Passive Oxide Film on Iron

Because of the very limited escape depths of conversion electrons (about 1.8 pm in water, 0.25 pm in metallic iron), their detection is somewhat difficult. This seeming drawback provides a unique surface sensitivity. In a rotating disc electrode arrangement Kordesch et al. [539] have used a disc-shaped electrode that slowly rotates with part of the disc immersed in the electrolyte solution. As a thin electrolyte film thin enough to permit escape of conversion electrons adheres to the metal surface, potential control is always maintained. Conversion electrons were detected using a suitable gas-filled detector mounted close to the upper emersed part of the disc. In a study of passive oxide films on iron, the advantage of this approach was demonstrated beyond an unmatched surface sensitivity, the measurement time was reduced to a small fraction of that needed for transmission measurements [543]. An inherent drawback of the setup is the poor current distribution inside the very thin electrolyte film (its thickness is around 4 nm as reported by Gordon [540]). 

Passive Oxide Films on Iron by In-Situ Detection of Optical Techniques... 

Thus inhibitive anions can retard the dissolution of both the T-FejO, and the magnetite layers of the passivating oxide layer on iron. This has the dual effect of preventing breakdown of an existing oxide film and also of facilitating the formation of a passivating oxide film on an active iron surface, as discussed in the previous section. 

Dissimilar metals. Galvanic corrosion occurs when two metals with different electrochemical potentials are in contact in the same solution [Figures 6.7 and 6.8]. In both cases, the corrosion of iron (steel) is exothermic and the cathodic reaction is controlling the corrosion rate. The more noble metal, copper increases the corrosion through cathodic reaction of hydrogen ion reduction and hydrogen evolution A passive oxide film on stainless steel for example can accelerate hydrogen reduction reaction. 

FIGURE 22.23 Thickness of passive oxide films on metallic iron in 0.15 kmol m 3 phosphoric acid solution and on metallic titanium in 0.1 kmol m-3 sulfuric acid solution as a function of electrode potential [30-32] L = film thickness, iFe = iron dissolution current, iTi — titanium dissolution current, and ia = anodic total current. 

Equation (4.7) corresponds to the potential variation of a metal electrode of the second kind as a function of pH. The Flade potential is used to evaluate the conditions for passive film formation and to determine the stabihty of the passive film. The reversible Flade potential of three important engineering materials is approximately +0.63 V for iron, +0.2 V for nickel, and —0.2 V for chromium [7,8]. The negative value of the Flade potential for chromium (—0.2 V) indicates that chromium has favorable Gibbs free-energy for the formation of passive oxide film on its surface. The oxide film is formed at much lower potentials than in other engineering materials. 

While the same basic mechanisms for passivity of pure metals also applies to alloys, the processes involved in the passivation of alloys have an added complexity. In many cases only one component of the alloy has the property of being passive in a particular environment. Alloys such as steiinless steels, which contain highly passive components (chromium in this case), owe their corrosion resistance to the surface enrichment of the passivating component Thus stainless steels resist corrosion in many acidic systems (where iron or carbon steel would be poorly passive or not passive at all) by a passivating oxide film containing Cr predominantly as Cr(III). Surface analytical techniques such as Auger electron and X-ray photoelectron spectroscopies reveal substantial enrichment of chromium in the passivating oxide film on these alloys " . There are only two ways by which this enrichment can... 

The passive film on nickel can be formed quite readily in contrast to the formation of the passive film on iron. Differences in the nature of the oxide film on iron and nickel are responsible for this phenomenom. The film thickness on nickel is between 0.9 and 1.2 mm, whereas the iron oxide film is between 1 and 4 mm. There are two theories as to what the passive film on nickel is. It is entirely NiO with a small amoimt of nonstoichiometry, giving rise to Ni cation vacancies, or it consists of an inner layer of NiO and an outer layer of anhydrous Ni(OH)2. The passive oxide film on nickel, once formed, cannot be easily removed by either cathodic treatment or chemical dissolution. 

Although there is general agreement today that anodic passivity of metals such as iron and nickel is associated with the formation of a three-dimensional oxide film on the surface and that breakdown of passivity is due to the disappearance of this protective film either locally or generally, there is still considerable controversy concerning the nature, composition, and structure of the passive film. Here the most prominent models for passivity will be presented and the nature of the passive oxide film on common metals such as iron and nickel will be discussed. 

As indicated earher, a clear understanding of the natirre, role, and stability of the passive oxide film on pitre iron and nickel is not easy to achieve. The difficulties are Copyright 2002 Marcel Dekker, Inc. 

The iron oxides have also been studied thoroughly by XPS [51]. From the XPS peaks obtained after deconvolution of Fe(2p.v2) and Fe(2pi/2) core level spectra, the identify of the passive oxide films on the mild steel surfaces was determined. Detailed results are listed in Table 31.12 and illustrated in Figs. 31.35 and 31.36. 

Figure 15. Steady current density in the passive potential region as a function of solution pH. The cd reached the stationary value in the solution at pH lower than 5 in 1 h oxidation at each potential, however, it does not reach at pH higher than 5 in which the cd was plotted after Ih oxidation. Reprint from N. Sato and T. Noda, Ion Migration in Anodic Barrier Oxide Films on Iron in Acidic Phosphate Solutions , Electrochim. Acta, 22 (1977) 839, Copyright 1977 with permission from Elsevier Science.

Figure 23. Thickness vs. time during the passive oxide growth for 900 s at 0.80 V vs. Ag/AgCk/ Sat. KCl in pH 8.4 horate solution containing Fe ions at concentration of 0.0, 0.25 and 0.50 mM. Reprint from T. Ohtsuka and H. Yamada, Effect of Ferrous Ion in Solution on the Formation of Anodic Oxide Film on Iron , Corrosion Sci., 40 (1998) 1131, Copyright 1998 with permission from Elsevier Science.

This effect of Fe concentration indicates that when iron is passivated by an anodic potential sweep from the active potential region, the thick hydrous oxide layer would be formed by the anodic deposition of Fe dissolved from the iron in the active potentials. Much thicker oxide films on iron have been reported by various authors in neutral solutions. The origin of the thick films may be Fe accumulated in the solutions. 

On the other hand, pit initiation which is the necessary precursor to propagation, is less well understood but is probably far more dependent on metallurgical structure. A detailed discussion of pit initiation is beyond the scope of this section. The two most widely accepted models are, however, as follows. Heine, etal. suggest that pit initiation on aluminium alloys occurs when chloride ions penetrate the passive oxide film by diffusion via lattice defects. McBee and Kruger indicate that this mechanism may also be applicable to pit initiation on iron. On the other hand, Evans has suggested that a pit initiates at a point on the surface where the rate of metal dissolution is momentarily high, with the result that more aggressive anions... 

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