STRUCTURAL STUDIES OF HOT DIP ALUMINIZED COATINGS ON MILD-STEEL

Hot dip aluminized layers on mild steel have been studied by optical microscopy, microhardness measurement, X-ray diffraction and electron diffraction. The investigation establishes the presence of an outermost, very soft, Al bearing layer followed by a soft intermetallic layer containing Fe3Al and a hard intermetallic region consisting of FeAl3 and Fe2Al5. The presence of an outer Al layer is responsible for the material's low degree of wear resistance and low stability at high temperatures. The possibility of eliminating or minimizing this layer by diffusion annealing was studied at 773, 873 and 923 K, for 10 min-10 h. Aluminum is found to diffuse, eliminating the pure Al layer and giving rise to FeAl3 initially. Subsequently the phase Fe2Al5 forms along with FeAl3 and finally only Fe2Al5 forms. However, this heat treatment is not sufficient to precipitate any of the favourable phases such as FeAl, Fe3Al which satisfy Pilling-Bedworth's ratio for providing good compatibility with the matrix. The intermetallic layers are adherent and can endure thermal shock and bending.