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CHEMICAL NEWS, Dec. 5, 1913

THE

CHEMICAL NEWS

VOL. CVIII., No. 2819.

ANODIC AND CATHODIC RETARDATION

THEORY OF PASSIVITY.*

By G. GRUBE (Dresden).
(Continued from p. 261).

WHEN we summarise the results of Treadwell and Escher we find that in solutions of nickel sulphate and ferrous sulphate, which further contain zinc sulphate, the electrolysis at ordinary temperature yields deposits consisting predominantly of zinc. When the electrolysis is performed at higher temperatures, it is possible under suitable conditions of low current density to obtain also deposits poor in zinc. These latter deposits are realised at low current efficiency and at potentials which would likewise be recorded in depositing pure iron or pure nickel from the same solution not containing any zinc; the formation of the former deposits takes place at considerably higher current efficiency, at the potential necessary for the deposition of zinc.

The explanation of these phenomena may first be given with regard to higher temperatures: at small current density the metals nickel and iron are deposited on the cathode at the potential required for their deposition from pure solutions; at the same time that amount of zinc must be deposited which corresponds to the concentration of the zinc ions at the cathode and to the cathode potential. This amount is in itself very small; since, however, both iron and nickel can absorb a good deal of zinc with formation of alloys, considerable amounts of zinc will have to be deposited in order to get into equilibrium as regards cathode potential and concentration of zinc ions. In this way an alloy poor in zinc is formed at the potential of the more noble metal, and the further deposition of iron or nickel on this alloy is effectually retarded. Consequently the hydrogen liberation will predominate. When now the current density is raised, the concentration of the hydrogen ions in the weakly acid solution will no longer satisfy the current, and polarisation is set up; thus the zinc potential is attained. The hydrogen liberation then diminishes, and zinc is chiefly deposited in addition to some more iron and nickel.

At ordinary temperature the separation of nickel and iron even from their pure solutions is much more difficult than at higher temperature (compare A. Schweitzer, loc. cit., and F. Foerster, loc. cit.). The retardation of their deposition from the solutions mixed with zinc sulphate will hence be so great from the very first, that only alloys rich in zinc will result at the zinc potential. Thus the experiments described offer the interesting phenomenon that the cathodic passivity of iron and nickel is so much strengthened by the original co-precipitation of zinc, that the deposition of these metals almost ceases and that, at a potential which is by o'2 volt less noble than that required for the deposition of pure iron and nickel, a cathode deposit will be formed which will essentially consist of zinc. The intensity of this polarisation apparently depends also on the concentration of the hydrogen ions in the electrolyte, because Escher has proved that the rapid rise in the potential takes place with greater difficulty in neutral than in weakly acid solutions. The acidity of the electrolyte hence acts in the sense of an increase in the zinc concentration, i.e., it intensifies the polarisation.

Communication from the Laboratory for Physical Chemistry and Electrochemistry of the Technical High School, Dresden. (Translated from the German). Contribution to the General Discussion on "The

Passivity of Metals" held before the Faraday Society, Nov. 12 1913.

The question now arises, how can we account for the polarisation phenomena accompanying the deposition of the iron metals which, as we have seen, are intensified by the presence of zinc sulphate or of hydrogen ions. Le Blanc has expressed the opinion that the metals form ionic hydrates, whose existence is also assumed for other reasons, and that the supplementary supply of metallic ions (for the deposition of the metal) from these hydrates takes place at a restricted velocity only. On this view the moderate velocity of the reaction

Men(H2O)++ Men+++H20

in the direction left to right would be the cause of the high polarisation in the deposition of the metals, and the increase in the polarisation, observed in the presence of zinc sulphate or acid, would occur because these additions much retard the decomposition of the ionic hydrates. Such an inhibitory action is possible; it appears little plausible, however. It seems preferable to look for the cause of the retardation of the cathodic reaction in some change of the cathode surface. We may suppose that alloys containing zinc or hydrogen are deposited on the cathode, and they possess a higher electrolytic solution pressure than the pure nickel or iron. The formation of the-for the present purely hypothetical-intermediate products would then be the cause of the marked retardation in the deposition of the metals.

The experiments which have been mentioned clearly demonstrate, at any rate, that a retarded cathodic process may undergo a further pronounced retardation when the cathode takes up foreign matter. If that be granted, the assumption which is made in explanation of the anodic passivity, namely, that slowly-proceeding anodic reactions are further and essentially retarded by the formation of oxygen alloys in the surface of the anode, gains a higher degree of probability. In addition to this passivity which is due to an oxygen charge, there will also be a possibility of passivity during the anodic dissolution of metals in cases where the cations emitted by the anode will be able to form a scarcely soluble compound with the anions of the electrolyte. We shall only then be justified in ascribing the passivity to surface films if such films may be expected to form in accordance with the chemical behaviour of the electrolyte near the anode. When the causes of the anodic passivity are discussed, the question will therefore not be quite general, whether any retardation in the emission of the ions may arise from the formation of an oxygen alloy on the surface of the anode, or whether the retardation be caused by the mechanical closure (covering up) of the surface of the electrode by the formation of an oxide skin or of some other scarcely soluble precipitate. According to the conditions of the experiment, the one or the other moment will cause passivity.

