the investigation of electroplating
and related solutions with the aid of
THE HULL CELL
This edited English translation © Robert Draper Ltd 1966
All rights reserved. This book, or parts thereof, may not be reproduced in any form without the previous written permission of the publishers
Printed in Great Britain
by Clare o' Molesey Ltd, Molesey, Surrey
PUBLISHER'S NOTE
I N R E c EN T years the Hull cell has come to be recognised as a most important tool in the control testing of electroplating and related solutions. The apparatus required is simple and inexpensive, the test takes only a few minutes to perform and does not require a particularly skilled operator. Yet with its aid much can be learnt about the composition and characteristics of a plating solution ; a chromium plating bath, for example, can be controlled and maintained by using only a hydrometer and a Hull cell. The hydrometer checks on the chromic acid while the Hull cell determines the catalyst ratio and impurity level. The Hull cell thus provides an extremely valuable plating control test.
What it does require however is skilled interpretation of the test panels obtained, comparisons being made with panels plated in solutions of known composition and purity and under known conditions. In the course of time most users of the Hull cell will accumulate a "considerable amount of comparative data. For the newcomer however the problem is how to establish the large number of necessary standards of comparison for which ideally one should take each plating solution in use and investigate the effects of altering each and every parameter-major and minor constituents, impurities and operating conditions-in small steps both singly and in combination.
This is where this book is of value because the Authors have themselves carried out these investigations and produced many hundreds of test panels from which nearly 200 have been selected and are reproduced in these pages for use by the operator in the interpretation of his own test results. Detailed written information on the Hull cell behaviour of plating solutions in various conditions of balance and under various plating conditions is also included together with notes on the use of the Hull cell for investigating covering power, current and metal distribution, metal surface condition, anodic processes including electropolishing and the suitability of tank linings.
This book was first published in Germany by the Eugen G. Leutze Verlag, Saulgau/Wiirtt., in 1956 and a second edition was published in 1965. The English translation was made by members of the staff of Electroplating and Metal Finishing who also edited the manuscript and included much additional information on the design and use of the Hull cell. Chapters 1 and 2 have in fact been completely re-written. We should like to express our thanks to R. 0. Hull & Co. Inc. for their co-operation in providing this additional information together with the whole of Chapter 9, and in particular to Dr. M. M. Beckwith, Vice-President, who also checked through the whole manuscript, for his most helpful comments and suggestions.
Teddington, June 1966
PREFACE TO THE FIRST GERMAN EDITION
. T H E I D E A of this book followed my own endeavours to form a photographic file of Hull cell panels illustrating various phenomena which arise in using the cell. Only during the course of this work did it occur to me that a collection of photographs of this type in book form would be of great help to others who use the Hull cell for testing solutions, and not least to the untrained apprentice. The basic material for this volume was therefore formed by many illustrations which I believe are capable of describing the appearance of Hull cell panels much better than the most comprehensive verbal descriptions. The reader who studies the illustrations and the accompanying text will save himself much effort in his attempts to interpret and evaluate the panels.
One problem which presented itself in reproducing the appearance of the panels photographically was how to show lustre and brightness. There appeared to be three ways of doing this. I could let the panel reflect a white light source. This showed fully bright surfaces as white and without structure while the less bright and matt surfaces appeared grey due to their lower specular reflectivity. If on the other hand the bright parts of a surface reflected a black colour, they would appear as black and without structural features. In this case again, the matt parts of the surface appeared grey due to the fact that they reflected diffuse light. I found that black gave a better contrast against less black surfaces than white against less white ones. For this reason I decided to use black reflections. This is also justified by the fact that a considerably larger number of areas would appear as light than as dark corresponding to bright and matt areas of surfaces. The third possibility, that of choosing a black-and-white pattern for the reflection source, was eliminated as giving rise to too much distortion and also because the pattern would mask structural features of the panels.
In order that this book should not be too long, I have restricted it to the most important plating solutions used in practice. My thanks are due to the Eugen G. Leuze Verlag for their careful attention to the compilation of the illustrations.
