Internal oxidation zone

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... 

In many investigations, F (= width of the oxidation zone) has been measured and the results have been compared with theoretical reaction rates [E. Verfurth, R. A. Rapp (1964)]. In technical applications, the internal oxidation zone sometimes forms below an external oxide scale. Analytical solutions for these cases are also available [C. Wagner (1968)]. 

The quasi-steady approximation requires the assumption that the dissolved oxygen concentration varies linearly across the zone of internal oxidation. Therefore, the oxygen flux through the internal oxidation zone (lOZ) is given by Pick s first law as Equation (5.3),... 

The latter gives the penetration depth of the internal oxidation zone as a function of oxidation time. The following points concerning Equation (5.9) are of interest. 

Figure 5.6 shows a polished and etched cross-section of an internally oxidized Cu-Ti cylinder showing the penetration of the internal oxidation zone. The 1102 particles are too small to resolve in the optical microscope. Measurements of r2... 

Extrusion of parent metal from the internal oxidation zone to form pure metal nodules on the surface. 

If/represents the mole fraction of BOv in the internal oxidation zone and Vm the molar volume of the alloy, then//Em will be the concentration in moles per volume and the number of moles in a volume element, AdX, will be (/7Em)AdX, where A is the cross-sectional area for diffusion. This quantity must be equal to the number of moles of B arriving at x = X in the time dt by diffusion from within the sample, i.e., from x > X. Therefore, we obtain Equation (5.21) ... 

As before, it is convenient to consider this system in order of increasing Cr content. At low Cr contents, no internal oxidation zone is seen. This is because the rate at which the external scale is formed is so rapid that the thickness of the internal oxidation zone is negligibly small. The system is best described in terms of the Fe-Cr-0 phase diagram shown schematically in Figure 5.18. The rhombohedral oxides FeiOs and Cr203 show a continuous series of solid solutions. The iron- and chromium-oxides react to form spinels which form solid solutions with Fc304. 

Figure 7.17 Oxygen diffuses through the internal oxide zone and reacts with sulphides in the front of the internal sulphide zone to form oxide and release sulphur that diffuses deeper into the alloy to form new sulphide.

When the concentration of B is so low that a protective scale of B O cannot form, a zone of internal oxidation of B O particles in a matrix of A will form. The surface of the alloy, effectively pure A, can now react with the complex atmosphere to form a scale of either A 0 or duplex A 0 and A, S. Where a duplex scale is formed, the metal-scale interface will be at equilibrium with A + A 0 + A S sulphur will dissolve in the metal and diffuse inwards through the internal oxidation zone to form internal B S particles. This forms as a second, inner, sulphide-based internal zone of precipitation below the outer internal oxidation zone. Since BpO is assumed to be substantially more stable than B S, sulphide formation is not expected to be seen in the outer internal oxidation zone. As oxygen continues to diffuse inwards it wiU react with the internal sulphide particles, forming oxide and releasing sulphur to diffuse further into the metal. This is shown in Figure 7.17. Thus, once the internal sulphide zone is established, it can be driven into the alloy by this cascading mechanism, effectively removing the metal B from solution in the alloy. 

The morphology of the internal oxidation zone and stabihty of transformation front were studied in [29]. Solid-state reactions with formation of two-phase zones were analyzed in [3, 30, 31]. 

Figure 2-17. Structure and composition of the oxide scale and the internal oxidation zone of a cast 25Cr-20Ni-Fe alloy after oxidation at 1000 C in air for 1000 h (Griinling et al., 1982).

The kinetics of internal oxidation are generally found to be diffusion-controlled. Accordingly, Wagner (1959) assumed that the depth of the internal oxidation zone, obeys the parabolic expression ... 

Figure 5-16. Ratio of the effective thickness of the internal oxidation zone in the presence of an outer AXy scale, to the thickness in the absence of an outer scale, 4°, as a function of (kpID ) for different values of A b and DxlD = 10 (after Gesmundo and Viani, 1986).

BSE image of the oxide scaie deveioped after 0.2 h of oxidation of a Y-Ni-27Cr-9AI alloy at p02 = 20 kPa and a temperature of 1373 K in a tube furnace (after Ref. [4]). For short oxidation times, the oxide scale consists of pure NiO and Cr203 layers on top of an internal oxidation zone of isolated a-Al203 precipitates. 

Figure 3.6 shows the surface and cross-section morphologies of the Ni-6Cr alloy after oxidation at 900°C under an oxygen partial pressure of 10 Pa for 2 min (Fig. 3.6a), 10" Pa for 40 hr (b) and 10 Pa for 40 hr (c). It can be seen that after both 2 min and 40 hr oxidation, oxide scales with small particles and big metal nodules appeared on the surfaces (Figs 3.6a and 3.6b) an internal oxidation zone can be seen in the cross-sectional image as well (Fig. 3.6c).

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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