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Passivity

The recognition of the " passive " or chemically inactive state of iron is one of long standing, but up to the present time no theory has been advanced which gives a thoroughly satisfactorily or complete explanation of the phenomenon. It is beyond doubt that passivity, however induced, is wholly associated with the surface film or layer of the metal, and the behaviour of passive iron may be due equally well to either a physical or a chemical change in this layer. Many instances might be quoted to illustrate the varying degrees of chemical activity which may be conferred upon a substance by alterations in its physical condition, the chemical composition and structure remaining, withal, unchanged.
A strict criticism of what is implied by the term " passive," i.e., chemical inactivity of the METAL, would necessitate its interpretation as a definite or peculiar physical condition ; for if passivity is the result of a change in the chemical condition, such, for instance, as the deposition of an oxide or gaseous film, then the activity of the underlying metal is obviously unimpaired, but is prevented from manifesting itself by the presence of a protective film or coating whose extremely low solution pressure renders it more or less permanent. Hence if passivity is due to such a protective film on the metal surface, the term cannot be rightly regarded as denoting a specific condition or property of the metal.
As generally understood, however, "passive" is the term used to indicate that condition of the metal in which the appearance of its surface is not visibly altered, but in which its chemical activity or solution pressure is reduced almost to nil. The condition is always fugitive, though the length of its duration may vary according to the method by which it is induced, and it is considerably lengthened if the passified iron is kept dry (Heathcote, J. Soc. Chem. Ind., 1907, 26, 899).
It is usual, for purposes of demonstration, to passify iron by immersion in strong nitric acid, and it was in this manner that the phenomenon was originally observed. It was soon found, however, that other acids, such as chromic, iodic, and chloric acids, would induce the same condition as well as mixtures of acids and salts, or even aqueous solutions of certain salts, for instance, lead and silver nitrates, permanganate, and bichromate of potassium, provided suitable concentrations were employed. Passivity may also be induced by momentarily heating the metal in air or by exposure to certain gases, such as nitric oxide and nitric acid fumes. Iron may also be rendered passive by making it the anode during the electrolysis of an aqueous electrolyte, usually caustic soda.
The outstanding property of passive iron is, of course, its reluctance to enter into those reactions which characterise the active metal. It will not rust, for instance, and it is insoluble in acids, unless made the anode for an electrolysing current. Its rust-resisting properties have naturally attracted attention with a view to producing a permanently passive condition, but so far without success, so that the passive condition cannot yet be turned directly to any practical account, though certain passifying agents are of considerable importance in the protection of iron and steel. The other properties are only of significance in illustrating the methods by which passivity may be destroyed, etc., and in providing evidence either for or against the several theories which have been suggested to explain the nature of the passive condition.
The oldest theory is, strange to say, the one which has proved to be the most compatible with the facts which subsequent research has brought to light, and is therefore the one which has received the most generous adoption. That it is not wholly beyond criticism will soon become apparent, and although it is the generally accepted explanation, yet it is felt that an entirely satisfactory theory has still to be evolved.
The Oxide theory assumes that passivity is the result of the production of a film of metallic oxide, which covers the entire surface of the metal and prevents it from coming into contact with reagents to whose attack it is susceptible. The film may-must, in fact-be so thin that it is impossible to detect its presence even microscopically. An iron mirror may be rendered passive without incurring any estimable reduction in its reflecting power. Provided it is continuous and unbroken, the film may be so tenuous that its thickness may be regarded as being of molecular dimensions. Its composition has been the cause of much speculation, and although several alternatives have been suggested it is generally considered to consist of ferroso-ferric oxide, Fe3O4.
A second theory attributes passivity to the formation of an adherent film of gas which acts similarly to the film of metallic oxide discussed above. The gas may vary in kind according to the method used in passifying; thus it has been suggested that passivity induced by anodic polarisation in sulphuric acid is the result of a film of gaseous oxygen, whilst passivity produced by immersion in strong nitric acid is due to an oxide of nitrogen film. This is known as the Gaseous Film theory.
The third, or Physical theory, presumes an alteration in the physical or electrical condition of the surface of the metal by passifying agents, such as the production of trivalent iron. How far this assumption is justified it is difficult to say, there is no evidence to show that trivalent iron is inactive chemically ; this is certainly not the case when trivalent iron occurs in combination with other elements. If, however, passivity is really a metallic property and not the result of protection as assumed by the Oxide and Gaseous Film theories, then the Physical theory would appear to be the most rational, but since we do not know whether passivity is a metallic property or not, this theory must be accepted or rejected on its merits as revealed by a careful examination of the evidence available. Since, however, the passivities produced by alternative methods are not in every case identical in their behaviours, it may be possible that each theory is applicable to certain forms of passivity.
