Microstructure of Batch Hot-Dip Zn-5 Wt Pct Al Coatings: Comparison of Ball-Milling Pretreatment and Conventional Pretreatment

Instead of pickling and solvent fluxing in the conventional batch hot-dip galvanizing, ball-milling pretreatment was used to study the crystal orientation of the Q235 steel surface and the structure growth of the Zn-5 wt pct Al coating. The results show that both pickling (P) and ball-milling (BM) treatments effectively eliminate rust without changing the phase composition and the structure of the Q235 matrix surface. The surface grains of both P-Q235 and BM-Q235 exhibit preferential orientation relationships. The ordering of the area fractions for grains with different orientations is (101) > (001) > (111). In this case, the area fraction of (101) crystalline decreases from 52 pct in P-Q235 to 39 pct in BM-Q235, which is caused by the rearrangement of grain internal crystallography induced by plastic deformation on the surface of the BM-Q235 matrix. The change in grain size on the surface of the matrix was negligible, decreasing from 6.9 ± 1.2 μm in P-Q235 to 6.0 ± 1.5 μm in BM-Q235. The free energy at Fe-Al interfaces with different crystallographic orientations is calculated based on first-principle simulations, and the influence of α-Fe grains with varying crystallographic orientations on the nucleation and the growth of Fe-Al alloy phases is studied. The results show that the bonding between Al and α-Fe is anisotropic. The free energy of the Fe-Al systems on the Fe crystal plane exhibit the order of (101) < (001) < (111), and the Fe(101) system demonstrates a significantly lower free energy than others. In comparison to the Al crystal plane, the Fe crystal plane exerts a more pronounced influence on the free energy of the Fe-Al system. The Fe(101) surface shows a tendency for the preferential nucleation of the Fe-Al alloy phase.