Passive Film on Iron

It is known that thin (-20 A) passive films form on iron, nickel, chromium, and other metals. In s ressive environments, these films provide excellent corrosion protection to the underlying metal. The structure and composition of passive films on iron have been investigated through iron K-edge EXAFS obtained under a variety of conditions, yet there is still some controversy about the exact nature of... 

The use of surface EXAFS in the study of passive films represents a natural application of the technique and, in fact, the studies by Kruger and co-workers70 73 on the passive film on iron represent the first reported. 

These results point to the importance of hydration effects on the structure of passive films on iron. However, these results were obtained ex situ and therefore are subject to some uncertainty. 

Hoffman and Kordesch74,75 have presented a series of studies on the passive films on iron with particular attention to cell design. They have employed a so-called bag cell that allows for the in situ passivation and/or cathodic protection of the iron films. These were deposited onto gold films deposited on Melinex. 

Oxide Formation on Metals The Nature of the Passive Film on Iron d 3.4.2—Conducting Polymers... 

Oxide formation on metals the nature of the passive film on iron... 

Schematic representation of the hydrated passive film on iron (From Pou et at., 1984)...

Larramona, G. Gutierrez, C. (1989) The passive film on iron at pH 1-14. A potential-modulated reflectance study. J. Electrochem. 

Sato, N. Kudo, K. Nishimira, R. (1976) Depth analysis of the passive film on iron in neutral borate solution. J. Electrochem. Soc. 123 1419-1423... 

Figure 1. A species graph (or diagram) for the formation of passive films on iron species are at nodes (not explicitly shown), reaction steps and flow are represented by directed edges. Reproduced from Ref. 3, Copyright (2001), by permission of The Electrochemical Society.

Fig. 12.65. A polymeric film of hydrate iron oxide consistent with Mossbauer spectra of passive film on iron.

Bonhoeffer, Vetter, and others (63) have made extensive studies on iron which indicate that the passive film is composed of one or more oxides of iron. Young (64). Vermilyea (65) and Johansen et at, (57) have shown that the Mott-Cabrera concepts are applicable for the thin films on Ta, Ti, Hf, and Hb. Petrocelli (58) has shown evidence that the dissolution of aluminum In sulfuric acid takes place through a thin film and that the process appears to follow the Motr-Cabret a theory. Stern (66) reports data indicating that the kinetic. for the anodic oxidation of stainless steel are similar to those for aluminum apd tantalum (67). Pryor (68) has recently reviewed the work on passive films on iron and suggests a single passive film of y contains non-uniform defect concentra-... 

Corrosion and Passivity. The inhibition of dissolution is important m the corrosion of metals and building materials. Passivity is imparted to many metals by overlying oxides, the so-called passive films the inhibition of the resolution of these passive layers protects the underlying material. Figure 13 eves a schematic model of the hydrated passive film on iron. 

The use of surface EXAFS in the study of passive films represents a natural application of the technique, and in fact, the studies by Kruger and co-workers on the passive film on iron represent the first reported. In their first studies, they employed vacuum-deposited iron films on glass slides and subsequently oxidized the films in either nitrite or chromate solution. They obtained the EXAFS spectra for the oxidized films, employing a photocathode ionization chamber (detecting the emitted electron current) and compared these with spectra for y-FeO(OH), y-Fe203, and Fc304. Although these studies were not in-situ, they did not require evacuation of... 

Hoffman and co-workers have carried out a series of studies on the passive films on iron, with particular attention to cell design. They have employed a so-called bag cell that allows for the in-situ passivation or cathodic protection of the iron films which were deposited onto gold films deposited on melinex (polymer film with excellent adhesive properties). In addition, they employed a setup in which the working electrode is partially immersed in solution and continuously rotated. In this way, they could expose the electrode to the x-ray beam with ostensibly only a very thin film of electrolyte. Under these conditions, they were able to obtain spectra of the film as prepared, a cathodically protected film as well as a film passivated in borate solution at 1.3 V. From an analysis of their data, they concluded that the passive film had an Fe—O coordination with 6 near neighbors at a distance of 2 + 0.1 A. The approach followed by these authors appears most appropriate since they were able to reduce the deposited films... 

Froment and co-workers " have employed REFLEXAFS (vide supra) for studying passive films on iron and nickel. Their early studies were concerned with demonstrating the applicability of the REFLEXAFS technique to electrochemical systems. Most recently, they have used this technique to study the structure of passive films on Ni and on Ni-Mo alloy electrodes. For the Ni electrodes, they performed studies after reduction at — 700 mV (vs. saturated mercurous sulphate electrode) as well as in the passive (-l-3(X)mV) and transpassive (-1-800 mV) regions. The Fourier transforms for the films in the passive region have a Ni—O peak at a distance that corresponds closely to that in bulk nickel oxide. However, no Ni-Ni interactions were observed. These investigators interpreted these results as consistent with a model that postulates an amorphous hydrated polymeric oxide. ... 

C.L. McBee and J. Kruger, Nature of Passive Films on Iron-Chromium Alloys, Electrochim. Acta, Vol 17, 1972, p 1337-1341... 

M. Janik-Czachor, AES Investigations of Passive Films on Iron and Iron Base Alloys, Corrosion, Vol 35, 1979, p 360-366... 

An early transfer system in which the cell is connected directly to the UHV chamber was described by Revie et al. [16]. The analysis chamber and the cell, both of glass, were joined by a straight-through UHV valve permitting the sample to be transferred vertically up from the cell using a magnetically operated windlass. This was used for AES analysis of passive films on iron and it does not seem to be readily extendable to use with well-defined electrodes. 

The kinetics of film growth can be studied in this manner without removing the electrode from the cell or interrupting the electrolysis. Figure 17.1.15 contains data showing three regimes of growth kinetics for the formation of a passive film on iron (42). 

Figure 17.1.15 Growth of passive film on iron at 0.8 V V5. SCE. [From J. Kruger and J. P. Calvert,...

As an alternative approach, gold particles have been deposited on iron electrodes in studies of passive films on iron [470]. A layered structure was found. The inner layer is composed mostly of Fe304 the outer layer contains Fe(III) species and may also contain y-Fc203. 

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