The composition, the structure and the growth of the alloy layer formed at iron-aluminum interface during the hot dipping process of steel in aluminum has been determined by using methods such as optical microscope, microhardness tester, X-ray diffractor meter and electronprobe microanalyzer.
The effects of carbon and silicon have also been studied on the growth rate of the alloy layer.
The results obtained as follows;
(1) The growth of the alloy layer follows the parabolic rate law.
(2) The thickness of the alloy layer is markedly decreased by adding the alloying element such as Si and C in the molten aluminum bath or iron specimen and the mechanism of thickness decrease of the alloy layer is also discussed.
(3) The interdiffusion coefficients in alloy layer formed in pure Al bath for the pure iron and commercial steel are
$\widetilde{D} = 11.0 \exp (-\frac{31.0\pm0.7}{RT} )$
$\widetilde{D} = 10.5 \exp (-\frac{31.1\pm0.7}{RT})$ respectively
The interdiffusivities for 4% Si-Al, 8% Si-Al and 12% Si-Al bath are
$\widetilde{D} = 2.6\times10^{-2} \exp (-\frac{24.4\pm4.3}{RT})$
$\widetilde{D} = 3.4\times10^{-4} \exp (-\frac{16.9\pm0.8}{RT})$
$\widetilde{D} = 5.6\times10^{-4} \exp (-\frac{18.4\pm2.2}{RT})$ respectively
(4) The microhardness of the alloy layer is independent of the Si additions in the molten aluminum bath.
(5) In the pure aluminum and Al-Si alloy coatings on steel, the alloy layer is mainly composed of $Fe_2Al_5$ phase.
(6) When aluminum coatings are made in Al-Si alloy bath, the inner boundary of the alloy layer is flatter than that in the pure aluminum bath. This phenomena is owing to the crystallographic structure of $Fe_2Al_5$ phase and Si motion in the $Fe_2Al_5$ phase.
(7) Finally some consideration on the mechanism of the formation and growth of the alloy layer is also discussed.
