Validation of the Modelling of a Solid-Liquid Reaction by A Solid-Vapor Reaction
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described as iron-saturated, is in fact "c-saturated". Therefore, as soon as iron concentration increases, an appreciable driving force for ý precipitation occurs. Another factor that favours the germination and growth of the c-phase is related to its crystallographic structure, which is much simpler than that of 81 [7], and to the atomic bonds which compared with those in 81, are more metallic as shown by the hardnesses: Hv 8 l=340 and Hv=l12 under 50g [8]. Therefore a nucleus is more likely to form than a 81 one. Previous thermodynamic calculations [3] show that, during the first immersion times, germination conditions are favourable to ý-crystallisation. Electron microprobe analysis and thermodynamical description of the Fe-Zn-Si system at 450'C [3] show that the solubility of Si in C-phase is vanishing. On the other hand, the 81 phase is likely to dissolve about 1 at% Si at 450°C and FeSi accepts about 1 at% Zn. Moreover, it is worth noticing that the C-phase cannot lead to any binary equilibrium with FeSi. The excess of silicon in the bath is likely to be reduced according to two ways: (i) precipitation of FeSi particules; (ii) nucleation and growth of 81. In this case, the nucleation of ý could be homogeneous and the texture of the c-layer is more diffuse. The high iron supersaturation leads to an important driving force for ý formation and therefore to an important reduction of the critical size for homogeneous nucleation of •. Modelling the mechanism Figure 1 presents the interpretation of the role of silicon when galvanizing industrial steels in pure zinc bath saturated with iron at 450'C for 9 minutes. For a low silicon content, which corresponds to "hypo-Sandelin" steels, the morphology does not greatly differ from that obtained with iron. The growth and the morphology of the coating are described for steel A. The silicon concentration in the zinc bath A' does not greatly impede the early formation of the C-phase in contact with the steel surface by a heterogeneous nucleation. When the iron concentration in the C-phase in contact with the steel increases, the conditions for the appearance of 81 are fulfilled. Later, a faint layer of F-phase is likely to be observed between ox and 81. Mechanisms are driven by diffusion in the solid state and the kinetics obeys a \t-relation. When the silicon concentration in steel is near 0.07 wt%, the amount of C compound in the coating dramatically increases and a thick two-phased C+r7 layer is overcoated by the Tl zinc layer corresponding to the solidification of the liquid which was wetting the sample when pulled out the zinc pool. This concentration of 0.07 wt% corresponds to the Sandelin peak. The thickness of the coating observed in the present case after 9 minutes dipping is about 500 gtm, roughly 10 times more than that obtained with pure iron. Silicon segregates in the liquid in the vicinity of steelcoating interface. With steel B, the liquid, of which the silicon concentration is about B' near the substrate, is no more in equilibrium with Cbut with 81 [3]. Howe
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