Cationic scale growth

Nucleation considerations may dictate that the first oxide to form will have an epitaxial relationship with the substrate. This constraint will result in stress development because of the difference in lattice parameter between the metal and the oxide. This mechanism would only seem to generate significant stresses when the oxide is very thin, i.e., for short oxidation times and low oxidation temperatures. However, there are proposals that the action of intrinsic dislocations, in what amounts to a semi-coherent interface, in amuhilating vacancies during cationic scale growth can lead to sizeable stresses. However, there are also interface dislocation structures proposed, which could annihilate vacancies without generating significant stress. ... 

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. 

Vacancy injection, a phenomenon usually considered to be characteristic of cationic scale growth, was a rather popular topic after the mid-1990s. This phenomenon is again often invoked to explain the formation of pores and cavities within the metallic substrate beneath an oxide scale such as NiO or FeO and, more recently, AI2O3 for the oxidation of intermetallic aluminides. In that case, the metal-oxide interface is assumed to be unable to move freely to follow the recession of the metal lattice. Reaction [2.3] must then be replaced by the following reaction ... 

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 ... 

When we consider continuous scale growth, we can expect that the mobile species from the metal (cations diffusing out) will be supplied by alloy grain-boundaries, bulk defects, and dislocations. These diffusivities are quite different from each other D(bulk) D(dislocation) < D(grain boundary) < D(surface). Therefore, we expect the formation of voids around the alloy grain boundaries and dislocations as the scale continues to grow. The chief concerns, here, is How can we prepare an inert state (kinetically and thermodynamically) for the point defects, for the grain boundaries, and especially for the dislocations in the alloy substrate ... 

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 2-8. Position of inert markers after oxidation when the scale growth is diffusion-controlled, a) Growth by metal cation diffusion, b) Growth by oxygen inward diffusion, c) Growth by simultaneous metal and oxygen diffusion (Kofstad, 1988).

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). 

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. 

Interfacial defects active in the growth of a cationic scale. 

The discoimection translation is a conservative movement but the misfit dislocations must be maintained along the scale/substrate interface to avoid the build-up of large interfacial stresses. Because of PBR values higher than 1, the growth of one molecular layer of semi-coherent oxide at the scale/ substrate interface involves fewer cations than the corresponding number of metal atoms in the substrate reticular plane in contact with the scale. Therefore, the translation of disconnections caimot be the only scale growth process as it would not permit the minimization of interface stresses and the maintainance of the interface in a stress-free state. Discoimection translation needs to be assisted by the climb of oxide misorientation dislocations and/or by the climb of misfit and misorientation dislocations on the metal side. Climb of oxide misorientation dislocations would result in an expansion of the oxide lattice normal to the interface, while the climb of metal dislocations would result in a recession of the metal lattice. 

Pieraggi B and Rapp R A (1988), Stress generation and vacancy annihilation during scale growth limited by cation-vacancy diffusion , Acta Metall, 36(5), 1281-1289. 

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... 

As indicated earlier, protective oxide scales typically have a PBR greater than unity and are, therefore, less dense than the metal from which they have formed. As a result, the formation of protective oxides invariably results in a local volume increase, or a stress-free oxidation strain" . If lateral growth occurs, then compressive stresses can build up, and these are intensified at convex and reduced at concave interfaces by the radial displacement of the scale due to outward cation diffusion (Fig. 7.7) . 

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