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
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.
Rigging
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