Growth rates, high temperature oxides

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

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. 

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 purpose of this review paper is to survey the principles of high temperature oxidation or high temperature corrosion. A typical situation is that of a metal exposed to a hot gas which can act as an oxidant. In many cases the oxidation product forms a layer which separates the reactants, the metal and the gas atmosphere. Under special conditions, the kinetics are diffusion controlled, i. e,, the rate of the reaction (the rate of oxide thickness growth) depends on the diffusion of species, ions and electrons, through the layer (sometimes called a tarnish layer). Actually when a metal or alloy is exposed to a corrosive gas, the reaction kinetics may be controlled by one or more of the following steps ... 

In this section, an important special case of high-temperature oxidation will be discussed. Up to now we have assumed that there is no appreciable dissolution of the electronegative component X in the metal during the oxidation process. However, in many important practical systems this is by no means the case, as one can easily appreciate by looking for example at the phase diagram for the Zr-0 system. What rate law should we then expect for the growth of the product layer when dissolution of the element X in the metal occurs simultaneously with the growth of the oxidation product, if local equilibrium can be assumed The situation for a onedimensional experiment when the compound MeX is formed is illustrated in Fig. 8-4. 

The wet oxidation process uses water molecules instead of dry oxygen as the oxygen source to oxidize sUicon. As a matter of fact, water molecules contact the silicon furnace in a normal wet oxidation process. Water molecules dissociate at high temperature and form hydroxide (HO) prior to reaching the silicon surface. Hydroxide has faster diffusion mobility in silicon dioxide than pure O2, which explains why wet oxidation has a higher growth rate than dry oxidation. Wet oxidation is used to form thick oxides such as the LOCOS oxide, masking oxide and field oxide. As shown in Fig. 7, several systems have been used to deliver water vapor into the process tube. The boiler system is the simplest setup which vaporizes ultra-pure water then drives the water vapor in to the process tube via heated gas lines. However, it is dif-... 

Figure 6-2. Schematic order of magnitude growth rates for some oxides of interest for high-temperature intermetallics (after compilations by Yurek, 1987 Doychak, 1994 Meier, 1996 Welsch et al., 1996).

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. 

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