Impressed Current Retrofit for a Coastal Sheet-Pile Bulkhead
A steel sheet-pile bulkhead 180 m long separates a brackish harbor (resistivity about 25 ohm-cm) from a sand backfill. The piles are uncoated below the mudline and coated above. A diver survey finds pitting on the harbor side at the mudline. The owner proposes a single rectifier and a remote anode bed 40 m offshore in 8 m of water. Available power is 480 V three-phase at the dock, 60 m from the proposed rectifier location. Target protection is -0.90 V (Ag/AgCl seawater) on the harbor side. Design current density for bare steel in brackish water is 1.1 mA/ft2, and the wetted area is 180 m x 4 m per side.
1. Size the current demand and comment on whether one rectifier and a single remote bed is a sound architecture.
Read the worked answer
Wetted area per side = 180 m * 4 m = 720 m2 = 7,750 ft2. At 1.1 mA/ft2 bare, demand is about 7,750 * 1.1 = 8.5 A per side; both sides roughly 17 A, plus coating losses and margin, so plan for a 25-30 A rectifier. That is easily within a single rectifier's capability.
The architecture question is different. A single remote bed 40 m offshore creates a long current path to the far end of a 180 m wall. Current density will be highest near the bed's projection and lowest at the ends, and the wall acts as a long conductor with its own attenuation. For a 180 m structure, consider a distributed anode array along the wall (multiple beds or a continuous sled) or a rectifier with multiple circuits so current can be trimmed per zone. A single remote bed is acceptable only if you verify, by calculation or a temporary CP trial, that the far ends reach criterion.
2. The harbor water is 25 ohm-cm and the backfill side is dry sand. What does that mean for anode placement and for the risk of overprotection?
Read the worked answer
Low-resistivity harbor water means the harbor-side anodes will have low resistance and will readily deliver current; the dry sand backfill side is high resistance and will receive little current from a harbor-side bed. That is consistent with the observed mudline pitting on the harbor side, which is the side to protect first.
Overprotection risk is real: at 25 ohm-cm, potentials can swing negative quickly near the anodes, driving hydrogen evolution on the steel and disbonding the above-water coating or embrittling high-strength components. Use distributed anodes, control rectifier output, and monitor potentials at multiple points along the wall rather than at one reference cell. Set the rectifier so the most negative reading stays within a safe limit (commonly not more than about -1.05 to -1.10 V Ag/AgCl on coated steel) and confirm with a 100 mV polarization decay test.
3. How would you verify the design before committing to the full installation?
Read the worked answer
Run a temporary CP trial: install a few anodes and a temporary rectifier or battery source, then measure potentials along the wall at the near end, mid-span, and far end, and at the mudline and below. Check:
- whether all points reach -0.90 V Ag/AgCl;
- the current required, to validate the 1.1 mA/ft2 assumption;
- the potential gradient along the wall, which tells you whether one bed is enough or you need distributed anodes;
- the 100 mV decay on a representative coupon to confirm polarization, not just a shifted reading from IR drop.
This trial is cheaper than discovering at commissioning that the far end is under-protected.