Anionic scale growth

Interface reactions depend on interface structure and vice-versa. The combined effects of these two elements of phase transformations involving the diffusion of reacting species results in interface dynamics as analysed the general case of diffusion-driven phase transformations (Pieraggi etal., 1990). The proposed models can be adapted to the growth of an oxide scale in distinguishing cationic and anionic scale growth. 

Mass transport measurements have shown that cation transport predominates in FeO (Fe ) and Fej04 (Fe, Fe ), whereas anion transport predominates in FejOj (0 ). This leads to the well-accepted growth scheme for multi-layered scale growth on iron shown in Fig. 7.3, with the governing equations for individual layer growth being ... 

The aforementioned requirements on surface stability are typical for all exposed areas of the metallic interconnect, as well as other metallic components in a SOFC stack (e.g., some designs use metallic frames to support the ceramic cell). In addition, the protection layer for the interconnect, or in particular the active areas that interface with electrodes and are in the path of electric current, must be electrically conductive. This conductivity requirement differentiates the interconnect protection layer from many traditional surface modifications as well as nonactive areas of interconnects and other components in SOFC stacks, where only surface stability is emphasized. While the electrical conductivity is usually dominated by their electronic conductivity, conductive oxides for protection layer applications often demonstrate a nonnegligible oxygen ion conductivity as well, which leads to scale growth beneath the protection layer. With this in mind, a high electrical conductivity is always desirable for the protection layers, along with low chromium cation and oxygen anion diffusivity. 

The theory of multi-layered scale growth on pure metals has been treated by Yurek et al The hypothetical system treated is shown in Figure 4.9. It is assumed that the growth of both scales is diffusion controlled with the outward migration of cations large relative to the inward migration of anions. The flux of cations in each oxide is assumed to be independent of distance. Each oxide exhibits predominantly... 

Figure 2>21. Schematic diagram of the situation concerning the development of geometrically induced growth stresses (arrows) in oxide scales on curved surfaces for scale growth by anion and by cation diffusion, respectively (Christl et al., 1989).

It has repeatedly been confirmed that in chromia-forming alloys, the mechanism of oxide-scale growth is changed in the presence of rare-earth elements from cation to anion control, and consequently the direction of growth also changes (Hussey et al. 1989, Graham 1991). 

Position of inert markers (e.g., Pt or AljOj partides) after oxidation when the scale growth is diffusion controlled (a) growth by outward metal cation diffusion, (b) growth by inward oxygen anion diffusion, and (c) growth by counterditfusion of both spedes. (From Kofstad, R, High Temperature Corrosion, Elsevier Applied Science, London, U.K., 1988.)... 

The term in relation [2.1] is the fraction of cationic scale growth occurring at the external interface fc is equal to the ratio of intrinsic diffusivity of cations and anions and, for the considered ideal case, is also equal to the ratio oxJ Oxi- Relation [2.1] can be easily checked from the displacement of the /(LQy plane for pure cationic (Fig. 2.5a) or pure anionic (Fig. 2.5b) growth. 

Interfacial defects active in the growth of an anionic scale. 

Ghelants and Precipitation Inhibitors vs Dispersants. Dispersants can inhibit crystal growth, but chelants, such as ethylenediaminetetraacetic acid [60-00-4] (EDTA), and pure precipitation inhibitors such as nitrilotris(methylene)tris-phosphonic acid [6419-19-8], commonly known as amino trismethylene phosphonic acid (ATMP), can be more effective under certain circumstances. Chelants can prevent scale by forming stoichiometric ring stmctures with polyvalent cations (such as calcium) to prevent interaction with anions (such as carbonate). Chelants interact... 

Some of the fastest photochemical processes occur on the ps time-scale, for instance electron transfer reactions. In the case of intermolecular electron transfers the actual reaction rate constants cannot be obtained when the diffusion of the reactants is the limiting factor. High concentrations must be used to ensure that encounters are faster than the reactions. Figure 8.7 shows the ps transient absorption spectra of the electron transfer between benzo-phenone and DABCO in acetonitrile. The triplet excited state of benzophe-none is seen to decay at 525 nm while the radical anion grows at about 700 nm to reach a maximum concentration after 1 ns. The decay and growth kinetics are shown in (b) of the same Figure. 

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