Electrodeposition of Nickel

The electrodeposition of nickel has been previously discussed. Y. Kohlschutter and E. Vuilleumier found that in the electrodeposition of nickel, a film is first formed on the cathode, and this is followed by a contraction of the film which causes a bending of the cathode. The contraction is dependent on the current density and the composition of the electrolyte; it is strongly influenced by the addition of other substances to the electrolyte, and is less in soln. which cause an evolution of hydrogen, and also when the deposit is fine-grained. When an already bent cathode is charged with hydrogen it straightens out again, but if the current is interrupted at this stage, the hydrogen is set free and the cathode takes up its original bent form. In soln. which do not evolve hydrogen the contraction occurs in jumps, which have the characteristics of delayed effects. The whole effect is probably due to the fact that the metal is at first deposited in a highly disperse form, and then the particles sinter with the formation of a denser material. This probably occurs along with the formation of a gas layer on the cathode, which plays the part of a dispersion medium. Since the electrolytic soln. pressure of the highly disperse form is necessarily greater than that of the denser metal, this view explains the increase in the deposition potential above that of the ordinary metal. According to H. Stager, there are two types of contraction; in one, the contraction gradually diminishes and in the other, it gradually increases. The temp, at which the layer is formed influences the structure, and consequently the mechanical behaviour, of the deposit; since the development of a film of gas at the electrode must be rendered difficult by rise of temp., the diminished dispersivity and the resultant diminished capacity to sinter which are observed may well be due to decrease in the deposit of hydrogen. When nickel is deposited from soln. containing depolarizers - such as hydrogen dioxide, nitrobenzene, potassium chlorate, and sodium cinnamate - the contraction of the nickel deposits is markedly diminished and their structure altered under these conditions, as should be the case if the discharged hydrogen is the ultimate cause of the effect. The subject was discussed by V. Kohlschutter and co-workers, H. Schodl, J. Prajzler, H. Marshall, O. A. Esin and A. Alfimova, R. Harr, V. P. Sacchi, and E. Bouty. For E. J. Mill's observations on the electrostriction of nickel, vide iron. K. M. Oesterle examined the crystal structure and physical properties of crystal deposits of nickel from A-NiCl2 on platinum or silver cathodes using nickel anodes, and a cathode density of 2.5 amp. per sq. dm. The character of the deposits depends on the ph value of the electrolyte, and also on the presence of gelatin, which, in serving as a protective colloid of the dispersed nickel hydroxide, causes a bright smooth layer to be formed. The data corresponding with those deposits obtained from soln. of pH=4.5 to 6.0 illustrate how great an effect a small change in may have on the quality of the layer. It is within this range that the electrolyte becomes contaminated with small amounts of nickel hydroxide.

The Effect of Current Density on the Simultaneous Deposition of Zinc and Nickel.

E. P. Schoch and A. Hirsch, and W. D. Treadwell found that, at 18°, a soln. 0.5 normal with respect to both zinc and nickel sulphates, and 0.01 normal with respect to sulphuric acid, gave the results summarized in Fig. 45 when electrolyzed. The percentage of nickel in the zinc-nickel deposit is very low at low current densities, attains a minimum, and then slowly rises. The current efficiency at high current densities is high, showing that little current is spent in the generation of hydrogen. The results at 80° are summarized in Fig. With low current densities, the deposit is rich in nickel, and the proportion of nickel decreases as the current density increases; at the same time, the cathode potential rises to values required for the deposition of zinc. At low current densities also, the current efficiency is below 20 per cent, and the current is spent principally in generating hydrogen. As the cathode potential rises with increasing current densities, so does the current efficiency also rise. The subject was investigated by S. Glasstone, A. Fischer, B. Bogitch, F. Forster and K. Georgi, M. Pavlik, and A. Thiel and A. Windelschmidt.

J. B. O'Sullivan found that the presence of small proportions of iron salts has no appreciable effect on the electrodeposition of nickel, owing to the preferential deposition of iron whereby this metal is removed from the cathode film before it can form colloidal compounds. On the other hand, aluminium can accumulate in the cathode film until colloidal compounds are formed. When this has gone far enough, the character of the deposit is profoundly modified; it becomes black or " burnt." The black deposit contains appreciable amounts of aluminium, the white deposit has none. The subject was discussed by L. D. Hammond, W. C. Ellis, P. K. Frolich and G. L. Clark, K. Engemann, and G. Fuseya and K. Murata.

A. B. Schiotz found nickel alone is deposited from soln. of nickel chloride in the presence of cerous chloride. V. Engelhardt and N. Schonfeldt studied the speeds of deposition at different distances from the anode; E. Becker, the composition of the anodes; A. W. Hothersall and R. A. F. Hammond, the effect of oxidizing agents - hydrogen dioxide and nickel nitrate; and E. S. Hedges, the periodic electrodeposition. H. Forestier observed that in a magnetic field of 5200 gauss, the rate of deposition of nickel from a 0.61 per cent. aq. soln. of nickel sulphate is considerably retarded when pH < 2.3, and is arrested when pH < 1.0. For higher values of there is a slight acceleration. For pH = 1.2, and increasing intensities of the magnetic field, the rate of deposition decreases asymptotically to a minimum.