Microstructural characterization and corrosion resistance of Ni–Zn–P alloys electrolessly deposited from a sulphate bath

Electroless Ni–Zn–P alloy coatings were obtained on an iron substrate from a sulfate bath at various pH values. The effects of changes in bath pH on alloy composition, morphology, microstructure and corrosion resistance were studied. Scanning electron microscopy was performed to observe the morphological change of the deposits with bath pH. Coating crystallinity was investigated by grazing incidence asymmetric Bragg X-ray diffraction and transmission electron microscopy. A transition from an amorphous to polycrystalline structure was observed on increasing the bath alkalinity, and thus decreasing the phosphorus content of the alloys. A single crystalline phase corresponding to face-centred-cubic nickel was identified in the alloys obtained from a strong alkaline solution. An increase in zinc percentage up to 23% in the deposits does not change the f.c.c. nickel crystalline structure. Corrosion potential and polarization resistance measurements indicated that the corrosion resistance of electroless Ni–Zn–P alloys depends strongly on the microstructure and chemical composition. The deposits obtained at pH 9.0–9.5 and with 11.4–12.5% zinc and 11.8–11.2% phosphorous exhibited the best corrosion resistance.     

Electrodeposition of ternary nickel—iron—cobalt alloys from acetate bath

Ternary nickel—iron—cobalt alloys of wide range composition have been deposited from acetate baths under a variety of conditions and the optimum conditions established are: nickel acetate 0.2828 M, ferrous sulphate 0.0359 M, cobalt acetate 0.2828 M, boric acid 0.1617 M, ascorbic acid 0.0056 M, pH 5.0, cd 1.5 A/dm² and temperature 30°. The bath gave bright, smooth and adherent deposits. Iron and cobalt contents decreased with an increase in cd and pH, X-ray studies of the deposits revealed fcc structure within the composition range studied (43.6–54.0% Ni). The results indicate that acetate bath can be successfully employed for plating purposes.     

Hydrogen evolution in alkaline water on cobalt electrodeposits prepared from baths containing different anions

Electrolytic cobalt deposits, as electrocatalytic materials, were prepared from baths containing chloride, sulphate or acetate. Electrodeposition was conducted at 200 A m^–2 with or without oxygen bubbling at the cathode. The hydrogen evolution reaction (h.e.r.) on the cobalt electrodes was performed in 30 wt % KOH at 25, 40, 50, 60 and 70 °C. Without oxygen bubbling during deposition, the cobalt electrodes prepared from the acetate bath gave superior performance for h.e.r. than electrodes deposited from the other baths. With oxygen bubbling, however, the lowest h.e.r. over-potential was obtained on the cobalt prepared from the chloride bath.     

Effects of boric acid on hydrogen evolution and internal stress in films deposited from a nickel sulfamate bath

The effects of boric acid additions on the pH close to the electrode surface, on the hydrogen evolution reaction and on the internal stress in the plated films were studied for the high speed electroplating of nickel from a nickel sulfamate bath at a current density close to the nickel ion limiting current density. The study was carried out at 50 °C and pH 4.0 using a 1.55 M nickel sulfamate plating bath containing boric acid at concentrations ranging from 0 to 0.81 mol L^–1. The variation of the internal strain in the plated nickel films was determined in situ using a resistance wire-type strain gauge fitted to the reverse side of the copper electrode substrate. The solution pH at a distance of 0.1 mm from the depositing nickel film was measured in situ using a miniature pH sensor assembly consisting of a thin wire-type antimony electrode and a Ag/AgCl/sat. KCl electrode housed in a thin Luggin capillary. The addition of boric acid was shown to effectively suppress the hydrogen evolution reaction at nickel electrodeposition rates (18.0 A dm^–2) close to the limiting current density (~20 A dm^–2). Consequently, the solution pH adjacent to the plating metal surface was maintained at a value close to that in the bulk solution and the development of high internal stresses in the deposited nickel films was avoided.     

Effect of cobalt deposits on nickel substrates on the oxygen evolution reaction in KOH

Cobalt deposits taking the form of cobalt nodules were obtained on nickel substrates in a bath containing dissolved cobalt sulphate and ammonium citrate at pH 9.8 and 23°C. Oxygen discharge was investigated on those electrodes in 40 wt % KOH solution at temperatures ranging between 40 and 80°C. Up to a cobalt loading of 12 mg cm^–2, the larger the loading, the larger the electrode/solution interface, which resulted in improved electrocatalytic activity with the loading. The electrocatalytic activity was also largely temperature-dependent.     

