THE FORMATION AND PROPERTIES OF PASSIVE FILMS ON IRON1

Transcription

1 THE FORMATON AND PROPERTES OF PASSVE FLMS ON RON1 ABSTRACT A unified mechanism for the formation of passive films on iron in aqueous solutions is presented. The effects of water, oxygen, and oxidizing and non-oxidizing ions are considered. The?-Fez03 film is formed first by the oxidation of water-formed magnetite and further thickening of the film takes place by the oxidation of diffusing Fe++ ion at the water surface of the oxide film. The main force leading to diffusion is the field set up by the adsorbed negative ion and the positive Fef+ ions at the surface of the metal. Some of the properties of the protective?-fez03 film and factors leading to its destruction are discussed. When iron is exposed to aqueous solutions containing certain oxidizing ions such as chromate or nitrite, or buffering agents and oxygen, it becomes passive. Electron diffraction measurements and film stripping experiments indicate that the iron is covered by a thin film, composed mainly of y-fe203 (1, 2). Although much has been published concerning the behavior and possible physical and chemical structure of these passive films on iron (3, 4, 5), very little data has been presented or possible mechanisms proposed to account for the formation of the films. Since the behavior of passive films on iron is much the same, independent of the solution in which they are formed, there is probably a common general mechanism of formation. n this paper such a mechanism for the formation process and composition of passive films is developed and used to explain the build-up and break-down of the films. 1. Reactions of ron with Water (a) n the Absence of Oxygen This reaction has been reviewed recently by V. J. Linnenbom (6). At room temperature he found evidence for ferrous hydroxide only. At higher temperatures some Fe304 was also found. n earlier work Schikorr (7) reports that Fe(OH)2 converted slowly to magnetite at room temperature but this was not confirmed by Linnenbom. t is thermodynamically possible for iron to react with water to form either Fe(0H)p or Fe304. When film-free iron is exposed to water, in all probability a film is formed which separates the iron from the water. f this film is formed by a heterogeneous reaction between the iron and the water then the composition of the film will probably approximate that of an anhydrous oxide. FeO is not stable below a temperature of 500" C, so that the most likely film is Fe304. This persists as a very thin film and the iron remains bright and silvery in appearance. The rate of reaction of iron with oxygen-free water is very slow, of the order of 20 pg/cm2 in 5 days (3). This rate is about the same in the presence of chloride ion. The reaction rate corresponds to the reaction of 56 A of metal/day or a current of 2X1OP1/ pa cm2. The potential of the specimen, which corresponds to the equilibrium potential of iron in a saturated solution of Fe(OH)?, inclicates high hydrogen polarization. The mechanism of the reaction is probably that of diffusion of both Fe++ and electrons through a thin magnetite film of the order of 1 or 2 unit cells (8-1G A) thicli with the 'il~an~~script received Jzdy 10, Co?ztributionfrotn the Divisio?~ of Applied Chewristry, Nafiotznl Rcsearcl~ Co~~ncil, Ottrrzua. Tlzis paher was presented (11 the Synzposizinz on Clzarge Transfer Processes l~dd at the li?~iversity of Toronto, Toro?zto, Ontario, Septe?,zber 4 and 6, ssued as N.R.C. No. &272. Can. J. Chem. Vol. 37 (1959)