Bielefeld, June 1956
WALTER NOHSE
PREFACE TO THE SECOND GERMAN EDITION
S IN c E THE publication of this book in 1956 the Hull cell has established itself widely both in the laboratory and in the factory. The need for a second edition, which finds its reason in the rapid sales of the original edition, demonstrates that today the Hull cell has become a standard method of control in the electroplating shop.
July 1962 saw the appearance of the German Standard DIN 50 957 which laid down the principles of the use of the Hull cell for testing electroplating solutions. In the meantime two different cells with capacities of 250 and 1000 ml respectively have come into use ; of these the 250 ml cell is the most frequently employed.
In describing the Hull cell and its method of operation, I have made good use of the information in DIN 50 957 and where it seemed necessary I have amplified this for the practical plater. The abbreviations for 'porous', ' rough ' and ' no deposit ' have been altered although their visual symbols remain almost unchanged.
In the second edition, further work has been added which is designed to· show the versatility of the Hull cell. The last two chapters in this edition consist of a chapter written in collaboration with Ing. G. Wagenblast on the investigation of nickel plating salts in the Hull cell and a section contributed by Dr. J. Heyes on the measurement of metal distribution in electroplating solutions.
My gratitude for support in further experiments and for many helpful suggestions is due to Mr. Richard Hull Jr., Mr. Giinter Wagenblast and Dr. J. Heyes.
Lippstadt/West.f, Spring 1965
WALTER NOHSE
CONTENTS
PUBLISHER'S NOTE
PRPREFACE TO THE FIRST GERMAN EDITION
EFACE TO THE SECOND GERMAN EDITION
THE HULL CELL
Why use a Hull cell - Hull cell design - Material of construction - Temperature and agitation - Anode - Cathode - Current supply, control and measurement - Hanging Hull cell - Modified Hull cell
THE HULL CELL TEST AND Irs INTERPRETATION
Current distribution on the test panel - Test procedure - Interpretation of the test results
CHROMIUM PLATING SOLUTIONS Test procedure - Test results
IMPURITIES IN BRIGHT NICKEL PLATING SOLUTIONS
Copper contamination - Iron contamination - Effect of hydrogen peroxide - Chromic acid contamination - Zinc contamination - Cadmium contamination - Lead contamination - Other impurities and plating defects
BRIGHTENERS IN NICKEL PLATING SOLUTIONS
CADMIUM PLATING SOLUTIONS
Hull cell operating conditions - Test results - Cadmium and cyanide contents - Brightener concentration - Caustic soda - Solution composition
ZINC PLATING SOLUTIONS
Test conditions - Effect of increasing hydroxide content - Effect of increasing cyanide content - Practical implications - Effect of impurities
COPPER PLATING SOLUTIONS
BRASS, TIN AND SIL VER PLATING SOLUTIONS (by R. 0. Hull and Company Inc.)
Brass plating baths - Alkaline tin baths - Cyanide silver baths - Miscellaneous plating baths
TESTING OF NICKEL SALTS
(by W. Nohse and G. Wagenblast)
Nickel chloride - Nickel sulphate - Nickel plating salts - Conclusion
MET AL AND CURRENT DISTRIBUTION
Part 1. Use of the Jet test in conjunction with the Hull cell to determine metal distribution
Part 2. Direct determination of current and metal distribution (by Dr. 'Josef Heyes)
SOl\1E MISCELLANEOUS APPLICATIONS OF THE HULL CELL 115
1. Testing the suitability of basis metals - 2. Covering power
3. Anodic oxidation - 4. Electropolishing - 5. Testing tank linings
FURTHER NOTES ON THE USE OF THE HULL CELL 120
TypeS of anode - Incorrect polarity of the experimental apparatus
INDEX
CHAPTER 1
THE HULL CELL
Why use a Hull cell ?
