Magnesium Anode Bank on a Coated Water Line
A 2-in. fusion-bonded epoxy coated steel water line runs 1,800 ft through soil averaging 6,000 ohm-cm. It is protected by eight 17-lb magnesium anodes installed in a single 40-ft cluster at the plant end. At commissioning, pipe-to-soil potentials (CSE) are -0.91 V at the drain point, -0.84 V at 500 ft, -0.77 V at 1,200 ft, and -0.72 V at the far end. The line carries potable water at 12 C. No insulating joints are installed. The operator wants to know whether the system meets the -0.85 V criterion and whether adding three more anodes to the same cluster is a sound fix.
1. Evaluate protection at each location against the -0.85 V CSE criterion and the 100 mV polarization criterion, stating what additional data are needed before declaring the line protected.
Read the worked answer
At the drain point, -0.91 V meets -0.85 V. At 500 ft, -0.84 V is 0.01 V short. At 1,200 ft (-0.77 V) and 1,800 ft (-0.72 V), the criterion clearly is not met.
More importantly, the listed values appear to be on-potentials, not instant-off readings. The -0.85 V criterion is an IR-free value, and the 100 mV criterion requires:
ΔE = E(instant-off) - E(native), with ΔE at least 100 mV more negative.
Without instant-off and native potentials at each location, protection is not demonstrated except possibly near the drain point. A close-interval survey with current interruption, or coupon instant-off readings, is needed before declaring the far end protected.
2. Diagnose the cause of the falling potential profile and recommend a technically sound remedy, including why adding anodes to the existing cluster is or is not appropriate.
Read the worked answer
The profile falls because all eight anodes are concentrated at one end in 6,000 ohm-cm soil. In high-resistivity soil, a single cluster cannot drive enough current to the far end; most current discharges near the drain point, where the pipe is already more negative.
Adding three anodes to the same cluster lowers local anode resistance somewhat, but it mainly increases current near the plant end. That risks overprotection and coating damage locally while leaving the 1,200-1,800 ft section starved.
A sound remedy is to distribute anodes along the line, such as at 300-500 ft intervals, or to install an impressed current system with a properly spaced groundbed. Before sizing either option, perform a current requirement test, close-interval survey, soil resistivity profile, and anode output checks. Confirm that the existing magnesium anodes are not already consumed.