Historical Articles

April, 1952 issue of Plating

 

Some Characteristics of Zinc Cyanide Plating Solutions
III. Limiting Anode Current Density and Solution Resistivity

Gustaf Soderberg, Editor, Plating

 

INTRODUCTION
The choice of bath composition and operating conditions for any plating solution depends not only upon how they affect the deposition at the cathode but also upon how they influence anode corrosion and the ability of the solution to carry the necessary current.

Three distinct stages of anode polarization can be observed when a metal of single valence is made anode in a solution. (Metallic impurities that do not dissolve at about the same rate as the anode metal proper may mask this behavior.)

(1) At the lower current densities, the anode dissolves at 100 per cent efficiency (or higher if it is chemically soluble in the bath) and takes on an etched appearance. The anode polarization is relatively low, up to a few tenths of a volt, and consists of pure concentration polarization.

(2) As the anode current density is increased, a region is reached within which periodic crystallization and re-solution of anode corrosion products (or oxides, or both) take place. This region, within which anodic brightening, or “electropolishing”, is observed, is characterized by large changes in anode potential with small changes in current density.

(3) At still higher current densities, the anode becomes permanently covered with anode corrosion products, and the only visible anode reaction is the evolution of oxygen, at almost constant anode potential, however high the anode current density.

In an electroplating solution like the zinc cyanide solution, one normally desires a high anode current efficiency. For this purpose, it is the safest to operate within the first-mentioned range of relatively low polarization. The “limiting anode current density” referred to here is the maximum current density at which there is no sharp rise in anode potential, from a low value of 0.08.34 volt to a high value of 2.182.60 volts, within about 8-25 minutes.

The ability of the solution to carry current depends on its specific conductance, the inverse value of which is the specific resistance. The higher the specific resistance, the higher must be the tank voltage to force the current through the solution, and the more will the solution become heated. Whereas low-resistivity is desired of all plating solutions to keep down the power cost, it is especially important in cyanide solutions which decompose to form ammonia, carbonates and’ other undesirable constituents if the temperature is allowed to rise too high.

EXPERIMENTAL
The same Haring cells were used as in the previously reported determination of throwing power(1). The cell arrangement was as described by Haring(2), with a-solid flat zinc anode, made by casting 99.99 per cent zinc in a steel mold, and a sheet-steel cathode at the opposite: ends of the cell and two heavily zinc plated steel-wire gauze electrodes dividing the cell into three equal compartments. With the cell filled to mark, each compartment had a 25-cm2 cross section and a length of 5 cm.

The cell was connected in series with an ammeter, a carbon-pile rheostat, and a 12-volt lead storage battery. Each of the four electrodes was connected through a 3-way mercury switch to a 5,000-ohm voltmeter, so that when desired, the voltage drop across each of the three compartments could be read successively within a very short period of time.

Fig. 1. Limiting anode current densities in zinc solutions at 77° F. Top graph: 2.4 oz/gal of zinc; bottom graph: open circles 4.5 oz/gal of zinc, filled dot 3.5 oz/gal of zinc. Each 1/8-inch elevator over the base plane represents 10 asf	Fig. 2. Resistivity of zinc solutions at 77° F. Top graph: 2.4 oz/gal of zinc; bottom graph: open circles 4.5 oz/gal of zinc, filled dot 3.5 oz/gal of zinc. Each 1/8-inch elevation over the base plane represents 1 ohm-cm

 

Limiting Anode Current Density
During the tests, the current density was held constant until a constant potential had been achieved across the anode compartment. If the’ potential was low, successively higher current densities were applied until a current density was reached which produced a much higher potential. If the high potential reading appeared during the first few minutes, the current was broken and the film on the anode allowed to dissolve. The test was then repeated at successively somewhat lower current densities, until no sharp rise in potential appeared.

The solution temperature rose during the tests at the high current densities. When this occurred, the solution was stirred gently with ice-filled test tubes until the temperature had dropped slightly below the standard value of 25° C (77° F). All reported readings were taken at 24.8-25.1° C (76.6-77.2° F), except those for the solutions containing 2.4 oz/gal (18 g/l) of zinc, 12 oz/gal (90 g/l) of total NaOH, and 3-5 oz/gal (22-37 g/l) of total NaCN, which varied between 25.4 and 28° C (77.7-78.8° F).

Specific Resistance
The solution resistances R in the cell were calculated from center-compartment voltages (IR drops) at a temperature of exactly 25.0° C (77.0° F) and a constant cell current I of 0.394 amp, and from these values of R, the specific resistances p in ohm-cm were obtained from the usual equation

where
A = cross section of the current path
   = electrode area in cm2
L = length of the current path
   = distance between electrodes in cm

RESULTS
Limiting Anode Current Density
The results of these tests are found in the three dimensional graphs of Fig. 1. The usefulness and use of such graphs were discussed in Part I of this series(3).

It will be noted that the coordinates for total-cyanide and total-caustic contents progress in directions opposite those usually employed. This was done in order that the graphs be more easily read.

The scope of these tests is limited in that only one solution containing 3.5 oz/gal (26 g/l) of zinc and very few solutions with 4.5 oz/gal (34 g/1) of zinc were tested.

As to accuracy, it will be noted that each reported value is intermediate between one at which no voltage “break” was observed and one at which such a break did occur. No attempt was made to locate the correct value closer than ± 2 asf (± 0.2 amp/dm2).

Resistivity
The resistivity values obtained are presented in the graphs of Fig. 2. The limitation on the scope is the same as for the limiting anode current density.

An attempt was made to obtain as accurate values as the Haring cell permits. For this purpose, the intermediate electrodes were made of screen with very fine wire, and the current density employed was low. There was no drift in the readings during extended tests.

DISCUSSION OF RESULTS
Limiting Anode Current Density
Within the narrow ranges that can be compared at constant total-cyanide and total-caustic contents, the limiting current density decreased with increased zinc content.

At constant zinc content, an increase in total-cyanide or total-caustic content or both- caused the limiting anode current density to increase.

Specific Resistance
Within the limited ranges that can be compared at constant total-cyanide and total-caustic contents, the resistivity increased with increased zinc content.

At constant zinc content, the resistivity decreased as the total cyanide or total-caustic content or both increased. It should be noted, however, that the decrease in resistivity is the more pronounced at the lower concentrations of total caustic and total sodium cyanide.

CONCLUSIONS
The next, and final, installment will deal with the ‘ conclusions drawn from all the experimental data presented in this and preceding installments.

ACKNOWLEDGMENTS
The experimental work reported in Parts I-III was carried out by Messrs. G. Kentta, W. Cheesman, D. B. Stockton and Dr. H. Brown in the laboratories of The Udylite Corporation, Detroit, Mich. - Thanks are due The Udylite Corporation for permission to publish

REFERENCES CITED
(1). G. Soderberg, Plating 39, 255 (March, 1952)
(2). H. E.: Harug, Trans. Electrochem. Soc. 49, 417 (1926).
(3). G. Soderberg, Plating 38, 928 (September, 1951).

Correction to Part II
The electrode area and solution cross section were 2 5 cm2 add not 1 dm2 as stated in line 10 from the bottom of column 1 of page 255, March, 1952 issue. The average deposit thickness on ‘the two sets of cathodes throughout the investigation was 4 times as high as stated in line 1 of column 2 of the same page, namely 0.00026 inch- (6.4 µ) instead of 0.000065 inch (1.6 µ).