Role of Oxide Films

The oxide film which helped create the cathodic areas in the first place may be destroyed by reductive dissolution, at least in the case of iron, because of the excess of electrons present at cathodic sites. This is possible for iron because of the accessibility of the iron(II) oxidation state. The extreme insolubility (and hence protective capacity) of the oxide film derives from its iron(III) content, but iron(ll) hydroxide has a significant solubility in neutral water, as noted above, and of course is readily soluble in acidic media. 

Equation 16.9 tells us that the protective oxide film on iron will be preserved in alkaline media, weakened in neutral water, and lost in acidic environments. Indeed, in very acidic solutions, the distinction between extended anodic and cathodic sites will be lost along with the oxide film, although local anodic and cathodic spots will persist, and so dissolution of iron with accompanying hydrogen evolution becomes general across the surface of the specimen. 

At the other extreme, the oxide layers on aluminum, beryllium, titanium, vanadium, chromium, nickel, and tantalum are very insoluble in water at intermediate pH values and do not have easily accessible reduced states with higher solubility. The oxide films on those metals are therefore highly protective against aqueous corrosion. 

Aluminum, for example, is a very reactive metal if it is freed of its oxide film  

There is no stable entity AP (aq) to compare with Fe +(aq) consequently, the mechanism that causes rust to be nonprotective because of migration of Fe +(aq) through the water before precipitation as FeO(OH) does not apply to aluminum, on which A1(0H)3 or AIO(OH) forms, at once, on the anodic site. Conversely, removal of the protective aluminum oxide film cannot occur by the reductive dissolution mechanism described for iron. 

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Biocorrosion of stainless steel is caused by exopolymer-producing bacteria. It can be shown that Fe is accumulated in the biofilm [2.62]. The effect of bacteria on the corrosion behavior of the Mo metal surface has also been investigated by XPS [2.63]. These last two investigations indicate a new field of research in which XPS can be employed successfully. XPS has also been used to study the corrosion of glasses [2.64], of polymer coatings on steel [2.65], of tooth-filling materials [2.66], and to investigate the role of surface hydroxyls of oxide films on metal [2.67] or other passive films. 

For this purpose, the authors used a special vacuum cell with a controlled focused electron beam incident on a zinc oxide film target. In these experiments, the role of the film was twofold. It served as an adsorbent and as a high-sensitivity detector of hydrogen atoms (10 at/cm ). Hydrogein atoms were produced due to surface dissociation of adsorbed molecular hydrogen. This process was induced by heating or bombardment of the adsorbed layer by an electron beam. 

While considering trends in further investigations, one has to pay special attention to the effect of electroreflection. So far, this effect has been used to obtain information on the structure of the near-the-surface region of a semiconductor, but the electroreflection method makes it possible, in principle, to study electrode reactions, adsorption, and the properties of thin surface layers. Let us note in this respect an important role of objects with semiconducting properties for electrochemistry and photoelectrochemistry as a whole. Here we mean oxide and other films, polylayers of adsorbed organic substances, and other materials on the surface of metallic electrodes. Anomalies in the electrochemical behavior of such systems are frequently explained by their semiconductor nature. Yet, there is a barrier between electrochemistry and photoelectrochemistry of crystalline semiconductors with electronic conductivity, on the one hand, and electrochemistry of oxide films, which usually are amorphous and have appreciable ionic conductivity, on the other hand. To overcome this barrier is the task of further investigations. 

Several reviews addressing the polarization behavior, d ion adsorption, competition between Cr adsorption and OH codeposition, oxide film formation, and cr ion discharge, as well as the kinetic aspects of the reaction on various oxide-covered and oxide-free surfaces that have been investigated during the past 15 years, have been published (55/, 333-338). Of these, particular mention should be made of Refs. 555, 335, 336, and 439-441, where the basic aspects of the properties of oxide electrodes and the kinetic aspects of oxide film formation in relation to Cl adsorption and the kinetics of Cr ion discharge were addressed. Mechanistic aspects of chlorine evolution were critically analyzed recently in an excellent article by Trasatti (338). In this article, the focus is primarily on the nature and characterization of the adsorbed intermediates partipatingin the course of CI2 evolution and their role in the electrocatalysis of the chlorine evolution reaction. As with the OER, in aqueous solutions CI2 evolution takes place on an oxidized surface of metals or on bulk oxide films, so that their surface states often have to be considered in treating the electrocatalysis of the reaction. 

