PRODUCT AND PROCESS DEVELOPMENT
Continuous Anodic Oxidation of Copper Wire DALLAS
T. HURD, JAMES G.
KRIEBLE, AND H. G. PFEIFFER
General Electric Research laboratory, Schenectady, N . Y .
T
H E trend toward higher operating temperatures for electrical apparatus has brought with it a need for an inexpensive, insulated magnet wire with good physical properties that will be stable in operation for extended periods a t elevated temperatures. In the development of magnet wire of this type, it has become increasingly apparent that some form of barrier substrate between the copper conductor and the sheath of insulating material is highly desirable. Such a barrier can perform a t least three import ant functions :
It may be used to retard the increase of electrical resistance caused by oxidation of a copper conductor by atmospheric oxygen a t operating temperatures 300" C., or higher in some types of apparatus. A suitable barrier layer between the copper and the insulating material can greatly prolong the useful life of the insulation a t elevated temperatures. Many insulating materials potentially capable of operation a t elevated temperatures, including silicones, Teflon, and poly F-1113, deteriorate much more rapidly above 150" to 200" C. when in contact with metallic copper. Many lastic materials, otherwise desirable, have relatively poor mecxanical properties in thin films or adhere only poorly to a copper conductor. Thus, wire insulated with such materials often will not withstand the rather severe mechanical treatment encountered in the fabrication of electrical apparatus. A suitable substrate can improve the adherence of the insulating material to the copper and, in some cases, mechanically reinforce the insulating film. T o be practical, the barrier substrate must be cheap, relatively stable, and easily applied to wire uniformly and on a continuous basis. One material that appears to meet the requirements is a film of black copper oxide applied to copper wire by continuous electrolytic oxidation in hot alkaline solution. Electric motors and generators wound with black-anodized copper wire insulated by a thin film of silicone resin have been operated for many hours a t temperatures above 300" C. I n electrical systems operating a t such relatively high temperatures, the performance of the insulation may be determined to a considerable degree by the quality and thickness of the barrier substrate; thus, specifications of quality must be met by the substrate as well as by the other components of the system. The properties of anodized copper wire and the performance of insulation systems employing such wire will be described in later papers. Anodization process is carried out in three steps
Batch electrolytic oxidation of copper has been used for forming ornamental finishes and for blackening the interior of optical instruments (6). I n its simplest form, this process comprises immersing the copper article t o be anodized in a bath of 15 t o 25% sodium hydroxide solution a t 95" to 100' C., then applying to the copper article (as anode) and to a suitable cathode, also in the bath or serving as a container for the bath, a direct current supply of 1.5 to 2.0 volts through a small series resistance. Under these conditions, a black layer begins to form on the copper. Generally the build-up of oxide film on the copper continues for a few minutes, then terminates rather abruptly with the evolution of oxygen a t the anodized surface. At this point, the copper article is removed from the bath, washed, and dried. Actually,
December 1955
the anodization of copper is somewhat more complicated; t h e course of the process and the nature of the coating formed depend on a number of different factors. Three-Step Mechanism. It is, perhaps, easiest to understand the effects of variables on the anodization process by considering a somewhat oversimplified mechanism comprising three steps : ( 1 ) the dissolution of copper from the anode as copper ions; (2) the precipitation of these ions as copper oxide by the hot alkaline solution; and (3) the solution of part of the cupric oxide in the film by the hot alkaline solution to form the sparingly soluble cuprate ion. All three steps are important in determining the rate a t which the oxide coating is formed and the character of the coating; which is the rate-controlling step will depend on the process conditions. The coating contains both cupric and cuprous oxides in varying proportion, depending upon the conditions of the anodization. Step 1 is a primary process, and the rate a t which it proceeds depends to a considerable extent on the current density. Were it not for step 2, the dissolution in step 1 would proceed until the copper anode was entirely eaten away. However, the copper ions react with the alkaline solution and are precipitated in the immediate neighborhood of the anode as copper oxide. The dense precipitate adheres closely to the anode and, as it is not a particularly good electrical conductor, this film acts to limit the current flow. At temperatures below about 85" C. the precipitate may appear bluish, indicating hydroxide or hydrated oxide. Thus, it is desirable to keep the operating temperature well above this value; 95" to 100' C. is a preferred range. It is immaterial, however, whether a hydroxide is first formed and then immediately dehydrated or the oxide is formed directly a t the operating temperature. Were it not for step 3, the layer