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In chromium electroplating for decorative applications, trivalent chromium based electrolytes are recently used as alternatives for solutions of chromic acid. Since the use of hexavalent chromium is restricted in serval countries, the demand on chromium(III) electrolytes increases and there is a need for understanding the deposition kinetics in order to improve the performance of the plating process. Electrochemical quartz crystal microbalance (EQCM) was used to study the mass-charge balance of the cathodic processes in chromium(III)-sulfate based plating baths. Current efficiency and deposition rate were determined as a function of time, additive concentration and current density. The addition of benzoic sulfimide (BSI) facilitates the reduction to chromium metal, as an optimum current efficiency of 8.1% was observed at 2.5 A dm−2. pH changes near the cathode surface during electroplating were evaluated in-situ by combining a pH electrode with a metal mesh and correlated with the results obtained by EQCM.

Aluminum was anodized in mixtures of aqueous sulfuric and chromic acid in different ratios, with overall concentration of 1.0 M. It was found that the logarithm of current density (and consequently, oxide growth rate) is a square function of the anodizing voltage. Moreover, the barrier layer thickness at the pores bottom was found to increase exponentially with the voltage, and increased as well with the fraction of chromic acid in the electrolyte. Additionally, interpore distance of anodic aluminum oxide, formed at the same voltage, was found to increase exponentially with the molar fraction of the chromic acid. Altogether, the high impact of the composition of the electrolyte on the morphological features of the nanoporous arrays is revealed.

In this study, we report the results obtained investigating the behavior of sulfuric and chromic acid alumina templates in typical supporting electrolytes frequently used for alternating current ( ac ) deposition of various nm-scaled materials. Qualitative analysis of voltammetric profiles taken for as-grown, ac -treated and annealed alumina films in a conventional tetraborate re-anodizing solution revealed dramatical changes in the properties of alumina barrier layer during ac treatment in these supporting electrolytes even at low current density. These changes were related here with the transport of protons through the barrier layer during ac treatment, discharge at the metal/oxide interface and hydrogenation of alumina material by hydrogen atoms in an upward way. This conclusion comes from the behavior of Pt/Hg|alumina|Me z + electrode and the valence band X-ray photoelectron spectra taken from the inner part of alumina barrier layer material before and after the ac treatment.

An electrochemical membrane reactor with three compartments (anolyte, catholyte and central compartment) based on in-house-prepared cation- and anion-exchange membrane was developed to achieve in situ separation and recovery of chromic acid and metal ions. The physicochemical and electrochemical properties of the ion-exchange membrane under standard operating conditions reveal its suitability for the proposed reactor. Experiments using synthetic solutions of chromate and dichromate of different concentrations were carried out to study the feasibility of the process. Electrochemical reactions occurring at the cathode and anode under operating conditions are proposed. It was observed that metal ion migrated through the cation-exchange membrane from central compartment to catholyte and OH− formation at the cathode leads to the formation of metal hydroxide. Simultaneously, chromate ion migrated through the anion-exchange membrane from central compartment to the anolyte and formed chromic acid by combining H+ produced their by oxidative water splitting. Thus a continuous decay in the concentration of chromate and metal ion was observed in the central compartment, which was recovered separately in the anolyte and catholyte, respectively, from their mixed solution. This process was completely optimized in terms of operating conditions such as initial concentration of chromate and metal ions in the central compartment, the applied cell voltage, chromate and metal ion flux, recovery percentage, energy consumption, and current efficiency. It was concluded that chromic acid and metal ions can be recovered efficiently from their mixed solution leaving behind the uncharged organics and can be reused as their corresponding acid and base apart from the purifying water for further applications.

