St. Rashkov

Lead–cobalt anodes for electrowinning of zinc from sulphate electrolytes

Abstract A new anodic lead–cobalt (Pb–Co) 0.5–6% (by weight) material for electrowinning of zinc from sulphuric acid electrolytes has been obtained. The properties of Pb–Co alloys anodes, based on an entirely different principal were studied by means of electrochemical methods. The results obtained were compared with those from lead–silver (Pb–Ag) anodes with a 1% Ag content as used in industry. It is established that above 0.5% Co, the corrosion resistance of the new anodes is better and at Co content of 3% the anodic overpotential is about 0.08–0.1 V lower than that of Pb–Ag (1%) alloy. The anodic layer of the Pb–Co anodes—dense and tightly attached to the surface—consisted of β-PbO 2 , MnO 2 and PbSO 4 .

Effect of brighteners on hydrogen evolution during zinc electroplating from zincate electrolytes

Hydrogen evolution during zinc electrodeposition on a steel substrate from zincate electrolytes containing different additives was studied using various experimental techniques.The hydrogen evolution reaction is limited by the electron transfer step. Hydrogen evolution is most intensive during the first seconds from the beginning of electrodeposition due to the lower overpotential of hydrogen on steel as compared with that on zinc. The evolved hydrogen is dissipated in three ways. Most is dissipated to the atmosphere via gas bubbles at a constant rate. Some is dispersed in the electrolyte some diffuses into the steel substrate, predominantly at the commencement of deposition. The additives affect both the total amount of evolved hydrogen and its distribution. The highest amount of hydrogen is evolved in the presence of the anisaldehyde bisulphite containing composite additive. The highest amount of hydrogen included in the substrate and remaining in the electrolyte corresponds to the use of the Na–N-benzylnicotinate containing additive. In this case blistering is observed.

Anodic behaviour of composition modulated Zn–Co multilayers electrodeposited from single and dual baths

The anodic behaviour of composition modulated Zn–Co multilayers (CMM) electrodeposited from single and dual baths was studied. Multilayers with thickness of the individual sublayers 0.3 and 3.0μm were electrodeposited galvanostatically. It was established that most of the CMM coatings obtained from dual baths dissolve at potentials that are close to those for pure Co coatings. CMM coatings obtained from a single bath dissolve at potentials between the dissolution potentials of pure Co and pure Zn coatings. With increase in the number of sublayers in CMM coatings with equal total thickness, deposited both from dual or single baths, regardless of their individual sublayer thickness and sequence, potentials of the stripping peaks are shifted positively.

On the nature of the ‘induction period’ during the electrowinning of zinc from nickel containing sulphate electrolytes

As a result of detailed voltammetric, impedance, electron microscopic and opticalin situ investigations of the peculiarities encountered in zinc electrowinning from nickel-containing acid electrolytes, a model for the ‘induction period’ is proposed and its dependence on the process conditions is elucidated. The model is based on the screening effect of hydrogen bubbles formed on the nickel-rich regions of the cathode which give rise to local galvanic cells.

Influence of current density and temperature on the morphology and preferred orientation of electrodeposited copper coatings

Abstract The texture of electrodeposited copper coatings has been investigated by X-ray (texture gonimeter) and galvanostatic methods. A dependence is found between the sequence of appearance of orientation axes and cd and temperature. The 〈111〉 texture axis appears only at high overvoltages (above 0·132 V), whereas the 〈110〉 is found below 0·126 V. The change in axis occurs within this very restricted overvoltage interval. A given 〈111〉 or 〈110〉 axis is preserved over a rather broad overvoltage range. In order to explain the probable way of change from a basic 〈110〉 orientation to a 〈111〉 one, the texture was studied in detail in the vincinity of the above overvoltage range. A mechanism is suggested for the change in orientation in which the 〈887〉 T 2 orientation, which is the orientation of twofold twinned crystal blocks on matrix crystallites with a 〈110〉 orientation, changes owing to epitaxial intergrowth to a 〈111〉 orientation. The 〈511〉 T 1 twin orientation observed is a proof that the 〈111〉 orientation changes from a pseudo orientation to a basic one, owing to epitaxy. In terms of the experimental material and the mechanism suggested for its explanation, mechanisms are suggested for other possibel changes (starting from different basic orientations) for several electrodeposited metals.

