Transition from internal to external

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 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-... 

Wagner (1959) proposed that the condition for the transition from internal to external BXj, formation occurs when a critical volume fraction of BX, /, is attained. Under this condition the influx of X is so restricted that sideways growth of the internal BXyprecipitates is kinetically favorable and BXy eventually forms as a continuous layer on the alloy surface. The expression Wagner... 

Substitution of Eq. (5-18) into the approximated Eq. (5-17) gives, after rearrangement, the following criterion for the transition from internal to external BX formation when the rate of metal recession is negligibly small ... 

Wagner s model of the transition from internal to external oxidation of alloy AB under the condition where only Scan be oxidized (a) Nb is less than the critical content for the transition (b) Nb is higher than the critical content for the transition. 

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. 

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. 

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