STUDY OF THE STRUCTURE OF ZINC COATING SURFACE DEFECTS FORMED IN THE TECHNIGALVA MELT

During hot-dip galvanizing of so-called "reactive" steels containing about 0.1 % (by weight) of silicon, the formation of a zinc coating is observed. To control the thickness of the zinc coating on steels, the Technigalva technology is widely used, which is the microalloying of the zinc melt with nickel in an amount of 0.05 % (by weight). Despite the popularity of the technology in question, in some cases defects form on the surface of the coating in the form of a stuck "grain". The purpose of this work was to study the structure and phase composition of zinc coating defects formed in a zinc melt with nickel microadditives, as well as to analyze the causes of their 

occurrence and find ways to prevent their formation. The microstructure of the coating in the area of defects was studied using a scanning electron microscopy. It is shown that the high-quality coating consists of standard intermetallic G-, δ- and ζ-phases, coated with a solid zinc solution with the n-phase. In the area of the defect in the n-phase, inclusions of regular geometric shape are observed, the location and dimensions of which indicate that they adhere to the coating at the moment of attraction of the product from the melt. The identification of the phase composition of inclusions was carried out using EDS (energy-dispersive X-ray spectroscopy) and EBSD (Electron backscatter diffraction) analysis. It was found that the inclusions are a ζ-phase (isomorphic to FeZn₁₃), containing about 0.8% (by weight) nickel. It is shown that the main causes of the formation of the "grain" defect are the appearance of floating waste particles in the melt caused by contamination of the zinc melt with iron, as well as a local excess of the recommended nickel concentration. Methods of preventing defects of the considered type of species during hot galvanizing are proposed.

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