Steel: Aluminum Coatings

Aluminum coated steel possesses excellent oxidation and corrosion resistance in sulfur and marine: environments and can substitute for expensive alloy of steels. Hot dip aluminizing (HDA) and pack cementation calorizing (CAL) are dealt with in detail. IN HDA coats, some alloying action takes place, when the substrate is dipped in molten Al at 973 K for 1–2 minutes. The coat consists of an outer pure Al layer, followed by a hard intermetallic layer consisting of FeAl3 and Fe2Al5, forming a serrated interface with the base. Isothermal holding of such samples at 773–933 K for 10 minutes leads to further diffusion and phase changes. This improves resistance to thermal shock and bending. In CAL coats, the process parameters (1173–1223 K/2–4 h and pack composition), were optimized, resulting in appreciable alloying. The surface layer consists of Fe3Al and FeAl, which is comparable to the inner alloy layer of HDA coats. The structure/ property correlation is carried out for both coatings and the results compared.