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THE CHEM

REVIEW AND INTERPRETATION OF RECENT EXPERIMENTS WHICH EXTEND AND ELUCIDATE THE DOMAIN OF THE PASSIVITY OF METALS.*

By Dr. D. REICHINSTEIN, Zurich.

1. Historical.

FARADAY (Letter to Brayley, July 8, 1836, Lond. and Edinb. Phil. Mag., 1836, ix., 122) ascribes the discovery of the passivity to Keir (Phil. Trans., 1790, pp. 374 and 379), who observed in 1790 that iron was not attacked in strong nitric acid, but assumed a "changed" state.

The problem was, however, investigated only in the thirties of the last century by Faraday and by Schōnbein; the latter introduced the term "passivity." The difference of opinion between the two originators of the problem is of high historical interest. Faraday's views as to the nature of passivity were interpreted by Schönbein (Pogg. Ann., xxxvii., 390; xxxviii., 444; xxxix., 342) to the effect as if Faraday saw the cause of the passive behaviour of a metal in the formation of a well-developed oxide on the metallic surface, and Faraday is still frequently being called the originator of the oxide theory of passivity. In my opinion Faraday may with equal right be regarded as the originator both of the oxide theory and of the most modern theories of gas charges and metal-oxygen alloys; and in this sense I regard the theory which I am going to present merely as a logical consequence of the views of Faraday.

In support of this statement I quote the beginning of a letter which Faraday addressed to R. Taylor (Phil. Mag., 1837, X., 175) on January 21, 1837 :

:

Dear Sir,-I am much obliged to you for a sight of Mr. Schoenbein's paper, the experiments and observations in which are excellent. The cause of the phænomena he has so well distinguished is indeed exceedingly difficult to be distinguished at present, and I was in hopes that the doubt on my mind when I ventured the view referred to would be evident from my words. My strong impression is," &c. (Phil. Mag., 1836, ix., 61). "Moreover, Mr. Schoenbein and also M. Alb. Mousson, in an attempt which he has made to explain the cause (Bibliothéque Universelle de Genêve, 1836, p. 165), have not given my view clearly. I have said that my impression is that the surface of the metal is oxidised, or else that the superficial particles of the metal are in such relation to the oxygen of the electrolyte as to be equivalent to an oxidation,

A Contribution to the General Discussion on "The Passivity of Metals" held before the Faraday Society, November 12, 1913. (Translated from the German.)

meaning by that, not an actual oxidation, but a relation.

Faraday further alludes to the researches of Nobili on the colours of thin plates, and he expresses the view that the latter went much further still in the opinion that skins of oxygen and acid may lastingly adhere to the surfaces of platinum, iron, and steel without entering into chemical combination with the metal.

The above quoted letter by Faraday to Taylor further contains, near its end, the following words: -"And my opinion of the cause of the phænomena as due to a relation of the superficial particles of the iron to oxygen. .. ."

From these views of Faraday has arisen the oxide theory of passivity which has been advocated up to the most recent days (e.g., by F. Haber and F. Goldschmidt, Zeit. Elektrochem., 1906, xii., 49).

The theory which I am going to develop is further connected with the views of M. Le Blanc (Zeit. Elektrochem., 1900, vi., 476), who first expressed the idea that it is velocity phenomena which condition passivity. This assumption has successfully been developed and specialised by Fredenhagen (Zeit. Elektrochem., xi., 857, and loc. cit.), who makes the occurrence of passivity dependent upon "a coherent, uniform gas charge"; finally Muth.nann and Fraunberger (Sitzber. Bayer. Akad. Wissensch. München: Math. Phys. Kl., 1904, 236) have recognised that this gas charge has the character of an alloy.

The question now, which I put to myself, and which is not discussed by any of the actual theories, is how the transition is taking place from the active state of a metal into the passive state. This question is to receive an answer which can quantitatively be tested by calculations,

Before we pass to the discussion of this question, however, we have to acquaint ourselves with a series of experiments of the very latest dates, some of which have extended the domain of passive phenomena, while others are able to introduce us into the labyrinth of passivity.

