different types of
Corrosion is one of
the biggest enemies of a yachtsman. There is a lot of
misunderstanding about corrosion. One often mistakes the corrosion
taking place under water for the corrosion taking place above deck.
It's very important to identify two types of corrosion:
or dry corrosion. This type of corrosion takes place when metals
are attacked by certain elements or combinations of elements. This
type of corrosion takes place above deck. A well know type of this
corrosion is rusty iron. The protection against this type of
corrosion is achieved by surface treatment.
corrosion or wet corrosion. This type of corrosion arises from the
creation of an electrical element in a watery environment. This
electrical element is build by two different metals with an
electrolyte in between. The best example for such an element is a
battery as used in every yacht or car. This electrochemical
corrosion is also known as electrolysis or galvanic corrosion. As
this corrosion is the biggest danger for all underwater metals we
will go into depth how avoid this.
Example of electrochemical corrosion caused by an electrical
connection of the rudder stock
with another underwater object like the propeller shaft.
principle of electrolysis:
The principle of
electrolysis is based on the fact that when a metal is put into
water it will create a non neutral voltage. On the surface a very
small quantity of positive charged metal ions will break out leaving
the negative charge in the form of electrodes behind in the metal.
The reaction is (Me=metal) Me -> Mez+ + z e. The metal
will be negatively charged. As long as this metal is not connected
to another object with a different charge, nothing will happen and a
balance will have been reached.
The reaction as described above is not enough for electrolysis to
arise. The problem starts because all existing metals have a
different charge when put underwater. Some positive, some
negative. The absolute charge varies and increases when the
temperature rises, the amount of salt in the water is higher and if
more oxygen is present in the water. The absolute worst place for
electrolysis is the Mediterranean as all factors are on it's worst.
The phenomena described above is still not enough for electrolysis
to arise. Only when the two different metals are electrically
connected, electrolysis can start. One metal (the one with the most
negative charge) will act as anode and sacrifice itself. The other
metal with act as cathode. As the anode is electrically connected to
the positively charged cathode, it can't reach it's natural
negatively charged balance as described above. But it will
constantly try to reach it. To get a more negative charge, the metal
will split constantly into positively charged metal ions breaking
out of the surface and leaving behind the negatively charged
electrons. The electrons will be transported through the wire
connection the two metals to the more positive charged cathode, and
the whole process will start over again. It will only end when the
complete anode is dissolved.
Electrolysis on GRP and composite yachts:
Lets now project
this theoretical phenomena to your sailing yacht. First we have to
determine which metal will start acting as anode. Following list is
a sum up of metals in order of electro negativity, starting with the
most negatively charged metal: Magnesium, Zinc, Aluminium, Steel and
Iron, cast Iron, Stainless Steel (active) , Lead, Brass, Copper,
Bronze, Stainless Steel (passive), Carbon. (One can make Stainless
Steel passive by chemical treatment after machining). Second, the
hull material is important. Lets start with composite hulls.
Typical rudder stock metals are aluminium and (active) stainless
steel. Typical propeller shaft metal is (passive) stainless steel.
Typical propeller metal is bronze. Typical keel material is lead or
cast iron or a combination of both. (The propeller shaft can be made
out of passive stainless steel as the geometry is often very simple,
no welding will have to be done and can be ordered as passive
Lets take a yacht with an aluminium rudder shaft, standard propeller
drive system and a cast iron keel. Some boat builders choose to
connect all tree parts with a copper wire (see red line in
illustration). They protect the complete system with a zinc anode on
the propeller or propeller shaft. The zinc anode has the lowest
electro negativity and will act as anode. The propeller shaft, made
of passive stainless steel, will act as cathode. As long as the zinc
anode is present, it will all go well. Problems will arise when the
zinc anode either is completely dissolved or falls off. The new
anode will be the aluminium rudder shaft, which will start to
sacrifice itself. Even if one had chosen to use a stainless steel
(active) rudder shaft, the same would happen: The rudder shaft will
start to dissolve in the water. A typical example is a Scandinavian
boat owner, having sailed years without any problems, now take's his
yacht to the Mediterranean and is confronted after one year in the
water with the fact that the zinc anode has been completely
dissolved in the water and his rudder shaft shows signs of
electrolysis. The extreme salty and warm environment highly
increases the electro potential difference between the different
metals, making the reaction to go much quicker.
