Many of us have never run our boats in salt
water, and there are many myths surrounding salt water and its corrosive
effects on outboards. It is important that we understand that using an
outboard in saltwater will not shorten its life anymore than using a
similar rig in fresh water as long as
the necessary care is taken to maintain the engine.
Yes, it’s true that salt
water carries with it increased rates of galvanic corrosion over fresh
water, however, galvanic corrosion itself is not just limited to the
saltwater environment. Galvanic corrosion can occur outside on a humid
day! Before we go further with this discussion, let us first understand
what galvanic corrosion is.
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Left: Mild corrosion forming on
the transom bracket assembly.
Middle: Severe corrosion has literally dissolved the aluminum
on the this midsection.
Right: Common corrosion points are seams and sharp edges, where
protection is always weakest. |
Galvanic corrosion is a
process that occurs when certain metals are connected to or grounded through
water. When this connection is present, electrons flow between the two
metals, which causes the most chemically active metal to change. This
change is known as galvanic corrosion.
For corrosion to occur, four
elements must be in place. These are:
-
Two
dissimilar metals.
-
A
connection between the two metals.
-
An
electrolyte, such as water.
-
A
potential difference between the two metals.
The more active the metal,
the more corrosion will take place when the above mentioned situation is
present. All metals range from very active, such as Magnesium and Zinc, to
the least active, such as Gold and Stainless Steel. Drive units and
engine parts could be made of stainless steel, however, the cost would be
enormous, and the weight would be unacceptable. Aluminum alloys seem to be
the best materials for this purpose, offering light weight, good strength,
and reasonable cost.
When galvanic corrosion occurs, it
is usually identified by characteristic paint blistering and white corrosion
around sharp metal edges. If left to its own, the corrosion process will
continue, and eventually pitting of the metal will occur. It is
important to identify corrosion at the beginning stages.
What can be done to prevent
corrosion?
The first line of defense
against corrosion is the protective paint covering your engine and
components. It is important that the integrity of this finish is
maintained, as it protects the aluminum from exposure to corrosion. Always
repair paint scratches and chips promptly and NEVER use a wire brush to sand
an engine surface. Doing so will embed small steel particles from the
brush into the aluminum, creating destructive “galvanic cavities” for
corrosion to take place.
Always make sure your motor
or drive unit is equipped with sacrificial zinc anodes. These inexpensive
and highly active plates are positioned in strategic places around the
engine and drive unit to prevent corrosion. Since the zinc plates are much
more active than the aluminum and stainless steel, they will corrode,
protecting the drive system. It is important to note that these anodes will
corrode quickly, and they must be replaced when 50% of the anode is
remaining. Also, the mounting surface for the anode must be free of paint
or any other contaminants to ensure a good electrical connection between the
zinc plate and the unit to be protected. Never paint or cover a zinc anode,
doing so will reduce or eliminate its protective potential.
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Left:
Detail of saltwater galvanic corrosion.
Right: Sacrificial zinc anode installed on Mercury Sportmaster
lower unit. |
Other corrosion methods,
such as continuity devices, are utilized to prevent corrosion. These
devices may be grounding straps and washers to maintain continuity
throughout the drive system, reducing corrosion. If your motor is equipped
with these devices, always make sure they are maintained and replaced when
broken. In many cases, continuity devices can be installed on drive
systems that were not so equipped. Such kits are generally available
from dealers and other marine supply outlets.
Finally, after every use in
salt water it is very important to thoroughly flush your engine by running
it on a garden hose with the appropriate lower unit adapter. Rinse the
exterior of your engine and surrounding areas with water. Pay
extra attention to corrosive parts, such as engine mounts and any exposed
non-stainless steel metals. It's a good idea to thoroughly flush and
lubricate the exposed surfaces of the upper and lower engine mounts, as
these areas are always hotspots for corrosion and accelerated rusting. A
complete washing using a mild detergent would be best, however this is not
always convenient, especially at public launch ramps. The key is to
remove as much salt water from the engine's cooling system and exterior
components as quickly as possible.
After the engine cools
down, a light spray of water displacing lubricant such as
CRC 6-5-6 or
WD-40
will help to protect metal surfaces and electrical connections from
corrosion. Mercury Marine's
Corrosion Guard is
a coating that can be sprayed on engines to form a light, waxy-like coating
that is waterproof. Although some people feel that these coatings are
messy, the protection they provide in areas of severe salt corrosion is well
worth it.
Some products, such as
Salt-Away and
Salt-X offer flush kits that claim to chemically remove salt
much more effectively that just water alone. Although these products
have been used extensively by owners of personal watercraft for years, OEMs
have not endorsed their use. We'll be testing Salt-Away this summer to
see first hand if this product lives up to its name and reputation.
The conclusion here is
simple: although galvanic corrosion is more of a problem in saltwater versus
freshwater environments, it can be prevented or greatly minimized when the
required extra preventive steps are taken.
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