FUNDAMENTAL ASPECTSOF ELECTROMETALLURGY

FUNDAMENTAL ASPECTSOF ELECTROMETALLURGY

Konstantin I. PopovUniversity of BelgradeBelgrade, Yugoslavia

Westaim CorporationFort Saskatchewan, Alberta, Canada

Branimir N. GrgurUniversity of BelgradeBelgrade, Yugoslavia

KLUWER ACADEMIC PUBLISHERS NEW YORK, BOSTON, DORDRECHT, LONDON, MOSCOW

eBook ISBN: 0-306-47564-2Print ISBN: 0-306-47269-4

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CONTRIBUTORS

Konstantin I. PopovFaculty of Technology and Metallurgy, University of Belgrade, Belgrade, Yugoslavia

The Westaim Corporation, Fort Saskatchewan, Alberta, T8L 3W4, Canada

Branimir N. GrgurFaculty of Technology and Metallurgy, University of Belgrade, Belgrade, Yugoslavia

v

FOREWORD

Electrometallurgy is a broad field but it is not a new one. It was the great Faradayin the 1830s who discovered laws covering the electrodeposition of metals and itsrelation to the current passed and equivalent weight of the metal undergoing deposi-tion. Since that time, applications and developments of his discoveries have spread tomany areas of technology. Electrowinning is the most well known, partly because itembraces the process by which aluminum is extracted from its ores. In electrorefining,the impure metal is made into anode and the pure metal dissolved therefrom isdeposited on a cathode. Electroplating is exemplified by its use in the manufacture ofcar bumpers. Finally, in electroreforming, objects may be metallized, often with a verythin layer of the coating desired.

The numerous technologies vary greatly in the degree to which they are intellec-tualized. Until the work of Popov et al., electrometallurgy has been regarded as largelyempirical, an activity in which there was much art and little science. This will allchange with the publication of this book. Several aspects of the background of itssenior author, Konstantin Popov, make him uniquely suited to the job of intellectualiz-ing electrometallurgy. First, he had as his mentor the great surelythe leading electrochemist in Eastern Europe since the death of Frumkin. Second, hehas had ample experience with the leading electrochemical engineer in America,Ralph White. And third, he has an admirable track record of a series of publicationsaimed at showing the remarkable variety of forms which may be made to arise inelectrodeposition.

Dr. Popov’s contributions are characterized by a comprehensive mathematicaltreatment of the phenomena he has discovered. Co-author too, has hadmuch relevant experience in applying the ideas so extensively developed in Belgradeat Canadian companies.

The contents of the present volume illustrate how certain fields of science becomesettled in certain countries. Industrial organic chemistry grew up in Germany, anddeveloped predominantly there until World War II. In electrometallurgy, the intellec-tual development has been largely in Serbia, with the most fundamental studies innucleation being carried out in the neighboring country of Bulgaria (Kaischev, Busev-ski). This is not to imply that there have been few American contributors (one recallsBrenner, Kardos, Lowenheim), but that the contributions made here in this field have

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viii Foreword

come from commercial laboratories and have been near the commercialization stage.University or research institute work on electrometallurgical topics in the United Stateshas been nonexistent since the 1950s.

There are many figures in this splendid book of Popov et al. which impress me.The first is the strong, broad contents of its arrangement. There is a fine first chapter onthe principles of application to electrochemical kinetics—the equations being writtenin a form modified for use in electrometallurgical situations (e.g., deposition on the tipsof growing crystals of minimal radius of curvature and on corners and edges). Here, itis encouraging to find authors applying the electrochemical version of Kelvin’s equa-tion relating vapor pressure as a function of the radius of drops to the phenomenaduring the electrogrowth of dendrites.

I personally find the treatments of the effects of current varying regimes (e.g.,pulse, reverse pulse, square wave, sinusoidal, etc.) the most exciting for I have longthought that instead of the use of chemical additives to the solution, the type of surfacefinally produced—even the crystal shape—could be achieved by electrical variationsonly. This book contains much toward the realization of this approach.

In the second half of the book, one finds the mathematical treatments of practicalsituations in electrowinning, electrorefining, electroplating, and electroreforming.What is the difference all this will make? It should enable to engineer to set up regimesto achieve what he wants with a minimum of prefatory experiments.

This book has no competitor. There are certainly books on electroplating, butthey are largely recipes for what to do which eschew the important question of why.

