Surface Technology, 17 (1982) 147 - 156 141

DETERMINATION OF THE FLOW RATE IN THE HYDROSON CLEANING SYSTEM

ROBERT WALKER and NICHOLAS S. HOLT*

Department of Metallurgy and Materials Technology, University of Surrey, Guildford GU2 5XH (Gt. Britain)

(Received April 30,1982)

Summary

In this paper the values of the rate of flow of aqueous solution in the Hydroson system as determined by a physical technique and by an electro- chemical method are compared. A brief description is given of the Hydroson system and its use in the cleaning of surfaces and the electrodeposition of zinc. The effect of agitation of the plating bath is also covered. Zinc was deposited from an alkaline zincate solution onto stationary steel cathodes in the form of a flat sheet in the Hydroson tank and onto a cylindrical rod which was rotated at different speeds in a static solution. The surface appear- ance and hardness of the different deposits were compared and deposits from the Hydroson system were found to be similar to those obtained with a rota- tion speed of 1500 - 3000 rev min- ‘. The flow rate through the Hydroson jets was measured and this was similar to the value calculated from the limiting current density for the cylindrical cathode rotating at 356 rad s-l.

1. Introduction

The Hydroson system was developed in 1973 and is now widely used for cleaning in the metal finishing industry. The system is based on the pro- duction of sound waves in a cleaning medium and involves the circulation of continuously filtered cleaning solution by using a controlled multistage cen- trifugal pump. The solution is pumped through a manifold system fitted with generators and is returned to the cleaning tank.

The generators convert the velocity of the solution into acoustic energy with a pulsed waveform in the emitted jet. This high velocity jet of liquid enters the tank and gives high turbulence and vortices in the boundary layer

*Present address: United Kingdom Atomic Energy Authority, Wimborne, Dorset, Gt. Britain.

0376.4583/82/0000-0000/$02.75 @ Elsevier Sequoia/Printed in The Netherlands

148

between the jet stream and the stationary liquid. A frequency analysis has shown that the generator produces frequencies varying from 2 Hz to 20 kHz at a reasonably consistent pressure level and ranging beyond 50 kHz at re- ducing pressure levels. It has been suggested [l - 31 that these sound waves produce cavitation in the solution although this has not been confirmed be- cause aluminium foil did not perforate [4] . Ultrasonic agitation of water, which produces cavitation, bubble collapse and shock waves at any immersed surface, gave a significant increase in the hardness of immersed copper [5] but this was not observed with the Hydroson system [4] .

Details of the Hydroson system have been given in the literature [l - 3, 61. The mechanism of cleaning has been described by Walker and Holt [6] who found that the rate of cleaning was increased by a factor of 500 com- pared with magnetic stirring. Wire can be passed through the generator itself and the removal of drawing lubricants and scale is enhanced [7]. The system has also been used to give agitation in the electrodeposition of zinc from a commercial zincate solution [8] and the deposition of zinc powder from dilute solutions [9] . Hydroson [B] and ultrasonic [lo] agitation during the electrodeposition of zinc from zincate solutions have a similar effect on the properties of the electrolyte and deposited metal [4] when compared with magnetic stirring. Because Hydroson agitation does not produce cavitation at the metal surface any effect is probably due to the high flow rates produced by the generators.

The flow rate at the surface of an immersed object is important because it can have a marked effect on reactions which occur at the metal-liquid interface such as cleaning, polishing and plating. The complex hydrodynamic nature of the Hydroson system, however, makes it difficult to determine quantitatively the flow rate of electrolyte across a surface. An agitation sys- tem of a better-defined hydrodynamic nature is therefore required to model this system and to allow a more theoretical approach to be made. The rate of flow should be controllable over a wide range up to a high value and, as the Hydroson jets impinge at 90”, the flow must be of a turbulent and not a lam- inar nature.

The rotating disc electrode is the standard method for reducing mass transfer rates in electrochemical studies, but it was not suitable for the pres- ent work because laminar flow is obtained even at high rotation speeds. The rotating cylinder electrode, however, satisfies the above criteria [ll] . A re- view of the literature shows that during electrodeposition the high speed rota- tion of the cathode has a similar effect to ultrasonic agitation. Cathode rota- tion during plating can give an increase in the maximum current density [12] with brighter [12,13] and harder [12] deposits and similar effects have been observed with ultrasound [14]. Kenahan and Schlain [15] have com- pared these forms of agitation in copper plating and found similarities. As cavitation is unlikely to occur at the surface of a rotating cylinder electrode, the effects with this type of arrangement are probably more comparable with the Hydroson system than with ultrasonic agitation and these are now inves- tigated.

