Internal Oxidation of Metals

Let us now compare the internal oxidation of nonmetallic (oxide) solid solutions with the internal oxidation of metal alloys. The role of the (neutral) point defect... 

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. 

Fiber degradation, dissolution of degradation products, and oxidation of metallic copper alter the Eh and pH of the corrosion solution so that bound (and also free) copper ions will react with H20 and available HCQr to form one or more copper minerals. These minerals will precipitate in situ within the template of the fiber internal structure. In this manner, the shape of the fiber will be retained, although the size of the pseudomorph can become larger than that of the original fiber. 

The intergranular oxidation can be followed or accompanied by internal oxidation of NiAl.Thc internal oxidation must be initiated by a rapid oxidation causing considerable Al-depletion but without formation of a protective scale [5], Then oxygen can diffuse into the NiAl matrix and reacts under formation of inward growing A1 03 precipitates. In the ampoule experiments with various metal/oxide or oxide/oxide mixtures the attack is mostly localized and inward and outward growth of pocks or cones is observed on the material (Fig. 3), but also general attack can occur, especially in the... 

B. A. Pint On the Formation of Interfacial and Internal Voids in a-AI203 Scales", submitted to Oxidation of Metals. 

Table 7. Mass change, maximum depth of internal oxidation and metal loss ((A B)/2) of Fe-A110-Cr2 and alloy 800H after 2016 hours of cyclic exposure in SO,/air at 650,750,850°C...

The rate has been obtained from the initial rate of internal oxidation of Ag-Cd alloys involving 1 % Cd. When a Ag-Cd alloy is exposed to oxygen at 400°C, oxygen is dissolved in form of atoms, diffuses into the interior of the alloy, and reacts with Cd atoms under formation of a precipitate of CdO in metallic silver as matrix. The reaction may be followed by volumetric measurements of the consumption of gaseous oxygen. Under quasi-steady-state conditions, the rate of formation of oxygen atoms by dissociation of O2,, must equal the rate of diffusion... 

Alloys of Nb with small additions of Zr exhibit internal oxidation of Zr under an external scale of Nb-rich oxides. This class of alloy is somewhat different from those such as dilute Ni-Cr alloys in that the external Nb-rich scale grows at a linear, rather than parabolic rate. The kinetics of this process have been analyzed by Rapp and Colson. The analysis indicates the process should involve a diffusion-controlled internal oxidation coupled with the linear scale growth, i.e., a paralinear process. At steady state, a limiting value for the penetration of the internal zone below the scale-metal interface is predicted. Rapp and Goldberg have verified these predictions for Nb-Zr alloys. 

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. 

Fra] Fratzl, P., Paris, O., Internal Oxidation of Cu-Fe-II. The Morphology of Oxide Inclusions from the Minimization of Elastic Misfit Energy , Acta Metall. Mat., 42(6), 2027—2033 (1994) (Theory, 15)... 

E.J. Opila, N.S. Jacobson, D.L. Myers, and E.H. Copland, Predicting oxide stability in high-temperature water vapor, Journal of the Minerals, Metals, and Materials Society 58 22-28, 2006 I. Kvernes, M. Oliveira, and P. Kofstad, High temperature oxidation of Fe-13Cr xAl alloys in air/water vapor mixtures, Corrosion Science 17 237-52, 1977 H. Asteman, J.-E. Svensson, M. Norrell, and L.-G. Johansson, Influence of water vapor and flow rate on the high-temperature oxidation of 304L Effect of chromium oxide hydroxide evaporation. Oxidation of Metals 54 11-26,2000 J.M. Rakowski and B.A. Pint, Observations on the effect of water vapor on the elevated temperature oxidation of austenitic stainless steel foil. Proceedings of Corrosion 2000, NACE Paper 00-517, NACE International, Houston, Texas, 2000 E. Essuman, G.H. Meier, J. Zurek, M. Hansel, and W.J. Quadakkers, The effect of water vapor on selective oxidation of Fe-Cr Alloys, Oxidation of Metals 69 143-162,2008 E.J. Opda, Oxidation and volatilization of silica formers in water vapor. Journal of the American Ceramic Society 86(8) 1238-1248,2003. 

The ratio is characteristic for an alloy for determining its ability to form a protective scale. In this case is the metal recession rate constant due to oxidation. This dependence of the critical concentration in the alloy on the k /D ratio is illustrated by the graph in Fig. 2-12. Field 1 in this figure shows the area in which no protective oxide scale can be formed, since the amount of chromium is insufficient for a continuous scale and instead internal oxidation of chromium in the base material may occur. As under practical conditions cracking and spalling of protective oxide scales may often occur, there must be a reservoir of the protective scale forming element in order to reestablish the scale after failure, which is dealt with by fields 2 to 4. The meaning of these fields is as follows ... 

H.E. Evans, Stress effects in high temperature oxidation of metals. International Materials Reviews, 40, 1, 1-40 (1995). 

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