Why Coating Selection Matters
Uncoated B7 alloy steel will show visible corrosion within weeks in outdoor or marine environments. Coating selection affects service life, friction coefficient for torque control, and compatibility with the service environment.
Hot-Dip Galvanising (HDG)
Per ASTM A153 / ISO 1461 — 45–85 µm sacrificial zinc coating applied at ~450°C.
- Corrosion life: 15–20 years in C3 atmosphere (ISO 9223)
- Salt spray: >500 hours (ASTM B117)
- Temperature limit: 200°C continuous
- Important: Requires oversize nuts (ASME B18.2.2). LOKRON applies post-galvanising hydrogen embrittlement relief (190°C / 4h) as standard.
PTFE / Xylan Coating
12–25 µm dry film PTFE dispersion coating, cured at 180–220°C.
- Key advantage: Controlled friction coefficient K = 0.10–0.15 — critical for accurate bolt preload in high-stakes flanged joints (API 6A, ASME Class 900+)
- Salt spray: 500–1,000 hours
- Temperature: −73°C to +230°C
- No oversize nuts required
- Superior galling resistance — essential for stainless-on-stainless assemblies
Coating Selection Matrix
| Environment | Coating | Reason |
|---|---|---|
| Onshore outdoor | HDG | Cost-effective sacrificial zinc |
| Offshore topsides | PTFE/Xylan | Superior salt spray + friction control |
| High-preload joints | PTFE/Xylan | Precise K-factor for torque calculation |
| High-temp >200°C | Electroless Nickel | Temperature-stable protection |
coatingHDGPTFEXylangalvanisingoffshorecorrosion protectionfastener coating
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