Electrochemical Processing
INTRODUCTION
Electrochemical processes involve the interconversion of electrical and chemical energy by means of a reaction at an electrode. Electrical charge may be fed to an electrolysis cell to induce chemical reactions (synthesis, metal winning or refining, etc), or chemical reactions may be run in a cell to generate electricity (batteries, accumulators, fuel cells) (see Batteries). Since the electrode reaction occurs at a surface, electrochemical techniques may also be used for surface treatment (electroplating (qv), electropolishing, anodizing) (see Metal surface treatments) or machining (see Electrolytic machining methods).
Electrosynthesis was first carried out by Davy in 1807 for the production of sodium and potassium, and in 1833 by Faraday who performed the first known example of the Kolbe reaction. Today this reaction remains one of the most useful in organic electrosynthesis. Generally speaking, all chemical reactions may be performed electrochemically and there are often great advantages to be obtained in doing so: electrochemical reactions allow control of selectivity and reaction rate through the electrode potential; are inherently pollution-free; have high thermodynamic efficiency; make possible reactions at ambient temperature; can often reduce the number of reaction steps; can often use cheaper starting materials; by means of electrolytic regeneration can use catalytic quantities of chemical oxidizing or reducing agents; and can often perform a synthesis electro generatively.
However, disadvantages are: electrochemical engineering and technology are far less developed than chemical engineering; many reaction variables may be involved with complex interdependences; long term stability of process components is often poor; and workup of product is often costly.
THE ELECTROCHEMICAL CELL
An electrochemical cell consists of at least two electrodes (anode and cathode) that dip into an electrolyte contained in a cell or reactor housing. The cell may be constructed so that the electrolytes at the anode and cathode are separated (anolyte and catholyte). One of the first cells studied was the Daniell cell and it is used as the basis of many introductions to the thermodynamics of electrochemical processes (2). It consists of a zinc anode and copper cathode dipped into solutions of their sulfates. A porous separator prevents mixing of the two solutions. Symbolically the cell is written :
Cu|Cu2+|Zn2 =|Zn
INORGANIC
Electrochemical process involve the transfer of electrons between an electrode and a substrate in solution. The energy required is in the range 0-3.5 h eV, and depends on the electrode material and the substrate. This is a moderate energy input in comparison to photochemical or radiation activation methods. Nevertheless, it is sufficient to produce the strongest oxidizing and reducing agents known, i.e. F2 and solvated electrons, respectively. It would indeed be difficult to develop manufacturing processes for chlorine, sodium, and fluorine that improve on the electrochemical ones (products; Chemicals from brine; Fluorine).
The importance of the electrochemical processing of inorganic chemicals for the production of either elements or compounds is easily appreciated from the fact that in the U.S. they consume 6% of all the electricity generated or about 16% of the electricity used by industry. Table 1 lists the most important processes that are now practiced, however, other industrial operations that involve an electrode reaction, e.g., metal refining, (see Aluminium; Extractive metallurgy), energy sources (batteries, fuel cells, electroplating (qv), electropolishing, anodizing, electrochemical machining are not discussed in this article.
Hardware for Electrochemical Processing
The distinguishing feature of an electrochemical process is the electrolysis cell and its power supply. Like many chemical processes, the feedstock must be made up by the addition of solvents/electrolytes and the product must be extracted or worked-up. This often involves much additional equipment as is demonstrated by the flow sheet shown in Figure 1 of a chloralkali process. However, only the electrochemical cell and its components are discussed, not the unit operations involved.
In an electrochemical cell, feedstock is transported to the electrode/electrolyte interface. The design of an electrolysis cell is therefore based, among other factors, on optimization of; transport of electroactive species from the cell volume to the electrode surface; materials and topography of the electrode; and possible need to separate the products or reactants of the anode and cathode reactions. The main design possibilities of electrolysis cells are summarized in Table 2.
