COATING IRON AND COPPER WITH NICKEL AND COBALT BY A CHEMICAL METHOD USING A SINGLE-COMPONENT SOLUTION

V. I. Latatuev, A. D. Denisov, O. L. Peshkov, N. I. Zakabunina

One of the effective methods of protection against corrosion is the coating of metals with various metals by the electrolytic method. But by this method it is impossible to solve many problems, for example: 1. The creation of coatings with special properties. 2. The production of coatings uniform in thickness on deeply profiled and precision parts.

New and very promising are chemical methods for obtaining metal coatings—in particular, chemical nickel plating (cobalt plating)—which are carried out without the application of electric current [1—3].

The process is based on the overall equation [1, 2, 3]

\[ \mathrm{Ni}^{++} + 2\mathrm{NaH}_2\mathrm{PO}_2 + 2\mathrm{H}_2\mathrm{O} \to \mathrm{Ni} + 2\mathrm{Na}_2\mathrm{HPO}_3 + 2\mathrm{H}^{+} + \mathrm{H}_2 \uparrow \]

The process is carried out at 80—95°C in solutions containing: a nickel salt, a reducing agent, complex-forming and buffering additives.

But such multicomponent systems have the following substantial disadvantages:

1. Instability of the working solution, which has an extremely negative effect on the process itself and on the quality of the coating.

2. Difficulty in correcting the solution, since several components have to be added simultaneously.

All these and other shortcomings compel the search for ways to improve the process of chemical nickel plating (cobalt plating), above all by reducing the number of components.

As early as 1844, Wurtz showed that an aqueous solution of nickel hypophosphite, heated to 100°C, undergoes decomposition with the liberation of nickel in the form of a powder [2, 5]. This circumstance was taken by us as the basis in developing a single-component solution for chemical nickel plating (cobalt plating). The process may be represented by the following overall equation:

\[ \mathrm{Ni}(\mathrm{H}_2\mathrm{PO}_2)_2 + 2\mathrm{H}_2\mathrm{O} \to \mathrm{Ni} + 2\mathrm{H}_3\mathrm{PO}_3 + \mathrm{H}_2 \uparrow . \]

All three products are identified quantitatively.

On the basis of the theoretical concepts proposed by K. M. Gorbunova and A. A. Nikiforova [2], the process may be represented in the form of the following stages:

$$ \begin{aligned} &\text{a) } \mathrm{Ni(H_2PO_2)_2 \to Ni^{+2} + 2H_2PO_2^-},\\ &\text{b) } \mathrm{2H_2PO_2^- + 2H_2O \to 2H_2PO_3^- + 4H},\\ &\text{c) } \mathrm{Ni^{+2} + 2H \to Ni + 2H^+},\\ &\text{d) } \mathrm{2H \to H_2 \uparrow}. \end{aligned} $$

In the present work the task was set of clarifying the fundamental possibility of using nickel (cobalt) hypophosphite for chemical nickel plating (cobalt plating) and of determining the optimum process conditions as a function of various factors: temperature, pH, etc. The rate of coating of the specimens with nickel (cobalt) was chosen as the determining criterion of the course of the process.

For the experiments, specimens of copper and iron measuring \(1.0 \times 1.0 \times 0.1\) mm were used. In carrying out the experiment the following sequence of operations was observed.

The specimens were degreased in benzine (3–5 min) and in an alkali solution (\(80\ \mathrm{g/l}\ \mathrm{KOH}\)) for 5 min at 80–85°C, followed by pickling in hydrochloric acid (1 : 10) at 50–60°C (0.5–1.0 min).

Preliminary experiments showed that copper is coated with nickel (cobalt) only in contact with more electronegative metals (for example, iron), which is apparently explained by the catalytic nature of the process. In conducting the experiments this circumstance was taken into account (for the experiment, copper and iron specimens were placed in contact).

For the experiments, a solution of nickel (cobalt) hypophosphite was prepared by dissolving nickel hydroxide (cobalt carbonate) in an aqueous solution of phosphorous acid in accordance with the recommendations [4, 6].