A P L A T I N G bath is a solution of a number of chemicals each of which has a certain effect on the properties. of the electrodeposited metal coating. In order, therefore, to obtain electroplated coatings of the desired properties it is necessary to know the composition. of the electrolyte. In as far as this concerns the concentrations of metal and other salts, there are a sufficient number of analytical methods known today which, with the aid of simple techniques, a little experience and care in operation, give results of sufficient
accuracy.* '> ·
However, such methods of analysis are sometimes time-consuming, for instance, the determination of catalyst concentration in chromium plating solutions. Moreover they are not generally suited to the control of organic 'addition agents. Although the type of compound employed fol.this purpose is often known from the patent literature, it is rare that the plater knows the exact chemical composition and it is seldom that analytical methods are available to him for determining the concentration in the plating solution. From the practical point of view this is dangerous because a small departure from the optimum concentration of such addition agents will not normally produce any visible change in the electrodeposit produced under the normal
* ' The Analysis of Electroplating and Related Solutions ' by K. E. Langford, Robert Draper Ltd., Teddington,
'Schnellanalysenmethoden fiir galvanische Bader' by R. Weiner and C. Schiele, Eugen G. Leuze Verlag, Germany.
THE HULL CELL
plating conditions and it is impossible for the plater to obtain advance warning in this way of a dangerous deterioration in the solution. Fortunately however changes are more easily. visible in deposits produced under other conditions of electrodeposition and the Hull cell is a very suitable tool for such plating tests.
It was first described by R. 0. Hull in a paper entitled ' Current density characteristics, their determination and application ' (Proc. Amer. Electroplaters' Soc., 1939, 27, 52 - 60) and its application in the electroplating industry has been well summarised by Armet* in the following words. " The Hull cell may be employed usefully for the control of most types of plating solutions. One of its great advantages is the fact that it is possible for the skilled worker to assess the deposit characteristics at varying current densities all on one test panel. It is furthermore possible to carry out tests at various temperatures and current densities, and so to get a good idea of the bath characteristics and of the changes due to the variables introduced.
" The following effects may be investigated using the Hull cell.
1. Variation in main solution constituents and addition agents.
2. Operational variations such as temperature and current density.
3. Effect of organic and/or metallic contamination of the solution.
4. Investigations of tank linings, i.e. rubber and plastics, which might be suspect. Hull cell panels may be conveniently stored in the desiccator, or in plastic envelopes as permanent records of solution characteristics.
" It is possible with regular Hull cell tests to anticipate plating solution faults well in advance of their becoming a production danger. This is because of the fact that it is possible to see results at current densities both higher and lower than those obtained in production working, and it is normally in these areas that troubles first manifest themselves. Such troubles, though very apparent over a range of laboratory plating tests, quickly extend to the production plating if not rectified.
" It is a fairly well established fact that all plating baths, even of the same solution, are individual and distinct from each other. When complex impurities accumulate, it is often very difficult to control such solutions as chromium plating solutions, by chemical analysis alone, and in cases such as this the Hull cell proves invaluable.
" In cases where chemical analysis points to the need for large additions to be made to a solution, the Hull cell test can be used to check whether it is safe tomake such additions in one go without upsetting the bath equilibrium. The Hull cell test can demonstrate the presence of plating solution impurities in quantities so small as to be difficult to detect chemically but which can
* R. C. Armet. 'Electroplating Laboratory Manual ', Robt. Draper Ltd., Teddington
H U L L C E L L -D E S I G N
nevertheless cause production difficulties. A very good example of this is the detection of chlorides in chromium plating solutions, which will cause a deterioration in covering power."
However, while chemical analysis yields quantitative results, a Hull cell test only presents the investigator with a picture, and in order to interpret the picture, standards are required for comparison. Such standard pictures have not, to the Author's knowledge, been published before and it is the aim of this book to present illustrations demonstrating the effects on the Hull cell deposit of changes in the main constituents of the more important electroplating solutions and thereby to save the reader the task of carrying out his own experiments.
