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120007747_E-ECHP_00_00_R1_072205
1
2 Electroplating
3 E
4 Helen H. Lou
5 Department of Chemical Engineering, Lamar University, Beaumont, Texas, U.S.A.
6
7 Yinlun Huang
8 Department of Chemical Engineering and Materials Science, Wayne State University,
9 Detroit, Michigan, U.S.A.
10
11
12
13
14 INTRODUCTION and then back to the cathode constitutes the current
15 in the external circuit. The metallic ions of the salt in
16 Electroplating is an electrodeposition process for the electrolyte carry a positive charge and are thus
17 producing a dense, uniform, and adherent coating, attracted to the cathode. When they reach the nega-
18 usually of metal or alloys, upon a surface by the act tively charged workpiece, it provides electrons to
19 of electric current.[1] The coating produced is usually reduce those positively charged ions to metallic form,
20 for decorative and=or protective purposes, or enhan- and then the metal atoms will be deposited onto the
21 cing specific properties of the surface. The surface surface of the negatively charged workpiece.
22 can be conductors, such as metal, or nonconductors, Fig. 1 illustrates a typical plating unit for plating F1
23 such as plastics. Electroplating products are widely copper from a solution of the metal salt copper sulfate
24 used for many industries, such as automobile, ship, (CuSO4). The cathode, which is the workpiece to be
25 air space, machinery, electronics, jewelry, defense, plated, is charged negatively. Some of the electrons
26 and toy industries. The core part of the electroplating from the cathode bar transfer to the positively charged
2þ
27 process is the electrolytic cell (electroplating unit). In copper ions (Cu ), setting them free as atoms of
28 the electrolytic cell (electroplating unit) a current is copper metal. These copper atoms take their place on
29 passedthroughabathcontainingelectrolyte,theanode, the cathode surface and copper plate it. Concurrently,
30 andthecathode.Inindustrialproduction,pretreatment the same number of sulfate ions SO42is discharged
31 andposttreatment steps are usually needed as well. onthecopperanodes,thereby completing the electrical
32 circuit. In so doing, they form a new quantity of copper
33 sulfate that dissolves in the solution and restores it to
34 its original composition. This procedure is typical of
35 BACKGROUND ordinary electroplating processes with sacrificial
36 anodes; the current deposits a given amount of metal
37 The workpiece to be plated is the cathode (negative on the cathode and the anode dissolves to the same
38 terminal). The anode, however, can be one of the two extent (of the same electrical charge), maintaining the
39 types: sacrificial anode (dissolvable anode) and perma- solution more or less uniformly.
40 nent anode (inert anode).[2] The sacrificial anodes are
41 made of the metal that is to be deposited. The perma-
42 nent anodes can only complete the electrical circuit,
43 but cannot provide a source of fresh metal to replace ELECTROCHEMISTRY FUNDAMENTALS
44 what has been removed from the solution by deposi-
45 tion at the cathode. Platinum and carbon are usually When a direct electric current passes through an
46 used as inert anodes. electrolyte, chemical reactions take place at the con-
47 Electrolyte is the electrical conductor in which tacts between the circuit and the solution. This process
48 current is carried by ions rather than by free electrons is called electrolysis. Electrolysis takes place in an
49 (as in a metal). Electrolyte completes an electric circuit electrolytic cell. Electroplating is one specific type of
50 between two electrodes. Upon application of electric electrolysis. Besides electroplating, electrolysis has
51 current, the positive ions in the electrolyte will move also been widely used for preparation of halogens and
52 toward the cathode and the negatively charged ions notably chlorine, and refining of metals, such as copper
53 toward the anode. This migration of ions through the and zinc. Understanding the electrochemical principles
54 electrolyte constitutes the electric current in that part of electrodeposition is essential to the development
55 of the circuit. The migration of electrons into the of electroplating technologies. Some basic concepts
56 anode through the wiring and an electric generator are presented below.[3]
Encyclopedia of Chemical Processing DOI: 10.1081/E-ECHP-120007747
Copyright # 2006 by Taylor & Francis. All rights reserved. 1
120007747_E-ECHP_00_00_R1_072205
2 Electroplating
1 Power SupplyPower Supply
2
3
4 __ ++
5
6
7
8
9
10
11
12
13 22+ 22+ Copper MetalCopper Metal
14 CuCu HH OO CuCu
15 22
16 CuCu22+ ,,SOSO22–
4
4
17 ElectrolElectrolyyttee
18 Copper Sulphate SolutionCopper Sulphate Solution
19 CathodeCathode AnodeAnode
20
21 Fig. 1 Principle of electroplating.
22
23
24 Oxidation/Reduction the anode. The anode material can be either a sacrifi-
25 cial anode or an inert anode. For the sacrificial anode,
26 In a wider sense, all electron-transfer reactions are the anode reaction is:
27 considered oxidation=reduction. The substance gaining
28 electrons (oxidizing agent, or oxidant) oxidizes the sub- M ! Mnþ þ ne ð2Þ
29 stance that is losing electrons (reducing agent, or
30 reductant). In the process, the oxidizing agent is itself In this case, the electrode reaction is electrodissolution
31 reduced by the reducing agent. Consequently, the that continuously supplies the metal ions.
