Passivation and Its Breakdown

Chemical passivation was discovered about 200 years ago. A piece of iron placed in concentrated nitric acid was found to be passive, while the metal dissolved readily in dilute HNO3, with copious evolution of hydrogen. This type of behavior can be demonstrated in a very simple, yet quite spectacular, experiment. Nitric acid of various concentrations, from 1 mM to 70%, is introduced into a series of test tubes, and an aluminum wire is placed in each solution. No reaction is observed in the most dilute solutions. As the concentration is increased, however, hydrogen evolution becomes visible. At even higher concentrations, reduction of the acid takes place, in addition to hydrogen evolution. This is evidenced by the liberation of a brown gas, NO2, which is one of the reduction products. When the concentration has reached 35%, the reaction suddenly stops. There is no gas evolution and the surface of the metal is not attacked. Accurate measurements show no weight loss when aluminum is kept in such solutions for months. Aluminum is passivated in concentrated HNO3. A thin oxide film is formed on the surface and further attack is prevented. 

The repassivation potential is less positive than the breakdown potential. Thus, it would seem that there is a potential region where an anodic passive film cannot be formed on a bare metal surface, although an existing film is chemically and electrochemically stable. A true hysteresis of this type may indeed occur, although it has been argued that the breakdown potential and the repassivation potentials are one and the same, and the apparent difference observed between them is just a manifestation of the long induction period needed for breakdown at potentials close to Eb- 

Where will the system actually settle That depends on the initial conditions. If the metal is initially passivated (by oxidizing it chemically or by increasing the potential in the anodic direction), it can remain passivated with the corrosion potential at D. If it is initially in the active region, it can establish its corrosion potential at B. This is referred to in the literature as unstable passivation, because passivation can be lost by transition from the passive region of potential at point D to an equally stable active corrosion potential at point B. 

Line 3 represents stable passivation. The anodic and cathodic lines cross at a single point and a corrosion potential is set up at point E, well inside the passive region. However, increasing the cathodic current even more could drive the potential to point F in the trans-passive region, where corrosion and pitting could occur. 

Passive films formed in aqueous solutions usually consist of an oxide or a mixture of oxides, usually in hydrated form. The oxide formed on some metals (e.g., Al, Ti, Ta, Nb) is an electronic insulator, while on other metals it has the properties of a semiconductor. Nickel, chromium, and their alloys with iron (notably the various kinds of stainless steel) can be readily passivated and, in fact, tend to be spontaneously 

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Murato, T., Sato, E, and Okada, H.. Passivity and its Breakdown on Iron-Base Alloys, (eds H Okada ahd R. W Staehle) NACE Houston Texas (1976) ... 

Y. Hisamatsu, Passivity and Its Breakdown on Iron andiron Base Alloys, R. W. Steahle and H. Okada, eds., p. 99, National Association of Corrosion Engineers, Houston, TX, 1976. 

Evans, U.R. and Stockdale, J. J. Chem. Soc. (1929), 2651. Staehle, R.W. and Okode, H. Editors. Passivity and Its Breakdown on Iron and non-Base Alloys, U.S.A.-Japan Seminar. MACE, Houston, Texas. 1976. 

Y. Hisamatsu in Passivity and Its Breakdown on Iron and Iron-Based Alloys (Eds. R. Staehle, H. Okada), NACE International, Houston, Tex., 1976, p. 99. 

The point defect model of passivity and its breakdown is a variant of the penetration mechanism [51]. The transport of cations from the metal surface to the oxide-electrolyte interface corresponds to an inward movement of cation vacancies Vm+- This inward transport of Vm+ is supported by their high concentration at... 

B.E. Wild, Chloride ion adsorption and pit initiation on stainless steels in neutral media, in R. W. Staehle, H. Okada (Eds.), Passivity and its Breakdown in Iron Based Alloys, NACE, Houston, 1976, pp. 129-130. 

H.H. Uhlig, Passivity and its breakdown on iron and iron based alloys, in NACE USA-Japan Seminar, NACE, Hoitston, 1976. 

Proceedings of the Symposium on Passivity and Its Breakdown, P. M. Natishan, H. S. Isaacs, M. Janik-Czachor, V. A. Macagno, P. Marcns, and M. Seo, editors. Electrochemical Society, Pennington, NJ, 1997. 

Sato, N. In Passivity and its Breakdown on Iron and from Base Alloys, U.SA.-Japan Seminar, Staehle, R. Okada, H Eds. NACE International Houston, TX, 1976 p. 1. 

I. Epelboin and M. Keddam, Electrochemical techniques for studying passivity and its breakdown, Passivity of Metals (R. R Frankenthal and J. Kroger, eds.), Electrochemical Society, Pennington, NJ, 1978, p. 184. 

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. 

A great many papers have been written about the important role that solution anions play in corrosion and passivation, especially of iron. An excellent ehapter was published some years ago by Hensler in Encyclopedia of Electrochemistry of the Elements [68]. Examples of other, more recent publications are artieles by Sato [69] and Kuznetsov and Valuev [70]. The coneept of solution anions interacting with the electrode surfaee and forming surfaee-ligand eomplexes, as well as influencing the potential distribution at the surfaee, is being developed. It is becoming apparent that in order to understand the meehanism of passivation and its breakdown, it is necessary to understand both the eleetrode and the electrolyte solution and the interaction between these two eomponents of the eorrosion process. 

On the basis of these ideas, Macdonald and eo-workers [13,14] developed their model of passivity and its breakdown involving the action of vacancies within the passive layer. It is assumed that cation vacancies migrate from the oxide-electrolyte to the metal-oxide interface, whieh is equivalent to the transport of cations in the opposite direetion. If these vacaneies penetrate into the metal phase at a slower rate than their transport through the oxide, they accumulate at the metal-oxide interface and finally lead to a loeal eoneentration. The related voids lead to stresses within the passive film and its final breakdown. The inward diffusion or migration of eation vacaneies is affeeted by the incorporation of Cl ions at the oxide-electrolyte interface aceording to the following mechanism The concentration c of metal ion V, and vacancies Fq2 are determined by the equihbrium of the Schottky pair formation at the oxide-eleetrolyte interface [Eq. (3)], which causes an inverse dependenee of their eoneentrations [Eq. (4)]. 

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