Release time：2020-01-16 18:44 Browse：
Standard Hot-Dip Galvanizing Process Flow (Applicable to Extra-Long Screws)
Hot-dip galvanizing is essentially a metallurgical reaction process. The basic steps are as follows:
Pretreatment (Critical Steps)
Degreasing (Alkaline Cleaning): Removes oil and grease from the screw surface. This is fundamental for the effectiveness of subsequent steps.
Pickling (Hydrochloric or Sulfuric Acid): Removes surface rust and mill scale, exposing the clean metal substrate.
Rinsing: Thoroughly rinses the pickled screws to remove residual acid.
Fluxing: The cleaned screws are immersed in a flux solution (typically zinc ammonium chloride). This forms a protective film to prevent re-oxidation before immersion in the zinc bath and promotes wetting of the steel by the molten zinc.
Hot-Dip Galvanizing
Preheating (Optional but Important): For extra-long screws, preheating before immersion is highly recommended. This prevents zinc spatter due to a cold workpiece and reduces thermal shock to the zinc bath, aiding in achieving a more uniform coating.
Zinc Immersion: The pretreated screws are immersed in a bath of molten zinc at approximately 445°C - 465°C. They are held for a sufficient time to allow the iron and zinc to react, forming iron-zinc alloy layers.
Post-Treatment
Vertical Withdrawal, Natural Dripping: The screws are withdrawn slowly and steadily from the zinc bath in a vertical position, allowing gravity to cause excess zinc to flow back into the bath.
Dedicated Vibration Equipment: During withdrawal, high-frequency vibration devices gently tap or shake the fixture to help dislodge zinc accumulated in the thread roots and other areas, preventing large zinc icicles.
Centrifuging/Vibrating (Excess Zinc Removal): This is a special step for galvanizing fasteners. For standard parts, a centrifuge is used to spin off excess zinc. For extra-long screws, centrifuging is not possible. The following methods are typically used:
Quenching: The galvanized screws are immersed in water for rapid cooling, which fixes the coating structure.
Inspection and Finishing: The coating appearance and thickness are inspected. Excessively thick zinc icicles may be manually trimmed with special tools, taking care not to compromise the coating integrity.
Special Challenges and Solutions for Galvanizing Extra-Long Screws
| Challenge | Specific Issues | Solutions and Precautions |
|---|---|---|
| 1. Galvanizing Bath Size Limitation | The screw length exceeds that of a standard horizontal zinc bath, making horizontal immersion impossible. | Vertical Dipping: This is the core solution. A deep "pit-type" or "deep-well" zinc kettle is required. The screws are lowered vertically into the bath using special fixtures. The kettle depth must exceed the screw length. This is the primary requirement when searching for a suitable processor. |
| 2. Deformation | Extra-long screws are highly susceptible to bending deformation under their own weight and thermal stress at high temperatures (~450°C). | - Special Fixtures and Support: Use multiple hanging points or custom jigs to ensure the screws remain vertical during immersion, withdrawal, and cooling, minimizing sagging. - Control Heating/Cooling Rates: Preheating and slow cooling reduce thermal stress. - Select High-Quality Material: The screws themselves should have good straightness; high-strength bolts perform better. |
| 3. Coating Uniformity | During vertical dipping, immersion time varies from top to bottom, potentially leading to uneven coating thickness. Thread roots are prone to zinc buildup (icicles). | - Control Immersion and Withdrawal Speed: Use a uniform, slow withdrawal speed to ensure even zinc runoff across the entire screw surface. - Vibration for Excess Zinc: As mentioned, vibration during withdrawal is key to achieving clear threads and minimizing icicles. - Zinc Bath Chemistry Control: Plants may adjust the alloy content (e.g., aluminum, nickel) in the zinc bath to improve fluidity and coating appearance. |
| 4. Thread Tolerance Disruption | The zinc coating thickness (typically 50-100μm+) alters thread dimensions, potentially preventing nut assembly. | - Allow for Coating Thickness (Most Critical): This is the fundamental solution. During screw machining, thread dimensions (major and pitch diameters) must be precision-cut smaller to预留 (pre-allocate) space for the zinc coating. This requires precise calculation and machining coordination. - Post-Tapping (Use with Caution): For through-hole connections, threads might be cleaned after galvanizing, but this damages the coating locally and is not recommended for critical applications. |
Practical Advice for You
If you need hot-dip galvanizing for extra-long screws, follow these steps:
Find a Specialized Processor: The primary task is to find a hot-dip galvanizer equipped with a deep-well zinc kettle and experience in processing long items. Standard fastener coaters usually cannot handle this.
Communicate Technical Details in Advance:
Provide the processor with precise drawings of the screw, including material, total length, thread specification, and strength grade.
Clearly state the requirement for "hot-dip galvanizing" and inquire if allowance for coating thickness is needed. A reputable processor will specify the required thread dimensions before galvanizing.
Discuss handling methods and anti-deformation plans.
Define Quality Requirements: Specify the coating standard (e.g., ISO 1461 or ASTM A123), average coating thickness, and appearance requirements (acceptable icicle size, etc.) in the contract or technical agreement.
Consider Alternatives (if Hot-Dip Galvanizing is not feasible):
Mechanical Galvanizing: Suitable for shorter screws; also has equipment limitations for very long parts. Coating durability is generally inferior to hot-dip galvanizing.
Dacromet / Zinc-Aluminum Flake Coatings: A dip-and-bake process with virtually no size limits, offers excellent corrosion resistance, and does not alter thread dimensions. Disadvantages include lower surface hardness and a matte gray-silver appearance.
Thermal Spray Zinc (Zinc Metallizing): Offers extremely long service life but is costly, results in a rough surface requiring sealing, and is typically used for large steel structures, not fasteners.
Summary:
Hot-dip galvanizing for extra-long screws is entirely feasible but is a non-standard, high-difficulty process. The key to success lies in finding a capable specialized processor and conducting thorough technical communication upfront, especially regarding thread dimension allowance and deformation control measures. This directly determines whether the final product will meet the usage requirements.
