Study of Certain Questions of Electrolytic Zinc Plating

O. S. Stepanova, A. S. Levina

(Presented by the scientific-methodological seminar of the Faculty of Chemical Technology)

Intensification of processes in electroplating is achieved mainly through the use of automatic and semiautomatic installations, an increase in the operating current densities, the selection of electrolytes with higher values of the electrochemical equivalent of the deposited metal, and also by increasing current efficiency [1].

In the practice of electroplating shops, simple sulfuric-acid zinc-plating electrolytes have found wide application. Their composition is simple, the current efficiency is close to 100%, and the permissible current densities are also high. However, a sulfuric-acid electrolyte has a substantial drawback: low throwing power of the electrolyte and a coarse-crystalline structure of the cathodic deposits. Consequently, the sulfuric-acid electrolyte can be used for zinc plating parts of simple configuration.

In the literature, for coating articles of complex configuration, electrolytes are recommended that possess increased productivity and good throwing power [2–4].

The quantitative characteristics of the principal properties of electrolytes turn out to be noncomparable because of differences in the methods of determining them and in the ways of expressing them.

On the basis of literature data, we selected a sulfate–chloride–ammonium electrolyte for zinc plating. This complex electrolyte is easy to prepare and is fairly stable in operation [3].

The absence of data on the regime for using a sulfate–chloride–ammonium electrolyte under specific conditions (for zinc plating various types of bases) led to the need to carry out a study of the principal technological properties of this electrolyte and of the quality of the resulting cathodic deposit in comparison with a sulfuric-acid electrolyte under comparable conditions.

In the course of the study, values were determined for the maximum permissible cathodic current densities, the dependence of the zinc current efficiency on current density, the throwing power of the electrolytes, their productivity under drum-bath conditions, and also the quality of the resulting deposits (color, structure, porosity).

Experimental Part

Two zinc-plating electrolytes were subjected to investigation: sulfuric-acid and sulfate–chloride–ammonium. The compositions of the electrolytes are given in Table 1.

The current efficiency was determined using a copper coulometer connected in series in the electric circuit with the electrolyzer. The throwing power of the electrolytes was determined by the method of G. Hering and V. Blum [5]. The thickness of the zinc coating was determined by the jet-volumetric method [6].

Table 1

Determination of the degree of porosity was carried out by the method of anodic polarization with an indicator.

Metallographic studies were performed using an MIM-8M-type microscope.

The study of electrolyte performance under drum-bath conditions was conducted on equal batches of identical parts, with equal volumes of electrolytes and a constant anode surface area.

Results of the Study and Their Discussion

1. Maximum Permissible Values of Cathodic Current Density and Zinc Current Efficiency

Zinc deposits of satisfactory quality are obtained from the sulfuric acid electrolyte in the current-density range of $1\ \mathrm{A/dm^2}$—$2.0\ \mathrm{A/dm^2}$ with a current efficiency of 99.7%, and from the sulfate-chloride-ammonium (SCA) electrolyte from $0.5\ \mathrm{A/dm^2}$ to $2.5\ \mathrm{A/dm^2}$ with a current efficiency from 100% to 90%, at an electrolyte temperature of 20–21°C.

Zinc deposits from the sulfuric electrolyte are light gray; those from the SCA electrolyte are somewhat darker, with a noticeable luster. It was observed that when the current density is increased to $5\ \mathrm{A/dm^2}$, the current efficiency of zinc from the sulfuric acid electrolyte decreases to 94%. This can be explained by the influence of the evolved hydrogen. In this case, the cathodic zinc deposit is porous. If the deposits are compared by degree of porosity, it turns out that the SCA electrolyte yields dense, nonporous deposits, whereas the sulfuric acid electrolyte yields porous deposits (there are from 2 to 6 pores per $1\ \mathrm{cm^2}$ of surface, and porosity increases with increasing current density).

2. Effect of Temperature on the Quality of Cathodic Deposits

From the sulfuric acid electrolyte, at working current densities of $1$–$2\ \mathrm{A/dm^2}$, as the temperature is increased from 30 to 50°C, the quality of the cathodic deposits

zinc deteriorates in the above-indicated range of operating current densities. The deposits become spongy and rough. In the current-density range from \(2.5\ \mathrm{A/dm^2}\) to \(5\ \mathrm{A/dm^2}\), light, porous deposits are formed.

