Microstructure and Corrosion Resistance of Zn-Based Coatings: a Comparative Study

Corrosion in steel leads to significant resource waste. With that in view, three kinds of anti-corrosive Zn-based coatings were prepared on carbon steel, including electrodeposited Zn and Zn–Ni coatings, and as-sprayed Zn + Al coating. The microstructure, morphology, chemical composition, phase structure, and microhardness of the coatings were investigated using scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, and a Vickers microhardness tester. The wettability of the coatings was determined using a contact angle tester. The corrosion resistance of the coatings was evaluated through electrochemical impedance and dynamic polarization. The results revealed that Zn coatings consisted of the Zn phase with a hexagonal close-packed crystal structure. Zn–Ni coatings were composed of the Zn and Ni5Zn21 alloy phases. The as-sprayed Zn + Al coatings consisted of the Zn phase and the Al phase. The grain size of Zn coatings was 78.4 nm, and the grains were relatively large and irregular in shape. Zn–Ni coatings had a small grain size of 54.96 nm and exhibited some pinholes on the surface. Zn + Al coatings showed a flake-like structure with some micropores. The average microhardness values of Zn, Zn–Ni, and Zn + Al coatings were 112.6 ± 8.2 HV0.5, 249.7 ± 9.8 HV0.5, and 140.9 ± 7.6 HV0.5, respectively. Zn and Zn + Al coatings exhibited hydrophilicity, while Zn–Ni coatings demonstrated hydrophobicity, which can effectively slow down the penetration of corrosive media. Zn–Ni coatings exhibited the best corrosion resistance, followed by Zn + Al coatings, while Zn coatings exhibited the poorest corrosion resistance.