troubleshooting, testing, & analysis

EXAMINING THE HULL CELL

BY JOE FOX, FINISHING TEST SUPPLY, INC., PAINESVILLE, OHIO

The hull cell was first outlined by R.O. Hull in a 1939 paper titled “Currentdensity characteristics, their determination, and application” (Proceedings ofthe American Electroplaters’ Society). The biggest advantage of the hull cell isits ability to allow a skilled operator to check the deposit properties at varyingcurrent densities, and all on one test panel.The hull cell can examine the effects of the following:

• Operational variables, such as pH current density, temperature, and

agitation

• Organics and metallic contamination

• Base salt constituents and addition agents

• Brightness range of plating deposit

• Covering power

Unlike quantitative analysis, the hull cell test gives the analyst a snapshot orpicture of the solution. In order to interpret the snapshot, other pictures arerequired for comparison.The aim of this article is to provide standards and training to educate theanalyst. More complete information will be available in Part III of this series,slated for the May issue of Metal Finishing. (Part II will appear in the April issue.)

DESIGN

The hull cell is a miniature plating tank that is trapezoidal in its configuration.The unique shape and dimensions of the hull cell are depicted in Figure 1 forvarious sizes. Different sizes exist mainly for the sake of convenience. Originalcells were based on 1,000 ml, but smaller cells were later developed. The 267-mlsize was chosen because a 2-g addition to the hull cell equals 1 oz/gal additionto the plating bath.The small cells are often criticized for being too small because rapid changesin composition and solution temperature can occur. For example, the 267-mlhull cell can have a 10% change in addition agent concentration in just one5-minute panel. This is dependent on the initial concentration and the consumptionrate of the addition agent system.The angle of inclination of the cathode, shape, and spacing are critical toprovide the current distribution that previous research has determined to beadvantageous. The depth of the solution can be altered, but the current mustalso be changed to allow for area and subsequent current changes. Figure 1shows a typical hull cell and temperature controller.

HULL CELL COMPOSITION

The hull cell should be constructed from a non-conductive material that ischemically inert from the solution being tested. Acrylic and polypropylene arecommonly used.

ANODE

The anodes are typically 2.25 × 2.25inches and are fitted close to thecell wall. In addition, the anodesare normally flat. If the solution issubject to high anode polarization,then the anode should be corrugated,ridged, or made from gauze toincrease the effective anode area.The anode material should becarefully chosen. Impurities in theanode composition can affect theoutcome of the tests. Therefore,the same anode material shouldalways be used for a series of tests.It is common practice to use filterpaper or a small anode bag to trapany anode fines or sludge that mayinterfere with the test. However,it is not sound practice to use filterpaper because it could impedeproper solution replenishment inthe anode film. If quantitative evaluationof the solution conditionsand the anode is required, then theanode area will usually need to beincreased to provide the proper anode-to-cathode ratio.

CATHODE

The cathode is used to judge the condition of the plating solution. The idealcondition would entail having the test panel made from the same compositionas the work being plated, however, this is easier said than done. Generally,polished brass panels and zinc-coated steel panels are used. The condition andcomposition of the cathode surface have a great effect on the outcome of thetest. Therefore, the cathode material and preparation must be standardized.Steel panels are zinc coated to prevent rusting during storage; usuallythe zinc is stripped in a 50%/volume solution of hydrochloric acid, rinsed,scratched with 400-grit wet/dry sandpaper, wiped with a clean paper towelto remove the smut, then rinsed and placed in the electrolyte for the platingtest. Operators must be careful not to touch or fingerprint the area that willundergo testing. Scratching the bottom of the test panel provides the operatorwith an idea of the leveling ability of the plating solution.Brass panels are generally coated with a peelable plastic. It is common practiceto remove the coating; scratch the bottom edge with 400-grit sandpaper;soak or direct-current clean the panel in a suitable brass cleaner; rinse; place in5% by volume sulfuric acid; rinse; and then place in the electrolyte for testing.

CURRENT SUPPLY

Direct current must be used for hull cell tests. The current supply should becapable of supplying a maximum of 12 V. Twelve volts is normally not required,but it is beneficial to have the capability. Amperage is normally 5 amps or less,but 10 amps is again beneficial to have if needed. Ripple should be less than5%. The ammeter should read in 0.2-amp increments or less to allow for propercontrol.

CURRENT DISTRIBUTION

For the tests to be effective, the current density should have a wider range than isencountered in practice. The equation below shows a logarithmic curve as follows:

Current density = I(C1 – C2 log L)

where L indicates the distance along the cathode, I indicates total cell current,and C1 and C2 are constants based on the nature of the electrolyte.Hull determined these constants from a number of electrolytes and foundthat they did not differ from one another. As a result, the values were averaged,and a general-purpose formula was derived:

For the 1,000-ml cell:Current density at any point = I(18.8 – 28.3 log L)

For the 267-ml cell:Current density at any point = I(27.7 – 48.7 log L)

This applies to the limits of L = 0.25 and L = 3.25 inches, where the currentdensity is in amps/ft2, I is in amperes, and L is in inches.Deviations from these are to be expected, particularly from cyanide electrolytes,as these solutions may have fluctuations in current due to cathodepolarization.