Electrodeposition of Zn–Co alloy coatingsfrom sulfate–chloride electrolytes

The composition, surface morphology and appearance of Zn–Co alloy deposits as a function of current density, electrode potential and Co2+ concentration in the electrolyte was studied. It was found that coatings of good quality with low (1%) Co content are formed at a current density of 0.2Adm−2 and with high (6.5%) Co content at 2Adm−2 from electrolytes containing 1.0M Co2+ under galvanostatic conditions. The potentiodynamic dissolution of coatings with Co content of 6.5% indicates successive deposition of Co enriched phases and a pure Zn phase. The Zn–Co alloys are more corrosion resistant than zinc but are less resistant than cobalt.

Investigation of the processes of obtaining plastic treatment and electrochemical behaviour of lead alloys in their capacity as anodes during the electroextraction of zinc I. Behaviour of PbAg, PbCa and PBAgCa alloys

Abstract The influence of the mode of obtaining and plastic treatment of binary PbAg and PbCa, and ternary PbAgCa alloys (used as anodes in Zn electro-extraction from sulphate electrolytes) on their electrochemical behaviour and corrosion resistance is studied. It has been established that the rolled PbAg alloys possess a higher corrosion resistance and lower anodic polarization compared to the cast lead-silver ones due to the structural fineness and homogeneity of the plastic deformed anodes. The plastic deformation of the ternary alloys with calcium content of 0.06% causes Pb3Ca precipitation in the solid solution. The hot-rolled alloys form an solid solution of Pb3Ca with fine-grained structure, deformed through the rolling direction. The cold-rolled alloys possess clearly expressed oriented structures also through the rolling direction. Cast and plastically deformed PbCa anodes possess better electrochemical and corrosion characteristics than pure Pb but a considerably higher anodic polarization and lower corrosion resistance than PbAg alloys. Both PbCa systems, with a calcium content of 0.08 and 0.11 % are very appropriate for the preparation of ternary PbAgCa alloys. These alloys (PbAg 0.5 %-Ca 0.11 %) possess better electrochemical and corrosion characteristics than binary PbCa ones. Only ternary rolled alloys have equal corrosion and electrochemical properties to those of the alloy PbAg (0.75%–1.0%) used in practice.

Adsorption of substances acting as brighteners in the electrolytic deposition of copper

The adsorption behaviours of polyethers of the polypropyleneglycol (P) type, derivatives of safronic dyes (B); sulpho-organic aliphatic compounds containing the disulphide group -S-S- (DS) and combinations of the three additives (KD) were investigated. For this purpose the dependence of the differential capacitance of a polycrystalline copper electrode on the potential was measured in 1 N H2SO4 solutions containing various concentrations of the additives. It was found that the adsorption activity of the substances investigated increases in the order P<B<DS. This order is directly related to changes in the axes of predominant orientation, internal stresses, deposition overvoltage and the brightness of the coatings in the presence of the various additives.

Anodic behaviour of one and two-layer coatings of Zn and Co electrodeposited from single and dual baths

One and two-layer Zn and Co coatings deposited from single and dual baths have been studied. During potentiodynamic stripping of a two-layer coating deposited from dual baths, containing either Zn2+ or Co2+, composed of a Co underlayer and a Zn overlayer, two separate peaks are observed, corresponding to the dissolution of both metals independently of one another. When the overlayer is of Co, the predominant part of the two-layer coating is stripped at the dissolution potential of pure Co coatings. In the anodic dissolution curve of a two-layer coating deposited from a single bath, containing both Zn2+ and Co2+, composed of a Zn layer with low (1.0%) Co content and a layer with high (6.5%) Co content, three anodic current peaks are observed. These are due to the dissolution of pure Zn and of Zn–Co alloy phases. The heights of the peaks and the potentials of their maxima do not depend on the order of layers but only on their thickness.

On the surface coverage with adatoms during the initial nucleation stages of nickel deposition onto glassy carbon

Abstract The surface concentration of nickel adatoms during the initial stages of nucleation onto glassy carbon substrates has been determined by galvanostatic and potentiostatic pulse methods. It is established that nickel adatoms carry a partial positive charge of approximately 25%. The value of the diffusion coefficient of nickel adatoms is calculated to be Da ≈ 1.25 × 10−10cm2 s−1, and the value of the diffusion overvoltage is found to be 0.5–1.5% of the overall overvoltage. The resistance of the surface diffusion process of adatoms Rd = 0.147 Ω cm2. The results obtained lead to the conclusion that at high overvoltages the rate-determining stage of the overall reaction of crystal growth might be the surface diffusion of adatoms, which is suppressed by the adsorption of 2-butyn-1,4-diol.