APPLICATION OF HYDRAZINE SULFATE FOR ELECTROLESS NICKEL PLATING (COBALT PLATING)

V. I. Latatuev, A. D. Denisov, O. L. Peshkov, V. P. Kazakova,
V. F. Biryukova

It is known that at present, in processes of electroless nickel plating (cobalt plating), salts of hypophosphorous acid (hypophosphites) are used almost exclusively as the reducing agent.

The use of these reducing agents, along with advantages (high chemical activity, ease of use, etc.), also has a number of significant drawbacks (high cost, low utilization coefficient, etc.).

This makes it necessary, in parallel with improving the “hypophosphite” method, to carry out studies aimed at testing other reducing agents free from the disadvantages associated with the use of hypophosphite.

From this point of view, a strong reducing agent such as hydrazine (and its salts) may be of considerable interest; as is known, in most oxidation–reduction reactions it is oxidized to free nitrogen, which leaves the reaction sphere.

In the literature [1] there is a brief mention of a proposal to use hydrazine hydrate for electroless nickel plating (Japanese patent). No detailed information on the process is given. In [2] the possibility is indicated of obtaining thin nickel layers by autocatalytic reduction with hydrazine. All this permits one to hope for the possibility of using hydrazine and its salts, and above all hydrazine sulfate, as a reducing agent for electroless nickel plating (cobalt plating).

The process of reducing the nickel ion to the elemental state may be represented by the following overall equation:

\[ \mathrm{Ni}^{+2} + \mathrm{N_2H_4} + 4\mathrm{OH}^{-} = \mathrm{Ni} + \mathrm{N_2} + 4\mathrm{H_2O}. \tag{1} \]

The course of the reaction is complicated by side processes (see below).

To carry out the experiments, iron and copper specimens measuring \(10 \times 10 \times 1.0\) mm were degreased and pickled by the usual method and immersed in the solution under test.

On the basis of work experience and the indication in [1], the following composition of the working solution was initially adopted:

\[ \mathrm{NiSO_4 \cdot 7H_2O} — 35\ \text{g/l}, \qquad \mathrm{N_2H_4 \cdot H_2SO_4} — 26\ \text{g/l}. \]

Studies on the effect of temperature showed that up to a temperature of 45°C the process does not proceed at all; at 45–65°C the reaction rate is insignificant. Only at temperatures above 70°C can the rate of the process be quantitatively evaluated. The temperature \(t = 85^\circ\text{–}90^\circ\text{C}\) was adopted as optimal.

To select the optimal acidity of the solution, several series of experiments were carried out in the pH ranges: 3–4, 6–7, 8–10, 10–11. The pH was adjusted with ammonia solution. The results of the experiments are presented in Table 1.

Table 1

Effect of solution pH on the rate of nickel plating

From the data in Table 1 it follows that the optimal pH of the process should be taken as pH = 8–10. Raising the pH above 10 does not increase the rate of the process and, as experiment showed, affects the quality of the coating (darkening).

During the process, considerable precipitation of a lilac-colored solid was observed. Analysis showed that the compound formed is nickel hydrazinate \([\mathrm{Ni}(\mathrm{N}_2\mathrm{H}_4)_3]\mathrm{SO}_4\), described in the literature [3, 4]. The formation of this compound is undesirable, since it reduces the concentration of the reacting substances. Consequently, the adopted ratios of the reagents are not the best, and it was necessary to carry out several series of exploratory experiments to find the optimal ratios of the reagents. The results of the studies are presented in Table 2.

As can be seen from the data in the table, the ratio of the components has a significant effect on the rate of the process but, unfortunately, does not eliminate the formation of nickel hydrazinate.

To find the optimal ratio of the components, the following experiment was carried out.