Cyclic voltammetry (CV), transmission electron microscopy (TEM) and 13 C nuclear magnetic resonance spectroscopy (NMR) were employed to investigate the particle size and surface morphology of fuel cell grade platinum black samples as received and prepared under three different cleaning methods. The Pt particle growth was most markedly caused by extensive CV treatment, moderately by cleaning with chromic acid, and negligibly by holding the potential at 250 mV. Even single anodic sweep CV removed Pt adatoms; in contrast, cleaning with chromic acid and holding the potential at 250 mV did not. Our results demonstrate that a cleaning method should be chosen with the knowledge of its effect on the metal surface and particle size. The possibility of detecting the particle size change due to cleaning procedures by 13 C NMR is also discussed.

International audience; In order to optimise anodisation surface treatment of titanium alloys, the electrochemical conditions of anodic oxidation of pure titanium and of a TA6V (Ti-6% Al-4% V) alloy were investigated. Both voltametric (potentio-dynamic) and chrono-amperometric (constant voltage, either applied directly or progressively increased during the first steps of the treatment) experiments are conducted. The electrolyte is a chromic acid (CA) (0.5 mol l(-1) Cr2O3) solution with and without hydrofluoric acid (HF) addition (9.5 x 10(-2) mol l(-1)). A thin oxide compact film is formed in CA electrolyte and a duplex film composed of a compact layer surmounted by a columnar porous layer grows from the fluorinated electrolyte. The voltametric results indicate a breakdown of the compact film (in non-fluorinated medium) for potential around 3 V/SCE, and a strong influence of the alloying elements of TA6V on the formation of porous films in fluorinated medium. The chrono-amperometric measurements reveal a complex growth process of the porous film, in which the residual current participates in the thickening of both compact and porous layers of the film. The overall electrochemical efficiency is small and decreases with the treatment time. A growth mechanism involving a poisoning (by Cr-VI-containing ions) and antidote (by F-containing species) competition is proposed.

Current oscillations have been found during anodic polarization of a stainless steel stationary electrode in concentrated H2SO4 solution or more dilute H2SO4 solution containing an appropriate amount of CrO3. The potential range of current oscillations and the frequency were dependent on the potential scan rate, solution composition and concentration, and temperature. The occurrence of current oscillations was rationalized in terms of a periodic sequence of dissolution and reformation of the M(OH)3 film, which was formed by the reaction of divalent metal ions dissolved in the earlier stage of polarization of the stainless steel. The microstructure of the stainless steel exposed to current oscillations was observed by a scanning electron microscope.

The morphology and structure of the anodic oxides, formed on aluminium in chromic and/or sulfuric acid aqueous solutions have been studied by the methods of high-resolution transmission electron microscopy, X-ray crystallography, ultrasoft X-ray spectroscopy and thermal derivatography. It has been shown, that the porous structure of oxides grown in the chromium acid solutions is less-ordered as compared with that for oxides formed in sulfuric acid. All the anodic oxides are amorphous and contain a mixture of AlO4 tetrahedra and AlO6 octahedra. The derivatography data indicate lower hydration of the anodic oxides grown in chromium acid but also their enhanced ability to absorb the water during the oxide sealing. The possible reasons of weak chromate incorporation into the anodic aluminium oxide are discussed. The decrease of cation transport number within the layer of incorporated anions is suggested to be the main reason.

The kinetics of growth and the chemical composition of anodic alumina films formed in chromium acid electrolyte and its mixtures with sulphuric acid are studied. It is shown that the oxide growth in the electrolyte, containing equal molar parts of chromium and sulphuric acids, occurs in a different way depending on the anodic current density. At j a lower than J ao = 5 mA/cm 2 porous oxide is formed, whereas higher current promotes barrier oxide formation. At j a ⋍ j ao oxide starts to grow in the porous oxidation mode and then switches to barrier oxide growth. However, the increase of the electrolyte temperature is found to inhibit this effect. The chemical composition of alumina films formed in chromate electrolyte exhibited the presence of Cr(VI) and Cr(III) ionic species at the oxide surface. The observed effect of the combined porous-and/or-barrier oxide growth in the mixed electrolyte is interpreted in terms of enhanced formation of insoluble aluminium chromates at high electric fields. The other possible interpretation employs the theoretically predicted effect of the extreme-possessing dependence of pore growth rate on the electric field applied.