As corrosion protection by active coatings such as Zn-rich [3], Mg-rich [11], or CP-coatings relies on electrical communication with the underlying metal, the nature of any intervening oxide layer will likely play an important role. As discussed in Section 15.6.3, the electrodeposition of CP films on oxideforming metals is also greatly influenced by the electrical properties of the oxide layer. A detailed understanding of oxide films requires aspects of materials science, solid-state physics, and electrochemistry, and such a discussion is beyond the scope of this chapter. For a detailed discussion of oxide films and their properties, the reader is referred to Ref. [16]. In this section, we provide a brief overview of the... 

Torchinskaya TV, Korsunskaya NE, Khomenkova LY, Dhumaev BR, Prokes SM (2001) The role of oxidation on porous silicon photoluminescence and its excitation. Thin Solid Films 381 88-93 Unagami T (1980) Oxidation of porous silicon and properties of its oxide film. Jpn J Appl Phys 19 231-241 Xu ZY, Gal M, Gross M (1992) Photoluminescence studies on porous silicon. Appl Phys Lett 60 1375-1377 Yon JJ, Barla K, Herino R, Bomchil G (1987) The kinetics and mechanism of oxide layer formation from porous silicon formed on p-Si substrates. J Appl Phys 62 1042-1048... 

The small amount of work, describing the addition of R salts to paint coatings as corrosion inhibiting pigments, indicates that they are capable of inhibiting corrosion at exposed areas of substrate. It can be reasonably assumed that the inhibition of corrosion results from the formation of a R oxide film. This concept has yet to be confirmed and the exact role of the film needs further clarification. 

Prevention. Fretting, because of the significant role of oxidation, is best combatted by oxidation-resistant coatings, for example, electroless nickel or softer self-lubricating coatings like silver or indium. Solid-film lubricants are also successfully employed. Additional information on prevention of fretting corrosion can be found in Chapter 2, Principles of Corrosion. ... 

Fukuda Y. (1974), Role of oxalate ion in the formation of oxide film on aluminum in oxalic acid electrolyte , Nippon Kagaku Kaishi, 10, 1868-75. 

The environment plays several roles in corrosion. It acts to complete the electrical circuit, ie, suppHes the ionic conduction path provide reactants for the cathodic process remove soluble reaction products from the metal surface and/or destabili2e or break down protective reaction products such as oxide films that are formed on the metal. Some important environmental factors include the oxygen concentration the pH of the electrolyte the temperature and the concentration of anions. 

The oxidation products are almost insoluble and lead to the formation of protective films. They promote aeration cells if these products do not cover the metal surface uniformly. Ions of soluble salts play an important role in these cells. In the schematic diagram in Fig. 4-1 it is assumed that from the start the two corrosion partial reactions are taking place at two entirely separate locations. This process must quickly come to a complete standstill if soluble salts are absent, because otherwise the ions produced according to Eqs. (2-21) and (2-17) would form a local space charge. Corrosion in salt-free water is only possible if the two partial reactions are not spatially separated, but occur at the same place with equivalent current densities. The reaction products then react according to Eq. (4-2) and in the subsequent reactions (4-3a) and (4-3b) to form protective films. Similar behavior occurs in salt-free sandy soils. 

It has been suggested that the role of nickel (as NiAlj) is to provide sites of low hydrogen overvoltage, where cathodically liberated hydrogen may be liberated without disrupting the protective oxide . The distribution of such sites is apparently critical however, since high corrosion resistance is associated with a fine dispersion of the second phase, while the electronic conductivity of the film is probably also important . 

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