2. Chemical Polarisation of Reversible Active Metallic Electrodes.

Before the year 1910 all the metals were distinguished as active and passive, and if there was a tendency to look for the cause of passivity in slowly progressing chemical reactions, i.e., in a slow rate of ion-formation, people held on the other side that with active metals anodically treated the formation of ions took place with infinite velocity, and that any polarisation observed with active metals had to be designated a concentration polarisation or a diffusion polarisation; that is to say, a polarisation which arose owing to the differences in the concentration of the electrolyte caused by the current flow, whilst

diffusion tended to balance this difference in concentration.

I should like to emphasise in this place already that these definitions of chemical or of concentration polarisation are inappropriate, and that they fail in complicated cases. For the occurrence of concentration differences is not characteristic for the diffusion polarisation. With one and the same electrode different polarisations are observed, always as results of different concentrations of the sub. stances in question. The point is rather, which process is opposing the differences in concentrations to which the electric current gives rise, i.e., whether this compensation is of a chemical nature or a diffusion phenomenon. Hence it would, in my opinion, be appropriate to introduce a definition like the following:-If in any observed polarisation the substitution of a real process by an imaginary chemical process, proceeding at infinite velocity, would make the polarisation vanish, we should have to deal with a chemical polarisation; if a diffusion proceeding at infinite rate should have the same effect, we should have to deal with a diffusion polarisation.

Now Le Blanc described in 1910 some experiments which have revolutionised our conception of active and passive metals ("Die elektromotorischen Krafte der Polarisation"). Two proofs can be adduced that there is only a quantitative, not a qualitative, difference between active and passive metals, and that there is, in active metals, a possibility not only of a diffusion polarisation, but also of a chemical polarisation.

These are the proofs :

In 1904 Thatcher observed that when quite small quantities of so called poisons are added to an electrode, at which an electrolytic oxidation is taking place, the anode potential rises considerably, which indicates that slowly progressing chemical reactions are at play Le Blanc and his assistants now demonstrated that an addition of poisons to the active electrode Cu | CuSO4 called forth the same effect. Thus I cc. of a faintly acid solution of brucine sulphate (of 1 per cent.), added to 100 cc. of a solution of IN-CuSO4+1N—H2SO4, raises the cathodic polarisation from 29 to 148, and the anodic from 14 to 40 millivolts. Strychnine raised the cathode polarisation by 115, the anode polarisation by 18 millivolts, &c.

When we further compare the polarisations of two electrodes under conditions, as equal as possible, of temperature, current density, concentration of the ions, &c., and when we consider that the diffusion rates must, under under those equal conditions, be of the same order of magnitude, we are entitled to conclude that large differences in the polarisation values of the two electrodes can only arise owing to the chemical polarisation of that electrode which possesses the higher polarisation value.

Thus the system Hg | o'1N-HgNO3+1N—HNO3 | Hg gave, at a current density of 0.09 amp./cm.2, a polarisation of about 6 millivolts, whilst under the same conditions the system Cu | IN-CuSO4+1N-H2SO4 | Cu gave, with electrodes of Cu quenched in methyl alcohol, 30 millivolts, and with Cu electrodes polished with emery II0 millivolts. Still higher polarisations, of the order of 500 millivolts, were obtained with the system

Fe | IN-FeCl2 + 1N-HCl | Fe.

The third proof for the existence of the chemical polarisation of active electrodes was given by Haber and Zawadzki (Zeit. Physik. Chem., 1912, lxxviii., 241) in 1912, in a paper on the polarisation of solid electrolytes. They found that "solid silver salts show, with a silver anode, a polarisation which increases considerably with falling temperature, and which, in the case of silver sul phate, becomes so pronounced at the temperature of solid carbon dioxide, that the silver anode behaves like an anode of platinum or graphite."