To determine the optimum concentration of nickel hypophosphite, a series of experiments was carried out at 90°C and pH 6. In the concentration range 5–25 g/l, the nickel-plating rate remains practically constant, 2–2.5 μm/h. With an increase in the concentration of hypophosphite, the coating rate decreases.

In subsequent experiments, \(C = 10\) g/l was adopted as the optimum concentration of nickel hypophosphite. In all experiments an increase in acidity was observed as a result of the formation of phosphorous acid.

The effect of the temperature of the working solution on the nickel-plating rate was clarified. At \(t = 50^\circ\mathrm{C}\) the process practically does not proceed. With increasing temperature, the coating rate increases in an almost linear dependence, which is explained by an increase in the rate of decomposition of the hypophosphite ion.

It is inexpedient to raise the temperature above 95°C because of vigorous boiling of the solution and decomposition of \(\mathrm{Ni(H_2PO_2)_2}\) (especially at 100°C), which is in agreement with the indication in [6].

Therefore, further experiments were carried out at \(95 \pm 2^\circ\mathrm{C}\). It is known that the acidity of the medium has a great influence on the rate of the process. It was found that the maximum nickel-plating rate is reached at pH 6–6.5 (3.5 μm/h at pH = 6 and 4 μm/h at pH = 6.5). On moving into the more acidic pH region the coating rate drops sharply, and at pH = 3 the process ceases. At pH = 6.5 precipitation of nickel hydroxide begins, which also stops the process (a good visual indication of the establishment of pH 6 is the appearance of a slight turbidity due to the nickel hydroxide being formed).

As the experiments showed, by correcting the pH and the nickel content it is possible to obtain a stable nickel-plating rate equal to 14–15 μm/h.

It should be noted that the nickel coatings obtained are low-porosity (1–2 pores per cm² at a coating thickness of 5–10 µm), lustrous, dense, and exhibit good adhesion to the base metal.

On the basis of the results obtained in work with nickel hypophosphite, analogous studies were carried out with cobalt hypophosphite to determine the fundamental possibility of coating metals (iron and copper) with cobalt. As was to be expected [6], the rates of cobalt coating were considerably lower than with nickel (cobalt hypophosphite begins to decompose at an appreciable rate only at 150°C). The experiments showed that varying the concentration of cobalt hypophosphite over wide limits (10–100 g/l) has practically no effect on the rate of the process, which remains very low (0.3–0.5 µm/h at pH = 6 ± 0.5 and \(t = 95\)—100°C). Below 90°C the process does not proceed at all. When the pH is changed to 4, it is possible to increase the coating rate to 1 µm/h. Apparently, in order to increase the reaction rate substantially it is necessary to raise the temperature above 100°C, which can be done only in nonaqueous media.

Adjustment of the pH and of the hypophosphite concentration did not give positive results.

Cobalt coatings are dense, have good adhesion, and are gray in color (the bright surface rapidly becomes dull in air owing to the formation of an oxide film).

Conclusions

1. The fundamental possibility has been demonstrated of using nickel (cobalt) hypophosphite for the chemical nickel-plating (cobalt-plating) of iron and copper surfaces.

2. Optimal conditions for carrying out the process have been found, providing a coating rate of 14–15 µm/h (for nickel) and 1 µm/h (for cobalt).

3. Nickel coatings obtained by this method may be recommended for corrosion protection and for decorative purposes.

4. Further investigations should be directed toward a more detailed study of the properties of nickel (cobalt) coatings, toward increasing the rate of cobalt coating, and toward studying the possibility of coating other metals and alloys with nickel (cobalt) by this method.

REFERENCES

1. B. Ya. Temkina. Progressive Technology for Applying Galvanic Coatings. Mashgiz, 73–89, 1962.

2. K. M. Gorbunova and A. A. Nikiforova. Physicochemical Principles of the Process of Chemical Nickel Plating. Academy of Sciences of the USSR, 1960.

3. N. V. Korovin. New Coatings and Electrolytes in Electroplating. Metallurgizdat, 106–122, 1962.

4. Van Wazer. Phosphorus and Its Compounds, 282–283, 1962.

5. Sheet Metal Industries, Vol. 33, No. 351, July, p. 439, London, 1956.

6. I. W. Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. VII, 870–891, 1931.