2 COHEN: PASSVE FLMS ON RON 287 rate of the reaction being controlled by the rate of formation of hydrogen. The very thin film of magnetite acts as a separator rather than a diffusion barrier and is stabilized by being in contact with the metal. The reactions can be expressed as: Fe -+ Fe++ + 2e, The rapidly formed magnetite film is formed by the reaction: The magnetite thicltens and becomes visible at higher temperatures or by the addition of certain other chemicals-that is when the rate of the reaction of Fe Fe301 in contrast to Fe Fe++ hydrated is increased. A diagram of the reactions of iron in oxygen-free water is shown in Fig.. FG. 1. Developnle~lt of steady-state corrosion of iron in water. X thin filrn of magnetite is formed by direct reaction with water. Reaction continues by diffusion of ferrous ion into the solution and hydrogen formation. (b) Absence of Oxygen but Presence of Oxidizing Anions Four oxidizing ions-namely nitrite, chromate, molybdate, and tungstate-have been found to form films on iron in oxygen-free water. Of these, chromate forms a passive film at the lowest concentration. This film is of the order of thick and contains both 7-Fe2Oa and CrzO3 (8). With nitrite solutions, at low concentrations thick films of Fe304 are formed. When the conce~ltratio~l of nitrite is increased to about 0.1 N a thin film of y-fe203, possibly containing some yfeo(oh), is formed (). Of the four oxidizing inhibitors the strongest oxidizing agent with respect to ferrous ion in neutral solution is chromate. The others react relatively slowly. All four inhibitors are poor cathodic depolarizers: nitrite ion is not reduced when exposed to cathodically polarized iron, while the reaction rate of iron with water, which is controlled by cathodic clepolarization, is the same irrespective of the presence of low concentrations of molybdate and tungstate (3). As in the presence of water alone, the instantaneous surface reaction of film-free iron is a heterogeneous one to form Fe30e. Adsorption of the oxidizing anions on the films during the formation of the film leads to further oxidation to y-fez03. n the case of the chromate ion, where the reduction product is insoluble Cr203, this leads to the build-up of a film co~ltaining both CraOa and y-fez03 in a ratio of about 1: 3. The amount 1 of chromium found in the film would correspond to a film about A thick (8). n the case of nitrite the reduction products are soluble and at sufficiently high con- centrations a film consisting mainly of 7-Fez03 is formed. At low co~lcentrations (0.001 iv) magnetite films of sufficient thickness to be visible are formed (9).

3 288 CANADAN JOURNAL OF CHEMSTRY. VOL. 37, 1959 With molybdate and tungstate visible magnetite films are also formed. However, if the iron is made anodic, y-fe203 films of a protective nature are formed. This may be due to an increase in oxidation rate by the discharge of hydroxyl ion to form oxygen and/ or discharge of the anions at the metal oxide - solution interface. This order of efficiency of the formation of passive films is probably related to the oxidizing power of the anions with respect to the oxidation of Fe++ to Fe+++. The reaction of chromate ion with ferrous ion is very rapid, while that of nitrite, molybdate, and tungstate is relatively slow. The y-fez03 which forms the main part of all the passive films is formed as the film grows. The first formed unit cell of Fe304 is oxidized to y-fe203 and further diffusion of Fe++ takes place through this y-fe203 film. The ferrous ion which reaches the oxidesolution interface is oxidized to ferric ion by the adsorbed oxidizing agent, thus leading to further thickening of the film. Although there is no direct contact between the iron and the solution the iron is oxidized to Fe++ by water in that the electrons are neutralized by H+. The main force responsible for the diffusion of Fe++ through the y-fe203 is supplied by the electrostatic attraction between the adsorbed negative anion and positive metal ions in the lattice. This force is sufficient to allow a film of about A thick to form. The geometry of these reactions is shown in Fig. 2. FG. 2. Conversion of FesOn to r-fep03 and further thickening of r-fe203 to attractive force between the anion and Fe++ depends on charge and thickness. form passive film. The f the concentration of the passivating oxidizing agent is too low, oxidation of the ferrous ion takes place to magnetite only and a proportion of ferrous ion escapes into the solution. The magnetite film thickens considerably, probably because the cationic diffusion rate through magnetite is greater than that through y-fe203. A number of oxidizing anions such as permanganate do not form passive films on iron. This could be due to either the physical nature of the reduction product and/or their ability to depolarize the cathodic reaction. The former is probably the more important in that a gelatinous reduction product interferes with the growth of the protective passive film. Because of depolarization of the hydrogen formation reaction some oxidizing anions may act as corrosion accelerators rather than passivating agents. (c) Presence of Oxygen ron reacts rapidly with dry oxygen at room temperature to form a film of y-fe A thick (10). At this thickness the reaction essentially ceases. f iron is exposed to water saturated with oxygen at normal pressures and temperatures the reaction continues at a relatively constant rate, the main products of the reaction being hydrated ferric oxides. f the specimen becomes covered with hydrated reaction product, so that there is a decrease in the oxygen concentration at the oxide-precipitate interface, visible films of magnetite are formed. The corrosion rate is very much higher than the rate in oxygen-free water, being of the order of 1.3 mg/cm2 5 days or 13 pa/cm2, or 65 times the rate in oxygen-free water.

