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How to choose fastener surface treatment? Zinc, Dacromet, hot-dip galvanising and black oxide compared
2026-09-12
Compiled by JIALI Industrial (164.hk) — a Hong Kong fastener supplier specialising in screws, bolts, nuts and selection know-how.
Why surface treatment is key in fastener selection
The same fastener can last several times longer — or fail within months — depending on its surface treatment. Outdoor humidity, coastal salt spray and chemical environments corrode unprotected carbon steel screws within months; electronics need low-permeability finishes that do not interfere with signals; automotive underbodies and rail vehicles prioritise heat and vibration corrosion resistance. Choosing the right fastener surface treatment is not just about looks: it keeps bolts, nuts and washers delivering clamping force over the intended service life. When requesting quotations, Hong Kong buyers should state the service environment, temperature range and appearance requirements, otherwise suppliers may default to the cheapest ordinary zinc plating.
Comparison of four common surface treatment processes
Electroplated zinc (white, blue-white or yellow passivated) is about 5–15 μm thick, low cost and bright, suited to dry indoor environments with neutral salt-spray life typically 24–96 hours. Hot-dip galvanising per GB/T 13912 reaches 45–85 μm or more, giving 500–1,500 hours salt-spray life for outdoor, bridge and power-tower applications. Dacromet at 8–12 μm delivers 500–1,000 hours salt-spray resistance and withstands 300°C, with no hydrogen-embrittlement risk, making it popular for high-strength bolts and automotive parts. Black oxide (blackening) changes dimensions negligibly, gives a matte black finish and good oil retention, but offers weak corrosion protection; it is used for indoor machinery, firearms or anti-glare applications. Selection should balance coating thickness, corrosion rating and temperature limit.
Practical selection by application environment
For general indoor machinery and furniture, electroplated zinc is sufficient. Outdoor construction, curtain walls and solar racking should prefer hot-dip galvanising or 304 / 316 stainless steel. Coastal, chemical and high-temperature sites suit Dacromet or stainless steel, and direct contact between dissimilar metals should be avoided to prevent galvanic corrosion. For high-strength 10.9 / 12.9 grade bolts needing corrosion protection, Dacromet is safer than electroplated zinc because it avoids hydrogen-embrittlement risk. At acceptance, always request coating-thickness reports and ASTM B117 or ISO 9227 salt-spray data to confirm the surface-treatment class matches the design life. Need advice on fastener surface treatment selection? Contact JIALI Industrial (164.hk) with your environmental conditions and specifications for a fast match.
Related Reading
- How to choose railway fasteners? Anti-loosening, material and acceptance essentials
- How to read a zinc-plating salt spray test report: NSS and coating thickness essentials
- JIALI launches Stainless Steel zone with full 304 / 316 range
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