Knowledgeof hot dip galvanizing surface zinc flower
Whatis zinc flower
Beautifulzinc flower is a main feature of traditional hot-dip galvanizedsheet. The most complete form of zinc crystal is similar tosnowflake or hexagonal star, so the zinc crystal obtained bycondensation on the surface of steel strip by hot plating process ismost likely to form snowflake or hexagonal star zinc flower.
Infact, zinc flowers only have ornamental value, not practical value.On the contrary, zinc flowers are not good in corrosion resistance,and the surface will appear convex and concave aftercoating. Therefore, the home appliance and automobile industriestend to choose small zinc flower (no zinc flower) products, and thebeautiful zinc flower products are also used in the constructionindustry.
Thehigh-purity zinc liquid cannot solidify and form zinc flowers when itis cooled because of the lack of necessary crystal nuclei in the zincliquid. To obtain a typical zinc flower, the precondition is toadd two kinds of alloy elements with different properties into theliquid zinc in an appropriate amount. One kind of alloy elements iscompletely soluble in the liquid zinc, but almost completelyinsoluble in solid zinc, such as lead and titanium; Another kindof alloy elements have certain solubility in liquid and solid zinc,such as aluminum, tin, antimony and so on. If only one kind ofalloy element is added, although the surface state of the zinc layerwill change after solidification, it is not enough to form a typicalzinc flower.
Methodof forming zinc flower
Theway to form zinc flower is to try to generate a large number ofcrystal nuclei as soon as possible, reduce the solidificationtemperature of zinc liquid, so as to prolong the growth time of zincflower crystals and facilitate the growth of zinc flower.
Adding other elements to the molten zinc
Whenan alloy element is added to the liquid zinc, the crystallizationprocess can be prolonged, and the liquid zinc on the surface will notsolidify before reaching its eutectic temperature.
Purezinc solidifies at 419.5 ℃.If tin (less than 0.5%) is added, the Sn Zn eutectoid alloy will notsolidify before it falls to 198 ℃. Whencadmium is added, the cadmium zinc eutectoid alloy will not solidifybefore it drops to 264 ℃. Ifantimony (less than 0.3%) is added, the antimony zinc eutectoid alloywill not solidify before it falls to 409 ℃. Afteradding lead, the lead-zinc eutectoid alloy will not condense untilthe temperature drops to 317 ℃.
Therefore,the addition of alloy reduces the melting point of molten zinc,prolongs the solidification time of pure zinc to varying degrees,greatly prolongs the growth time of zinc flower crystal, andfacilitates the growth of zinc flower, so larger zinc flower can beobtained.
Surface blowing method
Whenthe surface zinc layer is about to solidify, if the steam flow orsulfur dioxide gas flow passes through its surface, the zinc flowercrystal can grow smoothly, and a larger pattern can beobtained. Another purpose of blowing the sulfur dioxide gasstream is to form a dense oxide film and improve the corrosionresistance of the zinc coating.
Wire mesh method
Thismethod can only be used on the galvanized sheet, which is achieved bycontacting the steel wire mesh with the surface of the zinc layerwith a magnetic roller. Since the contact point of the steelwire mesh knot is cooled first, crystalline solid particles are firstformed there to become crystal cores, and the rest of the nonsolidified zinc liquid starts to crystallize outward to form zincflowers.
Spray mist method
Spraywater mist (mixture of water and steam or water and air) on thesurface of galvanized steel parts when the pure zinc liquid is notsolidified to form crystal nuclei. Thenon solidified zinc liquid diffuses outward from this starting pointto form zinc flowers.
Other methods
Inaddition to the above methods, if the surface of the plated parts issmooth, the zinc coating is relatively thin, the temperature iscontrolled to drop uniformly and the cooling time in the air issufficient, large zinc flowers can also be obtained.
Shapeof zinc flower
Theshape of zinc flower depends on the kind of alloy added in the zincliquid. For example, selecting lead and antimony can obtain PhoenixTail shaped zinc flower; When lead and tin are selected, sheeptooth shaped zinc flowers are obtained.
Thecrystal orientation of zinc flower on the surface of the coating isalso different in processing due to its different appearance. Forexample, in simple stretching and deep drawing, the processability offeather shaped zinc flower is the best, followed by theprocessability of sheep tooth shaped zinc flower and leaf shaped zincflower. In bending and tension processing, the processability of leafshaped zinc flower is the best, followed by the processability offeather shaped zinc flower and sheep tooth shaped zinc flower.
Zincflower size
Accordingto the European standard EN 10346:2015, the size of hot-dipgalvanized zinc flower can be divided into normal spangle andminimized spangle.
