Electrolytic Processes

22Electrolytic Processes

Uwe ErbUniversity of Toronto

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22-122.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22-222.2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22-2

Electromotive Force and Driving Force for ElectrolyticReactions Activity, Concentration, and TemperatureDependence; Pourbaix Diagrams Activation Overpotentialand Tafel Relationship Limiting Current Density, NernstDiffusion Layer, Reaction Kinetics Other FactorsContributing to Overpotential: Hydrogen Overvoltage Electrocrystallization

22.3 Surface Finishing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22-9Electrodeposition Electroless Deposition DisplacementDeposition Chemical Conversion Anodizing

22.4 Direct Manufacturing Using Electrolytic Processing . . 22-16Large-Scale Manufacture Small-Scale Manufacture

22.5 Primary Metal Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22-18Electrowinning and Electrorefining Molten Salt Electrolysis

22.6 Electrolytic Processing of Nanostructures . . . . . . . . . . . . . 22-20Monolithic Bulk Metal Nanostructures Structurally GradedNanomaterials Nanocomposites CMA Nanostructures Nanocrystalline Particles Nanocrystalline OxidePowders Template Nanotechnology by Anodizing

22.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22-26References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22-26

Abstract

This chapter deals with the synthesis of metals, alloys, and composite materials by electrolytic processes using

aqueous solutions. The first part of the chapter covers the fundamentals of electrochemical reactions including

electromotive force and driving force for electrolytic reactions, overpotential, reaction kinetics, and electro-

crystallization. This is followed by several examples of processes in the three major application areas: surface

finishing, direct manufacturing, and primary metal production. The final section deals with the application of

electrolytic processes in the synthesis of various nanostructured materials such as bulk metal nanostructures,

nanocrystalline particles, and template materials.

22-1

2007 by Taylor & Francis Group, LLC

22-2 Materials Processing Handbook

PrimaryProduction

ElectrochemicalSynthesis

SurfaceFinishing

DirectManufacturing

FIGURE 22.1 Main application areas of material processing by electrolytic methods.

22.1 Introduction

On the basis of the underlying fundamental chemical and physical principles, most material processingroutes can be broadly categorized into one of five major groups. These are (1) solid-state processing (e.g.,rolling, extrusion, severe plastic deformation, surface deformation); (2) liquid-phase processing (e.g.,casting, rapid solidification, atomization); (3) vapor-phase processing (e.g., chemical or physical vapordeposition, molecular beam deposition); (4) chemical processing (e.g., precipitation, replacement); and(5) electrolytic processing (e.g., electrodeposition, calcination, electrowinning). Each processing route hasunique features and is specifically designed to process distinct groups of materials in the most economicalway. This section deals with electrolytic processing of materials which, in many respects, is similar tochemical processing in terms of the chemical reactions involved. However, the distinguishing feature ofelectrolytic processing is that it involves a well-defined interface at which charge transfer takes place.Most electrolytic reactions have clearly separated cathodic and anodic reactions. In processes such aselectrodeposition or electrowinning an external power supply is required to drive the reaction, while incases such as electroless deposition or galvanic conversion the electrons for metal ion reduction comeeither from a reducing agent added to the base electrolyte or from electrochemical displacement reactions.

The three major areas in which electrolytic processing is applied in industry are primary production ofmaterials, surface finishing, and direct manufacturing (Figure 22.1). This chapter will first present some ofthe fundamental aspects of electrolytic processing by discussing issues such as electrode potential, chargetransfer, and structure formation (Section 22.2). This will be followed by a description of specific examplesfrom the three major application areas (Section 22.3 to Section 22.5). Finally, examples of applications ofelectrolytic processing in the synthesis of specific nanostructured materials will be presented (Section 22.6).

This chapter is mainly concerned with metallic materials such as pure metals, alloys, and metalmatrix composites. Electrolytic processing of other materials (e.g., semiconductors and ceramics) ismentioned whenever appropriate. However, a detailed description of electrolytic processing of thesestructures is beyond the scope of this review. Furthermore, this chapter deals mainly with electrolyticprocesses involving aqueous solutions. Electrolytic processing of materials using organic solutions, moltensalts, or ionic liquids will only be briefly addressed.

22.2 Background

This section summarizes some of the important electrochemical principles of relevance to electrolyticprocessing of materials. For a more in-depth treatment of the subject, the reader is referred to the many


Electrolytic Processes 22-3

excellent electrochemistry texts that have been published over the past several decades. (e.g., Reference 1to Reference 10). These give full explanations with figures and derivations of the equations. Importantterms often used in the literature which deal with electrolytic processing are given in italic print.

22.2.1 Electromotive Force and Driving Force for Electrolytic Reactions

When a metal is immersed in an aqueous electrolyte containing ions from the same metal there will bean exchange of ions between the electrolyte and the solid metal. After a certain period of time a dynamicequilibrium will be established at which the number of metal ions entering the solution from the solid isthe same as the number of metal ions entering the solid from the solution:

Mz+ + ze M (22.1)

where M is the solid metal, Mz+ is the charged ion, z is the number of electrons involved and e is thecharge of the electron. Reaction from the left to right is called a reduction reaction. Conversely, the right toleft reaction is referred to as oxidation reaction. At equilibrium the magnitudes of the currents associatedwith the anodic and cathodic reactions are the same, but in opposite directions. The current per unitarea is referred to as the exchange current density, i0. The consequence of this is the establishment ofcharge separation through an electrical double layer (Helmholtz layer) where the metal surface has a netnegative charge (inner Helmholtz plane) while the aqueous solution adjacent to it has a positive charge(outer Helmholtz plane). Outside the Helmholtz layer is the Gouy-Chapman layer, a diffuse region of excesselectric charge that can extend over considerable distances.

As in any other chemical reaction, the driving force for electrochemical reactions can be expressed interms of the Gibbs energy of the system. Reactions proceed in the direction that would decrease the Gibbsenergy. For processes that involve electron transfer, the driving force is directly related to the potential ofthe reaction of an electrolytic cell according to the equation describing electrochemical work:

G = z F E (22.2)

where z is the number of electrons taking part in the reaction, F is the Faraday constant(96,500 coulombs/mol) and E is the potential of the reaction.

The standard potential or electromotive force of a reaction can be measured using an electrochemicalcell consisting of two half cells. In the simplest form the first half cell consists of a solid metal rod, M,immersed in a solution containing Mz+ ions, usually at a concentration of 1 mol/L. This cell is connectedto a second half cell, the standard hydrogen electrode, via an electrically conductive, but chemically inertsalt bridge. The standard hydrogen electrode (standard half cell) consists of an inert platinum electrode ina 1 M solution of H+ ions and is saturated with hydrogen gas bubbling through the solution at a pressureof 1 atmosphere and a temperature of 25C. The shorthand to describe such an electrochemical cell isdemonstrated for the case of a cell consisting of a rod of zinc immersed in a solution of ZnCl2 (1 mol ofZn2+ ions) in cell 1, connected to the standard hydrogen electrode in cell 2:

Zn(s) |Zn2+ ... ... H+(aq), H2(g) |Pt(s). (22.3)

Here (s), (aq), and (g) describe the solid, aqueous, and gaseous states, respectively, and the twodotted lines indicate the salt bridge connecting the two half cells. When the two half cells are electricallyconnected via an external wiring circuit containing a volt meter, the potential difference between the twohalf cells can be measured in units of volt. Defining the standard potential of the hydrogen electrode aszero on the potential scale, the measured voltage against the Zn half cell is +0.76 V which is a measureof the electromotive force. Therefore, the half cell reactions in the two cells together with their standard



potentials can be written as follows:

