Abstract
The influence of Si addition to the coating bath on the growth of the Al–Fe alloy layer formed in the interface between the coating and steel substrate was studied utilizing a Zn–Al–Mg coating bath with 10–11 mass% Al content. While the Al–Fe intermetallic compound rapidly grew in the interface between the coating and steel substrate when dipped into a coating bath of Zn–10mass%Al–3mass%Mg at 550°C, the growth was remarkably suppressed by the addition of 0.2mass%Si to the coating bath. TEM observation revealed that, in the interface between the coating and the substrate, Si addition resulted in the uniform formation of very fine grains of 20–30 nm in diameter in the Fe2Al5 phase with dissolved Si and Zn in a solid solution. The mechanism of Si addition was postulated to be as follows: the large amount of Fe in the substrate could be dissolved easily into the liquid phase in the case of a ternary Fe–Al–Zn system, whereas in the case of a quaternary Fe–Al–Zn–Si system, the dissolution of Fe into the liquid phase was significantly suppressed by the presence of very fine, thin Fe2Al5 containing Si in the interface between the coating and the steel substrate. Thus, the growth rate of Fe2Al5 was greatly reduced.
