Oxidation rate laws, high-temperature

The oxidation rate laws described earlier are simple models derived from the behavior of pure metals. In contrast, practical high-temperature corrosion problems are much more complex and involve the use of... 

The equation 10.145 is valid only for pure photodegradation i.e. nonphoto-oxidative processes, which occurs at low temperatures. At sufficiently high temperatures (temperatures close to the glass transition temperature (Tg)) and radiation intensities, the diffusion of oxygen limits the rate of photo-oxidation. At these high temperatures both photo-oxidative and nonphoto-oxidative processes occur. For that case a power law adequately... 

If a compact film growing at a parabolic rate breaks down in some way, which results in a non-protective oxide layer, then the rate of reaction dramatically increases to one which is linear. This combination of parabolic and linear oxidation can be tenned paralinear oxidation. If a non-protective, e.g. porous oxide, is fonned from the start of oxidation, then the rate of oxidation will again be linear, as rapid transport of oxygen tlirough the porous oxide layer to the metal surface occurs. Figure C2.8.7 shows the various growth laws. Parabolic behaviour is desirable whereas linear or breakaway oxidation is often catastrophic for high-temperature materials. 

Figure C2.8.7. Principal oxide growth rate laws for low- and high-temperature oxidation inverse logarithmic, linear, paralinear and parabolic.

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. 

The second type of behaviour (Fig. 1.89) is much closer to that which one might predict from the regular cracking of successive oxide layers, i.e. the rate decreases to a constant value. Often the oxide-metal volume ratio (Table 1.27) is much greater than unity, and oxidation occurs by oxygen transport in the continuous oxide in some examples the data can be fitted by the paralinear rate law, which is considered later. Destructive oxidation of this type is shown by many metals such as molybdenum, tungsten and tantalum which would otherwise have excellent properties for use at high temperatures. 

At medium and high temperatures copper ultimately follows the parabolic law " . It has been shown " using radioactive tracers that the diffusion of copper ions in cuprous oxide is the rate-determining step at 8(X)-1 000°C, and there is considerable evidence in favour of the view that metal moves outwards through the film by means of vacant sites in the oxide lattice . 

Since the paper by Pilling and Bedworth in 1923 much has been written about the mechanism and laws of growth of oxides on metals. These studies have greatly assisted the understanding of high-temperature oxidation, and the mathematical rate laws deduced in some cases make possible useful quantitative predictions. With alloy steels the oxide scales have a complex structure chromium steels owe much of their oxidation resistance to the presence of chromium oxide in the inner scale layer. Other elements can act in the same way, but it is their chromium content which in the main establishes the oxidation resistance of most heat-resisting steels. 

Kinetics There have been few comprehensive studies of the kinetics of selective oxidation reactions (31,32). Kinetic expressions are usually of the power-rate law type and are applicable within limited experimental ranges. Often at high temperature the rate expression is nearly first order in the hydrocarbon reactant, close to zero order in oxygen, and of low positive order in water vapor. Many times a Mars-van Krevelen redox type of mechanism is assumed to operate. 

Finally, there are two special cases in which the rectilinear law is observed when the rate-controlling factor is the rate of supply of O2 and when the metal oxide is volatile at the temperature of oxidation. The latter case occurs in the high temperature oxidation of molybdenum, since M0O3 is quite volatile, and in this case dw/dt is negative. 

Examples of this category are oxides of cobalt, copper, nickel and tungsten. The oxidation of metal at high temperature such as iron at 1000°C and magnesium at 500°C obey rectilinear rate law. 

We have so far dealt with the rate laws that govern the oxidation of metals and the empirical nature of the protective properties of the oxides. It has been pointed out earlier that the oxidation of metals at high temperatures is analogous to galvanic corrosion phenomena. 

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. 

The oxidation of iron at high temperatures, where several iron oxide phases form, obeys the parabolic rate law whereas in CO-CO2mixtures above 900°C, it obeys a linear rate law with the exclusive formation of an FeO layer (18). This result is understandable if one considers the high defect concentration in FeO of approximately 10%, which ensures high diffusion velocity. The linear rate constant - exhibits the following de-... 

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