Industrial cells for inorganic processing seldom employ pumps to provide convection. Usually transport of the electroactive species to the working electrode is enhanced simply by the gas generation within the cell. Examples are the cells for fluorine (Fig. 2), chlorate (Figs. 3 and 4) and aluminium production (Fig. 5). Gas generation at the counter-electrode in the Krebs (Fig. 3) and Diamond Shamrock/Huron Chemicals (Fig. 4) chlorate cells are employed not only for local stirring within the electrolysis compartment but also to maintain circulation to the bulk reaction volume. The Alcoa cell for the production of aluminium by the electrolysis of AICl3 (Fig. 5) also relies on gas pumping of the electrolyte. In this case, the product of the secondary electrode reaction (chlorine) can exit only from one side of the electrolysis chamber. A clockwise flow of electrolyte is thereby induced.
Electrolytic Machinery Methods
A tool composed of a harder material is used to shape a given metallic sample of material mechanically according to a specified pattern. As in the aerospace industry, the ever-increasing development of high strength, high temperature alloys places extreme demands on the hardness of the material from which a tool may be fashioned. Consequently, one is forced to employ grinding techniques that are not only laboriously slow and highly expensive but also severely limited with respect to the intricacy of the part to be machined. It, therefore, becomes necessary to find new methods for machining these new, extremely hard alloys.
One of the promising novel machining methods is the atom by-atom removal of a metal by anodic corrosion which has been called electrochemical machining (ECM). Because the metal is removed atom-by-atom, ECM affords the opportunity to machine a given workpiece without work-hardening, burring or smearing the metal, without regard to the hardness of the metal being cut, and virtually without tool wear.
In a chemical reaction, electron transfer between the reacting species occurs by the oxidation (loss of electrons) and the reduction (gain of electrons) taking place at the same site, and the energy liberated appears as heat and possibly light. For an electrochemical reaction to occur, the oxidation must take place at a site remote from the reduction. This situation is accomplished by interposing an electrolyte between the conductor (anode) at which oxidation occurs and that (cathode) at which reduction occurs.
When iron (qv) dissolves, ferrous ions enter the electrolyte by donating electrons to the anode. Eventually, the anode become so negative that further dissolution of iron is prevented by the electrostatic attraction between negatively charged anode and the positive ions in solution. If a sink for the electrons on the anode is provided by a battery that is connected in an external circuit between the anode and cathode, continuous dissolution of the iron anode can be obtained. Current is carried in the circuit by electrons (electronic conduction), and internally by ions (electrolytic conduction) through the electrolyte. The dissolution of metal at an anode driven by an external source of current, such as a battery or rectifier, is termed anodic corrosion.
PRINCIPLES OF THE ECM PROCESS
In the ECM operation, the metal workpiece to be shaped is the anode, and the tool that produces the shaping is the cathode. The electrodes are connected to a low voltage source of direct current (dc). The anode and cathode are held in position by a properly designed fixture, and a solution of a strong electrolyte is pumped between the two electrodes. If there are no side reactions, the passage of each Faraday of electrical charge (96,500 C) results in the dissolution of an equivalent weight of metal. For the ECM process to be commercially competitive with conventional machining methods with a metal removal rate of ca 0.3 cm3/s, the electrical charge must be passed at a rapid rate requiring the use of very high current densities (50–500 A/cm2). In many cases, the upper limit to the current density that can be attained is determined by the availability of high capacity rectifiers. Voltages applied between the anode and cathode is normally 5-25 V.
The Solution Gap
At these high current densities, the solution path through which the current flows must be kept small to reduce an otherwise intolerably high IR loss. The typical gap between anode and cathode for acceptable electrochemical machining is 0.05-9.3 mm. Because metal is removed during the ECM process, the gap widens during the machining operation. The current density (ie, the metal removal rate) falls to low values unless relative motion between anode and cathode (ie, the feed rate) is maintained by a mechanically driven feed system. Either the anode is fixed and the cathode is moved or vice versa. Should the feed rate, for any reason, become too large for a given metal removal rate, a possible short circuit between the electrodes results, causing catastrophic damage to the tool and workpiece by spark erosion, thermal melting, welding, and tearing of the metal. Therefore, it is necessary to incorporate a protective device based, usually, on the detection of an increasing rate of current which shuts down the current within microseconds when the gap becomes smaller than a specified, safe distance. Because there is a balance established between opposing forces of feed rate, which tends to narrow the gap, and of current density, which tends to widen it, an equilibrium or steady-state gap distance between workpiece and tool is reached quickly for a given feed rate and current density within certain boundary values.
