Stray-Current Corrosion on a Buried Fire Main Near a DC Tramway
A 3 km DN600 cement-mortar-lined mild steel fire main runs parallel to a 1500 V DC tramway for 900 m at 8–15 m separation. The external coating is coal-tar enamel with known holidays from a previous survey. There is no cathodic protection. Pipe-to-soil potentials measured at test posts are −0.45 V CSE at night but swing between +0.10 V and −0.90 V CSE during tram service. A leak occurred last month at a coating holiday, and the excavated pipe shows bright, pitted metal with no general wastage. The water authority wants to know whether the leak was caused by soil corrosivity or by the tramway, and asks for a permanent fix that will not interfere with tram signalling.
1. Explain how you would confirm stray-current interference and distinguish it from natural corrosion.
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Signature of interference. A static potential of −0.45 V CSE is more positive than the −0.85 V CSE protection criterion, so the line is not protected. The decisive evidence is the synchronous swing from +0.10 V to −0.90 V with tram service: the pipe is alternately anodic, where current leaves the pipe and corrosion occurs, and cathodic, where current is picked up. Natural soil corrosion does not produce such regular, traction-correlated excursions.
Confirmation.
- Install data loggers at several test posts for at least 24 h, including peak service.
- Measure rail-to-earth voltage and soil resistivity along the route.
- Conduct a coating holiday survey and close-interval potential survey.
- Use coupons or ER probes to quantify current leaving the pipe.
- Compare weekday and weekend patterns to isolate the traction source.
Criteria: NACE SP0169 −850 mV CSE with CP applied, or a 100 mV cathodic polarization shift; the anodic swing is the key risk indicator.
2. Recommend a mitigation scheme, including its effect on the tramway.
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Hierarchy of measures.
- Repair coating holidays to reduce current demand at the points where current leaves the pipe.
- Install a forced drainage bond from the pipe to the running rail through a resistor and a diode or switch, sized so that the pipe is not over-protected. A switch or capacitor blocks current when the pipe would otherwise become cathodic.
- If drainage is insufficient, install an impressed-current cathodic protection system with anodes and permanent reference electrodes.
- Monitor with coupon and ER probes and a close-interval survey.
Tramway impact. Any bond changes the traction return circuit, so the bond resistance must be agreed with the rail operator, and signalling immunity must be verified. A drainage bond must never be installed without that agreement. The design should include fail-safe isolation and an alarm for loss of protection.
3. What acceptance criteria and records would you require after commissioning?
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I would require a close-interval potential survey showing at least −850 mV CSE with CP applied, or a 100 mV polarization shift, at all test posts, with no anodic excursion during peak tram service. Records should include as-built bond and anode details, rectifier settings, 24 h potential logs before and after, coating repair locations, and a monitoring plan with quarterly readings. A commissioning report signed by the designer and the operator should confirm that tram signalling and traction return remain unaffected.