STUDY OF CADMIUM ELECTRODEPOSITION FROM A POLYETHYLENEPOLYAMINE ELECTROLYTE

O. S. Stepanova, N. T. Rud’

(Presented by the scientific-methodological seminar of the Department of Technology of Inorganic Substances and Electrochemical Production)

One of the promising electrolytes for cadmium plating is the polyethylenepolyamine electrolyte [1, 2]. In order for this electrolyte to find broad application in the practice of electroplating shops, it is necessary to study its quantitative characteristics.

In the present work, the influence of various physicochemical factors on the electrodeposition of cadmium from a polyethylenepolyamine electrolyte has been studied, and data are given on the cathodic current efficiency, the rate of cadmium deposition, and the throwing power of the electrolyte at different current densities.

The investigations were carried out on an OH-20I-type installation. The kinetics of cadmium electrodeposition was studied by the method of polarization curves. The current efficiency was determined with the aid of a copper coulometer connected in series in the electrical circuit with the electrolyzer. The throwing power was calculated by the method of G. Hering and V. Blum [3]. The coating thickness was determined by the gravimetric method. The degree of porosity of the cadmium coating was evaluated by the method of anodic polarization with an indicator. Metallographic studies were carried out on an MM-8M-type microscope. The electrolytes studied had the compositions given in Table 1.

Of the first four electrolytes, the best deposits are obtained at a concentration of \(CdSO_4 \cdot 8H_2O\) equal to 100 g/l. Electrolyte No. 8 gives the best-quality deposits among the next five electrolytes. Coatings from electrolytes No. 11 and No. 12 are similar in quality. If

if electrolytes Nos. 2, 8, and 11 are compared with one another, the deposition of cadmium from electrolyte No. 11 is accompanied by the greatest polarization \((\Delta \psi = 174\ \text{mV})\) at \(i_k = 0.6\ \text{A}/\text{dm}^2\), Fig. 1.

Table 1.

Fig. 1. Polarization curves of cadmium electrodeposition from electrolytes.

1 — \(3CdSO_4 \cdot 8H_2O\) — 100 g/L;
2 — \(3CdSO_4 \cdot 8H_2O\) — 100 g/L;
  PEPA — 150 g/L;
3 — \(3CdSO_4 \cdot 8H_2O\) — 100 g/L;
  PEPA — 150 g/L;
  \((NH_4)_2SO_4\) — 300 g/L.

The structure and quality of the cadmium deposit from the latter electrolyte are considerably better than from electrolytes No. 2 and No. 8; therefore, the following data on the electrodeposition of cadmium are given for electrolyte No. II.

The effect of the electrolyte temperature ($t = 20–40^\circ\mathrm{C}$) on the rate of cadmium deposition was studied. The choice of the temperature interval was governed by the decomposition of PEPA at $t = 40^\circ\mathrm{C}$. The electrolyzer was thermostated in an I-8 type thermostat. It was established that increasing the temperature lowers the polarization in the investigated range of current densities (from $0.2\ \mathrm{A/dm^2}$ to $1.5\ \mathrm{A/dm^2}$), and this, in turn, leads to a deterioration of the structure and quality of the deposit, although the rate of cadmium deposition increases.

The effect of the cathodic current density ($i_k$) on the deposition rate ($v_{\mathrm{dep}}$) and coating thickness ($\delta$), the cathodic current efficiency (C.E.) of cadmium, and the structure and quality of the coating was investigated. The data are given in Table 2.

Table 2

It is seen from Table 2 that, as the current density increases, the rate of cadmium deposition increases, the current efficiency of cadmium decreases, and the quality of the deposit deteriorates: the coating becomes powdery and more porous. The decrease in current efficiency and the deterioration in the quality of cathodic cadmium are explained by the simultaneous evolution of hydrogen with cadmium.

Metallographic studies of the structure show that

dense, fine-crystalline cadmium deposits are obtained from the polyethylenepolyamine electrolyte in the current-density range from 0.2 to 1.2 A/dm\(^2\) at an electrolyte temperature of 25–30°C. Porosity of the deposit appears at a thickness of more than 6 μm.

As a rule, finished articles of complex configuration are subjected to cadmium plating; therefore it is of interest to evaluate the throwing power of the electrolyte. Data on the influence of the cathodic current density on the throwing power of the electrolyte (T) are given in Table 3.

Table 3

The data obtained show that the polyethylenepolyamine electrolyte has a fairly high throwing power and confirm the opinion of a number of authors [1,2] on the possibility of using this electrolyte instead of a cyanide one. The latter is of great practical importance. The throwing power of electrolyte No. II increases with increasing current density, which is in full agreement with the data on the cathodic current efficiency of cadmium (Table 2), which decreases with increasing current density.

On the basis of the results obtained, the following technological regime is recommended for depositing cadmium from a polyethylenepolyamine electrolyte of the composition:

\[ \begin{aligned} 3CdSO_4 \cdot 8H_2O &- 100\ \text{g/L};\\ \text{PEPA} &- 150\ \text{g/L};\\ (NH_4)_2SO_4 &- 200\ \text{g/L};\\ t^\circ C &- 20-30;\\ i_k &- 0.2-1.2\ \text{A/dm}^2;\\ T &- 29-38\%. \end{aligned} \]

References

1. A. V. Ryabchenkova. Cadmium Plating of Metals in a Complex Polyethylenepolyamine Electrolyte. Moscow, GOSINTI, 1967.

2. G. A. Khotmakher, N. P. Ponomareva. Metal coatings from polyethylenepolyamine electrolytes. “Exchange of Experience in the Radio Industry,” issue 7, 1970.

1. Reference Manual on Electroplating. Moscow, “Metallurgiya,” 1969 (translated from the German under the editorship of V. I. Lainer).