CHAPTER FIVE ELECTROCHEMICAL DEPOSITION OF METALS
Electrolytic industrial processes for metals
include:
electrolytic extraction of metals from
their ores; electrowinning electrolytic purification of metals electroplating of metals on conducting surfaces electroforming of metals in different desired shapes recovery of spent metals and production of metal powders and alloys. In all these types of electrolytic process, the
reactions are mainly reduction of ions of the
5.1 Theory of Electrodeposition A) Conditions for Starting Deposit:
A metal starts to deposit when the cathode potential becomes equal to the value given by the equation: E
actual
=E
theo
+ή
M
For two metals A and B to deposit simultaneously from their salts, the following condition should be satisfied: EA = E B E°A + (RT/zF) ln [A+] + ήA = E°B + (RT/ZF) ln [B+] + ήB Examination of the above equation shows that there are three ways by which the discharge potential for the two cations may be brought together:
1) If the standard potentials are approximately equal and polarization is small, the two metals will deposit simultaneously. 2) If the standard potentials of the two metals are different, but the overvoltage vary sufficiently to compensate for this difference, the two metals will deposit together at a given concentration: Eo
A
> Eo
B
but ή
A
< ή
B
3) If the standard electrode potentials are not equal and polarization is not negligible, the two potentials can be made close to each other through adjusting the metal ion concentration.
B) Examples from Practice on Electrochemical Deposition: 1) Lead and tin lie near each other in the electromotive series, (E°, Pb = -0.126 V and E°, Sn = -0.136 V). The two metals have a negligible polarization, but they can be deposited together by a slight adjustment of their ion concentration to compensate for the small difference in their E° values. 2) Zinc and hydrogen lie far apart in the electromotive series. Theoretically, it is impossible to deposit Zn form an aqueous ZnSO4 solution because H2 will evolve in preference to Zn deposition (hydrogen needs less potential). In practice, it is possible to deposit Zn from aqueous solutions since the difference in the standard potentials of the two elements is compensated for by the high hydrogen overvoltage on Zn.
• 3) Copper and zinc are far apart in the electromotive series; hence, it is impossible to deposit the two metals simultaneously from their simple source (a mixture of CuSO4 and ZnSO4). • It was found that the two metals can be deposited together if a cyanide complex of Cu and Zn is used as discussed below. • The degree of ionization of the Zn cyanide complex is relatively high compared with that of the Cu cyanide complex. • Therefore, the concentration of free Zn ions will be high compared to the concentration of free Cu ions. • The difference in E° of the two metals will be compensated for by the difference in the ion concentrations: Copper: K3[Cu(CN)4] → 3 K+ + {Cu(CN)4} {Cu(CN)4} -3 ↔ Cu+ + 4 CN-
-3
K = [Cu+][CN-]4 / [{Cu(CN)4} - 3] = 10 Zinc: K2[Zn(CN)4] → 2 K+ + {Zn(CN)4} -2 {Zn(CN)4} -2 ↔ Zn2+ + 4 CN-
-27
K = [Zn2+][CN-]4 / [{Zn(CN)4} -2] = 10
-18
C) Factors Affecting the Quality of Metal Electrodeposits:
Electrodepositive metals are crystalline in nature. The external appearance of the deposit depends on the rate of nucleation and the rate of growth of metal crystals. If the rate of nucleation is higher than the rate of crystal growth, a smooth, fine deposit is obtained. If, on the other hand, the rate of crystal growth is higher than the rate of nucleation, a coarse, rough deposit is obtained. The factors which affect the relative rates of nucleation and growth and hence affect the quality of the electrodeposit are discussed below.
1. Effect of current density: At low current densities, the rate of crystal growth is higher than the rate of nucleation. So, a rough (coarse) deposit is obtained. At moderate current densities, the rate of nucleation becomes higher than rate of growth, and a fine (smooth) deposit is obtained. At high current densities, the solution at the electrode surface becomes dilute in the electroactive ion and this leads to the formation of dendretic deposit (Figure 5.1). At the limiting current, hydrogen starts to evolve and interferes with crystal growth leading to the formation of powder metal. 2. Effect of the nature of electrolyte: It was found that complex salts produce fine, smooth deposits, while simple salts produce rough deposits e.g. Ag deposited from AgNO3 is coarse, while Ag deposited from silver cyanide complex: K[Ag(CN)2] is smooth and fine. Similarly, CuSO4 produces a rough Cu deposit, while copper cyanide complex: K3[Cu(CN)4] produces smooth deposit.
