In the late 18th century, Luigi Galvani (where the word “galvanic” comes from) discovered the prin- ciples of battery operation. Later, in 1800, Alessandro Volta (as in 12-volt) made the first battery, out
of alternating plates of silver and zinc
separated by paper soaked in saltwater,
and created a useful electrical current.
What does all this battery stuff have to
do with anodes and protecting metal on
your boat? Plenty. Galvanic corrosion
– the type that can destroy underwater
metal in your boat – is simply a giant battery cell that has been set up somewhere
on your boat’s hull.
When is a boat a battery?
As Volta discovered, a battery is composed of two dissimilar metals that are
electrically connected to each other and
surrounded by an electrolyte (saltwater
or acid, for example). In the case of a
boat moored in seawater, a stainless-steel
prop connected to an aluminum outdrive
floats in the world’s biggest electrolyte
All about anodes
There are several ways to stop your underwater hardware from becoming
the loser in your boat’s attempt to make electricity
BY CHARLES FORT
and – presto! You have a giant battery.
The energy created is not sufficient to
do anything useful. The more significant
problem, however, is that in this giant
battery cell, one of those dissimilar metals, for instance the aluminum outdrive
(the battery’s anode), will have a negative charge in relation to the stainless-steel prop (the battery’s cathode). Like
a household battery, the anodic metal
will be used up (read: destroyed) in the
process of generating electricity; this
destruction is called galvanic corrosion.
In a disposable battery, this is OK. On
a boat, the used-up metal is likely to be
a very expensive bronze propeller or an
Overcoming galvanic corrosion
How do you avoid galvanic corrosion?
First, avoid using dissimilar metals.
Unfortunately, every metal has unique
properties, and manufacturers take
advantage of them to create durable and
affordable boats, so mixing metals underwater is a fact of boat life.
Another way to avoid it is to not float
your boat in an electrolyte. Freshwater,
if not polluted, causes much less galvanic corrosion, though some minerals in
the water can create a weak electrolyte.
Unless you keep your boat in distilled
Impressed-current systems use an
in-water sensor and rely on a small
current from the boat’s battery to force
a counter-current into the water near
the less-noble metals, usually an outdrive housing. Unfortunately, because
of its complexity and reliance on a fully
charged battery, the effectiveness of
such a system can be assured only
through careful engineering, installation,
Without the proper anodes, your boat’s underwater metals are at risk of severe damage
GALVANIC CORROSION IS
SIMPLY A GIANT BATTERY
CELL THAT HAS BEEN SET
UP SOMEWHERE ON YOUR