Oxygen dissociation pressures

Figure 8.19 F.llingham diagram for the free energy of formation of metallic oxides. (After F. D. Richardson and J. H. F. Jeffes, J. Iron Steel Inst. 160, 261 (1948).) The oxygen dissociation pressure of a given M - MO system at a given temperature is obtained by joining on the lop left hand to the appropriate point on the M-MO frec-energy line, and extrapolating to the scale on the right hand ordinate for POi (atm).

The thermodynamic data for the platinum oxides are not well established. However, a reasonable value for the free energy of formation of the lower oxide, PtO, at 527°C. is —1 kcal./mole (5, 25). This corresponds to an oxygen dissociation pressure at 527°C. of 0.28 atm.—i.e., bulk PtO is unstable toward decomposition to bulk Pt for oxygen pressures below 0.28 atm. Bulk Pt02 is, of course, even less stable. Nevertheless, it has been reported that at this temperature and an oxygen pressure... 

Jolibois and Bosauet, Oompt. rend., 1922, 174, 386. The oxygen-dissociation pressures in vacuo, at temperatures between 625° C. and 1165° C., have teen determined lay Colani, see Ann. Ghim. Phys., 1907, [8], 12, 59. 

Flo] Floyd, J.M., Willis, G.M., Phase Relations and Oxygen Dissociation Pressures in the System Cu-Fe-O in Research in Chemical and Extraction Metallurgy , Woodcock, J.T., Jenkins, A.F., Willis, G.M. (Eds.), Australasian Inst. Min. Metall., 2, 61-64 (1967) (Experimental, Phase Relations, Thermodyn., 12)... 

The oxygen dissociation pressure at temperatures below those where metal containing species are significant have yielded thermodynamic data on the intermediate oxides of praseodymium and terbium as discussed in section 2.3. 

The pressure of oxygen developed by an intimate mixture of a pure metal and its oxide is conventionally referred to as the dissociation pressure (Po ) and it can be seen that at a given temperature T... 

At a fixed temperature, AGr is constant, and so the equilibrium oxygen partial pressure, po2, will also be constant. This oxygen pressure is called the decomposition pressure or dissociation pressure of the oxide and depends only upon the temperature of the system. 

For oxidation of iron to occur at high temperatures, the oxygen partial pressure must be above that of the dissociation pressure of the appropriate corrosion products. For example, at ca. 700 °C, an oxygen partial pressure of greater than 10 Pa is required for wiistite to form. In air, of course, this condition is readily satisfied, at least initially. As oxidation continues and the film thickens and becomes coherent, an oxygen gradient across the film is established and the composition of the corrosion layer changes. 

---