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Fig. 1. A Hydroson tank (80 1).

2. Electrodeposition studies

The Hydroson system is shown in Fig. 1. Deposition onto steel cathodes was carried out from a zincate solution containing 15 g ZnO 1-l and 105 g NaOH l-‘, with a bath pH of 14 and a temperature of 20 + 1 “C, using anodes of AnalaR grade zinc foil. The cathodes were lacquered on one side and posi-

tioned at the focal point facing the upper jets. Deposition was also carried out on a cylindrical steel rod cathode at-

tached to a variable speed electric motor, the speed of which was measured with a tachometer. The d.c. power was supplied by sprung carbon brushes. The cylinder had a diameter of 6.5 X lop3 m and a working surface area of 2.04 X lop4 m2 when the surface appearance was being studied and 5.37 X lo- 5 m2 for polarization studies. 5 1 of plating solution were used and rota- tion speeds were varied between 100 and 3000 rev min ‘. Polarization studies were carried out with a Wenking potentiostat using a platinum counterelec- trode as an anode, a calomel reference electrode and a mild steel cathode.

The surface appearance of the zinc deposited at a current density of 2 A dmp2 to a thickness of 10 I.trn was examined and photographed using a scanning electron microscope. The microhardness was measured on deposits of thickness 70 pm using a load of 50 gf. Ten readings were taken on each sample and the means and standard deviations were calculated.

2.1. Results and discussion The appearance of the surface of the electrodeposited zinc (Fig. 2)

changes with the form of agitation and the speed of rotation. At low rates of 100 and 300 rev min- ’ the size and shape of the crystallites are very similar to those obtained when the bath was magnetically stirred. Less facetting oc- curs at 700 and 1100 rev min-’ and there is a sharp reduction in the crystal- lite size between 1100 and 1500 rev min-‘. The appearance of the surface obtained between 1500 and 3000 rev mini’ was very similar to that with Hydroson agitation.

(a)

(e)

(b)

* c

25pm

(d)

(h)

(i)

Fig. 2. Variation in the surface appearance with agitation for (a) - (h) a rotating c$inder electrode at various rotational speeds and (i) a Hydroson tank: (a) 100 rev min ; (b) 300 rev min-‘; (c) 700 rev min -I; (d) 1100 rev min ‘; (e) 1500 rev min-l;(f) 1900 rev min-‘; (g) 2300 rev min-’ ;(h) 3000 rev min-‘.

The microhardness values are very similar for deposits produced with Hydroson agitation (80 + 7 HV) and with a rotation speed of 3000 rev mini’ (76 t 5 HV). They are significantly different from those obtained with mag- netic stirring (68 ? 5 HV) and ultrasound (51 * 6 HV).

Both these findings confirm the view gained from the literature that a rotating cylinder electrode would model adequately the Hydroson system.

3. The flow rate at a jet

The velocity of a solution produced by a Hydroson generator at a given distance from a jet may be studied by considering the work of Davies [16] on free turbulent jets. A jet is regarded as “free” when its cross-sectional area is less than 20% of the total cross-sectional flow area of the region through which it is flowing. A free jet is shown diagrammatically in Fig. 3 and is

151

Fig. 3. Diagram of a free turbulent jet.

thought to have four different flow regions: (1) a region of flow establish- ment extending to about 6.4 nozzle diameters d, (the fluid in this region has a core velocity that is about the same as the discharge velocity V, from the nozzle); (2) a transition region between 6.4& and 8d,; (3) a region of estab- lished flow, extending out to about lOOd,; (4) a terminal region where the centre-line velocity decreases steeply towards zero.

The half-angle of a cone formed by the locus of the half centre-line velocities is defined by Davies [16] as the jet angle, which is about 5”. Ap- proximately 45% of the total volume flow is within this half-speed cone. The outer limits of the jet are less well defined, but can be approximated by a cone of half-angle 10”.

The centre-line velocity V, of the jet, beyond the constant-velocity core, increases with distance X from the nozzle according to

V, X” __ = __ VII X

or

v, = -g v, = q5 v, (1)

where V, is the velocity at the nozzle, X, is the length of the constant- velocity core and d, is the nozzle diameter.

