Internal oxidation of alloys

For the dilute reactive constituent R which forms the oxide RO , let the thickness of the internal oxidation zone be then for the inward diffusion of oxygen, mole fraction No 

Rapp (1961) has confirmed this equation in a study of the oxidation in air of Ag-In alloys at 550°C. The reaction proceeds with tire internal formation of In203 particles over a range of indium concenuations, but at a critical mole fraction of indium in the alloy, external oxidation occurs with the growdr of a layer of In203 covering the alloy. The n airsitioir from internal to external oxidation was found by Rapp to occur at the mole fraction of indium cone-sponding to 

Maak (1961) has obtained the equation governing the oxidation rate of a metal to form both an external oxide and an internally oxidized dilute solute, as for example in Cu-Be alloys, coiTesponding to tire equation given earlier 

The thickness of the internal zone can be calculated from the equation quoted earlier 

The oxidation of nickel-copper alloys provides an example of die dependence of the composition of the oxide layer on the composition of the alloy. Nickel-copper alloys depart from Raoult s law, but as a first approximation can be taken as ideal. The Gibbs energy change for the reaction 

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Alloy oxidation processes are far more complex than the oxidation of metallic elements. Let us also distinguish between external and internal oxidation. In external oxidation, a layer forms by way of a heterogeneous reaction as discussed in Chapter 7. In this section, however, we are concerned with the internal oxidation of alloys. Pure metal A can only be oxidized externally. The simplest system for the study of internal oxidation is the binary metal alloy (A,B), to which we shall confine our discussion. 

Quantitative models for internal oxidation of alloys were developed by various authors in the case of oxidation by oxygen [RHI 40], [WAG 59]. We will choose this gas on an alloy (A, B) as an example in our presentatiom... 

Transition between external and internal oxidation of alloys 37... 

The transition phenomena between external and internal oxidation of alloys are very complex. For simplification, such transitions can be divided into two main types according to the composition of the oxides formed on or in the alloys [6]. In the first type of transition, the external oxide scale and the internal oxide particle have the same composition, as shown in Fig. 3.1. This type of transition takes place between the growth processes of internal oxide particles in the alloy substrate (Fig. 3.1a) and an exclusive oxide scale on the alloy surface (Fig. 3.1b), under the condition that only the solute metal in the alloy can oxidize. In the second type of transition, the external oxide scale has a composite structure, as shown in Figs 3.2a and 3.2b. The composite oxide scale (AO + BO) represents mixtures, compounds or layers of AO and BO under different oxidizing conditions. This type of transition takes place between the formations of the internal oxide particles beneath the composite oxide scale (Fig. 3.2a) and a composite oxide scale on the alloy surface (Fig. 3.2b), under the condition that oxidation of all elements in the alloy can occur. In addition, there is a possibility of another type of transition between the growth processes of a composite oxide scale (Fig. 3.2b) and an exclusive oxide scale on the alloy surface (Fig. 3.2c) [7]. 

The principal features and theoretical models for the transitions between external and internal oxidation of alloys have been studied under relatively simple oxidation conditions. Wagner [2] was the first to develop a theoretical... 

He and his co-workers [14-19] studied the oxidation behaviours of Ag-In, Co-Cr, Ni-Al and Ni-Cr binary alloys under ultra-low oxygen pressure atmospheres in which the solvent in the alloys does not oxidize. They found that continuous external oxide scales could form on aU alloy surfaces, especially for the specimens after a short exposure. More importantly, the formation and growth of external oxide scales may or may not be accompanied by the formation of internal oxides in the alloy matrix. Based on these experimental results, they proposed that the transition between the oxidation models of an alloy should be from external to internal oxidation [18,19]. In this chapter, their works on this subject will be summarized and discussed briefly to get a better understanding of the transition between external and internal oxidation of alloys. 

Comparing the two important theories developed by Wagner, it can be found that, for the first type of transition, these two theories actually describe the same criterion, i.e. the criterion for such a transition is equal to the criterion for the formation of a stable external oxide scale. However, these two theories are established on different bases. The first is based on the nucleation and growth of oxides in the alloy substrate, and the second is based on the growth of oxides on the alloy surface. Therefore, in order to obtain a better imderstanding of these two theories and the oxidation transition behaviour of an alloy, a theory describing the transition between external and internal oxidation of alloys should be established. This model should be based on full consideration of the nucleation and growth of oxides both on the alloy surface and in the alloy substrate. 

Secondly, in Wagner s theory, a critical volume fraction, g, of the internal oxides is needed for the transition from internal to external oxidation. In the present model, is no longer needed. However, it gives a better understanding on why, under the critical condition for this transition, there is a critical volume fraction of oxide g ) formed in the internal oxidation zone. The reason is that the characteristics of nucleation and growth of oxides on the alloy surface and in the alloy substrate are quite different. The nucleation and growth of oxides on the surface of an alloy are easier than inside the alloy the oxide volume fraction of a continuous BO scale formed on the alloy surface must be 1, while the internal oxide volume fraction must be less than 1. For the transition from temporary external oxidation to internal oxidation of alloys, the internal oxide volume fraction must be changed with the alloy composition, since the transition from permanent external oxidation to internal oxidation is an extreme case of the transition from temporary external oxidation to internal oxidation, and occurs under a special condition, i.e., with a certain alloy composition, and the internal oxide volume fraction must be a constant, g, which is smaller than 1. 

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