Cell 1: Zn(s) Zn2+(aq)+ 2e E01 = 0.76V (22.4)

Cell 2: 2H+(aq)+ 2e H2(g) E02 = 0V (22.5)

The overall cell reaction is given as:

Zn(s)+ 2H+ Zn(aq)2+ +H2(g) (22.6)E = +0.76V+ 0V = +0.76V (22.7)

where E is the standard potential of this electrochemical cell.The cell reaction with a positive cell potential is consistent with the fact that the zinc metal dissolves

spontaneously when immersed in an acid.It should be pointed out that the oxidation potential of E0 = 0.76V for Zn(s) Zn2+(aq) + 2e is

opposite in sign as for the reduction reaction Zn2+(aq)+ 2e Zn(s) which is E0 = 0.76V.A relative ranking in terms of their electromotive force can be obtained by measuring cell reaction

potentials for various elements relative to the standard hydrogen electrode, as described above for thecase of zinc, and summarizing the results in a table of standard potentials at 25C such as shown inTable 22.1. The values given in this table are for the reduction reactions. In this table the metals at the top

TABLE 22.1 Standard Electrode Potentials for Various Elements

Standard Standardpotential, E potential, E

Element Electrode reaction (volts), 25C Element Electrode reaction (volts), 25C

Gold Au+ + e Au +1.68 Iron Fe2+ + 2e Fe 0.44Gold Au3+ + 3e Au +1.50 Gallium Ga3+ + 3e Ga 0.53Oxygen O2 + 4H+ + 4e 2H2O +1.23 Tantalum Ta3+ + 3e Ta 0.60Platinum Pt2+ + 2e Pt +1.20 Chromium Cr3+ + 3e Cr 0.74Iridium Ir3+ + 3e Ir +1.00 Zinc Zn2+ + 2e Zn 0.76Palladium Pd2+ + 2e Pd +0.98 Chromium Cr2+ + 2e Cr 0.91Mercury Hg2+ + 2e Hg +0.85 Niobium Nb3+ + 3e Nb 1.10Silver Ag+ + e Ag +0.80 Vanadium V2+ + 2e V 1.13Tellurium Te2+ + 2e Te +0.57 Manganese Mn2+ + 2e Mn 1.18Copper Cu+ + e Cu +0.52 Zirconium Zr4+ + 4e Zr 1.53Ruthenium Ru2+ + 2e Ru +0.45 Titanium Ti2+ + 2e Ti 1.63Oxygen O2+ 2H2O+ 4e 4OH +0.40 Aluminum Al3+ + 3e Al 1.66Copper Cu2+ + 2e Cu +0.34 Hafnium Hf4+ + 4e Hf 1.70Arsenic As3+ + 2e As +0.34 Uranium U3+ + 3e U 1.80Rhenium Re3+ + 3e Re +0.30 Beryllium Be2+ + 2e Be 1.85Bismuth Bi3+ + 3e Bi +0.20 Neodymium Nd2+ + 2e Nd 2.10Antimony Sb3++ 3e Sb +0.11 Cerium Ce3+ + 3e Ce 2.34Tungsten W3+ + 3e W +0.10 Magnesium Mg2+ + 2e Mg 2.37Hydrogen 2H+ + 2e H2 0.00 Yttrium Y3+ + 3e Y 2.37Lead Pb2+ + 2e Pb 0.13 Sodium Na+ + e Na 2.71Tin Sn2+ + 2e Sn 0.14 Calcium Ca2+ + 2e Ca 2.87Molybdenum Mo3+ + 3e Mo 0.20 Strontium Sr2+ + 2e Sr 2.93Nickel Ni2+ + 2e Ni 0.25 Rubidium Rb+ + e Rb 2.93Cobalt Co2+ + 2e Co 0.28 Potassium K+ + e K 2.93Thallium Tl+ + e Tl 0.34 Arsenic As+ + e As 2.93Indium In3+ + 3e In 0.34 Lithium Li+ + e Li 3.05Cadmium Cd2+ + 2e Cd 0.40 Cesium Cs+ + e Cs 3.05

Note: Depending on the choice of cell diagram, the sign of the potential may be reversed in other references.



are chemically inert (e.g., gold, platinum, and palladium). Moving down in the table, the metals becomemore active (i.e., more susceptible to oxidation). Lithium and cesium show the highest tendencies foroxidation. Using the values given in Table 22.1, the cell potential of any pair of redox half reactions can becalculated. For example, for Zn(s)+ Cu2+ Zn2+ + Cu(s), the two half reactions are:

reduction half reaction: Cu2+ + 2e Cu(s); E0 = +0.34V (22.8)oxidation half reaction: Zn(s) Zn2 + + 2e; E0 = +0.76V (22.9)which gives: E = (+0.34V)+ (+0.76V) = +1.103V. (22.10)

The table of standard potentials is of considerable technological importance in several areas. It canbe used as a starting point in assessing the corrosion tendency of materials in the absence of corrosioninhibiting surface effects. It is also widely used as a first guide in the design of electrolytic materialsprocessing methods to be discussed later in this chapter. However, there are a number of issues to be awareof when using tables of electromotive force values such as shown in Table 22.1. The first has to do withthe convention regarding the sign of the potentials. Many European texts show the same values as shownin Table 22.1, but with the opposite sign. This was discussed in great detail by Bockris and Reddy.6 Whilethis issue is beyond the scope of this chapter, it should be pointed out that the sign of the potentials inTable 22.1 is consistent with the recommendation given by the International Union of Pure and AppliedChemistry (IUPAC). Second, the values listed in the table are only valid for standard conditions. There area number of factors that can result in considerable changes in the potential values, such as the formationof complex ions, formation of adsorbed layers, and the like. Third, while the standard potentials give goodinformation regarding the relative energy states, they provide no information with respect to the kineticsof processes (i.e., the rates of reactions).

22.2.2 Activity, Concentration, and Temperature Dependence;Pourbaix Diagrams

The potential of the Mz+/M electrode as a function of metal ion activity in solution, a (Mz+), andtemperature, T , is given by the Nernst equation:

E = E0 + RTzF

ln a(Mz+) (22.11)

where R, T , F , and z are the gas constant, absolute temperature, Faradays constant, and number ofelectrons in the reaction, respectively. For many practical applications using dilute solutions the activity,a(Mz+), can be replaced with the metal ion concentration, c(Mz+), without introducing substantial errors.Thus, Equation 22.11 can be approximated as follows:

E = E0 + RTzF

ln c(Mz+). (22.12)

At 298K (25C), the term (RT/F) has the value of 0.0257 V, giving:

E = E0 + 0.0257z

ln c(Mz+) (22.13)

or when converting the natural logarithm to decimal logarithm (factor of 2.303):

E = E0 + 0.0592z

log c(Mz+). (22.14)



1.6

1.2

0.8

0.4

0

0.4

0.8

1.2

1.6

2

0 2 4 6 8 10 12 14 16pH

Pote

ntia

l, V

(SHE

)Cu+

Cu

Cu2+

CuO

Cu2O

FIGURE 22.2 Pourbaix diagram for the watercopper system.

Using this equation it can be shown, for example, that the electrode potential at 25C of a copperelectrode immersed in aqueous copper sulfate solution (z = 2) changes from 0.34 V in a 1 M solution to0.25 V in a 0.001 molar solution, that is, by 0.0592/z for each order of magnitude decrease in the ionicconcentration.

When the Nernst equation is considered in conjunction with all possible electrochemical equilibriawhen a metal is in an aqueous solution containing its own ions, phase stability regions of various speciescan be plotted in potential pH diagrams, also known as Pourbaix diagrams.11,12 An example of aPourbaix diagram for the watercopper system is shown in Figure 22.2. These diagrams are useful forelectrolytic processes (as well as in corrosion studies) to identify all thermodynamically possible phases.However, again it is not possible to draw conclusions from such diagrams when it comes to reaction rates.

