A Hydrogen Permeation Study of Electroplated Cadium on an Iron Substrate

Acknowledgement List of Tables List of Figures Chapter

Many of these alloys are susceptafile to hydrogen embrittlement, and cadmium electroplating has been shown to Be a very ef£iciene method of reducing hydrogen induced failures. However, industrial cadmium plating wastes are toxic and expensive to process for proper disposal. Current government regulations regarding this waste disposal have encouraged commercial platers away from cadmium as an amphoteric coating . Zinc is frequently used and aluminum looks encouraging as an alternative to cadmium electroplates. In order to accurately assess the effectiveness of these alternative coatings to cadmium, it is first necessary to quantatively determine the permeation rate of hydrogen through cadmium electropl~ted coatings.
Hydrogen permeation experiments can be done in an electrochemical cell with electrolytically generated hydrogen using a technique known as the electrochemical hydro-gen permeation method. Previous work at th_ e University of Rhode Island, using this method, has determined the necessity for a controlled sample preparation te chnique in order to get reproducibility of data. It has also been determined that it is necessary to coat the inlet and exit surfaces of the sample membrane with an inert coating to ii prevent reaction of the test sample material or c oating with either th.e inlet or exit electrolyte . Also , this inert coating must not be the rate determining step for the hydrogen permeation rate. In this project, an electroplating bath and technique was developed that would provide a thin flash of palladium over the inlet and exit surfaces of the sample membrane. Electron microscopy was used to check the integrity of these inert coatings.
The base metal chosen for this project was a high purity "Ferrovac E" iron. The first phase of the tests was to replicate earlier experiments on a pure palladium-ironpalladium membrane in order to confirm the proper function ·ing of the experimental technique .
After this, it was necessary to determine the rate controlling step of the sample membrane with a cadmium electroplate just inside the palladium coating on the inlet side of the membrane. Cadmium should be rate controlling , and this is determined by varying the thickness of the iron substrate and observing the variations in steady state permeation rates .
Finally, the effects of various thicknesses of cadmium electroplated coatings was determined hy observing the changes in steady state permeation rates .
The results of tests with samples without cadmium coating closely resembled those of earlier researchers at University of Rhode Island , which confirme d the proper iii function of the equipment . The tests per.for.med wi.th a constant cadmium plating thickness and vari ous i ron suB. ·strate thicknesses showed a relatively small difterence in steady state permeation rate, which proved the cadmium layer to be the rate controlling step. The tests with various thicknesses of cadmium on a constant iron substrate thickness proved again that the cadmium was rate control~ ling. iv LJST OF TABLES Page Table I Hydrogen permeation rate summary 14 through iron Table II Sample preparation technique 15 Table III Cadmium bath composition 24 Table IV Ferrovac "E" composition 28 Table v Sample cleaning and plating schedules 31          The detrimental e£fects of hydrogen on metals was first documented by Deville and T'roost in 1853 (1). Since that time much research and several conferences have been specially devoted to hydrogen in metals (2)(3)(4)(5). In 1941 , Zapffe and Sims (6) referenced 104 papers on this subject and documented then the debates over proposed mechanisms of failure, many of which now have been discarded. To illustrate the mechanical effects of hydrogen embrittlement , Figure 1 represents the nature 0£ a metals response to hydrogen and shows the delayed fracture typical for a hydrogen induced failure.
Several proposed mechanisms of hydrogen embrittlement over the years have been updated to more inclusive theories (7). Loutham and McNitt (8) presented a comprehensive review of modern mechanisms. and Latanison. et . al. (9) assigned the labels to the sunnnaries presented below.
Pressure Model. This mechanism was originally pro-  f!ydrogen-stimulated plastic deformation . Beachem (14) in 1972 presented this mechanism, which supposes that the lattice is locally enhanced to be plastic due to absorbed hydrogen generating an increase in dislocation mobility.
Hydrogen-rich phase. This was presented by Westlake (15) in 1969. This mechanism assumes the presence of a hydride layer with different mechanical properties than that of the matrix.
Hydrogen-dislocation interactions. These were first presented by Bastien and Azou (16) in 1951 and subsequently discussed by others in 1968 and 1972. Hydrogen is assumed to react with dislocations to restrict dislocation mobility or to generate local high accumulations of hydrogen which both embrittle the lattice.
From these models it is evident that an all inclusive mechanism that is compatible with the observed phenomenon has yet to be developed and generally accepted. Regardless 3 of the lack of a comprehensive mechanism of hydrogen embrittlement, the deleterious effects of hydrogen in steel are well documented.
There are many sources of hydrogen to cause subsequent embrittlement.
Hydrogen containment vessels provide the most direct source of hydrogen in metals. Hydrogen is exposed to metallic surfaces in molecular form in ooth high pressure and high temperature vessels .
The most potent source of hydrogen in metals is from an electrolytic reaction which cathodically deposits atomic hydrogen on the metallic surface. The fugacity (virtual pressure or concentration) of hydrogen in iron is 10 5 to 10 8 atmospheres (17).
Electrolytic reactions include the galvanic corrosion of iron in seawater and the corrosion reaction of steel in a hydrogen sulfide solution conunon in the petroleum industry. The reduction reaction in cathodic protection can lead to the evolution of hydrogen on the protected electrode.
Electroplating and chemical cleaning are also potent sources of hydrogen on a metal surface. In electroplating , Conflicting data ex.ists in the literature regarding the effectiveness of these coatings toward increasing or reducing the susceptability of delayed failure due to hydrogen embrittlement (21). Much of this is due to the co-deposition of hydrogen in the plating process. Post plating bakeout procedures (22) should re ·move the bulk of entrained hydrogen to tolerable concen·trations.
Hot dipped coatings include zinc and zinc aluminum alloys. These coatings have been found to cause hydrogen embrittlement by one investigator (23) and have been de ·-termined as an effective diffusion barrier by another (18).
Attempting to justify the difference . • Townsend (23) (2 7) delved into the side effects of hydrogen entry during phosphate plating and determined the dependence on pH, temperature and oxidants (28). The hydrogen permeation rate was found to have a linearly decreasing value with increasing pli.
Of these types of coating , metallic coatings axe the most widely used, and of the metallic coatings~ electroplated cadmium is frequently the coatings of choice b.y 7 designers and specification writers 0£ high st:r:ength_ fasteners (_29). However, cadmium is recently receiving much discussion and attention , Because ot its toxicity in handling (25) and added expense in processing of the electroplating wastes (29). Current government regula·tions (30) regarding electroplating waste disposal have provided motivation to electroplaters and researchers to reevaluate cadmium as an amphoteric coating and to consider alternatives to cadmium. The following are the advantages and disadvantages of cadmium and its properties .
The advantages of electroplated cadmium are that cadmium has historically been used as a protective coating on high strength fasteners and other structural parts (31) and produces a uniform, adherent coating. An alternate method is vacuum deposition which requires the use of large chambers, has difficulties coating recessed areas, and has poor adhesion characteristics (31}. The electroplated cadmium from a cyanide bath is a cost efficient technique that has been used for many years (J9).
Cadmium is an anodic coating to ferrous substrates in aqueous environments (32). It serves as a sacrificial coating in the presence of holidays.
The wear resistant properties of cadmium are superior to alternative coatings (29). The ability to maintain close dimensional tolerances of the coating make cadmium the preferred galvanically active coating for many uses. 8 The disadvantages of electroplated cadmium are primarily toxicity in handling and its susceptability to induce hydrogen generated failures.
It is connnon knowledge in the cadmium plating industry that hydrogen is generated and contained in cadmium plated parts (23,31,33,34). This is attributed to plating inefficiencies which co-deposit hydrogen on the plating surface (21). It has also been extensively studied to optimize plating baths to maximize their efficiencies.
Commercial practice specifies depositing a thin or porous coat of cadmium followed by a thermal Bake out procedure, typically this is 3750 F for 8 hours or so (33}.
There are mixed opinions regarding the risk involved in

