Phase Structures of Galvanized Alloyed Coating and Their Corrosion Resistance

Galvanized alloyed (GA) coating has a complex multiphase structure. In order to characterize the phase distribution in GA coating, the morphology of the coating was observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the thickness, element distribution and phase structure of the coating were analyzed by glow discharge spectroscopy (GDS), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The coatings were stripped by constant current polarization, and the electrochemical properties of different phases in the coatings were studied. The results show that the GA coating was composed of several phases, which were ζ phase, δ phase, Γ1 phase and Γ phase from top to bottom. Fe content increased with the increase of coating thickness. ζ phase and δ phase had good corrosion resistance, and the potential difference between them was small, so their galvanic corrosion effect was small. After δ phase was completely consumed, the Γ1 phase, Γ phase and substrate potential were corroded. In the three phases, the free corrosion potential of Γ phase was more negative and corrosion occurred first in this phase. Therefore, the δ phase should be kept uniform and thick to protect the substrate from corrosion.