Hull cell design
The Hull cell is a miniature plating vat having a particular trapezoidal plan (see Fig. 1 et seq). The shape and dimensions are most important as
Fig. 1. Commercial (British) Hull. cell apparatus showing the cell, water bath and current control panel. [Courtesy M. L. Alkan Ltd.
THE HULL CELL
Fig. 2. 267 ml Hull cell. The 250 ml and 320 ml cells differ only in the depth of solution employed.
Fig. 3. 534 ml Hull cell recantmended [or extended testing because of the relatively smaller change in bath composition and temperature during a series of tests.
Fig. 4. 1000 111/ Hull cell.
HULL CELL DESIGN
will be seen below. It is manufactured in various sizes, 250, 267, 320, 534 and 1000 ml, mainly for operating convenience. The intending user would buy either the cell itself and make his own arrangements for current supply, etc., or a complete test apparatus with current supply, control panel and water bath (or immersion heater) as shown in Fig. 1.
The original cell upon which Dr. Hull's calculations were made was the 1000 ml cell but while it proved quite successful in practice, it was felt. that a smaller capacity cell might prove more convenient to use. So the 267 ml cell was developed, this capacity being chosen to assist in the rapid calculation of required· additions to the plating bath since 2·0 g of any addition per 267 ml are equivalent to 1 ·0 oz per U.S. gallon. For the same reason the 320 ml cell was developed for use in Britain (2·0 g of additive per 320 ml are equivalent to lO oz per Imperial gallon) while the 250 ml cell is popular on the Continent of Europe.
However, these small Hull cells have been criticised for being too small in that solution composition and temperature can change rapidly in spite of precautions and the designers now recommend the 534 ml cell which is not only double the volume. but also of a different shape, allowing alternative test procedures. The increased volume allows more tests to be conducted before the condition of the solution changes sufficiently to nullify · the results. Generally, on a bright nickel plating bath, the maximum number of tests recommended (without any correction of the solution for pH) in the 267 ml cell is three and in the 534 cell five. Temperature control is also better. Frequently, tests are run for 5 min at 25 to 30°C with cathode currents of 1 to 3 amps and in practice the temperature of the 534 ml bath does not appreciably increase under these conditions. Also if the anode for the system being tested has a relatively low polarizing current density, then the 534 is preferable because two or three anodes can be used with a single cathode thus bringing the anode current density below the limiting value. The 534 Hull cell uses the same size panel and the same current density scale as the 267.
People who use the 1000 ml cell generally do SO· because they prefer the even smaller temperature rise which results from the passage of a given amount of current. Also, a larger cathode is employed on which it is easier to detect minute effects occurring in the test bath. This larger panel is also used in the Hanging Hull Cell to which reference will be made later.
In the Hull cell the cathode is fixed at an angle to the anode and, in most models, both anode and cathode occupy the full cross section of the cell. The shape and actual dimension of the cell are most important in determining the cell current distribution ; the cathode angle of inclination was arrived at only after careful thought and research and, if this is changed, current distribution will no longer remain independent of the nature of the electrolyte.
THE HULL CELL
Cathode
267 ml Solution Level
Fig. S. 267 ml capacity Hull cell. All measurements given are internal and in inches. The 250 ml and 320 ml cells have the same plan measurements but the solution level is lowered and raised respectively to yield the appropriate capacity.
Fig. 6. 534 ml Hull cell. All measurements given are internal and in inches.
CONSTRUCTION
ANODE
Fig. 7. Dimensions of the 1000 ml Hull cell.
The various cells, their dimensions and test lay-outs are shown in the illustrations. The 250, 267 and 320 ml cells are identical except in regard to the depth of electrolyte employed and, of course, to the relation between the applied cell current and the scales or printed curves relating distance to primary current, density (see Chapter 2).
Hull cells are manufactured under one or more of the following U.S.
Patents: 2,149,344 ; 2,760,928 ; 2,801,963 ; 3,121,053.
