1.How to completely remove oil stains and avoid "rejection of zinc layer"?
Oil contamination: Rolling oil, lubricating oil, and other contaminants on the substrate surface can form an "isolation film," preventing the zinc solution/electrolyte from fully contacting the substrate. During galvanizing, the zinc layer is thin or even absent in the oily areas, resulting in a "mottled and uneven" appearance.
Safeguards:
Select a degreasing process based on the type of oil stain: alkaline degreasing (e.g., NaOH or Na₂CO₃ solution, 60-80°C, immersion/spraying for 1-3 minutes) is recommended for mineral oil-based degreasing, while emulsification degreasing (adding an emulsifier to remove the oil stain through oil-water emulsification) is recommended for animal and vegetable oil-based degreasing.
Control degreasing parameters: degreasing solution concentration (alkaline degreasing agent concentration is typically 5%-10%), temperature (too low degreasing efficiency is low, while too high wastes energy and may corrode the substrate), and treatment time tailored to the thickness of the oil stain (for thick stains, the treatment time can be extended to 5 minutes, or a combination of spraying and immersion can be used).
Post-cleaning: Rinse thoroughly with running water after degreasing to prevent residual degreasing agent from forming a new, uneven film on the substrate surface (residual alkaline substances may cause localized, overly rapid reaction during subsequent pickling).
2.How to remove rust/scale?
Impact of Oxide Scale/Rust: Oxide scale (Fe₃O₄, Fe₂O₃, etc.) or rust (FeO (OH)) on the surface of the hot-rolled substrate is loose and unevenly bonded to the substrate. During galvanizing, the zinc layer cannot adhere to the oxide scale (or easily falls off after it adheres), resulting in "partial zinc absence" or "zinc layer thickness fluctuations." Safeguards:
Pickling (mainstream method): Soak in hydrochloric acid (15%-20% concentration) or sulfuric acid (10%-15%) to remove impurities through a chemical reaction between the acid and scale/rust. Control the pickling temperature (20-40°C for hydrochloric acid, 50-70°C for sulfuric acid; higher temperatures can lead to excessive corrosion of the substrate) and duration (adjusted according to scale thickness, typically 5-15 minutes to avoid over-pickling, which can lead to a rough and uneven surface.
Auxiliary treatment: For thick scale, mechanical rust removal (such as sandblasting or wire brushing) can be performed before pickling to reduce pickling variations. After pickling, neutralize the substrate with a weak alkaline solution (such as Na₂CO₃, pH 8-9) to eliminate any residual acid corrosion. Rinse with water until the substrate is neutral (pH 6-7).
3.How to adjust the substrate surface roughness and plate shape control?
Roughness Impact: Excessively rough substrate surfaces (e.g., Ra > 5μm) or with local scratches or indentations can result in a thicker zinc layer in concave areas and thinner zinc layer in convex areas. A smooth surface with localized micro-pitting can cause solution retention, leading to a thicker zinc layer in certain areas.
Strip Shape Impact: Strip steel (e.g., galvanized steel sheet) with wavy or cambered strips can lead to uneven distances from the zinc solution/electrode during galvanizing, resulting in variations in zinc layer thickness.
Safeguards:
Substrate Rolling Control: Ensure uniform surface roughness during the cold rolling process (e.g., by uniforming the roll roughness to Ra within 1-3μm) and avoid local scratches (rollers require regular grinding to remove surface defects).
Strip Shape Correction: Before entering the galvanizing line, the strip is corrected in a "stretching and leveling machine" to eliminate wavy or cambered strips and ensure consistent distances from process equipment (e.g., air knives and anodes) across the entire strip width during operation.
4.How to adjust parameters in hot-dip galvanizing process?
Zinc bath temperature and galvanizing time: Avoiding uneven reaction rates
Excessively high molten zinc temperature (>480°C) can cause the iron-zinc reaction on the substrate surface to accelerate, thickening the alloy layer and making it porous. Furthermore, the molten zinc is too fluid, leading to large fluctuations in the amount of molten zinc carried overboard by the strip/workpiece. Excessively low molten zinc temperature (<430°C) results in high viscosity and poor fluidity, leading to uneven accumulation on the substrate surface (e.g., stagnation in corners).
The temperature should be maintained at a stable temperature of 445-455°C (within ±5°C). The galvanizing time should be tailored to the substrate thickness (1-3 seconds for thin strip, 5-10 seconds for thick workpieces). Precise control should be achieved through line speed (for strip) or workpiece suspension conveyor speed (for workpieces) to avoid excessively long or short galvanizing times in certain areas. Air Knife Control (Core of Hot-Dip Galvanizing): Precisely "Scrape Off" Excess Zinc
After the strip is drawn from the molten zinc, excess zinc adheres to the surface. This excess zinc needs to be removed by an "air knife" (a high-pressure air nozzle). Air knife parameters directly determine the uniformity of the zinc coating thickness:
Air Knife Pressure: Too low a pressure will prevent excess zinc from being scraped off (resulting in a thicker zinc coating), while too high a pressure can easily lead to "zinc breakdown" (partial exposure of the base material). Air knife pressure should be set based on the target zinc coating thickness (e.g., for a zinc coating of 80-120g/m², the pressure is typically 0.2-0.4MPa). Furthermore, "pressure must be uniform across the entire strip" (by designing the internal flow channel of the air knife to avoid pressure differences greater than 0.02MPa between the edge and the center).
Air Knife Distance and Angle: The air knife should maintain a fixed distance from the strip (typically 10-20mm. Too close can cause scratches, while too far can cause pressure drop and uneven scraping). The air knife angle should be perpendicular to the strip (within ±1°) to avoid "side blowing." This can result in a thinner zinc layer on one side.
Air knife gap: Maintain gap uniformity (±0.1mm). If the gap widens locally, airflow weakens at that location, resulting in a thicker zinc layer. Regularly inspect the air knife nozzle to remove any clogging caused by zinc slag (iron-zinc alloy slag in the zinc bath easily adheres to the nozzle, causing uneven gaps).
Zinc bath composition: Avoid "fluidity fluctuations."
Excessive Fe content in the zinc bath (>0.03%) will form a large amount of iron-zinc alloy slag, reducing the fluidity of the zinc bath. This slag adheres to the substrate surface, forming "slag spots" and resulting in an uneven zinc layer. Zinc bath purification (e.g., adding Al to form Al-Fe alloy slag, which needs to be regularly removed) is necessary to control the Fe content to <0.02%. Furthermore, the Al content (typically 0.15%-0.25% for hot-dip galvanizing) must be stable. Al improves the fluidity of the zinc bath; fluctuations in the Al content can lead to unstable zinc layer adhesion.
5.How to ensure "stable output of process parameters"?
Hot-dip galvanizing air knives must be equipped with a "closed-loop pressure control system" (real-time monitoring of air knife pressure, with automatic adjustment if deviation exceeds 0.01 MPa). The nozzles must be regularly calibrated (check the gap uniformity with a feeler gauge once a month).
Electrogalvanizing power supplies must use a "high-frequency switching power supply" (current fluctuation ≤ ±0.5%) to avoid uneven deposition caused by current fluctuations associated with traditional power supplies. Anode rods must be well insulated to prevent leakage and localized current abnormalities. Regular inspection of the connection between the anode and power supply is required to prevent excessive contact resistance from causing current distribution deviations.