Historical Articles

March, 1952 issue of Plating

 

Some Characteristics of Zinc Cyanide Plating Solutions
II. Haring Cell Throwing Power

Gustaf Soderberg, Editor, Plating

 

INTRODUCTION
The word throwing power, or “throw”, has been used to denote several different concepts. As employed here it pertains solely to plate thickness.

In the Haring Cell(1), one cathode is placed farther away from the anode than a second cathode, and the throwing power is expressed as follows:

For example, if one cathode is five times as far from the anode as the other cathode, and the deposit on the near cathode weighs four times as much as the deposit on the far cathode, the distance ratio is 5 and the thickness ratio, 4. Hence the throwing power, in percent, is

As pointed out by Blum et al.(2), the Haring-cell throwing power is not a “characteristic” of the solutions in the strict meaning of the word; it includes both solution variables and cell-size factors. The cell is useful, however, in classifying solutions with respect to their ability to deposit coatings of uniform thickness on articles with recesses.

EXPERIMENTAL
A Haring cell made of hard rubber was employed. At each end was placed a tightly fitting cold-rolled SAE 1010 steel cathode, which had been cleaned and weighed in the same manner as the efficiency-test cathodes of Part I of this series of articles(3). A tightly fitting fine steel gauze electroplated with zinc was placed as the anode between the cathodes. After the cell had been filled to the mark with plating solution, its dimensions were as follows:

Electrode area and solution cross section . . . 1 dm2
Distance from anode to far cathode. . . . . . .12.5 cm
Distance from anode to near cathode . . . . . 2.5 cm
Distance ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

The cell was connected in series with an ammeter, a carbon-pile rheostat, and a 12-volt lead storage battery. Plating was continued only as long as the temperature throughout the solution remained at 77 ± 2°F (25 ± 1°C). For that reason the average thickness of zinc on the cathodes was always very low. It averaged 0.000065 inch (1.6 µ) throughout the investigation, but varied a great deal from one determination to another.

At the end of the plating period, the cathodes were removed, rinsed under the tap, in distilled water and in alcohol. After they had dried in the air, they were placed in a dessicator until they were ready for reweighing. The Haring-cell throwing power was then calculated.

RESULTS
The results are presented in the three-dimensional graphs of Fig. 1. The usefulness and use of such graphs were discussed in Part I(3).

Some of the limitations on the scope and accuracy are set forth in Part I. In addition, it will be noted that in this study of throwing power only a few measurements were made with solutions containing 3.5 and 4.5 oz/gal (26 and 34 g/l) of zinc, limiting the scope still further, and that the accuracy is lower because of the larger number of measurements involved in each determination.

Fig. 1. Haring-cell throwing power of zinc solutions at 77° F. Top graphs: 2.4 oz/gal of zinc; bottom graphs: 4.5 oz/gal of zinc, except for the filled dots which are for 3.5 oz/gal of zinc. From left to right: 10, 30 and 50 asf average current density on the two cathodes in the cell. Each 1/8 inch elevation over the base plane represents 10 per cent throwing power.

 

DISCUSSION OF RESULTS
The changes in throwing power with changes in solution composition or cathode current density are not as straight-forward as the corresponding changes in cathode current efficiency reported in Part I(3). The reason is, of course, that throwing power is not a simple characteristic. Also, the errors are undoubtedly larger.

Within the limited ranges that can be compared, with total-cyanide and total-zinc contents remaining constant, the throwing power decreased rather sharply with increasing zinc content. However, at high current density (50 asf, or 5.4 amp/dm2) and high total cyanide content (12 oz/gal, or 90 g/l) there was no change in throwing power with increased zinc content.

The general effect of an increase in total-cyanide content, at constant zinc and total-caustic contents, was an increase in throwing power. Exceptions were found at low zinc content (2.4 oz/gal, or 18 g/l) and high current density (50 asf, or 5.4 amp/dm2) where an increase of total cyanide from 8 to 12 oz/gal (60 to 90 g/l) caused a decrease in throwing power. This decrease was the cause of the lack of effect of increased zinc content mentioned above.

No general rule can be formulated for the effect of total-caustic content. A trend is discernible, however, at low zinc content (2.4 oz/gal, or 18 g/l). When the total-cyanide content was low (5 oz/gal, or 38 g/l) an increase in total NaOH from 3 to 12 oz/gal (22 to 90 g/l) caused the throwing power to rise from 47 to 57 per cent at 10 asf (1.1 amp/dm2), but at 30 and 50 asf (3.2 and 5.4 amp/dm2) the throwing power decreased from 55 to 36 per cent and from 63 to 46 per cent respectively. As the total-cyanide content was increased this trend tended to reverse itself. Thus, at 12 oz/gal (90 g/l) of total NaCN, as the total-caustic content was raised gradually from 3 to 12 oz/gal (22 to 90 g/l) the throwing power first dropped and then rose at 10 and 50 asf (1.1 and 5.4 amp/dm2) and first rose and then dropped at 30 asf (3.2 amp/dm2).

Within the range explored at 4.5 oz/gal (34 g/l) of zinc, an increase in the total-caustic content had little effect.

Broadly speaking, at a zinc content of 2.4 oz/gal (18 g/l), an increase in cathode current density was effective in raising the throwing power when the total cyanide and total-caustic contents were low. High total cyanide content, however, partially reversed this effect, and the lowest throwing power was had at the highest current density (50 asf, or 5.4 amp/dm2).

Within the narrow range studied at 4.5 oz/gal (34 g/l) of zinc, the throwing power dropped with increasing current density at the lowest total cyanide content (8 oz/gal, or 60 g/l), but rose with increasing current density at the highest total-cyanide content (16 oz/gal, or 120 g/l). At intermediate total-cyanide contents (9-12 oz/gal, or 68-90 g/l) the throwing power first dropped to a minimum at 30 asf (3.2 amp/dm2) and then rose again.

Although only relatively few tests were carried out in solutions containing 4.5 oz/gal (38 g/l) of zinc, they do show that high-zinc solutions can have equally high throwing power as low-zinc solutions, but only when the total-cyanide content is very high and then particularly when the higher current densities are employed.

CONCLUSIONS AND ACKNOWLEDGMENTS
These will be presented in the final installment of this series.

REFERENCES CITED
(1). H.E. Haring and W. Blum, Trans. Electrochem Soc. 44, 313 (1923).
(2). W. Blum, A. O. Beckman and W. R. Meyer, ibid. 80, 256 (1941), “Modern Electroplating”, The Electrochemical Society, New York (1942) p. 14.
(3). G. Soderberg, Plating 38, 928 (1951).