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HaiGang Yunyi
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Tianjin Haigang Steel Coil
Service first, quality assured. More dedicated and focused on crafting even better boards.
08
2020
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09
The entire process of hot-dip galvanizing.
Hot-dip galvanizing process.
Workpiece → Degreasing → Water Cleaning → Acid Pickling and Passivation → Water Cleaning → Immersion Coating with Organic Solvent → Air Drying and Heating → Hot-Dip Galvanizing → Combing → Cooling → Passivation Treatment → Rinse Bath → Drying → Inspection
Description of the relevant processing procedures
(1) Degreasing
You can use organic chemical degreasers or water-based metal degreasing cleaners to remove oil, ensuring that the workpiece is thoroughly saturated.
(2) Acid pickling and passivation
H2SO4 at 15% and thiourea at 0.1%, at temperatures ranging from 40 to 60℃, or HCl at 20% and hexamethylenetetramine at 1–3 g/L, at temperatures ranging from 20 to 40℃, can be used for acid pickling and passivation. The addition of a desulfurizing agent can prevent over-etching of the substrate and reduce hydrogen absorption by the iron matrix.
Poor degreasing and pickling passivation treatments can lead to insufficient adhesion of the coating, preventing zinc plating altogether or causing the zinc layer to peel off.
(3) Immersion Plating Agent
Also known as a flux, it can impart certain specific properties to the workpiece prior to plating, thereby enhancing the adhesion between the coating and the substrate.
(4) Air-drying and heating
To prevent workpieces from deforming due to a significant temperature rise during immersion plating, and to remove residual moisture, thereby avoiding zinc explosions that could cause the zinc bath to splatter.
(5) Hot-dip galvanizing
It is crucial to carefully control the temperature of the zinc bath, the dipping time, and the rate at which the workpiece is removed from the zinc bath. If the temperature is too low, the zinc bath will have poor fluidity, resulting in a thick and uneven coating that is prone to dullness and poor surface quality. Conversely, if the temperature is too high, the zinc bath will have excellent fluidity, but the zinc may easily detach from the workpiece, exacerbating dullness and skin-creasing defects. While this leads to strong adhesion and a thin, high-quality coating with superior appearance and higher production efficiency, excessively high temperatures can cause severe iron loss in both the workpiece and the zinc pot, generating large amounts of zinc dross, degrading the quality of the zinc coating, increasing zinc consumption, and even rendering the plating process completely unfeasible. At the same time, under the same temperature, a longer dipping time will result in a thicker coating. For different temperatures, when aiming for the same coating thickness, the dipping time required at higher temperatures will be longer. Typically, manufacturers choose a temperature range of 450–470°C and a dipping time of 0.5–1.5 minutes to avoid high-temperature deformation of the workpiece and reduce the formation of zinc dross caused by excessive iron loss. Some factories use higher temperatures for large workpieces and die-cast aluminum parts, but they carefully avoid temperature ranges where iron loss peaks. To improve the fluidity of the hot-dip coating bath at lower temperatures, prevent excessive coating thickness, and enhance the coating’s surface appearance, 0.01% to 0.02% of pure aluminum is often added. The aluminum should be added in small quantities and multiple times.
(6) Sort out
After plating, the primary purpose of brushing the workpiece is to remove residual zinc and zinc nodules from the surface; this can be done either by vibration or manually.
(7) Passivation treatment
The purpose is to enhance the zinc coating’s resistance to atmospheric corrosion on the workpiece surface, reduce or extend the time it takes for white rust to appear, and maintain the coating’s excellent appearance.
(8) Cooling
Generally, tap water is used for cooling, but the temperature must not be too low, as excessively rapid cooling could cause thermal shock and lead to cracking in the matrix structure, especially in castings.
(9) Detection
The coating exhibits a bright, fine surface with no signs of dripping or wrinkling. For thickness measurement, a coating thickness gauge can be used—a method that is relatively simple. Alternatively, the coating thickness can also be calculated based on the amount of zinc adhesion. To assess bonding strength, a bending and stamping machine can be employed: bend the sample at an angle of 90° to 180°; there should be no cracks or coating delamination. Another method involves testing by striking the sample with a heavy hammer.
Strip steel, galvanized strip steel, cold-rolled hard strip steel, hot-dip galvanized strip steel, galvanized steel strip
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We primarily produce various products including galvanized strip steel, pickled strip steel, and cold-rolled strip steel. These products are widely used in industries such as construction, mechanical manufacturing, automotive, and household appliances.
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Company address:
Shuangtang High-end Hardware Products Industrial Park, Jinghai District, Tianjin City
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tjhaigang@tjhaigang.com
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