Figure 5.1: I-E Plot for Metal Deposition
3. Effect of electrolyte concentration: Relatively high concentrations of metalcontaining electrolyte give firm, adherent deposits while dilute solutions give dendretic, loose deposits. 4. Effect of temperature: The operating temperature of the electrolytic cell has two opposing effects. The first effect is that increasing the temperature increases the rate of diffusion of the electroactive ion to the cathode surface. This leads to the prevention of the formation of dendretic deposits which are formed at high current densities where the solution at the cathode surface is very dilute. The second effect is that increasing the temperature favors crystal growth, i.e., the formation of rough deposits.
The first effect predominates at moderate temperatures, while the second effect predominates at high temperatures. 5. Effect of adding colloidal materials: Addition of a very small amount of a colloidal material (in the order of 50 mg/L) to electroplating baths produces smooth, fine and bright deposit. Such colloidal materials include gelatin, agar, glue and gum. The mechanism of action of colloidal matter is that this material becomes adsorbed on the surface of the nuclei, thus reducing the rate of growth of these nuclei; hence, the solution is forced to form new nuclei. In other words, the rate of nucleation becomes higher than the rate of growth and this result in the formation of smooth and fine deposits.
5.2 Industrial Applications of Metal Electrodeposition
Electrowinning of Metals Electrowinning of metals is the recovery of metals from their compounds or natural ores by electrolysis and it is the oldest industrial electrolytic process. Sodium metal was first prepared in 1807 by the Humphrey Davy, who obtained it using electrolysis of molten sodium hydroxide. Several industrially important metals (the active metals which react with water) are produced commercially today by electrolysis of molten salts. The process of electrowinning consist of three steps (see Figure 5.2 for zinc production from ZnS ores): 1- Leaching by dissolving the ore in a suitable acid. 2- Purification of the leach liquor by one of the following methods: a) fractional crystallization b) fractional precipitation c) solvent extraction d) cementation; which is the addition of pure zinc dust to precipitate the impurities and to free zinc ions: Zn (powder) + CuSO4 (aq) → Cu (solid deposit) + ZnSO 4 (aq)
Figure 5.2 Electrowinning of Zinc
3- Pure electrolyte is electrolyzed in a cell under the suitable conditions to obtain the required metal and a spent acid which is circulated to the leaching tank. In electrowinning, usually an insoluble anode is used (e.g. Pb, carbon and Pt). At the cathode, the following reactions take place: Zn2+ + 2 e- → Zn & 2H+ + 2 e- → H2 At the anode: 4 OH→ O2 + 2 H2O + 4 e• Metals which are more noble than H2 in the electromotive series such as Cu, Ag and Au deposit with a current efficiency of 100% while metals which are less noble than H 2 deposit with a current efficiency less than 100 % owing to the simultaneous evolution of H2 with the metal. • Metals with very high oxidation potential such as Mg and Al cannot be deposited from aqueous solutions because H 2 is discharged from these solutions instead of the metals. Such metals are electrowon from molten baths.
Electrowinning of Magnesium. • This is the most commonly used industrial process for magnesium production. • The raw material for magnesium production is well brines or seawater, which is about 0.13% magnesium by mass. • The Mg2+ ion is precipitated from seawater by addition of CaO (lime): CaO + H2O → Ca (OH)2 and Ca(OH)2 + Mg2+ → Mg(OH)2 + Ca2+ • The insoluble Mg (OH)2 is removed by filtration. • Acidification of a slurry of this solid and an aqueous MgCl2 solution with HCl converts the Mg(OH)2 to soluble MgCl2, which is recovered as solid MgCl2 by evaporation. • This is dried to the hydrate MgCl2.1.5H2O, which is used as the feedstock to an electrolytic cell.
• The cell electrolyte is a molten mixture containing about 25% MgCl2 - 15% CaCl2 - 60% NaCl, and the cell is operated between 700oC and 750oC. • The cell electrolytic reaction is MgCl2 → Mg (l) + Cl2 (g) • The magnesium produced is about 99.9% pure as it comes from the cell, since neither sodium nor calcium is reducible more easily than is magnesium. • The chlorine produced in the cell is converted to HCl for use in the acidification process.
