Oxide films rate laws

Although our simple oxide film model explains most of the experimental observations we have mentioned, it does not explain the linear laws. How, for example, can a material lose weight linearly when it oxidises as is sometimes observed (see Fig. 21.2) Well, some oxides (e.g. M0O3, WO3) are very volatile. During oxidation of Mo and W at high temperature, the oxides evaporate as soon as they are formed, and offer no barrier at all to oxidation. Oxidation, therefore, proceeds at a rate that is independent of time, and the material loses weight because the oxide is lost. This behaviour explains the catastrophically rapid section loss of Mo and W shown in Table 21.2. 

If K = 1 K, a = 0.25 nm, and z = 3, X = 30nm at 300 K, so that for a film 1 nm thick, the field increases the rate of growth by a factor of about 10 The term in the growth law due to the field, namely exp (K/X), is large only when X is small. Because of this a thin oxide film can form even at low temperatures where the ordinary rate of entry of ions into the oxide, is negligible. As the film thickens, the factor exp /X) decreases rapidly, and the rate of growth soon falls to such a low value that, for practical purposes, oxidation has ended. 

The corrosion rate of a metal, which depends for its protection on a passive oxide film, may be predicted from a simple empirical adsorption law (Freundlich) ... 

The literature on the oxidation of nickel-copper alloys is not extensive and emphasis tends to be placed on the copper-rich materials. The nickel-rich alloys oxidise according to a parabolic law and at a rate similar to that for nickel Corronil (Ni-30Cu) exhibited a parabolic rate behaviour below 850°C but a more complex behaviour involving two parabolic stages above 900°C. Electron diffraction examination of the oxide films formed on a range of nickel-copper alloys showed the structures of the films to be the same as for the bulk oxides of the component metals and on all the alloys examined only copper oxide was formed below 500°C and only nickel oxide above 700°C . 

The surface oxidation of a metal such as copper is accompanied by the growth of an oxide layer, the thickness of which may be measured by the method of colour interference when due allowance is made for the refractive index of the oxide formed, or by the decrease in electrical resistance of a thin wire or tube of the metal as oxidation ensues. Investigations have been made on the rate of such oxidations by Tammann (Zeit.f anorg. GJiem. cxi. 78 cxxiii. 196 cxxiv. 196), Hinshelwood (Proc. Mog. Soc. A, cii. 318), Palmer Proo. Roy. Soc. A, cm. 444) and Dunn (unpublished, see also Pilling and Bedworth, Jour. Inst. Metals, xxix. 629, 1923). It is found that the rate of increase in thickness of the oxide film oc) obeys under ideal conditions the ordinary diffusion law or CG = kt. ... 

Figure 5.6 Rate laws for formation of oxide films, (a) Parabolic rate law. (b) Effect of film cracking (successive parabolic segments), (c) Limiting case of (6). (d) Logarithmic rate law.

With this electric potential Poisson equation (A

el = net charge density) to eventually obtain the concentration of electrons at the film surface (A ). It further follows that Ne(A ) varies with the film layer thickness as A -2. If we now assume that the (catalyzed) rate of dissociation of the adsorbed X2 molecules is proportional to the surface concentration of electrons, and that this dissociation process is rate determining, a cubic rate law for the film growth can be expected (A — At 2 At - t in). In fact, during the oxidation of Ni at temperatures between 250 and 400 °C, an approximately cubic rate law has been experimentally observed. We emphasize, however, that the observed cubic oxidation rate does not prove the validity of the proposed reaction mechanism. Different models and assumptions concerning the atomic reaction mechanism may lead to the same or similar dependences of the growth rate on thickness. 

The kinetics of a similar oxidation reaction was studied by Pritchard and Dobson (236). These authors studied the oxidation between 450 and 560 K of a metallic-iron foil (0.02 mm thick electroplated with 1 mg cm 2 57Fe) by deoxygenated water. The resulting Mossbauer spectra (at room temperature) showed Fe304 to be the only detectable reaction product, and from the ratio of the Fe304 spectral area to that of metallic iron, the magnetite film thickness y can be calculated. Assuming that the rate law is of the form... 

Faraday s Law applies to the anode as well as to the cathode ie, the total reaction at the anode is proportional to the current, and much like the cathode, the anode efficiency varies with the current density. As the current on the anode is increased, the anode efficiency decreases, slightly at first, until it reaches a point at which the anode metal cannot dissolve fast enough through the anode film. The first stage of dissolution for the soluble anode is the oxidation of the metal followed by dissolution of the oxide. When the oxide dissolution rate is less than the oxidation rate, polarization of the anode takes place. The oxide film builds up in sufficient thickness to form an insulating coating, and the current decreases rapidly. The thick anode films can dislodge at... 

Combinations of the above factors may thus lead to very different rates of attack on metals. The most common method of studying high-temperature oxidation of metals is to analyse the pattern of film (scale) growth and then assess which physical/chemical mechanisms would fit those rate laws. In this way, the effects of adding alloying elements to the metal can clearly be seen. 

From 327-400 °C, a firmly adherent blue and thin oxide film will be formed. It acts as a protective layer. Its formation rate is determined by diffusion and proceeds according to a parabolic rate law. The film composition is given as WO2.75, but this is in contradiction to above ESCA findings. The color is not real but is caused by interference. 

---