Metal parts underwater are
subjected to two basic types of corrosion: galvanic corrosion and stray
current corrosion. Both can harm your boat, propeller, and motor if not
correctly monitored and avoided.
What is Corrosion?
There is nothing mysterious about corrosion. The process metal goes
through in changing is slightly complicated, but not especially complex.
To best describe corrosion, let’s start with the most common type, rust.
We all know rust, but to understand rust, we have to go back to the very
beginning. Iron ore has a chemical composition of two iron atoms bonded
with three oxygen atoms. As it is mined out of the ground, it’s a
brownish-red powder useless to us. But by refining, purifying, and
smelting, we create iron, which is useful. We can use it as plain iron, or
we can process it further and combine it with other elements to get
different types of steel.
Let’s say the iron is made into hinges for your backyard fence. Everyone
knows that if you leave iron out in the rain, it rusts. If it rusts long
and badly enough, the metal disappears and you’re left with a pile of
brownish-red powder-rust or iron oxide, which has the same composition as
Here’s why. Iron atoms want to return to their normal state as iron ore,
iron oxide, or rust. Which are all the same things. That’s the state in
which iron is most comfortable and most stable. Left alone, it won’t turn
into anything else. And most metals used in manufactured products want to
do the same—return to their natural state.
In looking at
this chart, you can also see that gold is the
least active metal of all metals, however magnesium is the
most active, therefore the most likely to protect your boat
Iron left out in the rain results in a specific kind of corrosion. It’s
called an electrochemical reaction, meaning there is an electrical change.
Here’s how that works:
For two iron atoms to really interlock with three oxygen atoms and make
iron, they have to share some electrons, which releases a few electrons.
Since electricity is just a flow of electrons, those free electrons become
a little bit of electricity when the chemical change takes place.
Remember the iron wants to corrode into iron oxide because that is its
natural, most stable state. And all it needs for this to take place is
oxygen. Water is a supply of oxygen, so iron rusts fastest when it gets
wet. You knew that already but now you know why. And that same scenario
applies to aluminum and aluminum oxide. Those are the deep, dark secrets
of corrosion as they apply to metals. Those are also the basics of an
electrochemical reaction, which is known as galvanic corrosion. All
galvanic corrosion is an electrical reaction. Not all electrochemical
reactions, however, are galvanic corrosion.
Galvanic corrosion is an electrochemical reaction between two or more
different metals. The metals must be different because one must be more
chemically active (or less stable) than the others for a reaction to take
place. When we talk about galvanic corrosion, we’re talking about
electrical exchange. All metals have electrical potential because all
atoms have electrons, which have an electrochemical charge.
Galvanic corrosion of the more chemically active metal can occur whenever
two or more dissimilar metals that are "grounded" (connected by actually
touching each other, or through a wire or metal part) are immersed in a
conductive solution (any liquid that can transfer electricity). Anything
but pure water is conductive. Saltwater, freshwater with high mineral
content, and polluted freshwater are very conductive, and conductivity
goes up with water temperature. That’s one reason why boats in Florida
experience more corrosion than boats in Maine.
The simplest example of galvanic corrosion, and the most applicable, is an
aluminum lower unit with a stainless steel propeller. The aluminum is the
more chemically active metal (the anode), and the stainless steel is the
less chemically active metal (the cathode). Several things happen at the
At the Anode
1. Electrons flow from the anode, the metal that is more chemically active
(the aluminum drive unit), via the external conducting path to the
cathode, the metal that is less chemically active (the stainless steel
2. When this happens, the more chemically active metal atoms become ions
(an atom with one or more electrons either missing or added) and break
away into the water, where they can bond to oxygen ions, with which they
can share electrons and produce aluminum oxide. This is the same process
iron ions go through when combining with oxygen ions in water to form iron
3. The newly formed aluminum oxide molecules either drift away in the
water or settle on the surface of the aluminum. Your lower unit is
literally dissolving through galvanic corrosion.
At the Cathode
1. Electrons are accepted from the anode; however, they cannot simply
accumulate, they react with ions in the electrolyte.
