Segment Bolt Corrosion in Metro and Sub-Sea Tunnels: Hot-Dip Galvanizing vs Dacromet

Segment bolts are a lining's most exposed steel. In sub-sea tunnels, chloride-rich groundwater and oxygen gradients drive pitting at bolt heads and nuts; in metro running tunnels, stray traction currents accelerate electrochemical attack. Latin American metro and water programs now combining volcanic and sedimentary mixed ground add abrasion, variable pH and aggressive sulphates, so a bolt that passes a standard salt-spray test can still corrode well within the design life.

Hot-dip galvanizing gives a thick, sacrificial zinc layer, typically 45–85 µm, with strong abrasion resistance, but the molten bath risks hydrogen embrittlement in high-strength bolts and can thicken threads, shifting torque–tension behaviour. Dacromet and zinc-flake coatings apply thin, uniform, embrittlement-free layers with excellent chloride performance and controlled friction, though sacrificial reserve is smaller. Zinc-rich primer with a PTFE topcoat, or stainless grades, suits the most aggressive sub-sea and cathodically protected sections.

When specifying, fix coating class and thickness, salt-spray hours, post-coating thread tolerance, friction coefficient, hydrogen embrittlement relief, and compatibility with cathodic protection to avoid overprotection. Buyers on Latin American metro and water programs should also request batch traceability and consistent friction values across deliveries. OUSUN supplies interchangeable segment bolts and lining hardware from Deyang, Sichuan — China's largest TBM cluster — with spares lead times of roughly two to four weeks at [email protected].