The Passivation of Iron

They arc bronze and iron They are all stubbornly defiant  

In Ha-mm-urabrs Babylon iron was the next most expensive element after silver two shekels of silver cost eight of iron and 120-140 shekels of copper. Hie iron column near Delhi is more than 1500 years old, is 7.66 m in height and weighs 6 t. It consists of 99.72 % pure iron (as well as traces of C, Mn, S and P) and has retained its purity throughout the centuries. And it is symbolic that the Atomiiim built in Brussels la 1958 consists of nine iron spheres which represent the cubic body-centered lattice structure of the stable modification a-iron. 

Two glass vessels (10 x lO x 5 cm), spatula, piece of sheet iron fitted with a handle, safety glasses, proLective gloves. 

The sheet iron is immersed in the concentrated nitric acid contained in the first glass vessel, and after about 10 seconds transferred to the second glass vessel which contains the copper nitrate solution there is no visible change at the sur lace. If however the iron surface is damaged, for example by scratching it with the spatula, a layer of copper rapidly covers the surface. The same process also occurs when the iron is treated first with diluie nitric acid and then with the copper nitrate solution. Tlie passivation process can be repeated several limes. 

The treatment of the iron with concentrated nitric acid HNOj (or chromic acid, H2Cr04) leads to the fonnatjon of an oxide layer which covers the whole surface of the metal this layer prevents the diffusion of oxygen and moisture from the air into the iron lattice. However, if the surface is untreated, corrosion sets in. This is greatly favored by the high cell voltage which is built up between the base metal iron and the hydrated copper ions Cu or when the iron is treated with dilute acids (II3CV ions) as a result Fe ions go into solution. 

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Figure 60. Christian Friedrich Schonbein (1799-1868). (Courtesy E. Berl.) Discovered guncotton, 1846. Discovered ozone, worked on hydrogen peroxide, auto-oxidation, the passivity of iron, hydrosulfites, catalysts, and prussic acid. Professor of Chemistry at Basel from 1829 until the time of his death. He published more than 300 papers on chemical subjects. Reproduced from original in Kekule s portrait album.

H. Beyers and M. Darrin have studied the passivity of iron in phosphoric acid ... 

Much of our knowledge of passivity is concerned with the passivity of iron. The reason of course is that this is the most important layer technologically. However, there is an increasing amount of work being done on other passive layers (for Al, see Section 12.3) and on Cr and Ti. For details of the structure on Cr and T, see the further reading sections. 

Bergmann (Opuscula Chimica et Physica, 1779-88, 3, 140. See also de Benneville, J. Iron Steel hist, 1897, II., 40. Senderens (Bull Soc. chim., 1896, 15, 691 1897, 17, 279) had already observed, prior to the discovery by K.eir of the passivity of iron, that this metal does not precipitate silver from concentrated aqueous solutions of silver mtrate. 

W. Hittorf concluded that the passivity of chromium is not due to the formation of an oxide film, but rather to the metal assuming a different electrical. state the metal in the passive state is in a strained or coerced condition— Zwangzustand—so that instead of dissolving as a bivalent element it dissolves as a sexivalent element. The film hypothesis, discussed in connection with the passivity of iron, best fits the facts. C. W. Bennett and W. 8. Burnham stated that the film is best regarded as a film of oxide which is rendered stable by adsorption into the metal. The oxide is usually unstable, but becomes stable when adsorbed by the... 

Other Non-Electrochemical Methods. B. Cahan(56) is examining the passivation of iron and ferrous alloys by a... 

Ferritic stainless steels exhibit IGSCC in hot nitrate, caustic, carbonate, and other environments. The phenomenon is potential dependent as discussed above. Susceptibility has been attributed to carbon and phosphorus segregation [94, 95]. Levels as low as 2 to 3 at. % can alter the passivity of iron in hot nitrate... 

R. Frankenthal, On the passivity of iron-chromium alloys I. Reversible primary passivation and secondary film formation, J. Electrochem. Soc. 114 (1967) 542—547. 

The application of the technique in the time and frequency domains is illustrated hereafter for the passivation of iron in acidic solutions. 

R. Kirchheim. B. Heine, H. Fischmeister, S. Hofman, H. Knote, and U. Stolz, The passivity of iron-chromium alloys, Corros. Sci. 29.S99, (1989). 

Interest in passivity started with the studies of Faraday [1] and Schonbein [2] over 150 years ago. The lack of metallic corrosion in the case of iron immersed in certain solutions was attributed to either the presence of an oxide film or an electronic change in the metal. This basic argument has persisted in various forms to this day, although the majority of scientific evidence suggests protection by a three-dimensional oxide film. Much has been published on passivity and its breakdown over the last 50 years. This chapter does not attempt to cover all the literature but concentrates on work over the past 10-15 years, emphasizing the passivity of iron, nickel, iron-chromium, and iron-nickel alloys in aqueous environments. Examples are given fi om the authors and other selected laboratories. 

The strong influence of nonhalide anions on the passivation of iron is illustrated by experiments in which borate was added to sulfate solution. In pure sulfate, passivation occurs only after formation of a salt film, which requires the passage of considerable anodic charge. In borate solution, as mentioned earlier, the passive film forms with essentially 100% current efficiency, the addition of borate to sulfate Copyright 2002 Marcel Dekker, Inc. 

Figure 32. The Mott-Schottky type plot of capacitance (C vs. E) and potential modulation reflectance (PMR), [(d/ /d )(l/f o)] vs. E for the iron electrode covered by passive oxide formed at 1.55 V vs. RHE. The PMR measurement was done at frequency of 500 Hz by light wavelength for 350 nm. Reprint from D. J. Wheeler, B. D. Cahan, C. T. Chen, and E. Yeager, Optical Study of the Passivation of Iron , in Passivity of Metals, Ed. by R. P. Frankenthal and J. Kruger, The Electrochem. Soc. Inc., Pronceton, 1978, p. 546, Copyright 1978 with permission from The Electrochemical Soc.

Passivity has been known for several hundred years. Uhlig mentioned in his review on passivity that Lomonosov was the first in 1738 to detect that iron does not dissolve in concentrated nitric acid [3]. Ostwald described in his history of electrochemistry [4] that Keir observed in 1790 the passivity of iron again in concentrated nitric acid [5]. Similar observations were made in 1782 by Wenzel according to citations of Gmelin [6. In 1807, Hiesinger and Berzelius found that one could achieve passivity by anodic polarization [6], which was confirmed by Schonbein for iron in diluted nitric, sulfuric, and phosphoric acids [7], In their correspondence, Schonbein and Faraday discussed the nature of these observations and Faraday came to the conclusion that a thin film provides protection to the metal. He postulated its electronic conductivity based on his own experiments [8]. 

This section presents a detailed discussion of the passivity of iron. It is intended as an example of the procedure and strategy in which the results of several electrochemical and surface analytical methods can be combined to get to a clear view and a sound model of the nature of passivity. For other pure metals and alloys, the same methods have been applied. However, the results for the other systems will be presented in a more condensed form to avoid redundancies and to keep the chapter sufficiently short. References to a more detailed description in literature are given. 

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