2. The resulting hydroxide ion is alkaline, and makes the electrolyte
alkaline in the area of the cathode. This detail is especially important
for wooden boats, as an alkaline solution will attack cellulose (i.e.
It's important to understand that for each positive metallic ion released
at the anode, electrons in the cathode react to form a negative ion in the
electrolyte. Electrically the anodic and cathodic reactions must be
equivalent. Increases or decreases in the rate of the cathodic reaction
will have a corresponding increase or decrease on the anodic reaction.
This is a basic fact in understanding and controlling corrosion. This fact
can also be
demonstrated by the effect of size ratios between anodes and cathodes. If
there is a very large anode connected to a small cathode, the anode will
corrode very slowly. However, if a very large cathode is connected to a
small anode, the anode will corrode very rapidly. Marine drive components
have many aluminum parts. If you do not control galvanic corrosion, over
time the aluminum will corrode away.
Galvanic corrosion can also occur without any stainless steel components
on your boat. For example, you have an aluminum drive unit and an aluminum
propeller, but you dock at a pier with steel pilings or a steel seawall,
then plug into shore power. The ground wire, which is grounded, connects
your aluminum components with the submerged steel because the steel is
also grounded. Considering the mass of a seawall or even a single piling,
your drive and propeller can sustain serious damage. This damage could be
prevented with a galvanic isolator.
What to Look For
The first sign of galvanic
corrosion is paint blistering (starting on sharp edges) below the water
line—a white powdery substance forms on the exposed metal areas. As the
corrosion continues, the exposed metal areas will become deeply pitted, as
the metal is actually eaten away.
Typical signs of corrosion on marine lower drive
units and propellers
include blistering paint and the formation of a white powdery
substance on the exposed metal areas
Galvanic corrosion of aluminum drive units—or any underwater aluminum on
your boat—is accelerated by attaching stainless steel components like
propellers, trim planes (if connected to engine ground), and aftermarket
steering aids. In doing this, you have introduced a dissimilar metal to
which electrons from your drive unit will follow. Another condition that
will increase the speed or intensity of galvanic corrosion is the removal
or reduction in surface area of sacrificial anodes. But you don’t need
stainless steel components for galvanic corrosion to take place. Galvanic
corrosion continually affects all underwater aluminum, but at a reduced
rate when no dissimilar metals are connected to your aluminum parts. When
in contact with an electrolyte, most metals form small anodes and cathodes
on their surfaces due to such things as alloy segregation, impurities, or
We have used stainless steel (cathode) and aluminum (anode) in this
discussion as an example, however other metals coupled with aluminum also
produce galvanic corrosion cells. For example, zinc connected to aluminum
will form a corrosion cell, but in this case, the aluminum becomes the
cathode and the zinc (anode) corrodes. One of the worst couples with an
aluminum drive would be connecting it with copper or a copper alloy
(bronze). Another cause of galvanic corrosion is the shore power hookup.
When you plug in, you tie your aluminum drive unit to other boats using
shore power through the green grounding lead. Your aluminum drive unit is
now part of a large galvanic cell (a battery) interconnected with onshore
metal that is in the water—as well as other boats—and corrosion may be
Galvanic corrosion caused by nearby grounded steel
can occur when you dock your boat and use shore power
Stray Current Corrosion
We’ve discussed what galvanic corrosion can do, using just the electrical
potential in metals. Imagine what happens if you add more electricity.
That’s exactly the basis for stray current corrosion.
Stray current corrosion occurs when metal with an electrical current
flowing into it is immersed in water that is grounded (such as in any
lake, river, or ocean). The current can leave the metal and flow through
the water to ground. This will cause rapid corrosion of the metal at the
point where the current leaves. Stray direct current (or battery current)
is particularly destructive. Stray current corrosion can cause rapid
deterioration of the metal. If the metal in question happens to be an
aluminum part like your drive unit, it can be destroyed in a matter of
Stray current corrosion is different from galvanic corrosion in that
galvanic corrosion is caused by connections between dissimilar metals of
your boat’s drive components, and utilizes the electrical potential of
those dissimilar metals. Electrons flow from one dissimilar metal (the
anode) to another dissimilar metal (the cathode). In stray current
corrosion, electricity from an outside source flows into your boat’s metal
components and out
through the water for a ground.