of oxide formed as a result of steps 1 and 2 would effectively stop the anodization process after a very thin film of oxide had formed on the surface of the anode. However, cupric oxide dissolves to a certain extent in strongly alkaline solutions t o form cuprate ion, and the cuprous oxide component of the coating is continually being converted, in part a t least, t o cupric oxide during the anodization a t a rate dependent on the voltage and current density. As a result, some of the oxide is removed continuously from the anode as it is formed ; thus the oxide layer that remains is somewhat porous, and the electrolyte penetrates through the film to the copper metal underneath. When the coating gets sufficiently thick, the passage of current through the porous coating becomes more difficult, the resistance of the cell rises, the cell voltage rises with it, and the pores become restricted owing t o deposition of oxide; eventually, as the voltage rises, the evolution of oxygen begins. These latter stages appear to be particularly desirable, in t h a t they act to seal the pores in the coating and probably play a considerable part in converting cuprous oxide to cupric oxide. The proportion of cupric oxide to cuprous oxide differs within the film, depending upon distance from the metal interference, where the film is largely cuprous; a t the surface of the film, the oxide may be largely cupric. The anodization process will depend upon a balance among the three steps outlined. If the initial current density is high,
I N D U S T R I A L A N D E N G I N E E R I N G C E-E M I S T R Y
2483
PRODUCT AND PROCESS DEVELOPMENT
A process for applying a film of black copper oxide to copper wire to achieve an insulating barrier for magnet wires to be used at high temperatures
I
the rate at which copper oxide is formed is much greater than the rate of solution. Therefore, the process is essentially stopped after the formation of a very thin film of oxide. If the current density is so low that the rate of solution of oxide is greater than the rate of formation, the oxidation process never terminates, and the anode eventually dissolves entirely in the solution. Within these limits, the anodization process can be extended over rather wide limits in time, and substantial layers of oxide (up to several mils) can be deposited by adjustment of the operating conditions. Step 3 is also dependent upon the concentration of sodium hydroxide in the bath. If the concentration is too low, the oxide coating will be only a thin layer. If it is very high, so t h a t the solubility of the oxide is increased, the current density must be raised to compensate for this effect, However, as the current density usually can be adjusted as desired, the sodium hydroxide concentration can be varied within rather wide limits. The process of solution of copper from the anode can be observed in a fresh bath of alkali. During the anodization process, blue streamers of hydrated cuprate ion can be seen extending from the anode. With continued operation of an anodization bath, the solution becomes blue, and metallic copper deposits at the cathode, usually in the form of a spongy soft mass. Despite this process of solution, the changes in dimension of a copper piece on anodization usually are negligibly small except for very thin sheets, fine wires, or very thin copper platings on other base materials. Because of the requirements of the electrolyte, sodium hydroxide appears to be the most economically practical material. I AME/cmz 06 AMP. /
(NO COATING1
M!
0.01 AMP 1 cmf .017AMP. 1 c m f
ool AMP,
I!
_______-___----
a -0 4
_______________--I
0
(NO LIMIT REAMED!
----- C 5 0 / CUO (THEORETIC&)
5
--CU
/CU,O
(THEORETIWL)
I
I
I
10
15
20
COULOMBS /om?
Figure 1. Relation of anode potential to charge transfer at different current densities
Of the potential changes that occur during the anodization process, only anode-to-solution potentials are considered here, as the over-all cell potential depends not only upon the anode process, but also on the nature of the cathode surface and on reactions occurring a t the cathode. It is desirable to measure the anodeto-solution potential directly with a suitable reference electrode. ;Of the reference electrodes tried, a mercury-mercury oxide cell with 5N sodium hydroxide electrolyte has been the most useful. This electrode is very stable, does not polarize readily, and works 2484
well up to 105’ C., and potential measurements are essentially independent of electrolyte concentration and the presence of copper ion. The potential of the mercury-mercury oxide electrode is +O.l volt measured against a hydrogen electrode in the basic solution. When a piece of clean copper is placed in a hot alkaline solution, it assumes a potential of -0.26 volt against the solution, owing to the formation of a very thin surface film of cuprous oxide. This oxide is practically insoluble in the anodizing solution. If current is forced to flow from the strip by connecting it to the positive terminal of a voltage source, and attaching the negative terminal to a suitable cathode in the bath, the potential of the piece against the solution becomes less negative-Le., more positive-by an amount depending on the current density. After a certain amount of current has flowed, the potential of the
0
10
30
20 TIME
Figure 2.
IN
40
SECONDS
Relation of anode potential to anodization time a t different current densities
Sign convention is electrkal.