22.2.3 Activation Overpotential and Tafel Relationship

When an external potential is applied across the double layer, the electrode becomes polarized and currentwill flow either as anodic or cathodic current. The difference of the equilibrium potential, E(i = 0), andthe potential of the same electrode as a result of flowing current, E(i), is usually referred to as the activationoverpotential a:

a = E(i) E(i = 0). (22.15)The relationship between overpotential and current is usually expressed in the form of the Tafel equations:

a = a b logi (22.16)

where the constants a and b for cathodic () and anodic (+) reactions are given as:

anodic: aa = (2.303RT/zF) log i0 (22.17)ba = 2.303RT/zF (22.18)

cathodic: ac = (2.303RT/(1 )zF) log i0 (22.19)bc = 2.303RT/(1 )zF . (22.20)

In these equations i0 is the exchange current density and the geometrical transfer coefficient. Thisactivation overpotential is a direct measure of the interference of the equilibrium conditions at the electrode.Larger exchange current densities require smaller overpotentials to produce a given net external anodic orcathodic current.



22.2.4 Limiting Current Density, Nernst Diffusion Layer, Reaction Kinetics

The Tafel equation describes the currentpotential relationship for cases where charge transfer is the rate-determining step. However, for both anodic dissolution and cathodic deposition of metals the mobilityof the ions in the solution presents a practical limit that results in a deviation from linear Tafel behavior,in an E-log i plot, at what is usually referred to as the concentration overpotential. Here, we only considerthe case of cathodic deposition. If the deposition is carried out too fast, the ions Mz+ from the bulk ofthe electrolyte cannot diffuse fast enough to the solid surface to become incorporated into the growingdeposit. A current density limit will be reached at which the ions are deposited as soon as they arrive atthe electrode. At this point the metal ion concentration at the electrode approaches zero and any furtherincrease in current density will result in other electrochemical processes, such as the evolution of hydrogenbubbles. The limiting current density, iL, for a system is given by:

iL = nFD

c (22.21)

where is the thickness of the so-called Nernst diffusion layer, D the bulk diffusion coefficient for Mz+ions in the solution and c the concentration of Mz+ ions in the bulk solution.

In Figure 22.3, the change in the metal ion concentration from c, the bulk concentration in theelectrolyte to zero at the electrode surface is schematically indicated. The thickness of the transition layeris typically on the order of micrometers. With Equation 22.21 the concentration overpotential, c, can begiven as:

c = 2.303RTnF

log

(1 i

iL

)(22.22)

where the current density i is a direct measure of the actual rate of the reduction process.Both activation and concentration polarization occur at the same electrode. At low reaction rates

(small current densities) activation polarization is predominant while higher reaction rates (high currentdensities) are concentration polarization controlled.

The combined effect is summarized in the ButlerVolmer equation:

reduction = log(

i

i0

)+ 2.303RT

i0log

(1 i

iL

)(22.23)

where is the expression 2.303RT/zF . This equation allows us to determine the kinetics of manyreduction reactions from only three parameters: , i0 and iL .

Growth

Nucleation Mez+

Surface DiffusionCMez+

Cx

ze ze

Mez+

Mez+

FIGURE 22.3 Schematic diagram showing various steps involved in electrodeposition.



22.2.5 Other Factors Contributing to Overpotential: Hydrogen Overvoltage

There are many other factors that can affect the overpotential such as adsorped species (adsorptionoverpotential), ionically conducting films or oxide films (film or resistance overpotential), formation ofcomplexed ions (complex ion overpotential), ad-atom diffusion, and surface nucleation (crystallizationoverpotential).

One specific type of overpotential, sometimes referred to in short as the overvoltage, is the overpotential(required polarization) at which gas evolution appears at the electrode as a result of oxidation (e.g.,oxygen evolution) or reduction (e.g., hydrogen evolution). In particular the hydrogen overvoltage is ofgreat importance during electrodeposition processes (Section 22.3.1.1).

22.2.6 Electrocrystallization

When a metal is deposited from an aqueous solution onto a cathode, the hydrated ions in the solutionmust go through a series of complex steps before they can be incorporated as an atom into the growingelectrodeposit. As pure metals always deposit in crystalline form (amorphous structures are only formedin certain alloy deposits), electrocrystallization is usually used as the term to describe this series of processes.Electrocrystallization can be considered to consist of two distinct stages.5,6 The first stage, referred to asthe deposition stage, is the mass transfer through the Nernst diffusion layer. Mass transfer is a function ofthe surface and bulk concentrations of ions, diffusion of the ions, and total overpotential. All of these arestrongly dependent upon process parameters such as electrolyte composition, pH, temperature, agitation,and applied current density or potential. Fundamental mathematical expressions exist to describe thesephysical phenomena that give fairly good control during electrolytic processing.

The second stage of electrocrystallization is the actual crystallization process that involves the formationof adions on the cathode surface, adsorption and desorption reactions, surface diffusion of adatoms, aswell as nucleation and growth of crystals. Some of these are shown in Figure 22.3 in very simplified form.To date, crystallization phenomena are less well understood than the deposition stage phenomena. Theyare usually described by using empirical relationships.

When deposition begins, the surface conditions of the cathode are very important not only in termsof cleanliness, adsorped layers, or oxide films, but also with respect to atomistic structure. All surfacescontain numerous defects such as steps, ledges, kinks, vacancies, dislocations, and intersecting grainboundaries. Processes such as adatom attachment and diffusion, as well as nucleation and growth arestrongly influenced by these defects. Chapter 9 provides more details on surface growth.

Crystalline metal electrodeposits exhibit several growth forms including layers, blocks, pyramids, ridges,spiral growth forms, dendrites, powders, and whiskers.13 These morphologies have been studied extens-ively and various models have been proposed to correlate specific growth forms with process parametersand substrate microstructure. For example, with increasing overvoltage the deposit structure changesfrom compact to powdery.

The internal microstructure evolution of the deposit in terms of grain size and shape is strongly depend-ent upon crystal growth inhibition that can be controlled, for example, by adding certain bath additives thatreduce the surface mobility of adatoms or block certain sites for deposition. With increasing inhibition, thedeposit structure changes from field oriented (FI) to basis oriented and reproduction type (BR), to twintransition types (TT), to field-oriented texture type (FT) and finally to unoriented dispersion type (UD):14

FI: Field-oriented crystal types such as whiskers, dendrites, or loose crystal powder.BR: Basis reproduction type consisting of coherent deposits in which grain size and surface roughness

increase with deposit thickness.TT: Twin transition type showing high twin densities, usually in materials with low stacking fault

energies.FT: Field-oriented texture type showing coherent deposits with constant small grain size throughout

the deposit.UD: Unoriented dispersed type with very small grain size.



Substrate

Gro

wth

Dire

ctio

n

Substrate

Poly Ni

250 m

(a) (b)

FIGURE 22.4 Cross-sectional structure of a typical Ni electrodeposit. (a) Schematic diagram and (b) thick depositshowing transition from equiaxed fine-grained to columnar textured structure.

