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

Segmental lining rings are only as durable as the bolts that clamp them, and across Southeast Asia's humid metro and karst corridors those bolts sit in warm, chloride-bearing groundwater. Thread roots behave as crevice-corrosion sites, and high-strength bolts are also vulnerable to hydrogen embrittlement when pickling or cathodic protection is applied without control. Failure is rarely dramatic: it appears as lost clamping force, gasket relaxation and water ingress years after the TBM has been dismantled and the site demobilised.

Hot-dip galvanizing remains the default choice, giving a thick sacrificial zinc layer that tolerates handling damage and suits permanently wet, buried conditions. Its limits are dimensional: zinc build-up can interfere with thread fit and torque-tension control, so threads often need dressing or retapping. Dacromet-type zinc-flake coatings are thinner, applied by dip-spin without the acid pickling that risks hydrogen embrittlement, and they keep threads clean for repeatable torque — but offer less sacrificial reserve under aggressive chloride attack.

Practical specification means matching coating to environment class rather than to habit. For inland metro segments, galvanized or zinc-flake bolts with a sealing washer are usually adequate;