Material of construction
The cell must be manufactured from a non-conducting material which is also completely inert to the electrolyte being tested. As the Hull cell is. frequently used to test for impurities in solution, this latter requirement is most important. Perspex is the most usual material of construction and is suitable for all plating solutions other than chromium. Its transparency is an advantage in that one can see better what is happening to the cathode during the test and also that one can see the back of the cathode, which is close to the cell wall. Polythene is also used and PVC and PVC-lined steel are sometimes encountered. A new development by the R. 0. Hull & Co. Inc. is a moulded polypropylene cell for chromium plating solutions which is preferable to the porcelain enamelled iron cell previously recommended.
Since stability of size and shape are important even when hot solutions are being tested, the plastic material must be carefully joined with adhesive or by welding. ·One piece moulded plastic cells are best.
THE HULL CELL
Fig. 8. 2SO ml Hull cell of German manuiacture with electrically heated water bath.
Fig. 9. 534 ml American Hull cell fitted with quartz immersion heater and thermostatic control.
USE
Temperature and agitation
For temperature control .either a water bath (see Fig. 8) or thermostatically controlled quartz immersion .heaters (see Fig. 9) can be employed.
The R. 0. Hull Co. also make a variable speed reciprocating agitator to closely simulate cathode rod agitation (Fig. 10).
Anode
As can be seen from the illustrations, the anode, which is 2! in X 2i in, is fitted closely to the cell wall. Normally it is flat and occupies the full cross section of the cell. Where the solution being tested is likely to give rise to high anode polarization, the anode will need to be constructed from gauze or from corrugated material to give an increased superficial area (see Fig. 11 ; see also Figs. 198 - 200). Its thickness must not exceed 0·2 in and its height should be such as to leave sufficient area over the solution level to take the electrical connections. The choice of anode material depends on the solution under test ; anodes are in fact made of brass, cadmium, copper, lead, nickel, tin and zinc and, for precious metal plating, in stainless steel, platinum and silver.
Fig. 10. Hull cell agitator to simulate cathode rod agitation in the Hull cell.
THE HULL CELL
Fig. 11. Flat and corrugated Hull cell anodes complete with lead wire.
The same anode should be used for all experiments on a given solution in order to eliminate the possible effect of differences in the anode material. It has become common practice (in Germany) to surround the anode with filter paper, and if this is done at all it should be done for the entire series of experiments. However, it is generally preferable not to use filter paper but to clean and activate the anode in a suitable stripping solution for the metal.
When testing an electrolyte for the conditions of solution of the anode, the latter replaces the cathode panel in the cell. If a quantitative evaluation is required, the anode panel must be made to the full size of the standard cathode for the particular cell in use.
Cathode
The condition of the plating solution is judged from the appearance of the cathode after the test. Accordingly, the cathode material and its surface condition should ideally be the same as that of the work which is plated in practice. Generally however polished brass or steel is quite suitable, particularly if the brightness of the deposit is the only point of interest.*
It is well known that the surface composition and condition of the basis metal can have a most considerable effect on the plating obtained. It is important therefore that these factors be standardised. The R. 0. Hull Co. supply standard cathodes in zinc plated semi-bright steel (No. 3 finish) which is ideal for cadmium and zinc or in highly polished brass, protected by peel-off
ELECTRODES
Fig. 12. Cathodes for the Hull cell.
lacquer, which is suitable for testing bright brass, bright copper and bright nickel. The zinc coating is put on at the mill to prevent rusting of the panels before use and it is removed by dipping in 50% (vol) hydrochloric acid followed by rinsing, wiping of the panel with a clean cloth or wet paper towel and final rinsing just before use. In the case of polished brass panels, the protective plastic coating is first removed by grasping the corner edge with a knife or fingernail, then soaking the panel for approximately 20 seconds in a hot non-tarnishing brass cleaner or rubbing with a cotton wad or sponge soaked in trisodium phosphate or alkali cleaner, followed by rinsing in cold running water. When cleaning test panels, care must be taken to handle the panels by the edges only so as to prevent finger print smudges. If necessary, rubber gloves may be worn. ·
It is inadvisable to use a cathode more than once since stripping will in all probability materially alter the surface characteristics of the test piece.