With increasing deposit thickness considerable structural changes can be observed as a result of thecompetition between crystal nucleation and crystal growth. Under certain deposition conditions (e.g.,BR growth conditions) the structure can change from initially fine grained, equiaxed structure to coarsegrained columnar structure with extensive grain shape anisotropy and crystallographic texture. Figure 22.4shows a schematic diagram of this transition as well as a cross-section through a nickel electrodeposit withsuch a change in the microstructure. It is generally accepted that this transition is a direct result of thegrowth competition between differently oriented grains, such that grains with lower surface energy growfaster than grains with high surface energy.14

Over the past decade, considerable progress has been made in the understanding of the early stagesof electrodeposition, microstructural evolution with increasing deposit thickness and the formation ofdendritic, densely branched, fractal, stringy, and needle-like morphologies.15,16 Many studies in theseareas apply powerful computer simulations (e.g., tight binding approximations, embedded atom models,Monte Carlo simulations) or experimental methods (e.g., in situ techniques such as scanning tunnelingmicroscopy, small-angle neutron scattering or microwave reflectivity) to study these phenomena.

22.3 Surface Finishing

The broadest area of applications of electrolytic processes deals with various types of surface-finishingmethods in which a coating is deposited or formed on the surface of finished or semifinished products.The reasons for applying coatings are to improve/modify surfaces in terms of appearance, corrosion resist-ance, wear resistance, oxidation resistance, heat and light reflectivity or absorption, electrical conductivity,magnetic properties, or to build up undersized or worn parts. The various process types include elec-trodeposition, electroless deposition, metal displacement, oxidation (anodizing) processes, and chemicaltreatments.

22.3.1 Electrodeposition

In electrodeposition the workpiece to be plated is made the cathode in an electroplating tank containingthe metal ions in aqueous solution. The workpiece is electrically connected via a power supply with theanode that is usually made of the metal to be plated either in the form of a solid rod/sheet or, morecommonly, pieces of the metal contained in an inert titanium basket. During electrodeposition, the anodedissolves slowly and replenishes the plating bath with metal ions as they are reduced on the cathode toform the coating. In some applications, dimensionally stable anodes are used (e.g., platinum or platinizedtitanium) that do not dissolve during the plating process.



TABLE 22.2 Watts Nickel and Sulfamate Nickel

Watts Nickel Sulfamate Nickel

Nickel sulfate, NiSO4 6H2O [g/l] 220400 Nickel sulfamate, Ni(SO3NH2)2 4H2O [g/l] 300450Nickel chloride, NiCl2 6H2O [g/l] 3060 030Boric acid, H3BO3 [g/l] 3045 3045Temperature (C) 4065 3060Cathode current density [mA/cm2] 30100 5300pH 24.5 3.55Anode Nickel Nickel

22.3.1.1 Pure Metals

Electrodeposition of pure metals from aqueous solutions is the most widely used surface-finishing method.The basic process at the cathode follows the reduction reaction given in Equation 22.1, for example forthe case of Ni:

Ni2+ + 2e Ni. (22.24)A large number of electroplating baths have been developed for nickel plating14 which differ mainly interms of metal salt types and concentrations, pH and bath additives used for very specific purposes. Twocommonly used plating solutions for nickel are the Watts and the sulfamate baths, Table 22.2. Nickelsulfate/nickel sulfamate are the main sources of the nickel ions in the solutions. Nickel chloride alsosupplies some nickel ions, but its primary functions are to promote the dissolution of the anode nickeland to increase the electrical conductivity of the plating solution. Boric acid acts as a buffer in the solution.There are a number of other bath additives (e.g., saccharin, coumarin) that may be added for specificpurposes such as stress relief or grain size refinement in the deposit. Many of these bath additives areproprietary chemicals supplied by many electroplating supply companies.

Potentially all metals with standard potentials more positive than the hydrogen reduction reaction(Table 22.1) could be electrodeposited from aqueous solutions. However, there are other factors that limitthe number of electropositive metals that are routinely electrodeposited. These include the quality of theresulting deposits in terms of growth form, physical integrity and properties, the plating bath stability,ease and economics of operation, as well as environmental and health risk concerns when dealing withtoxic chemicals.

On the other hand, there is also a relatively large number of metals with more negative potentialthan hydrogen that can still be plated, including nickel, cadmium, zinc, and chromium. The reason forthis is the hydrogen overvoltage relative to the metal deposition overvoltage. The hydrogen overvoltageis a direct measure of the required activation energy for hydrogen reduction that varies greatly frommetal to metal. The following example will illustrate that the metal zinc can be electroplated from anaqueous solution onto steel even though this would not be expected from its standard potential relative tohydrogen. By comparing only the two standard potentials in Table 22.1 for zinc (0.76 V) and hydrogen(0 V) one would expect hydrogen evolution to occur instead. For zinc deposition, for example, from anacidic (pH = 1) 1 molar zinc sulfate solution, the reversible hydrogen potential as per Nernst equation isE = 0.059 pH 0.06V. On the other hand, the activation overvoltage for zinc deposition is very smalland zinc can be plated at a potential very close to 0.76 V (Table 22.1). The hydrogen overvoltage onsteel at pH = 1 increases from 0.56 V at a current density of 10 mA/cm2 to 0.82 V at 100 mA/cm2.14Therefore, at a current density of 100 mA/cm2, the onset of hydrogen evolution is at a more negativepotential than the potential required for zinc deposition, thus allowing for zinc deposition onto steel.

Nevertheless, with increasing negative potential it becomes more and more difficult to deposit metalsfrom aqueous solutions. Chromium and manganese can still be plated. However, there are no practicalaqueous solutions for electrolytic processing of elements such as Zr, Ti, Al, or other elements with evenmore negative standard potentials. For these metals electrolysis requires the use of molten salt or ionic



TABLE 22.3 Important Pure Metal Electrodeposits and Their Applications

Metal Application Typical thickness(m)

Cadmium Electrochemical protection of steel and cast iron


TABLE 22.4 Examples of Nickel, Cobalt, and Zinc Alloy Electrodeposits

Material Application

NickelChromium Corrosion resistant coatingsNickelCobalt Decorative, mirror like depositsNickelCopper Compositionally modulated alloysNickelIndium Low friction and microelectronics coatingsNickelIron Soft magnetic coatings; decorative coatingsNickelManganese High ductility nickel coatingsNickelPhosphorus Abrasion and corrosion resistant coatingsNickelTungsten High temperature oxidation/resistant coatingsCobaltNickel Magnetic coatings for storage devicesCobaltMolybdenum High temperature oxidation resistant coatingsCobaltPhosphorus Wear-resistant coatingsCobaltTungsten High temperature strength coatingsZincCobalt Corrosion resistant coatingsZincNickel Corrosion resistant coatingsZincTin Coatings on fasteners for aluminum panels

Reaction 22.29 shows that phosphorus is reduced together with nickel, which explains the NiP alloydeposit formation. In this case, the phosphorus in the alloy deposit comes from the phosphorous acid inthe plating bath. The phosphoric acid does not provide P for the alloy formation. Its primary role is toadjust the pH of the solution.

The composition of the electrodeposit may be quite different from the ratio of the relative concentrationsof bath constituents. For example, for electrodeposited ZnNi electrodeposits, the ratio of the less noblemetal (Zn) to the more noble metal (Ni) in the deposit is larger than in the bath. This effect is known asanomalous codeposition.19

The electrochemistry of alloy plating is much more complex than for simple metal deposition. Thecomplexity is due to the presence of more than one ionic species in the solution, their mutual influencewhen it comes to double layer formation, electrochemical potentials, electrocrystallization, and depositstructure formation. Alloy deposition is beyond the scope of this chapter and the reader is referred to thereferences that treat alloy formation by describing simultaneous codeposition of several species for veryspecific alloy systems.13,19,25

22.3.1.3 Composite Deposits

Metalmatrix composites can be produced by adding second phase particles to the plating bath that arethen codeposited with the metal matrix. An example is shown in Figure 22.5 which represents a nickelsilicon carbide composite consisting of a nickel matrix with about 5 vol.% SiC particles (average particlesize: 0.3 m). The coating was produced from a Watts type plating bath containing about 50 g/l of SiCpowder. The purpose of the SiC particles is to increase the wear and scratch resistance of the nickel coating.Other hard particles that find application in wear-resistant coatings include alumina, titania, thoria, anddiamond.21 On the other hand, particles such as Teflon or molybdenum disulfide can be incorporated intothe metal to make low friction surfaces. Several models have been proposed26 that describe the particleincorporation in the deposits in terms of the various steps involved including initial weak adsorption ofparticles on the cathode surface, field-assisted adsorption, ionic double layer formation on the particles,and encapsulation in the metals matrix.

