Aluminum
corrosion
Boat
Corrosion 101
The singularly
most misunderstood, but frequently discussed, problem in boating
is corrosion. Corrosion of metal components below the waterline
will be a problem all boaters will eventually encounter and
inevitably discuss with their boating neighbors. By far, the
best source of information is a specialist such as a marine
electrician, electrical engineer or metallurgist. Unfortunately
most of them speak an incomprehensible language. Furthermore,
the few specialists who are able to speak in simple, understandable
terms don’t stop in time. They continue their explanations
about ions, electrons and current carrying conductors until
the basic explanation gets lost in the mental overload that
follows. For those who wish to understand this subject in basic
terms, you are in luck. The following explanation is written
by an electrical simpleton and remains basic from start to finish.
Let’s
begin with some very basic concepts. Alternating current (AC)
is the type of electricity from the shore power cord, generator
or alternator. Direct current (DC) is the type of electricity
from the batteries. Circuit breakers are designed to prevent
the overheating of wires and fires. Ground fault circuit interrupter
devices, such as G.F.C.I. protected AC electrical outlets are
designed to protect people. It is more important to protect
people and prevent fires than it is to prevent corrosion. Bonding
is connecting metal components with wires, resulting in electrical
continuity. This is done with most below waterline metal components,
including through hulls, struts, propeller shafts and rudders.
Testing
for voltage and current can get complicated fast. The only testing
we will discuss is electrical continuity. On virtually all electrical
meters there is a continuity test, the symbol is an ohm sign
(horse shoe). To determine if one metal component has continuity
to another, set the tester on this setting, touch one metal
with one of the tester’s leads and touch the other metal
component with the other lead, continuity is indicated by a
beep or a number. A number near one is good continuity. This
test will allow you to determine if your rudder is continuous
with a sacrificial anode or if a through hull is included in
the bonding system.
It is unlikely
that the cause of your corrosion problem is the “stupid
wiring” at the marina. The wiring on your dock is almost
certainly to an electrical code. The wiring on your boat is
almost certainly not to code, as the only legal requirements
apply to commercial, not recreational vessels. There is no “code”
or Coast Guard requirements for the electrical system on your
boat. While there are standards and recommended practices, compliance
with them is voluntary. Regardless, metal corrosion in boats
is a result of the laws of nature or a failure aboard the boat
and rarely a result of a problem with the wiring on the dock.
The normally
green AC ground wire used by the marina does provide a path
for an electrical current that can contribute to corrosion.
It also provides a path to ground in the event of an electrical
problem and is designed to save lives. Don’t cut it! There
are two common choices that can be used to allow the safety
aspects of the AC ground wire and reduce or eliminate its contribution
to corrosion, galvanic isolators and isolation transformers.
Galvanic isolators are relatively inexpensive devices that prevent
most problems associated with the AC ground wire. A more sophisticated,
expensive and reliable method to prevent AC ground wire related
corrosion is an isolation transformer. Without falling into
the abyss of electrical theory, both devices reduce the potential
for corrosion on your vessel. By using these devices and leaving
the AC ground wire connected as designed, the boater can reduce
or prevent corrosion while allowing the safety components to
function as designed.
There are two primary types of corrosion experienced by boaters:
galvanic and stray current. Galvanic corrosion is a natural
process which occurs when dissimilar metals have electrical
continuity and are immersed in the same electrolyte. In other
words, different metals are either in contact with each other
or connected by a wire and they are in the same body of water.
This is by far the most common cause of corrosion. Stray current
corrosion is caused by an electrical current leak and in the
boater’s world is almost exclusively direct current (DC).
This simply means that the primary source of the power driving
this type of corrosion is your batteries not your shore power
cord.
Another
type of corrosion experienced mostly by high performance vessels
is called cavitation corrosion or impingement. This primarily
is seen on high speed propellers or rudders and is not addressed
in this article. Anaerobic corrosion or oxygen starvation damage
effects stainless steel where it is wet and starved of oxygen.
Sailboat rigging and propeller shafts are common victims of
this type of corrosion; this is also not addressed in this article.
Both of these types of corrosion are unusual.
Galvanic
corrosion is the common cause of corrosion on boats. The prerequisites,
electrical continuity between the dissimilar metals and common
water are the only things we need to understand to fight the
good fight against it. If we bond all below waterline components,
the least noble metal will corrode faster. While the theory
is most easily understood if we use two separate pieces of metal
in our examples, a single metal component can galvanically corrode.
A bronze propeller is an alloy of several metallic elements,
including copper and zinc. In this example, if left unprotected,
the zinc will corrode out of the alloy, leaving a higher concentration
of copper. This results in a pink color, versus bronze or gold.
