Coastal tidal flat and island PV power stations suffer severe chloride ion corrosion. Traditional hot-dip galvanizing only reaches 600 hours salt spray resistance, failing to meet the 25-year design life requirement. ZAM zinc-aluminum-magnesium continuous coating becomes designated tender process for PV projects relying on self-healing anti-corrosion performance. PV bolts require high thread precision; excessive coating thickness leads to slipping and excessive fitting clearance during assembly, while thin coating shortens outdoor service life. This article fully standardizes full-set process parameters including continuous strip hot dipping and post-cut bolt passivation, clarifies mandatory coating thickness standards for inland and coastal PV projects.
| PV Project Scenario | Min Average Coating Thickness(μm) | ZAM Alloy Ratio | Standard Alternating NSS Hours(h) | Reserved Thread Machining Allowance(mm) |
| Inland Mountain Ground PV | 85 | 95%Zn 3.5%Al 1.5%Mg | ≥1000 | 0.06~0.08 |
| Coastal Island Tidal Flat PV | 100 | 93%Zn 4.0%Al 3.0%Mg | ≥1500 | 0.08~0.10 |
1. Strip Pre-treatment: Continuous pickling & multi-stage degreasing to fully remove rolling scale and eliminate local missing plating on cut bolts; 2. Constant 460℃ molten ZAM bath dipping, stable strip running speed to guarantee uniform coating thickness; 3. Precise air knife thickness control: High-pressure nitrogen air knife scrapes excess alloy liquid to accurately match reserved thread allowance; 4. Closed-loop air cooling: Segmented gradient cooling restrains coarse ZAM grains and improves coating adhesion; 5. Chrome-free self-healing passivation: Special PV anti-corrosion passivation chemical forms self-repair protective film to enhance chloride ion barrier capacity; 6. Cutting & secondary thread cleaning: Slightly remove redundant coating on thread surface to avoid jamming during assembly.
1. Thread slipping & jamming during assembly: Insufficient air knife pressure leads to excessive coating thickness and small reserved thread allowance; Rectify: Raise nitrogen air knife pressure, enlarge reserved thread coating allowance and perform light thread cleaning for finished goods. 2. Local dot missing plating without alloy coating: Incomplete strip pickling & degreasing with residual surface scale; Rectify: Extend multi-stage pickling & degreasing procedures and monitor strip surface cleanliness in real time. 3. Large-area coating peeling & insufficient adhesion: Excessive temperature difference in cooling gradient causes grain stress cracking; Rectify: Adjust segmented air cooling cooling rate, reduce temperature gap between start and end sections and optimize constant dipping temperature range.
1. Min 100μm ZAM coating is mandatory for bolts of coastal island projects, min 85μm for inland mountain projects; 2. Randomly extract 20 pieces per batch for alternating salt spray test, full batch rework if index below national standard limit; 3. Uniform thread coating without local accumulation, matching standard nut smoothly by hand without jamming in full travel; 4. Coating delivers self-healing performance, automatic passivation protective film forms within 48 hours after slight scratch without substrate rust spread.