---
6N 9N
-
NaOH NaOH Multiply by 1 for thermodynamic values
-
copper against the solution rises to a value somewhat more positive than -0.06 volt, which represents the potential due to the formation of a film of cupric oxide on the surface. The actual value of the potential again will depend on the current density. As the anodization process proceeds a t an anode potential more positive than -0.06 volt, the film of oxide increases in thickness, and both cuprous and cupric oxides are formed. I n the usual disposition of apparatus, with a fixed voltage source and series resistance supplying the anodization cell, the anodization process is self-limiting. T h e end of the process is characterized by a rise in anode potential to about 0.7 volt and by evolution of oxygen a t the anode. If the electrolysis is allowed to proceed beyond this point, however, the oxide layer on the anode eventually will be removed by the solution, leaving only a thin dark film on the metal. Figure 1 is a plot of anode-to-solution potential us. the amount of electricity passed through the cell under the conditions of anodization described above. Figure 2 shows a few curves of anode potential and anode current density us. anodization time for representative anodizations in two different concentrations of sodium hydroxide. The initial steps in the process are accomplished rapidly, and the main part of the anodization process occurs on what is almost a constant voltage-constant current curve. With a sufficiently “stiff” voltage source and a cathode that stays constant voltagewise with respect to the solution, it can be shown t h a t satisfactory anodization can be accomplished as a unipotential process; however, a rise in anode potential a t the end of the process is desirable. If the anode is lifted from the electrolyte before the anodization process limits itself, the oxide coating is rather soft and brown and is largely cuprous oxide. However, the process can be made t o stop at any point by raising the current density sharply; in such case, the final coating is black. Coatings formed with
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 12
-
PRODUCT AND PROCESS DEVELOPMENT 4 CLEANING 4 0 CATHODIC
CLEANER
SECTION 4.2 ELECTROLYTIC
WAT WA
5 AIR ORVER D C CONTACT SHEAVE ELECTRICAL GROUND
8
HOT AIR
DRYER
AND
NSlON
CONTROL
DRAIN
LE3 TC FL
Figure 3.
anode-to-solution potential is dropped below the potential of oxygen evolution, the film of oxygen is dissolved by the solution and the process can proceed. Similarly, the oxygen film can be removed by agitation or by rubbing the anode. I n some respects, the passivity of copper seems t o be similar t o the passivity effects observed with other metals in electrolytic solutions. The anode material should be as clean as possible before anodization begins. Indeed, the quality of the oxide film depends on the thoroughness of the initial cleaning. The anodized part must be washed free of residual electrolyte following the anodization process; the stability of the oxide coating at elevated temperatures is markedly reduced by small amounts of residual sodium hydroxide.
TAP WATER MOLE FOR WIRE RUBBER GROMMET INSERT I WATER
h
SUMP PUMP
1
I
SCALE
The variation in cathode-to-solution potential may be restricted by making the cathode relatively small compared to the anode, so that the cathode current density is always very high. Under such condition, the cathode process is hydrogen ion discharge, and potential changes caused by the action of the electrolyte on the cathode are greatly minimized. It would be possible to keep the cathode surface discharging hydrogen regardless of the anodization current density by inserting into the bath a secondary anode on a separate voltage supply. Such a system does not interfere with the anodization of the primary anode, if the potential distribution about this electrode is not seriously disturbed by the second anode.
-
If a copper anode is immersed in the bath with the supply voltage applied to the electrodes, the copper appears to be passive; oxygen is evolved a t the surface, but no blackening occurs. This effect is enhanced by the presence of a reducible oxide film on the cathode, which, in effect, increases the proportion of the supply voltage represented by the anode potential. The anodization process can be started, however, by shortcircuiting the cell momentarily, by disconnecting and reconnecting the supply voltage, by vigorously stirring the bath, or by scratching or rubbing the surface of the passive anode. This effect of inhibition of the coating process is believed to be due to the evolution of oxygen a t the anode and the maintenance of an adsorbed layer of oxygen on the copper metal. Feitknecht and Lenel (3) have shown t h a t oxygen passivates copper in sodium hydroxide solution for the process Cu ---c CuO. If the December 1955
SUMP PUMP CONTROL B VALVE
Over-all detail of continuous anodization process
relatively long anodization times appear to be somewhat harder and more abrasion-resistant than coatings made in relatively short times. This is believed to be due to a contraction and aging of the black oxide precipitate. Reaction of the cathode with the hot electrolyte makes use of the over-all cell potential as a measure of the anodization reaction uncertain and often meaningless. During anodization, the cathode becomes covered with copper by deposition from the cuprate ion in solution. The hot alkali slowly attacks this copper Iayer t o form a surface crust of copper oxide. During anodization, this dark gray oxide film, which builds up while the bath stands idle and also during anodization if the cathode current density is not sufficiently high, is reduced at a potential lower than that a t which hydrogen normally is evolved a t the cathode. Therefore, for a given voltage applied across the cell, the presence of the reducible oxide film on the cathode increases the positive potential a t the anode, and, consequently, the rate of anodization. If the cathode oxide layer becomes entirely reduced, hydrogen evolution proceeds normally a t the cathode, and the anodization is slower for a given applied voltage. As this cathode effect depends on the area of the cathode as well as time and the passage of current, it is difficult to control. Unless the anode-to-solution potential is measured directly with a reference electrode, the anode current density is the most reliable and probably the most practical means of measurement and control.
-
CENTRIFUGAL TEMPERATURE FLOW METER
b
I
MATERIAL OF CONSTRUCTION
h- I F T +
Figure 4.