Brass test panels are usually polished on the front surface only but in some circumstances it may be necessary to polish the back as well, as for example
* R. 0. Hull & Co. Inc. are not in entire agreement with this and always prefer to carry out a test on a substrate, be it basis metal or electrodeposit, as close as possible
to that used in practice. Steel is recommended for testing cadmium and zinc. For nickel either a steel panel or a bright polished brass panel is used. When testing chromium solutions the panel is first plated with bright nickel and then rinsed and used in the Hull cell test. If it is a question whether the chromium or the bright " nickel is the source of the problem, the panel is first nickel plated in the Hull cell using the bright nickel plating solution in question, and then tested in the chromium plating solution in another Hull cell.
THE HULL CELL
when it is required to observe the tendency to etching on very low current density areas.
Hull cell test panels are' made in two sizes, one to fit the 250, 267, 320 and 534 cells and the other for the 1000 ml cell and the Hanging Hull cell. It fits closely to the cell wall and its height should be such as to stand out of the solution sufficiently to make the electrical connections. Its thickness should preferably be 0·5 ± O·l mm but it can be up to 1 mm.
Current supply, control and measurement
For operation of the Hull cell a source of direct current is required capable of supplying 3 amps (or 5 amps in the case of 1000 ml cell) ; the maximum voltage ever likely to be required is 18 volts though normally it will be very much less than this.
To supply this direct current use can be made of either car batteries or small transformer/rectifier units. Since current ripple can affect the structure of an electrodeposit, it is important if a rectifier is used that the amount of ripple does not exceed 15% ; an ordinary battery charger is therefore unsuitable. Three-phase rectifier units incorporating valve smoothing circuits have been widely employed in the past. However, even these are affected by mains voltage fluctuations such as may occur in any works laboratory and the modern tendency therefore is to use transistorised mains units which supply a constant voltage direct current, independent of either loading or mains voltage and which is practically free from ripple. An arrangement for constant voltage or constant current density control is sometimes used as an additional refinement.
When a lead accumulator is used to supply the power for the Hull cell it is recommended to arrange two 6 volt car batteries as shown in Fig. 13. This arrangement will usually provide sufficient current for the 250, 267 or 320 ml Hull cell but is less convenient than a transformer/rectifier unit.
Whichever method is employed means must be provided for controlling and measuring the current passed by the cell. The regulator should operate over 100% of the range and be stepless. Variable resistances can be used though in the case of a rectifier unit 100% stepless control through an autotransformer is to be preferred.
Resistances should be selected in such a way that
R = t.V ~ (U)
lmin
CURRENT SUPPLY
Fig. 13. Example of a circuit for Hull cell current supply using lead accumulators.
R1 Variable rheostat with 'off' position, 6 ohms, 6 amperes R;i Variable rheostat with 'off' position, 30 ohms, 5 amperes A1 Ammeter, 10 amps, divided into 0·2 amp units
~ Ammeter, 6 amps, divided into 0·2 amp units
V1 Voltmeter, 15 volts, divided into 0·2 volt units
V2 Voltmeter, 6 volts divided into 0·2 volt units.
where
!J.. V = the largest required voltage drop in volts and Imin =the lowest required current in amperes.
The maximum current which can pass is taken as the loading.
THE HULL CELL
To O.C. Source
Fig. 14. Circuit diagram of resistance comm! in the Hull cell apparatus shown in Fig. 1.
All the experiments described up to p. 90 were carried out using car batteries connected as shown in Fig. 13. The experiments on quality testing of nickel salts were conducted with the aid of a mains unit while Dr. Heyes (see p. 109) conducted his investigations with an electroplating rectifier with a ripple smoothing arrangement.
Hanging Hull cell
A newer development, currently not available outside the United States, is the Hanging Hull cell (Fig. 15).
