-The nature of the intermetallic layer which forms on the steel surface during immersion in typical galvanizing baths for galvannealed (GA) sheets production has been investigated on two commercial Titanium-stabilized Interstitial-Free (Ti-IF) steel substrates galvanized in baths with different Al contents. Results from this study show that in both cases the inhibition layer is biphasic and composed of a very thin Al-rich phase layer, identified as Fe 2 Al 5 Zn x , and a thicker Zn-rich phase layer on top of it, identified as δ. Experimental results also show that the Fe 2 Al 5 Zn x phase layer becomes discontinuous when decreasing the bath Al content. Discussions about the mechanisms of formation and the final microstructure of this inhibiting layer are also tackled in this paper by means of the Al-Fe-Zn ternary phase diagram at 460 • C and assumptions to justify any deviation from thermodynamic equilibrium are as well proposed.
In the galvannealing process, steel strips are immersed in molten zinc containing 0.100 to 0.135 wt pct Al at 450°C. The coating obtained is composed of a thin intermetallic compounds' layer called the inhibition layer (200 nm) covered with a thick zinc layer (10 lm). The nature of this inhibition layer has been investigated here for a galvanizing bath with a low Al content. The inhibition layer formed on industrial low-alloyed steels was characterized by transmission electron microscopy and atom probe tomography. The inhibition layer is composed of a thin Fe 2 Al 5 Zn x layer (20 nm), covered with a thicker d layer (200 nm). The Fe 2 Al 5 Zn x layer is discontinuous at the lowest bath Al content. Small precipitates (20 nm in diameter) with a stoichiometry corresponding to Fe 3 Al-containing Zn were also found for the first time in the d phase. The microstructure of the inhibition layer can be described with diffusion paths drawn in the Al-Fe-Zn ternary section at 450°C. This means that all interfaces of the inhibition layer are at thermodynamic equilibrium. The Fe 2 Al 5 Zn x layer is formed on the steel surface before the d layer. The nucleation and growth of the Fe 3 Al-Zn particles probably occur in the liquid metal at the same time as d.
The aim of this paper is to present a model that predicts the transition from internal to external oxidation. This variant is based on the simultaneous resolution of the diffusion equations and the equilibrium equation that stems from the assumption of the local instantaneous thermodynamic equilibrium. It accounts for the possible formation of large precipitates fractions that may act as diffusion barriers. This effect is modeled by introducing a dependence of the diffusion coefficients upon the mass fraction of precipitates. As a counterpart, it is generally impossible to solve the non-linear equations of the model analytically. Thus, a semi-analytical and a finite elements models are presented.
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