CATHODIC AND ANODIC CLEANING ELECTROLYTIC ACID DIP WATER WASU
IRON STAINLESS STEEL 347 COPPER
Detail of wire-cleaning tanks
A segment of copper wire can be coated with several microns of copper oxide in 5 to 10 minutes by the process described. A wire segment so coated can be heated in air a t 200" C. for months without loosening or visible change of the oxide film, and the wire can be bent about its own diameter without any visual signs of cracking of the coating. A coating of this sort not only retards oxidation of the underlying copper by air a t elevated temperatures, but it also serves as an effective barrier between the copper and semiorganic insulating materials, greatly prolonging the useful lives of such materials as electrical insulation a t elevated temperatures. A good film of anodic copper oxide offers substantial improvement in the adhesion of plastic materials t o copper, imparts some degree of reinforcement t o silicone resins, and provides an excellent base for the attachment of rubbers to copper or to steel electroplated with copper. Vulcanization of a rubber compound in contact with a well anodized copper surface generally results in a rubber-to-metal bond stronger than the rubber.
INDUSTRIAL AND ENGINEERING CHEMISTRY
2485
'
PRODUCT AND PROCESS DEVELOPMENT NEOPRENE
/NEOPRENE
GASKE? ST'D
IRON P I P E
THREADED FLANGE
DC
RUBBER
CELL
f'
T E R M I N A L S CATHODE TUBING
COPPER
TEXTOLITE
WASHERS
CELL INSULATION DETAIL
WIRE
SCALE
Figure 5.
FT+
Detail of 12-cell continuous anodization machine
Anodization may be adapted to continuous processes
T o be practical industrially, the anodization of copper wire for electrical purposes must be on a continuous basis. A continuous anodization process should be capable of operation over long periods of time with a minimum change with time in operating variables and thickness and quality of the oxide layer produced on the wire. It is desirable t o operate a continuous apparatus a t as high a rate of production as possible, particularly in the anodization of very fine wires, where many thousands of feet of wire may be incorporated into a single manufactured component. A continuous apparatus must be capable of operation with a minimum of attention and manual control, and should handle a variety of wire sizes with relatively minor adjustments in the setup. The adaptation of static anodization procedure to continuous anodization presented certain problems. I n an ideal system for continuous anodization each point on the wire would enter the anodization bath a t a potential low enough for the oxidation process to be initiated, undergo treatment a t desired potential levels along a predetermined curve as the coating process proceeded, and leave the bath immediately a t the completion of the process. However, passivity, enhanced by the cathode effect, usually prevented the electrolytic oxidation process from occurring if the copper was introduced into the bath a t a level of applied cell voltage at which anodization normally proceeds. Particularly with fine wires, the potential drop along the length of the wire, representing a t any given point an integration of anodization currents from all points on the wire beyond the point considered, affected the course of the anodization. The rate of production in a given system will depend upon the length of wire within the bath and the desired oxide thickness, since, over a wide range of operating conditions, the thickness of the anodic coating increases with the time of anodization, which in turn is a function of the anode current density. The general shapes of the anodization potential and current density curves indicate that continuous anodization might be accomplished successfully in a single bath such as a long metal trough or other metal compartment acting as a cathode. Such an apparatus has been operated in this laboratory. However, a serious problem in control is the elimination of undesirable 2486
+I
potential gradients due to potential drop in the wire being coated or to chemical action of the electrolyte on the cathode. It i s possible to use wire of such diameter t h a t the potential drop along the length of the wire has negligible effect on the anodization process. It is possible to compensate to some extent for the potential drop in the anode wire by making the cathode of:a metal having appreciable electrical resistance, such as Nichrome, and imposing across the length of the cathode a voltage sueh that the potential drop along the cathode just compensates for the potential drop along the anode lying parallel to it (6) Segmented cathode anodization apparatus has been constructed
The segmented apparatus incorporates the factors that must be considered in the design and operation of a continuous a n d z a tion process. The cathode is divided into a series of individual cathode compartments, spaced along the anode wire and %parately fed with voltage. Contact is made with the wire a t the entrance end of the system. The use of individual cathode compartments allows a wide variety of operating conditions, as necessitated by different wire sizes, wire speeds, cathode conditions, etr., and makes possible correction for the potential drop along the wire due to the anodization currents. As the cathode compartments are made shorter and more numerous, the degree of control is increased. The desirable rise in voltage a t the end of the coating process also is accomplished within the system. Within the limitations imposed by the length and number of the individual cathodes, any desired cycle of potential variation with time may be imposed a t a given point on a wire moving through the bath. The problem of variation in cathode-tosolution potential may be solved either by checking the anodeto-solution potential in each compartment, or by measuring anode current density. I n such measurements, correction must be made for the progressive summation of currents and potential drops along the length of the anode. I n this type of apparatus, as in all the others described, the voltage source should have internal resistance low in comparison with that of the anodization cells, or other form of constant 'current supply. The process flow sheet is shown in Figure 3, where the order of the various steps in the process is indicated by the numbers.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. I2
*
PRODUCT AND PROCESS DEVELOPMENT Hard-drawn, shaved, round copper wire was used. As received from the drawing operation, this wire is hard and springy and is covered with a thin film of lubricating compound together with dirt picked u p from the surroundings. An oxide film may also be present, indicated by the tarnished appearance of the copper. The first part of the process was designed to correct these conditions prior to anodization.
