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Anodes by CMP
Preventing Galvanic Corrosion
What is galvanic corrosion?
Galvanic corrosion (sometimes erroneously called
electrolysis) can damage or destroy underwater
metal parts of boats, dock hardware and other
equipment. When two different metals are touch-
ing each other or are electrically connected by
a conductor, and are immersed in an electrolyte
(an electrically conductive fluid, like saltwater), an
electro-chemical reaction can occur. One of the
metals (the "least noble" metal, called the anode)
will corrode faster than it normally would, and the
other (the "most noble" metal, or the cathode) will
dissolve more slowly.
The seawater Galvanic Sequence lists metals in
order, based on their voltage potential and ten-
dency to corrode. More active metals, the faster-
dissolving anodes, are at the top of the series.
Passive metals, the cathodes, are located at
the bottom. Your boat might have a collection of
submerged, electrically connected metal parts
immersed in saltwater surrounding the boat,
in bilgewater or the water in the engine cooling
system. The stainless prop shaft, for example,
because it is more active (less noble) than the
bronze propeller, will begin to dissolve, leaving
the prop intact.
How to prevent it
Breaking the electrical circuit between exposed metals by connecting them
to a sacrificial anode will prevent galvanic corrosion. A sacrificial anode is an
inexpensive piece of metal that is less noble than any other metal found on
the boat, and it is electrically connected, bolted or wired into electrical contact
with the other metal components. Bolting a sacrificial anode made from zinc,
aluminum or magnesium (located near the top of the Galvanic Sequence)
to the stainless prop shaft will protect it from corrosion. The anode and the
stainless shaft will form a galvanic couple, and the anode will dissolve, keep-
ing your prop shaft spinning happily. Sacrificial anodes can extend the life
of your boat's hull, engine, rudder, propeller shaft, engine cooling system,
refrigeration condenser and other metal components by protecting them from
the deterioration caused by galvanic corrosion.
When to replace an anode
The effectiveness of an anode depends on a good electrical connection, and
is directly proportional to its surface area. As it corrodes away its surface
area and effectiveness diminishes. Some manufacturers suggest replacing
anodes when they are two thirds gone, but we recommend replacing them
when half-eroded or half-dissolved. If an anode is allowed to dissolve com-
pletely, the next least noble piece of metal in the circuit will start to dissolve.
And that might be a part of your engine. Remember that polluted water, warm
water temperatures and stray current corrosion can cause your anodes to
wear away at an accelerated rate, and it's smart to check them regularly.
The importance of marine-grade fasteners
Galvanic corrosion can also happen in a single piece of hardware, particularly
alloys, which contain more than one metal. Interactions between the differing
metals in the alloy will dissolve the least noble, the most common example of
this being the dissolution of zinc from many zinc/copper alloys (such as brass
and some bronzes), leaving a weak, spongy copper residue. That's why it's
so important to use marine-grade fasteners (bronze, monel and 316 stainless
steel) in underwater applications so they are not destroyed, allowing a hose
clamp to fail or your propeller to fall off. The higher the salinity and temperature
of the water, the greater the likelihood of corrosion.
Galvanic isolators protect from dockside power problems
Attaching anodes protects the metal parts of the boat from onboard sources
of electrical current. Unfortunately, other boats and improper shoreside wiring
are sources of galvanic current originating outside the boat. If your boat is
hooked to shore power, a galvanic isolator or isolation transformer will protect
it from rapid dissolution of your anodes and the corrosion that will follow.
Getting rid of cadmium and zinc
Zinc and cadmium are elements used in traditional saltwater anodes that are
suspected of causing environmental damage. CMP Global, our primary sup-
plier, has re-formulated their product, removing these alloys for safer and more
"green" anodes. The alloy components of the new Martyr II (Cadmium Free)
anode do not include cadmium (one of six substances banned by the Euro-
pean Union's Restriction on Hazardous Substances directive) and contain a
fraction of the zinc used in making the traditional zinc anode (concentrations
of zinc as low as two parts-per-million adversely affects the amount of oxygen
that fish can carry in their blood). The two Martyr II components, aluminum
and indium, are considered non-toxic. A rare bit of good news for the boater:
these new formulations actually work better! They're lighter and last longer, so
they're a plus for your boat and for the underwater life in your marina.
Which anode is right for you?
Zinc or aluminum for saltwater:
If you are a saltwater boater, you should
install zinc or aluminum anodes to prevent galvanic corrosion on the engine
and underwater parts of your boat.
Magnesium for freshwater ONLY:
Since freshwater is much less conductive
than saltwater, magnesium anodes are the best choice because they're more
active (less noble) than zinc or aluminum, so they will protect your engine
parts more effectively. Caution: do not use magnesium anodes in any appli-
cation other than freshwater because they will corrode rapidly, exposing your
boat and engine to possible damage.
Aluminum for brackish water:
More active than zinc, aluminum anodes are
a good compromise where fresh- and saltwater mix, such as the brackish
water of river deltas, or if you use your boat in a variety of water types. Zinc is
too passive for brackish water, where it gets covered in a zinc oxide film and
becomes inactive. Magnesium corrodes too rapidly (due to the water's salt
content), leaving your boat without adequate protection.
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