The evidence from which an estimate of the comparative validities of these theories may be formed may be summarised in the following manner by considering some well-established properties of passive iron. When passivity results from immersion in a liquid media the latter is invariably an oxidising agent. This provides considerable evidence in favour of the Oxide theory, but it does not necessarily reflect adversely on the other two theories. The gaseous oxygen which may be liberated on the surface of the metal would conform to the Gaseous Film theory, since this oxygen need not combine with the iron. It is known that the metal may remain in contact with oxygen, under certain conditions, for an indefinite period without any combination occurring between the two elements. We are also bound to regard the film of gas as sufficiently continuous and impenetrable to prevent contact between the iron and the liquid medium, and in the absence of such contact reaction between the metal and the oxygen will not take place. Again, contact with an oxidising liquid may be looked upon as particularly favourable to the production of a physical change in the metal, such as the formation of trivalent iron or other modification in the electrical condition of the metal.
Passive iron is readily restored to its original active condition by various chemical and mechanical means. Immersion in dilute acids destroys passivity, and the rate of destruction is accelerated by those external influences, such as elevation of temperature and mechanical agitation, which usually increase the rates of chemical reactions. Lightly scratching the surface, or rubbing, renders passive metal active, as does also the galvanic effect produced by contact, whilst in an electrolyte, with a piece of a more electro-positive metal, i.e., zinc. These observations lend support to the Oxide theory and also to the Gaseous film theory. The action of dilute acids and the acceleration of this action by the agencies referred to may be explained by the fact that the tenuous film of iron oxide is dissolved by the acids, or that the hydrogen liberated on the iron, when the latter is acting cathodically in contact with zinc, reduces the oxide film or the gaseous oxygen film, thus removing the cause of passivity, whichever this may be. Mechanical abrasion would, of course, destroy either form of protection, if not wholly, by at least so damaging it that its continuity is broken, thereby exposing the electro-positive metal beneath it to attack, when such action will be accelerated by differences in potential between the remaining portions of the film and the exposed metal. The remainder of a film of such excessive thinness as is assumed to suffice to impart passivity would, under these conditions, be almost instantaneously removed, and hence the apparent result that .scratching, etc., renders all portions of the surface active simultaneously. It has been shown, however, that the untouched areas are not actified in this way. If a piece of passive iron is scratched at one end then the immersion of the other end in some reagent shows that it is still passive, but if the level of the liquid is raised until the damaged end is submerged, then the whole piece at once exhibits activity.
It has already been pointed out that passivity is fugitive, and this fact has been quoted as evidence against the Oxide theory. Such evidence is not conclusive, however ; it is clear that moist conditions will accelerate the activation of passive iron, but even in the absence of moisture an oxide film of molecular dimensions as regards thickness may be anticipated to be extremely susceptible to such influences as slight variations in temperature, etc., which could result in the flaking off of the film in much the same way as hammer scale is known to peel away from iron articles.
Another method by which passivity may be destroyed is by heating the passive metal in a reducing atmosphere, and this may obviously be acclaimed as substantial evidence in favour of the Oxide and Gaseous Film theories, for in a reducing atmosphere it is equally easy to imagine the reduction of either a metallic oxide film, a gaseous oxygen film, or a gaseous oxide of nitrogen film. Galvanic activation may be _ explained in the same way, since when the passive iron is made cathodic, gaseous hydrogen will be liberated on its surface and may reduce the passivifying film as above. This process of destroying passivity does not appear to afford much support to the physical theory, except that it might be argued that if an oxidising medium is conducive to the production of that physical condition which confers passivity on the metal, then the antithesis of this, i.e., a reducing medium, may be expected to operate in the reverse direction, by restoring a normal physical condition associated with normal chemical activity.
Heathcote (J. Soc. Chem. Ind., 1907, 26, 899) threw considerable doubt on the tenability of the Gaseous Film theory by carefully conducted experiments, in which he showed that, contrary to previously recorded observations, passivity is not destroyed by high vacua. In his experiments the passive material was subjected to pressures of the order of 1/50 of a millimetre, and it is only reasonable to suppose that any film of gas on the surface of the metal would have been broken up by such treatment, especially as it was observed that the metal gave up occluded gases. Yet on removal from the apparatus the iron was still passive.
Further observations might be given to supplement the foregoing, but they would serve no further purpose than to emphasise that the cause of passivity has still to be definitely established, though it may ultimately be found that each theory is correct in that passivity may not necessarily be the result of one particular cause only, but may be induced by a number of alternative causes

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