CONSTANT
VOLTAGE
CIRCUIT
VOLTAGE DIVIDER - 6 0 M I L CU WIRE
RESISTOR CONTROLLED
r
CONSTANT
i
CURRENT CIRCUIT
1
IO- I20 V.D.C. 0-50 AMP.
WIRE
R CELL *I 2
1
STEINYETZ
CONSTANT
CURRENT CIRCUIT
JiVI9
GE 6RS14HSBI
60m
T Y P E IO POWERSTAT 250 VA. G E 61019
Figure
WIRE
-
I
-
R
CE LL*I
6. Voltage and current control circuits for continuous anodization machine
The spools of copper wire were mounted on racks, which permitted them t o rotate freely. Tension on the wire was maintained by a braking mechanism involving the use of a stationary leather belt sliding over a flat-surfaced sheave keyed t o the shaft holding the spool. The wire was drawn from the spool by a capstan, rotating a t any desired constant speed to give wire speeds of 2 to 36 feet per minute. The first operation was annealing, in which the hard-drawn wire was heated almost to red heat by passing alternating current through the wire between two copper contact sheaves over which the wire ran. The wire, while hot, was passed through a glass tube, the exit end of which was dipped in water. Steam, generated by the hot wire as it entered the water, filled the tube and prevented oxide formation. Another important contribution of the annealing was the burning off of the lubricating materials introduced in the drawing operation. A series of three electrolytic cleaning baths followed the annealer. Each bath consisted of a tank for the electrolyte, followed by a water tank in which the electrolyte was washed from the wire. Figure 4 shows the main feature of tank construction and arrangement. Each tank was divided into three compartments, with holes through which the wire passed horizontally. The center compartment contained the electrolyte. The solution flowing from the center compartment through the wire entrance and exit holes was caught in the end compartments and sent to the storage tank to be pumped back to the center compartment. With such a construction, no sheaves were needed for guiding the wire in and out of the tanks, a definite advantage in a process of this type. The first cleaning section was the cathodic cleaner, in which the December 1955
wire was made the cathode and the tank the anode. Traces of oils and greases were removed by the action of the alkali (4 ounces per gallon of Type BN cleaner obtained from the Wyandotte Chemical Co., bath temperature 140" F.) and by the loosening effect of the violent evolution of gas at the wire surface. The electrolytic bath was followed b y a water wash. Lightning mixers placed in the water-wash tanks kept the water in constant motion. The second cleaning section, containing the same electrolyte, was the anodic cleaner in which the wire was made the anode. I n this bath, oxygen was evolved from the wire, and any metallic impurities picked up in the cathodic cleaning were removed. The cleaning solution was removed from the wire by another water wash. The last cleaning step was the electrolytic acid dip. The solution was made up of a 10% sulfuric acid water solution to which approximately 4 ounces per gallon of sodium dichromate (technical grade) had been added. The dichromate acts both t o inhibit the corrosion of the stainless steel construction material and to accelerate the removal of oxides from the wire. The wire was the anode, and two stainless steel plates inserted in the center compartment were the cathodes. The current was adjusted to give a smooth matte finish on the surface of the wire. The electrolytic acid dip operated at room temperature. The etched copper, after leaving the acid dip, was washed, and then dried with a jet of compressed air. The wire then passed around a large brass sheave and into the anodizer. This sheave was used to make positive electrical contact to the wire from the direct current power supply, and the wire was also grounded a t this point by grounding the sheave Following the sheave, the wire entered the 12-cell anodizer, shown in detail in Figure 5. The individual cells in this apparatus were constructed of 1-foot lengths of 1.5-inch standard iron pipe, which served as both electrolyte compartments and cathodes. Each cell was electrically insulated from the adjacent cells by means of neoprene gaskets, held between screw-on pipe flanges, as shown in Figure 5. A length of S/,-inch copper tubing extended vertically from the center of each cell. These tubes were used both as electrical leads to the cells and as vents for the gases formed during operation. The electrolyte, hot (98" to 100" C.) aqueous sodium hydroxide, was pumped a t a constant measured rate into the first cell (cell 1 ) of the anodizer. From here it flowed concurrently with the wire to the exit end of the anodizer located a t cell 12, and was recycled back through a 5-gallon tank to the first cell. The electrolyte was maintained a t a constant temperature in the storage tank by a separate recycle system in which a pyrometercontrolled 5-kw. heater was situated. The sodium hydroxide solution was maintained at a constant concentration by the addition of small amounts of water to make up for losses by evaporation. The anodized wire emerging from the anodizer was thoroughly washed in another water-wash tank and then dried with a blast of hot air. The dry oxide-coated wire next passed between two 1-inch copper pulleys which were positioned