Historical Articles

May, 1952 issue of Plating

 

CALIBRATION OF PLATING RANGE CELLS

J. B. MOHLER
Director of Research, Johnson Bronze Company, Newcastle, Pa.

INTRODUCTION

Plating-range cells have become widely accepted as laboratory tools for electroplating control and experimentation.

Calibrations were reported by Hull(1) for the Hull cell, the best known and most popular plating-range cell. These calibrations were made by stripping and weighing small sections of the deposits and by measuring the current density at points along the cathode with a current-density meter. An equation for general use was derived by averaging data for nickel, acid copper, cyanide zinc and cyanide cadmium solutions, but it was pointed out that individual calibrations are necessary for accurate interpretations of results.

By the use of thickness measurements a cell may be calibrated not only for plate thickness, but where the cathode efficiency is 100 per cent or where the variation in cathode efficiency with current density has been measured, also for current density.

Measurements carried out under project No. 7 of the American Electroplaters’ Society(2) show the relative accuracy of various methods for measuring the thickness of metal deposits. These studies may be used as a guide in the calibration of plating-range cathodes. If a rapid calibration is required the Magne-gage is a very convenient instrument.

Many factors can influence the metal distribution over a cathode, as shown by Tucker and Flint(3). Not only does cathode efficiency and covering power vary with the preparation of the test panel, but in some baths, such as the alkaline tin bath, it also varies with the thickness of the deposit when thin layers are concerned. Therefore, if a plating-range cell is to be used for precise work it is sometimes necessary to calibrate it under the expected conditions of use.

For those baths that have a cathode efficiency of 100 per cent, however, it is likely that the calibration of a plating-range cell for a particular bath will apply under various conditions of time and total current. For that reason, in the work reported here, it was decided to use two such baths in the calibration of a slot cell(4) with a slot at one end as shown in Fig. 1.

THE SLOT CELL
There are two distinct advantages to this cell. First, the anode is outside of the cell and thus has no influence on the cathode. Second, the cell may be immersed in a plating bath to overcome the difficulties encountered with temperature control in box-type plating range cells.

Another useful feature of the cell is that it is dimensionless. The cell shown in Fig. 1 is 4 inches (10.2 cm) long with a 1/4-inch (6.4-mm) slot 1 inch (25.4 mm) from the cathode. A 2-inch (5.1-cm) cell was constructed with a 1/8-inch (3.2-mm) slot inch (12.7 mm) from the cathode Test cathodes were plated in these cells using an acid tin bath at an average cathode current density of 10 asf (1.1 amp/dm2). For a 5-minute plating time the plate pattern in each case agreed very closely with that shown at the top of Fig. 2 when the slot-cathode distance was used as the unit for measuring the distance on the panel from the slot end. For long plating times this relation would not hold, because the ratio of cathode area to cell volume changes with the size of the cell, and the solution in a small cell would become depleted of metal more rapidly than the solution in a large cell.

Fig 1. Slot type plating range cell; at top, seen from front (dashed horizontal line indicating solution level) and at bottom, in cross section

CALIBRATION OF A SLOT CELL
Two baths were used to calibrate the 4-inch slot cell, a silver cyanide bath and an acid tin bath. These baths were chosen because they both deposit metal at essentially 100 per cent cathode efficiency and because they are representative of baths of relatively high and relatively low throwing power, respectively.

Fig. 2. At top, appearance of plate, and at bottom, current distribution, on panel plated in slot-type plating-range cell immersed in an acid tin solution

The acid tin bath had the following composition:

A 2 1/2 x 4-inch (6.4 x 10.2-cm) polished steel cathode was used with 2 inches (5.1 cm) immersed in the bath. A current of 0.56 amperes was allowed to flow for 40 minutes. The plate was then stripped from the cathode, and vertical sections were cut from the tin sheet. These were weighed, and from the weight the current densities at points along the cathode were calculated. Above a current density of 35 asf (3.8 amp/dm2) the deposit was quite rough and was not used.

Three panels of the same size were cleaned, pickled and plated in the same manner, but for minutes. Magne-gage readings were taken on the three panels and averaged. From the thickness determinations the average current densities were calculated.

Fig. 3. At top, appearance of plate, and at bottom, current distribution, on panel plated in slot-type plating-range cell immersed in cyanide silver solution

The current densities obtained by the two methods are shown in Fig. 2.