It also results in a much softer and weaker metal. Galvanic
corrosion can also occur where two dissimilar alloys are in
contact above the waterline, such as a steel fastener in an
aluminum toe rail. In this case the aluminum will corrode. The
least noble metal will lose this fight every time. This is a
fundamental law of nature.
To protect
against galvanic corrosion we use paint, sacrificial anodes
(usually zinc), galvanic isolators or isolation transformers.
The most effective form of protection is paint. If a piece of
metal is completely isolated from the ocean by paint, it will
not corrode. This is the initial method for protecting outdrives
and outboard engines. The paint has to be designed for this
purpose; anti-fouling paint is not designed to prevent corrosion.
Metal boats are painted to reduce corrosion.
For a sacrificial
zinc anode to protect a metal component from corrosion, it needs
to have electrical conductivity and immersion in the same body
of water. In other words your zinc anode must be touching or
connected by a wire to the metal you are trying to protect and
they both have to be on the bottom of your boat or in the same
component in your engine. Simply speaking, connect all submerged
metal to the zinc anode. The anode on your transom is not protecting
your rudder unless your meter beeps when you touch both components.
The sacrificial anode in the engine’s heat exchanger (a
big round tube on the back of your engine) does not prevent
corrosion in the exhaust manifolds. And, by the way, you should
replace that anode in your heat exchanger.
The plate
zinc on your transom rarely protects the propeller shaft and
the propeller. It is difficult to maintain continuity with a
propeller shaft that spins several thousand revolutions per
minute. The most practical means to protect the propeller shaft
and propeller from galvanic corrosion is a collar zinc installed
directly on the shaft. You do not need to put a zinc anode on
every piece of submerged metal. However, if you wish to provide
galvanic corrosion protection, the metal does require continuity
with a zinc anode located somewhere on the bottom of the boat.
By their
nature, outdrives have corrosion challenges. Aluminum is low
on the galvanic chart thus subject to corrosion more easily
than bronze or stainless steel. Zinc anodes can reduce corrosion
on an outdrive but the rubber bushings isolate the various components
including the hydraulic rams, steering components and transom
assemblies. Electrical continuity must be maintained between
each component part and a sacrificial zinc anode in order for
all parts to have protection. Small cables used for this purpose
often become disconnected and the zinc anodes are more difficult
to see and maintain than transom plates or propeller shaft collar
zincs.
Trim tabs
corrode. Deal with it.
Stray current
corrosion is a much more rapid and damaging condition. The good
news, it is unusual. The bad news, it is much more destructive
and more difficult to diagnose. Extreme cases of stray current
corrosion can result in a good propeller taking the form of
Swiss Cheese in a matter of days. A through hull flange can
disappear and solid steel components such as propeller shafts
and rudder shafts can be severed. As previously mentioned, the
root cause of this problem is a DC electrical stray current
leak. An electrical short circuit has occurred and the current
is being discharged into the water. One piece of metal will
inevitably be coated with a white compound and another piece
of metal will suffer metal loss. What the boater will experience
is a sudden change in the condition of a submerged metal component,
often found by a diver. The root cause will often end up being
a failed electric motor, bilge pump or battery charger.
Sacrificial
zinc anodes will not protect against stray current corrosion.
Superman is defenseless against Kryptonite and metal is defenseless
against a stray current. If a stray current problem is suspected,
call a professional. The source must be eliminated. Take immediate
action, boats have sunk after the through hulls were destroyed
by stray current corrosion in a few days.
We do not
believe that all metal components necessarily require bonding
and protection with a sacrificial zinc anode. While there are
standards which recommend this practice, we have seen many quality
alloys last for several decades, virtually undamaged. We do
believe that a bonding system should be complete if attempted,
a complete bonding system or no bonding.
Fiberglass
boats can not be “over zinced”, or install so much
zinc that damage is caused. Wood and metal boats can be “over
zinced”. This issue is particularly important to wooden
boat owners. The amount of zinc installed on wooden and metal
vessel is important and should be measured and maintained actively.
There are
devices available to measure the amount of protection provided
by zinc anodes. They are seldom understood or used as designed.
If your boat has one, either learn how it works and use it actively
or abandon it and use the simple system of bonding everything
to a zinc.
There are
more sophisticated corrosion prevention systems which are generally
used in commercial applications. While zinc anodes are “passive”
some outdrives and many steel ships and structures use “active”
systems. Some of these systems impress a current into the water
to prevent corrosion. These systems are beyond the scope of
this article.
Corrosion
of that toe rail we discussed earlier (and other above waterline
metal) can be reduced by washing away the salts left after a
day sailing. We won’t broach the subject of tank failures,
just keep them dry.
Remember
to protect metal with an anode; it needs electrical conductivity
and a shared body of water. Sudden corrosion or significant
changes in the metal components require immediate attention
by a qualified expert, and follow the advice of that crusty
boating neighbor at your peril.
Posted
2/1/2012