in such a way that one was in contact with the top and the other with the bottom of the wire. Leads from an ohmmeter were connected, one to each pulley shaft, and the resistance of the coating was measured continuously during operation of the anodizer. The resistance reading was found t o be a reliable indication of t h e quality and thickness of the oxide film on the wire. A high resistance reading indicated an oxide coating of good quality. Following the resistance measurement, the wire was lightly buffered and then, passed through the "dancer roll,'' a device which regulated the speed of the take-up spool and maintained a constant tension on the wire. The wire was level-wound on the take-up spool. The electrical system originally employed for supplying power t o the anodizer was a voltage-divider type of circuit from which supply voltages of different values were fed t o each of t h e cells in the anodizer, so that at a given spot on a wire passing through
INDUSTRIAL AND ENGINEERING CHEMISTRY
2487
PRODUCT AND PROCESS DEVELOPMENT the apparatus, the change of potential with time would be similar to that encountered in a static anodization procedure. The circuit is shown in Figure 6. The constant-voltage system was successfully used in the early stages of development, but as wire speeds were increased, several disadvantages became apparent. The most important wm the variability of the current caused by small changes in the resistance of the electrical contact t o the wire, and in the cathode potentials, which thus affected the anode potential. Small changes in the applied voltage led to disproportionately large changes in the cell current. For these reasons, a constant-current system was built and used to replace it. A circuit diagram of this system, called the Steinmetz circuit, is also shown in Figure 6. Fluctuations in the resistances of the cell circuits have only a negligible effect on the current flowing, and each cell circuit is independent of the other circuits. The power losses are relatively small. T h e system has proved t o be an excellent solution to the problem of current control. All the variables were measured using individual meters for each cell voltage and each cell current, and a tachometer actuated 'by a generator coupled to the main capstan drive for measuring wire speed, A l l the drive motors were Thymotrol-controlled, a n d a continuous range of speeds from 2 to 36 feet per minute was available. The design also included facilities for processing three wires in parallel in the apparatus, which tripled the production from the unit. Wire sizes from 2.5- to 60-mil were successfully anodized. I n the later stages of development, the process was run continuously for more than 8 hours with no difficulty.
-~
Two methods were used for evaluating oxide films on wire. One was essentially a use test, in which 8-inch samples of the anodized wire were coated with a silicone resin, then baked for 64 hours a t 250" C. After baking, each sample was wound around a mandrel whose diameter was five times t h a t of the wire under test and closely scrutinized under a microscope. Any blemishes, such as cracks, blisters, or craze marks, were rated according to a point system. I n this way, the oxide coating was given a quality ranking and its acceptability for use in the production of magnet wire was established. The amount of oxide deposit was also determined. I n this test, the oxide on a weighed and measured length of wire was dissolved in concentrated hydrochloric acid. A weight loss value expressed as milligrams of oxide per square centimeter thus was determined. Early in the study it was found that the amount of oxide deposited did not necessarily correlate with the results of the heat-aging use test; however, this test wm an important indication as to proper operation of the anodizer. Other important tests were the scrape abrasion of the enameled wire, tensile strength, diameter of wire, and cuprous-cupric ratio of the oxide coating. Process variables are important in controlling course of anodization
I n the early stages of process research, the production of anodized wire of sufficient quality to be used as magnet wire was most discouragingly a matter of chance. As experience was gained, however, it became apparent t h a t certain process variables, presented in Table I, were most important in controlling the course of the anodization.
WIRE S P E E D - l 6 F T / M I N SPEED 19FTlMlN NoOH CONC -18 TO 35% NaOH TEMP -98 IOO'C. WIRE SIZE SYMBOL 25 3 0 40 3
-
LIQUID
-
-
Table 1.
7 8 9
/ dL (NOOH)
7. Relation of limiting current to function of sodium hydroxide concentration
Range Studied 2 . 8 t'
Total current Temperature of electrolyte Current profile Surface condition of copper wire
6
Figure
Process Variables
Variable diameter
80" t o 105' C.
All these variables, except the current profile and the surface properties, were easily measured and given numerical values. The general type of current profile-Le., variation of current density with time for a given segment of wire passing through the apparatus-that was required had a definite form which IO 9
! 5 4
3
2
2
10
1
6 5 4
3 2
c'
I
10'
2
3
4
56789103
2
3
4
5 6789104
2
3
I -
"d
Figure 8. Relation of oxide film thickness to function of wire speed and current flow
2488
Figure 9.