Electrowinning of Aluminum. • Aluminum compounds, primarily the oxide in forms of various purity and hydration, are fairly widely distributed in nature. • The major ore of aluminum is bauxite, a hydrated aluminum (III) oxide (Al2O3.xH2O). • In the industrial Bayer process, bauxite is concentrated to produce aluminum hydroxide. • When this concentrate is calcined at temperatures in excess of 1000oC, anhydrous aluminum oxide, Al2O3, is formed. • In the well-known Hall process (Figure 6.3), purified Al2O3 is dissolved in molten cryolite, Na3AlF6, which has a melting point of 1012oC and is an effective conductor of electric current. Graphite rods are employed as anodes and are consumed in the electrolysis process. • The cell electrolytic reaction is: 2Al2O3 + 3C → 4 Al (l) + 3 CO2 (g)
Figure 5.3 Electrowinning of Aluminum
The quantities of bauxite, cryolite, graphite, and energy required to produce 1000 kg of aluminum.
Because molten aluminum is more dense than the molten mixture of Na3AlF6 and Al2O3, the metal collects at the bottom of the cell. The cells are designed to use 8,000 A and upwards, and a given cell requires about 5 V although only 2.1 V are theoretically required to decompose aluminum oxide. The excess 2.9 V, plus the heat of combustion of carbon, is used as heat to keep the cell warm. The production of one ton of aluminum requires about 6570 GJ (18-20 MWh) and about half a ton of carbon. The process is generally nonpolluting but is a heavy consumer of electricity with approximately 36% of the electricity used in the Faradaic process, the rest being lost as heat.
The Downs cell used in the commercial production of sodium
Electrorefining of Metals In electrorefining (electrolytic purification) processes the crude metal (impure metal) is made as the anode in an electrolytic cell, while a cathode of the same metal is used. Conditions are adjusted in order to deposit that metal alone at the cathode surface. To illustrate the process of electro refining, the example of copper refining is considered (Figure 5.4). Crude copper, which is obtained by thermal reduction, consists of two types of impurities: a) Impurities which are more noble than copper (e.g. Ag, Au, Se and Te). b) Impurities which are less noble than copper (e.g. Zn, Pb, Fe and Ni). If the anode potential is adjusted on the value required to dissolve (oxidize) Cu, only the less noble impurities will dissolve with copper, while the more noble metals will not dissolve and will precipitate at the cell bottom as slimes.
Figure 5.4: Parallel Plate Reactor for Electrorefining of Copper
Cu → Cu2+ + 2 eE anode = E° anode - (RT/zF) ln [Cu 2+] •
At the cathode, the potential is adjusted at the value required to deposit (reduce) Cu. The less noble impurities which exist in the solution will not deposit because their deposition potential is not yet reached:
• Cu2+ + 2 e- → Cu E cathode = E° cathode + (RT/zF) ln [Cu
2+
]
•
The theoretical cell voltage is the sum of the two electrode voltages above. To determine the actual cell voltage, concentration and activation overpotentials as well as voltage drop across various cell resistances should be added: Eactu = Etheo + ήc + ήa + IR
•
The overpotentials and voltage drop are usually small compared to the theoretical voltage required. Since the theoretical cell voltage is equal to zero, it is concluded that the voltage required to operate electrorefining cells is small; in other words. This is a low operating cost process.
Electrolysis cell for refining of copper: As the anodes dissolve away, the cathodes on which the pure metal is deposited grow in size.
Electroplating of Metals In all aspects of our lives we are surrounded by products with electroplated surfaces. Whether we are looking at a silver-plated watch through gold-plated glasses, watching television, using the washing machine, getting into a car or boarding a plane: electroplating plays an important part in all of these situations. The objective is to prevent corrosion and wear,produce hardness and conductivity, and give products an attractive appearance.
The principle: thin metallic layers with specific properties are deposited on base materials including steel, brass, aluminium, plastic and die-cast parts.
Silver electroplating was the first large scale use of electrolysis for coating base metal objects with a higher value decorative finish.
Electroplating Electroplating is one kind of surface finishing. Everyone has seen and handled electroplated objects, even if they didn't know it. Examples include kitchen and bathroom faucets, inexpensive jewelry and the trim on some automobiles. There are thousands of examples.
◦ In fact, it is certain that nearly every piece of metal you have ever seen has been through some kind of surface finishing process. The most significant metal electroplated today is tin. A tin can used to contain food or other preserved items is actually made of thin steel plate which has been electroplated with a very thin layer of tin to protect the steel from corrosion reactions between the steel and the can contents or environment.
