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Marine corrosion survey

By: Scott Fratcher - Marine Engineer/Captain. Adapted from Metal boat repair and Maintenance


Your boat may be suffering from a variety of corrosive electrical nasties – and you’re probably unaware of them. Scott Fratcher describes easy ways to trace the sources – and warns you shouldn’t delay.

Electrolysis vs. galvanic action

The two most common types of metal corrosion in the marine environment are galvanic action and electrolysis. These terms can be confusing, so let’s have a quick review:

Galvanic action is the electrical current formed when dissimilar metals are placed in (or around) saltwater. A common example is a bronze propeller on a stainless shaft.

I like to bundle together “stray current”, “voltage leaks” and “induced current” under the term electrolysis. The source may be a faulty pump winding, a generator in the harbor or a reversed polarity appliance.

At first glance, the destructive effects of galvanic action and electrolysis look similar, but the source is completely different, and we can observe different effects. Galvanic corrosion tends to be a slow process, while electrolysis can destroy large amounts of equipment quickly. A poorly-wired alternator grounding (through the prop and back into the bonding system) can cause thousands of dollars worth of damage very quickly. We can use this “speed difference” to our advantage when searching for the source of metal corrosion.

For example, electrolysis tends to leave bright patches in the corroded metal, while galvanic action tends to leave encrusted corrosion. Shiny metal is present because electrolysis is corroding the metal quickly, leaving a raw surface exposed.

Tracking the source of electrolysis can be diabolical, and it can show up as a random event. Maybe there is a voltage leak in one appliance, or only at one marina. And the effects may only be noticed during the annual haul-out, so locating the source is even more challenging.

Tracing the problem

A good first step is to monitor the vessel’s surroundings. We take a series of electrical measurements that allow us to monitor for change. Any change alerts us to a potential problem.

To conduct a corrosion survey (searching for both electrolysis and galvanic corrosion) you’ll need the following tools:

  • Quality multi-meter
  • Hertz meter (might be a function of the multi-meter)
  • Silver half-cell
  • AC/DC amp clamp

Galvanic corrosion survey

Everyone knows zinc protects the boat’s underwater metal, but how much zinc is enough and is it doing its job? A simple test will tell you: you measure the voltage of the metal being protected, and compare it to that of a silver half-cell.

A silver half-cell is a piece of high quality silver. We dunk it in the water next to the boat to provide a reference voltage. We attach the silver to one multi-meter probe and the other probe to the boat’s bonding system, or metal in question.

Tip: A silver half-cell is really just a piece of silver wire. You’ll find a piece at any jewelry shop in the form of silver soldering wire. Buy a 100mm piece and attach it to one multi-meter lead. Submerge the silver wire in water near the boat and measure voltage between it and the suspect metal.

  • With the meter set on DC we should expect the following voltages in common yacht metals protected by zinc:
  • * Silicon bronze 0.24-0.27 volt
  • * Brass 0.3-0.4 volt
  • * Bronze 0.5-0.7 volt
  • * Cast iron 0.6-0.7 volt
  • * Steel 0.7-0.85 volt
  • * Aluminum 0.8-1.00 volt

Less voltage means we are “freely eroding”. This means the surface of the protected metal is disappearing into the water and we need more zinc. More voltage means we may be over-protected, with too much zinc. The protected metal surface may form a hydrogen gas, causing the paint to blister.

This basic project tests the zinc protection due to galvanic action. To expand the test, switch the volt-meter to AC. We should see zero AC volts. If there is a reading – we have to find and eliminate the stray volts.


Tracing stray AC volts

To begin tracing stray AC volts, switch your meter to Hertz and check the frequency of the AC voltage – 50Hz means commercial power such as a power cord in the water is leaking voltage back to the dock – 60Hz means an American boat is leaking the current.

Frequencies in the thousands mean a big DC alternator with a bad diode is running in the area. A frequency close to 50Hz (say 49.8 Hz) means a local portable generator is leaking voltage into the water.

