasing the microhardness but does not cancel the dependence of coating composition and properties on the electrode surface area.

The effect of the anode material on the rate of electrodeposition of Fe−W alloy coatings from a citrate bath is studied. Both Fe and Ni soluble anodes and Pt and graphite insoluble anodes are addressed. The effects associated with the anode material are attributed to anodic oxidation of an Fe(II)−citrate complex involved in electrodeposition. In addition to its likely oxidation at the anode, this complex catalyzes reduction of W-containing species and acts as precursor to Fe deposition; these processes unfold via the formation of corresponding intermediates, their surface coverage determining the alloy composition. X-ray photoelectron spectroscopy characterization of deposited alloys indicates that the intermediate FeOHads is oxidized by water to form surface oxides. This process can explain the previously reported macroscopic size effect, i.e., the effect of the volume current density on the microhardness of deposited alloys. By using a soluble iron anode, we achieve an unprecedentedly high rate of alloy deposition (25 μm/h at a current density of 20 mA/cm2).

The electrode processes occurring during electrodeposition of nanocrystalline Fe-W alloy coatings from a citrate bath containing iron(II) sulfate and a tungstate (pH 6.9; 80°C, graphite anode) are studied by cyclic voltammetry. The current efficiency of alloy electrodeposition is up to 30%, if the applied current density is confined to the range of 2–5 A/dm 2 . The limitation on range of applied current densities is twofold: the lower limit is dictated by the diffusion-limited current density due to the reduction of the oxidized form of iron-citrate complex that forms at the bath preparation stage as a result of oxidation of Fe(II) species in a citrate solution; while the upper limit is imposed by the occurrence of side reactions such as the hydrogen evolution reaction and/or reduction of organic components of the bath. The use of an iron anode seems to be promising in this deposition process (the current efficiency of anodic dissolution of Fe in this bath is 93 ± 2%). The deposited coatings contain ~25 at % tungsten and their microhardness (which can be up to 900 kgF/mm 2 ) depends on the volume current density. The studied system holds promise for application in mask-free localized electrodeposition.

The cause of the microscopic size effect in the microhardness of electroplated binary alloys between iron group metals and tungsten is identified by studying electrodeposition of Co-W alloys. The effect is caused by the presence of oxygen-containing impurities in electroplated alloys, the impurity content growing with increasing the volume current density, which leads to a reduction in the coating microhardness. We find that the nature of anodes used affects the properties of deposited coatings, in particular, microhardness, because a deposition-inducing metal complex, which is a complex of an iron group metal, is consumed during electrolysis not only at the cathode, but it may also undergo oxidation at the anode, which identifies the way the anode affects the coating properties.

The given study overview the results obtained for Co–W alloys electrodeposited from gluconate bath. Namely, the influence of different parameters (the concentration of the bath components, pH, temperature, cathodic current density, volume current density, hydrodynamic conditions, insoluble and soluble anodes) on the rate of electrodeposition and microhardness of the coatings has been investigated. The given research determine the conditions ensuring high deposition rate, microhardness and bath efficiency. It was shown, that the deposition rate, tungsten content and microhardness reach the maximum values at pH 6.5, regardless on the concentration of bath components. Also, it was detected the macroscopic size effect of microhardness, which is linked to the fact that for a fixed cathodic current density microhardness depends on volume current density. In addition, different anodes (Pt, graphite, W and Co–W) were investigated to reveal the anode influence on reaching the maximum values of the current efficiency and microhardness.

Conditions were determined in which an active anodic dissolution of tungsten is observed in a borongluconate electrolyte used to obtain Co–W coatings (pH ~6.5) and the nature of critical currents of transition to the passivation was found, which makes it possible to use the tungsten anode as a soluble electrode. The anodic dissolution of tungsten occurs under these conditions with a current efficiency of 90–100%, which, in contrast to the case of a graphite anode, does not lead to an additional oxidation of the electrolyte components and polymerization in solution; in combination with the decrease in the concentration of tungstate ions, this reduces the electrolyte performance. It was shown that the use of a soluble tungsten anode in obtaining nanocrystalline cobalt–tungsten coating can improve the electrolyte performance due to the rise in the current efficiency of electrodeposition and to the increase in the microhardness of the coatings in comparison with the case of an insoluble graphite anode.