As its name suggests, this is a version of the Hull cell designed to be hung directly on the carrier bar or cathode rod of the plating tank and which will produce on the test panel a deposit showing the characteristics of the plating bath over the entire operating current density range, thus combining most of the features of the standard Hull cell with the advantage of testing under production conditions. It consists of a 0 to 50 amp Perspex-encased ammeter hermetically sealed to a Perspex-encased 24 in or 32 in copper stem which, at the top, forms the cathode bar hook and at the bottom holds a Perspex Hull cell which in this case comprises a V-shaped box holding the larger size cathode as used in the 1000 ml Hull cell.
It is operated in much the same way as the standard Hull cell and the results are comparable for comparable current densities. It should however be remembered that the Hanging Hull cell panel may be plated at considerably higher currents than the standard Hull cell panel, thereby widening the current density range on the panel. For testing barrel solutions, the empty stationary barrel is submerged, the loading panel removed, and the Hanging Hull cell
HANGING HULL CELL
held in the barrel, electrical contact being made with a flexible lead to the cathode rod.
Since the Hanging Hull cell operates directly in the plating solution, a much closer picture of the actual plating range present in the bath is obtained on the test panels than in the laboratory Hull cells. This is especially true in chromium 'plating solutions where the current densities found in production may not be duplicated in the 267 and 1000 ml Hull cells.
Among the most important uses of the Hanging Hull cell is the determination of the source of trouble in a sequence of plating operations. By comparing the panels obtained by cleaning and pickling the test panels in the plating set-up with those obtained using freshly stripped steel or freshly cleaned brass panels, any difficulty due to poor cleaning or pickling can be detected and traced directly to its source.
Fig. 15. The Ha11gi11g Hull cell.
THE HULL CELL
By a similar set-up, the Hanging Hull .cell may be used to test the effectiveness of post-plate bright or proprietary dips by comparing the panels obtained with those made in fresh· dip solutions in a small container.
Another important use of the Hanging Hull cell is to make a current survey of a plating tank. A stripped panel is inserted into the cell head and the unit hung on various positions of the cathode rod. With the total voltage at a fixed value, the meter readings on the cell should be the same throughout the length of the cathode bar. If any variations in current reading are noticed, the cause may be poor anode contact or improper bussing of the cathode rod.
Modified Hull cell
The Modified Hull cell was developed for particular application to high current density solutions such as chromium plating solutions.* It consists of the standard (250, 267 or 320 ml) Hull cell in which t in diameter holes have been introduced into the two parallel sides as shown in Fig. 16. On the long side of the cell the holes are kept towards the anode end, thus retaining the restrictive effect of the low current density areas. Exact spacing and size of the holes do not seem to be at all critical.
%"Dia Holes
Fig. 16. Modified Hull cell.
The Modified cell is used inside another vessel, such as a 7 in diameter crystallising dish filled to the cell solution mark with the solution under test or it can be placed directly in the plating tank. Operation of the cell at currents of 10 to 15 amps and temperatures of 115°F are possible with little temperature fluctuation as natural solution movement by convection currents produces a constant interchange of. solution through the holes. Slow stirring helps and, unless of a very violent nature, does not appear to have any effect on the deposit.
* J. Branciaroli, Plating, 1959, 46, No. 3, 253 - 60.
CHAPTER 2
THE HULL CELL TEST
AND ITS INTERPRETATION
Current distribution on the test panel
To BE effective the current distribution on the test panel should cover a wider range of current density than that which is encountered in practice with the solution under test.
The primary current distribution on the Hull cell cathode follows a logarithmic curve and can be represented as
C.D. at any point = I (C1 - C2 log L)
where L = distance along the cathode, I = total cell current and C1 and CJ represent . constants which depend on the nature of the electrolyte. Hull determined these constants for a number of electrolytes, finding in general that they altered little from one electrolyte to another. In consequence, the values were averaged and general purpose formulae applicable to all electrolytes have been arrived at as follows :
for the 1000 ml cell
C.D. at any point = I (18·8 - 28·3 log L) for the 267 ml (and 534 ml) cell
C.D. at any point = I (27·7 - 48·7 log L)
(between the limits L = 0·25 and L =·3·25 in) where the current density is in amp/sq ft, I is in amperes and L is in inches.
