Thomas J. O’Keefe

The Phase Stability of Cerium Species in Aqueous Systems

Cerium compounds have been identified as leading candidates to replace hexavalent chrome as conversion coatings on aluminum alloys to improve corrosion resistance. Cerium also shows promise for use as an inhibiting pigment in paint systems. The cerium conversion coatings can be deposited using either spontaneous or nonspontaneous electrolytic processes. In both cases the protective cerium oxide film forms by a precipitation mechanism that is very dependent on electrochemical potential and pH. The oxidation state and phase of the condensed cerium has been shown to be an important aspect of the corrosion protection properties provided by the film. Because of the strong influence of the solution chemistry and operating parameters on film performance, a basic knowledge of the system stability is essential. Toward this end, a revised E-pH diagram was developed for the Ce-H 2 O system. The Ce-H 2 O-HClO 4 system was chosen as an example system in which hydroxy ions are the only significant complexing species for the Ce ions. A stability diagram was constructed using more recent thermodynamic data on cerium species not available for the original diagram published in Pourbaixs atlas. Significant differences were noted between the previously published Ce-H 2 O diagram and the one presented here. Precipitation tests were carried out to verify the trends indicated in the new diagram. The importance of the updated E-pH diagram in understanding the formation processes of cerium conversion coatings are discussed.

Evaluation of Lead Anode Reactions in Acid Sulfate Electrolytes II. Manganese Reactions

Lead alloys containing calcium-tin, silver, or similar combinations are the primary insoluble anodes used for copper and zinc electrowinning. One potential concern with these anodes is the detrimental effect on cathode purity from incorporation of lead corrosion products. Other factors which are important to process economics are the reaction potential and overpotential required for oxygen evolution. In addition to the inherent overpotential, impurities in the electrolyte can have a significant polarization impact, such as the films that may form when manganese is present. In order to gain a better understanding of the fundamental behavior of various concentrations of manganese ions at lead anodes, the polarization behavior of lead calcium-tin and lead-silver alloys in sulfuric acid was evaluated. A common depolarizing agent, Co(II), was also used in some cases to determine possible interactions with manganese. The data show that manganese ions are active on the lead anodes and may precipitate manganese oxide in addition to electrochemical deposition. One possible precipitation mechanism seems to occur by the disproportionation of Mn(III). Co(II) lowers the electrode potential at given current densities which decreases Pb corrosion but favors MnO 2 formation.

The effects of certain impurities and their interactions on zinc electrowinning

The effects of germanium, antimony, arsenic, cobalt, glue, and free acid concentration were studied on both commercial and synthetic electrolytes. The effects of a single factor and the combined effects of multiple factors were elucidated. The temperature, zinc concentration, and current density were varied for some of the tests. It was found that the acid content was the most critical factor when impurity levels were at normal plant solution concentrations. The ranges where the effect of the impurity became apparent were: greater than 20 ppb (parts per billion) for antimony; 40 ppb for germanium; 120 ppb for arsenic; and 7 to 8 mg/1 for cobalt for a solution containing 65 g/1 zinc and 100 g/1 free sulfuric acid. At higher levels of acid, the acceptable level of impurity declined markedly. Glue additions were found to counteract the effects of antimony and germanium, but did little to counteract the effects of cobalt and arsenic. The level of acid was found to be especially critical when cobalt and arsenic were in the electrolyte. Cobalt and arsenic exhibited synergism, and lower current efficiencies were obtained for arsenic-cobalt combinations than expected. A factorially designed experiment was conducted to quantify the effects observed by one-factor-at-a-time testing. The structures and morphologies of the deposits were examined using X-ray diffraction and scanning electron microscopy.

Evaluation of zinc sulfate electrolytes by cyclic voltammetry and electron microscopy

A cyclic voltammetric technique has been developed for approximating the quantities of active chemical species present in zinc sulfate electrolytes. The experimental apparatus consisted of a Pyrex “H” cell, an aluminum cathode encased in a Teflon holder, a carbon anode and a mercurous sulfate reference electrode. Voltammograms were obtained using industrial, purified neutral leach solution (Cominco Ltd., Trail, BC) acidified to give a final concentration of 0.77 M Zn++ and 1 M H2SO4. The polarization curves were then evaluated and used as reference standards to compare with results obtained when various organic and inorganic additions were made. The deposit morphologies obtained for short-time cathodic cycles were also studied with the aid of a Scanning Electron Microscope. Changes in concentrations of glue in the 5 to 10 ppm range and of antimony in the 5 to 10 ppb range were detected using the techniques described.

Evaluation of organic additives for use in zinc electrowinning

Organic additives are used extensively in zinc electrowinning to assist in controlling the process. A cyclic voltammetry technique has been developed to provide a rapid, quantitative evaluation of the effectiveness of selected organic additives in minimizing the deleterious effects that impurities, such as antimony, have on zinc deposition. Results have indicated that animal glues are more effective than the other organic additives tested, which included several gums, enzymes, and amino acids, in relation to the current efficiency of zinc production. Of the various animal glues compared in this research, the most effective appeared to have average molecular weights in the 25,000 to 30,000 range. The effects of certain process variables on the test results have also been evaluated; these included the acid concentration of the zinc sulfate solution and the cathode preparation.

