The Effect of Bi on the Kinetics of Growths, Microstructure and Corrosion Resistance of Hot-Dip Galvanized Coatings

This paper presents the results of studies on the growth kinetics, microstructure (SEM/EDS) and corrosion behavior of coatings obtained by hot-dip galvanizing process in baths containing Bi additive. The coatings for testing were produced on low-silicon steel in a Zn bath containing 0.04, 0.12 and 0.4 wt.% Bi. The corrosion resistance of the coatings was determined comparatively in standard Neutral Salt Spray Tests (NSST) (ISO 9227) and sulfur dioxide test (SDT) in a humid atmosphere (ISO 22479). Potentiodynamic tests and electrochemical impedance spectroscopy measurements were conducted. It was found that the addition of 0.04 and 0.12 wt.% Bi reduces the total thickness of the coatings and the thickness of intermetallic layers, while the content of 0.4 wt.% Bi in the bath increases the thickness of the layers forming the coating. Direct corrosion tests (NSST and SDT) and electrochemical tests showed that the addition of Bi to the zinc bath reduces the corrosion resistance of the coatings. The corrosion resistance of the coatings decreases with increasing Bi concentration in the zinc bath. In the microstructure of the coatings, it was found that Bi precipitates mainly on the surface of the coating, but also on the cross-section of the outer layer and zeta intermetallic layer. Bi precipitates, due to their cathodic nature, affect the reduction of the corrosion resistance of the coatings with the increase of their content in the bath.