Relation of mass transfer to liquid flow, showing effect of different wire speeds
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 4'1, No. 12
PRODUCT AND PROCESS DEVELOPMENT could be set up quickly by an experienced operator. The surface conditions of the wire were more difficult to control, although the annealing and cleaning steps outlined went a long way toward making uniform the surface of the wire on any given spool. During a run, the wire size and speed, the electrolyte concentration, temperature, and flow rate, and the surface condition of the wire were set a t previously chosen values. The formation of the oxide coating then was controlled most easily by adjusting the currents t o each of the cells until the proper shade of oxide color was obtained on the wire. At low current levels, the wire emerging from the anodizer had a metallic gray or copper color, depending on the speed of the wire. Excessive wire speeds also favored this condition. As the total current was progressively increased, the color of the coating progressed from copper, through metallic gray, bright yellow, greenish yellow, olive drab, bron-n, and finally black, as the thickness of the oxide and the cuprous-cupric ratio changed with the current density. When the anodizing current was raised to a level that exceeded the limiting current for the particular process conditions, the process becanie unstable. The electrical resistance of the oxide film began t o drop rapidly and the black coating gradually disappeared, until finally the wire emerged from the anodiaer as bright copper, well cleaned and with no oxide coating. The weight of oxide deposited per unit area increased to a maximum as the total current approached the limiting current. The cupric oxide in the coating increased to a maximum at the point of limiting current and ranged from 0 (brown oxide film) to 60 (black films) 7 0 for anodized wires that would pass the heat-aging use test. The operating region in which the color changed from yellow to black corresponded to a very narrow range of total current. For example, in one controlled anodization, the weight of oxide deposited per unit area doubled when total current changed by a factor of only 1.13. Thus, it was important t o control all the process variables rather closely, in order to produce an oxide coating having acceptable properties. The cell currents and voltages measured during successful runs always had definite patterns, or profiles, which differed from run to run only in level and depended only on the set conditions for the run. The profiles were similar to those observed in the static bath anodizations (see Figure 2).
rearranging Equations 1 and 2 and substituting the area of the anode TdL, for A , the following equations are obtained:
(3)
(4) Equations 3 and 4 form the basis for estimating the apparent mass-transfer and efficiency coefficients, K L and K B for the continuous anodization of copper. The dimensions used are recorded in the nomenclature at the end of this paper. The constant conversion factors are included in constant IC.
Figure 10.
Continuous anodizer used in process development
I n applying these equations t o the anodization of copper, most of the quantities were available from direct measurements and
from determinations of the oxide weight per unit area. The value of the expression (C C,) could not be measured directly, but should be related to the concentration of sodium hydroxide. It was assumed that C , was essentially zero or would approach zero as the total current approached the limiting current, as t h k quantity would obviously depend on the current density. To check these assumptions, several anodizations were run at constant wire speed, wire size, liquid flow speed, and temperature, but with varying concentrations of sodium hydroxide. Using the sodium hydroxide concentrations in per cent by weight for (C Ca), the quantities obtained from these runs were plotted on the basis of Equat,ion 3. Figure 7 shows some of the results obtained. Data obtained at a constant concentration of hydroxide, but with varying wire speeds, are plotted in Figure 8. A linear relationship was obtained, which indicated that Equation 4 also is valid. I n all cases, the correlations obtained indicated that the basic assumptions are sufficiently accurate for correlating the data. Fluctuations from the correlation are possible, however; one such fluctuation arises from the assumption of a constant value of M , the molecular weight of the oxide coating. Chemical analyses of the oxide films show that it consists of cuprous oxide together with varying amounts of cupric oxide from extremes of 0 to about 6070,with most of the samples examined containing around 30% cupric oxide. The magnitude of the effect on K E caused by this variability in M is decreased, however, by a compensating change in the efficiency coefficient, IC,. Another source of variation arises in the calculation of KL,as a precise value of K L is obtained only when C,, the concentration of hydroxyl ions at the liquid film next to the wire surface, is aero. The condition
-
Correlation of process variables'presents experimental daia in useful form
-
During the course of the development many experimental data were accumulated. I n order t o organize them into usable form, a method of correlation was developed. It was assumed a priori that the diffusion of hydroxyl ion from the electrolyte t o the anode nras the controlling step in the the anodization reaction. (This is by no means certain, but is an assumption that agrees well with the phenomena observed in both static and continuous anodization.) Mathematically, for a stagnant electrolyte, this Etep may be approximated by the equation ( 1 , 4 ) :
-
where k~ is proportiona,l to the mass-transfer coefficient, (C C.) is the concentration gradient, Z is the total current, A is the area of the anode, n is the number of electrons per molecule, and F is the Faraday constant. Another relationship found applicable ie:
where k , is the true efficiency coefficient, W A is the weight of oxide per unit area, V w is the wire speed, M is the molecular weight of t h e oxide, and L is the length of the anodizer. By December 1955
-
INDUSTRIAL AND ENGINEERING CHEMISTRY
2489
PRODUCT AND PROCESS DEVELOPMENT
Figure 1 1.