To begin your trace, shut off the shore power at the dock box and check for voltage change in the silver half-cell meter. Any change means the boat being tested has a fault. Turn the dock box back on and go to the boat’s main electrical board. Turn off all branch circuits at the breaker and continue to monitor for voltage change.

If turning off the branch circuit caused a change in the meter, note that branch circuit and work downstream till you find the offending piece of equipment.

Note: For this test to be effective the downstream devices must be turned on and drawing current. For example, to test an oven for voltage leak the oven must be on, with all elements working.

If you can’t find a leak on your boat, search the surrounding boats. Ask permission to turn off the neighboring boat’s dock box breaker, and watch your meter. Sometimes the marina operator will turn off the breaker feeding the entire dock for the quick test. A change in meter reading shows the source of the leak is in the dock power. If there is no change, the problem lies elsewhere.

This same series of tests should be run with the meter set on DC. Turn on every DC device aboard the vessel in two stages – first the breaker, and then the device itself. This will show whether the device or the feed circuit is at fault.

Be sure to start the engines and gensets and let them charge at full capacity. Keep switching your meter between AC and DC.


Do you have a challenging corrosion problem? Why not call in an expert? Scott Fratcher is available to help solve your marine corrosion issues. Scott has a long history of quickly solving corrosion problems on large and small vessels alike.
Click here to contact Scott Fratcher, Marine Engineer



Bonding is a system to protect many underwater metals with a single zinc block. The idea is to inter-connect through-hull fittings and a bronze depth sounder (for example) with a wire. This saves having to zinc every piece of underwater metal. Any excess voltage will bleed off through the single sacrificial zinc.

Bonding used to be the answer for almost all electrical corrosion, but this is no longer the case because of the sheer amount of electrical equipment near the water. A poorly maintained bonding system is particularly dangerous because the owner believes he is protected.

Electricians often say: “If a bonding system is not giving any problems then leave it, but when electrical corrosion is found, suspect the bonding system first”.


Inspecting a bonding system

A suspect bonding system is easily checked. Start at the zinc and check the resistance between the zinc block and each protected metal. For example, if a string of through-hull fittings is protected by one zinc, connect one probe to the zinc and one to the through-hull. Use the sharp tip of the probe to work your way into the metal of the through-hull.

Note: Check the metal of the through-hull itself and not just the bonding wire.

You should see less than one ohm between all points in the bonding system. But you’ll often find readings in the hundreds of ohms, or no continuity at all. This is a clear indication the underwater metal is not protected and the zinc not sacrificing itself. The boat may be at risk due to a weakened through-hull, and it’s unknown to the crew.


Bonding system current check

The bonding system should be checked for continuity from bow to stern, and it should also be checked for current. Use a clamp amp, and look for both AC and DC. Hunt for amperage in the bonding system connection wire. There should be no amperage at any time. If there is, a ground loop has formed and current is traveling through the bonding system. This is bad.

A ground loop forms when an electrical device finds a second path to the battery’s negative terminal. The zinc bonding system may be connected to battery ground through a variety of paths. For example, the SSB radio ground plane is often connected to the zinc bonding system and this provides a path to battery ground through the back of the radio.

Another common destructive electrical path is out the zinc, into the prop, back up the prop shaft, through the engine and to the battery ground. In corrosion surveys I always clamp the prop shaft and often find errant current.

Inverters are often connected to the bonding system. The inverter is especially suspect because it has a connections to both the AC and the DC system. Be sure to clamp all inverter leads and monitor the silver half-cell test during inverter operation.


Deck fittings

Corrosion above the water line (deck fittings) is also a problem. Sea spray landing on metal and drying leaves a small amount of concentrated salt crystals that hold moisture. This salt deposit allows galvanic action to take place in small cracks and crevices – and they cannot be protected by zinc.

A classic example is a galvanized chain attached to a steel anchor with a stainless steel shackle. The galvanizing soon bubbles away, leaving the last few links of chain to rust. And a chain is only as strong as the weakest link.

