Diffusion oxide scale growth

Schematic showing the effect of oxide scale growth by counterdiffusion leading to compressive intrinsic oxide growth stresses (a). If diffusion in one direction is suppressed by doping (e.g., REE) the scale grows only on one side and no intrinsic growth stresses are built up (b). (From Kofstad, P., High Temperature Corrosion, Elsevier Applied Science, London, U.K., 1988.)...

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) . 

When exposed in air or cathode-side environment, active elements, e.g., Cr, in alloy substrates are preferentially oxidized forming an oxide scale on the alloy surface to protect it from further environment attack. Though the scale growth on an alloy substrate is affected by factors such as scale vaporization and grainboundary diffusion [163-165], it is often approximated by a parabolic relationship with time t ... 

Hydrogen permeation tests on ferritic stainless steels indicated that hydrogen can diffuse through the alloys, though the permeation was drastically decreased by formation of chromia scale on the alloys. - The mechanisms by which the presence of hydrogen or protons at the air side affects the oxide scale structure and growth are not clearly understood at this time. Several mechanisms have been proposed to tentatively explain the observed anomalous oxidation behavior. ... 

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. 

Critical for the coating system lifetime is the formation of a thermally grown oxide scale (TGO) at the interface BC/TBC during service, which mainly consists of alumina [3]. The possible oxygen diffusion in zirconia itself and the open columnar structure of the ceramic coating allows oxidation of bond coat aluminum. The scale s growth and the difference in... 

Stage 2 also follows logarithmic kinetics, reflecting competition between parabolic oxide growth and short circuit diffusion down preferred channels. Initially, the short circuit paths account for the early observed rapid scale growth. A transition is later observed to parabolic kinetics, which marks the onset of the third stage of scale growth in hot salt accelerated oxidation of -y-TiAl. 

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... 

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. 

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