Electrolytes
The current is conducted across the gap by the ions of a suitable electrolyte. Solutions of acids (HCI, H2SO4) have been used, but these electrolytes are very corrosive to the structural material of the machine. In addition, metal dissolved from the anode can be plated onto the cathode, thus causing the shape of the tool to change, and producing an unwanted change in shape of the workpiece. Alkaline electrolytes (NaOH, KOH) form protective anodic films on the workpiece surface and prevent metal dissolution. An exception is tungsten (qv), which can be machined only in strong alkali. The only practical electrolytes are neutral salt solutions in which a metal ion precipitates as a sludge. The most universally used salt in NaCI because of its availability and low cost. To reduce the IR drop in the gap, strong NaCI solutions of 5-25 wt % must be employed.
Any parameter that changes the conductivity of the electrolyte changes the current density, and hence, the metal removal rate.
Electroplating of Aluminium
Many metals are electroplated on aluminium to obtain various decorative and functional finishes. Aluminium has many desirable engineering properties. Aluminium has replace copper in electrical switch gear industry, because of having excellent current carrying capacity and relative cheapness. The high contact resistance of aluminium can be overcome by treating with the Alstan/Zincate process with overplating of copper/tin/silver.
Aluminium is difficult to electroplate than the common heavier metals because :
(a) It has high affinity for oxygen,
(b) Most metals used in electroplating are cathodic to aluminium, so flues in the coating lead to localised galvania corrosion. The electrodepotentials of several common metals against pure aluminium are given in Table 1.
Table 1 : Electrode Potential of Several
Metals against Aluminium
Metal Potential (mv)
Magnesium 850
Zinc 350
Cadmium 20
Pure Aluminium 0
Mild Steel + 50 to 150
Tin +300
Brass +500
Nickel +500
Copper +550
Silver +700
Gold +950
Aluminium will protect it. Those below cause aluminium to corrode preferentially.
Hence, in order to get satisfactory deposition on aluminium, the natural oxide film which forms on its surface should be removed. It can be done either mechanically or chemically or suitably strengthened or modified.
The few common practices are given below :
1. Surface roughening or roughening plus metal deposition by immersion, before electroplating.
2. Anodic oxidation followed by electroplating.
3. Direct zinc plating before plating with other metals.
4. Immersion deposits of zinc before plating with any other metal (zincating)
5. Immersion coating of tin or nickel before any other coating (Alstan/Bondel Process)
Surface Roughening
It is done either by mechanical abrasion or by chemical etching to assist in bonding the electrode deposits to the aluminium surface. This process is used generallys for the application of hard chrome deposits to the aluminium engine parts, such as piston, etc.
Anodising
This process is also used as method of surface preparation prior to electroplating. However, the adherence of the subsequent electro deposit is limited. When subsequent plating is involved, control on anodising process is critical.
Zincating
The most widely used immersion process of coating on aluminium is Zicate process it is necessary that cleaning and conditioning treatments produce a surface of uniform activity for deposition of zinc film.
When it is ensured that the surface is free from oil/grease, by solvent/vapour degreasing, the metal is subjected to milk etching type of alkaline cleaner for 1-3 minutes.
Electroplating of Cobalt
Cobalt coatings are very similar to nickel coatings. Since coabalt metal costs several times as much as nickel the interest in cobalt plating has been relatively small and intermittent. According to forester cobalt plating replaced nickel plating in Germany during World War I but disappeared again thereafter. Berger indicates some commercial use of cobalt on printing plates because of its hardness and on mirrors and reflectors because of its high reflecting power and its resistance to oxidation. On this last point. Blum and Hogaboom state that cobalt is less resistant than nickel to corrosion attack by printing inks for example and it oxidizes more readily at elevated temperatures.
Isaac Adams, the father of commercial nickel plating, recommended double salts of cobalt with ammonium or magnesium and laid down the same rules for the preparation and operation of cobalt baths as for nickel baths.
Because of the nickel shortage, cobalt salts were added to commercial bright nickel baths in 1968 and 1969 to produce alloy coatings containing up to 50% cobalt. These alloys have better mechanical properties than the individual metals and are said to be equivalent to sulphur containing nickel when combined with chromium for outdoor protection. Bright levelling unalloyed cobalt decoratively plated with chromium was also used successfully for some indoor and mild exposure applications.