HULL CELL TEST
Fig. 17. Plating range of 267 ml Hull cell.
[H. ']. Sedusky and ']. B. Mohler
CURRENT DISTRIBUTION ON PANEL
Since the 250 and 320 ml Hull cells differ from the 267 only in the depth of the solution employed, the formulae for these cells become :
for the 250 ml cell
C.D. at any point and for the 320 ml cell 267 I (z7.7 _ 48·7 log L) 250
C.D. at any point = ~~~ I (27·7 - 48·7 log L)
Deviations from these are to be expected, particularly for cyanide electrolytes. If for instance the above equations lead one to expect a current density of say 30 amp/sq ft, this current density will generally not apply over the whole period of the test. When the current is first turned on the current density at this point will be 30 amp/sq ft but later cathode polarization wil! cause a change in the current density, or alternatively it may cause periodic current fluctuations. However, it has been shown in practice that these equations do give sufficiently close approximations of the local current density for most purposes.
In any case it must be remembered that for nearly all purposes the Hull cell test is a qualitative one and the specification of current density is usually quite unnecessary except by such general terms as low, medium or high.
When it is desired to refer to the actual current densities, graphs such as that shown in Fig. 17 may be constructed to avoid the often lengthy calculations involved in applying the above equations. If for instance one is using a 3 amp cell current in a 267 ml Hull cell, the current density at a point on the test panel 1 inch from the high current density end can be read off as 85 amp/sq ft, and at 2 inches from the high current density end, it is approximately 39 amp/sq ft and so on. These values can in turn be plotted on scales such as that shown in Fig. 19, which are even more convenient to use.
Hull cells can be left or right handed so that the high current density area can be at either end of the test panel. Generally, however, the cells are as shown in Figs. 2 - 4, p. 4 and the high current density area is on the left of the panel.
Metal distribution is again different from the current distribution since it also depends on the throwing power of the solution. The one exception is the acid copper plating bath in which the metal distribution and primary current distribution are always closely similar and secondary polarization effects are very small.
Fig. 19. Current density scales [or the 267 ml, 534 ml and 1000 ml Hull cells (actual size). C11rrc11t density in amp/sq ft. (R. 0. Hull & Co. Jue.)
Test procedure
The first essential in making a Hull cell test on a plating solution is to obtain a truly representative sample. The best way of doing this is to use a sampling tube which is simply a Perspex, polythene or PVC tube i in inside diameter and perhaps 4 ft long brought to a coarse jet at the lower end. Still solutions must be mixed with a plunger to avoid layering. It is important to measure the solution depth at the time of sampling in order that the volume in the vat can be estimated accurately before calculating the amounts of any additions that have to be made.
If chemical analysis is to be carried out (and Hull cell plating tests are not to be regarded as eliminating the necessity for occasional chemical analysis)
TEST PROCEDURE
such analysis should be made at this stage and the composition of the bath corrected accordingly before commencing the Hull cell test.
Different operators prefer slightly different ways of making a Hull cell test (the method given here differs slightly from that described in DI.N 50 957). First the cell is cleaned ; if more than one kind of plating bath is tested regularly, one cell should be used exclusively for each type of bath to avoid contamination of one bath sample by another. Next the temperature of the cell, and that of the water jacket if used, is adjusted to the test temperature. At the same time the electrolyte is warmed in a beaker to the same temperature and transferred to the cell to reach just below the solution level mark.
The anode should now be cleaned and the cathode prepared as already described. Both electrodes are then placed in position in the cell, connected electrically and a small current applied immediately ; in some cases it will be better to connect and switch on the current before placing the cathode in the electrolyte. The arrangement of electrodes in the various Hull cells is shown in Fig. 20.
~ anode
250,267and 320ml Hull cell
534 ml Hull cell
lOOOml Hull cell