22.3.1.4 Semiconductors

Because of the very large negative reduction potential silicon cannot be electrodeposited from aqueoussolutions. However, several compound semiconductors can be deposited from suitable aqueouselectrolytes.14,27 Examples of the main bath constituents for several IIVI and IIIV compoundsemiconductors are given in Table 22.5.



10 m

(a)

100 nm

(b)

FIGURE 22.5 NiSiC composite electrodeposit: (a) scanning electron micrograph and (b) bright-field transmissionelectron micrograph.

TABLE 22.5 Main Bath Constituents for Semiconductor Deposition

Semiconductor Bath Constituents

InAs Indium chloride (InCl3); Arsenic chloride (AsCl3)InSb Indium chloride (InCl3); Antimony chloride (SbCl3)PbS Lead nitride (Pb(NO3)2); Sodium sulfite (Na2SO3)CdTe Cadmium sulfate (CdSO4); Tellurium oxide (TeO2)CdSe Cadmium chloride (CdCl2); Selenium oxide (SeO2)ZnSe Zinc sulfate (ZnSO4); Selenic acid (H2SeO3)GaAs Arsenic oxide (As2O3); Gallium oxide (Ga2O3)

22.3.2 Electroless Deposition

Electroless deposition14,28,29 was originally known as electrode-less deposition, describing the fact thatno external power supply is connected to positive and negative electrodes. Also known in the literatureas autocatalytic reduction or chemical reduction, the process still requires anodic and cathodic reactions.However, these occur on the same material, and the electrons required for these are supplied by a reducingagent in the plating bath according to the following general reactions:14

Mz+ + Red M+Ox (22.30)

where Red is the electron-providing reducing agent and Ox is the oxidation product of the reducing agent.The partial cathodic and anodic reactions are as follows:

cathodic: Mz+ + ze M (22.31)anodic: Red Ox+ n e (22.32)

where n is the number of electrons produced during the oxidation of the reducing agent. The two halfreactions occur on the same surface which could be either a metal substrate or a nonconductive substrate(e.g., glass, polymer) made active by first depositing catalytic nuclei such as tin or palladium. In otherwords, electroless deposition is an electrolytic method that can be used to deposit a metal coating ontononconductors such as polymers or glass.



TABLE 22.6 Reducing Agents for Electroless Metal Deposition

Metal Reducing Agents

Nickel Sodium hypophosphite (NaH2PO2H2O)Nickel Sodium Borohydride (NaBH4)Nickel Dimethylamine borane, DMAB ((CH3)2 NHBH3)Nickel Hydrazine (N2H4H2O)Copper Formaldehyde (HCHO)Gold Potassium borohydride (KBH4)Gold Dimethylamine borone, DMAB ((CH3)2 NHBH3)Palladium Hydrazine (N2H4H2O)Palladium Hydroxylamine hydrochloride (NH2OH HCl)

Some of the commonly used electroless metals that are commercially of considerable importanceare nickel, copper, gold, and palladium. Table 22.6 shows several of the reducing agents used in theirdeposition.

The electrochemical reactions during electroless deposition are usually more complex than shown inEquation 22.30, and involve several intermediate steps. Without presenting all intermediate reactions, oneimportant step during the deposition of electroless nickel using sodium hypophosphite as reducing agentis as follows:

H2PO2 +H P+H2O+OH. (22.33)This reaction shows that phosphorus is codeposited with Ni, similar to the case of electrodepositionof NiP alloys (Section 22.3.1.2). This explains why electroless nickel can contain substantial amountsof phosphorus, up to 20 at.%, depending on the bath composition and temperature. A direct consequenceof this is seen in the microstructural evolution of the deposits. For small phosphorus contents, the depositsare usually crystalline. With increasing phosphorus, the grain size continuously decreases, and for highphosphorus the deposits become amorphous.14,28,29

22.3.3 Displacement Deposition

Displacement deposition, also known as immersion plating or galvanic plating is similar to electrolessdeposition in that it does not require an external power supply. However, in contrast to electrolessplating, no reducing agent is required for this process. In displacement deposition, the electrons for metalreduction come from the substrate itself. For example, a strip of zinc can be plated with copper froma copper sulfate solution at room temperature according to the reactions given in Equation 22.8 andEquation 22.9. In other words, the more electropositive metal copper replaces the more electronegativemetal zinc on the zinc substrate. Of course, the thickness of galvanic Cu deposits is very limited, typicallyto a few micrometers, as this reaction needs direct contact between the zinc substrate and the coppersulfate solution. Once a closed layer of copper has been deposited the reaction will cease. Immersionplating solutions are commercially available17 for a number of metals (e.g., Cu, Cd, Au, Pd, Pb, Pt, Rh,Ru, Ag) as well as several alloys (e.g., brass, bronze) in particular to plate substrate materials such as steel,aluminum, zinc, and copper. Of course, the choice of deposit/substrate combination is largely dictated bythe standard potentials given in Table 22.1.

22.3.4 Chemical Conversion

From a processing point of view, chemical conversion techniques are borderline processes falling betweenchemical and electrochemical synthesis. In many cases, no clear electrified interface can be defined at whichcharge transfer occurs. Nevertheless, the most important chemical conversion processes are includedhere, because of (1) their current importance in surface finishing and (2) their as of yet unexplored



future application potential in surface nanotechnology. To date, microstructural information on chemicalconversion layers is very limited.

22.3.4.1 Antiquing of Brass, Copper, and Bronze

This process is a popular method in the manufacture of decorative hardware fixtures for many indoorand outdoor applications.17 The surface color of copper, brass, and bronze articles is modified to variousshades of brown to black or green, including verde green patina (verdigris) similar to natural patina onaged copper roofs, by an immersion technique. Typical solutions are based on polysulfide salts, seleniumsalts, or mild acids, and the color of the conversion layer comes from the compounds that form duringthe chemical reaction with the surface. Articles made of steel or zinc die casts are first electroplated witha thin layer of copper, brass, or bronze and then antiqued using the chemical conversion treatment.

22.3.4.2 Phosphating

Phosphate coatings are chemical conversion coatings applied to the surfaces of steels and aluminum alloysfor articles such as nuts, bolts, and screws, as well as sheet steel for automotive and consumer productapplications.18 These coatings provide good corrosion and wear protection and, in many cases, are appliedas a base for subsequent paint application. Using dilute solutions of phosphoric acid containing zinc, iron,or manganese salts, coating layers (typical thickness: 1 to 50 m) of strongly adherent phosphate crystalsare formed either during immersion or spray operations.