Getting the intellectual side over to the practical engineer, of course, requiresgreat lucidity, for he will not puzzle over material delivered over his head. I think therequired clarity has been attained herewith, particularly in the early chapters wherethe concepts of exchange currents and overpotential are being added to the wearythermodynamics which covers most of what engineers are likely to know aboutelectrochemistry.

A great strength is in the photographs of electrodeposited crystals in all theirvariety. Such photographs can be found in the usual journals, but I have not previouslyseen such a collection accompanied by textual rationalization.

Lastly, I was impressed by the application of the theory to areas which normallyreceive little more than a definition. I would cite electropolishing, where theory isseldom presented; electromachining; and electroless plating.

This book is a feast simply to read, but I believe its main importance is that it givesfor the first time an educational tool. It will surely lead to translations and its use willfeed back upon the economics of electrometallurgical processes—with a reduction incost many orders of magnitude greater than the total in purchases of the books.

John O’M. BockrisTexas A&M University

PREFACE

In their preface to the book Fundamental Aspects of Electrocrystallization,Bockris and Razumney in 1966 wrote: “Electroplating, the electrochemical extractionof metals from ores and mixtures, and electrodeposition from nonaqueous solutions,together make up a large area in technology. It is relevant to distinguish two classes oftechnologies: those in which, the fundamentals and theory were understood first andthe applications followed, and those in which the applied side began as a kind of art andthe theory limped behind the art, sometimes dragged back by it and occasionally givena little push. Of course, the atomic energy and electronic industry are ideal example ofthe first two. The electrodeposition and electroextraction industry is a fair example ofthe second. It is clear enough that the rate of development of the first type of industrywas much greater than that of the second, as the possibilities could be estimated or atleast the direction in which to push defined, whereas the second type relied in the paston what might be called “inspired groping.” It is hoped that this small monograph mayprovide a basis for the training of research workers who can then perform the conver-sion of what remains an art in technological electrocrystallization into a technologywith a largely rational basis.” In the meantime, although the electrochemistry of metalshas significantly advanced, the message of Bockris and Razumney, especially in thelight of education professionals, is practically the same. The bridge between high leveltheory and highly developed practice is still necessary, probably even more thanpreviously. The purpose of this book is to enable this. It is written in order to explainthe principles of electrometallurgical technologies, not to give their technical details. Itshould be easy to follow by a reader with a graduate or an undergraduate degree ineither engineering or science. It assumes the knowledge of the basic facts required onthe level of Modern Electrochemistry by Bockris and Reddy.

The book comprises twelve chapters, which can tentatively be divided into threeparts: Chapters 1,2,6, and 7 are written by Grgur and Popov, Chapters 3–5 and 8 byPopov and Grgur and Chapters 9–12 by

In the first part (Chapters 1 and 2) the significance of electrometallurgy in scienceand technology is discussed. The fundamentals of electrochemistry necessary for anunderstanding of electrometallurgical processes are also given. In the second part(Chapters 3–6) the mechanisms of metal deposition are discussed at a high scientificlevel although efforts were made to simplify them in an approachable way. For

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x Preface

example the text related to the morphology of electrodeposited metals is separated intothree levels: the mathematical approach, the physical model and, finally, a realisticsystem. The current distribution in electrochemical cells is described in a new way.Using this approach, classical complicated mathematical methods are not onlyavoided, but also calculations of typical examples for electrochemical cell are per-formed using simple relations of electrochemical kinetics (described in Chapter 2).The reader is also introduced to the effects of additives and periodically changing rateat a level which allows an understanding of metal deposition under these conditions. Inaddition, in the third part (Chapters 6–12), theoretical aspects of the electrowinningand electrorefining of metals, as well as electro- and electroless deposition of thin filmsused in electronics, automotive and aerospace applications are discussed. Electro-deposition of thin films of metals, alloys or composite coatings under direct orperiodically changing current conditions from aqueous or non-aqueous solutions, aswell as molten salts is described. Attempts have been made to up-to-date the currentknowledge with kinetics and mechanisms of electroless deposition and to criticallyevaluate the similarities and differences of deposition processes with and without anexternal current source.

In the next stage of technological development, we hope, this book will initiateefforts for the advancement of theoretical fundamentals of electrometallurgy at a level,which will permit the planning of technological processes without or with a minimumof experimental data. This is in a way similar to challenge of Bockris and Razumney, ahalf-century ago, which led to the appearance of this book.