B. Prior Work on Project
The electrochemical hydrogen permeation technique used in this study was first developed in 1962 by Devanathan and Stachurski (42). It has been subsequently modified by others and has been extensively employed by many researchers (20,41,43). The technique entails potentiostatically monitoring the ionization current required to maintain the exit surf aces potential of a sample membrane to a preset electrochemical potential. When a hydrogen atom is exposed to the exit surface of the membrane, it is reduced to an ion due to that preset membrane potential.
The hydrogen ion gains an electron, which alters the potential of the membrane. The potentiostat supplies enough current to the membrane to maintain the sample membrane at the preset potential.
A microarnmeter monitors the potentiostatic current supplied to the membrane, and Farraday 1 s Law h.olds for converting the current flux to moles of hydrogen . Prior experience on iron has led to the use of palla~ dium on the inlet and exit surfaces in order to prevent surface reactions i .n the electrolyte from altering the inlet flux or exit potential.
Pourbaix stability diagrams of iron in aqueous solutions, see Figure 2, indicate the wide ranges of oxide stability, and therefore, the necessity of providing a clean surface for hydrogen entry.
6. 600 grit silicon carbide wet polish and rinse. ...  Anodic metal alternatives to cadmium consist of zinc , aluminum and magnesium. Zinc has a similar problem to cadmium when plated from a cyanide bath as it will codeposit significant quantities of hydrogen (21). However , zinc can be successfully plated from a high efficiency acid bath (45) with a reduced level of co-deposited hydrogen.
Aluminum can be vapor deposited on a metal part (45).
Aluminum also has potential for coating from an electroplated bath on certain metals ( 46) . ·The vapor deposition and the electroplating aluminum coatings both are free from co-deposition of hydrogen (_45 . 46) however . both aluminum and zinc create stronger gal vanic c ouples when coupled with steel as compared to cadmium . This potenti a l difference could result in an increased production of hydrogen at voids due to the galvani c coupl i ng .
Because of its . greater rest potential differen ce . . from steel, and its greater reactivity, magnesium poses even a greater threat to the production of hydrogen duri.ng service corrosion than does cadm:lum. It i.s not a likely alternative to cadmium.

