Langfang DingYang

How Does The Pure Zinc Coating of Galvanized Pipe Crystallize?

fittingflange | 30 December, 2018 10:34

The surface condition of the plated workpiece is very important for the quality of hot-dip galvanizing. In order to obtain a good galvanizing coating, it is necessary to treat the surface of the workpiece seriously, which is the necessary condition to ensure the effect of the whole process. In the electroplating operation, even if the grease scale and oxide scale on the surface of the workpiece are not cleaned up, the zinc coating may still deposit on it. Of course, the interlayer between the coating and the iron base may form, leading to bad conditions such as peeling, foaming or speckles. In hot-dip galvanizing, if the surface of the workpiece is not cleaned thoroughly, the adhesion of the zinc layer will be hindered. This situation, in which no zinc can be plated on the workpiece, is called "zinc deficiency" in hot-dip galvanizing. If the surface of the plated workpiece is rough and uneven, although it does not affect the hot-dip galvanizing work, but the silver out of the workpiece is also rough, uneven, of course, not bright. The corrosion resistance of rough galvanized coating is lower than that of flat and smooth galvanized coating. Obviously, the rough galvanized coating can not achieve the ideal anticorrosive and decorative purposes. Therefore, before hot-dip galvanizing, it is necessary to clean the workpiece as cleanly as possible by mechanical or chemical methods to meet the requirement of surface smoothness for hot-dip galvanizing. Hot-dip galvanizing is simpler than galvanizing for surface treatment of workpiece. Because machined parts can be directly hot-dipped without pre-plating treatment. For the workpieces processed by other methods, only chemical treatment is needed; for the workpieces formed by casting, mechanical treatment is needed. In hot-dip catalpa plating process, the residual oil, oil film and other organic matter on the surface of the workpiece during the processing do not hinder galvanizing, because these substances are burned or volatilized due to the high temperature during galvanizing. The main factors affecting the quality of hot-dip galvanizing are oxide scale and iron sulfide layer on the surface of the workpiece. As for the thinner rust, it will not have a great impact on the hot-dip galvanizing construction. Nevertheless, the pretreatment of hot-dip galvanizing should not be neglected. Because the quality of the workpiece before plating determines the quality of the whole hot-dip plating process. There are two stages in the process of metal crystallization: nucleation and growth. That is to say, the nucleus with a certain size is formed in the liquid first, and then the liquid atoms around the nucleus are continuously condensed on the nucleus, which makes the nucleus grow into a grain. When the temperature of liquid metal drops below the solidification point, there are more embryos in the liquid, but whether it can grow to a certain size and become nucleus is affected by various external factors. The number of nuclei determines the size of crystalline grains after crystallization. The more nuclei, the more grains, but the smaller the size. The ways to form nuclei are as follows: First, the impurities attached to the surface of steel strip, such as iron powder, oil residue, alkali stain, etc. The nuclei formed by these ways are often not ideal, resulting in uneven zinc flowers or poor zinc plating. Secondly, the protrusion attached to the surface of the steel strip is the reason why the surface roughness of the steel strip affects the size of the zinc spark. If the surface roughness of the original plate is human, the zinc spark is smaller. Third, the lower temperature in the coating. If there is no external core in the coating, the undercooling of the coating will increase. From the microscopic point of view, the phenomenon of temperature inhomogeneity in the liquid will increase, that is, the temperature fluctuation will increase, and the lower temperature will be conducive to the formation of crystalline core. Comparing these three ways, the first two ways are easier to nucleate, but the adhesion of the coating is poor. The third crystallization rate is the slowest and the most ideal crystallization state. The coating after crystallization has strong adhesion and good uniformity of zinc flower. With nuclei in the coating, it is very easy for the nuclei to grow up to crystallize completely. With the decrease of temperature, liquid atoms continuously gather on the nuclei and grow up according to certain rules. Finally, all the liquids are transformed into solids, and the whole crystallization process is completed. The final result is that a crystal nucleates into a grain, which is a zinc flower. In fact, the so-called large zinc flower, small zinc flower and zinc-free flower are just the number and size of grains. At the junction of zinc flowers, i.e. grain boundaries, the atomic arrangement is irregular. During crystallization, some alloy elements and impurities that are insoluble in the coatings will be driven to this place. The aggregation formed at the grain boundaries is called intergranular segregation, so the corrosion usually begins at the grain boundaries. The larger the grain size, the more obvious the segregation and the greater the influence.


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