A further proof for the existence of the chemical polarisation of active electrodes is afforded by the fact that

additions of free sulphuric acid and of its salts to the Cu | CuSO4 electrode much increase the cathodic polarisation (R. Goebel, "Dissertation," Dresden, 1912, researches carried out under F. Foerster; also D. Reichinstein, Zeit. Elektrochem., 1913, xix., 520, and D. Reichinstein and A. Zieren, Ibid., 530). These two accounts do not yet contain the results of all the experiments made. In order to keep the effect of an addition of a neutral salt quantitatively reproducible, the electrolyte should possess a certain minimum of H ions; for below a certain H⚫ ion concentration the cathodic current intensity-potential curve of the Cu | CuSO4 electrode takes a qualitatively quite different course from the curve found above this minimum. That indicates that it is purely chemical pro. cesses which are responsible for the current intensitypotential curve. Since now all the neutral salts as well as the sulphuric acid itself have the same effect, the increased polarisation is evidently due to the fact that the addition diminishes the concentration of the Cu ions in the electrolyte. What kind of a polarisation is this them -a chemical or a diffusion polarisation ?

The first-mentioned case must not be confounded with another, in which the electrolyte contains the same small concentration of Cu ions, but in which the neutral salt and the noteworthy concentration of undissociated CuSO4 are missing in the electrolyte.

ΙΟ

a ΙΟ

In contrast to this simple case the more complicated case may be discussed as follows:-Imagine an ideal case, in which the Cu electrode has a considerable concentration of the Cu ions (a), the required minimum of Hions, and a very small (practically none) concentration of undissociated CuSO4 molecules. At a certain current density at which the cathodic polarisation is still zero, a large amount of some alkali salt is added to the electrolyte, in order to reduce the concentration of the Cu ions to, e.g., and to set up a certain definite polarisation at the same time. Let us further assume the existence of an electrode which, in the presence of the concentration of its ions (without, however, possessing the orga concentration of the undissociated salt) will mark, ceteris paribus, the same polarisation as the Cu electrode referred to. The comparison between this hypothetical electrode and the really existing Cu electrode would suggest :-On the one hand (case 1) the polarisation of the Cu electrode would be caused by a slow (i.e., secondary) transition of the ions into the metallic state; it is assumed that while the current is flowing the concentration of the ions is, by vigorous stirring, kept the same at the electrode surface as in the middle of the electrolyte. On the other hand (case 2) the polarisation of the Cu electrode might be caused by the difference in the concentration near the electrode and in the middle of the electrolyte. The comparison between the Cu electrode and the hypothetical electrode now teaches

The latter case (2) is only possible when the rate at which the spent Cu ions are replenished by the reaction CuSO4 CuSO4" is infinitely small compared with the rate of diffusion of the Cu ions from the middle of the electrolyte to the surface of the electrode.

The following criteria enable us to distinguish between case I and case 2 :

If we can find an electrode such that the concentration of its ions be only a/10, that it contain no undissociated salt, and that it do not display any polarisation, eeteris paribus, then that Cu electrode would belong to case 1.

If we further succeed in constructing an electrode such hat, with an ion concentration of a/10, it show ceteris paribus the same polarisation as our Cu electrode, but practically do not, in an electrolyte of the composition o‘Ia concentration of its ions + o'ga concentration of the undis

CHEMICAL NEWS,

Jan. 2, 1914 sociated salt, possess any polarisation, that would be our case 2. It should be added that case I represents a pure example of chemical polarisation, whilst case 2 may be regarded as chemical polarisation and as diffusion polarisation as well. For, according to the definition formulated above, in case 2 the polarisation would vanish: by substitution of the real rate of dissociation of the CuSO by an infinitely rapid rate of dissociation, and also by substitution of the real rate of diffusion of the Cu ions by an infinitely rapid rate of diffusion.