c. Purpose
The purpose of this thesis is to quantify the effectiveness of electrodeposited cadmium for restricting the entry of hydrogen in a metal by using the electrochemical hydrogen permeation technique to monitor the hydrogen permeation rate through a metallic membrane without and with various thicknesses of a cadmium coating.
The project is divided into two major parts. The first part is to develop a methodology to plate the exposure samples and then successfully generate hydrogen evolution flux transients on the samples. This will provide the technique and samples to do the second part of the project.
The experimental effort, second part of the project, is broken into three parts. The first part is to determine the effects of the iron substrate on the hydrogen permeation rate. This will be the base line for comparisons with cadmium coatings, and will also verify the repeatability with previous U.R.I. experiments.
The second part is to determine the eftects of various iron substrate thicknesses with a constant cadmium coating layer on the inlet side.

19
The third part is to vary the thickness of the cadmium   Table III and Appendix II respectively list the composition of these two solutions .
Attempts to use a cyanide and acid zinc bath were unsuccessful as were the efforts by a connnercial plater (46)

B. Hydrogen Permeation System
The hydrogen permeation process system consists of the combined use of the sample preparation techniques, the plating procedures to produce a sample membrane and the use of the electrochemical cell .
Ferrovac "E", a double vacuum arc remelted high purity magnetic purpose iron , was used as the base sub·strate in the sample membrane. Table IV presents the chemical composition of the metal used. The as ·-rolled bar stock was cut along the longitudinal grain axis and 1I1achined into 2.54 cm square plates of various thicknesses (see Figure 4) . Ferrovac "E" was chosen as the substrate because of its high purity and ex. pected reproductibility between replicate samples.
The samples. as received £rom the machine shop , were surface ground to an approximate surface roughness of 120 rit paper. They were then rough polished on silicon carbide water lubricated discs of 350 and grit grid successively, followed by a 6 micron diamond wheel medium polish. plate and agitated with a magnetic stirrer. Figure 6 shows the plating system used.

