Grade 10.9 torsional shear bolts for outdoor prefabricated factories and high-rise steel frame main beams mandate hot-dip galvanizing long-acting anti-corrosion process, with superior salt spray durability compared to electro-galvanizing. However, zinc dipping process easily causes defects such as excessive thread coating thickness leading to thread stripping, local missing plating and zinc layer peeling, directly affecting anti-slip coefficient and connection safety of steel structures. All process temperature, soaking time and coating judgment thresholds in this article are derived from national standard GB/T13912, covering standardized full-link control from bolt incoming pre-treatment to finished coating inspection, no overlapping content with sheet metal press-fit or imported locking fasteners.
| Process Step | Control Temperature | Standard Soaking Time | Core Control Target |
| Hydrochloric Acid Pickling For Scale Removal | 20~28℃ Ambient | 6~10min | Fully remove rolling scale without substrate corrosion from over-pickling |
| Zinc Ammonium Flux Coating | 60~70℃ Constant Temp | 2~3min | Uniform flux film formed on workpiece surface to eliminate missing plating |
| Molten Zinc Bath Dipping | 445~455℃ Constant Temp | 30~60s | Uniform zinc layer adhesion, control thread coating thickness |
| Closed-loop Water Cooling | Ambient Clean Water | 1.5~2min | Terminate zinc layer growth rapidly to restrain zinc lump protrusion |
1. Excessive thread coating thickness causing thread stripping with matched nut: Over 60s zinc dipping time, low zinc bath temperature; Rectify: Shorten dipping time within 40s, stabilize zinc bath at standard 450℃, re-thread rolling to clean tooth after galvanizing. 2. Local missing plating & dot bare metal: Residual oil stain on workpiece after pickling, dried flux film; Rectify: Add two-stage ultrasonic degreasing, send workpiece into zinc bath within 1min after flux coating to avoid air drying. 3. Large-area zinc layer peeling & insufficient adhesion: Intergranular corrosion of substrate caused by over-pickling; Rectify: Limit pickling time within 10min, replenish corrosion inhibitor regularly, reduce hydrochloric acid concentration. 4. Protruding zinc lump & rough surface: Slow cooling speed leading to continuous zinc accumulation; Rectify: Lift workpiece out of zinc bath rapidly after dipping, instant high-pressure clean water spray cooling.
1. Coating thickness test: Average zinc layer thickness ≥85μm for main beam connecting bolts, 20 pieces randomly sampled per batch with magnetic thickness gauge, separate measurement for thread pitch diameter and shank; 2. Adhesion impact test: 1kg steel hammer impact at 45° without zinc layer peeling or cracking; 3. Neutral salt spray durability test: ≥600h no red rust for inland projects, ≥1000h no red rust for coastal high-humidity projects; 4. Thread matching verification: Matching standard nut of same size can be screwed smoothly manually without jamming or stripping during full travel.
1. Thread coating thickness shall not exceed 110μm, over-thickness directly judged unqualified for assembly requiring full batch rework thread cleaning; 2. Warehousing prohibited if zinc peeling & missing plating workpiece ratio exceeds 3%; 3. Third-party 1000-hour alternating salt spray test report mandatory for coastal prefabricated projects, goods rejected without report; 4. Goods can only be packed and shipped 48 hours after hot-dip galvanizing to reserve stabilization time for zinc layer and avoid peeling from transportation collision.