Magnetic properties of cobalt and cobalt alloys are of interest in electronic applications such as memory drums, disks, cards and tapes, particularly in the computer industry.
Several cobalt alloys have been investigated for electroforming parts for the aerospace industry. In addition to the properties import for electroforming the low coefficient of friction and excellent wear resistance of hexagonal close-packed (hcp) cobalt and its alloys the high hot hardness of cobalt-molybdenum and cobalt-tungsten alloys the interesting mechanical properties (especially strength after heat treatment) of cobalt nickel and other alloys are opening new fields for industrial cobalt alloy plating. Dispersions of oxides (Al2O3 ThO2) or intermetallic compounds (carbides etc.) in cobalt alloy electro deposits promise a new type of material with unusual mechanical properties such as resistance to creep, high-temperature oxidation and sulfidation, wear and galling.
Principles
The baths commonly considered are the single and the double cobalt salt baths; both baths can be operated at much higher current densities than the corresponding nickel salt baths.
Baths containing fluorides instead of chlorides have been suggested with claims for better buffering and for whiter deposits than can be obtained with straight cobalt chloride baths. However, the simultaneous use of boric acid and hydrofluoric acid in a double cobalt bath results in a tendency toward pitting, pin holes and poor adherence, where as hydrofluoric acid alone without boric acid is beneficial.
Several other cobalt baths have been reported, including cobalt triethanolamine and cobalt sulfamate baths, both of which may merit further study, cobalt thicocyanate baths, which appeared discouraging, coordination compounds and fused anhydrous cobaltous chloride baths. Cobalt flakes have been prepared by adding 0.4 to 0.6 g/1Co(NH2)2 to the sulphate bath, then breaking up the deposit by impact grinding.
Electroplating of Iron
Iron plating is used principally for applications which depend on the desirable physical properties of Iron and on its low cost. The early literature on iron deposition, which was concerned with both its commercial applications and its electro-refining, has been comprehensively in a monograph.
A present application is a process reported in 1930 in which intaglio plates for printing government currency and bonds are made by depositing a nickel face backed by a heavy deposit of electrolytic iron from a hot chloride bath.
Iron plating was used during World War II to make electrotypes and to coat stereotypes in order to conserve nickel and copper. During the same period the United States Rubber Company electroformed iron molds for rubber, glass and plastics. Sodering tips are plated with iron commercially and undoubtedly there are many other small-scale applications.
Electrodeposition of iron as a means of producing iron powder for powder metallurgy is an application.
There are several reasons for this persistent interest in iron plating. Iron is cheap and abundant. It can be deposited as a hard and brittle metal which by heat treatment, can be rendered soft and malleable, or as a soft and ductile metal to which surface hardness can be imparted by carburizing, cyaniding, or nitriding. The fatigue strength of surfaces prepared by case hardening electrodeposited iron has been reported equivalent to the best of commercial rolling - element bearing material. On flat or rounded surfaces that are not too deeply indented, deposits of any reasonably thickness can be produced. Electrodeposited iron can be welded readily other metals can be easily plated on it and in the soft state if has superior drawing properties. Electrodeposited iron is relatively resistant to corrosion as would be expected from its high purity the contrary opinion is probably due to failure to rinse deposits completely free of electrolyte traces. The throwing power of iron baths is comparable to that of nickel baths.
An important problem with iron plating that has limited its usage to specialized or to high volume applications is that despite usually lower costs for anodes or solutions, the expenditures for capital equipment and maintenance may be high for iron plating that for other more commonly used plating baths. Special high temperature or corrosion resistant equipment may be required to heat, agitate, filter, or ventilate the iron plating bath. Also, unless used regularly, the solution will oxidize gradually. The time and effort required to restore the electrolyte to an operable condition may overweigh the economics of depositing a lower cost metal.
Principles
Practically all iron is plated from acidic solutions of iron (ferrous) salts. The presence of iron in the iron (ferric) state in these baths, in an appreciable concentration, is undesirable because it lowers the cathode efficiency for depositing the metal and it may cause deposits to be brittle, stressed and pitted. In practice it is not difficult to maintain the concentration of iron ion at a harmless level. Until recently, practically all iron was plated from baths containing iron sulphate, iron chloride, or mixtures of the two. In recent years iron fluoborate and iron sulfamate baths have been used to some extent.