For 1 hour at 90°C and pH = 8–9, a solution of the initial composition was held:

\[ \mathrm{N}_2\mathrm{H}_4 \cdot \mathrm{H}_2\mathrm{SO}_4 — 26\ \text{g/l}, \quad \mathrm{NiSO}_4 \cdot 7\mathrm{H}_2\mathrm{O} — 35\ \text{g/l}. \]

A considerable precipitate of nickel hydrazinate formed. In the filtered solution the following concentration of components was found:
\(\mathrm{N}_2\mathrm{H}_4 \cdot \mathrm{H}_2\mathrm{SO}_4 — 13\ \text{g/l}, \ \mathrm{NiSO}_4 \cdot 7\mathrm{H}_2\mathrm{O} — 29\ \text{g/l}\) (II).

This ratio corresponds almost to the stoichiometric ratio of the components according to equation (1).

At this ratio, the plating rate was stable and was approximately \(\sim 2\ \mu\text{m}/\text{h}\). Any deviation from the indicated ratio leads either to precipitation of hydrazinate (with an increase in hydrazine sulfate) or to a decrease in the quality of the coating, etc. (with an increase in the concentration of nickel salt).

Table 2

Effect of the component ratio on the rate of the process.
Experimental conditions: time—1 h; \(T = 94 \pm 2^\circ\text{C}\); \(\mathrm{pH} = 8\text{–}9\).

A proportional increase in the concentration of the components did not increase the rate, and in fact reduced it considerably. Thus, for example, when the concentration of reagents (II) was doubled, the plating rate decreased to \(\sim 0.8\ \mu\text{m}/\text{h}\).

Thus, the component ratio (2) should be considered optimal.

On the basis of experience with electroless nickel plating using hydrazine sulfate, studies were carried out on cobalt plating of iron and copper. For the experiment, the optimal composition used was:
\(\mathrm{N_2H_4 \cdot H_2SO_4}\)—13 g/L, \(\mathrm{CoSO_4 \cdot 7H_2O}\)—29 g/L, \(\mathrm{pH} = 11\text{–}12\), \(t = 85 \pm 2^\circ\text{C}\). Time—1 h.

At temperatures below \(80^\circ\text{C}\) the process does not proceed at all; at temperatures above \(85^\circ\text{C}\) the solution begins to boil because of the high ammonia concentration (200–300 ml/L). The pH of the solution had to be maintained strictly within the range 11–12, since at pH below 11 the ammonia complex breaks down with precipitation of hydroxide, while at pH above 12 there is noticeable dissolution of copper and its deposition on the iron (coppering).

The rate of cobalt plating under the indicated conditions reached 11–12 µ/h, which is 5–6 times greater than the rate of nickel coating. Such a difference in coating rate may be explained by the different values of the instability constants of the complex ions [3] (for \([\mathrm{Ni}(\mathrm{NH}_3)_6]^{+2}\), \(K = 1.86 \cdot 10^{-9}\); for \([\mathrm{Co}(\mathrm{NH}_3)_6]^{+2}\), \(K = 7.75 \cdot 10^{-6}\)). The coatings were dense, with good adhesion, and gray in color.

Conclusions

1. The fundamental possibility has been demonstrated of using hydrazine sulfate as a reducing agent for the processes of electroless nickel plating and cobalt plating of iron and copper surfaces.

2. The optimal conditions for carrying out the process have been found, making it possible to obtain a coating rate of:

nickel — 2 µ/h,

cobalt — ~11–12 µ/h.

1. Nickel plating by this method may be recommended for precision parts where a small thickness and uniformity of the coating are required.

2. It is necessary to continue research toward finding conditions that ensure a higher nickel-plating rate and better coating quality (cobalt plating).

Literature

1. Korovin. New Coatings and Electrolytes in Electroplating. Metallurgizdat, 115, 1962.

2. Electrochem. Technol., 1, 1–2, 38–42, 1963.

3. K. B. Yatsimirskii, V. V. Vasil’ev. Instability Constants of Complex Compounds. Academy of Sciences of the USSR, 93, 1959.

4. V. A. Tolovnya, I. A. Fedorov. Basic Concepts of the Chemistry of Complex Compounds. Academy of Sciences of the USSR, 55, 1961.