Continuous anodization unit showing wire-winding equipment and control panel
Figure 12. 2490
Continuous anodization machine
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 12
PRODUCT AND PROCESS DEVELOPMENT of C, = 0 is satisfied only a t the point of limiting current oE imminent instability, however, which is not a very satisfactory condition for operation. I n tbe discussion so far, the liquid velocity, VL,has been held constant. The rate of liquid flow could, however, be correlated with the other process variables. The method of correlating the mass-transfer coefficient with liquid flow is t h a t suggested by Chilton and Colburn (2), in which the j factor
controls. The panel at the far end of the apparatus shows the Thymotrols and tachometer. Figure 11 gives a close-up view of the winding table, tension control device, annealer, capstan, and cathodic cleaning tank. Mounted on the panel board in the background are the ammeters and voltmeters for the cells, as well as the receptacles and switches for controlling the various pumps, motors, and heaters. Figure 12 is a view of a 12-foot manufacturing unit located in the Motor and Generator Division, Schenectady Works, General Electric Co.
is plotted on log-log coordinates as a function of the Reynolds
Acknowledgment
number, DpvL, in a form analogous t o that of the friction factor
The authors wish t o acknowledge the invaluable cooperation of E. M. Boldebuck, T. E. Etherington, W. F. Gilliam, and A . E. Schubert in various phases of this development.
P
of the Fanning equation for pressure drop in fluid flow. Under these circumstances, j is a unique function of the Reynolds number for a particular mechanical system. I n applying this correlation t o the anodieation data, the j factor was modified to
Nomenclature
A C
by the omission of DL,the diffusivity coefficient, as actual values for this quantity were not available. The error introduced by this omission is small, however, because the electrolyte concentration and temperature were limited t o a narrow operating range. is the viscosity and p is the density of the electrolyte at the temperature prevailing, while D is the inside diameter of the anodizer cell. Using this method, the correlation of some of the data is presented in Figure 9. The effect of the liquid velocity on the masstransfer rate is small u p to a Reynolds number of about 3000, indicating that, in the viscous region of flow, little increase in mass-transfer rate is obtained by increasing the liquid speed. For Reynolds numbers over 3000, in the turbulent region, the mass-transfer coefficient increases as the liquid flow increases. Thus, increasing the rate of liquid flow can act t o increase the maximum thickness of oxide obtainable. From Figure 9, i t can be seen that the effect of wire speed is very pronounced, as indicated by the separate correlations obtained at different wire speeds. Increasing the wire speed a t constant liquid flow increases the mass-transfer coefficient very markedly. Thus, the continuous anodization process is not limited t o moderate wire speeds. The continuous anodization of three or more wires in parallel presents no difficulties other than minor mechanical problems. It will be obvious that a continuous anodizer of almost any size and capacity can be built and operated satisfactorily. Figure 10 is an over-all view of a continuous anodizer located in the General Electric Research Laboratory. The 9-inch brass direct current contact sheave is in the foreground. The electrolytic acid dip, water wash, and air dryer are also visible. The anodizer itself cannot be seen, but runs down the far side of the table. The metal trough on top contains all the electrical leads required. The first panel on the right has the direct current generator controls. The next panel contains the anodizer heater
December 1955
surface area of wire in anodizer = xnF concentration of hydroxyl in bulk of solution C, = concentration of hydroxyl ion at surface of wire D L = diffusivity coefficient set equal to 1 F = Faraday constant, 96,500 coulombs per equivalent I = current, amperes k K E = efficiency coefficient = M -* k K L = apparent mass-transfer coefficient = k ~ k L = length of anodizer, feet M = molecular weight (NaOH) = concentration of NaOH, per cent by weight = (C = =
Cd
-
Re = Reynolds number, DpvL, __ dimensionless P V L = velocity of electrolyte, feet per minute V W = speed of wire, feet per minute W A = mg. of copper oxide per sq. cm. d = diameter of wire, inches k = constant equal to m F plus dimensional conversion factors k, = true efficiency coefficient k~ = true mass-transfer coefficient n = number of electrons per molecule T = pi, 3.14 = density of electrolyte, pounds per cubic foot p lb mass p = viscosity of electrolyte, f t . hr. = J' factor, s L ) 2 / a , DI, assumed constant equal to 1 j
2(
~~
literature cited (1) Agar, J. N., Discussions Faraday Soc., 1 , 26 (1947). (2) Colburn, A. P., Trans. Am. Inst. Chem. Engrs., 29, 174 (1933). (3) Feitknecht, W., and Lenel, H. W., Helv. Chim. Acta, 27, 775 (1944). (4) Lin, C. S., Denton, F. B., Gaskill, H. S., and Putnam, G. L., IND.ENQ.CHEM.,43, 2136 (1951). (5) AIcLean, J. D., and Young, C. B. F., Metal Finishing, 43, 247 (1945). (6) Radke, D. F., and Fraser, K. C . , private communication. RECEIVED for review February 10, 1955.
INDUSTRIAL AND ENGINEERING CHEMISTRY
ACCEPTEDAugust 20, 1955.
2491