29
Early efforts with palladium plating on cadmium were unsatisfactory and led to the use of the high speed palla·dium bath shown in Appendix II . Because of the high react·ivity of the pure iron in the acid bath, it was necessary to flash on a thin film of palladium to seal the iron from the palladium electrolyte. In order to produce this thin flash, the samples were electrically connected to the power supply and dipped into the palladium bath, with the power on, for approximately one second of plating at 9.0 ma/cm 2 (SO ASF). This process was followed by a 30 second plate at 0.9 ma/cm 2 (5 ASF) in order to deposit 1000 A of palladium on the iron. Appendix VIII shows the calcula·tions used to determine the plating current and time for the various plates.
The Hull cell was employed to determine a bright and efficient current density range to plate the palladium on the different metals.    Engineering. Appendix I shows representative pictures of.
the exposed and unexposed surfaces of palladium on iron , cadmium on iron and palladium on cadmium, respectively , at the magnifications listed.
The elctrochemical cell consisted of a charging (inlet) chamber. a potentiostatic (exit) chamber and a sample membrane between the two, see Figure 7.
The hydrogen charing on the inlet surface was done via a direct-current constant-current power supply. The sample membrane was made the cathode for the hydrogen evolution and platinum clad niobium expanded metal strip was used as the anode , see Figure 7 . The sample membranes were charged at 0.5 milliamps.
Nitrogen gas purging and magnetic bar stirring were both used to thoroughly mix and prevent oxygen from contaminating the electrolyte.
A potentiostatically controlled , three electrode cell monitored the exit side of the sample membrane . The membrane was made the working electrode and a platinum cla d niobium strip was used as the auxillary electrode. A calomel reference electrode monitored the membrane poten ·tial set at -250 mV relative to the calome l e lectrode vi a a Luggin probe, see Figure 8 .
A micro annneter in series with the working electrode (sample) measures the current from the pot entiostat. c. Experimental Plan

B. Tests with Iron Substrate
Following the development of plating baths and procedures, the next step was to replicate the permeation experiment of Jones, using iron samples coated only with palladium. Figure 9 shows the results of this test with two replicate samples. Both the data of Figure  Since all of the data using samples with a cadmium coating was taken at a charging rate of 0.5 ma, an uncoated, i.e. no cadmium, reference sample was run at the 0 .. 5 ma charging rate. Figure 10 shows data from the replicate samples of iron with only palladium on the inlet and exit surfaces. This data was the reference for comparison with the samples coated with cadmium.

C. Effects of Anodic Coatings on Substrate
Following the tests on palladium-iron-palladium samples, experiments were run with palladium-cadmium-ironpalladium samples. The first group of these were run at a constant cadmium plate thickness and various iron substrate thicknesses.

C. Cadmium Effects
The first group of samples tested with a cadmium so coating were those run to determine the effects of the iron substrate base thickness. Figure 16 is a composite of all tests with a cadmium coating thickness of 0.2 mils .
It shows a large scatter of steady state rates that do not appear to be dependent upon the iron substrate thickness.
This lack of rate dependence is reasonable since the cadmium layer should be the rate determining step in the diffusion process.
Comparison of the 0.2 and 0.5 mil cadmium plating thickness sample rates, Figures 14 & 15 respectively, shows that the cadmium is the rate determining step. Figure 17 has the inverse thickness versus the steady state rate for the samples tested in the teflon cell and shows the expected inverse thickness dependence.
Comparison of the steady state flux rates of the samples without and with cadmium, Figures 9 and 16 respectively, show that the presence of the cadmium has dropped the flux rates by three orders ·of magnitude, therefore the cadmium is the rate controlling step .
Hydrogen transport is a diffusion process and Figure 18 shows the expected diffusion concentrations gradients through the sample thickness (48).  Reciprocal Thickness (m~lsJ The complexity and quality of composite coat~ ings required for this research is higher than the average commercial shop is accustomed to producing, therefore discretion must be exercised in selecting the shop.