Solution to avoid electrolysis:
This is the reason
that we can only advise to make sure the rudder shaft is completely
electrically disconnected from the rest of the vessel. Often this
connection in made without the awareness of the boat owner. Other
parts on the yacht, like the steering system and autopilot drive,
are linked to the rudder shaft. Parts like the engine control and
instruments are again linked to the steering system. Without any
precautions a connection to the engine and propeller shaft is easily
made. As the general minus pole of the power supply on the yacht is
mostly connected to the engine, every electrical function on board
is connected to the propeller shaft. (An exception to this is when a
trust bearing in combination with an electrically isolated flexible
coupling is used in the propeller drive system). Lets closely look
at the parts that could connect to your rudder shaft:
Autopilot drive: As
the minus pole of the autopilot drive is mostly continuously
connected to the battery and thus to the propeller shaft, we have a
very dangerous potential connecter. One should thereby always check
if the housing and drive shaft of the autopilot drive is insulated
from the minus pole (most autopilot drive manufacturers will not
guarantee an electrical insulation!) . Even when there is no
connection (an infinite resistance on the resistance meter),
eventually a connection inside the electrical motor could arise due
to carbon dust, worn from the carbon brushes in the motor,
connection the minus pole to the housing.
The best solution for disconnection is to insulate the autopilot
drive shaft from the tiller arm connected to the rudder. As most
autopilot drives use a rose joint - pin connection, one should use
an insulated rose joint. As most rose joints used in this
application are standard industry products, insulation is not a
subject. Mostly the housing and the ball are made of stainless
steel. Between the ball one can either find a bronze bush or a thin
Teflon layer. The first one is completely non insulation, the
second is insulating up until the Teflon is worn.
Steering system: If
you use a hydraulic steering system, in combination with nylon
flexible hoses, there is no problem. As the use of a hydraulic
system on a sailing yacht is very rare, one should concentrate on
mechanical steering systems. Two methods of steering can be used:
cable steering or rod steering. Cable steering systems can't be
electrically disconnected from the rudder. Rod steering systems
always use a rose joint to connect to the rudder. The same rules of
disconnection are valid as shown in the above section on
autopilots. If complete insulation of the connecting rod (draglink)
is not possible, one should take the same measurements as on cable
systems; electrically insolate the steering system and pedestal
from the rest of the yacht.
mounted on steering pedestal: As the control cables of the engine
control are made of steel and stainless steel one should carefully
insulate the engine control housing from the pedestal.
and switches. Often the pedestal is covered with instruments and
switches. One should make sure the cables attached to these
instruments and switches are not in contact with any metal parts of
the steering pedestal.
Compass wire: In
most cases, the compass wire will run through the pedestal. A cable
joint will be made in the top of the pedestal or binnacle. Please
check if no contact is made to the pedestal or binnacle.
When the yacht is
ashore, these connections can be tested by using an accurate
resistance meter. One should connect one pole to the rudder shaft
and the other pole to the propeller shaft or keel. The resistance
should be infinite. If the resistance is less, one has a current
leak and should test all above possible connections.
When the yacht is in the water, one can do the same test. A second
test is to measure the voltage difference between the rudder and
propeller shaft. Depending on the factors water type, temperature,
used metals as described above, the voltage difference can vary from
a couple of millivolts to tenths of volts. If a voltage difference
is present, there will be no connection between the measured parts.
aluminium and steel yachts:
The rules for
avoiding electrolysis on aluminium and steel yachts are the same as
on composite yachts: Disconnect the rudder from the rest of the
ship. Mostly metal ships will have metal rudder blades. It's
advisable to protect these blades with a separate zinc anode on the
rudder blade. Especially when different types of metals or metal
mixture are used. For example a stainless steel rudderstock with a
steel blade, or a aluminium rudder stock with an aluminium blade.