4 COHEN: PASSVE FLMS ON RON 289 The initial reaction is the formation of a thin layer of magnetite as in the case of oxygenfree water. This thin layer of magnetite can then be oxidized to y-fe203 by reaction with adsorbed oxygen. At room temperature and pressure the solubility of oxygen in water is only 8 parts per million and owing to incomplete coverage by oxygen both the rate of oxidation to y-fez03 and further thickening of the y-fez03 film is slow. Fe++ ions continue to diffuse through the oxide into the solution where they are oxidized by dissolved oxygen to FeO(0H). These hydrated oxides are precipitated on the metal surface and form a diffusion barrier for the movement of oxygen to the metal surface. As in the case of the oxidizing anions, the lowered concentration of oxygen at the metal oxide - solution interface allows further growth of magnetite but does not oxidize the magnetite to y-fezo3. The precipitate, being laid dowil uneveilly, will tend to allow the formation of separate anodic and cathodic areas on the surface and the over-all corrosion current is sufficiently high and the ph sufficiently low to cathodically reduce any already formed y-fezoa (11). The above sequence of formation of Fe304, etc., will then be repeated. As well as oxidizing magnetite to y-fez03 the oxygen can also react with (e + H+) to form OH- ions. This both accelerates the rate of corrosion and, at the normally low concentrations of oxygen in the solution, decreases the per cent of the surface area covered by oxygen. This lowered coverage leads to a decreased probability of the formation of a protective y-fez03 film. At sufficiently high concentrations of oxygen it is possible to obtain protective passive 1 films. This is related to a more rapid replenishment of the cathodically removed oxygen which leads to y-fez03 film growth rather than corrosion. The formation of a protective film thicker than A may be due in part to the field set up by the adsorption of some OH- ions. (d) Presence of Oxygen and Oxidizing on n general the presence of oxygen decreases the concentration of oxidizing anion required to give passivity. Because of the presence of the anion the film still thickens to A. The main function of the oxygen in the case of the nitrite, molybdate, and tungstate is probably the oxidation to y-fez03 of some of the magnetite formed by the oxidation of ferrous ion by the anion. (e) Presence of Oxygen and Non-oxidizing Buffering on ron is passive in air-saturated solutions containing relatively high concentrations of alkaline buffering agents such as NazC03, Na3P04, and C6HjCOONa. The passive film is mainly composed of y-fez03 with some hydrated oxide, and in the case of the phosphate, hydrated ferric phosphate (12). Rost of the hydrated material appears to be located in rather thick inclusions which protrude above the oxide film (9). The salts would appear to serve two purposes. Firstly adsorption of the negative ion, by increasing the field strength across the oxide, leads to a sufficient thickening of the y-fez03 film formed by the reaction of Oz with Fe304 to prevent further diffusion of iron ions. Secondly, where the film has not thickened, so that Fe++ ions continue to escape into the solution, the anodic pores are kept on the basic side. This decreases the rate of cathodic reduction and consequent thinning of the adjacent oxide. Since diffusion of ferrous ions through the 1! porous precipitated plug of hydrated material is probably more rapid than through the i oxide the plugs grow relatively thick before the reaction stops. 2. Reactions in the Presence of the Air-formed Film As mentioned above, the air-formed film is y-fez03 of the order of 20 A thick. n the presence of the' passivating agent this film thickens, by the formation of more y-fez03

