1 Is the boat’s cathodic protection system
(assuming it has one) doing its job?
2 What is the overall condition of the boat’s
bonding system?
First, let’s review the difference between a
cathodic protection system and a bonding
system. Simply put, a cathodic protection system
protects a boat against galvanic corrosion
through the use of zinc anodes. A bonding system
protects the boat against stray-current corrosion
by electrically tying together all the metals on
the boat and then connecting the bonding wires
to the boat’s common ground point.
Cathodic Protection and
Galvanic Corrosion
When two dissimilar metals are immersed in
an electrolyte, their differing electrical poten-
tials will produce a voltage difference. Any-
time a connection is made between these two
metals, an electrical current will flow from the
higher-voltage metal (the cathode) to the
lower-voltage metal (the anode), leading to
the deterioration of the anode. Since the cath-
ode in a galvanic cell does not dissolve, it is
also referred to as the most noble metal, while
the anode is referred to as the least noble.
Note that this relationship is relative; depend-
ing on its position in the galvanic series table
(see page 106), a metal will be cathodic with
respect to metals that have a lower electrical
potential but anodic with respect to those
with a higher potential.
Applying this to boats and themarine envi-
ronment, it’s clear that since metal fittings and
components on boats are made of dissimilar
metals and are usually underwater, you have
the makings of a galvanic cell on your boat.
Add an electrical connection, and galvanic
corrosion will result.
Cathodic protection is the answer to basic
galvanic corrosion. By installing zinc anodes
a basic corrosion survey 105on your boat, you provide an anode that will
be consumed (sacrificed) to protect the boat’s
metal fittings and components.
The scenario becomes a bit more compli-
cated when you hook up to dockside power.
Now the external circuit becomes the green
grounding wire, which basically connects
every boat to one another. As a result, you
could find your boat becoming the cathodic
protection for your neighbor’s boat, and all of
your zinc anodes will start corroding very
quickly. Once they are gone, the next least
noble metal on your boat will follow.
The solution to this problem is a galvanic
isolator. Basically, this device interrupts the
circuit between your boat and the other
boats tied into the dockside power. It has a
pair of diodes that block low DC voltage, up
to 1.5 V, but allow higher AC voltage to
pass, which is an important feature because
as mentioned in Chapter 10, even low-level
AC voltage in the water is a danger to swim-
mers. Thus while providing a solution to gal-
vanic corrosion, the galvanic isolator also
provides protection from dangerous AC
voltage.Bonding and Stray-Current Corrosion
Stray currents can come from faulty electrical
circuits or from any situation that provides
electrical current with a path that has a lower
resistance to ground than the appropriate one
back to the battery. For example, if you have a
connection sitting in a puddle of water, the
water provides a path of lower resistance than
the wire going back to the battery. As this cur-
rent follows its path of least resistance through
metal into the water, it will corrode the metal.
It doesn’t matter if the metal is more anodic
or more cathodic (zinc anodes won’t work
here)—the current will corrode the metal, and
the direction of the current flow determines
which metal will corrode.