Structural and Mechanical Characterizations of Top Dross in a Molten Zinc Bath

In a molten zinc bath in a continuous galvanizing line (CGL), top dross particles crystallize as Fe–Al–Zn intermetallic compounds. These particles easily adhere to the steel sheets causing surface defects. Therefore, controlling the top dross particles is a key issue. Our study focused on the structural and mechanical characterizations of the top dross particles using an electron probe micro analyzer, X-ray diffraction, electron back scattering diffraction, Vickers hardness measurements, and nano-indentation measurements. The following results were obtained: (1) The crystal structure of the top dross particles Fe2Al5Znx having 37–38 wt% Fe, 44–45 wt% Al, and 18–19 wt% Zn belongs to the orthorhombic system with lattice constants of a = 7.61 Å, b = 6.48 Å, and c = 4.23 Å. The a-axis of Fe2Al5Znx becomes shorter, while its b- and c-axes become longer compared to those of the binary Fe2Al5. (2) The coarsening of the top dross particles with the faceted interface was postulated to occur as a result of the driving force provided by the anisotropic interface energy between the top dross particles and molten Zn, rather than via the aggregation mechanism. (3) The hardness and the elastic modulus of the top dross particles are the lowest in the [001] orientation, similar to Fe2Al5, and are lower than those of Fe2Al5. (4) The fracture toughness of the top dross particles is approximately 1.1 MPa·m1/2, which is slightly lower than that of Fe2Al5.