B. Alternative Coatings
The most likely commercial alternative to cadmium is a non-cyanide zinc.
Aluminum has a good potential if the plating difficulties can be overcome. It is recommended that aluminum and both cyanide and non~cyanide zinc be evaluated and compared to cadmium for determining the relative effectiveness of the various barrier coatings.  (49), it is obvious that hydrogen can rapidly diffuse through palladium. The iron hole is smaller than the palladium hole yet larger than hydrogen atom. Cadmium has an interstitial hole size of Q.46A which can explain why it is such an effective hydrogen barrier. Of the alternate metals under consideration, zinc has an interstitial hole size slightly larger than cadmium and should perform quite well and when a palladium plating process is developed. If the interstitial holes size analysis is valid.
Conversely, the aluminum and nickel interstitial hole sizes are substantially larger than hydrogen and should not prove to be effective hydrogen barriers as pure metals. Doping could fill the interstitial sites and should therefore be tested as pure and doped metals and alloys. PALLASPEED is an organically-brightened high speed palladium ath which is capable of operating over a temperature range from oom ambient to over 130°F. The deposits are bright, highly ducjle, and exceptionally tarnish-resistant. Platinized. Anode-to-cathode area ratio should be at least 1:1 see notes 12° Baume' minimum

Recommended
The current efficiency of this bath is in the range of 70-80% t room temperature, and 85-95% at 120°F. At an efficiency of 90%, eplenish 36 ml PALLASPEED concentrate · (1.8 grams palladium metal) er ampere-hour. pH should be maintained at 5.5 -6.5. pH may be ajtisted upwards if necessary with potassium hydroxide, or down-ar~s with~hosphoric acid.
Specific gravity may be increased by a~1ng monopotassium phosphate. Under ordinary conditions, the r1ghtening agent in this bath is self-maintaining.

