1.How do the coating's own characteristics affect the weather resistance of galvanized coils?
The thickness of the zinc coating is the foundation of corrosion resistance: the thicker the zinc coating, the longer it takes corrosive media (such as oxygen, water, and chloride ions) to penetrate the substrate (steel).
Uniformity determines the "short plate effect": if the zinc coating is locally too thin (such as at edges or scratches), it can become a starting point for corrosion, accelerating overall failure. During production, the galvanizing speed and zinc bath temperature must be controlled to ensure a double-sided zinc coating deviation of ≤±10%.
Coating Composition
Pure zinc coating (GI): Relies on the sacrificial anodic effect of zinc, but has limited resistance to chloride ion and high humidity corrosion.
Zinc alloy coating: Optimizes corrosion resistance through alloying elements.
Zinc-aluminum-magnesium (ZM): Aluminum and magnesium form dense corrosion products, blocking the penetration of corrosive media. Corrosion resistance is 3-5 times that of pure zinc, making it particularly suitable for coastal high-chloride environments.
Zinc-nickel alloy: Nickel increases coating density, resulting in corrosion resistance 5-10 times that of pure zinc. It also offers high-temperature resistance (suitable for high-temperature and high-humidity areas).
2.How does the surface treatment layer performance affect the weather resistance of galvanized coils?
Passivation Layer Type and Quality
The passivation layer's function: isolates the zinc layer from air/water, inhibiting zinc oxidation (white rust).
Chromium-free passivation (silane, titanate): Relying on silicon-oxygen bonds to form a dense film, its corrosion resistance is weaker than that of chromates (salt spray white rust lasts ≥ 72 hours), but it is environmentally friendly.
Chromate passivation (gradually phased out): Chromium ions (Cr³⁺/Cr⁶⁺) form a self-healing film, offering even greater corrosion resistance (salt spray white rust lasts ≥ 120 hours), but it presents environmental concerns.
Key to quality: Passivation layer thickness (50-200nm) and uniformity (avoiding missed coatings that could lead to localized corrosion).
Pre-coated coating characteristics
Coating materials:
PVDF (polyvinylidene fluoride): High fluorine content, UV resistance, acid and alkali resistance, outdoor weathering life ≥ 20 years;
Polyester (PE): Low cost, moderate weathering resistance (lifespan 10-15 years), suitable for typical urban environments;
Silicon-modified polyester (SMP): Excellent temperature resistance, suitable for high-temperature regions. Coating parameters: thickness, degree of curing, adhesion (insufficient bonding with the zinc layer will cause peeling and loss of protection).
3.How do environmental factors affect galvanized coils?
Atmospheric Corrosive Media
Chloride Ions (Cl⁻): High concentrations of Cl⁻ in coastal areas and winter snowmelt (in northern cities) can penetrate zinc oxidation products, forming soluble zinc salts and accelerating zinc degradation.
Sulfur Dioxide (SO₂): In industrial cities, SO₂ reacts with water to form H₂SO₃. The acidic environment destroys the zinc passive film and promotes electrochemical corrosion.
Humidity and Temperature: When relative humidity exceeds 60%, a water film forms on the zinc surface, accelerating corrosion. The corrosion rate increases approximately 100% for every 10°C increase in temperature.
4.How do mechanical properties affect weathering resistance?
Plating/Coating Adhesion
Zinc layer to steel substrate: A bend test is required to prevent the zinc layer from peeling off during processing, exposing the steel substrate.
Coating to zinc layer: Color-coated coils must pass a cross-cut test with an adhesion level of 4 or higher to prevent the coating from peeling off and losing its barrier properties.
Impact and Abrasion Resistance
Impact Resistance: No coating or zinc layer is exposed under a 5J impact, preventing damage to the protective layer caused by collisions during installation.
Abrasion Resistance: The color coating has passed the Taber abrasion test, minimizing damage to the coating caused by friction during transportation and installation.
5.How to control the production process?
Galvanizing Process: The zinc bath temperature, strip feeding speed, and air knife pressure must be stable. Otherwise, the zinc layer may be too thin, there may be missing plating, and zinc nodules.
Passivation/Coating Process: The passivation bath concentration, coating speed, and drying temperature must be precisely controlled (for example, the drying temperature for chromium-free passivation is 120-150°C. Insufficient temperatures will result in the passivation film not curing, reducing corrosion resistance by 30%).
Substrate Quality: The substrate surface must be free of oil, dirt, and scale. Otherwise, the zinc layer will have poor adhesion, leading to blistering and flaking.