The aluminium of the rudder stock is not the same as the aluminium
of the blade. A small galvanic element is created that should be
protected with an anode.
Electrolysis between the yacht and the outside world:
This type of
electrolysis is the most feared type. A complete aluminium hull can
dissolve in one month making the yacht a total loss. Instances are
known even on composite yachts that lost a complete stainless steel
rudder in one month time.
As this explanation of electrolysis is mainly based on protecting
the rudder system, and as long as the rudder shaft is electrically
disconnected from the rest of the yacht, the rudder shaft can't
suffer from electrolysis.
The only way to prevent electrolysis between your yacht and the
outside world is to completely insolate the power supply via a
David H. Pascoe
keep your boat in a crowded marina and there is a lot of talk about
corrosion and bad wiring.
The reason why is due to a general lack
of understanding of what happens in a marina full of boats. After
all, this is not an easy thing to understand, and it took a lot of
research to find out what was going on here. Yes, the marina's
wiring is involved with all such corrosion problems, but it may or
may not be the cause.
The key to
understanding corrosion problems in marinas involves several things.
First, all the boats in the marina are tied into the marina's
grounding system. Second, all the boats are also grounded at a
second source -- the water.
Thirdly -- a major
point here -- the AC power system on the boat (green wire) is also
grounded to the boat's common ground point, usually established at
the engines. This means that your boat's common grounding and
bonding system is electrically connected with the dock's grounding
Before we get any
further into this, let's be sure that we understand our terms here.
The 125 VAC shore system has three wires (except for 125/250 VAC
which has four), one of which (black) is obviously the current
carrying conductor. The others are the ground (white), or neutral,
and the grounding wire (green), which is the safety ground intended
to deal with short circuits and protect people from electrical
shock. Also called a "bond," it is attached to the frames of all
appliances and other electrical devices. The ground, which is the
negative (-) side of the system, is supposed to be at earth
potential, but this is a normally current carrying conductor,
whereas the grounding wire isn't.
means that a metal is less highly charged than others. Stray current
will seek a path to an anode, and from there travel to ground, the
water. When an anode is energized by an outside source, as the
electricity leaves the anode, it carries molecules of metal away
with it. This electrical erosion is true electrolysis. Cathodic
means that a metal is more positively charged; current will flow
from it, and on to one that is anodic, with no resulting metal loss
or erosion. Thus, with a zinc attached to a stainless shaft, the
zinc erodes while the stainless is protected. Adding an outside
source of current accelerates the process.
or galvanic current, is caused by dissimilar metals, metals with a
greatly different electrical potential that are electrically joined.
Galvanism plays the primary role in the "boat battery" problem
that all the boats in the marina are connected together electrically
by both of these wires, the neutral and the ground or bond, one of
which is also joined to the underwater metals of the boat (the
bond). The boat's neutral is never grounded to the boat itself, but
always earth potential of the dock ground. The underwater metals of
the boats, of course, can vary between such things as brass, bronze,
stainless steel -- and aluminium, as in outboard motors, aluminium
boats and stern drives. All of these metals can develop different
electrical potentials galvanic ally in the same manner as a battery
We could have, for
example, ten boats on a pier all in a row, and all tied together via
these ground systems. Each boat in this chain creates a galvanic
"cell," just as the plates in a battery does. The average inboard
boat develops about ? volt or less via its submerged metals, so that
ten boats connected together has the potential to develop 5 volts
since cells in series increase overall voltage in a line by the
value of each cell.
obsessed with finding a path to ground and will always do so. It
will follow the path of least resistance. If the 5th boat in the row
were an aluminium stern drive boat, as shown in the illustration
below, what do you think would happen? Yes, all those boats with
bronze and stainless parts on the bottom, which are far more noble
than aluminium, are going to set up a nifty little battery with the
stern drive or aluminium boat as the sacrificial anode! The inboard
boats will do just fine, but the stern drive or aluminium boat will
see its drives or hull damaged or destroyed.