NOTES:
At a concentration of 30 grams palladium per gallon, the workrange of this bath is from 1 to approximately 25ASF at room e~perature; from 1 to approximately 40 ASF at 120°F. Deposit r;ghtness is greatest at the lower operating temperatures. At OF, the deposit will hold mirror-brightness when plated to thickesses exceeding one mil. Throwing power at low current densities P.LT.J\SPEED -continued ...----2s also greatest at low temperatures. For barrel work, a temperaure of 80 -90°F is recommended. Deposit ductility increases ith ificreasing bath temperature. Knoop hardness of the deposit s in the range of 200 -220 at room temperature, and in the range f 140 -160 at 120°F. For e~gineering p~rposes, a t7mper~ture of bout 120°F represents an optimum compromise of deposit brightness d plating speed.
This bath when first installed. undergoes a brief period of working-in" during which the bath color will change from very pale o a bright yellow, and the deposits will increase in brilliance, articularly at high current densities. During the working-in eriod it is best not to exceed a current density of about 10 ASF hen the bath is being operated at 120°F. When the bath is fully orked-in, a Hull cell panel plated at 1 ampere for 2 minutes at 20°F should be bright and haze-free to an indicated current denity of about 40 ASF. Note: The development of a dark amber or rownish coloration in the bath indicates an excessive buildup of rganics which tend to reduce the current efficiency.
In order to revent this the bath should be periodically carbon-treated or arbon-f il tered. A PLATING bath will operate successfully only when all sources of trouble are under proper control. This is an obvious fact, but the sources of trouble are not always obvious. Troubles may arise from improper chemical concentrations, drag-in of impurities, contamination from the atmosphere, contamination from the plating racks, products of decomposition of the chemicals in the bath, impurities in the water, or impurities from t~ anodes and chemicals themselves.
A number of foreign metals may enter the bath from several sources. If these metals are more electronegative than the metal being deposited they will cause trouble after they reach a critical concentration. Undesirable organic material may enter the bath from rack coatings, stop-offs, or by decomposition of addition agents.
There is one method to test for these and other troubles that produce an undesirable appearing plate. The way to do this is to plate a few pieces of work and observe the results. These tests may indicate freedom from trouble but upon plating at another current density or plating an article of an entirely different shape, troubles may appear. Such troubles may be detected by plating a series of test specimens over a range of current densities.
If plating at several current densities reveals a troublesome plating range then it is logical to use a plating lest that covers the entire plating range in one operation. The Hull cell was developed specifically to provide such a test. A plate from this test covers the normal plating range plus a higher and lower current density range. It is in this extra range particularily that troubles may be predicted before they appear in the normal plating range. Let us take an example to show how the plating test reveals troubles: A bright nickel bath begins to develop a smoky deposit on production pieces. Oiemical analyses show that all essential chemicals, including the primary brightener, are within the proper limits. Sufficient anti-pit agent is known to be present by measurement of surface tension.
A plating test is run and the same smoky deposit appears over part of the plating range. From previous ex~rience with prepared standards it is known that this typical test plate indicates an excessive amount of anti-pit agent present. The surface tension measurement did not locate the trouble because the surface tension test does not reveal an excessive amount ol anti-pit agent, but only establishes that a sufficient amount is present.
The bath may now either be treated with activated carbon or electrolyzed with dummy cathodes to remove all or part of the surface active material. After treatment and readjustment of the bath a second test may be run to confirm expected bright plating.
The point to this hyopthetical case is that the best way to make an over-all test is by a plating test and Lhe best plating test to use is one that covers the entire plating range required in production.
The Hull test is a universal plating range test. It may be used to predict results for the variation in current density on all but the most complicated shapes. The variation in current density during plating is the greatest of any of the common variables. It does not change at a given point with time but it does vary with the shape of the article being plated. It is low in recesses and high on corners and edges. Temperature, voltage, chemical composition and agitation are not variable for the normal time used to plate one rack but the current density varies at almost every poinl on every piece. Usually, however, the current density ~tays within the allowable plating range.
A Hull test may he run and the results show that  he width of the plating range available is at an ptimum value. At the same time current density roubles may be experienced in the bath (such as burnng in high current density areas). An attempt to overome this trouble may be made by lowering the total urrent used. The burning may cease but new diffiulties may now be encountered by the pieces not overing in low current density areas. The plating est showed that the bath was functionin g at its opti· num range. However, experience with the bath showed hat the range was not wide enough for the pieces being lated. The problem thP.n is one of racking and must i>e solved by robbing, shadowing or anode arrange· nenL The plating test was of specific value in that it mowed that attempts to change the bath anrl extend he plating range would be useless.
The Hull cell may be .used to measure plating ranges ~ut its greatest value is as an analytical and control nstrument. As such, it may be used both to detect resent troubles and avoid future difficulties.
It is possible to control a plating bath with nothing 11ore than a Hull cell and a hydrometer. A chromic cid bath is an example of a bath that may be contrnled in this manner without chemical analyses. A bath IS complicated as the brass bath may also be controled to a great extent by the Hull test. For most baths however, it is best to run both the plating test and hemical analyses since the latter gives definite, deirahle, quantitative information. The cell used in the Hull test is shown in Figure l. he cell is so constructed that the current density hanges regularly for every point along the width of the cathode. For this volume, an addition of 2 grams of solid is equivalent to one ounce per gallon. An anode is placed at the square end of the cell so as to cover the entire end. A 2% " x 4" cathode is placed at the opposite inclined end of the cell. The total current used depends on the type of bath being tested. The current density at any point on the cathode can be obtained by referring to the graph in Figure 2.
The graph of Figure   e width of the plating range available is at an >timum value. At the same time current density oubles may be experienced in the bath (such as burng in high current density areas). An attempt lo overime this trouble may be made by lowering the total irrent used. The burning may cease but new diffi-1]ties may now be encountered by the pieces not wering in low current density areas. The plating st showed that the bath was functioning at its optium range. However, experience with the bath showed at the range was not wide enough for the pieces being l ated. The problem th1m is one of racking and must ~ solved by robbing, shadowing or anode arrange · ~enL The plating test was of specific value in that it owed that attempts to change the bath anrl extend e plating range would be useless. The Hull cell may be .used to measure plating ranges 1t its greatest value is as an analytical and control strument. As such, it may he used both to detect esent troubles and avoid future difficulties.
It is possible to <:ontrol a plating bath with nothing ore than a Hull cell and a hydrometer. A chromic id bath is an example of a bath that may be contrnld in this manner without chemical analyses. A bath complicated as the brass bath may also be control· cl to a great extent by the Hull test. For most baths wever, it is best to run both the plating test and lemical analyses since the latter gives definite, de· rahle, quantitative information. The cell used in the Hull test is shown in Figure l. he cell is so constructed that the current density anges regularly for every point along the width of e cathode.  Th e preferred cell1 h olds 26 7 milliliters of solution. For this volume, an addition of 2 grams of solid is equival ent lo one ounce per gallon. An anode is placed at the square end of the cell so as to cover the entire end. A 21/:z" x 4" cathode is placed at the opposite inclined end of the cell.