Another common destructive path of electrical current is through the anchor chain. Electrical voltage seeking ground will travel down the chain and back into a through-hull fitting. This path may occur when a positive lead comes into contact with deck metal.

An “energized” pulpit is a common culprit (with positive leads feeding the running lights run inside the tubing). The pulpit itself begins to seek battery ground, and the anchor chain can provide the path through the water.

All exposed deck wires are suspect. Small amounts of chafe or damage to the cover sees salt water entering the wire and forming an electrical connection to surrounding metal.
There is quick test to see if a wire is leaking electrical power.

Simply energize the wire and clamp the duplex lead (duplex wire is where the positive and negative leads are contained in one plastic sheath). A clamped duplex wire should show zero amps. You can even grab whole bundles of wires and clamp them. The bundle should read zero. When you find a reading you have found a leak.


Induced current

Induced current is where an electrical charge is picked up, merely by being in the vicinity of leaking electricity. A live electrical cord laid across a metal deck, or tied along a lifeline, can cause induced current. The magnetic field from the power cord “jumps” or is induced into surrounding metal. Luckily, the silver half-cell test on AC is an easy way trace this leak.

Another form of induced current is where the entire boat is in the path of a larger electrical leak. For example, the next boat along the pier may be leaking electricity into the water, and it’s seeking ground on the other side of your vessel.

If a vessel disrupts such an electrical path, its bonding system may become the path of least resistance, causing the stray current to enter one end of the boat, follow the path of least resistance (the bonding system’s copper wire) and exit a forward through-hull.

This form of electrical corrosion is easy to find with the silver half-cell test (or clamp amp), but you have to be watching the meter when the current passes. So if another boat’s battery charger causes the leak and you monitor the silver half-cell when the charger is off, you may miss seeing the corrosive warning.



Stray current in a rig is common – and easy to find. A metal mast will inevitably have a path to battery ground through the VHF coaxial cable. The coax screws to the back of the VHF (with a SSB there is similar path to battery ground). Clamp the coaxial cable of the VHF, turn on all mast-mounted devices such as spreader lights, radar, and navigation lights, and check the clamp amp for current.

Also clamp each piece of standing rigging and examine it for visual corrosion. Clamp the standing rigging while the mast is fully energized with the lights on. There should be zero amperage on both AC and DC, but you’ll often find current passing through the stainless rigging.

At the base of the mast you should have some type of junction box. Clamp the wire bundle. You should show zero at all times. Any reading shows an unbalanced load and thus a leak. While checking the wires at the base of the mast take note of the condition of the mast base. This is a common area of corrosion and should be inspected regularly.

A final tip: Electricity can travel through just about any metal. This means it is important to clamp engine control cables, copper hydraulic steering lines and handrails. I routinely find errant current in the most unlikely places. Only with proper testing can this corrosion threat be minimized.


Permanent silver half cell installation

Many yachts have installed a silver half-cell permanently in the hull that leads to a meter at the electrical panel. This is good, but I have not found any system that also monitors for stray AC and hertz. For that reason some boat owners install the silver half cell into the hull, and use a standard multi meter as the testing device. This is an inexpensive method of providing monitoring protection to just about any hull.


Build a electrical leak tracker

To track leaking current in a marina build a simple, portable, mobile corrosion skiff corrosion monitor. Place dissimilar metals in the water about two meters apart and monitor the voltage. In short we will have built a submerged electrical field monitored from inside a moveable skiff. In other words, a directional antenna. When the space between the sensors encounters another electrical field the meter voltage changes. Rotate skiff until the highest voltage is found and follow the line to the source of the leak. To the vessel owner this means an inexpensive method of tracking errant electrical leaks. To the harbormaster this means a skiff can drive through the marina and detect stray current.


Do you have a challenging corrosion problem? Why not call in an expert? Scott Fratcher is available to help solve your marine corrosion issues. Scott has a long history of quickly solving corrosion problems on large and small vessels alike.
Click here to contact Scott Fratcher, Marine Engineer




Marine corrosion survey

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