1.How to control the quality of galvanized surface treatment?
Degreasing: Use alkaline cleaning agent (pH 10-12) or organic solvent (such as acetone) to remove rolling oil and fingerprints on the surface of the zinc layer, and ensure that the contact angle is ≤30° (water drop test).
Do not use strong acids and alkalis to avoid damaging the zinc layer. It is recommended to use a special zinc alloy cleaning agent.
Surface roughness treatment: Sandblasting: Use angular sand, the roughness is controlled at Ra 6.3-12.5μm, and the anchor pattern depth is ≥25μm.
Alternative solution: If sandblasting is not possible, high-pressure water jet or mechanical grinding can be used, but it is necessary to ensure that there is no zinc salt residue on the surface.
Zinc layer defect repair: Scratched or missed plating areas need to be sprayed with zinc powder (purity ≥99.9%), thickness ≥38μm, or zinc alloy repair agent.
2.What is the surface cleanliness standard of the galvanized layer?
Visual inspection: The surface is free of oil, rust, zinc salt (white powder) and dust, reaching ISO 8501-1 Sa2.5 level (near white level).
Salt pollution detection: Wipe the surface with deionized water, and test the conductivity ≤50μS/cm.
3.How to manage raw materials?
Primer (epoxy zinc-rich): Storage temperature 5-35℃, humidity ≤75%, avoid direct sunlight, shelf life 6 months (unopened).
Zinc powder content must be ≥80% (weight ratio), and the zinc powder sedimentation rate must be ≤5% before use.
Topcoat (fluorocarbon/PVDF): ◦ Mix the curing agent and the main agent in a ratio of 5:1 (weight ratio), use a special diluent (butyl acetate: xylene = 3:1), and control the viscosity to 25-30s (23℃) after applying - 4 cups.
4.What are the key points of the mixing process?
Curing time: Epoxy primer needs to be cured for 30 minutes after mixing, and fluorocarbon topcoat needs to be cured for 15 minutes. It needs to be discarded after exceeding the applicable period (usually 4 hours).
Filtration requirements: Use a 120-mesh filter to filter to remove impurities and avoid nozzle blockage leading to uneven coating.
5.What are the key steps in two-coat and two-bake construction?
Primer application (first coat)
Spraying parameters: air pressure 0.4-0.6MPa, nozzle diameter 1.5-2.0mm, spraying distance 20-30cm, gun travel speed 30-50cm/s.
Dry film thickness control 80-100μm (wet film thickness × 0.4 = dry film thickness, real-time monitoring with a wet film thickness gauge is required).
Bake curing: Convection heating: 80-100℃×30 minutes (heating rate 5℃/min), to ensure that the epoxy primer is completely cross-linked.
Topcoat application (second coat)
Surface treatment: After the primer is cured, lightly grind with 320# sandpaper to remove burrs, and blow the surface with compressed air (dew point ≤-20℃).
Spraying parameters: Fluorocarbon topcoat needs to be sprayed with high-pressure airless spray (pressure ≥ 15MPa), nozzle diameter 0.3-0.5mm, film thickness 150-200μm (wet film).
Baking and curing: Step heating: 120℃×15 minutes→180℃×20 minutes→200℃×10 minutes (cooling rate ≤10℃/min), ensuring that PVDF molecules are fully crystallized (gloss ≥90°, GB/T 9754 standard).
