Binary alloys, internal oxidation

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. 

Pauling File The PA ULING FILE Binaries Edition, ed. P. Villars, ASM International, Materials Park, Ohio, USA, 2002, ISBN 0 87170 786 1 Binary Compounds including oxides, alloys and phase diagrams ... 

The classical treatment of the internal oxidation of binary alloys was first developed by Wagner (1959) and reviewed later by others (Rapp, 1965 S yisher, 1971 Stott and Wood, 1988 Douglass, 1995). Consider a binary, single-phase alloy A-B in which B is the solute and more reactive element. The necessary and sufficient criteria for the internal precipitation of BX, where X is the oxidant, are that the amount of B in the alloy must be below the critical value necessary for the transition from internal to external BX formation, and that the solubil-... 

Internal oxidation of single-phase binary alloys... 

Figure 16.5. Concentration gradient in internal oxidation of a binary A/B alloy by oxygen...

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. 

This is shown schematically in Fig. 3.4, in which and Pbo are the dissociation pressures of AO and BO, respectively. For a given binary alloy system at a particular temperature, Ng° is a constant. Therefore, alloys with a composition of Nb < Ng°, in zone a, will form AO preferentially, whereas alloys with a composition of jVg > Nb°, in zone b, will form 50preferentially. However, from the thermodynamic diagram shown in Fig. 3.4, it is impossible to determine such oxidation processes as being internal or external. 

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]. 

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). 

Extensive experimental observations demonstrate that on a binary A-Bd oy oxidized in an atmosphere with a low oxygen partial pressure A will not be oxidized), a continuous external 50 scale always forms on the alloy surface after a short exposure. The formation of this external oxide scale may or may not be accompanied by the formation of internal oxides in the aUoy substrate. In other words, the oxidation process starts on the alloy surface to form an external BO scale, and then it may or may not proceed into the aUoy substrate to develop internal 50 particles, depending on the alloy composition and the oxidation condition. When the content of 5 in the A B aUoy is higher than a critical value, a protective external BO scale can form and stay on the alloy surface, and the formation of internal 50particles in the alloy substrate can be completely avoided. 

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. 

Smeltzer W W and Whittle D P, The criterion for the onset of internal oxidation beneath the external scales on binary alloys , J. Electrochem. Soc., 1978 125, 1116. 

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