These complicated polarisation phenomena may prob. ably be more easily investigated in practice than it would appear from this deduction; for I consider that the compensation process of case 2 will, in the predominating number of examples, practically be conditioned only by the dissociation of the undissociated salt. We shall thus either have case I, or have case 2, in which the dissociation of an undissociated salt will represent the compensation process. The further investigation of the Cu elec trode, undertaken for the purpose of deciding the question whether the chemical polarisation owes its existence to the dissociation process indicated, or perhaps to the reduction of the Cu ions (slow because of the small concentration of the latter), has suggested a solution of the problem in favour of case 1 (loc. cit.). This consideration is of principal importance for electrodes whose electrolytes consist of complex salts, e.g., Ag | Ag(CN)2K | electrode. Interesting examples of chemical polarisation, again not due to slow diffusion of the electrolyte, can be observed when the Cu electrode is rubbed with a little mercury. These cases will be explained lower down.

In looking for an example of an electrolysis which would supply a simple analogy to the described cases of chemical polarisation, that is to say, which would render clear how chemical polarisation can occur with an active electrode, without the metallic electrode losing its capacity of primarily (see Note) furnishing ions-in looking for such an analogy (which has been found) a whole series of phenomena was recognised as exemplifying negative depolarisation.

(NOTE.-By a primary electrochemical reaction we understand a reaction with which the passage of the current through the boundary electrode-electrolyte is connected, e.g., the charging of a metallic atom with an electron, or the discharge of an ion. Since this reaction proceeds in synchronism with the electric current, it is more rapid than all chemical reactions-practically of infinite velocity. If the anodic formation of Cu ions progressed primarily, undisturbed, that would not cause any chemical polarisation. The fact that chemical polarisation does occur indicates that another primary reaction is proceeding, and that the Cu ion formation is secondary).

3. Negative Depolarisation.

(Reichenstein, Zeit. Elektrochem, 1913, xix. 520). What is a depolariser? Imagine a circuit, consisting of a source of electric current having a constant e.m.f. independent of the load, and of a polarisation cell, e.g., of two platinum electrodes in clay cells charged with diluted sulphuric acid, these pots themselves immersed in a common beaker also filled with H2SO4. We close the circuit after the current intensity has reached its stationary value, and we add to the electrolyte about the cathode a solution of CuSO4. Then a partly new process will set in at the cathode, and the current intensity will rise. Any material capable of producing these two effects simultaneously is usually called a depolariser. The action of the depolariser appears especially clear to us when it is soluble in the solvent which constitutes the electrolyte, and particularly when it does not chemically react with the electrolyte to which it is added, and does not disturb the state of this electrolyte.

Are there now such materials which, when added to the electrolyte, will not chemically react with it, and will diminish the current intensity instead of increasing it?

3

In other words, are there any negative depolarisers? The reply to this question must decidedly be in the affirmative. Let the cathode of the described circuit consist, not of Pt, but of a Pd-H2SO4 electrode. When the electrode has, from the beginning, a small H2 concentration, the electrolysis at not too high current densities will proceed in such a way that the whole hydrogen will be occluded by the electrode, and the polarisation will be very small. We have probably to deal with a very rapid chemical compensation process constituted perhaps by the reaction between the Pd atoms and the primarily formed H atoms.

If we now add, during the electrolysis under suitable conditions, a zinc salt to the electrolyte, the polarisation will increase, the current intensity in the circuit diminish, and a new process will set in: zinc is deposited on the cathode (Zeit. Elektrochem., 1910, xvi.).

The surprising feature in this phenomenon is that, of all the possible processes, it is generally the more rapid which takes place. Which means in the case of the zinc addition that that process which occurs more easily will be impeded, namely, the deposition of hydrogen from the electrolyte by the zinc which would be deposited in minimal quantities (not to be determined by weighing), even at open circuit owing to local action. The zinc hence strengthens the chemical inertia of the reaction between the Pd and H atoms, and acts as a depolariser in a negative sense. I should like to point out in this place already that, if we desire to explain the chemical polarisation of a metallic electrode-let it scarcely be recognisable, or, on the other hand, sufficiently marked to lead to a generation of oxygen at the anode (passivity)— without denying to the metallic atoms the capability of forming primary ions, then the discharge of OH' ions, which with anodic treatment is simultaneous with the process of charging the atoms, may be regarded as the first step of a negatively depolarising reaction. The analogy is patent; we need only build up the mechanism of the chemical polarisation in a correct way.