ACKNOWLEDGMENTS

Chapters 1–8 of this book are based on a few classical studies and on research inthe field of metal electrodeposition performed in the Department of Physical Chemis-try and Electrochemistry at the Faculty of Technology and Metallurgy, the Universityof Belgrade, Yugoslavia. K. I. Popov and B. N. Grgur would like to acknowledgeProfessor who initiated this research, and colleagues Professors

andas well as numerous other colleagues and students who participate

in it.Chapters 9 to 12 represent a discussion of various aspects of electrodeposition of

alloys, composite coatings, electroforming, electrochemical oxidation of metals, elec-troless deposition and electrodeposition from molten salts, found in the open literature.

acknowledges the cooperation of numerous colleagues and coworkers fromSherritt Inc., The Westaim Corporation, and Dr. B. E. Conway at the University ofOttawa. For the help in the literature search and manuscript preparation, isthankful to and Mrs. C. Lepard.

Finally, we wish to express our thanks to Mr. Ken Howell for helpful advice andto for the preparation of the camera-ready manuscript.

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CONTENTS

1. WHAT IS ELECTROMETALLURGY 1

3

5

55

677

1.1 Further Readings

2. DEFINITIONS, PRINCIPLES AND CONCEPTS

2.1 Basic Facts2.1.1 Electrodes and electrochemical reactions, cell and circuit2.1.2 The electrochemical double layer and possible

electrochemical reactions2.2 Self Driving Cells

2.2.1 The Nernst equation and energy producing cells2.2.2 Cementation

2.3 Electrolysis2.3.1 Decomposition voltage2.3.2 A cell with an insoluble anode2.3.3 A cell with a soluble anode2.3.4 Current efficiency2.3.5 Faraday’s law2.3.6 The current density—overpotential relationships

2.3.8

2.3.6.1 Basic equations2.3.6.2 Some approximations

2.3.7 The cell voltage

2.4 Some Aspects of Electrocrystallization

3. SURFACE MORPHOLOGY OF METAL ELECTRODEPOSITS

2.5 Conclusions

3.1

2.6 Further Readings

3.1.1

Specific energy consumption

Thin Compact Surface Metal FilmsCrystallization overpotential

xiii

101010111314141414171920202626

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xiv Contents

3.1.2 The nucleation exclusion zones3.1.2.13.1.2.2

Basic definitionsPhysical simulation

3.1.33.1.43.1.53.1.6

Nucleation rate and deposition overpotentialDeposition from simple salt solutionsDeposition from complex salt solutionsDeposition in the presence of adsorbed additives3.1.6.13.1.6.2

Inorganic compoundsOrganic compounds

3.1.7 Conclusions3.2 Thick Compact Metal Electrodeposits

3.2.1 Coarse surfaces3.2.1.13.2.1.23.2.1.3

Mathematical modelsPhysical simulationReal systems3.2.1.3.1 Optimum current density for compact metal

deposition3.2.1.3.23.2.1.3.3

Cauliflower like formsCarrot like forms

3.2.2 Smooth surfaces3.2.2.13.2.2.23.2.2.3

Basic factsModel of levelingQuantitative treatment

3.2.3 Bright surfaces3.2.3.13.2.3.23.2.3.33.2.3.4

Silver mirrorElectropolished surfacesElectrodeposited surfacesConclusions

3.3 Disperse Deposits3.3.1 Spongy deposits

3.3.1.13.3.1.23.3.1.3

Mathematical modelPhysical modelReal systems

3.3.2 Dendritic deposits3.3.2.13.3.2.23.3.2.3

Mathematical modelPhysical simulationReal systems

3.3.33.3.43.3.53.3.6

Powdered depositsGranular depositsWhisker depositsConclusions

3.4 Further Readings

343436384143444445474949495456

5657596161636566666771727272727476787882838992939595

Contents xv

4. THE CURRENT DISTRIBUTION IN ELECTROCHEMICALCELLS

4.1 Two Equal Plane Parallel Electrodes Arrangement4.1.14.1.24.1.34.1.4

Ohmic resistance of the cellThe very edge ohmic resistanceThe edge effectThe depth of the penetration of a current line betweenthe electrode edges and the cell side walls4.1.4.14.1.4.24.1.4.3

Mathematical modelCell voltage-current density dependenciesDetermination of the current density distribution

4.1.5 Quantitative treatment4.1.5.1 Calculation of the cell voltage-current density

distribution dependences4.1.5.2 The critical current density for dendritic growth

initiation at the edges4.2 Cells with Low Anode Polarisation

4.2.1 The dependence of the current density at the tip of astationary wire electrode on the current density in the middleof the electrode