I I
The total current used depends on the type of bath being tested. The current den sity at any point on the cathode can be obtained by referring to the graph in Figure 2.
The graph of Figure    50-60° c.
in order to I.ume fcuniliar with the typical appear· ance of a plac,. Preparation of the standards . should aimulate as d.ely as possible production operations including dr.ing, pickling and striking steps in order to ohtainmaximum information from the plates.
If the hath in1rjiated in actual practice, as in the case of the high diiiency cyanide copper hath, then mild agitation with• stirring rod should be used during the test. It is not~ best to run the test at the same average operatizg m.rrent density since some other total quantity of c:unmt may give greater sensitivity in seeking infonmOuir on the effect of a definite addative.
An example d , presently be given to illustrate tliis point but 6ntit· us look at a typical Hull cell plate in Figure 3.
A plate • isii Figure 3 might be obtained from a cold nickel Wusing a brightener. The high current density end ofdle plate is at the left. The area A is ~ark and ra.p,B is dull, C is bright and at D thPre IS no deposit •rt is very thin. The lines fro.111 the top to the Lotamr. of the plate marking the zones between A and ~md between B and C curve toward the low cum:nt~sitJ end. This is caused by inter· ruptioti of the flow of solution along the cathode L.y 1he bottom of the cell, but it does not interfere with interpretation of the results. A good scheme for Phowing typical results is to sketch the appearance of only the part of the plate observed between the dotted lines . A notebook entry would then appear as in Figure 4.
If the purpose of the test is to evaluate the cficct of addition agent concentration in a new bath, the total currP.nt used should be that which will give ~he widest possible bright range. For instance, the plates shown in Figure 5 might be obtained for a total current of 1, 2 and 3 amperes.
It is seen that as the total current is increased the ~··· ...

DIRECTIONS FOR HULL CELL PLATING TESTS
The Hull Cell (U.S. Patent 2,149,344) is a iniature plating unit designed to produce a cathde deposit that records the character of electrolate obtained at all current densities within the perating range. The character of deposit so made s dependent upon the condition of the plating bath ith respect to the primary components, addition gents, and impurities. The :{-foll Cell enables the xperienced operator to determine the following acts regarding plating baths: . The approximate limits of bright density range. This is accomplished by comparison of bright plated areas on the panel with the current densities given in the chart. Thus, if the bright or operable range is between 1 ~ inches and 2 ¥.? inches as measured from the left side of the plate, and the total current applied is 3 amps., the corresponding respective current densities from the curve are 70 amps./sq. ft. and 25 amps./sq. ft. Since these values represent extreme limits, it does not follow that either of these current densities can be used in a plating bath without obtaining poor areas of deposit, but some intermediate current density such as 50 amps./sq. ft. should work best. As a general rule, the acid or non-cy~nide baths should show bright or otherwise acceptable ranges from the low to high current density end of the plate over at least three-fourths of the cathode plate, alkaline or cyani®-baths over at least one-half of the catiiOc1e plate, and baths for barrel platmg over at least the lower one-third of the plate.
The approximate concentrations of the primary constitutents, such as cadmium content! so-dium cyanide content, nickel metal content, 'ere:-·Geiierally, the higher the me of a oa , e ig er ut not necessarily wider}-is-the operable bright current density range. The voltage across the cell also indicates the bath composition, i.e., cyanide in cadmium, or impurities and trivalent chromium in chromium.

Addition agent concentration. Although a few
addition agents can be determined by analysis, usually the Hull Cell provides the only satisfactory means for controlling the addition of these highly important materials, provided they exercise a visible effect upon deposits.
4. Metallic or organic impurities. Foreign metals or other harmful impurities in a plating bath exercise a definite effect on the appearance of the Hull Cell deposit, and their presence or absence can be established without difficulty.

The Hull
Cell is also an indispensible instrument for experimental plating investigations such · as addition agents, "covering power" or the lowest current density at which a deposit is produced, average cathode efficiency, average metal distribution or throwing power, and effects of pH, temperature, and decomposition products. The clear lucite Hull Cell enables the operator to observe the plating on the back of the panel to determine relative covering power at very low current densities. Clear lucite also makes it possible to insure complete solution of additions.