The silver bath had the following composition:

A 2 1/2 x 4-inch (6.4 x 10.2 cm) polished steel cathode was used with 2 inches (5.1 cm) immersed in the bath. A current of 0.22 amperes was allowed to flow for 1 hour. The plate was then stripped from the cathode, and vertical sections 1/8 inch (3.2 mm) wide by 1 inch (25.4 mm) long were cut from the silver sheet. These were weighed, and from the weight the current densities at points along the cathode were calculated.

Another cell was constructed, of- the same proportions as the 4-inch (10.2-cm) cell but of three times its size, i. e., the slot was inch (1.9 cm) wide and placed 3 inches (.6 cm) from a 12-inch (30.5-cm) long cathode. The depth of immersion during plating was 2 inches (5.1 cm). Three cathodes were cleaned, pickled and struck with silver prior to plating for 10 minutes at 0.67 amperes. Magne-gage readings were taken, and the current densities were calculated from the averages for the three cathodes.

Fig. 3 shows the two sets of data for the silver bath plotted on the same graph where the distances from the high current density end are in terms of the slot-to-cathode distance as the unit.

The data show that a rapid calibration can be made with the Magne-gage, and that the calibration for a cell of one size can be used for another of equal proportions.

There is an apparent difference in the shape of; the curves obtained by the two methods of calibration, particularly for the silver solution at low current densities. An investigation to determine whether this was due to the difference between the two methods of thickness measurement or to differences between the bonded and unbonded deposits would have required considerable more experimentation.

CURRENT DENSITY DISTRIBUTION EQUATIONS
Approximating equations were derived by plotting the logarithm of the average current -density against the distance from the slot end and solving for the point-slope formula. These equations are as follows:

For the acid tin solution, c = antilog 0.50 (1.84 - z)

For the silver solution, c = antilog 0.36 (1.78 - x)

where c = Times average current density

x = Distance along cathode in slot-cathode-distance units

Equations were also derived by using the increase in thickness as one moves 1 unit of distance from any point as a denominator power function and the ratio of the theoretical thickness at zero distance to actual thickness at the point of average current density as the numerator. These equations are as follows:

For the acid tin solution,

For the silver solution,

Fig. 4. Current distribution curves obtained in slot-type plating-range cells. Note the more uniform current distribution on panels plated in solutions of high throwing power

The curves represented by these equations follow very closely the solid-line curves in the graphs.

The equations did not hold near the high-current density end of the panel, because there the current density tended to level off, and the results were erratic. Whether or not the other end of the cell would interfere with the shape of the curve was not determined, but it is suspected that it does; this difficulty can be overcome, however, merely by using a cell that is longer than the distance under study.

CURRENT DISTRIBUTION VS. THROWING POWER
Most of the curve obtained in this type of cell approximates a simple exponential curve. With more accurate calibrations it may be possible to obtain an indication of the effect of polarization over the entire range, or in more popular terms, to obtain an indication of change in throwing power with change in current density over the entire range. The curves in Fig. 4 were obtained from data from various sources and are only approximate. They do, however, provide a good graphical illustration of the effect of throwing power.

APPLICATION OF THE CELLS
A cell that can be used to deposit metal of a thickness that changes continuously in a known manner is a useful laboratory tool. In addition to its use as a plating-range cell in which the behavior of a bath can be studied, it may be employed where variable plate thickness on a single cathode offers an advantage. For example, it was desired to determine the minimum thickness of silver required to prevent rust spots, during air drying following a hot rinse, on steel ground to 100-microinch (2.5-µ) roughness. By processing blanks in a slot-type cell this thickness was readily determined, visually, to be 0.3 mil (8 µ). Similar obvious applications would be found in studies of porosity and corrosion.

REFERENCES CITED
(1). R. O. Hull, Proc. Am. Electroplaters’ Soc. 27, 52 (1939)
(2). H. J. Read and F. R. Lorenz, Plating 38, 255 (March, 1951) and 945 (September, 1951).
(3). W. M. Tucker and R. L. Flint Trans. Electrochem. Soc. 88, 338 (1945).
(4). J. B. Mohler and R. A. Schaefer, Monthly Rev. Am. Electroplaters’ Soc. 34, 1361 (December, 1947).