22.3.4.3 Chromate Conversion Coatings

Chromate conversion coatings are applied on various metals by chemical or electrochemical treatmentwith hexavalent chromium (chromic acid) or chromium salts (e.g., potassium chromate). During thistreatment the surface of the metal is converted to a protective film that contains complex chromiumcompounds.17,18 The thickness of the film is typically on the order of 1 m and can vary in color fromclear-bright, blue-bright and yellow to brown, olive, and black, depending on the substrate material. Themain purpose of the chromate coating is to provide corrosion protection, hardness, and wear resistanceand to serve as a base layer for subsequent paint application. Chromate coatings are routinely applied onaluminum, magnesium, and zinc alloys. More recently, trivalent chromium conversion layers have beendeveloped, in view of the environmental and health risk concerns associated with the use of carcinogenichexavalent chromium compounds.17

22.3.4.4 Blackening of Ferrous Metals

This conversion process uses various bath chemistries to produce attractive black finishes on ferrous metalsthat also provide moderate corrosion resistance.17 In the hot alkaline nitrate black oxidizing solution, theblack color comes from the black iron oxide (magnetite, Fe3O4)which is formed in boiling alkaline nitratesolution. A black zinc phosphating process has been developed in which steel, cast iron, or malleable ironis blackened before coating with a zincphosphate top layer.17

22.3.5 Anodizing

Anodizing of aluminum is a widely used surface-finishing process to improve the surface characteristicsin terms of appearance (color), durability, and corrosion resistance of finished aluminum parts.30,31

In this process, the natural protective aluminum oxide layer that forms in air to a thickness of only afew nanometers is artificially built up to considerable thickness (up to several tens of even hundreds ofmicrometers) by making the aluminum part the anode in a suitable electrolyte, connected via the powersupply to a dimensionally stable (inert) cathode such as graphite or stainless steel. While anodic treatmentin boric acid, borate, or nitrate baths produces only slightly thicker oxide films than air-exposure, sulfuricacid and chromic acid baths produce initially a thin oxide barrier, followed by the growth of a thicker,



(a)

(b)

(c)

TopView

SideView

Alumina

Alumina

Pigment

100 nm

Aluminum

Aluminum

FIGURE 22.6 Nanoporous aluminum oxide produced by anodizing: (a) top view showing hexagonal pore structure,(b) cross-sectional view and (c) coloring by pigment addition.

porous layer on top of the initial layer. The reactions are as follows:

cathodic reaction: 6H+ + 6e 3H2(g) (22.34)

anodic reactions: 2Al+ 3O2 Al2O3 + 6e (22.35)2Al3+ + 3H2O Al2O3 + 6H+. (22.36)

It should be noted that there are two anodic reactions that produce Al2O3. The first Reaction 22.35 isat the Al/Al2O3 interface, while the second Reaction 20.36 takes place at the Al2O3/electrolyte interface,which requires the diffusion of Al3+ ions through the Al2O3 layer already formed on the base material.

Under suitable conditions this porous layer can be produced as irregular or regular arrangements ofhexagonal columns with pores extending from the free surface down to the initial barrier layer. Figure 22.6aand Figure 22.6b show schematic diagrams of the top and side views of a regular oxide structure withoutgiving details of the original barrier layer. For requirements such as scratch and corrosion resistance, theporous aluminum oxide is usually sealed, for example, by boiling in water, which hydrates and expands theoxide to close the pores. In color anodizing, the pores are impregnated prior to sealing with various dyeingcompounds, mineral pigments, or microparticles formed from the microconstituents of the aluminumalloy during the anodizing process, to produce a wide variety of colors, in particular for architecturalapplications of aluminum (Figure 22.6c).

22.4 Direct Manufacturing Using Electrolytic Processing

Electrodeposition is not restricted to surface-finishing applications, but is also widely used to producefinished or semifinished products through direct manufacturing. Generally known as electroforming,3235



VE VE

ExtractionCollar

Stainless SteelMandrel

Nickel DepositForming

NickelElectroform

During Electroforming After Extraction

FIGURE 22.7 Net shape manufacturing of an electroformed product (courtesy of Nickel Development Institute,Toronto, Canada).

these processes are used to produce various product forms by electrodeposition onto suitably shapedcathodes, designed as temporary reusable mandrels during the deposition process (Figure 22.7). Followingelectrodeposition of the product, the mandrel is separated from the electroform by simple mechanicalmeans or heating/cooling cycles. To allow for easy separation, mandrel materials such as titanium orstainless steel are commonly used that have a thin oxide layer on the surface to prevent strong adhesionof the electroformed product. Many metals have been used in electroforming, including nickel, copper,iron, gold, platinum, cobaltnickel, and cobalttungsten alloys.

22.4.1 Large-Scale Manufacture

Direct net shape manufacturing by electroforming is applied in areas where other processing techniquesare limited in their capabilities or simply too expensive. The most widely known example of electroformednickel are the large printing rolls used as mandrels in textile printing in which the pattern is embeddedin the surface of the electroformed nickel. Other examples in which the same principle was used are thestampers for records and CDs and the embossing tools for holographic images.

Sheet and foil products with thicknesses


TopView

SideView

MaskPMMAMold

OriginalMask

CavityEtchedMold

PlanarizedDeposit

MetalDeposit

FinalProduct

MEMS gear

X-Ray Irradiation(a) (b)

(d)

(f)

(c)

(e)

500 m

Metal Substrate

Metal Substrate

Metal Substrate

Metal Substrate

FIGURE 22.8 Various steps in the production of a MEMS gear by electrodeposition: (a) mask showing the shape of thegear, (b) irradiation of PMMA through mask, (c) etched mold with cavity, (d) mold cavity filled with electrodepositedmetal, (e) planarization step and (f) final product removed from mold.

applications.36,37 One specific area of MEMS manufacturing that deals with electrodeposition of small(


lead) anodes. For this reason the anodic and cathodic reactions for both processes are quite different.For example, if the electrowinning electrolyte, coming from a sulfuric acid leaching of a metal oxide, isMSO4(aq) then the reactions are as follows:

cathodic reaction: MSO4(aq) + 2e M+ SO24(aq) (22.37)anodic reaction: H2O 2H+ + 1/2O2(g) + 2e (22.38)

In other words, the main reaction at the anode is the decomposition of water. On the other hand, duringelectrorefining the reactions are simply as follows:

cathodic reaction: Mz+ + ze M (22.39)anodic reaction: M Mz+ + ze (22.40)

In both cases, the cathode materials are often starter sheets of the same material that is to be deposited.Assuming that there are no other cathodic reactions, the weight, W , of the deposit can be calculated usingFaradays law:

W = I t Az F (22.41)

where I is the current through the cell in amps, t the time in seconds and A the atomic weight of thedeposited material.

22.5.2 Molten Salt Electrolysis

As pointed out in Section 22.3.1.1, reactive metals more electronegative than manganese cannot beprocessed electrolytically from aqueous solution. For this reason electrolytic methods for such metalsrequire either water-free organic electrolytes38 (not discussed in this chapter) or molten salt electrolytes.The latter are sometimes also referred to as fused salt or high temperature inorganic melts. Examplesof materials that have been produced by this method include many refractories such as molybdenum,niobium, vanadium and tungsten, titanium, zirconium, magnesium, and aluminum, and several rareearth elements. While many of the fundamental concepts developed for aqueous electrolytic processescan be applied to molten salt electrolysis, considerable modifications and new considerations are requiredbecause the molten salt solvent is quite different from water as the solvent. In fact, the electrochemistry offused salts is an entire research field on its own.39,40

Probably the most widely known molten salt process is the HallHroult process for aluminum pro-duction introduced in 1866.41 In this process, molten cryolite (Na2AlF6) is used as the solvent for alumina(Al2O3) which is usually chemically purified from bauxite in the Bayer process. Cryolite melts at about940C, and the HallHroult process operates at temperatures between 960 and 1000C. The anodematerial used is carbon and a typical electrolytic cell operates at 6 V. The cell is lined with carbon that actsas the cathode onto which Al is deposited. The overall cell reaction is:

2/3Al2O3 + C 4/3Al+ CO2. (22.42)

During the process, the carbon from the anode is continuously consumed to produce CO2. A typicalcell produces about 300 kg of aluminum daily at an energy consumption of about 15 to 26 kWh/kg ofaluminum. Aluminum with a purity of 99% or higher can be produced by this process. It is important tonote that the fused salt electrolyte does not chemically react with the Al deposit.



