I. Krastev

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.

Composition and Structure of Silver-Indium Alloy Coatings Electrodeposited from Cyanide Electrolytes

Forschungsinstitut fu¨r Edelmetalle und Metallchemie, 73525 Schwa¨bisch Gmu¨nd, GermanyThe electrodeposition of Ag-In alloys from cyanide electrolytes was investigated. The resulting coatings were characterized bycyclic voltammetry, scanning electron microscopy, energy-dispersive X-ray, and X-ray diffraction. The effect of current density onthe structure and coating phase composition was established. With increasing current density heterogeneous, more indium-richalloy coatings were deposited, containing different phases such as Ag, Ag

Structure and properties of electrodeposited silver–bismuth alloys

The elemental and phase compositions, the deposition rate, the structure, and some physico-mechanical properties, such as hardness, wear resistance, roughness and internal stress of electrodeposited coatings of silver–bismuth alloy have been studied. The possibility of deposition of alloy coatings of desired composition depending on the electrolysis conditions and on the composition of the electrolyte used has been demonstrated. With increase in the Bi content in the coatings, hardness and wear resistance decrease, and at Bi contents higher than 50 wt.% the wear resistance remains unchanged. The roughness of the coatings is independent of their Bi content. The alloy coatings reveal a negative (compressive) internal stress. They consist of two phases and are heterogeneous both in the bulk and in the surface layer. A clearly expressed columnar structure is observed, the columns being of different phase composition. This structure is sometimes accompanied by a lamellar structure. The simultaneous deposition of phases with different Bi content leads to self-organization phenomena and formation of wave, spiral and target structures on the electrode surface.

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.

Effect of electrolysis conditions on the deposition of silver–bismuth alloys

The electrochemical deposition and dissolution of silver, bismuth and silver–bismuth alloy from a cyanide–tartrate electrolyte were studied by means of cyclic voltammetry. The influence of the electrolyte composition on the electrochemical reactions is discussed. The deposition potentials of the two metals could be maintained close to each other by means of appropriate complex forming agents, leading to their codeposition. Silver deposition is the predominant reaction in the electrolyte studied and the bismuth content in the coating increases with increased current density. The dissolution potentials of the two metals are quite distinct; they differ by more than 0.5 V. In the presence of a free complex forming agent, both the deposition and the dissolution potentials of silver can be shifted in the negative direction. Depending on the type and amount of the complex forming agent, they can become more negative than the deposition and dissolution potentials of bismuth. Predominant deposition of bismuth is realized in this case and the codeposition of silver is enhanced at higher current densities. By varying the amount of the complex forming agent, silver–bismuth coatings of any desired composition can be obtained.

Pattern formation during electrodeposition of indium-cobalt alloys

The investigations on the effect of the electrolysis conditions, including high speed electroplating, on the content, structure, morphology and some properties of electrodeposited In-Co alloys from citrate electrolytes are presented. It was shown that indium and cobalt could be successfully deposited from acid citrate electrolytes and deposition of alloys with indium content between 20 and 80 wt. % is possible. At high cobalt content, heterogeneous multiphase coatings with spatio-temporal structures are obtained. Spatio-temporal structures could be observed also during electrodeposition under intensive hydrodynamic flow and improved mass transport conditions at high current densities. The structures are obtained for the first time from silver- and cyanide-free non-alkaline stable electrolytes of a relatively simple composition.

Properties of silver–antimony alloys electrodeposited from ferrocyanide–thiocyanate electrolytes

The influence of electrolysis conditions on the internal stress, microhardness, electrical contact resistance, wear resistance, roughness, and friction properties of silver–antimony alloys deposited from ferrocyanide–thiocyanate electrolytes is studied. The internal stress of the coatings depends strongly on their antimony content. Stress changes and transition from compressive to tensile stress at increased antimony content in the alloy are observed. By internal stress measurements, conclusions can be drawn concerning the homogeneity or heterogeneity of the coating, that is, whether the coating consists of one or two phases. The changes in microhardness are related to variations of the internal stress and to the phase transition. The codeposition of antimony leads to a reduction of the coating roughness. Increased Sb content of the alloy leads to an increase in the electrical contact resistance and to reduction of the contact forces and wear resistance of the coatings. At large deviations from equilibrium, an oscillating electrochemical reaction is observed leading to space-time structures on the electrode surface.

A cyclic voltammetric study of ferrocyanide-thiocyanate silver electrodeposition electrolyte

The electrochemical behaviour of ferrocyanide-thiocyanate electrolytes for silver electrodeposition was studied by cyclic voltammetry. The differences in the electrolyte preparation procedure do not affect their electrochemical behaviour at identical silver concentrations. The silver electrodeposition is characterized by two cathodic current maxima and by charge transfer limitations with a preceding chemical reaction. The dissolution of the electrodeposited silver is characterized by the formation of AgCN on the electrode and further dissolution by passivation and formation of oxide products, which are reduced during the next cathodic scan. The redox processes of iron from the ferrocyanide complex on Pt- and Ag-substrates, as well as the effect of the complex forming agents like SCN−- and CN−- ions are shown.

Pattern formation during electrodeposition of alloys

The increase in the content of the alloying element in electrodeposited alloys reflects in the changes of their phase composition, when the saturation limit of the lattice of the basic metal is reached. At higher percentages, the excess amount of the alloying element forms one or more new, richer in this element phases. The coatings become multi-phase, heterogeneous and their physical–mechanical properties change. Sometimes an ordered distribution of the different phases of the heterogeneous alloy coating could be observed. Examples of self-organization phenomena during electrodeposition of different alloy systems, such as Ag-Sb, Ag-Bi, Ag-In, Ag-Sn, Ag-Cd, Cu-Sb and In-Co, resulting in pattern formation and formation of spatio-temporal structures on the surface of the obtained coatings are presented and compared. Instabilities resulting in potential or current oscillations are registered in most of the investigated systems. The phase composition of the alloy coatings and especially of the observed pattern is determined and some similarities in the structure of the phases forming the pattern are registered. The pattern formation is registered on the cathode not only in cyanide silver alloys electrolytes, but also during deposition of other alloy systems in acidic electrolytes like Cu-Sb and In-Co. The effect of the natural convection in non-agitated electrolytes on the pattern formation is discussed. The possibility of formation of periodic structured coatings without applying external electrical pulses which could result in appropriate modification of some properties of the electrodeposited alloys is demonstrated. The hypothesis that similar pattern formation could be observed in agitated electrolytes at different hydrodynamic and electrolysis conditions, when the same percentage or the same phase composition of the alloy is reached was examined for Ag-Cd, In-Co and Ag-Sb alloys in jet-plating experiments.

A Rotating Disc Electrode Study of Zinc Electro-deposition from Alkaline Zincate Solutions

The method of the rotating disc electrode was used for the potentiodynamic investigation of zinc deposition on steel from alkaline zincate electrolytes. The effect of two types of additives -basic and brighteners, as well as their combinations on the cathodic characteristics of zinc electrodeposition was established. The measurements under controlled hydrodynamic conditions show that zinc deposition from electrolytes without additives is always mass transport limited and that deposits are powdery. In the presence of additives the deposition is kinetically controlled and smooth compact zinc is formed. The inhibition influence of the additives on the electrochemical reduction of zinc was also estimated. The data were correlated with Hull cell experiments.