Oxide films continued protective

The passivity of metals like iron, chromium, nickel, and their alloys is a typical example. Their dissolution rate in the passive state in acidic solutions like 0.5 M sulfuric acid may be seriously reduced by almost six orders of magnitude due to a poreless passivating oxide film continuously covering the metal surface. Any metal dissolution has to pass this layer. The transfer rate for metal cations from this oxide surface to the electrolyte is extremely slow. Therefore, this film is stabilized by its extremely slow dissolution kinetics and not by its thermodynamics. Under these conditions, it is far from its dissolution equilibrium. Apparently, it is the interaction of both the thermodynamic and kinetic factors that decides whether a metal is subject to corrosion or protected against it. Therefore, corrosion is based on thermodynamics and electrode kinetics. A short introduction to both disciplines is given in the following sections. Their application to corrosion reactions is part of the aim of this chapter. For more detailed information, textbooks on physical chemistry are recommended (Atkins, 1999 Wedler, 1997). 

In addition, with high solid content of the cooling water and at high flow velocities, severe corrosive conditions exist which continuously destroy surface films. Cathodic protection alone is not sufficient. Additional measures must be undertaken to promote the formation of a surface film. This is possible with iron anodes because the anodically produced hydrated iron oxide promotes surface film formation on copper. 

The Ni-base alloy surface is exposed to an oxidizing gas, oxide nuclei form, and a continuous oxide film forms (Ni) (Cr203, etc.)- This oxide film is a protective layer. The metal ions diffuse to the surface of the oxide layer and combine with the molten Na2S04 to destroy the protective layer. Ni2S and Cr2S3 results sulfidation) ... 

In general, however, for titanium immersed in acid solutions, potentials above zero on the saturated calomel scale are conducive to the formation of protective oxide, while at certain negative potentials hydride films, which also confer some protection, can be formed. Between the potential at which a continuous hydride film is formed and that at which protective oxide films appear, soluble titanium ions are produced and rapid corrosion ensues. 

Since the natural passivity of aluminium is due to the thin film of oxide formed by the action of the atmosphere, it is not unexpected that the thicker films formed by anodic oxidation afford considerable protection against corrosive influences, provided the oxide layer is continuous, and free from macropores. The protective action of the film is considerably enhanced by effective sealing, which plugs the mouths of the micropores formed in the normal course of anodising with hydrated oxide, and still further improvement may be afforded by the incorporation of corrosion inhibitors, such as dichromates, in the sealing solution. Chromic acid films, in spite of their thinness, show good corrosion resistance. 

ANODIC OXIDATION. Oxidation is defined not only as reaction with oxygen, but as any chemical reaction attended by removal of electrons. Therefore, when current is applied to a pair of electrodes so as to make them anode and cathode, the former can act as a continuous remover of electrons and hence bring about oxidation (while the latter will favor reduction since it supplies electrons). This anodic oxidation is utilized in industry for various purposes, One of tire earliest to be discovered (H, Kolbe. 1849) was the production of hydrocarbons from aliphatic acids, or more commonly, from their alkali salts. Many other substances may be produced, on a laboratory scale or even, in some cases, on an economically sound production scale, by anodic oxidation. The process is also widely used to impart corrosion-resistant or decorative (colored) films to metal surfaces. For example, in the anodization or Eloxal process, the protection afforded by the oxide film ordinarily present on the surface of aluminum articles is considerably increased by building up this film by anodic oxidation. 

Chemically, the film is a hydrated form of aluminum oxide. The corrosion resistance of aluminum depends upon this protective oxide film, which is stable in aqueous media when the pH is between about 4.0 and 8.5. The oxide film is naturally self-renewing and accidental abrasion or other mechanical damage of the surface film is rapidly repaired. The conditions that promote corrosion of aluminum and its alloys, therefore, must be those that continuously abrade the film mechanically or promote conditions that locally degrade the protective oxide film and minimize the availability of oxygen to rebuild it. The acidity or alkalinity of the environment significantly affects the corrosion behavior of aluminum alloys. At lower and higher pH, aluminum is more likely to corrode. 

I would like to comment on the application of the Pilling-Bedworth rule. In most cases of oxidation, it is the metal ion that migrates in the oxide film. If this is true, then the metal ion will go through the oxide layer and form new oxide on the outer surface. Since this new oxide is not constrained, there seems to me to be no reason why the difference in the volume of the metal oxide to the volume of the metal should be used to predict the continuity of the oxide film. For example, for sodium, one would predict, by this rule, a porous oxide when actually a dense, protective oxide is found. For tungsten, one would predict a compact oxide while the opposite is found, ft seems to me that the continuity of the oxide layer is determined more by the possible transformations or reactions of the film which forms initially. 

The continued oxidation of the metal substrate beneath the protective oxide layer must become a diffusion-controlled process for thick enough oxide films in which either metal atoms or oxygen atoms diffuse through the metal oxide layer to the appropriate interface where reaction proceeds. Let us assume a thick enough oxide layer on a plane metal surface where a steady state has been achieved. Then we can write for the rate of formation of metal oxide, MO, per unit area (assuming metal ion diffusion) ... 

Since protection of electrodes against corrosion in the photoelectrolysis cells is a question of vital importance, many attempts have been made to use protective films of different nature (metals, conductive polymers, or stable semiconductors, eg., oxides). Of these, semiconductive films are less effective since they often cause deterioration in the characteristics of the electrode to be protected (laying aside heterojunction photoelectrodes specially formed with semiconducting layers of different nature [42]). When metals are used as continuous protecting film (and not catalytical "islands" discussed above), a Schottky barrier is formed at the metal/semiconductor interface. The other interface, i.e., metal/electrolyte solution is as if connected in series to the former and is feeded with photocurrent produced in the Schottky diode upon illuminating the semiconductor (through the metal film). So, the structure under discussion is but a combination of the "solar cell" and "electrolyzer" within the photoelectrode Unfortunately, light is partly lost due to absorption by the metal film. 

The discussion of alloy oxidation in this chapter and the discussion of pure metal oxidation in the previous one have indicated that resistance to high-temperature oxidation requires the development of an oxide barrier which separates the environment from the substrate. Continued resistance requires the maintenance of this protective barrier. Therefore, stress generation and relief in oxide films and the ability of an alloy to reform a protective scale, if stress-induced spalling or cracking occurs, are important considerations in the high-temperature oxidation of alloys. This subject has been discussed in reviews by Douglass, Stringer, Hancock and Hurst, Stott and Atkinson, and Evans. " ... 

In order to account for these results Padalia et al. (1976) have proposed the following model of oxidation In Yb the oxide was assumed to be nucleated in islands and the linear kinetics was attributed to island growth. In the case of the other heavy rare earth metals the oxide was assumed to form a continuous film at the surface, which grows steadily deeper into the bulk thus accounting for the logarithmic relationship between exposure and O Is peak intensity. After an exposure of about 40 L O2 the oxide film was thought to form a protective layer which slows down the further oxidation as can be recognized in fig. 7. 

Metals vary greatly in their corrosion-resistance - chromium and titanium have good resistance, while steel readily corrodes. The oxide film formed on chromium and titanium closely adheres to the surface and protects the metal from further oxidation. In the case of steel, the oxide film in the form of rust is loose, allows moisture to be retained, and promotes further corrosion. If corrosion is allowed to continue, the steel will eventually be completely consumed, i.e. the metal will have returned to the condition of the ore from which it was extracted. 

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