Impressed Current Corrosion Protection on Yachts: How It Works and How to Keep an Eye on It

Locate the control panel (usually in the engine room or a technical space), its display or meter, and the circuit breaker or fuse that powers it. Ask your yard or the commissioning report where the hull anodes and reference electrodes are; on a haul-out you'll see them as flush-mounted disks or plates, often with a shield of special coating around each anode. Write it all down in BoatMend with a photo.

Most panels show at least the hull potential measured by the reference electrode (in millivolts) and the current going to the anodes (in amps), and many have alarm lights. Compare today's readings with the target and the readings in the commissioning report. Your system's manual explains what each light means; keep a copy aboard.

Write down hull potential, anode current, any alarm, the date, and whether shore power was connected, on a regular schedule (weekly while you're aboard is a good habit) and after any event like a haul-out or new equipment. Over time the log shows the normal pattern. Two changes are worth a call:

• Current climbing steadily while potential stays the same can mean more bare metal is exposed (paint damage) or something else is draining protection, such as a stray current or a neighbor's boat through the shore power ground.
• Current at zero or an alarm means the system isn't protecting the hull.

The system protects only while it has power. Check that its breaker stays on and that batteries stay charged while the boat sits in the water, especially in winter storage afloat or when you're away for weeks. If the boat's batteries are regularly allowed to go flat, the hull loses protection. Ask your installer how it is wired and whether a battery charger on shore power backs it up.

The system trusts its own reference electrodes. If one drifts or fails, the panel can show "normal" while the hull is under- or over-protected. Do an independent check a few times a year:

1. Unplug shore power at the boat (not just switched off), as in the hull potential survey guide.
2. Connect your portable silver/silver-chloride reference electrode and multimeter the same way as in that guide, with the positive lead on a clean point of the hull or bonding system your specialist recommends.
3. Lower the electrode over the side, away from the system's anodes, and take readings at several places along the hull, including near and away from each anode.

Compare your readings with the panel's and with the target range for your hull. Readings close to the anodes are usually more negative; that is one reason Evac uses a second reference electrode near the anode, to watch for local over-protection.

On an aluminum hull, D'Antonio's guidance is that readings should never be more negative than about -1,100 mV (Ag/AgCl). Watch for:

• portable readings more negative than the target range, anywhere on the hull
• paint blistering, lifting or peeling, especially around the anode shields, or bare "halos" in the bottom paint around fittings, shafts and props
• white, powdery or etched aluminum under failing paint
• bubbles or a fizzing look around the anodes in calm water

Any of these means call the specialist (step 12). Don't adjust the control settings yourself.

• readings less negative than the target range (closer to zero)
• pitting on the hull, the shaft, the propeller or around fittings
• sacrificial backup anodes wearing much faster than before

Under-protection often follows a power failure, a failed reference electrode, damaged anodes or a new stray current source.

The system was set up for the metal on the boat and the anodes it has. Adding sacrificial anodes, changing a propeller from bronze to stainless (or the reverse), adding a bow thruster or new through-hulls changes the balance. Ask your specialist before adding or removing anodes. Practical Sailor's rule on anodes applies here too: more is not always better.

A boat plugged into shore power is electrically connected to every other boat on the dock through the green ground wire. A galvanic isolator or isolation transformer keeps those currents off your hull (see the galvanic isolator test and isolation transformer basics guides). If the system's current jumps when you plug in, or when a new boat moves in next to you, the isolation may not be working.

When the boat is out of the water:

• switch the system off as its manual describes; out of the water it cannot work and may raise an alarm
• tell the yard about the system before any hull work, sanding, welding or painting
• inspect each anode, its shield and each reference electrode with a flashlight; look for cracks, missing coating, impact damage or growth, and clean growth off only as the manual describes (usually gently, with no scraper or pressure washer on the faces)
• check the paint around each anode shield for lifting or blistering, a sign of local over-protection
• make sure the painters keep paint off the anode and reference electrode faces, unless the maker says otherwise
• inspect any backup sacrificial anodes and replace them as needed

After launch, switch the system on as the manual describes and let it settle; readings change for a while as the hull polarizes. Log the readings for the first few days, then do a portable check (step 5). If anything doesn't return to the old normal, have the specialist look before the boat sits for weeks.

Have a marine electrician or corrosion specialist with experience in impressed current systems check the whole system on a regular schedule (the maker or installer can suggest how often), and always after hull repairs, new underwater metal, a lightning strike or a long power outage. They test each reference electrode, check anode output and shields, review your log and adjust the settings if needed.

Store the commissioning report, the manual, your reading log, haul-out photos of anodes and electrodes, and every professional survey in BoatMend. A future buyer's surveyor will ask for them, and a specialist can diagnose a problem much faster with a history to compare.