The effect of additives on the nucleation and growth of copper onto stainless steel cathodes

A potentiostatic technique has been used to study the effects of chloride ion, glue, and thiourea on the initial electrodeposition of copper. A stainless steel (AISI 304) rotating disc electrode (RDE) with an electrolyte containing 40 g/1 Cu2+ and 180 g/1 H2SO4 at 40 °C was employed. The current transients from the potential step measurements for the additive-free electrolyte could be fitted to a model that assumed progressive nucleation followed by growth of three-dimensional (3-D) centers under diffusion control. The growth mechanism and the type of nuclei were also confirmed by scanning electron microscopy (SEM) of the deposit. Chloride ions (40 ppm) affect the rate of the reaction, decrease the number of nuclei, and enhance the growth process. The particular glue (TPC 69, 5 ppm) used in this work is a polarizer and increases the number of nuclei formed on the surface. For the experimental parameters used in this research, the nucleation and growth mechanism is not changed by the presence of chloride ion or glue in the electrolyte. However, thiourea (0.5 ppm) additions caused the mechanism to change to instantaneous nucleation with 3-D growth under kinetic control, and a large number of equal sized nuclei are observed on the SEM micrographs, tending to verify the proposed mechanism.

Evaluating and monitoring nucleation and growth in copper foil

The electrodeposition of copper foil for use in electronic materials applications is a complex and demanding process. The specific aspects of producing and controlling the structure-property-performance requirements of the foil are important because of the stringent demands placed on their use in printed circuit boards and similar products. In this paper, a brief review of the electrodeposition process for raw copper foil is presented. Since electrolyte additives play such a significant role in the copper-depositionprocess, the effects of two essential additives, chloride ion and an organic (e.g., glue or gelatine), on the foil are described. Also, the influence of other operating parameters on the initial nucleation, growth, and subsequent electrocrystallization are discussed. Selected characterization methods, such as polarization and scanning electron micrography techniques, are described as a means of monitoring the process, but universally accepted methods of evaluating and controlling the additives and foil quality during electrolysis are still being sought.

A microscopy study of lead sinter

Samples of up-draft sinters produced at three different Missouri lead smelters were studied, to determine the chemical phases present, as a first step toward a more complete understanding of the lead blast furnace operation. The sinters seemed to be basically similar, and all the results reported are from samples from one smelter (Herculaneum smelter, St. Joe Minerals). Analytical tools used included the electron microprobe and scanning electron microscope with a non-dispersive X-ray spectrometer attachment Major phases that were positively identified are metallic lead, lead sulfide and zinc ferrite. The predominant lead phase was tentatively identified as a lead oxide-silicate compound. Other compounds probably present are calcium, iron, zinc-silicates or some combination of calcium, iron or zinc with silica. Contrary to the results of other investigators no lead oxide or basic lead sulfate compounds were identified. It appears that the majority of the lead is present in the sinter as a lead oxide-silicate phase.

The Fe−Ho binary system

The Fe−Ho phase diagram was determined on the basis of data obtained by X-ray diffraction, metallographic and differential thermal analysis techniques. Since emphasis was centered in the region where intermetallic compounds predominate, neither the iron nor holmium terminal regions were included in this study. Eutectic reactions were found to occur at 16.5 wt pct Fe and 875° C, 61 wt pct Fe and 1284° C, and 79 wt pct Fe and 1338° C. The congruent melting points of the compounds Ho6Fe23 and Ho2Fe17 were found to be 1332° and 1343° C, respectively. Two other intermetallic compounds were found, HoFe2 and HoFe3, and had peritectic decomposition temperatures of 1288° and 1293° C, respectively.

The effects of lead on the electrochemical and adhesion behavior of zinc electrodeposits

The presence of excessive lead in zinc sulfate electrolytes can lead to problems related to both processing efficiency and the properties of the metal produced. For example, poor adhesion can occur in electrogalvanized steel when it is heated in the temperature range of 215 °C to 280 °C if lead is present in the deposit. The duration of heating necessary to induce the peeling of the zinc was found to be dependent on the temperature, time, and concentration of lead in the electrolyte and the plating parameters. The presence of lead slowed the formation of the intermetallic, and the peeling occurred between the zinc and the iron-zinc intermetallic layer. In order to gain a better fundamental understanding of the role of lead, rotating disc electrodes were used to measure the diffusion coefficient of lead in the zinc sulfate electrolyte. The experimentally determined mass transport data on lead can be used as an aid to set an acceptable limit of lead allowable in the electrolyte or to evaluate the electrochemical characteristics of an electrolytic zinc system. By the addition of strontium carbonate to the plating solution followed by filtration, the lead concentration in the electrolyte could be reduced to an acceptable level, preventing the poor adhesion on heating.