This is the first article in a two-part series about Hull cells and their use in plating operations. Click here to proceed to part 2
Most platers use a Hull cell to find out what is wrong when a plating station begins to produce rejects. But Hull cells can do far more than help to get you out of trouble. Properly used, they can prevent problems. You can use them in routine daily maintenance, establishing operating parameters, and in considering modifications or improvements of a plating process.
Because a Hull cell produces a deposit that is a true reproduction of the electroplate obtained at various current densities within the operating range of a particular system, it allows experienced operators to determine multiple process parameters, including:
Approximate bright range.
Approximate concentration of primary bath components, such as metal and electrolyte.
Approximate concentration of addition agents.
Presence or absence of metallic and organic impurities.
“Covering power”—the lowest current density at which plate is deposited.
“Throwing power”—metal distribution.
Effects of temperature variations.
Effects of pH variations.
The Hull cell is a time saver. Hull-cell panels are inexpensive when compared with experimenting in a production tank plating actual parts. You can introduce variables quickly and safely in the small tank represented by the cell. And you can see the results of several different tests at the same time.
Hull cells accurately duplicate all the variables present in a production-plating tank size, shape, width, depth and time parts are exposed to the process. The simplest Hull cell is a clear, trapezoidal Lucite chamber that is filled with plating solution and equipped with electrical contacts—essentially, a miniature plating station. Its design allows users to angle the cathode panel so that current flow is directly related to the distance from the anode. The current applied increases as the distance decreases. The clear plastic lets you see what is happening while metal is being deposited, if the plating solution does not block out visual observation.
The cell holds 267 ml of plating solution. At that volume, there are direct correlations of milliliter or gram additions to 100 gal of plating solution, without the need for a math degree. A 2-g or 2-ml addition to the 267-ml cell equal an addition of 1 oz/gal in your operating bath.
Types of Cells
From this basic cell concept, many variations, modifications and auxiliary aids have been developed over the years. For chrome bath evaluations, for example, porcelain is substituted for Lucite, because the Lucite deteriorates with prolonged exposure to chromium plating solutions. There are many other variations in cell volume and configuration, some of which are described below.
The Gornall cell was developed in an attempt to duplicate conditions in a printed-circuit-board plating operation. It consists of two, 534-ml cells welded together so that air and heat can be introduced to both sides of a perforated plastic panel. The larger cell volume allows more tests per bath sample with solutions whose chemistry will be altered too much by plating more than two or three panels (such as bright nickel). Larger volume also allows you to run several test panels before replacing the chemically depleted test solution.
Still larger cells—with volumes to 1,000-ml—can be equipped with heaters and agitation. The larger size allows more precise evaluation of baths.
A hanging Hull cell can be hung in an operating tank anywhere a rack of parts can be hung. Capable of reading from 0 to 50 amps, the cell hangs on a cathode bar below the solution and is an excellent tool for finding “dead” areas, poor electrical contact, no current, reduced current, and other conditions.
The Haring cell is a 1,000-ml cell that is used for more precise evaluations of throwing power. This cell is a rectangle with grooves that are different distances from the
anode.
Jiggle cells are mechanically actuated to move the panel up and down via connection to an offset cam. Panels run in this cell are one inch wide by approximately seven inches long, formed on a wooden mandrel that provides expanded areas (approximately one inch square) of various common asf areas commonly looked at in more precision evaluations. The areas are 1-20, 20-40, and 40-60 asf, by mandrel design. Also, right-angle bends in panels provide shelf areas for roughness observations.
The anode in a jiggle cell is (conveniently) a spear from the production bath, and the anode bag is readily made. Jiggle cells also provide hook-ups for air agitation and heat.
Analyze First!
While Hull-cell panels can provide a quick and factual picture of what is happening in the production tank, never forget that analysis should be a routine precursor of Hull cell evaluation. Looking at a panel without first knowing the chemistry of the bath can result in very misleading interpretations. Correcting bath chemistry often will eliminate a problem before you run a Hull cell. Having done this, you can count on the Hull cell to give you a much truer picture of what is happening in the large tank.
Please note that if, for example, your supplier suggests a 1-to-10 ratio of zinc metal to caustic, and outlines it as 0.8-1.2 oz/gal of zinc and 8.0-10.0 oz/gal of sodium hydroxide, running the bath at 0.8 oz/gal zinc and 12.0 oz/gal sodium hydroxide is not a 1-10 ratio. And while all around you are screaming for the answer to what’s wrong, remember that no bath is smart enough to cause rejects only on one particular part, or parts only in one area of a rack, or one side of a part, or every other barrel load. If you feel that’s what is happening...go take a walk, come back and start over again!