The flow velocity V, at the nozzle for one of the Hydroson generators was determined experimentally by holding the end of a wide bore rubber tube over one of the nozzles and measuring the volume L of solution col- lected in a given time t. The value of V, was then obtained from

V, ZZ ;L.. n

where

d, = 4 X 1O-3 m

t=60s

L = 9 X 10e3 m3

Therefore

V, = 11.9 m sP1

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This value permits the calculation, from eqn. (l), of the centre-line velocity at any distance from the nozzle. At a distance 3c = 0.17 m, which corresponds to the distance from the Hydroson generator nozzle at the focal point of the two jets, the value of V, is given by

6.4 X 4 X 10 3 X 11.9 v, = __-_ - rns-’

0.17

= 1.79 m s1

This calculation, however, assumes that the jet is emerging into a stationary fluid. It does not take into account the effect of the other jets and therefore the value can only be considered as an approximation.

4. Polarization studies

One of the problems in studying the kinetics of electrochemical reac- tions is to eliminate the random effects due to free convection produced by changes in temperature and density in the electrolyte. A common method of reducing these is to use the rotating disc electrode in which the electrode surface is circular and rotated in a horizontal plane. This arrangement gives laminar flow over a wide range of rotation speeds. A rotating cylinder elec- trode has a similar effect in reducing free convection. Although the hydro- dynamic conditions are not as clearly defined, the rotating cylinder has the major advantage that laminar flow only occurs at very low rotation speeds (usually less than 10 rev min ‘).

The relationship between the velocity flow at the cathode surface and the limiting current density with a rotating cylinder electrode has been dis- cussed by several workers. An equation of the following form has often been used :

i, = KU” (2)

where iL is the limiting current density, K is a constant, and U is the periph- eral velocity. The value of n was 0.67 - 0.7 according to early workers [17 - 211 but could be as low as 0.5 according to Euchen [22,23] or as high [24] as 0.9. For the dissolution of zinc and magnesium [25 - 271 the value of n was 0.7 at rotation speeds below 1000 rev min ’ but above this speed the value approached unity.

Eisenberg et al. [28] have studied the ionic mass transfer and concen- tration polarization on rotating cylinder electrodes. For the ferrocyanide- ferricyanide redox reaction they derived an equation relating the limiting current density i, to the peripheral velocity U:

(3)

where z is the number of electrons involved in the reaction, F is Faraday’s constant, co is the concentration of the bulk solution, R is the radius of the

153

electrode, W is the kinematic viscosity (the coefficient of viscosity divided by the density) and D is the diffusion coefficient. Hence if the relevant data were known the limiting current density for the rotating cylinder electrode could be calculated.

4.1. Experimental details To ensure constant values of the overpotential, a layer of zinc was ini-

tially deposited onto the steel cathode at a potential of -1.29 V. The over- potential values represent the applied potential above the rest potential of zinc (--1.21 V).

Cathodic polarization curves were produced for rotation speeds of 300, 700,1100,1500,1900,2300 and 3000 rev mini’. A similar curve was plotted for the electrolyte subjected to Hydroson agitation. Data for the Eisenberg equation were determined. The viscosity of the solution was measured with an Ostwald viscometer and the density was determined with a standard den- sity bottle.

4.2. Results and discussion The value of the overpotential is plotted against the logarithm of the

current density for different speeds of rotation in Fig. 4. The plotted lines consist of two curves: the lower curve is for the deposition of zinc and the upper curve is due to the cathodic evolution of hydrogen at the higher values of the overpotential. The curves for zinc deposition were extrapolated to give the limiting current density for the different rotation speeds. The polar- ization curve for the Hydroson agitation was superimposed on this graph and the limiting current density obtained in a similar manner.

The values of the limiting current density and angular velocity are plotted in Fig. 5. The theoretical curve in Fig. 5 was produced from the Eisen- berg equation (eqn. (3)) and calculated by using the experimentally measured values for the viscosity of 1.79 X lop3 kg m-l sK1 (1.79 cP) and the density

20 30

LOG CURRENT DENSITY (Am-’ I

Fig. 4. Overpotential us. log(current density) for rotating cylinder electrodes and elec- trodes in a Hydroson tank.