The Iron Chloride Bath
In rapid plating, where a hot and corrosive bath may be tolerated, the chloride bath is often used. Thick deposits can be obtained if care is exercised in the control of the bath. A finer-grained deposit could be obtained by the addition of manganese chloride to the bath. The following composition is typical.
Table 1 : Formulation for Iron Chloride Bath
Ingredients Quantity
Ferrous Chloride 300 g/1 or 40 oz/gal.
Manganese Chloride 5 g/1 or 0.67 oz/gal.
Temperature 160 - 220°F
Current Density 50 - 75 amp./sq.ft.
Anodes Ingot iron
pH 1.5 - 2.0
The bath can be easily prepared from readily soluble ferrous chloride. Commercial ferrous chloride, however, contains some ferric salts that are detrimental to the bath. In the presence of ferric ion, a rough deposit will be obtained that is unsuitable for any iron-plating application. By working the bath in the presence of a small excess of hydro chloride acid, the ferric ion can be reduced to ferrous iron. If too much acid is present, a deposit cannot be obtained on electrolysis.
But if high purity iron is hung in the bath, the acid will have a reducing action on the iron to eliminate the ferric ion. Complete reduction can be recognized by the change in colour of the bath from a brownish green to a pure green. After the bath is reduced and worked, the pH is adjusted and the bath is ready for operation. High temperatures must be used since the deposit becomes brittle with decrease in temperature.
Rubber, or plastic-lined tanks or ceramic-lined tanks must be used because this high-temperature, low-pH bath is extremely corrosive to metals.
The pH is the critical control factor in the iron chloride bath. If acid is not added frequently the pH will rise and a brittle deposit will be obtained. However, if too much acid is added, the cathode efficiency will decrease rapidly but excess acid is used up by chemical reaction with the iron anodes so that small excesses are rapidly consumed.
This variation in the rate of chemical attack with change in pH makes it practical to add acid continuously. A small amount of acid present at all times, even though the bath is not being used, keeps the iron in the reduced state.
If the bath is operated within the recommended conditions of high temperature and controlled pH, excellent results will be obtained at high cathode efficiency.
High purity iron anodes must be used. If the carbon content of the iron is much over 0.02%, carbides will be left on the surface. The carbides will be loose, will become suspended in the bath and will eventually cause a rough plate. In any case, it is recommended that the anodes are bagged in some suitable material.
Through variation of the bath conditions, Stoddard obtained tensile strengths of 50,000 to 1,10,000 pounds per sq. inch and elongations of 6 to 50%.
Small amounts of noble metals, such as copper or arsenic will contaminate the iron bath. These can be removed by electrolysis. Organic impurities can be removed by treatment with activated carbon in the same manner as from acid baths in general.
Electroplating of Nickel
Ni coatings have long been applied to substrates of steel, brass, zinc and other metals in order to provide a surface that is resistant to corrosion, errosion and abrasive wear besides platings plastics component in order to give an attractive metallic appearance.
The importance of nickel plating industry can be gauged by the estimated 60,000 tonnes of Ni consumed each year in the western world i.e. 1 tonne in every 8 tonnes consumed in all application.
Types of Ni Solutions
Most nickel electroplating is carried out in solutions based on a mixture of NiSO4, NiCl2 and H2BO3. Typical solution is :
Table 1 : Watts Solutions for Nickel Electroplating
Ingredients Quantity
Nickel Sulphate NiSO4. 7H2O 240 - 30 g/1
Nickel Chloride NiCl2. 6H2O 40 - 60 g/1
Boric Acid H3BO3 25 - 40 g/1
Operating conditions for the Watts Soln.
Temperature 25-50°C
Agitation Usually Air
pH 4.0-5.0
Cathode Current Density 3-7 A/dm2
Mean Deposition Rate 40-90 µm/n
The watts solution is relatively cheap and simple solution and is easy to control besides its ability to be relatively less affrity towards impurities.
Engineering Application
Nickel electro deposits are applied for their physical and mechanical properties and they are rarely applied brightness and leveling. Engineering Ni coatings are used on new parts and also in the reclamation of worn, corroded or mis. machined parts.