Electrodeposition of nickel-cobalt-zinc alloys from a borate bath

The electrodeposition has been studied of nickel-cobalt-zinc alloys from a borate bath containing nickel sulphate (120–140 g dm^–3), cobalt sulphate (30–46 g dm^–3), zinc sulphate (144–168 g dm^–3), boric acid (30 g dm^–3) and ammonium chloride (2 g dm^–3). The operating conditions were: current density, 2.0–5.0 A dm^–2; temperature, 30–40°C and pH, 2.4 to 5.4. Light grey, semibright, stressed films have been obtained. However, the deposits consist partially of black powder when the concentration of the various components is increased. The brightness is found to increase with decreasing temperature and pH of the solution. The total cathode efficiency increases when the pH and temperature of the solution decrease, whereas at any particular pH and temperature it first decreases, reaches a minimum and then increases with increasing current density.     

Electrolytic nickel—molybdenum—vanadium alloy coatings as a material with a decreased hydrogen overvoltage

The process of electrodeposition of Ni-Mo-V alloys from an alkaline tartrate bath was studied. The effect of the cathodic current density on the chemical composition, phase composition and surface morphology of Ni-Mo-V alloy deposits, as well as on the current efficiency of the deposition process, was determined. Codeposition of molybdenum and vanadium with nickel and formation of the Ni-Mo-V alloy is possible due to the effects of depolarization and overpolarization occurring in the process of codischarge of the complex ions of these metals. It was observed that the use of cathodes electrodeposited with Ni-Mo-V alloy containing 9–12% Mo and 0.1–0.2% V for water electrolysis resulted in a decreased overpotential for hydrogen evolution.     

Study of a mechanism of hard gold electrodeposition

Electrodeposition of nickel hardened gold was studied from a proprietary bath (Renovel N). Linear sweep voltammetry (LSV), chronoamperometry and chronopotentiometry were employed and Tafel curves determined. LSV studies revealed formation of a current peak connected with the inhibition of the deposition reaction. In the hard gold bath Tafel curves are characterized by two slopes: –0.47 V dec.^–1 between –0.5 and –0.8 V and –0.19 V dec.^–1 at more negative potentials; in the soft gold bath (without Ni) these slopes are: –0.35 V dec.^–1 and –0.15 V dec.^–1, respectively. Current efficiency of hard gold plating in galvanostatic conditions was on the average 54–57%, depending more on current density than on the charge passed (thickness of the deposit). No influence of oxygen on the process was found. It was also found, that the bath must be activated before reproducible results are obtained.     

The preparation of high surface area nickel oxide electrodes for synthesis

A study of the electrodeposition of nickel onto copper and graphite is described; the objective was to prepare high surface area, i.e. microrough, but stable deposits which could be used as nickel oxide anodes in aqueous base. It is shown that such electrodes may be prepared by electrodeposition at a high current density (200 mA cm^–2) from a Watts bath and that they give useful apparent current densities for a model synthetic reaction, the oxidation of ethanol to acetic acid.     

Electrodissolution of nickel sulphide concentrates in ferrous chloride solutions

A 3 M ferrous chloride solution of pH 1.5 and operated at 72° C was found suitable to electro-oxidize nickel sulphide concentrates containing the following constituents in wt.%: nickel 20, iron 8, magnesium 10, silicon 8, sulphur 11 and copper 0.83.The percentages of dissolution for 16 A h at current passed (current density 20–120 mAcm^–2 with respect to graphite) were in the ranges: Ni 90–70; Cu 70–50 and Mg 20–15. The changes in mol % of the final (mainly elemental) sulphur to initial sulphide-sulphur were in the range of 50 to 25. On the other hand the mole ratio of initial to final iron decreases, and this relates to the precipitation of hydroxides. Corresponding to these changes, the X-ray diffraction patterns detected the presence of elemental sulphur and hydrated oxides of iron and magnesium. The anolyte pH and electrode potential were followed during the electro-oxidation experiments.Experiments were carried out to establish the feasibility of separating copper from nickel chloride-copper chloride mixtures at 75° C. It was possible to separate copper whose concentration was 5 gl^–1 or above while that of nickel was 75 gl^–1. The cathodic polarization curves for copper, nickel and hydrogen discharge from their individual and binary mixtures were used to predict the conditions required to avoid copper-nickel alloy deposition, and the efficiencies of deposition.     