For example, your boat may be sitting between a boat leaking DC current
and the best ground for that current. Rather than the DC current moving
through the water to ground, your boat could provide a path of lower
resistance. The DC current could enter a through hull fitting, travel
through the bonding system, and leave via your drive to the ground.
Remember that corrosion occurs at the locations where DC current leaves
Stray current can come from an outside source either internal or external
to your boat. Internal sources involve a short in your boat’s wiring
system, such as a poorly insulated wire in the bilge, an electrical
accessory that may be improperly wired, or a wire with a weak or broken
insulation that is intermittently wet.
External sources are almost always related to shore power connections. A
boat with internal stray current problems can cause accelerated corrosion
to other boats plugged into the same shore power line if they provide
better ground. The stray current would be transmitted to other boats
through the common ground wire, but can and should be blocked by
installing a galvanic isolator.
A much more subtle, but potentially more damaging cause of stray current
corrosion can occur without any electrical problems. Supposed you cruise
back to your marina after a weekend on the water, and plug into shore
to recharge batteries using your automatic trickle charger. Then you go to
work for the week. On Monday, a large steel hulled boat (with scratched
and scraped paint) ties up next to your boat. This boat is also plugged
into shore power and goes visiting onshore for a few days. A battery has
just been formed—the large steel hull and your small aluminum drive
connected by the shore power and ground wire. Depending on the proximity,
relative sizes, and how long your neighbor is ashore, when you go out the
next weekend you may find your drive highly deteriorated. This unfortunate
scenario can also be prevented by the installation of a galvanic isolator.
There is greater danger for boats that connect to AC shore power:
destructive, low-voltage galvanic currents (DC) passing through the
shore power ground wire. Normally, AC is not a corrosion problem, but
because the boat, pier, and wire are all connected, or due to a leakage,
there can be direct current (DC) also present. This is potentially very
damaging and requires additional protection.
Safety regulations require a three-wire cable for carrying shore power
aboard any boat, and that one of these leads grounds all electrical and
propulsion equipment to shore. This safety procedure reduces the danger of
shock, but also connects the underwater metal components on your boat with
metal on neighboring boats using shore power, steel piers, and metal
objects on shore that extend into the water. This interconnecting of
metals allows destructive galvanic currents to flow between them. If these
currents are allowed to continue, your drive unit will experience severe
corrosion damage in a very short time—as little as a few days.
There is a common misconception that you can overprotect your drive by
using too many zinc or sacrificial aluminum anodes. This is not true. The
corrosion potential of any metal is a voltage that can be measured by a
reference electrode. Such measurements in water commonly are made with a
silver/silver chloride reference electrode. The corrosion potential of a
sacrificial anode is a characteristic value for that metal, and it does
matter if you have one piece of the metal or 100 pieces. The corrosion
potential stays the same. Of course, 100 anodes would be expensive, heavy,
and a considerable drag under water. Only by increasing the corrosion
potential by using a different anode material (such as magnesium in
seawater) can you overprotect your drive.
There is also a form of corrosion that affects many metals, particularly
stainless steel, called crevice corrosion. A crevice may be formed under
any of the following: deposits (such as silt or sand), plastic washers,
fibrous gaskets, or tightly wrapped fishing line. It can also form where
moisture can get in and not back out, forming a stagnant zone. Stainless
steel is an iron-based alloy containing chrome and nickel. The quality
it to be stainless (no rusting) is its formation of a thin, tightly
adhering surface layer of chrome oxide. If this surface is deprived of
oxygen, the oxide layer breaks down and the stainless steel will rust just
like plain steel. In other words, stainless steel is only stainless when
it has access to oxygen. In a crevice where there is moisture depleted of
oxygen, stainless steel rusts. The simplest prevention for this condition
is to seal out the moisture
or clean off any deposits.
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