1. Appearance Inspection
Coating Uniformity: The zinc layer should be completely covered, with no missing coating, nodules, or peeling. There should be no zinc slag residue on the threads (this can be observed with a magnifying glass).
Color and Gloss: Hot-dip galvanized bolts have a dark grayish-gray surface, while cold-dip galvanized bolts have a bluish-white or yellow-green color. Any abnormal color (such as blackening) may indicate inferior quality.
Head and Thread: The bolt head should be free of skew, the thread profile should be clear and burr-free, and the hexagon socket should be centered and free of eccentricity.
2. Coating Performance Inspection
Zinc Coating Thickness:
National standards require a thickness of ≥35μm (bolts under M16) or ≥55μm (bolts above M16). This can be measured with a magnetic thickness gauge.
A coating that is too thin is susceptible to rust, while a coating that is too thick may affect thread fit. Corrosion Resistance:
Salt spray testing (e.g., GB/T 5267.1-2023) must ensure no red rust for 72 hours.
For coastal areas, it is recommended to select products with a zinc coating ≥ 80μm.
3. Mechanical Property Verification
Strength Grade Marking:
Grade 8.8 bolts must have a tensile strength ≥ 800MPa and a yield strength ≥ 640MPa, and the head must be engraved with "8.8."
High-strength bolts, such as grade 10.9, require additional testing for hydrogen embrittlement risk (hardness ≤ 390HV).
Hardness Testing:
Rockwell hardness (HRC) or Brinell hardness (HB) must comply with GB/T 3098.1-2010.
4. Document and Certification Verification
Test Report: The manufacturer is required to provide data such as zinc coating thickness, salt spray testing, and tensile strength. Standard compliance: Look for standards such as GB/T 5267.1-2023 (electroplating layer) and GB/T 3098.1-2010 (mechanical properties).