22.6 Electrolytic Processing of Nanostructures

As a result of their outstanding physical, chemical, and mechanical properties, nanostructured materialshave received considerable attention over the past 10 years. Since their first introduction in the early 1980s,several hundred synthesis methods for their production have been described in the literature.4252 Manyof these methods are simple extensions of well-established production routes while others were specificallydesigned for various types of highly specialized nanomaterials. Many of these techniques are based onelectrolytic processes.

In the following sections, examples will be presented for the synthesis of a variety of nanostructuredmaterials by electrolytic processing including monolithic metals and alloys, structurally graded nano-metals, nanocomposites, compositionally modulated alloy (CMA) nanostructures, and nanocrystallineoxide powders.

22.6.1 Monolithic Bulk Metal Nanostructures

Synthesis methods for the production of monolithic bulk electrodeposits with grain sizes


200 nm

FIGURE 22.10 Darkfield electron micrograph parallel to the substrate of bulk nanocrystalline nickel (average grainsize: 12 nm) in planar cross-section.

Electrodeposition processes have been developed for the synthesis of nanocrystalline deposits in manydifferent shapes and forms.54,55 These range from thin and thick corrosion and wear-resistant coatings tosheet and plate products for structural, magnetic, and electronic applications. Furthermore, the techno-logy can be incorporated with little extra costs in conventional electroplating plants in processes such asrack plating, reel-to-reel plating, continuous strip plating, barrel plating, and brush plating.

One of the earliest industrial applications of this type of nanocrystalline deposits was the in situelectrosleeve nuclear reactor steam generator tubing repair technology.60,61 This technology was the firstlarge-scale structural application of nanomaterials in the world, specifically developed for Canadian andU.S. nuclear reactors since the early 1990s. In this process, steam generator tubes (e.g., Alloy 600 or 400)whose structural integrity was compromised by localized degradation phenomena (e.g., intergranularcorrosion, pitting) were repaired by coating the inside of the tubes with a thick (1 mm) nanocrystalline Nimicroalloy to restore a complete pressure boundary. The grain size of the materials was adjusted to be inthe 50 to 100 nm range to give the required combination of strength, ductility, corrosion resistance, andthermal stability for this application. Figure 22.11 shows some of the tremendous property improvementsthat can be achieved when reducing the grain size in nickel electrodeposits from 10 m to 10 nm.59,62

22.6.2 Structurally Graded Nanomaterials

While monolithic nanometals with relatively narrow grain size distributions have excellent propertiesin terms of hardness, tensile strength, and wear resistance, their tensile ductility is usually comprom-ised regardless of the processing route.59,63 In the past few years, it has been recognized that considerableductility can be restored in these materials through broader or bimodal grain size distributions.63 Recently,electroplating conditions have been developed for the synthesis of a variety of structurally graded nano-metals (Figure 22.12), including bimodal distributions, alternate layers of different grain sizes, and grainsize gradient deposits.59,64 In such structures, a good compromise between strength and ductility canbe achieved, whereby the smaller grains provide the strengthening in the metal while the larger grainsallow for sufficient dislocation activity to result in reasonable ductility values. It should be noted that thedeformation mechanisms in these materials are currently not completely understood.

22.6.3 Nanocomposites

Over the past several years, the concept of composite coatings (Section 22.3.1.3) has been extended to nano-crystalline materials.54,55,59,65 Figure 22.13 shows several examples of submicrocrystalline/nanocrystallinesecond phase particles or fibers embedded in a nanocrystalline matrix. Typical examples of second phase



Grain Size (nm)

Vick

ers

Har

dnes

s (G

Pa)

0

1

2

3

4

5

6

7

8

Yiel

d St

reng

th (M

Pa)

0

200

400

600

800

1000

Grain Size (nm)100 101 102 103 104 105

Wea

r rat

e (m

m3/m

m)

105

0

25

50

75

100

125

150

Grain Size (nm)

Coef

ficie

nt o

f Fric

tion

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

(a) (b)

(c) (d)

100 101 102 103 104 105

Grain Size (nm)100 101 102 103 104 105 100 101 102 103 104 105

FIGURE 22.11 Effect of grain size in electrodeposited Ni on (a) Vickers hardness, (b) yield strength, (c) sliding wearrate and (d) coefficient of friction.

materials are Al2O3, SiC, or B4C for improved hardness and wear resistance, or Teflon and MoS2 forreduced coefficient of friction. Composite coatings can also be produced in situ by first codepositing analloy as a supersaturated solid solution, followed by heat treatment to precipitate nanosized second phaseparticles in a nanocrystalline matrix.62,66

22.6.4 CMA Nanostructures

The idea of producing CMAs by electrodeposition (Figure 22.14) is not new. Some of the earliest workwas apparently already done by A. Brenner in his 1939 Ph.D. thesis (as cited in Reference 67). Becauseof the unusual microstructure (i.e., individual layer thickness


(a) (b)

(c)

FIGURE 22.12 Schematic diagrams showing various types of structurally graded nanomaterials: (a) bimodal grainsize distribution, (b) alternate layers with different grain sizes and (c) grain size gradient structure. (Courtesy ofIntegran Technologies, Toronto, Canada.)

(a) (b)

(c) (d)

FIGURE 22.13 Examples of nanocomposite electrodeposits showing various configurations with different shapesand sizes of the reinforcing phase in a matrix of electrodeposited nanometal. (Courtesy of Integran Technologies,Toronto, Canada.)

into an electrolyte containing both species to be plated. By stepping the potential between predeterminedvalues, the relative deposition rate of one species over that of the other for certain time intervals iscontrolled. Using these approaches, numerous compositionally modulated nanostructures have beenproduced, including AgPd, CuNi, CuPb, and NiP59,67 with modulation wavelength down to a fewnanometers.

Even nanomodulated ceramic superlattices have been produced by the potential-stepping method. Forexample, layered structures consisting of TlaPbbOc semiconducting oxides, with varying values for a, b,c in the alternating layers, were produced with modulation wavelengths between 6 and 13 nm.68 Thesematerials are of considerable interest for optical and electrical applications.



NiCu

NiCu

NiCu

Multilayers

Substrate

FIGURE 22.14 Schematic diagram showing a compositionally modulated material with alternating nanometer thicklayers of Ni and Cu.

22.6.5 Nanocrystalline Particles

A number of electrolytic methods have been developed for the synthesis of various nanoparticles eitheron substrates or in colloidal solutions. For example, using the galvanic displacement approach (Section22.3.3), very intricate structures of silver nanoparticles were recently grown on (111) and (100) orientedsingle crystals of germanium according to the following displacement reaction:69

Ge Ge4+ + 4e (22.43)4Ag+ + 4e 4Ag. (22.44)

The hexagonal nanoparticles deposited in the form of vertical stacks on the substrate in what wasdescribed as nanoinukshuks, an analogy to the inukshuk structures made by the Inuit people in the Arcticusing flat slabs of rock.

Another example of nanoparticle production using a combination of electrolytic and chemical reac-tions is the synthesis of -CuI semiconductor nanocrystallites.70 The three-step process involved (1) theelectrodeposition of copper nanocrystals on a basal oriented single crystal of graphite from a Cu2+ con-taining solution, followed by (2) electrochemical oxidation of Cu to Cu2O and (3) displacement of oxygenby iodine in an aqueous potassium iodine solution to obtain semiconducting -CuI nanocrystallites withparticle sizes up to about 20 nm.

