E. Chassaing

Mechanism of nickel-molybdenum alloy electrodeposition in citrate electrolytes

The kinetics of the induced codischarge of Mo with Ni in citrate-ammonia electrolytes was investigated by means of polarization and a.c. impedance measurements. Three potential ranges were considered. At low polarization, hydrogen evolution resulting from citrate reduction is the main reaction. The impedance plots exhibit a large capacitive loop with a small high frequency inflection characteristic of the development of a porous layer and a low frequency inductive feature. At intermediate polarization, the partial currents for Ni and Mo discharge increase in the same proportion; the hydrogen evolution is first constant and then rapidly decreases. Then a large low-frequency capacitive feature is observed on the impedance plots, whose size decreases with increasing polarization. At still higher polarization, the Mo discharge becomes increasingly controlled by diffusion which generates an additional capacitive loop. A reaction scheme is proposed which accounts for the polarization data and the major impedance features.

Mechanism of copper-nickel alloy electrodeposition

The codeposition kinetics of copper and nickel alloys in complexing citrate ammonia electrolytes has been investigated by means of polarization and electrochemical impedance techniques. It is confirmed that the two-step discharge of the complexed cupric species Cu(II)Cit is diffusion-controlled during the alloy deposition, resulting in an increase in the nickel content of the alloy with electrode polarization. Impedance spectra are also consistent with a two-step discharge of Ni(II) cations involving an intermediate adsorbate, Ni(I)ads, originating from the reversible first step. A reaction model is developed for the parallel discharge of Cu(II)Cit and Ni(II) in which the reactions for nickel deposition are catalysed by active sites permanently renewed at the surface of the growing alloy. The surface density of these sites, slowly nucleated from Ni(I)ads and included in the deposit, varies with the electrode polarization, thus generating a low-frequency feature specific of Cu−Ni codeposition. This reaction model reproduces to a reasonable extent the potential dependence of the partial current densities for nickel and copper discharge, the current dependence of the alloy nickel content and also most of the experimental relaxation processes observed on impedance spectra.

Electrocrystallization mechanism of ZNi alloys in chloride electrolytes

Abstract The electrocrystallization kinetics of zinc and nickel alloys in chloride solutions has been investigated for ion ratio Ni2+/ZnII between 0 and 3, by means of polarization and electrochemical impedance techniques. During the codeposition, interactions between the two elements occur and both zinc and nickel discharges are inhibited. A reaction path has been developed which involves the slow blocking of the electrode surface by a hydrogenated adsorbate inhibiting the discharges of zinc and nickel ions. The reduction of ZnII species occurs in two steps via the intermediate ZnIads and it is slowed down by the codischarge of the Ni2+ ions. The specific feature of the codeposition mechanism is the formation of a mixed intermediate (NiZn)2+ads which catalyses the Ni2+ reduction. This model reproduces to a reasonable extent the potential dependencies of the partial current densities for zinc and nickel discharges and of the relaxation processes observed on the impedance spectra.

Kinetics of copper electrodeposition in citrate electrolytes

The kinetics of copper electrocrystallization in citrate electrolytes (0.5M CuSO4, 0.01 to 2M sodium citrate) and citrate ammonia electrolytes (up to pH 10.5) were investigated. The addition of citrate strongly inhibits the copper reduction. For citrate concentrations ranging from 0.6 to 0.8 M, the impedance plots exhibit two separate capacitive features. The low frequency loop has a characteristic frequency which depends mainly on the electrode rotation speed. Its size increases with increasing current density or citrate concentration and decreases with increasing electrode rotation speed. A reaction path is proposed to account for the main features of the reduction kinetics (polarization curves, current dependence of the current efficiency and impedance plots) observed in the range 0.5 to 0.8 M citrate concentrations. This involves the reduction of cupric complex species into a compound that can be either included as a whole into the deposit or decomplexed to produce the metal deposit. The resulting excess free complexing ions at the interface would adsorb and inhibit the reduction of complexed species. With a charge transfer reaction occurring in two steps coupled by the soluble Cu(I) intermediate which is able to diffuse into the solution, this model can also account for the low current efficiencies observed in citrate ammonia electrolytes and their dependencies upon the current density and electrode rotation speed.

