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

Metro programmes across South Asia are driving tunnels through high water table alluvial soils, where chloride- and sulphate-bearing groundwater reaches the segment ring long before the lining shows visible distress. The segment bolt is usually the first component to fail: it is small, highly stressed, easily damaged at the thread during installation, and completely inaccessible once the tail skin has passed. Where capital cost dominates the tender, coating selection is often deferred to the contractor and then settled on price rather than exposure class — which is exactly where long-term durability quietly leaks away.

Hot-dip galvanizing remains the default choice. Its zinc layer, commonly 55–85 µm, provides genuine sacrificial protection and tolerates site handling well, but it is harder to control on high-strength bolts: acid pickling carries hydrogen embrittlement risk, and heavy zinc buildup can interfere with thread fit and torque–tension repeatability. Dacromet-type zinc-aluminium flake coatings deposit only around 8–12 µm, avoid acid pickling, and suit grade 8.8 and 10.9 fasteners — but they lean more on barrier protection, offering a smaller sacrificial reserve once chloride ingress begins.

The practical takeaway: define the exposure class first, then the coating. For sub-sea and aggressive chloride conditions, specify a zinc-flake system backed by documented torque–tension and salt-spray testing, and consider isolating washers to limit stray-current attack from metro traction systems. Record coating thickness by batch and re-check threads on delivery. OUSUN, based in the Deyang TBM cluster in Sichuan, supplies segment bolts, disc cutters, seals and screw conveyor parts factory-direct, with typical spares lead times of two to four weeks.