The value of the maximum permissible current densities in the SHA electrolyte expands to \(3.5\ \mathrm{A/dm^2}\) when the temperature is raised from 30 to \(50^\circ\mathrm{C}\). The deposits are fine-crystalline, dense, and have a beautiful gray color. At current densities above \(3.5\ \mathrm{A/dm^2}\), “burning” occurs along the edges of the part.

The current efficiency of zinc is close to 100% from the SHA electrolyte in the range of the temperatures and current densities studied.

At \(30^\circ\mathrm{C}\), the range of operating current densities is from 0.5 to \(2.5\ \mathrm{A/dm^2}\).
At \(40^\circ\mathrm{C}\)—from 0.5 to \(3\ \mathrm{A/dm^2}\).
At \(50^\circ\mathrm{C}\)—from 1 to \(3.5\ \mathrm{A/dm^2}\).

3. Throwing Power

The throwing power of the electrolytes was determined at various current densities: from \(1\ \mathrm{A/dm^2}\) to \(3\ \mathrm{A/dm^2}\). As the current density increases, the throwing power of the sulfuric-acid and SHA electrolytes improves, with the throwing power of the SHA electrolyte being approximately 4 times higher than that of the sulfuric-acid electrolyte, which is in agreement with [2]. On the dependence curves (throwing power—current density), two regions are observed that agree well with the dependence “current efficiency—current density.” When the current efficiency of zinc decreases, the throwing power increases; at a constant value of the current efficiency, the throwing power of the electrolyte does not change. Thus, for depositing zinc on articles of complex configuration it is more expedient to use the SHA electrolyte.

4. Productivity of Electrolytes under Conditions of a Barrel Electrolyzer

Testing was carried out in a barrel-type bath with a capacity of 5 l; the surface area of the parts being coated was \(4.8\ \mathrm{dm^2}\). The ratio between anodic and cathodic current density was 1 : 1.

The rotation speed of the barrel was 15 rpm. The results are presented in Table 2.

From the data of Table 2 it is evident that, when the same quantity of electricity is passed through both electrolytes, the rate of zinc deposition is higher from the SHA electrolyte by approximately a factor of 1.4, while the quality of the zinc deposit from the SHA electrolyte is better (the deposits are dense, pore-free, light gray in color, with luster).

Table 2

Consequently, by using the SHA electrolyte for zinc plating in barrel electrolyzers instead of the sulfuric-acid electrolyte, it is possible to increase the productivity of the bath. At present, in the practice of electroplating shops, in a barrel bath the average current density does not exceed-

is \(0.2\ \mathrm{A/dm^2}\) in a sulfuric-acid electrolyte. To obtain a zinc coating \(6\ \mu\mathrm{m}\) thick, deposition is carried out for \(1.5\)—\(2\) hours.

Using the results of this work, a zinc coating \(6\ \mu\mathrm{m}\) thick can be obtained from a CZA electrolyte in a drum-type bath in \(30\)—\(45\) min at an average current density of \(0.9\)—\(1.3\ \mathrm{A/dm^2}\), i.e., the rate of zinc deposition increases approximately threefold.

REFERENCES

1. L. I. Kadaner. Electroplating. Kiev, 1964.

2. B. Sh. Shteinberg, N. A. Bogakovskii. In: Protective Metallic and Oxide Coatings, Corrosion of Metals, and Research in the Field of Electrochemistry. Nauka Publishers, Moscow—Leningrad, 1965, pp. 92—98.

3. B. A. Purin. Electrodeposition of a Number of Metals from Complex (Noncyanide) Electrolytes. Leningrad, 1968. (Leningrad House of Scientific and Technical Propaganda).

4. E. S. Naidina, A. A. Gerasimenko. Zinc Plating of Steel Parts in a CZA Electrolyte. GOSINTI, Moscow, 1966.

5. Ya. V. Vainer, M. A. Dasoyan. Technology of Electrochemical Coatings. GNTI, Moscow—Leningrad, 1962.

6. E. G. Kruglova, P. M. Vyacheslavov. Control of Electroplating Baths and Coatings. Moscow—Leningrad, 1958.