Effect of Steel Substrate Microstructure on Intermetallic Layer Growth during Hot Dipping in Aluminum

In this study, aluminizing of carbon steel with different microstructures was investigated by hot dipping into molten pure aluminum. For this purpose, low carbon and medium carbon steel samples were annealed, normalized and quenched to achieve different microstructures. Then, samples were hot dipped in molten aluminum at 730, 770 and 830 °C. Scanning electron microscope indicated an outer layer of aluminum and an inner compound layer on the surface of the hot dipped samples. The compound layer was composed of two intermetallic phases of Fe2Al5 in a columnar and tongue-like morphology and FeAl3 in a needle-like morphology. The growth behavior of the intermetallic layer was studied after various times of immersion by measuring the thickness of the compound layer. Furthermore, effect of steel substrate composition and microstructure on the morphology of the compound layer was investigated. Results indicated that steel substrates with higher carbon content reveal smoother coating layer. However, the substrate grain size did not show a considerable effect on the rate and morphology of coating. Kinetic studies proved a semi-parabolic behavior that implies diffusion controlled growth of the compound layer. Finally, the activation energy of the layer thickening was estimated for the low carbon steel substrate with different heat treatment conditions.

Keywords:

Aluminizing , Hot dip , kinetics , Intermetallics , Fe2Al5