A study has been made of the pulse plating of chromium from the standard aqueous chromic acid baths containing sulfate catalyst in the regime of pulse periods from 0.1 ms to 1.0 s and duty cycles from 10 to 100%. The coulombic efficiences of the chromium deposition and hydrogen evolution reactions at the cathode and of the oxygen evolution at a lead anode were measured with a gravimetric and gas-volumetric analysis. The results indicate that pulse current (pc) reduced the coulombic efficiency of the cathodic chromium deposition reaction. Pulse current modified the morphology of chromium deposits. With pc the chromium deposits from the dilute chromic acid bath at 22°C were of a needle-like structure as compared to the hemispherical growth in dc plating. A flake-type chromium deposit was obtained with pc from the concentrated chromic acid bath at 65°C. Pulse current increased the grain sizes of chromium deposits. For the hydrogen and oxygen evolution reactions, pc behaved as a depolarizer and considerably increased the coulombic efficiencies of both reactions.

The electrochemical activity of the platinum electrode for hydrogen reaction in 0.5 M H2SO4 has been studied with special reference to the state of the electrode surface. The electrode, prepared by electrodepositing a small amount of platinum on a platinum substrate from H2PtCl4 solution, is characterized by extremely high and stable activity, but this is decreased remarkably by annealing in vacuum. The activity decay on annealing is associated with the settling down of superficial metastable platinum atoms to stable lattice positions. The annealed electrode may be activated by usual oxidation-reduction treatment, ie by soaking in oxidizing reagent such as chromic acid solution or by anodization, followed by cathodic reduction in each case. The platinum electrode is also activated merely by exposing the electrode to air for a short time. The activation by these treatments seems to result from the formation of a superficial active monolayer of platinum atoms, which have been forced to re-arrange to new positions by oxygen adsorbed during the oxidation or the exposure to air.

Possible reactions accounting for the oxidation of the lead anode in CrO3 solutions are formulated and their theoretical potentials calculated from free energy data. The implications of these equations are discussed and certain conclusions are drawn. The passivation of lead in CrO3 solutions of different concentrations has been studied using both the cyclic galvanostatic and potentiostatic techniques. Oxidation with constant current gives rise to potential/time curves having four distinct arrests. These correspond to the consecutive formation of PbO, Pb(OH)2, PbCrO4 and PbO2 before the evolution of oxygen. The same conclusion is obtained from forward potentiostatic experiments in which polarization is directed towards positive potentials. A fifth oxidation arrest is recorded in backward potentiostatic polarization curves and is ascribed to the oxidation of PbO and Pb(OH)2 to a higher valent lead oxide, PbOn, where 1 < n < 2. Anode decay curves obtained after current interruption show two arrests at potentials very near to those of the systems PbOn/PbO2 and Pb/PbCrO4 under the prevailing conditions. Following a change in slope at ca +0·85 V, the potential tends ultimately to that of the Pb/Pb(OH)2 system. Sulphate additions do not affect the shape of the decay curves and it is concluded that the behaviour of the lead anode in chromium-plating baths is practically the same as that in pure chromic acid solutions. X-ray diffraction patterns of material formed upon oxidizing lead in CrO3 solutions show characteristic d-lines for PbO, PbCrO4 and PbO2.

Previous work has shown that a low level of sulphate impurity in chromic acid can significantly change the growth rate and the morphology of porous anodic films formed on aluminium at a constant voltage. The changes were associated with incorporation of sulphate into the films. The present study employs electron microscopy to reveal the growth of larger pores and cells at longer times of anodizing than previously used, leading to a duplex film morphology, with finer pores in the outer region. The change in pore size correlates with a reduction in the concentration of sulphate in the film. However, the behaviour of a tungsten tracer indicates that the sulphate does not alter the mechanism of pore generation significantly. From the results of sequential anodizing in sulphate-containing and sulphate-depleted electrolytes, it is suggested that incorporated sulphate ions lead to the generation of a space charge layer, which has an important role in determining the current density.