METHOD OF TEST
1 Before making a Hull Cell plating test, the fol-O\Ving points must be observed. l. ~ring the plating bath to be tested to operatmg level in the plating tank.
2. ~ither stir the bath thoroughly or use a sampling tube extending to the bottom of the tank, going over the tank uniformly from one end to the other. 3 · Be sure that the sample to be tested is brought to and kept at the proper operating tempera-ture during the test. The best method for testing samples at high temperature is by use of .: the Model WT or HT Hull Cell, in which are incorporated provisions for heating element with thermostat control. 4. Use a clean Hull Cell and clean cathode. If more than one kind of plating bath is to be tested regularly, one cell should be used exclusively for each type of bath to avoid contamination of one bath by the other.

METHOD OF TEST (Continued)
ce Hull Cell plating tests are not to be re-·ded as eliminating the necessity for occaal chemical analysis, such analysis should n made before the plating test so that the th sample c:an be a.djusted to th.e optimum nposition either pnor to or during the seence of Hull Cell tests.
e zinc plated steel cathodes provided with , set must be stripped of zinc by dipping in ; 0 1ution of half hydrochloric acid and halL ter and cleaned with a wet. clean cloth or [rpaper towel, Just before use. Remember ~ Hull Cell is an actual plating tank and properly prepared specimens will not rend satisfactorialy in the cell, just as in iillllercial practice.
ating times in the Hull Cell should be timed ctly for duplication of results. These times not all the same and are specified in the tion which follows for the different soluns. A convenient timer can be supplied as accessory to the Hull Cell Set. ~e proper volume of sample of plating bath 267 ml. for the 267 ml. Hull Cell, 534 ml. for e 534 ml. Hull Cell or 1000 ml. for the 1000 Hull Cell. Two grams addition to 267 ml. grams addition to the ·534 ml. or 7.5 grams dition to 1000 ml. Hull Cell is equal to one ce per gallon in the plating tank. To test riath sample hot, use the Model WT or HT Hull Cell. Never put a lucite Hull Cell on a hotplate.
9. The Filtered Output Rectified Model B-267 for the 267 ml. Hull Cell or Model B-1000 for the 1000 ml. Hull Cell is the preferred current source. A single-phase rectified without a properly designed filter circuit must not be used. Two 6-volt storage batteries connected in series usually give sufficient voltage for the 267 ml. Hull Cell but this source is less convenient than the rectifier mentioned.
10. Steel cathodes have a semi-bright and uniform surface. Cathodes should be used once and filed for future reference and not stripped for re-use, to avoid destruction of the surface. Experience has shown that poor or non-uniform steel surfaces are very misleading in interpretation of results. Replacement cathodes are available upon ordey=Irom-.us that duplica.t_e tlie original z~lated cathnde plates provided -Witb the set Polished brass panels are a so ava ilable for copper, nickel, and chrome plating tests.
11. Do not make too many tests on one sample of plating bath. Generally 6 to 8 plates can be made on a single mckel plafang bath sample -4Htless the pH is checked after each plate. Here the HT 534 is desirable because of the high volume-low current ratio.

SPECIFIC BATHS
far the best means of using the Hull Cell test efficiently is first to determine and the effects of every variation in each type of bath used in production. Thus, if a bright .bath is used, each constituent should be m turn, and effects noted on the cathode t, including nickel content, sulfate content, e content, brightening agents, and the probetal impurities such as copper, lead, zinc, ~~onducting such tests, it will be found very "'6eous to make up dilute solutions of agents so that unit volumes added to the ll Y?lume of 267 ml. are equivalent to abadd1tions to the plating bath. Practically all th concentrations are figured or stated in Per gallon.
. For solid chemicals the addition of 2 grams to 267 ml., 4 grams to 534 ml., and 7.5 grams to 1000 ml. Hull Cells are equivalent to one (1.0) ounce per gallon to the plating tank. Liquid chemicals such as brighteners or addition agents are normally specified or controlled on the basis of liquid ounces per gallon.
When the concentrated addition agent is a liquid, a diluted solution of the concentrate is recommended for Hull Cell additions and testing; The concentrate should be diluted to a 20% by volume solution. The addition of one (