Internal/external oxidation transition

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 the internal formation of ln203 particles over a range of indium concentrations, but at a critical mole fraction of indium in the alloy, external oxidation occurs with the growth of a layer of ln203 covering the alloy. The transition from internal to external oxidation was found by Rapp to occur at the mole fraction of indium corresponding to... 

Even if the transport product cB-DB of component B in the alloy (A,B) cannot be neglected in comparison to that of oxygen, internal oxidation may still occur. The amount of BO precipitates will then be enhanced toward the alloy surface. In this way, a transition from internal to external oxidation becomes more and more likely. This transition (/>., the formation of a dense external BO layer) is expected to occur if... 

The solute concentration of the alloy must be lower than that required for the transition from internal to external oxidation. 

When a is large, we expect the accumulation and lateral growth of the internal oxides to form a continuous layer, i.e., the transition to external oxidation. Figure 5.12 shows this occurring in a Co-7.5 wt% Ti alloy. Wagner states that, when the volume fraction of oxide, g = /(Vox/ Tm), reaches a critical value, g, the transition from internal to external scale formation should occur. Insertion of/, in terms of g, in a in Equation (5.23) then gives the criterion for external oxidation as shown... 

Figure 5.12 Optical micrograph showing transition from internal to external oxidation in a Co-7.5 wt% Ti alloy oxidized for 528 h at 900 °C.

The parameter /can be used as a criterion for the transition from selective external oxidation to selective internal oxidation if />1, external oxidation leads to formation of a compact layer if /<7, the internal oxidation occurs. 

Fig. 2. Wagner s model of the transition from the internal to external oxidation of alloy A-B under the condition where only B can oxidize a) B is less than the critical amount of B required for the transition and b) B is higher than the critical content of B required for the transition from the internal to external oxidation [44,45,110-112].

N. Otsuka, Y. Shida, H. Fujikawa, Internal-external transition for the oxidation of Fe-Cr-Ni austenitic stainless steels in steam, Oxid. Met 32 (1989) 13. 

A method to calculate the critical aluminium content for breakaway oxidation has been proposed which is based on Wagner s ideas on selective oxidation in ternary alloys. It focusses on the transition between internal and external oxidation, because after spallation of the oxide the bare metal is exposed again to the atmosphere. The calculation shows that the critical aluminium content is temperature dependent and varies from 0 wt.% at about 1000°C to 3 wt.% at 1300°C. This corresponds to the scatter range reported in the literature for the critical aluminium content. 

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]. 

Models of transitions under the condition that all elements in the alloy can oxidize transition from internal oxide particles (a) to external composite oxide scale (b) and transition from external composite oxide scale (b) to external exclusive oxide scale (c). 

However, it should be noted that Eq. (3.1) describes an extreme condition of the internal oxidation process, i.e., the extent to which the internal oxidation can develop under the critical condition of the transition. When a stable exclusive external oxide scale forms, internal oxides might not have formed in the alloy at all. Therefore, the formation of internal oxides might not be necessary for the formation of an external oxide scale. In other words, the internal oxidation process can be completely avoided by the formation of an external oxide scale. 

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. 

Here, it is necessary to distinguish two concepts (1) the transition from temporary external oxidation to internal oxidation, i.e., the external oxide scale forms on the alloy surface during the initial oxidation stage (Fig. 3.10a), then oxides form in the alloy within a longer exposure time (Fig. 3.10b) and (2) the transition from permanent external oxidation to internal oxidation, i.e., an external oxide scale forms on the alloy surface without any internal... 

Transition models from external to internal oxidation of a binary alloy A-B the transition from temporary external oxidation (a) to internal oxidation (b), and the transition from permanent external oxidation (c) to internal oxidation (b). 

Criterion for the transition from permanent external oxidation to internal oxidation... 

Different types of oxides can form. Under the conditions of Po, > P t and Eq. (3.30), a composite oxide scale can form on the alloy surface, as shown in Fig. 3.2b. Under the conditions of Pbo < P>2 - Po ° and Eq. (3.30), a transition from temporary external oxidation to intemad oxidation can take place (Fig. 3.10b). Under the conditions described by Eq. (3.29), however, this is the transition from permanent external to internal oxidation (Figs 3.10c to 3.10b). Therefore, under the condition of - Po 4 0- (3.28) can be used as the criterion for the transition between the growth processes of an exclusive oxide scale and a composite oxide scale while under the conditions of Pbo < Vq, fo °, it can be used as the criterion for the transition from... 

Comparing the present analysis with Wagner s theory on the transition from internal to external oxidation of alloys [2], it can be seen that there are several important differences. Firstly, in Wagner s theory, the formation of an external oxide scale is related to the oxygen diffusion in the alloy, while in the present model it is not, since the transition actually takes place from external oxidation to internal oxidation. The diffusion of oxygen in the alloy substrate has no influence on the transition before it occurs. Only after the transition does the diffusion of oxygen affect the internal oxidation rate. 

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. 

Finally, although the critical concentrations of B in the alloy A-B fox both transition models are inversely proportional to where Dis the diffusivity of B in the alloy, their physical meanings are quite different. In Wagner s theory, the diffusion boundary for B is at the internal oxidation front an increase of D can increase the internal oxide volume fraction in the alloy substrate. On the other hand, in the present analysis, the diffusion boundary for B is at the oxide scale/alloy interface an increase of D increases the formation of external oxide scale on the alloy surface. For example, surface micro- or nano-crystallization of M-Cr alloys can increase the outward diffusion of Cr along grain boundaries with a higher density, thus the diffusivity in Eq. (3.28) increases, which decreases Ng and promotes the formation of permanent Cr203 scale on M-Cr alloys [31,32]. 

Rapp R A, The transition from internal to external oxidation and the formation of interruption bands in silver-indium alloys , Acta. Metall., 1961 9, 730-741. 

The transition from non-protective internal oxidation to the formation of a protective external alumina layer on nickel aluminium alloys at 1 000-1 300°C was studied by Hindam and Smeltzer . Addition of 2% A1 led to an increase in the oxidation rate compared with pure nickel, and the development of a duplex scale of aluminium-doped nickel oxide and the nickel aluminate spinel with rod-like internal oxide of alumina. During the early stages of oxidation of a 6% A1 alloy somewhat irreproducible behaviour was observed while the a-alumina layer developed by the coalescence of the rodlike internal precipitates and lateral diffusion of aluminium. At a lower temperature (800°C) Stott and Wood observed that the rate of oxidation was reduced by the addition of 0-5-4% A1 which they attributed to the blocking action of internal precipitates accumulating at the scale/alloy interface. At higher temperatures up to 1 200°C, however, an increase in the oxidation rate was observed due to aluminium doping of the nickel oxide and the inability to establish a healing layer of alumina. 

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