PART II

TESTING METHODS FOR

ELECTROPLATING BATHS

The jiggle cell is unique in that it is the best test equipment for mimickingactual plating tank conditions. The jiggle cell utilizes a bentcathode panel, and it can be used for most plating solutions providedthat the materials of construction are adequate for the solution beingtested. Most jiggle cells are made of either polypropylene or acrylic.

JIGGLE CELL CATHODE

The jiggle cell cathode is usually 1 inch wide × 7 inches long. The jiggle cellcathode panel is usually bent on a mandrel supplied by the equipment manufacturer.Figure 1 shows brass- and zinc-plated steel panels as purchased, afterbending on a mandrel.Figure 1 depicts the various 1-inch-square areas after bending the panel. There is a bottom, a face, a recess,and, finally, another face area. Theseimages closely reproduce the types ofcontours and areas generally foundon production parts in the platingtank.The bottom of the panel is usefulfor representing the lower portionof parts in the plating tank. Typicalproblems include pitting or air patternhaziness on the bottom ofthepanels.The two faces are good for representingthe vertical areas of partsin the plating tanks. The recessedarea mimics a typical recess on platingtank parts. It also representsthe shelf or top of parts in a platingtank.

JIGGLE CELLS

Jiggle cells are usually supplied invarious configurations:

Air or mechanical agitation with heatand temperature control:

This type ofunit will allow the operator to checkboth air and mechanically agitatedplating baths. The heating controlsallow the operator to also checkheated and unheated plating solutions.The motor on the right willmove the cathode up and down, providing the mechanical agitation (Fig. 2).

Air agitation with heat and temperature control: This unit will do air-agitatedsolutions but not mechanically agitated solutions. The unit does not have amotor to move the cathode panel up and down. See the example in Figure 3.

JIGGLE CELL ANODES

Jiggle cell anodes are usually 1 inch wide × 7 inches or longer. The anode materialwill usually match the anode material used in the plating bath to be tested.The anode should be bagged to prevent anode fines or sludge from entering thesolution. Figure 4 shows examples of various jiggle cell anodes.Test ProcedureThe test procedure should be standardized to allow for reproducibilitybetween tests. All steps should be taken to ensure that the conditions in thejiggle cell closely match the conditions in the plating tank. Particular attentionshould be paid to the agitation, current density, and temperature.The finishing test equipment supplier will usually provide the user withrecommended tips for the various plating tests.The cathode will need to be carefully prepared for each test with a stan- dardized procedure. Care shouldbe taken to ensure that the cathodepanel is not contaminated from fingerprintsor handling marks, poorcleaning, or poor activation.The test cell will need to be rinsedout after each test. It is recommendedto have jiggle cells devotedto each type of plating solution toprevent cross contamination fromdifferent solution types.Unique anodes and anodes bagsshould also be used on differentsolution types. An example wouldbe semibright sulfur-free nickel andbright nickel plating solutions.The plating tests are normallyrun on the solution from the platingbath and then evaluated. The solutionchemistry or operating conditionsare then changed and anotherplating test is performed. The twopanels should then be compared todetermine if there is any improvement.

SPECIALIZED TESTS

It is common practice to use thejiggle cell for testing the ductilityof nickel deposits. Users will use anunbent jiggle cell cathode and cut orshear off the bottom of the panel toreduce the surface area (and subsequent solution additive depletion). This iscommonly used for semibright and bright nickel ductility evaluations.

CONCLUSIONS

The jiggle cell is the single piece of test equipment that represents the actualplating conditions in the plating tank. It is especially useful for under shelfand shelf area troubleshooting.

 PART III

TESTING METHODS FOR

ELECTROPLATING BATHS

The hull cell and jiggle cell are the two most widely used test equipmentparaphernalia for electroplaters and electroplating addition agent suppliers.The interpretation of these test results is important. The operator must makea judgment based on previous tests on known acceptable plating solutions.The tables that follow list the common causes and the corrective actionsneeded to improve a particular plating bath problem. The operator shouldlook at one of the possible causes at a time to determine the correct action. Thecauses and necessary remedies are listed from the most to least frequent causes.It should be noted that when running more than one test on a solution, thecomposition in the hull cell can change rapidly. Therefore, it is recommendedthat the concentrations of the solution be adjusted if fresh solution is not availablefor each test.

OTHER HULL CELL APPLICATIONS

Covering power. Hull cells can be used to also test the covering power of solutions.Tests are usually run at 0.2 amp for 1 minute. This gives a current densityof approximately 12 to 0.4 ASF. This current density will usually provide anaccurate determination of covering power. The effects of addition agents canalso be investigated

Cleaning and base metal compatibility. Hull cells can be used to evaluate the basemetal’s ability to accept electroplating. Hull cells can also be used to check thecleaning process prior toelectroplating.When proprietary addition agents are used, it is good practice to contact the supplier for specific recommendations.