Conventionalzinc flower refers to the size and morphology of zinc grain formedafter normal solidification of zinc liquid, while no zinc flower(also known as small zinc flower) is the size and morphology of zincgrain obtained by special control of the solidification process ofzinc. If the user needs to specify the size of zinc flower, itneeds to be negotiated when ordering.
Thereare many factors affecting the size of zinc flower. When thecomposition of zinc liquid meets the conditions for forming zincflower, the following factors will affect the size of zinc flower:
•(1) raw materials: the thicker the raw steel plate, the smaller thesurface roughness, and the larger the zinc flower.
• (2)cooling speed: the faster the cooling speed, the shorter the crystalgrowth time and the smaller the zinc flower.
• (3)number of crystal nuclei: the size of zinc flower is closely relatedto the number of crystal nuclei of zinc flower. When the surfaceof the steel base is rough or the surface cleanliness is poor, thezinc flower is relatively small. Zinc grains are formed on thesteel base, and the more nuclei there are, the smaller the zincflowers are. This principle is used in the production process ofsmall zinc flowers, water spray and fog.
• (4)other factors (production environment, etc.).
Largezinc flower
Commonzinc flower
Smallzinc flower
Inorder to obtain small zinc flowers, when the zinc solution is nearthe solidification temperature during galvanizing, water dropletswith a diameter of 0.1mm or less are immediately sprayed onto thesurface of the galvanized sheet to form fine and uniform crystalnuclei; In order to obtain the smooth zinc flower, flat rollingis carried out with a reduction of less than 1%. Small zincflower is suitable for painting, and smooth zinc flower is suitablefor deep drawing.
Adhesionof zinc layer
Whenthe heated steel strip passes through the molten zinc bath (zincbath), an iron zinc alloy layer is first formed on the substrate, anda pure zinc layer is covered outside the alloy layer. Thethickness of the iron zinc alloy layer is uneven, loose, brittle,poor ductility and easy to crack.
Ifa certain amount of iron-based salt is left on the surface of thesteel plate before galvanizing, the resulting iron-zinc alloy haspoor adhesion, poor ductility and is easy to peel off. When thepure zinc coating contains harmful impurities, such as zinc oxide,zinc slag, cadmium and lead, large zinc flowers will be formed, orthe continuity of the zinc layer will be damaged, and small cracks ofthe pure zinc layer will easily occur.
Whenthe zinc coating is too thick, its bendability becomes poor. Whenaluminum is added to the zinc liquid, the aluminum in the zinc liquidreacts with the substrate to first generate fe2al5 or fe2al3, whichthins the iron zinc alloy coating and improves the adhesion of thezinc layer.
Unevenzinc flower and its influencing factors
Theuneven zinc flower on the surface of the coating is a surface defectof hot-dip galvanized products. Although it will not have a greatimpact on the use of most products, it will affect the beauty oftheir surface and limit the application of products in some fields ofthe market. Some products with uneven zinc flower also have pooradhesion of zinc layer, which directly affects the use of products.
Theuneven state of zinc flower on the surface of hot-dip galvanizedsheet can be summarized as follows:
• (1)there is zinc flower on one side and no zinc flower on the otherside. Thediameter of the zinc flower on the side with the zinc flower is about6mm.
• (2)along the longitudinal direction of the strip steel, the size of zincflower fluctuates regularly.
• (3)along the transverse direction of the strip steel, one side has largezinc flower and the other side has small zinc flower.
• (4)the zinc flower on the edge of the strip steel is small and the zincflower in the middle is large.
• (1)there is zinc flower on one side and no zinc flower on the otherside. The diameter of the zinc flower on the side with the zincflower is about 6mm.
• (2)along the longitudinal direction of the strip steel, the size of zincflower fluctuates regularly.
• (3)along the transverse direction of the strip steel, one side has largezinc flower and the other side has small zinc flower.
• (4)the zinc flower on the edge of the strip steel is small and the zincflower in the middle is large.
Themain factors affecting the uneven zinc bloom include: the proportionof two kinds of alloy elements in the composition of molten zinc, theair knife control process, the temperature of the strip steel, theimpurity content in the gas and combustion supporting air, thesurface cleanliness of the strip steel and the reduction state in thefurnace, the furnace roll nodulation, the scratch of the strip steeland the oxide scale on the surface of the raw material, thetemperature of the strip steel entering the zinc pot, the temperatureof the zinc melt, and the cooling temperature after plating.
Themain factors affecting the uneven zinc bloom include: the proportionof two kinds of alloy elements in the composition of molten zinc, theair knife control process, the temperature of the strip steel, theimpurity content in the gas and combustion supporting air, thesurface cleanliness of the strip steel and the reduction state in thefurnace, the furnace roll nodulation, the scratch of the strip steeland the oxide scale on the surface of the raw material, thetemperature of the strip steel entering the zinc pot, the temperatureof the zinc melt, and the cooling temperature after plating.