32 reduction process is sometimes called electronation,
33 and the oxidation process is called ‘‘de-electronation.’’
34 Because a cathode is attached to the negative pole of Faraday’s Laws of Electrolysis
35 the electric source, it supplies electrons to the electro-
36 lyte. On the contrary, an anode is connected to the In 1833, the English scientist, Michael Faraday, devel-
37 positive pole of the electric source; therefore, it accepts opedFaraday’slawsofelectrolysis. Faraday’s first law
38 electrons from the electrolyte. Various reactions take of electrolysis and Faraday’s second law of electrolysis
39 place at the electrodes during electrolysis. In general, state that the amount of a material deposited on an
40 reduction takes place at the cathode, and oxidation electrode is proportional to the amount of electricity
41 takes place at the anode. used. The amount of different substances liberated by
42 a given quantity of electricity is proportional to their
43 electrochemical equivalent (or chemical equivalent
44 Anode and Cathode Reactions weight).
45 In the SI system, the unit quantity of electricity
46 Electrodeposition or electrochemical deposition (of charge and the unit of electric charge are coulomb
47 metals or alloys) involves the reduction of metal ions (C); one coulomb is equivalent to one ampere flowing
48 from electrolytes. At the cathode, electrons are sup- for one second (1C ¼ 1Asec). The electrochemical
49 plied to cations, which migrate to the anode. In its sim- equivalent of an element is its atomic weight divided
50 plest form, the reaction in aqueous medium at the by the valence change involved in the reaction. For
51 cathode follows the equation: example, for the reaction, Fe2þ ! Fe0, the valence
52 change is 2, and the electrochemical equivalent of iron
53 Mnþ þ ne ! M ð1Þ is 55.85=2 ¼ 27.925 in this reaction. Depending on the
54 specific reaction, one element may have different
55 with a corresponding anode reaction. At the anode, equivalent weights, although it has only one atomic
56 electrons are supplied to the anions, which migrate to weight.
120007747_E-ECHP_00_00_R1_072205
Electroplating 3
1 In detail, to reduce one mole of a given metal from a Current Efficiency, Current Density,
2 metal ion with the valence charge of nþ, n moles of and Current Distribution E
3 electrons are required. That is, the total cathodic
4 charge used in the deposition, Q(C), is the product of Faraday’s laws give theoretical prediction of electrode-
5 the number of gram moles of the metal deposited, m, positioninanidealsituation.Inarealapplication,many
6 the number of electrons taking part in the reduction, factors influence the coating quantity and quality.[4]
7 n, Avogadro’s number, Na (the number of atoms in a
8 mole), and the electrical charge per electron, Qe(C). Current efficiency
9 Thus, the following equation gives the charge required
10 to reduce m moles of metal: It is stated in Faraday’s laws that the amount of
11 chemical charge at an electrode is exactly proportional
12 Q ¼ mnNaQe ð3Þ to the total quantity of electricity passing. However, if
13 several reactions take place simultaneously at the elec-
14 The product of the last two terms in Eq. (3) is the trode, side reactions may consume the product. There-
15 Faraday constant, F. Therefore, the number of moles fore, inefficiencies may arise from the side reactions
16 of the metal reduced by charge Q can be obtained as: other than the intended reaction taking place at the
17 electrodes. Current efficiency is a fraction, usually
18 m ¼ Q ð4Þ expressed as a percentage, of the current passing
19 nF through an electrolytic cell (or an electrode) that
20 The Faraday constant represents the amount of accomplishes the desired chemical reaction. Or,
21 electric charge carried by 1mol, or the Avogadro’s
22 number of electrons. The Faraday constant can be Current efficiency ¼ 100 WAct=WTheo ð9Þ
23 derived by dividing Avogadro’s number, or the num-
24 where W is the weight of metal deposited or
ber of electrons per mole, by the number of electrons Act
25 per coulomb. The former is approximately equal to dissolved, and WTheo is the corresponding weight to
26 6.02 1023 and the latter is approximately be expected from Faraday’s laws [Eq. (7)] if there is
27 6.24 1018. Therefore, no side reaction. Note that the cathode efficiency is
28 the current efficiency applied to the cathode reaction,
29 ð6:02 1023Þ and the anode efficiency is the current efficiency
30 F ¼ ¼ 9:65 104C=mol ð5Þ applied to the anode reaction.
ð6:24 1018Þ
31
32 On the other hand, the total charge used in the Current density
33 deposition can be obtained as the product of the cur-
34 rent, I(A), and the time of deposition, t(sec), if the Current density is defined as current in amperes
35 deposition current is held constant. Or, if the current per unit area of the electrode. It is a very important
36 varies during the deposition, variable in electroplating operations. It affects the
37 character of the deposit and its distribution.
