Transition between external and internal oxidation of alloys

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

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. 

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]. 

Ni-Cr and Co-Cr alloys are typical single-phase binary alloys. Ni-6Cr, Ni-8Cr, Co-5Cr and Co-lOCr alloys were selected to study the first type of transition between external and internal oxidation under atmospheres with ultra-low oxygen partial pressures Pq in which the solvent Ni or Co in the alloys does not oxidize [14,19]. 

The criterion for the minimum solute concentration, Ng, of a binary A B alloy required for the second transition process has been derived based on thermodynamic and kinetic analyses. Using this equation the effects of oxygen partial pressure, addition of rare earth elements, surface micro-crystallization, and the gettering effects on the transition between external and internal oxidations can be explained. 

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... 

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. 

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