Engineering of zinc-bismuth binary microspheres and evaluation of ZnBi-rich primer for corrosion protection of steel in marine environment

Zinc-rich primers (ZRPs) are highly effective for steel corrosion protection. In this study, two types of zinc-bismuth binary microspheres (ZnBi1 and ZnBi2) were produced via co-evaporation of zinc and bismuth under gas flow rates of 4 Nl/min and 0.3 Nl/min. Scanning electron microscopy and energy-dispersive spectrometer, X-ray diffraction and differential scanning calorimetry analyses showed that both ZnBi1 and ZnBi2 particles were heterogeneous solidified mixtures, with Zn and Bi non-uniformly distributed in individual particles. ZnBi1, with finer particle size, exhibited higher self-corrosion and galvanic corrosion rates in 3.5 wt.% NaCl solution in its pulverized state. The particles were incorporated into ZRPs, and their sacrificial effects were evaluated using a salt spray exposure test, open circuit potential (OCP) and electrochemical impedance spectroscopy measurements. Compared to conventional ZRP containing 78 wt.% commercial zinc dust, the coating with 78 wt.% ZnBi1 particles exhibited enhanced cathodic protection at an early stage of exposure due to promoted sacrificial oxidation of zinc at damaged areas. However, the accelerated zinc oxidation led to a shortened cathodic protection duration as observed in the OCP result. A combination of ZnBi particles and commercial zinc dust mitigated the intense galvanic function, resulting in a more stable galvanic activity and improved anticorrosion performance.