5 290 CANADAN JOURNAL OF CHEMSTRY. VOL. 37, 1959 and possibly other hydrated products. Under borderline passivating conditions some of the film inay be removed by cathodic reduction ancl either new film or plugs are formed in the manner outlined above. 3. Effect of an Applied Cz~rrent f the iron is made cathodic the y-fe203 film is reduced. At low cathodic current cleilsities corrosion continues, probably through a layer of magnetite. At higher cathodic currents corrosion essentially ceases, probably with sone redeposition of iron. At low anodic currents the corrosio~l of iron is increased. At higher current densities passivity is often observed. This can be explained by a combination of the suppression of the cathodic reaction with its possibility of cathodic reduction of the y-fezos and the direct discharge of oxygen on the metal surface. Because of the high overvoltage for oxygen evolution the equivalent pressure of oxygen at the metal surface is high and conditions for the formation of 7-Fe,Os favorable. Oxidizing ions such as tungstate and molybdate will also be discharged at the anodic areas with a similar effect of apparent increased activity. 4. Properties of the Passive Film Passive filins have been stripped from iron by both chemical (13) and electrochemical (14) dissolution of the underlying metal. They are found to be mainly anhydrous y-fe3o3. They are chemically inert and dissolve in acid only with difficulty. The hydrated parts of the film are less inert than the anhydrous oxide. The film may be cathodically reduced to Fe++ ion which, at neutral and low ph's, is quite soluble. The film is a relatively good electronic conductor but a poor cationic conductor and the growth of the film appears to be controlled by the rate of cationic conduction. The brealcdown of the passive film probably takes place through the brealcdown of pores in the anhydrous oxide film. Electron microscope and diffractiotl studies indicate that most of the passive films have discontinuities in the anhydrous oxide filins which contain the less inert hydrated ferric oxides or other insoluble ferric salts. Diffusion of the solution through these rather porous precipitated plugs leads to the formation of acidity in the pores-particularly in the presence of such ions as chloride or sulphate. The ferric salts can be removed by the formation of soluble complexes with many ions. The combination of anodic current ancl acid conditioils leads to undermining or cathodic reduction of the protective y-fe2oa film. t is rather surprising that there is such a large difference in the cationic diffusion properties of magnetite and y-fe20a as indicated by possible thicknesses of the two types of film. t is generally assumed that y-fe20a has essentially the FeaO4 structure with cation vacancies (15). The diffraction patterns of the two oxides are very similar. The two oxides, however, behave quite differently chemically. Fe304 dissolves in acid very easily, while y-fe20, dissolves oilly with difficulty. On cathodic reductioii y-fez03 is reduced completely to the ferrous state with high current efficiency, while magnetite is reduced with difficulty and very low current efficiency. n some X-ray diffraction studies in this laboratory it has been found that y-fezoa produced in several ways has d-values greater than 6 A as well as two lines which do not appear in the Fe304 patterns. This would point to a somewhat different kind of ordered structure and may account for the different chemical and diffusional behaviors of the two oxides. This would also indicate that some recrystallization takes place in the conversion of Fe304 to y-fezos. A second factor which may contribute to a difference in the growth rate and hence final thickness of the two oxides could be a difference in the rate of transfer of Fe++ ions

6 COEN: PASSVE FLMS ON RON 291 across the metal-oxide interface. This would be partly related to a difference in the crystallographic arrangement of the oxides on the metal. Thin films of Fe304 show preferred orientation on iron, while y-fe20a films do not. The bulk of evidence shows that y-fe203, the main constituent of the passive film, is an inert material which in passivating solutions grows to a maximum thickness of A. REFERENCES 1. COHEN, M. J. Phys. Chem. 56, 451 (1952). 2. MAYNE, J. E. 0. and PRYOR, M. J. J. Chem. Soc (1949). 3. PRYOR, M. J. and COHEN, M. J. Electrochem. Soc. 100, 203 (1953). 4. WADE, W. H. and HACKERMAN, N. Trans. Faraday Soc. 53, 1636 (1957). 5. HOAR, T. P. and EVANS, U. R. J. Electrochem. Soc. 99, 212 (1952). 6. LNNENBOM, V. J. J. Electrochem. Soc. 105, 322 (1958). 7. SCHKORR, G. Z. anorg. u. allgem. Chem. 212, 33 (1933). 8. COHEN, M. and BECK, A. F. Z. Elel;trochem. (n press). 9. MELLORS, G. W., COHEN, M., and BECK, A. F. J. Electrochem. Soc. 105, 332 (1958). 10. GULBRANSEN, E. A. J. Electrochem. Soc. 81, 327, OSWN, H. G. and COHEN, M. J. Electrochem. Soc. 104, 9 (1957). 12. PRYOR, M. J., COHEN, M., and BROWN, F. J. Electrochem. Soc. 99, 542 (1952). 13. VERNON, W. H. J., WORMWELL, F., and NURSE, J. T. J. Chern. Soc. 621 (1939). 14. EVANS, U. R. Nature, 126, 130 (1930). 15. WELLS, A. F. Structural inorganic chemistry. Oxford University Press, London p. 381.