Corrosion is the most common cause of fastener failure in industrial service. Mechanical strength (4.8 / 8.8 / 10.9 / 12.9) tells you how much load a bolt can carry; surface treatment determines how long it can carry that load before corrosion compromises the joint. Yet buyers often default to "zinc plated" without checking whether the environment demands more.
Before specifying a finish, evaluate: corrosion environment (indoor dry, outdoor humid, marine, chemical), required service life, operating temperature, torque/friction behaviour, environmental compliance (RoHS/REACH), and total cost of ownership.
The Four Main Options
Electro-galvanizing (zinc plating) deposits a thin 5–12 μm zinc layer electrolytically, usually with a passivate topcoat. It is the most economical choice for indoor or low-humidity service, general machinery, and painted assemblies. Salt-spray to red rust is typically 240–480 h (ASTM B117). Limitations: limited protection outdoors, hydrogen-embrittlement risk on Grade 10.9+ (bake-out required), and minor thread-fit adjustment. Reference standards: ISO 4042, ASTM B633.
Hot-dip galvanizing (HDG) immerses fasteners in molten zinc at ~450 °C, building a metallurgically bonded 40–100 μm coating. Red-rust resistance of 800–1,500+ h makes it the default for outdoor steel structures, bridges, transmission towers, and metal buildings. Caveats: nuts must be overtapped to fit the thick coating (matched sets required), and many specifications prohibit HDG on Grade 10.9/12.9 due to embrittlement risks. Reference: ISO 10684, ASTM A153.
PTFE coating is a sprayed or dip-spun fluoropolymer, often over a zinc or phosphate base, offering a very low friction coefficient and chemical inertness up to ~260 °C. Best for oil & gas flanged connections where low, consistent break-out torque matters, and chemical-processing service. Typical salt-spray: 500–1,000+ h. It costs more and is hard to repair in the field. Reference: ISO 10683.
Dacromet (zinc-flake / Geomet) applies a water-based zinc-and-aluminium-flake slurry baked at ~300 °C—no electrolysis, hence no hydrogen embrittlement. This makes it the safe finish for high-strength fasteners (10.9/12.9), automotive underbody parts, and wind-energy hardware, with 500–1,500+ h salt-spray resistance and RoHS-compliant formulations. Reference: ISO 10683, DIN 267-27.
Quick Comparison
| Property | Zinc Plating | HDG | PTFE | Dacromet |
| Thickness | 5–12 μm | 40–100 μm | 15–40 μm | 6–12 μm/layer |
| Salt-spray (red rust) | 240–480 h | 800–1,500+ h | 500–1,000+ h | 500–1,500+ h |
| Max temp. | ~120 °C | ~200 °C | ~260 °C | ~300 °C |
| H₂ embrittlement risk | Yes (≥10.9) | Yes (high grades) | No | No |
| Friction | Moderate | High | Very low | Low–moderate |
| Relative cost | Lowest | Moderate | Higher | Higher |
Matching Finish to Industry
- Steel structures / metal buildings / power transmission: HDG—proven long-life outdoor protection.
- Oil & gas: PTFE or Dacromet—chemical resistance plus predictable torque–tension behaviour.
- Wind power: Dacromet—no embrittlement on high-strength bolts, salt-spray resistance for coastal sites.
- Photovoltaic: HDG (ground-mount) or Dacromet (tracker mechanics), depending on environment.
- Automotive: Dacromet—OEM-approved and hexavalent-chromium-free.
What to Require from Your Supplier
- Salt-spray test report (ASTM B117 / ISO 9227) for the actual coating batch
- Coating thickness measured on the threaded portion
- Adhesion test (ASTM D3359 tape test or bend test)
- Hydrogen-embrittlement test (ISO 15330 / ASTM F519) for plated Grade 10.9+
- EN 10204 3.1/3.2 material certificate
At FLRS (Shaanxi Flourish Industrial Co., Ltd.), every shipment includes dimensional inspection reports, salt-spray results, and EN 10204 3.1/3.2 certificates; hardness, tensile, and yield testing is performed in our own laboratory, and SGS, BV, or TÜV third-party inspection can be arranged on request.
Conclusion
There is no universally "best" surface treatment—only the right one for the environment, strength grade, and life-cycle cost. Zinc plating suits benign conditions, HDG dominates outdoor infrastructure, PTFE excels in chemical and low-torque service, and Dacromet protects high-strength, corrosion-critical assemblies. Define the operating conditions first, then specify the finish.
Contact: flrs@mechanical-fasteners.com | Tel: +86-29-86613868 | https://mechanical-fasteners.com