The behaviour of real alloys on the anode may further be recognised as an example of negative depolarisation. A. Bürger and I (loc. cit.) have anodically investigated the alloys Cu-Ag, Cu-Hg, Cd-Hg, and Cd-Ag.

The noble metals, in amounts as small as possible, are All the alloyed with the surface of the baser metals. alloys examined have qualitatively the same property: they raise the polarisation whilst the equilibrium potential remains equal to that of the unalloyed chemically pure electrode. Thus the polarisation of a Cu electrode which has been rubbed with a little mercury rises ceteris paribus from 33 to 161 millivolts. The anodic current densitypotential curve of the alloys shows under certain conditions a point of inflection. The alloy is decomposed when the electrolysis is prolonged, and the nobler constituent aggregates or drops down from the electrode.

Of special interest for all these alloys is a strange relation between the velocity of formation of the alloy and the degree of polarisation attainable with the resulting alloy. The Cu electrode may simply be amalgamated by holding it horizontally, the surface free from paraffin facing upward, and pouring some mercury on it, finally adding a few drops of concentrated nitric acid. When the electrode is quickly rinsed with distilled water, a bright mercury surface is left; the nitric acid in this operation plays the part of the soldering liquid in the soldering process.

An electrode prepared in this way, however, hardly marks any greater polarisation than the pure unamalgamated Cu electrode.

In order to prepare an electrode which will show the effect described of the amalgamated Cu electrode, every trace of HNO3 must be avoided, and the greatest care be observed to deal with pure metals. The mercury is rubbed in the dry into the chemically pure copper surface; this is a tedious operation, and the slower the alloy formation the higher will afterwards be the polarisation realised under anodic treatment.

To the phenomena of negative polarisation might perhaps be joined another class of phenomena. In the cathodic polarisation of mixtures of nickel sulphate and zinc sulphate, of the sulphates of nickel and iron, and of zinc and iron, the polarisation rises frequently up to the value of the baser metal, and this latter metal is likewise deposited (A. F. Walter von Escher, "Kathodische Vorgänge bei der Elektrolyse gemischter Lösungen von Zink- und Eisersulfat," Dissertation, Dresden, 1912; a full literature list will be found there; compare also F. Foerster," General Electrochemical Behaviour of Metals," Zsit. Elektrochem., 1908, xiv., 153). These processes are not pure examples of negative depolarisation, however, because in the absence of the less noble metal the more noble one is deposited quantitatively, i.e., with a current efficiency of 100 per cent; at the same time hydrogen is generated. These cases are worthy of attention when one of the two metals at least in the salt mixture which is being electrolysed belongs to the cathodically passive

metals.

To these experiments, which, as we shall see, introduce us into the ante-room of the passivity phenomena, others may be added, which initiate us directly into the secrets of passivity.

a

4. Direct Experiments on the Passivation of Metals. Surprise has often been expressed at the fact that a metal turning passive does not, at higher current density, show at least that anodic solubility which belongs to it at small current density. Let a passive metal be dissolved anodically at a low current density a to the extent of 50 per cent, and let it liberate oxygen, so that its dissolution takes place at amp./cm. of metallic surface. When we increase the current intensity tenfold, the dissolution will not proceed at the rate of 5a amp./cm.2; it does not even take place at the former rate amp., but at a much smaller rate. How is this to be interpreted chemicokinetically? Let us assume that a passive metal Me generates primarily oxygen. Two reactions are possible :

2

a

2

1. Me+O+2H Me · + H2O.
2.0+0 02.

The commencement of reaction (2) is designated "passivity." The example teaches us that passivity is not a result of two competing reactions, for both of which the velocity would increase with increasing oxygen concentration, but: the passivity is caused by two reactions such that the velocity of the one reaction decreases with increasing O concentration, whilst the velocity of the second reaction regularly increases with increasing Ŏ con

centration.