Much the same thing
can happen to boats that underwater metals of inferior alloys such
as low grade bronze or stainless steel such as the kind that often
comes from the orient. Boats with cheap "active" alloyed metals are
going to be anodic to those with high quality passive alloys. Alloys
are often termed as active or passive based on the amount of
alloying metal that raises or reduces its electrical potential. Only
passive metals should be used on boats. Aquamet 22 stainless shafts
are passive, whereas other alloys may not be, or less passive.
What we have then is
a veritable "boat battery" where each boat on the circuit acts like
a battery plate. So it is that this has nothing to do with the
dock's wiring, though the marina often gets blamed for these
Long and Short of it
is only the beginning of the electrical problems experienced in
marinas. From here, it gets quite a bit more complicated. Let's use
our aluminium stern drive boat again as an example. Let's say this
little beauty is really hot. Or maybe it's an aluminium hulled boat.
Again, it's connected to all the other boats via the grounding wire.
So the current that it is generating is also being fed back into the
docks grounding system, and affecting all the other boats in the
chain. Would that be the boats upstream or down stream on the wire
circuit? See what I mean? It gets complicated.
The size of the
immersed metal, and the distance upstream or downstream on the
grounding circuit has a lot to do with it. A larger area of metal
will dissipate current to the water with far less corrosion damage
than a smaller area of metal. This is because of resistance in the
metal that disperses the energy over a wider area. Yet the very same
thing happens over a distance of wire. The longer the wire, the more
resistance in it. The boat out on the end of the pier is at the end
of a very long wire, so it has a less effective the ground because
there is more resistance in it.
This can translate
to good news or bad news, depending on the electrical potential of
your underwater metals. Keep in mind that sea water is also a
grounding source. It's bad news if the resistance on the dock is
high enough to cause the sea water to become the ground path. This
will happen if the resistance in the dock ground is greater than
that of the water, and your metals are incompatible. The increased
electrical potential in the circuit has now caused them to become
highly anodic. Oddly enough, this may actually help our stern drive
boat as the increased resistance in the line makes it less anodic,
but probably not that much. Because the amount of surface area of
metals plays a role, it will likely be small boats mixed with large
boats that will experience the greater degree of corrosion.
The grounding wire
is supposed to be at earth potential. If it's being charged, then
it's not at ground potential. It has become a current-carrying
conductor, meaning that all the boats on the dock are experiencing
more impedance in the ground circuit. This is going to hurt the
boats upstream, but help the boats downstream. Here's why: the
resistance in the line reduces the current flow to earth, thus
causing underwater metals to become anodic. This is because our dock
wire is not the only ground source; a boat in sea water is also
grounded via the water. When the resistance in the wire exceeds that
of the water, the water then becomes the ground path. The longer the
wire, the greater the resistance, so the end boats are most
affected. When the
water becomes the ground, the underwater metals become anodic.
This is when the
general corrosion problems start. It will first attack the weaker or
less noble metals such as zinc and aluminium until they disappear.
If you have a sea cock, perhaps a gate valve that came from a
plumbing supply store that is a poor alloy, or the wrong alloy
fasteners holding things on the bottom, then these items will go
next. By this time, your problem will have announced itself loud and
clear as serious leaks develop.
Water Versus Fresh
Sea water always
presents a more serious problem, right? Wrong. In most cases it
does, but there is one important exception. Fresh water is more
resistive by a factor of around 70:1 With the second ground
potential (fresh water) substantially lessened, the problem comes
about when the dock wiring is defective, when the ground and bonds
have high resistance due to corroded connections, etc.
