In industrial fastening applications, surface coating is not an afterthought — it’s a critical engineering specification. The right coating enhances corrosion resistance, reduces friction during installation, prevents galvanic corrosion, and ensures long-term structural integrity under thermal, chemical, or mechanical stress. Failure to specify appropriate coatings can lead to premature failure, safety hazards, and significant lifecycle cost increases.
Industrial Screw Coating Technologies Compared
Below is a technical comparison of the most commonly specified screw surface treatments used in heavy machinery, automotive, construction, and offshore industries:
| Coating Type | Corrosion Resistance (Salt Spray Hours ASTM B117) |
Friction Coefficient | Max Operating Temp | Hydrogen Embrittlement Risk | Typical Applications |
|---|---|---|---|---|---|
| Zinc Plating (Clear/Yellow) | 72–120 hrs | 0.12–0.18 | ~120°C | Yes (Post-bake required for HRC >39) | Indoor machinery, electronics, low-cost assemblies |
| Dacromet® (Zn-Al Flake) | 500–1000+ hrs | 0.10–0.15 | ~300°C | No | Automotive chassis, marine, bridge structures |
| Geomet® | 500–800 hrs | 0.10–0.14 | ~250°C | No | Environmentally regulated projects, EU exports |
| Hot-Dip Galvanized | ≥800 hrs | 0.20–0.35 | ~200°C | No | Structural steel, outdoor towers, utility poles |
| Xylan® 1070 (PTFE) | 250–500 hrs | 0.06–0.09 | ~260°C | No | High-sliding, non-conductive, food-grade equipment |
| Cadmium (Restricted) | ≥1000 hrs | 0.08–0.12 | ~230°C | Yes | Aerospace, military legacy systems (with exemption) |
Where & Why Each Coating Is Specified
- Zinc Electroplated Screws – Cost-effective solution for dry, controlled environments. Requires chromate conversion coating for enhanced passivation.
- Dacromet & Geomet Coated Screws – Solvent-free, Cr6+-free alternatives offering high corrosion resistance without hydrogen embrittlement — ideal for high-strength bolts (Grade 10.9+).
- Hot-Dip Galvanized Screws – Thick coating necessitates oversizing internal threads — use DIN 267-24 or ISO 10684 standards for tapped hole adjustment.
- Fluoropolymer (Xylan, Teflon) Coated Screws – Used where lubricity, electrical insulation, or chemical inertness is required (e.g., semiconductor, food processing).
How to Specify the Right Coating: A Decision Framework
- Define Environment – Marine? Chemical exposure? Temperature cycling?
- Determine Life Expectancy – 5 years vs. 25+ years? Dictates salt spray hours needed.
- Check Galvanic Compatibility – Avoid coupling dissimilar metals without isolation coating.
- Verify Regulatory Compliance – RoHS, REACH, FDA, NSF — especially in export or medical applications.
- Specify Testing Requirements – Include ASTM B117 (salt spray), ASTM B571 (adhesion), and torque-tension curves per SAE J995.
Relevant Industry Standards for Screw Coatings
- ISO 4042 – Fasteners — Electroplated coatings
- ASTM F1941 – Standard Specification for Electrodeposited Coatings on Mechanical Fasteners
- ISO 10683 – Fasteners — Non-electrolytically applied zinc flake coatings
- DIN 267-11 – Technical delivery conditions for hot-dip galvanized threaded parts
- SAE AMS2451 – Aerospace coating specifications including Dacromet variants
🔧 Engineering Tip: Avoid Costly Field Failures
Always include these key coating specs in your RFQ or drawing:
- Coating system name (e.g., “Dacromet 320A per ISO 10683”)
- Minimum thickness (microns or mils)
- Salt spray performance requirement (e.g., “≥720 hours to white rust”)
- Post-coating baking requirement if applicable (for hardened steels)
- Appearance acceptance criteria (blotches, runs, chromate color)
Without explicit specs, suppliers may deliver minimal compliance — risking field corrosion within months.