In 1911 I set myself the task to investigate how the rate of increase of the anodic polarisation potential accompanying the flow of continuous current through a Pt-H2SO4 electrode differs in the two cases when the Pt electrode is free from oxygen and when it has previously been charged with O2 (Reichinstein, Zeit. Elektrochem., 1911, xvii., 89; 1913, xix., 672). I made the following observations:-A current impulse is imparted to the combination

Pt | 28 per cent H2SO4 || clay cell-KI-I | Pt

in the direction of the arrow. Its anodical amplitude is o'1 amp./cm. of the Pt electrode, its duration o'026 second. After the expiration of this period follows a spontaneous diminution in the electrode charge lasting 0.028 second. The rest of the anodic charge is then destroyed by shortcircuiting the cell for o'025 second. These three processes succeed one another. (The arrangement comprises a

rotating commutator which closes the primary circuit, interrupts it, and closes the short-circuit; the latter could be opened and closed at will during an experiment with the aid of a cut-out). In this way it was possible to obtain, on a photographic plate, oscillographs of the time potential curves at open and at closed short-circuit.

At open short-circuit this time-potential curve rises steadily up to an asymptotic potential value, which is independent of the time. At closed short-circuit the curve marks a point of inflection corresponding to the moment 0'004 second after closing the circuit. This point is well marked on the oscillogram, and the experiment is easily to be reproduced. I have been able to show this with the aid of the oscillographs at different times to several

scientists.

This point of inflection can only be interpreted as indicating that we have to deal with a chemical reaction, the velocity of which first increases and then decreases, as the current quantity (i.e., together with the oxygen concentration) is increasing. During a period which is smaller than 0004 second the time-potential curve has the distinct tendency of approaching the time ordinate and a stationary value at low potentials. Every approach to a stationary condition now is connected with an increase in the velocity of the compensation process, i.e., of that process in the absence of which the stationary conditions could not be reached. Now we observe a point of inflection within 0004 second after closing the circuit, when the potential rises; the velocity of the compensation process is beginning to fail.

Thus the curve velocity-oxygen concentration possesses a maximum.

The compensation process consists of the formation of platinum oxides (see below).

During a period which is smaller than o'004 second, therefore, a Pt | H2SO4 electrode behaves, at a current density of o'r amp./cm., like an inattackable electrode. The amount of Pt oxide which is formed per sq. c., when the continuous current is not interrupted, is hence of the order of magnitude 0·004. 01/96540=4. 10-9 grm.equivalent.

When afterwards the electrode is polarised for some time with the short-circuit closed, the electrode becomes covered with a yellowish layer which consists of Pt oxides. This does not occur when the short-circuit is open. We remember that platinum is dissolved with a high current yield in KCN solutions by alternating currents, but not by continuous currents. A combination, continuous currents superposed on alternating currents, is utilised in the gold industry.

A lucky accident has led us to the direct experimental recognition of the described maximum of the curve, in which the velocity is plotted as a function of the oxygen

concentration.

In Russia a large amount of gold is gained by dissolving the ores in aqueous KCN solution in the presence of air as oxidising agent. This process being very slow, there has been no lack of attempts of trying other oxidising agents.

In this connection Andrejew (Journ. Russ. Phys. Chem. Soc., 1907, xxxix., 1637; Nachricht. Polyt. Inst., Petersburg, 1908, ix., 447; Zeit. Elektrochem., 1913, xix., 667) and later Michailenko and Meschterjakow (Journ. Russ. Phys. Chem. Soc., 1912, xliv., 567)—on the instigation of Kistiakowsky-determined the rate of solution of gold in KCN in the presence of oxidising agents. They all found that this dissolution takes place in such a manner that the rate first increases and then decreases with the concentration of the oxidising agent. The velocity-concentration curve displays a well-marked maximum.

We may now pass to the exposition of the theories which aim at bringing all the cases of chemical polarisation under one point of view.

(To be continued).