The Hull Cell: Accessories, Equipment and Supplies
Running Hull cell tests will be a lot easier and more accurate if the proper accessory equipment and supplies are available. Here are some of the items you’ll need.
Rectifier A 115-volt, 60-cycle, single-phase rectifier is the norm. It is available in 0-5, 0-10 and 0-30 amperes, direct current. The electronic state of the art assures very little “ripple.” Therefore close Hull cell investigation of covering and throwing power as well as bath chemistry is very viable. A built-in timer allows the operator to do other things while a panel is being run. Timers normally regulate up to 30 minutes.
Hull cell Ruler Usually supplied as a “freebie” by the supplier of your plating chemicals. It is probably the most useful support tool for tank-side or laboratory testing. It is a little over six inches long and one to two inches wide (depending on how much information is being offered).
On a Hull cell ruler one can find a scale for interpreting what you are looking at as it relates to the current at which you are plating (1,2, 3, 5 amperes) and actual amperes per square foot (asf) for 267-, 534- and 1000-ml test cells. There are conversion numbers and tables for converting from grams to ounces, ml to fluid oz, cu ft to gal, Fahrenheit to Centigrade, etc. Finally there are charts for suggested settings to run tests of the majority of plating processes.
Sandpaper Using 60- to 80-grit paper, apply uniform pressure to abrade along the bottom half inch of the entire ampere range of the Hull cell cathode panel. After the plating test is performed you will see the degree of leveling or lack thereof. This is particularly useful in evaluating copper and nickel plating.
Air supply If your process uses air agitation, you will want air to duplicate conditions in the Hull cell. A fish-aquarium bubbler is quite adequate. There is also an air-agitated Hull cell. It guarantees more uniform air distribution and a reproducible air pattern.
Alligator clips These assure more positive contact in the electrical circuit. They also position both the anode and cathode pieces (the anodes have a long stem that can be bent over the edge of the cell to assure positioning, and are designed for Hull cell testing).
Agitator If your system(s) use manual agitation, you should buy a Hull cell agitator, as it will allow for readily duplicated continuous solution movement in all your testing. It’s true that a glass stirring rod and a strong and patient arm will work, but with a powered agitator you can be doing other things rather than standing over an in-process panel.
Anodes They come in many types. Where possible you should use anodes that are metallurgically as close to as possible to those used in production. Here is a listing of the more common types, and where appropriate, particular applications:
Aluminum. For evaluating electropolishing, they are ideally cut from the same stock as that being processed.
Brass or bronze. Use the same alloy used in production. Bag the brass or bronze anode when appropriate. You do not need a professional seamstress; just a loose cloth bag for the anode.
Cadmium. Can be used for acid, cyanide, and “neutral” systems. Where anodes are bagged in production, the Hull cell anode also should be bagged.
Copper. Oxygen Free High Conductivity (OFHC) copper anodes are generally acceptable; however, for printed-circuit testing, exact duplication of the anode used in production is more critical. Anode bagging should again duplicate that in the process system.
Lead. More appropriately tin/lead (10% tin) is used in chromium plating, since it provides good efficiency and allows for reoxidizing of hexavalent to trivalent chrome -- a vital function in the chromium plating process.
Nickel. Almost without exception, nickel plating requires single- and often double- bagging of anodes. Whatever is used for the outer bag will normally suffice for Hull-Cell testing.
Stainless Steel. Be aware that 300 stainless contains some or a lot of chrome. In testing electropolishing, 400 stainless is less likely to introduce a little chromium contaminant. Just a little causes a lot of problems.
Steel. If an alkaline zinc system is being fed from a zinc generator, a plain steel anode that does not have a bunch of numbers to describe the steel is best. All those numbers designate potential contaminating metals. 1010 works very well.
Tin. Pure electrolytic tin, and bagged.
Titanium. Prime application is in electropolishing.
Zinc. By all means, bag if being used in the tank that way.
Note: In the case of the many new alloys that are electrodeposited, use the same type anode as that used in the process.
Anode Bags Cloth, nylon, or whatever allows duplication of the conditions in the operating system. In the case of double bagging, use the same material as is used for the exterior bag in production.
Cathode Panels There are many sizes and basis materials. In the printed-circuit industry, specific non-conductors are readily obtainable, pre-coated. Standard panel size is 7 by 10 cm. The “Jiggle cell” panel is approximately one inch wide by eight inches long before forming.
Having obtained the anodes, bags and cathode panels required, you’ll need some additional laboratory equipment to do the most effective and accurate testing.