154

IBOO- ix--

1600. P ."-

14lxl. . 'e

,' ;I 1200~ Iyc

1MO. ,-,c

BOO- ,'

,':

6OOj /'

40 8b 120 160 2bo 240 2ko no 360

ANGULAR VELOCITY W irad cl)

Fig. 5. Angular velocity us. limiting current density for the rotating cylinder electrode: -x--, experimental; g--, predicted.

of 1.105 X lo3 kg mp3. The value of the diffusion coefficient D was obtained from the published literature: a value of 6.8 X 10p’o m s--r was given by Des- pit et aZ. [29] for a solution of 0.65 g Zn 1-r in 10 g KOH 1-l which is in good agreement with the value given by McBreen [30] and similar to the value of 9 X 10~~‘” m s ’ found by Popova et al. [ 311 for zincate ions in 56 g KOH 1 ‘. No value was found for zincate in a KOH solution.

The value of the logarithm of this limiting current density for deposi- tion in the Hydroson tank is 3.26 so that the current density is 1820 A m 2. This value corresponds to a rotating cylinder electrode at an angular velocity of 356 rad s-l. This can be converted to a linear peripheral velocity U accord- ing to

U= WR

where W is the angular velocity and R = 3.25 X lop3 m is the radius of the cylinder. Therefore

U=356X3.25X10p3ms1

= 1.16 m s-l

Hence the experimentally determined value of the flow velocity in the Hy- droson tank at the cathode surface is 1.16 m so ‘.

Mizushina [32] has suggested that certain criteria must be observed when relating the surface velocity to the limiting current density and these are now considered.

(1) The reaction must be diffusion controlled. Kudryavtsev [33] showed that concentration polarization was the predominant type of polarization in zincate solutions and this means that the reaction must be diffusion con- trolled.

(2) The reaction must be stoichiometric and the number of electrons involved in discharge must be known and constant. According to Kamat et al. [ 341 the possible reactions involving deposition from the zincate bath are

ZnO,’ + 4H’ + 2e- -i; Zn + 2H,O

HZnO, + 3H’ + 2e- + Zn + 2H,O

155

Zn(OH)42- + 4H’ + 2eP -+ Zn + 4Hz0

All these reactions involve two electrons. (3) The cathodic current efficiency must be 100%. Although the cur-

rent efficiency was not measured for the rotating cylinder cathode owing to the necessity for specialized equipment, it should be close to that for the Hy- droson which was 98% at a current density of 2 A drn- 2. At higher current densities the efficiency decreased owing to the evolution of hydrogen but this was taken into account by the extrapolation.

(4) A limiting current density must be clearly read from a potential- current density polarization graph. This was easily determined from the extrapolated curves although some small error may be involved.

Hence the use of the Eisenberg equation was considered justifiable. In the Eisenberg equation the limiting current density is dependent on

the angular velocity to the power of 0.7 (iL = KU0.7). Our experimental curves give a value of 0.7 for values of the angular velocity less than or equal to 120 rad-’ and 0.5 for values of the angular velocity greater than 160 rad-‘. There is reasonable agreement between the experimental and theoretical curves (Fig. 5).

The value of the velocity of the solution flowing across the cathode positioned at the focal point between the upper two jets in the Hydroson tank was 1.16 m s-l. This is lower than the theoretical value of 1.79 m sP1 which was obtained with only one jet and no allowance was made for the action of the other jets. In practice the liquid from the pair of jets would impinge, from perpendicular directions, at the same point on the cathode surface and reduce the flow velocity.

Naybour [35] has reported on the morphology of zinc deposited from a flowing zincate electrolyte containing 19.1 g ZnO ll’ and 325 g KOH 1-l. For a flow rate of 1.16 m sP1 the change in appearance from a flat to a boul- der type of growth occurred at about 25 - 30 A dmP2. In the present work the change occurs at 15 - 20 A dm-2. It should be noted that Naybour used more concentrated solutions, so that the current densities would be expected to be higher.

5. Conclusion

The flow rate of liquid at the focal point between two jets in the Hydro- son tank was determined electrochemically using rotating cylinder electrodes and limiting current density measurements and found to be about 1.16 m s-l. This is in good agreement with a physical measurement from a single jet of 1.79 m sP1 which is too high because it does not include any interaction from the other jets.

The morphology and microhardness of zinc electrodeposited from the Hydroson tank support these findings.

156

Acknowledgments

We thank Professor M. B. Waldron for his interest and the provision of research facilities at the University of Surrey and Mr. A. P. Thomson, Manag- ing Director, Nickerson Ultrason Ltd., Cradley Heath, Warley, West Midlands, for supplying the Hydroson system. N.S.H. also thanks the Science Research Council for financial support.

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