Current Density in Nickel Salts Solutions

E. P. Schoch and C. P. Randolph studied the connection between the anode potential and the current density for N-soln. of nickel sulphate, and chloride, and potassium sulphate. For the current densities less than 4 milliamperes per sq. dm., the nickel shows normal anodic behaviour, and the potentials are readily reproducible. This is not the case if higher current densities are used, and with excessive densities the potential falls continuously until the evolution of oxygen sets in. The chemical action which takes place as long as the potential for the evolution of oxygen has not been reached, is a quantitative soln. of nickel. There is neither a critical voltage nor a critical current density which marks the termination of the normal anodic behaviour and the transition to the passive condition. The potential and density values corresponding with this transition depend on the previous electrolytic treatment. When the current is discontinued, the potential at the anode begins to rise immediately, the rate of rise diminishing with increase in the extent of the preceding electrolysis and with diminution in the accompanying potential. It was concluded that nickel has a very small ionization velocity; in consequence of this, comparatively small current densities result in the liberation of oxygen. Whether the nickel in the passive condition is protected by a layer of oxygen or covered by an oxide film cannot be decided on the basis of the observed facts. Expressing the current density, D, in milliamps. per 100 sq. cm., and the potential E relative to the normal calomel electrode in volts:

D	0	0.04	0.2	2	4	22	24	32	24	185
E	0.475	-0.463	-0.445	-0.39	-0.36	-0.27	-0.123	-0.05	+1.06	+1.131
	Active								Passive

C. A. L. de Bruyn found that with nickel nitrate the current density required to produce passivity is quite small owing to the catalytic influence of the nitrate ions. The greater tendency of nickel to become passive than is the case with iron is also shown by the behaviour of the metal in soln. of the nickel halides. With iron the disturbance produced by the halides is small, and with nickel large. W. Liebreich observed that there is a potential range in the feeble cathodic polarization of nickel in which the metal has a relatively great tendency to dissolve in the electrolyte and form a surface film of a basic salt. B. Strauss denied the theory that surface oxidation is the cause of the passivity of nickel or of nickel-chromium steels. S. Triandafil studied the effect of temp, on the polarization of nickel.

The current density required to render nickel passive is materially increased in a magnetic field. U. Sborgi and A. Borgia examined the influence of magnetism in passivity. H. G. Byers and A. F. Morgan found that the current density required to produce the passive state with a nickel anode, C without a magnetic field, and Cm with a magnetic field in nitric, sulphuric, and phosphoric acids, as well as a soln. of sodium nitrate and potassium sulphate, was:

	HNO3			H2SO4		H3PO4		NaNO3		K2SO4
	1%	5%	10%	5%	10%	5%	10%	N	0.1N	0.5N
C	247	297	892	450	471	46	39.7	3.71	2.81	2.38
Cm	324	385	1051	550	627	51	42.1	4.46	3.13	3.00
Diff	77	88	159	100	156	5	2.4	0.75	0.32	0.62

Activation Curve of Passive Nickel.

C. A. L. de Bruyn found that the activation curve of nickel does not show a portion almost horizontal as in the case of iron. The oxygen taken up by nickel during anodic polarization is gradually removed by hydrogen until the nickel surface contains only hydrogen, not oxygen. The curve with nickel nitrate soln. is shown in Fig. Analogous results are obtained with soln. of the nickel halides. E. S. Hedges studied the periodic passivity of nickel.

G. Grube showed that in neutral soln. the variation of the potential of the anode with current density is much more marked with nickel than with platinum, and the curves show how very strong the polarization is with the nickel anode which forms a surface film of oxide - vide iron.

A. P. Rollet studied the electrolysis of dil. alkali-lye, or dil. sulphuric acid with nickel anodes and an alternating current, and observed that the electrode is alternately coated with oxide and powdered metal. L. Tronstad, and F. Kriiger and E. Nahring observed that the X-radiograms agree with the assumption that a surface film is formed when nickel becomes passive, and, added L. Tronstad, the oxide film is not totally destroyed on activation, but it becomes porous and spongy; on re-passivation, the holes are refilled with oxide, and the film becomes thicker. W. J. Miiller and co-workers studied the passivating film of oxide on nickel, and said that the greater resistance of nickel to corrosion than of iron is due to the greater stability of the oxide film. E. Muller and J. Janitzki found that rubbing the nickel makes its potential nearly reversible when in soln. of acids, bases, or neutral salts. P. K. Frolich and G. L. Clark discussed the mechanism of the deposition - vide swpra. L. McCulloch observed cases of passivity in which a sparingly soluble film of sulphate was present. The surface film of oxide was studied by L. Tronstad, and U. R. Evans and C. Stockdale. U. R. Evans investigated the effect of passivation films on the anodic process; W. Frese, the effect of films on the photochemical effect, and passivity - vide the passivity of iron; and E. S. Hedges, the periodic passivity of nickel anodes. G. C. Schmidt discussed the hydrogen theory of passivity - vide passive iron; A. Barattini, and E. Werner, the solubility of different kinds of nickel anodes - cast, rolled, hammered, sintered, and electrolytic nickel. E. Newbery, and W. Ogawa studied the rectifying qualities of nickel and galena on the alternating electric current.

J. B. O'Sullivan measured the deposition voltages of nickel in soln. of NSO₄.7H₂O, 24 grms. per 100 c.c.; NaCl, 1.56 grms. per 100 c.c.; and 0.2N-NH₄- or acetate ion; and the 0.2M-borate ions, for current densities of 2,5,10, and 15 milliamps. per sq. cm. at 17° and 35°, and calculated values for unit current density. The deposition potentials were measured against a saturated calomel electrode. Here the pH voltage is the reversible hydrogen potential in the soln. against the same standard electrode. The difference between the deposition potential and the corresponding pB voltage gives the hydrogen over- voltage. G. Sartori, R. Saxon, H. T. S. Britton, N. V. Emelianova and J. Heyrovsky, J. Heyrovsky, N. A. Isgarischeff and H. Ravikovitsch, C. Marie and N. Thon studied the subject. M. Pavlik found that nickel deposits reversibly only from soln. having concentrations of calcium or lithium chloride over 6.4N-soln. at room temp., or from soln. containing less of these salts at 100°. The cathode deposition potential of nickel from soln. containing large amounts of dehydrating chlorides was about 0.3 volt more positive than the potential at which nickel is deposited from pure soln. of nickel chloride with the same concentration of nickel. The deposition of nickel was not affected by H^•-ions; a slight influence of acidity was attributed to the removal of the products of hydrolysis in dil. soln. The irreversible deposition of nickel from dil. soln. is attributed to the slow dehydration of nickel ions in the inner sphere of co-ordination, whereas the reversible deposition of nickel in conc. soln. is assumed to occur from complexes [NCl_n], from which the deformable chlorine ions are readily removed in the strong electric field of the cathodic surface - vide supra, cobalt chloride. F. Forster and K. Georgi found that the deposition of nickel does not, as supposed by S. Glasstone, commence at a definite potential which differs from the equilibrium potential, and then after proceed without retardation; for, under specified conditions, with a current density of 0.56×10^-4 amp. per sq. cm. no deposition occurs during the first 30 mins., but after 24 hrs. sufficient nickel is deposited to calculate the current yield. R. Muller and co-workers, and G. Devoto and A. Ratti examined the effect of organic substances. F. Braun studied some phenomena connected with the electrolysis of nickel salts. T. Kinbara observed the effect of a current with a nickel electrode on a photographic dry plate.