Characterization of zinc–nickel alloys obtained from an industrial chloride bath

Zinc–nickel electrocoatings obtained from an industrial bath containing two additives were characterized by analysing their morphology, structure, microhardness, residual stress and corrosion resistance. Changes in deposit characteristics following the addition of saccharin were also studied. All coating, with or without saccharin, presented a good appearance, good mechanical properties and a high corrosion resistance. Moreover, by varying certain plating conditions it was observed that the deposits maintained their properties over a wide interval of experimental conditions, which suggests their suitability for use in batch bath.     

The hydrogen evolution reaction in an acid medium on nickel electrodeposited with SiW12O 40 4− or SiMo12O 40 4− from electrolytes of various anionic compositions

Nickel was electrodeposited from baths based mainly on dilute aqueous solutions of nickel chloride, sulphate or acetate containing SiW₁₂O ₄₀ ^4– or SiMo₁₂O ₄₀ ^4– anions. Each bath contained one of the ions. The other chemical constituents of the solutions were the same in all of the baths. It was shown that improvement in the overpotential () and in the exchange current density (i ₀) of the electrodes depends both on the composition of the deposition bath and on deposition time. The h.e.r. electrocatalytic activity of the electrodeposits was then analysed and related to their chemical composition. The results show that some extremely active electrocatalysts are produced when combinations of nickel-tungsten (Ni–W) or nickel-molybdenum (Ni–Mo) are formed. The discussion is extended to include typical hypo-hyper-d-electronic transition metals, and it is concluded that alloying these metals significantly increases electrocatalytic activity.     

Physico-chemical properties of Zn-Fe alloy deposits from an alkaline sulphate bath containing triethanolamine

The composition, properties, structure and morphology of Zn-Fe alloy deposits obtained from an alkaline sulphate bath have been investigated. The bath containing triethanolamine produced smooth, uniform and bright Zn-Fe alloy deposits having the desired 15–25% Fe. The deposition potentials of Zn-Fe alloy lie between the potentials of the individual metals. Increased current density lowered the Fe percentage in the alloy deposit. The structure and morphology of the alloy deposits were found to depend on the Fe percentage in the alloy.     

The hydrogen evolution reaction in an acid medium on nickel electrodeposited with PMo12O 40 3− or MoO 4 2−

Nickel was electrodeposited from dilute aqueous solutions of nickel chloride, containing MoO ₄ ^2– or PMo₁₂O ₄₀ ^3– ions. It was shown that improvement in the overpotential () and in the exchange current density (i ₀) of the electrodes depends on the composition of the deposition bath. The h.e.r. electrocatalytic activity of the electrodeposits was then analysed and related to their chemical composition. The results show that some extremely active electrocatalysts are produced when combinations of nickel-molybdenum (Ni–Mo) are formed. The influence of the MoO ₄ ^2– or PMo₁₂O ₄₀ ^3– concentration in the electrode deposition electrolyte on the electrocatalytic properties of these deposits for the hydrogen evolution reaction (h.e.r.) was investigated. The effects of the deposition current density on the electrocatalytic properties of the electrodes were also studied. The best electrocatalytic properties for the h.e.r. were obtained with electrodes electrodeposited with 2 g dm^–3 of MoO ₄ ^2– or PMo₁₂O ₄₀ ^3– . The kinetic parameters for the h.e.r. in H₂SO₄ 1 M were deduced for temperatures ranging from 298 to 378 K.     