The problem with
electrical shocking is heightened when things like metal framed
floating docks are involved, which provide yet a third grounding
source. It is the difference in potential between these two or three
grounding sources that causes our problems. If the potential within
the boat's metallic systems is greater than the grounded neutral,
then sea water will become the ground. If in fresh water, you may
end up getting shocked when touching metal parts on both boat and
dock. The current flow is going to be between your body and the
dock. Therefore, stray current can be more serious in fresh water
than salt because of poor grounds on the dock.
As you can see,
the issue of corrosion problems in marinas can be complex, and even
more difficult to solve from the marina end. You probably find all
this very confusing. Fortunately there are simple solutions.
Faults in the dock
wiring should be corrected, of course. Yet as we have seen, this may
be only part of the problem. The only reasonable solution for the
marina "boat battery" problem (if you have one) is to use galvanic
isolators on the shore power system of the vessel. Transformers
eliminate direct electrical contact by transferring electricity
magnetically. Some of the more high end marinas these days that
cater to large aluminium yachts have such transformers on the dock.
This is very expensive since there's also a great deal of power loss
with transformers, so you won't find many -- if any -- marinas
catering to smaller boats like this. It's up to you, the boat owner,
to protect yourself.
For small boats with
aluminium drives, galvanic isolators are available that work only
with the grounding circuits, and not the main power feeds. These are
okay for boats that are only running something like a battery
charger on AC current. For larger boats with higher power demands,
it is necessary to have a full sized isolation transformer.
In case you've heard
of that crazy solution of disconnecting your green, grounding wire,
consider that quite a few people have been electrocuted as a result
of this half-baked idea. The green is there to protect people
against electric shock and electrocution, so don't defeat its
If the dock system
takes a beating, how about your own equipment, like the connectors
on your shore cord and boat? When was the last time you opened them
up and inspected the condition of these things. It is very common
for shore connections to get wet and corroded, both on the male and
female connectors, and at the wire connections to the prongs within
the connector head. Corrosion and damage at these points not only
interferes with grounding potential, but also is one of the largest
causes of fires on boats. High resistance connections in the power
feed causes overheating that can start fires. This equipment should
be serviced at least annually, depending on the amount of use it
Corrosion on the
grounding connectors cause faulty grounds on your boat which is very
damaging to electrical equipment. Do you know why people so often
replace refrigerators and air conditioning compressors on boats?
Yep. Faulty grounds (the neutral) wreck compressors. Refrigerators
at home last forever, but not on boats for this reason.
This common problem
is one that should be thoroughly understood by all boaters. Reversed
polarity can exist on the dock or within your own boat, which should
be equipped with a reverse polarity indicator light on you main
electric panel. Regardless of whether the reversing point is on the
dock or in you own boat, this is going to energize the neutral
ground and create an electrical shock hazard. It will not find its
way into your bonding system because these circuits should never be
joined. Most experienced boaters keep a small polarity tester handy
and use it occasionally just to be sure. It's a wise thing to do,
especially if you ever run power tools off a dock outlet.
chargers are a common cause of corrosion in boats, particularly
small boats without shore systems. Auto chargers often provide no
isolation between the ac and dc windings and can energize the
negative terminal, which also energizes the boats grounding system.
Portable auto chargers should not be used on boats, and are a
frequent cause of stern drive damage.
should not be used on board boats. Period. Unless it is equipped
with a ground fault current interrupter, or plugged into an outlet
so equipped, and then only for temporary use of power tools. Why
not? Because of the acute electrical shock hazard that accompanies
the basically unprotected connector. Get some water in the
connection and these things short across the terminals to energize
all circuits, including the protective (green) bond.
and more marinas have become aware of the dangers of faulty dock
wiring, and there are far fewer with bad systems these days. As you
may have noticed, marina electrical facilities take a beating and
quickly degrade. If the power outlets just look bad, they probably
are. If outlets are damaged and exposed to weather, if you find
circuit breakers broken or missing, damaged or wasted conduits (Look
on the underside of the dock). Are there wires and conduits hanging
down, maybe in the water?) If so, then take a pass and don't connect
If you're skilled in
the use of a voltmeter, one thing you can do is to measure the
current flow between your boats grounding system and the dock
grounding system. Which way is the current flowing, and how much is
it? Your boat is safe if the reading is on the positive side with
red lead to the boat, black to dock. If it's reversed, and you're
getting more that ? volt, you shouldn't stay connected to if for
very long. ? volt is common, but if it reaches up towards 1 volt,
there will be an overnight affect on your zincs. In a week, they can
If you have an
aluminium boat, or a boat with stern drives or outboards that is
connected to shore power in a marina, consider it a must to have an
isolation transformer installed.
