Ts. Dobrev

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.

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 .

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.

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.

Developing and studying the properties of Pb–TiO2 alloy coated lead composite anodes for zinc electrowinning

Abstract A new anodic material for zinc electrowinning from sulphate electrolytes has been developed. This is a lead–titanium alloy coated lead composite electrode containing titanium nanoparticles. The titanium is present in the lead matrix as n-type TiO2. It has been established that the optimal content of the Ti in the composite coatings is about 0·5 wt-%. The behaviour of the lead–titanium anodes during the zinc electrowinning has been studied by means of galvanostatic polarisation investigations. The depolarising of the anodic reaction at lead–titanium composite electrodes is attributed to the increased anode area during continued polarisation. The processes occurring on the lead–titanium anodes during zinc electrowinning have been studied by cyclic voltammetry. The surface morphology of the composite lead–titanium electrodes was investigated by scanning electron microscopy.

Investigations of new anodic materials for zinc electrowinning

Abstract A new group of anodic materials and lead–silver alloys, produced in various ways, used as anodes for zinc electrowinning have been investigated. The new anodic materials are composite coatings, deposited on lead–calcium rolled substrates, consisting of a lead matrix and a cobalt–titanium phase. The cobalt and titanium are present in the lead matrix as CoTiO3 nanoparticles. The behaviour of lead–cobalt–titanium anodes during zinc electrowinning was studied by means of galvanostatic polarisation investigations. The processes, occurring on the anodes during zinc electrowinning, have been studied by cyclic voltammetry. The surface morphology of the composite electrodes was investigated by scanning electron microscopy. It has been established that the anodic potentials of the composite electrodes investigated are negligibly higher than those of the classical lead–silver alloy. It has been shown by cyclic voltammetry that the curves of the new electrodes possess the same characteristic peaks as those of pure lead electrode.

Electrochemical behaviour of lead alloys as anodes in zinc electrowinning

Abstract The electrochemical and corrosion properties of the ternary and quaternary lead alloys Pb–0·18Ag–0·012Co, Pb–0·2Ag–0·06Sn–0·03Co, and Pb–0·2Ag–0·12Sn–0·06Co (wt-%) have been investigated. The formation of oxide layers on the surface of these alloys was traced by cyclic voltammetric methods and the composition of the anodic oxide layers was determined by X-ray and SEM analyses. The cobalt inclusions reduce the anodic polarisation and improve the corrosion resistance of the alloy. The Pb–0·2Ag–0·12Sn–0·06Co alloy displays electrochemical and corrosion properties similar to those of Pb–1Ag and can be used as a substitute for the latter as a material for anodes in the electrowinning of zinc from sulphate electrolytes.

Evaluation of the corrosion protection properties of multilayer Cu/Ni/Cr deposits

Abstract Investigations of multilayer Ni/Cr deposits with conventional and with microporous chromium, in some cases incorporating bright copper sublayers, are reported. The microporosity of the chromium deposit is determined by the Dubpernell test. The polarity, the potential differences and the thicknesses of the individual nickel deposits are determined using the simultaneous thickness and electrochemical potential (STEP) test. Accelerated corrosion determinations are carried out using the Corrodkote method. The superposition of the duplex nickel (semibright nickel (SBNi) or bright nickel (BNi) and nickel seal (Ni s )), microporous chromium and bright copper (BCu) sublayer on nickel strike (Ni str ) in the system Ni str (2 μm)/ BCu(20 μm)/SBNi(9 – 12 μm)/Ni s (3 – 5 μm)/Cr(0.3 μm) leads to the formation of multilayer protective and/or decorative plates with the best corrosion protection properties. It is possible to use the fast STEP test in combination with the Dubpernell method to forecast the corrosion behaviour of multilayer Cu/Ni/Cr deposits.

Influence of Co2+ ions on cathode behaviour during zinc electrowinning

The effect of Co2+ ions on zinc deposition from acidic sulfate electrolytes is investigated on the basis of steady-state polarization curves and impedance measurements. Hydrogen evolution is shown to be enhanced by adsorbed cobalt on both the zinc deposit and the aluminium substrate, and this stimulation occurs in combination with the catalytic effect of adsorbed species originating from anodic products. Hydrogen evolution involves a diffusion-controlled step and a slow adsorption process which is the rate-determining step. It also induces the redissolution of the zinc deposit observable from the potential dependencies of current, charge transfer resistance and double layer capacitance.