The effects of nickel and cobalt and their interaction with antimony on zinc electrowinning from industrial acid sulphate electrolyte

The effects of nickel and cobalt and their interaction with antimony on the electrowinning of zinc from industrial acid sulphate electrolyte were studied using X-ray diffraction, scanning electron microscopy and cyclic voltammetry. Concentrations of cobalt as high as 20 mgl^–1 had no effect on the zinc deposition current efficiency. The current efficiency decreased rapidly when the electrolyte contained >5 mgl^–1 nickel. Neither cobalt or nickel had an effect on the morphology of the 1-h zinc deposits. Nickel and cobalt caused characteristic changes in the cyclic voltammograms for zinc deposition. As a result this technique might provide a rapid means for evaluating the electrolyte prior to zinc electrowinning. The combined presence of cobalt and antimony in the zinc electrolyte was more deleterious to zinc electrowinning than was the combined presence of nickel and antimony. The presence of 0.08 mgl^–1 antimony in the electrolyte counteracted the effect of nickel both on the current efficiency for 1-h deposits and on the zinc deposition polarization curves.     

Ni–Zn–P alloy deposition from sulfate bath: inhibitory effect of zinc

Electroless Ni–Zn–P alloy deposition from a sulphate bath, containing sodium hypophosphite as reducer, was investigated. To increase the plating rate, the deposition parameters were optimized. The effect of process parameters (T, pH and [Zn²⁺]) on the plating rate and deposit composition was examined and it was found that the presence of zinc in the bath has an inhibitory effect on the alloy deposition. As a consequence, the percentage of zinc in the electroless Ni–Zn–P alloys never reaches high values. Using cyclic voltammetry the electrodeposition mechanism of Ni–Zn–P alloys was investigated. It was observed that the zinc deposition inhibits the nickel discharge and, as a consequence, its catalytic activity on hypophosphite oxidation. It was also found that increase in temperature or pH leads to the deposition of nickel rich alloys.     

Electrodeposition of copper and nickel on InBi alloy electrodes

Electrodeposition of copper and nickel was investigated on the 67–33 w/w InBi soft alloy, which has a low melting point (72°C). The electrochemical behaviour of this alloy was found to be similar to that of pure indium in a Watt's type bath. Direct and pulsed current techniques were compared. For copper, a current density ranging from 500 to 1000Am–2 was found to be suitable to obtain regular and shiny deposits at 25°C. For nickel, at 45°C, the current density range used was 500 to 1250Am–2 to obtain good deposits, that were not affected by hydrogen evolution. Pulsed and direct currents were found to be equally efficient. The main interest of electrodeposition on InBi alloy was the ease of removal of the alloy by simple melting, and consequently to make electrodes of unusual shapes which are normally difficult to produce industrially. Only a small amount of InBi alloy remained in the inner part of the deposit.     

Electrodeposition of nickel on vitreous carbon: Influence of potential on deposit morphology

The initial stages of the deposition of nickel on vitreous carbon from aqueous NiCl₂ (10^–2M adjusted to 1 M in chloride) has been studied using voltammetric and potentiostatic methods. The morphology of the deposits was observed by scanning electron and optical microscopy. A discussion of the relation between the deposition mechanism and the morphology is presented.     

Electrowinning of zinc from alkaline solutions

The objective of this work was to determine the best conditions for minimizing energy consumption in zinc electrowinning from alkaline solutions. The effects of several variables, i.e. hydroxide concentration (300–500 gl^–1), current density (50–1000 A m^–2), temperature (24–74°C), cathode material (magnesium, nickel, lead, stainless steels 304 and 316) and impurities (copper and arsenide) on current efficiency and cell voltage were investigated. The current efficiency was always 100% on magnesium except in the presence of arsenide (78% at 100 mg l^–1). With cathode materials such as stainless steels 304 and 316, nickel and lead, hydrogen evolution was observed at the beginning of electrolysis. Hydroxide concentration did not have a significant effect on cell voltage.Specific energy was low, even at 1000 A m^–2, and decreased with rising temperature, being only 2.17 kW h kg^–1 at 74°C. No redissolution of the deposit was observed. Decreasing distance between electrodes and using active anodes permitted a further reduction of specific energy to 1.75 kW h kg^–1. Decreasing space between electrodes was possible as no dendritic deposits were observed.     

Effect of halide ions on electroless nickel plating

The effect of halide ions (F^–, Cl^–, Br, I^–) on nickel deposition in acidic electroless nickel plating baths is investigated. Halide ions were found to have a significant effect on the nickel deposition and the results could not fully be explained using mixed-potential theory. A correlation between the stability constants of halide ions with palladium (ii) ions and the plating rate is proposed to explain the observations. Various parameters, such as the activation energy, deposit microstructure and phosphorus contents of the plating bath in the presence of various halide ions, were also studied.