Abstract

This study systematically investigates the effects of Ni and Al addition to a zinc bath on the microstructure, corrosion resistance and Vickers hardness of hot–dip galvanized coatings on Q235 steel. The Zn, Zn–0.04 wt.% Ni and Zn–0.04 wt.% Ni–1 wt.% Al coatings are characterized by scanning electron microscopy (SEM), X–ray diffraction (XRD), neutral salt spray (NSS) testing, electrochemical measurements and Vickers hardness testing. The addition of Ni refines and thins the ζ phase, increases the thickness of the δ phase, reduces the total coating thickness to 56.33 ± 0.89 μm and improves the corrosion resistance. With further Al addition, a continuous and dense inhibition layer forms at the interface, and the coating consists solely of this inhibition layer and the η phase; the total thickness is drastically reduced to 17.66 ± 1.78 μm, accompanied by a substantial improvement in both corrosion resistance and hardness. Electrochemical analysis reveals that the Zn–0.04 wt.% Ni–1 wt.% Al coating exhibits the most negative corrosion potential (0.61 V) and the smallest corrosion current density (2.07 μA·cm−2). Characterization of the corrosion products reveals that Al helps to stabilize Zn5(OH)8Cl2·H2O and Zn5(OH)6(CO3)2, thereby enhancing the corrosion resistance of the coating.

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