These tend to be the
worst offenders, where inexperienced owners will do some of the
craziest things. It's not uncommon to see boats connected with
extension cords, or unprotected three-prong male connectors and
adapters which, when they get wet short across the terminals.
Lacking installed battery chargers, they use portable automotive
chargers, connected to extension cords. I've even seen people take
extension cords and splice a male connector on the female end and
plug it into a receptacle on the boat! Be extra wary of electrical
faults at these marinas.
Answers to Common Questions
How about a stern
drive boat with a shore power system at a private dock? If the
wiring system is good, then you shouldn't have a problem with it.
When in doubt, have the impedance of the ground checked to be sure
that the boat isn't the ground source rather than the dock. This is
simple and inexpensive to do.
If the zincs on your
boat are disappearing very rapidly, at a rate which leaves shiny,
instead of oxidized metal, you should suspect both the dock wiring
and other boats on your dock. The question becomes who is the
culprit and what can be done about it. Rates of zinc loss are normal
when a layer of oxide develops on the zinc, not when pits appear
that leave bright or hard metal with no oxide. Rates of zinc loss
are not predictable in terms of time as there is a wide range of
Does current travel
through the water from boat to boat? No, it doesn't. Only via the
complexity of the issue, boats with galvanically compatible
underwater metals usually will not be affected (unless the problem
is extreme), while the ones with lesser compatible metals, the ones
that become anodic, suffer the consequences. Oriental boats with a
lot of submerged and questionable stainless steel and poor quality
bronze propellers are often victims as these metals are active
rather than passive, and often contain impurities in the metal that
further adds to the corrosion problem.
Can a boat with a
stray current problem affect those nearby? Yes, but. Remember that
all our boats are wired together. Assuming that the ground
connections to each boat are solid, but the dock ground isn't, then
the grounding wire is going to be energized. High resistance in the
bond will feed current back into all boats with good connections,
those without isolation transformers. Again, because distance and
surface area of metals dissipates current, those closest are most
likely to be affected.
What about marinas
with shorts in the system? Won't they cause corrosion on my boat? In
the case of marinas with short circuits in the wiring, usually
someone is going to get badly shocked, in addition to which there
will be a power drop that will affect everyone. This sort of thing
usually gets discovered rather quickly as electric equipment doesn't
function properly and the circuit breakers start popping.
What happens when
there is a fault in the neutral ground? Your lights are dim and
electrical appliances run slower, and eventually burn up. You will
get lower readings when you measure the voltage.
Will my panel meters
reflect the problem in any way? Usually not, although if there is a
ground fault you may get lower voltage readings.
What is a
cathodic protection system, and what about them? These systems
protect a boat's metal hull by using an electrified anode. The
problem with these things is that when something goes wrong with the
system, they can end up destroying the hull rather than protecting
it. This can also affect other boats nearby. It's not a reasonable
solution and is not recommended.
In summary, the "hot
marina" is really either one that has faults in the grounding
system, or one or two boats in an electrical series that are
galvanically deficient. If it's your boat that is suffering the
damage, most likely it's your boat that is the offender. This is why
usually only a few boats on the pier are affected, and not all
This describes but a
few, but most significant factors involved. There can be all sorts
of unusual conditions that can make discovery of the problem very
difficult. On the other hand, if you install a galvanic isolator, at
least the corrosion problems won't be yours. Whether you need
isolators depends on the conditions where you keep your boat.