Segment Bolt Corrosion: Galvanizing vs Dacromet for Metro and Sub-Sea Tunnels

Segment bolts are the quiet weak point in a lined tunnel's corrosion defence. They sit in joints that stay damp, wick chloride-laden groundwater, and in metro running tunnels also collect stray current. In Middle East desert and mixed-face drives — sand over rock, sabkha sulphate soils — attack on couplers often runs ahead of the concrete itself. Once a shank loses section, joint clamp drops, gaskets relax, and leakage starts.

Two coating systems dominate. Hot-dip galvanizing lays down a thick zinc layer, typically 50-85 µm, giving barrier plus sacrificial protection and good resistance to handling and grout abrasion. Its constraint is heat: the molten dip can soften high-strength bolts and raises hydrogen-embrittlement questions, so threads are over-tapped and torque values re-tested. Dacromet, a zinc-aluminium flake coating, applies at low temperature, avoids embrittlement and coats threads cleanly at roughly 8-12 µm, but offers less wear resistance.

For metro and sub-sea specifications, state the exposure class first — chloride, sulphate, stray current, immersed or splash zone — then require coating thickness measured on the finished thread, not just the shank. Ask suppliers for torque-tension and salt-spray data on coated fasteners, and consider duplex systems or duplex stainless grades where chloride is high and maintenance access is nil. Treat consumables as scheduled items: OUSUN supplies segment bolts, seals and TBM spares factory-direct from Deyang with about 2-4 week lead times ([email protected]).