L. Mirkova

Insoluble anodes used in hydrometallurgy. Part I. Corrosion resistance of lead and lead alloy anodes

A literature review on the corrosion resistance of Pb and Pb alloy anodes used in hydrometallurgy is presented. Data from many authors show that Pb alloys that contain Ag, Co or Tl are the most resistant. The increased corrosion resistance of these anodes is explained by the decreased current density and the decreased potential of the lead component of the alloy due to the higher conductivity of the alloying metals as compared with that of lead. As a result, a greater part of the current is distributed on the alloying metal in spite of its high positive potential.

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.

Hydrogen coevolution and permeation in nickel electroplating

Nickel coatings were electrodeposited onto a steel membrane in a conventional Devanathan cell in order to measure the diffusion rate of hydrogen into the steel substrate during electrodeposition. In most cases a Watts solution containing various organic additives was used: butyne-2 diol-1, 4; saccharine or thiourea. The structure of the electrodeposits was studied by X-ray and Transmission electron microscopy (TEM). It was shown that the electrodeposition parameters (pH, composition of the bath, additives) have a strong effect on hydrogen permeation. The use of organic additives during Ni plating increased the penetration of hydrogen into the substrate. In particular, sulfur-containing additives cause a fast initial increase of the permeation rate, which is attributable to a high surface concentration of Hads when steel is not totally covered with nickel. By performing permeation experiments with Ni coatings during hydrogen charging from a H2SO4 solution, it was shown that hydrogen permeation through nickel coatings is influenced by their fibre texture and by their grain sizes. A low permeation rate was observed in coatings plated in the presence of butyne-2 diol-1,4, which exhibit a strong 100 texture with large grains and a low density of defects. Conversely, the hydrogen diffusion rate is very high in coatings plated in the presence of thiourea or saccharine. These coatings exhibit a weak texture with very small grains.

Electrochemical properties of Pb–Sb, Pb–Ca–Sn and Pb–Co3O4 anodes in copper electrowinning

Three types of anode, Pb–Sb, Pb–Ca–Sn and Pb–Co3O4, for copper electrowinning were investigated. The corrosion resistance, as evaluated by cyclic voltammetric (CV) measurements was higher for Pb–Co3O4 than for Pb–Sb and Pb–Ca–Sn. During prolonged electrowinning under galvanostatic conditions, the anodic reaction on the Pb–Co3O4 anode was depolarized by 0.053 V as compared to Pb–Sb, and by 0.106 V with respect to Pb–Ca–Sn. The composition and structure of the anodic layer were determined by XPS, X-ray and SEM analyses. The surface layer on the three anodes examined was composed mainly of PbSO4, α-PbO2 and β-PbO2. Different structure of the surface layer was observed: loose and highly spread coral-like structure in the case of Pb–Sb; fibrous structure in the case of Pb–Ca–Sn and dense, fine-grained structure in the case of Pb–Co3O4.

Anodic behaviour of the Pb–Co3O4 composite coating in copper electrowinning

Abstract The electrochemical and corrosion properties of Pb–Co 3 O 4 (about 3% Co) composite anode for copper electrowinning without additives as well as in the presence of two combinations of organic additives in the electrolyte have been investigated and compared with those of Pb–Sb 5.85% anode. The formation of PbO 2 layer on the surface of these electrodes was traced by cyclic voltametric measurements using rotating disc electrode (RDE) method. The Pb–Co 3 O 4 composite anode shows a depolarizing effect on the process of oxygen evolution as compared to Pb–Sb anode. The corrosion rate of Pb–Co 3 O 4 anode during prolonged polarization is approximately 6.7 times lower than that of Pb–Sb anode. The influence of the tested organic additives on the anodic behaviour of both anodes is negligible.

Hydrogen evolution and permeation into steel during zinc electroplating; effect of organic additives

The Devanathan and Stachurski diffusion membrane method was used to study the evolution of hydrogen and its permeation into a steel sheet during cathodic charging from a chloride electrolyte or during zinc electroplating. The influence of four different organic compounds, which are the components of various formulations derived to improve zinc electrocoatings, were also tested. At a high-charging current density, the permeation transients obtained in a chloride electrolyte without zinc ions exhibit a maximum attributed to hydrogen trapping in the subsurface layer on the entry side. The concentration of adsorbed hydrogen on the steel surface depends not only on the cathodic current density and the composition of the solution, but also on the influence of the organic additives on the recombination of hydrogen atoms. During zinc electrodeposition, the coating covers the substrate in a few seconds and acts as a barrier for hydrogen absorption. The permeation rate depends on the cathodic current density but also on the concentration of ZnCl2 in correlation with the porosity of the coating. It is shown that steel substrate hydrogenation (beneath the zinc coating) is strongly reduced in the presence of a combined additive, composed of four compounds in appropriate amounts as well as in the presence of PEG6000 in the plating bath. This effect, which is correlated to the modification of the hydrogen evolution process, can be used to hinder the severe drawbacks caused by hydrogen penetration into the steel substrate.

The effect of some surface active additives upon the quality of cathodic copper deposits during the electro-refining process

Abstract The effects of four organic formulations on morphology, texture, dendrite formation and impurity content of electro-refined copper deposits were determined and compared. The influence of these additives on the quality of deposits during current reversal was also examined. The experimental conditions resemble those prevailing in industry. It was demonstrated that two of the investigated formulations were effective in producing smooth, fine grained, dendrite-free copper deposits with high purity.