Forming spangles method–galvanized iron sheet
The method of forming spangles is to generate a large number of crystalline nuclei as early as possible and reduce the solidification temperature of zinc liquid to prolong the growth time of spangle crystals and facilitate the growth of spangles.
(1) Add other elements to the zinc solution
When a specific alloying element is added to the zinc liquid, the crystallization process can be prolonged, and the surface zinc liquid will not solidify before reaching its eutectic temperature.
Pure zinc solidifies at 419.5°C. If tin (less than 0.5%) is added, the tin-zinc eutectoid alloy will not solidify until it drops to 198°C. After adding cadmium, the cadmium-zinc eutectoid alloy will not solidify until it drops to 264 °C. For example, after adding antimony (below 0.3%), the antimony-zinc eutectoid alloy will not solidify until it drops to 409℃. After adding lead, its lead-zinc eutectoid alloy will not condense until it drops to 317℃.
Therefore, adding alloys to reduce the melting point of zinc liquid will prolong the solidification time of pure zinc to varying degrees, which significantly prolongs the growth time of spangle crystals and facilitates the growth of spangles, so larger spangles can be obtained.
(2) Surface blowing gas method
When the surface zinc layer is about to solidify, if steam or sulfur dioxide gas flows through the surface, the spangle crystal can grow smoothly, and a larger pattern can be obtained. Another purpose of blowing sulfur dioxide gas flow is to form a thick oxide film and improve the corrosion resistance of the galvanized layer.
(3) Wire mesh method
This method can only be used on galvanized sheets, and it uses magnetic rollers to make the wire mesh contact the surface of the zinc layer to achieve this purpose. Because the contact point of the steel mesh is cooled first, crystalline solid particles are first formed there to become crystallization nuclei, and the remaining unsolidified zinc liquid, that is, the crystallization nuclei, begin to crystallize outward to form spangles.
(4) Water spray method
When the pure zinc liquid on the galvanized iron sheet surface is not solidified, spray water mist (a mixture of water and steam or water and air) to cause crystallization nuclei. The unsolidified zinc solution spreads out from this starting point to form spangles.
(5) Other methods
In addition to the above methods, if the surface of the plated parts is smooth, the galvanized layer is relatively thin, the temperature is controlled to drop evenly, and the cooling time in the air is sufficient, larger spangles can also be obtained.