22.6.6 Nanocrystalline Oxide Powders

Known as electrochemical deposition under oxidizing conditions (EDOC), one specific technique toproduce nanostructured oxide powders is based on metal dissolution and deposition.59,71,72 As shown inFigure 22.15, this method consists of three basic steps. At the sacrificial anode, metal is dissolved in anorganic electrolyte (e.g., 2-propanol) containing a salt to increase the electrical conductivity of the bath.The second step is the cathodic reduction of metal ions to form metal clusters. These clusters are coatedwith a stabilizer (e.g., quarternary ammonia salts, betains, or ethoxylated fatty alcohols, not shown inFigure 22.15). In the final step, the colloidal metal particles are oxidized by introducing oxygen or air intothe bath with simultaneous strong agitation or sonication. Subsequently, the oxide particles are filtered,washed with ethanol and dried. This method can produce simple oxides (e.g., ZnO, MgO, CuO, Fe2O3,SnO2) or mixed oxides (e.g., CoFe3O4, In2O3/SnO2)with high purity and particle size distributions muchnarrower than what can be achieved with other nano-oxide powder synthesis methods such as inert gascondensation, sol-gel, or flame pyrolysis. EDOC is a relatively inexpensive synthesis method and easy toscale up for large-scale production.

22.6.7 Template Nanotechnology by Anodizing

Over the past decade, the synthesis of highly ordered structures based on using ordered arrays of nanopor-ous honeycomb structures of anodic aluminum oxide (Section 22.3.5) has evolved rapidly as part of the



PowerSupply

Oxygen

+

+

Sacr

ificia

l Ano

de

Cath

ode

e

Mex+Mex+

Oxygen

Metal Cluster

MeO e

e

e

FIGURE 22.15 Schematic diagram showing an experimental setup for the production of nanocrystalline powdersvia EDOC. (Modified from Dierstein, A. et al., Scripta Mater., 44, 2209, 2001.)

Deposition of Various Materials

Substrate

Substrate

Substrate

Free-StandingAluminaTemplate

100 nm Nanodots

Nanowires

(a)

(b)

(c)

+

FIGURE 22.16 Use of nanoporous alumina template in nanodot and nanowire synthesis.

general area of template synthesis of nanostructures.59,7375 In these processes, the porous alumina filmsare extracted from previously anodized aluminum sheets (usually by dissolving the aluminum base) toserve as a template (Figure 22.16a) in further synthesis steps for a variety of nanostructures. For example,Figure 22.16b shows that deposition of other materials (e.g., metals, semiconductors) through such atemplate can be used to produce regular arrays of so-called quantum dots, which show very interestingelectrical properties resulting from electronic state confinement in crystals with very small external sizes(


Besides nanowires, template synthesis has been used to produce nanocables with a radialmetal/semiconductor junction.76

22.7 Conclusions

Electrochemical reactions provide the basis for an extensive number of electrolytic processes to synthesizematerials in many different shapes and forms. Numerous processing routes have been developed overthe past century for primary metal production, direct manufacturing processes, and surface-finishingapplications. Today, many of the chemical and physical fundamentals of electrolytic processing are fairlywell understood. However, numerous aspects of electrocrystallization require further study to achieve abetter control of microstructure, and therefore properties, of the final material through appropriate choiceof processing parameters such as electrolyte composition, pH, temperature, current density, and electrolyteagitation. While electrolytic processing is overall a relatively mature technology for conventional materialsynthesis, research over the past 20 years has shown that there are tremendous new opportunities forelectrolytic processes in the general area of nanotechnology. Several nanostructured materials producedby electrolytic processes have already found application in industry, and many more processes are veryclose to commercialization. The outlook for electrolytic processing is excellent, and it is expected thatmany of the questions regarding electrocrystallization will be answered in the next few years through theextensive use of powerful computational as well as nanoscale chemical and microstructural analysis toolscurrently applied in the development of electrolytic nanomaterials processing technologies.

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New York, 1967.[6] Bockris, J.O.M. and Reddy, A.K.N., Modern Electrochemistry, Vols. 2, Plenum Press, New York,

1970.[7] Bloom, H. and Gutmann, F., Electrochemistry The Past Thirty and the Next Thirty Years, Plenum

Press, New York, 1977.[8] Bard, A.J. and Faulkner, L.R., Electrochemical Methods, Wiley & Sons, New York, 1980.[9] Reiger, P.H., Electrochemistry, 2nd ed., Chapman and Hall, New York, 1993.

[10] Bockris, J.O.M. and Khan, S.U.M., Surface Electrochemistry A Molecular Level Approach, PlenumPress, New York, 1993.

[11] Valensi, G., Van Muylder, J., and Pourbaix, M., Atlas of Electrochemical Equilibria in AqueousSolutions, Pergamon Press, New York, 1966.

[12] Pourbaix, M., Lectures on Electrochemical Corrosion, Plenum Press, New York, 1973.[13] Gorbunova, K.M. and Polukarov, Y.M., Electrodeposition of alloys, in Advances in Electrochemistry

and Electrochemical Engineering, Vol. 5, Tobias, C.W., Ed., Interscience Publishers, Wiley & Sons,New York, 249, 1967.

[14] Schlesinger, M. and Paunovic, M., Eds., Modern Electroplating, John Wiley & Sons Inc., New York,2000.

[15] Andricacos, P.C. et al., Eds., Electrochemical synthesis and modification of materials, Mat. Res.Soc. Symp. Proc., 451, 1997.

[16] Merchant, H., Ed., Defect Structure, Morphology and Properties of Deposits, TMS, Warrendale, PA,1344, 1995.

[17] Stivison, D.S., Ed., Metal Finishing 2004 Guidebook and Directory, Elsevier, New York, 2004.



[18] Wood, W.G., Ed., Metals Handbook 9th ed., Vol. 5: Surface Cleaning, Finishing and Coating,American Society for Metals, Metals Park, OH, 1982.

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Chapter 22: Electrolytic ProcessesAbstract22.1 Introduction22.2 Background22.2.1 Electromotive Force and Driving Force for Electrolytic Reactions22.2.2 Activity, Concentration, and Temperature Dependence; Pourbaix Diagrams22.2.3 Activation Overpotential and Tafel Relationship22.2.4 Limiting Current Density, Nernst Diffusion Layer, Reaction Kinetics22.2.5 Other Factors Contributing to Overpotential: Hydrogen Overvoltage22.2.6 Electrocrystallization

22.3 Surface Finishing22.3.1 Electrodeposition22.3.1.1 Pure Metals22.3.1.2 Alloy Deposits22.3.1.3 Composite Deposits22.3.1.4 Semiconductors

22.3.2 Electroless Deposition22.3.3 Displacement Deposition22.3.4 Chemical Conversion22.3.4.1 Antiquing of Brass, Copper, and Bronze22.3.4.2 Phosphating22.3.4.3 Chromate Conversion Coatings22.3.4.4 Blackening of Ferrous Metals

22.3.5 Anodizing

22.4 Direct Manufacturing Using Electrolytic Processing22.4.1 Large-Scale Manufacture22.4.2 Small-Scale Manufacture

22.5 Primary Metal Production22.5.1 Electrowinning and Electrorefining22.5.2 Molten Salt Electrolysis

22.6 Electrolytic Processing of Nanostructures22.6.1 Monolithic Bulk Metal Nanostructures22.6.2 Structurally Graded Nanomaterials22.6.3 Nanocomposites22.6.4 CMA Nanostructures22.6.5 Nanocrystalline Particles22.6.6 Nanocrystalline Oxide Powders22.6.7 Template Nanotechnology by Anodizing

22.7 ConclusionsReferences

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Electrolytic Processes