Experimental evidence for gravity induced motion in the vicinity of ramified electrodeposits

Abstract Electrodeposition of copper from CuSO 4 solutions under high electric field gives rise to ramified deposits. We present optical measurements of the concentration maps in the electrolyte during copper electrodeposition, showing the role of gravity driven motion in the solution. Together with cell potential measurements, our experiments permit to give a semi-quantitative description of the onset of ramified electrodeposition.

Interface characterization in electrodeposited Cu–Co multilayers

Multilayers of Cu–Co made by electrodeposition are characterized. The interface width and layer roughness are measured by the Fresnel technique in electron microscopy. It is shown that the quality of the interfaces is comparable to that of layers made by physical deposition techniques. By depositing the layers on a glass substrate, we also show that it is possible to measure the magnetoresistance of the multilayers without removing the substrate. The values obtained are discussed in relation to the roughness of the layers and the electrodeposition conditions.

Electrodeposition of Ni-Mo alloys with pulse reverse potentials

With the aim of improving the protective properties of Ni-Mo alloy layers, pulse reverse electro-deposition has been investigated. The anodic pulses were applied in the potential range where hydrogen desorption and oxidation occur. The alloy composition was shown to depend on the pulse parameters, especially on the anodic pulse duration. For long anodic pulses a preferential dissolution of molybdenum in the electrodeposited alloy occurs, while bulk Ni-Mo alloys do not undergo any dissolution. For anodic pulses longer than a certain threshold the electrocrystallization process becomes blocked. The morphology and microstructure of the layers are mainly determined by the molybdenum content rather than by the pulse parameters.

Effect of Organic Additives on the Electrocrystallization and the Magnetoresistance of Cu-Co Multilayers

The electrodeposition of Cu-Co multilayers was investigated in the presence of two additives, sodium dodecylsulfate (SDS) and saccharin. Cyclic voltammetry, steady-state polarization, impedance spectroscopy, and potential pulses were carried out using an electrochemical quartz crystal microbalance. Magnetoresistance measurements were performed on Cu-Co multilayers deposited on indium tin oxide glass. Saccharin strongly inhibits the discharge of cobalt whereas SDS has a depolarizing effect. However the reaction path is not markedly changed. The deposition of cobalt starts with a strong evolution of hydrogen due to diffusion-controlled reduction of protons. The giant magnetoresistance ratio is markedly reduced by these additives.

Modelisation of Ni–P electroless deposition in ammoniacal solutions

Abstract A reaction path for the electroless deposition of Ni–P layers is developed. The steady-state polarisation and impedance responses are calculated. The comparison between the experimental results and the calculation shows the adequacy of the model. The mechanism accounts for most of the experimental features observed for the steady-state and the impedance behaviours and their dependencies with pH, hypophosphite and nickel sulphate concentrations.

Electrochemical behaviour of the titanium chlorides in various alkali chloride baths

Abstract In order to determine the best conditions for the electrolytic production of titanium we have studied the electrochemical behaviour of TiCl 4 in various molten chloride baths. We have shown that the potentials of the reduction steps of TiCl 4 depend strongly on the nature of the cations. When the melt contains no Ti(IV) complexing agent, as in BaCl 2 -CaCl 2 -NaCl, the IV → III step proceeds via the reduction of gaseous TiCl 4 . When the bath contains a complexing cation, such as K + , Rb + or Cs + , TiCl 4 gives the complex anion TiCl 2− 6 . In BaCl 2 -KCl-LiCl at 450 °C, the IV → III step occurs in two waves; the first is related to the reduction of the TiCl 2− 6 anion, the second to the reduction of gaseous TiCl 4 . The subsequent steps III → II and II → 0 arise at the same potentials as in BaCl 2 -CaCl 2 -NaCl. As we progressively substitute for KC1 with CsCl, the reduction of TiCl 2− 6 becomes more pronounced and the other reaction decreases. In CsCl-LiCl, Ti(III) is strongly complexed and the reduction to the metallic state proceeds in a single step Ti(III) → Ti(0).