1.What is the material basis for zinc layer crystallization?
Physical and chemical properties of zinc
The melting point of zinc is 419.5℃, the boiling point is 907℃, and it has good ductility and corrosion resistance at room temperature. During the galvanizing process, when liquid zinc solidifies on the surface of the steel plate, its crystallization behavior is affected by factors such as temperature gradient, composition supercooling, and interface energy.
The crystal structure of zinc is hexagonal close-packed (HCP), with lattice constants a=0.266nm, c=0.495nm. This structure determines its anisotropic growth characteristics during crystallization, that is, the growth rates of different crystal faces are different.
2.What are the thermodynamic and kinetic principles of crystallization phase transition?
Thermodynamic drive: When liquid zinc solidifies, the free energy of the system decreases, and the driving force of the crystallization process comes from supercooling (the difference between the actual temperature and the melting point). The greater the supercooling, the stronger the driving force for crystallization and the higher the rate of grain nucleation.
Kinetic process: Crystallization includes two stages: nucleation and growth. Nucleation is divided into uniform nucleation (spontaneous nucleation inside liquid zinc) and non-uniform nucleation (nucleation based on impurities, substrate surface defects, etc.); grain growth is achieved by the diffusion of atoms at the solid-liquid interface, and the growth rate is affected by temperature, solute diffusion coefficient, etc.
3.What are the core factors that affect the crystal morphology of the zinc layer?
Undercooling: As the undercooling increases, the nucleation rate increases and the grains become finer; when the undercooling is insufficient, coarse grains or columnar crystals are easily formed.
Substrate surface state: The roughness, oxide film, and alloy element distribution of the substrate surface affect the nucleation position and growth direction, and non-uniform nucleation occurs preferentially at defects.
Alloy element addition: Alloy elements such as Al, Ni, and Mg can change the melting point, surface tension, and diffusion rate of zinc liquid, inhibiting or promoting the growth of specific crystal planes.
Cooling rate: Rapid cooling (such as air knife injection and water cooling) increases the undercooling, promotes nucleation, and inhibits grain growth; slow cooling is conducive to grain coarsening.
Solute distribution and segregation: During solidification, solutes (such as Fe-Zn alloy phases) segregate at grain boundaries or between dendrites, affecting interface stability and resulting in differences in dendritic or equiaxed crystal morphology.
4.What is the crystallization principle of zinc flowers?
Zinc flowers are the most common crystalline form in the galvanized layer. In essence, they are the macroscopic appearance of zinc grains. Their formation is closely related to the addition of alloying elements (such as Pb and Sn) and the control of cooling rate.
Mechanism: When liquid zinc solidifies, alloying elements are enriched at the solid-liquid interface to form a supercooled zone, which promotes the growth of zinc grains in a dendrite manner, and finally forms radial or petal-shaped zinc flowers.
5.What is the crystallization control principle of zinc spangle-free?
Adding Al to inhibit the formation of zinc spangles: Al preferentially forms fine particles such as Al₂O₃ in the early stage of zinc liquid solidification, which serves as the core of non-uniform nucleation, greatly improving the nucleation rate, making the grains extremely fine, and zinc spangles cannot be seen macroscopically. ◦ At the same time, the rapid cooling process (such as increasing the air knife pressure and lowering the substrate temperature) further refines the grains and strengthens the zinc spangle-free effect.