1.How does zinc layer weight affect weathering resistance?
The zinc coating weight for architectural galvanized coils must match the corrosiveness of the environment:
Normal rural/urban environments: ≥Z140 (zinc coating weight 140g/m², corresponding to a zinc coating thickness of ≈20μm);
Industrial/coastal urban areas (C3): ≥Z275 (275g/m², ≈39μm);
High-humidity industrial/coastal areas (C4): ≥Z350 (350g/m², ≈50μm);
Extreme marine environments (C5, e.g., ≤1km offshore): ≥Z450 (450g/m², ≈64μm) or a zinc alloy coating (such as zinc-aluminum-magnesium, which has corrosion resistance 3-5 times that of pure zinc) is required.
2.How does salt spray resistance affect weathering resistance?
Time to avoid red rust: Z275 ≥ 500 hours, Z350 ≥ 700 hours, zinc-aluminum-magnesium coating (ZM100) ≥ 1500 hours.
If white rust (a corrosion product of zinc) appears, it must pass the "white rust conversion test" (e.g., rainwater erosion does not accelerate substrate corrosion).
3.How does weathering of surface treatments enhance protection?
Passivation Film:
Chromate passivation (gradually phased out) or chromium-free passivation (such as silane and titanate) must meet the following requirements:
Corrosion Resistance: White rust appearance time in salt spray test after passivation ≥ 72 hours (chromium-free passivation) or ≥ 120 hours (chromate passivation);
Adhesion: Bond strength between passivation film and zinc layer ≥ 3N/cm (no peeling in tape peel test);
Moisture and Heat Resistance: No blistering or peeling after 1000 hours at 40°C and 95% humidity.
Color Coating (for Color-Coated Galvanized Coil):
Architectural color-coated galvanized coil must meet the following requirements:
UV aging resistance: After 1000 hours of QUV aging testing (simulating sunlight UV rays), the coating must show no chalking, no flaking, and a color difference of ΔE ≤ 3.
Acid and alkali corrosion resistance: After immersion in 5% sulfuric acid or 5% sodium hydroxide solution for 24 hours, the coating must show no blistering or discoloration. (For coastal areas, enhanced salt spray resistance is required; the coating must show no rust after a 500-hour salt spray test.)
Temperature fluctuation resistance: After 50 cycles of -40°C to 80°C, the coating must show no cracking or peeling (to withstand diurnal and seasonal temperature fluctuations).
4.What is the mechanical tolerance and damage resistance?
Coating/Zinc Adhesion:
Bend Test: After 180° bending (bend radius = 2 x plate thickness), the zinc/coating must show no peeling or cracking.
Impact Test: After a 5J impact, the coating must show no detachment, and the zinc layer must not be exposed (to prevent rapid corrosion at the damaged area).
Abrasion Resistance:
Coatings (such as pre-painted coatings) must pass the Taber abrasion test, with a wear loss of ≤50mg (1000 revolutions, 500g load) to prevent installation friction from causing the protective layer to fail.
5.What are the differences in weather resistance requirements for different building scenarios?
Rural/ordinary city: Weather resistance requires the zinc layer to be ≥Z140, ordinary passivation (chromium-free) can be used, and no additional coating is required (such as for steel structure base).
Industrial cities: Weather-resistant zinc layer ≥ Z275, acid-resistant coating (such as polyester) is recommended, salt spray test ≥ 500h without red rust.
Coastal areas: Weather resistance requirements include a zinc layer ≥ Z350 or zinc-aluminum-magnesium alloy coating (Zn-Al-Mg, which is more than three times more resistant to chloride ion corrosion than pure zinc). Color coating requires PVDF coating (weather resistance level ≥ 20 years).
High temperature and high humidity areas: Weather resistance requires the addition of mildew inhibitors (such as copper powder) to the coating, and the heat and humidity resistance test (40°C, 95% humidity) must be carried out for 2000 hours with no mildew or bubbling.