4.2.2 Experimental evidence4.2.2.14.2.2.2

The effect of ohmic resistanceDeposition in the presence of strongly adsorbedorganic additives (effect of increased cathodic Tafelslope)

4.2.2.3 Deposition from a complex salt solution (effect ofexchange current density)

4.3 Corner Weakness Phenomena in Electroforming4.3.14.3.24.3.3

Ohmic controlled depositionMixed activation - diffusion - ohmic controlled depositionActivation - diffusion controlled deposition

4.44.5

ConclusionsFurther Readings

5. ELECTRODEPOSITION AT A PERIODICALLY CHANGINGRATE

5.1 Basic Definitions5.1.15.1.25.1.3

Reversing currentPulsating currentAlternating current superimposed on direct current

101

103103106108

109109111115118

119

122124

124127127

129

131133133136140141142

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5.1.4 Pulsating overpotential5.2 Surface Concentration of Depositing Ions in the Periodic

Condition5.2.1 Electrodeposition with periodically changing rate in the

millisecond range5.2.25.2.3

Capacitance effectsReversing current in the second range

5.3 Prevention of the Formation of Spongy Deposits and the Effect onDendritic Particles

5.4 Compact Deposits5.4.15.4.2

Surface filmElectrode surface coarsening

5.55.65.7

Current Density and Morphology Distribution on a MacroprofileConclusionsFurther Readings

6. ELECTROWINNING

6.16.2

Theoretical Aspects of ElectrowinningTheoretical Aspects of Zinc Electrowinning6.2.1 Technological scheme of zinc electrowinning

6.3 Further Readings

7. ELECTROREFINING

7.1 Theoretical Aspects of Electrorefining7.1.1 Selectivity of metal dissolution and deposition

7.2 Theoretical Aspect of Copper Electrorefining7.2.1 Technological scheme of copper electrorefining

7.2.1.17.2.1.2

Refining of the electrolyteProcessing of the anodic slime

7.3 Further Readings

8. OPTIMUM CONDITIONS FOR ELECTROPLATING

8.18.28.3

Cementation and Deposition from the Complex Salt SolutionsThe Porosity of Metal ElectrodepositsThe Condition for the Deposition of a Coating with a MinimumPorosity

8.4 Further Readings

148

149

149154156

158161161163169172172

175

175176177180

181

181182183186188188189

191

191192

195196

Contents xvii

9. ELECTROPLATING AND SURFACE FINISHING

9.19.29.39.49.59.69.79.89.9

Electrodeposition of AlloysElectrodeposition of Composite MaterialsElectroformingElectroplating from Non-Aqueous ElectrolytesElectroplating from Room Temperature Molten SaltsElectropolishingElectromachiningElectrochemical Oxidation of MetalsFurther Readings

10. METAL DEPOSITION WITHOUT AN EXTERNALCURRENT

10.1 Basic Definitions10.1.110.1.210.1.3

Displacement depositionContact depositionAutocatalytic deposition

10.210.3

Solutions for Electroless DepositionMechanistic Aspects of Electroless Deposition10.3.110.3.210.3.310.3.410.3.5

The atomic hydrogen mechanismThe hydride ion mechanismThe electrochemical mechanismMetal hydroxide mechanismThe universal mechanism

10.4 Applications and Properties of Electroless Deposited Films10.4.110.4.210.4.310.4.4

Metallization of non-conductive surfacesElectroless deposition of composite coatingsElectroless deposition of goldElectroless deposition of other metals

10.5 Further Readings

11. ELECTRODEPOSITION OF METALS FROM MOLTEN SALTS

11.111.2

Ionically Conducting MeltsElectrochemical Studies in Electrodeposition from Molten Salts11.2.1 Electrolysis of alumina-cryolite melts

11.311.4

The Anode EffectNonconsumable Anode Materials in Molten SaltsElectrodeposition

11.5 Further Readings

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197210216219223226233236243

249

250250251251252259259260261263265266267267268269270

271

273275276286

287289

xviii Contents

12. ENVIRONMENTAL ISSUES

12.1 Environmental Concerns in the Electrowinning and Electrorefiningfrom Aqueous Solutions

12.212.312.4

Environmental Concerns in the Molten Salts ElectrolysisEnvironmental Concerns in the Electroplating TechnologiesFurther Readings

291

292293294299

INDEX 301