Studies of boron-gluconate electrolyte for obtaining nanocrystalline Co–W coatings and its separation by molecular weights of the components is carried out by gel filtration. It is shown that boric acid (as a buffer agent) and sodium chloride (as the agent increasing conductivity) after being introduced into the electrolyte form certain gluconate complexes in the solution. The boron-gluconate complex has a larger molecular weight. Three fractions with different molecular weights including the fractions that contain Co-boron-gluconate and W-boron-gluconate complexes were obtained. It is shown that the formation of certain macromolecular complexes is a slow process, the consequence of which is the dependence of the bulk properties of the electrolyte on time.

Study of the electrochemical activity of fractions of boron-gluconate electrolyte used for the manufacturing of nanocrystalline Co–W coatings (the fractioning is performed based on the different molecular weights of the solution components) shows that the electrodeposition results in the formation of an alloy with a very low concentration of tungsten (~1% at.) only from the fractions with a high molecular weight. Electrodeposition from the low-molecular fraction that contains chloride, boron-gluconate, and wolframate ions with cobalt inclusions is not observed. The obtained results are found to be described correctly by the Podlaha–Landolt model of the induced codeposition of iron-group metals with refractory metals.

The purpose of the given research has been to specify the route of formation of Co-W coatings. To this end, the gel-chromatographic separation of gluconate electrolyte has been used, as well as the photo-colorimetric determination of the composition of the separation products, and the evaluation of the electro-chemical activity of these complexes in the pH range of 4.0 to 8.0. As a result, it has been found out that the composition of the Co-W coatings thus formed is defined by the relation of the rates of the reduction of those heterometallic complexes (with the Co-W ratio 1: 1) and Co complexes in solution, as well as the reaction of hydrogen reduction, dependent on both the electrolyte pH and hydrodynamic conditions.

The purpose of the given research has been to specify the route of formation of Co-W coatings. To this end, the gel permeation chromatographic separation of borongluconate electrolyte has been used, as well as the photocolorimentric determination of the composition of the separation products, and the evaluation of the electrochemical activity of these complexes in the pH range 4.0 to 8.0. As a result, it has been found out that the composition of the Co-W coatings thus formed is defined by the relation of the rates of the reduction of those heterometallic complexes (with the Co-W ratio 1:1) and Co complexes, as well as the reaction of hydrogen reduction, dependent on both the electrolyte pH and hydrodynamic conditions.

It is shown that the anomalous electrodeposition of Co-W coatings in a citrate electrolyte occurs as a result of the formation in the bulk of the solution of multinuclear heterometallic complexes with their structure (composition) defined by the solution and near-electrode layer pH values.

For the first time thick (~ 8 μm) Cu/Co-W multilayered coatings with individual layers ranging from 5 to 200 nm were electrodeposited from a single bath. The content of tungsten in rich-in Co-W layers was controlled by varying current densities in a citrate-borate bath. Continuous Multi-Cycle (CMC) nanoindentation technique was used to analyze mechanical properties of those deposits. Optical examination of the indented zone revealed the absence of cracks inside and outside the indentation area in the interval of the normal loads used. The hardness of Cu/Co-W multilayers varied with the bi-layer period and the electrodeposition parameters. The Cu/Co-W multilayers showed an increased hardness compared to that of Co-W coatings electrodeposited under the same conditions.

It is shown, that anomalous electrodeposition of Co-W coatings in citrate electrolyte occurs as a result of formation in a bulk solution of the multinuclear heterometallic complexes with structure defined by solution and near-electrode layer pH.