Stable oxide scale

Table 1 shows the equilibrium oxygen partial pressure of several metal oxides, i. e. the minimum oxygen partial pressure which is required for oxide formation. A comparison with Table 2, in which typical oxygen partial pressures of industrial processes are listed, clearly demonstrates, that in coal gasification processes and petrochemical plants stable oxide scales can only be expected on alumina formers. Chromia becomes unstable at the low oxygen partial pressures encountered in such processes. 

Stable oxide scale Corrosion affects SOFC stacks in two distinct ways. First, it increases the electrical resistivity. In normal configuration of cells, the electrical path usually penetrates across the oxide scale, which implies that growth of oxide scale makes a contribution to increase the area-specific resistivity. Another aspect of oxidation is related to mechanical stability. Growth of an oxide scale is inevitably accompanied with a volume change. 

To fully understand the formation of the N13S2 scale under certain gas conditions, a brief description needs to be given on the chemical aspects of the protective (chromium oxide) Ci 203/(nickel oxide) NiO scales that form at elevated temperatures. Under ideal oxidizing conditions, the alloy Waspaloy preferentially forms a protective oxide layer of NiO and Ci 203 The partial pressure of oxygen is such that these scales are thermodynamically stable and a condition of equilibrium is observed between the oxidizing atmosphere and the scale. Even if the scale surface is damaged or removed, the oxidizing condition of the atmosphere would preferentially reform the oxide scales. 

Iso-UP has ester bonds only in the main chain where hydrolysis occurs, so a part of reaction products from the main chain dissolves into the solution. While the crosslink formed by styrene remains unaffected because of its stable C-C bonding. As a result, the corroded surface layer resists the diffusion of NaOH solution. This mechanism is just like an oxidation of the metal at high temperature with formation of thick, cohered oxide scale, and can be expressed by similar relation of Wagner s parabolic law as shown in Equation 2. The concept of corrosion in metals can be applied in this case too. 

The oxides formed with soft acids like Ag+ and Hg2+ are of very low thermal stability, and the metal can be obtained from them by moderate heating. Gold, the most noble metal of all, does not form any stable oxide. Contrariwise, the bromides and iodides of elements whose cations are classed as hard acids can often be thermally decomposed at moderate temperatures. Such a procedure is rarely economic for industrial production, but can be useful for small-scale laboratory preparations, especially if the elemental substance is required in a state of high purity. 

Although originally AISI347 stainless steel seemed a reasonable choice, it was soon shown that it is not suited for long term operation over several years. The best corrosion protection would be provided by materials that can passivate. AISI 316 or AISI 310 stainless steel would in principle provide such a relatively stable and protective oxide scale in the presence of molten carbonate at least under cathodic conditions. 

Metallic nickel is stable at —1100 mV and -900 mV. Therefore no oxide scale is formed on the surface at these potentials. 

In the Nb-containing alloys the near-continuous nitride layer was found under all conditions studied after air oxidation (Fig. 10b, d).This can be explained by the better protective properties of the oxide scale formed on these alloys, which results in a lower p02/pN2-ratio at the oxide/ alloy interface in case of the binary alloys. Consequently the near-continuous nitride layer remains stable up to very long oxidation times. A shift from oxidation stage II into III, which occurs for the Nb-containing alloys in a nitrogen-free environment (Fig. 10a, c), does therefore not occur during air oxidation. 

The met prefix refers to the ferric heme form of Mb/Hb, and, in fact, metMb/Hb is the thermodynamically stable oxidation state in an aerobic atmosphere. Reaction (3) typically occurs on the time scale of several hours to days at room temperature. MetMb/Hb reductases are present in tissues that keep the steady level of the met forms at a few percent or less. 

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