38 Q ¼ Z Idt ð6Þ
39 Current distribution
40
41 The weight of the deposit, W(g), thus can be The local current density on an electrode is a function
42 obtained by multiplying the number of moles of metal of the position on the electrode surface. The current
43 reduced with the atomic weight, Mw, of the deposited distribution over an electrode surface is complicated.
44 metal: Current will tend to concentrate at edges and points,
45 Z and unless the resistance of the solution is very low,
46 Mw it will flow to the workpieces near the opposite elec-
47 W ¼ nF I dt ð7Þ trode more readily than to the more distant work-
48 pieces. It is desired to operate processes with uniform
49 Ideally, the deposition thickness, d (cm), can be current distribution. That is, the current density is
50 solved by: the same at all points on the electrode surface.
51 Z
52 d ¼ W ¼ Mw I dt ð8Þ
53 rA nFrA Potential Relationships
54
55 where r is the density of the metal (g=cm3) and A is the In electroplating, sufficient voltage should be provided
56 area of deposition (cm2). by the power source. The voltage–current relationship
120007747_E-ECHP_00_00_R1_072205
4 Electroplating
1 follows Ohm’s law. The concepts of electrode poten- change; a the activity of the metal ion. In approxima-
2 tials, equilibrium electrode potential, overpotential, tion, the concentration of the metal ion can be used
3 and overvoltage are of fundamental importance. instead of the activity.
4 If numerical values are substituted for R and F, and
5 The voltage–current relationship: Ohm’s law T is at 25C (298K), and base 10 logarithm is used
6 insteadofbasee,theNernstequationcanbeexpressedas:
7 The current is driven by a potential difference, or
8 voltage through the conducting medium, either electro- 0:059
9 lytic or metallic. The voltage necessary to force a given E ¼ E0 þ log a ð13Þ
10 n
current through a conductor is given by Ohm’s law:
11 In the above equation, if a ¼ 1, then E ¼ E0. The
12 E ¼ IR ð10Þ standard potential of an electrode E0 is the potential of
13 an electrode in contact with a solution of its ions of
14 where E is the voltage (V) and R the resistance of the unit activity. The standard potentials are always
15 conductor (O). expressed against the standard hydrogen electrode,
16 the potential of which is zero by definition. The stan-
17 Electrode potentials dard potentials are a function of temperature; they
18 are usually tabulated for 25C. Standard electrode
19 Theelectrode potential is the electrical potential differ- potential is also called normal electrode potential.
20 ence between an electrode and a reference electrode.
21 The absolute potential of an electrode is not directly
22 measurable. Therefore, the electrode potential must Overpotential and overvoltage
23 always be referred to an arbitrary zero point that is
24 defined by the potential of the reference electrode. The equilibrium is dynamic with metal ions being
25 discharged and metal atoms being ionized, but these
26 Equilibrium electrode potential twoeffects cancel each other and there is no net change
27 in the system. For the realization of metal deposition
28 When a metal is immersed into a solution containing at the cathode and metal dissolution at the anode,
29 ions of that metal, equilibrium is set up between the the system must be moved away from the equilibrium
30 tendency of the metal to enter solution as ions and condition. An external potential must be provided
31 the opposing tendency of the ions to lose their charge for the useful electrode reactions to take place at a
32 and deposit on or in the metal. practical rate; this external potential may have several
33 causes.
34 M $ Mnþ þ ne ð11Þ Overpotential is the difference in the electrode
35 potential of an electrode between its equilibrium
36 Depending on the conditions of the system, this can potential and its operating potential when a current
37 occur in either direction. At equilibrium, the driving is flowing. The overpotential represents the extra
38 forces for metal ions being discharged and metal atoms energy needed to force the electrode reaction to
39 being ionized are equal. The potential difference proceed at a required rate (or its equivalent current
40 between the metal and the solution phases under these density). Consequently, the operating potential of an
41 conditions is the equilibrium potential difference. anode is always more positive than its equilibrium
42 The equilibrium electrode potential is the electrical potential, while the operating potential of a cathode
43 potential of an electrode measured against a reference is always more negative than its equilibrium potential.
44 electrode when there is no current flowing through the The overpotential increases with increasing current
45 electrode. It is also called open circuit potential (OCP). density. The value of the overpotential also depends
46 The equilibrium potential between a metal and a solu- on the inherent speed of the electrode reaction. A slow
47 tion of its ions is given by the Nernst equation as reaction (with small exchange current density) will
48 follows: require a larger overpotential for a given current den-
49 sity than a fast reaction (with large exchange current
50 E ¼ E0 þ RT ln a ð12Þ density). Overpotential is also referred to as polariza-
51 nF tion of the electrode.
52 An electrode reaction always occurs in more
53 where E0 is the standard electrode potential, which is a than one elementary step, and there is an overpotential
54 constant characteristic of the material of the electrode; associated with each step. Even for the simplest case,
55 R the gas constant (8.3143J=k=mol); T the absolute the overpotential is the sum of the concentration
56 temperature (K); F the Faraday constant; n the valence overpotential and the activation overpotential.
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