I. Visual Inspection
This is the most direct and commonly used method.
Normal Condition: The galvanized coating surface should be uniform, intact, and without obvious color difference. Hot-dip galvanizing is dark gray, while electro-galvanizing is bright white or bluish-white.
Signs of Damage:
The appearance of white powdery substances (basic zinc carbonate) is an early sign of zinc layer corrosion.
The appearance of reddish-brown rust spots indicates that the steel substrate has been exposed and begun to oxidize, and the protective function of the galvanized coating has failed.
The surface has blistering, peeling, cracks, or uncoated areas, especially at the root of threads, corners, and other stress concentration areas.
✅ Recommendation: Regularly inspect critical connection points in well-lit environments, paying particular attention to bolts in outdoor or high-humidity environments.
II. Cross-cut Adhesion Test (Applicable to new parts or random inspections)
The cross-cut adhesion test can assess the bonding strength between the coating and the substrate, determining whether there is a risk of easy detachment.
Use a sharp tool to score a 1mm grid of squares on the bolt surface; then apply adhesive tape tightly and quickly peel it off; if the coating peels off in large pieces, it indicates poor adhesion and a risk of early damage.
This method is destructive testing and is suitable for quality sampling before installation. It is not recommended for bolts already in service.
III. Measuring Coating Thickness with a Magnetic Thickness Gauge
Coating thickness is an important indicator of its protective capability. A thickness significantly lower than the standard value is considered potential damage or a manufacturing defect.
Use a magnetic thickness gauge to measure multiple points on different parts of the bolt (avoiding the thread tips);
Reference Standards: Electro-galvanized: 5–15μm; Hot-dip galvanized: 50–80μm, up to 120μm for M20 and above; If the measured value is lower than the lower limit of the standard, or if there are significant differences between different parts of the same bolt, it indicates localized thinning or incomplete plating.
IV. Salt Spray Test (Laboratory Assessment)
Used to evaluate the overall corrosion resistance of the galvanized layer, often used for quality verification.
Neutral salt spray testing should be conducted according to standards such as GB/T 10125. Qualified hot-dip galvanized bolts should withstand more than 500 hours without red rust.
If a large number of rust spots appear in a short period, it indicates poor coating integrity or substandard processing.
This method is a professional testing technique suitable for quality verification before bulk purchases.
V. Comprehensive Judgment Based on the Usage Environment
In high-humidity, near-shore environments such as Wuxi, high chloride ion content accelerates zinc coating consumption.
If bolts show obvious rust within one year of outdoor use, even without obvious mechanical damage, it should be considered that the galvanized coating's protective ability is insufficient or damaged. Excessive torque from power tools or tilted bolt tightening during installation can also cause micro-cracks that are difficult to see with the naked eye, leading to subsequent localized corrosion expansion.