Beakers It’s helpful to have 600-ml beakers for dipping panels in cleaners and acid- neutralizing chemicals prior to Hull cell testing. They are also for post-plate immersions such as chromating. A separate beaker may be necessary for a one per cent nitric acid dip (its width should be approximately half that of the lower cathode panel’s horizontal width). If the luxury of bench air is available, a Buckner funnel and flask is a wonderful time saver, especially if you are filtering a viscous liquid.
Polished brass panels In testing nickel deposits, you should have a polished brass panel covered by a thin plastic sheet to prevent tarnish. Scraping the edge of one panel with another will usually break the bond so that a piece of scotch tape will remove the protective film in one piece. These panels may be used on bright chrome systems as well, but often are plated with the nickel solution being used on the line. This is especially true when chasing the elusive “cloud” pattern.
IPC test panels If you plate through-hole printed circuits you should have an IPC (Institute of Printed Circuits) test panel. It is used with the Gornall Cell.
Zinc-coated panels A hot-dip-zinc-coated panel is a workhorse. The list of applications reads like a metal-finishing text. Its use for testing electropolishing is less known, but it does provide a quick way to determine the need for acids or wetting agents and to find gassing-pattern problems, all by simple visual examination. Hull cell examination also will help to predict the “right” amp/volt range for best results.
Eyedroppers A small Hull cell with limited volume sometimes requires very small additions of liquid chemicals. Calibrated eyedroppers make it easy to add 0.1 to 2.0 ounces.
Filtration Laboratory filtration equipment finds particles that should not be there. For most operations a stemmed funnel with a paper filter will suffice. Pay particular attention to the filter-size rating of the paper used. White paper is the usual, but a pack of black helps in those few instances when you are looking for a suspected white precipitate.
Heaters In duplicating conditions in a plating bath, you often must heat the solution. There are hot plates with built-in temperature regulators and vibrators that work in conjunction with a magnetized pellet for agitation of a solution. The temperature setting should be checked periodically against a lab thermometer. Built-in heat and/or air “sparge pipes” are available for most models. There is even one with holes in the vertical sides for both heating and volume control. It can be placed inside a larger Pyrex dish that can withstand the direct heat from a hot plate. A small quartz heater-- approximately ½ inch in diameter and three or four inches long, is another way to duplicate operating temperature. It can be plugged into any 110-volt outlet.
Hydrometers Specific gravity of a solution being tested, using a hydrometer, is often a useful measurement. Use a precision “weighed” glass tube, calibrated to show the specific gravity. Chromium plating baths and iron content in acid zinc are just two examples of applications.
Thickness testers You’ll often want to check electroplate thickness. A solution-drop test allows a corrosive solution to drop on a confined spot so that thickness can be determined by recording the time to remove the electroplate, exposing the basis metal. It works well on Hull cell panels. Magnetic thickness testers are nondestructive, and they also provide good information in the hands of an experienced operator.
Litmus paper You often need to know something about acidity and alkalinity of the solution. A treated paper that can be dipped into a solution to tell you quickly (by color change), whether a solution is acid or alkaline is adequate for most uses.
pH papers If you need to know acidity or alkalinity more accurately, there are chemically pre-treated papers for closer reading of pH. These strips are calibrated in 0.1 increments, dependent upon what “indicator” they contain.
Scale You’ll need to weigh additions of the various chemicals that may be required. A simple postal scale may be adequate, but for precision work a chemical balance is required. Both should be protected from the multitude of fumes found in a plating lab.
Scotch tape You’ll need it to peel the protective coating from polished brass cathode panels, and to check adhesion of deposits (where does the blister start?).
Record keeper A spreadsheet is useful for maintaining a history of an operating line. Keep a record of daily occurrences, by operating shifts, additions and problem-solving. Consider a photo album, with the slots holding a pictorial history in the form of preserved panels as part of the good, bad and the ugly.
Squeeze bottle A most useful tool for spraying off panels. It consists of a poly bottle fitted with a one-hole cork; a right-angled bent piece of glass tubing; a small length of rubber tubing; and an eyedropper with the squeeze bulb removed. Put them all together, fill the bottle with D.I. water, and you are in business.
Stalagmometer Some baths operate properly only at a certain surface tension (bright nickel for example). A stalagmometer reads surface-tension numbers that can then be translated into additions required.
Tong tester This current interpreter is not used in the Hull cell. But by getting readings of eddy currents at the rack, it can tell you where to set current to duplicate the production operation in the Hull cell.
Portable voltmeter The plant engineer or electrician has one. Borrow it. You’ll be looking for breaks in the circuitry, and as with the tong tester, where to set your rectifier.