I now wish to refer to some experiments of mine (Zeit. Elektrochem., 1912, xviii., 189) which show that, at one and the same electrode metal in the same electrolyte, there may, according to the experimental conditions, be a passivity of the surface films, as well as a passivity due to an oxygen charge, and that, under suitable conditions, the two phenomena may continuously pass into one another. The experiments were undertaken, in the first instance, for the purpose of inquiring into the suitability of various metals as electrodes for the electrolytic preparation of ferripotassium cyanide. I determined current densitypotential curves with anodes of the several metals in

neutral and in alkaline solutions containing both ferro- and ferripotassium cyanide. The sheet metals were dipped into the electrolyte; the potential was measured first without current; the electrode was then anodically polarised while the electrolyte was well stirred, and the current density was raised, and the potential was determined in the usual way.

We are on this occasion interested only in the results obtained with the metals platinum, gold, iron, nickel, cobalt, and copper. The polarisation phenomena varied to an extraordinary degree with the experimental conditions. When the electrolyte was a neutral solution with a total content (of ferropotassium cyanide + ferripotassium cyanide) of 0.5 mol., whilst the ratio K4FeCy6: K3FeCy6 in the solution was 25: 75 or 75: 25, the potential found, when the current was not flowing, with platinum and gold electrodes was the oxidation potential of the respective solution, whilst much less noble potentials were observed with iron, cobalt, nickel, and copper; these latter potentials gradually became more noble, however. The latter electrodes at the same time became covered with films of their sparely soluble ferrocyanogen compounds. Thus gold and platinum were completely passive in neutral solutions, whilst iron, nickel, cobalt, and copper behaved at first like soluble electrodes which gradually became covered with surface films and assumed the nobler, passive potential. When the electrodes were now anodically polarised, gold and platinum too showed the

small polarisation (due to changes in concentration), whilst an extraordinarily intense polarisation was noticed on the other electrodes, which were covered with surface films. The current density-potential curves observed with iron, nickel, and platinum in neutral solutions are reproduced in Fig. 4. We see how very strong the polarisations are in the first two cases (anodes with surface films) compared with the concentration polarisation of the platinum anode. The aspect changed very materially when the same solution of ferro- and ferricyanide was rendered o'r N alkaline by the addition of KOH. In this solution not only gold and platinum but also iron, cobalt, nickel, and copper marked, when not under current, the oxidation potential of the electrolyte; the appearance of a surface film was no longer noticeable in any case; all the metals remained perfectly bright. When the electrodes were anodically polarised in this instance, the same small concentration polarisation was shown by all the metals except iron. Iron yet indicated in the alkaline solution a pronouinced polarisation which could considerably be dim nished, it is true, by increasing the alkalinity of the electrolyte and by raising the temperature, but which could never be reduced to the low values measured at the

other metals. In these experiments, however, the eye could not discern the formation of any surface film on the iron. But the film was visible when the experiment was made in a o'or N alkaline solution. In this solution the iron electrode marked, a short while after immersion, a potential which was by 8 millivolts less noble than the oxidation potential of the solution. Within half an hour, however, the iron potential had risen to this value, while the electrode became covered with a faintly discernible layer of Prussian blue, which was gradually decomposed by the alkali of the electrolyte and converted into an oxide film. This oxide film could only be seen in some spots; the rest of the surface appeared quite bright. Under anodic current this electrode gave a still higher polarisation than had been noticed in the o.1 N alkaline solution. The existence of an invisible oxide film had, however, to be presumed also in more strongly alkaline solutions, since the polarisation was gradually seen to increase in them when the current intensity was kept constant and the electrolyte remained unchanged. Thus in a o'r N alkaline solution, e.g., the polarisation rose, at o'002 amp./cm.2, in 265 minutes from 90 up to 134 millivolts. These observations indicate that in all probability the passivity of the iron electrode in alkaline ferro-ferricyanide solutions was also due to the formation of an oxide film.

On the contrary, the passivity of electrodes which showed merely concentration-polarisation in alkaline solutions could not be attributed to an oxide film of similar kind as with the iron electrode. This was concluded, in the first instance, from the non-observance of any increasing polarisation, and further from the following experiments:

When by suitable previous treatment a very thin translucid oxide film had been produced on electrodes of nickel or cobalt, these electrodes showed, in alkaline ferro-ferricyanide solutions, at once the same polarisation phenomena of the same order of magnitude as iron electrodes. If, therefore, the passivity had on nickel, cobalt, and the other metals which were not polarisable in alkaline electrolytes, been caused by the same kind of oxide layers, this passivity should have called forth an intensified polarisation. As this was not the case, we should have been forced-provided we declined to abandon the surface layer as cause of the polarisation-to assume that these surface films responsible for the passivity were good conductors, capable indeed of impeding the emission of cations, but not of opposing any resistance to the passage of the current. The existence of such well-conducting films is, however, extremely improbable, since the oxide skins formed by purely chemical means introduced a considerable polarisation.

If, in spite of that, it should be suggested that the passivity films were of the same kind as those chemically prepared, but of so insignificant a thickness that they could not give rise to any pronounced polarisation, we should have to imagine that the oxidations of ferrocyanide were taking place under the co-operation of some peroxide in the oxide layer. We could not conceive then, however, why such peroxides, whose existence we should have to assume for the different metals gold, platinum, nickel, cobalt, and copper in alkaline solutions, should all be able to effect the oxidation of the ferrocyanide at the same potential and with the same velocity. On account of this equality of the potential, at which the oxidation of the ferro-ferricyanide solutions is taking place with different metals, we shall have to suppose that there is no oxide film on these metals (which cannot anodically be polarised), but that these metallic surfaces are in perfect contact with the electrolyte. Since therefore the presumption of oxide films does not afford any unstrained explanation of the cause of the passivity of platinum, gold, nickel, cobalt, and copper, it appears apposite to trace their passivity back to a retardation in the emission of cations consequent upon the charge of the anode with oxygen.

Such oxygen charges must doubtless exist, to judge