Ni And Ur Plating Butterworths
Electroplating Baths used
Watts Nickel Bath
Most commercial Ni plating solution are based on the one named after Watts who first introduced a bath having the formulation :
NiSO4 7H2O : 240 g/l
NiCl2 6H2O : 20 g/l
H3BO3 : 20 g/l
The Rame Watts bath now used to cover a range of solutions whose composition vary within the range as shown in the table 2.
Table 2 : Composition of Rame Watts Baths
Chemical Cons. Range (g/1)
NiSO4 6H2O 150 to 400
NiCl2 6H2O 20 to 80
OR
Sodium Chloride 10 to 40
H3BO3 15 to 50
The chloride ion sometimes being introduced in the form of NaCl. NaCl being cheaper and is satisfactory for most purposes.
NiSO4 is the principle ingredient. It is used as main source of nickel ion because it is readily soluble, relatively cheap commercially available and is a source of uncompleted nickel ion.
In Ni plating solution the activity of the nickel ions is governed by conc. of nickel salts in solution. The presence of chloride has two main effects. It assist anode corrosion and increase the diffusion coefficient of Ni ions thus permitting a higher limiting C.D. Boric acid is used as a buffering agent in watts Ni solution in order to maintains the pH of the cathode at predetermined value. Boric acid solutions of the strength used in watts solution have pH of about 4.0 due to the Ni-ions.
Hard Watts Bath
This is mainly used for engineering proposes. The increase in hardness in achieved at the expense of other properties. The incorporation of ammonium ions or organic additions in the plating solution results in modification of structure and certain properties such as ductility are adversely affected. On the other hard this solution does provide a melens of producing a hard deposit when this is the most important feature required for a particular purpose.
Nickel Sulphate Bath
A simple solution of NiSO4 in water has little commercial application, but sound deposits can be produced at reasonable efficiency if boric acid is added as buffer. However, for commerical application the sulphate plus chloride soln. of the watts type is superior in performance, except with inert anodes.
Nickel Sulphonate Bath
Small volumes of plating solution based on Ni salts other than Ni sulphate and Ni chloride are used for certain purposes and the commonest of these is the Nickel sulphate solution. The advantages of this soln. are in the high rates of deposition possible and low in the deposit. It is more expensive plating solution than sulphates but is used where the aforementioned properties are important for purpose such as electrotypes in the printing industry and for making gramophone record stampers.
The process is mainly used for heavy Ni deposition and electro forming. The essential features of the solution are similar to those for nickel sulphate the pH of the bath being between 3 and 5 and most often between 3.5 and 4.5
Electroplating of Alloy
Electroplating - It normally indicates depositing of one metal one substrate metal. The deposit is of high quality rather of high purity. But sometimes the phenomenon of deposition of more than one metal takes place termed as alloy plating. To avoid rusting NiFe, Ni-W, CD-Zn and Zn-Sn alloys are used.
The most important and characteristics factor in the alloy plating is the metals with common electrochemical properties can only be employed and electroplating of alloys is much difficult.
Alloy plating for solution of simple ions is limited because of the deposition potential of the metals. These potential can be changed to certain extent by changing concentration, acidity, temperature, current density etc.
In alloy plating one metal is termed as noble metal i.e. one metal deposits more readily than the other. It is necessary then to use the term noble metal in the potential system and is independent of the cone ration of the two metals. Potential system obtains the percentage of metals in the deposit is dependent on the ratio of metal in the soln.
In alloy plating there are certain problems which are not in the single metal plating. For alloy plating in solution salt of both metals are mixed with different quantities e.g. Cu and Zn make an alloy called brass and its chemical equivalent are different. Due to slight from the process can lead either of the metal getting deposited. Similar difficulties may arise while selection of anode.
Electrodeposition of Zinc-Iron Alloy
In recent years, many developments have taken place in production of galvanised coatings on steel and efforts have been made from time to time to enhance the corrosion resistance of the coating. Zinc-Iron alloy is such development. These alloys were mainly deposited from sulphate or chloride baths. It has been reported that a Zn alloy containing 15-25% iron has good weldability and corrosion resistance and is electroplated commercially on steel strip. For automobile commercially on steel strip. For automobile application Further Zn-alloy containing 50% or more iron provided better paissntablity.
Table 1 : Bath Composition
Ingredients &n