• Cadmium, chromium, copper, lead, nickel, silver, and zinc are also metals for which large-scale electroplating processes are used. • Nonmetal surfaces such as those of plastics can be electroplated with metals after a conductive surface is placed on them. • This can be done by evaporation or chemical reaction of a metal, usually copper or silver, or by adhesion of a conductive powdered material such as graphite.
Figure 5.5: Simple Electroplating Cell
A. Objectives of Electroplating: The main reasons for carrying out an electroplating process are: 1. To give a good appearance to the metal (ornamental or decorative purposes). 2. To protect the metal against corrosion by covering it with a metal which resists corrosion. 3. To confer a new property to the metal, e.g. copper is electroplated with Cr to confer hardness.
The object to be plated is made the cathode of the plating cell. The cell anode is often a bar of the metal to be plated from solution, so that the anode reaction of metal dissolution replaces the metal ion lost from solution by cathodic deposition. (see Figure 5.5). The quality of the plate obtained is critically dependent on surface preparation. It also depends on many other factors, including temperature, metal ion concentration, current density, time, electrolyte pH, and the nature and amount of all species present in the electrolyte solution.
B. Pretreatment of the Article before Plating. • Electroplating of metals requires careful preparation of the conductive surface of the cathode, which must be as clean and uniform as possible. The article to be plated should be cleaned from (i) greases and (ii) metal oxides.
– De-greasing is carried out by treating the article with hot NaOH to remove the saponifiable greases (e.g. lead stearate). Non-saponifiable greases (e.g. petroleum lubricating oils) are removed by organic solvents such as CCl4 or chloroform. – Scales and oxides are removed either mechanically by treating the article with emery paper, or chemically by acid pickling. Acid pickling is carried out by placing the article in dilute acid (e.g. 5 % HC1 or H2SO4) for few minutes. Degreasing should be carried out before acid pickling.
C. Constituents of a Plating Bath: A typical electroplating bath would contain the following: 1 - A salt containing the ion to be deposited (e.g. NiSO4 for plating with nickel). 2 - A conducting salt (e.g. Na2SO4 to improve the conductivity of solution containing NiSO4). 3 - A buffer solution to adjust the pH of the plating bath, otherwise the alkalinity which is produced in the bath as a result of H2 evolution at the cathode may deposit the metal ions as hydroxides 4 - Colloidal matter (e.g. glue) to give a smooth and bright deposit. 5 - Chloride ions to prevent the formation of an oxide film on the anode arising from oxygen evolution reaction.
D. Properties of an Ideal electrodeposit: • An ideal electrodeposit should be coherent, adherent, continuous (free of pores) and uniform. • Uniformity (or good throwing power) describes the ability of the electroplating bath to give a deposit of uniform thickness on a cathode of irregular shape. – If the bath is able to give a uniform deposit on the irregular cathode, the bath is said to have a ‘good throwing power’. – If the bath is unable to give a continuous uniform deposit on the irregular cathode, the bath is said to have a ‘bad throwing power’ as shown in Figure 5.6. • A good throwing power (uniform deposit) is obtained by – designing the electroplating cell in such away that the anode geometry is similar to the cathode geometry, as illustrated in Figure 5.7.
E. Controlling the Properties of Electrodeposit: • The properties of the electrodeposit can be controlled by – adjusting the current density and – the plating bath properties. • Plating bath properties are – salt concentration, – bath composition (including addition of colloidal matter), the solution pH and – temperature. • A flowchart of a general electroplating process is shown in Figure 5.8. Many rinsing steps are involved which indicates a necessity to manage the environmentally hazardous process effluent.
Figure 5.8: EP Process Flowsheet
Electroforming of Metals 1. Articles such as seamless tubes are fabricated by electrodepositing of metal on a form (or mold) made of wax which is subsequently removed. The surface of the wax mold is first rendered electrically conducting by spraying powder graphite or depositing a layer of silver mirror on it from ammoniacal solution of AgNO3 and a reducing agent such as formaldehyde. The metalized mold is placed in an electrolytic cell to deposit the required metal. Finally, the mold is taken out of the cell, heated to melt the wax and obtain the metallic seamless tube (Figure 5.9).
2- Electroforming is to used to build out worn parts of machines by depositing metal on the worn part, while other parts are insulated (Figure 5.10). 3- Metal sheets or foils with the desired dimensions can also be made by electroforming (See Figure 5.11).
