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Controlling Galvanic Corrosion
By Dave Gerr, © Dave Gerr, 1991 & 2011
Last issue, we explored the corrosion process, and a method
to test a boat and its fittings to determine if they're properly
protected. In this issue, we'll take a detailed look at estimat-
ing the amount of protection a boat needs, and how it
should be installed.
How Much Protection?
In part one, we saw that the way
to protect metals from galvanic
corrosion is to flood them with
excess electrons. These electrons
come from zinc anodes. How
much zinc is required? How much
is too much? A good way to deter-
mine this is by testing on board.
When your
Dry Roller
's going to be
snugged up at its mooring for a
few days, take several zinc an-
odes—about as many as you think
should do the job—and connect
them temporarily with wires to its
hull or the fittings to be protected.
(If it’s a wood or FRP/fiberglass
craft, connect to the bonding sys-
tem. If your
Dry Roller
is metal,
connect to the hull itself.)
Dangle the zincs in the water.
Twelve to 24 hours later,
Dry
Roller
's hull and/or its fittings will
have fully polarized from these
anodes. Referring to the galvanic
series and the testing method de-
scribed last issue, you can go
around checking results with a
multimeter. If you need more or
less zinc it will be obvious from the
voltage readings. (Cover one side
of the anodes with tape or plastic
during the test. This side will, of
course, be against the hull after
the anodes are installed. If you forget to do this, the anodes
will have excess surface during testing.)
Estimating The Anodes Required
Most builders, yards and boat owners don’t approach the
anode quantity problem so systematically. Many simply slap
a few anodes on haphazardly—a shaft collar, a zinc on the
strut, perhaps a pair of teardrops on the rudder—and hope
for the best. In fact, even without running an actual test to
determine zinc quantity, reasonable estimates of anode
quantity can be made.
Now, there's a basic subdivision between boats—metal-h
(steel and aluminum) and non-metal (wood and fiberglas
On metal hulls, the anode’s (zin
job is to protect the hull from g
vanic attack by the other metal
fittings—almost invariably of m
noble metals—and from attack
from itself. (Small variations in
amount carbon, oxygen or hydr
gen—the effects on the metal o
welding, and cold working—cha
the composition of local region
the hull. This creates differing p
tentials that can cause galvanic
corrosion within the plating itse
By contrast, on wood and FRP v
sels, the anode's job is to prote
differing metals among the fitti
from attacking each other.
Anodes for Wood and FRP Craf
The principle underwater fitting
on wood and FRP craft are the,
propeller and shaft assemblies,
and the rudder. (Parts of the ex-
haust, through-hulls, tanks, and
on may also need protection.) T
zincs that come installed on m
boats, aren't the best system. T
do, however, provide basic prot
tion. Customarily, zinc collars a
fitted around the shaft and occ
sionally—for additional protecti
a zinc prop nut is fitted to the e
of the shaft. Zincs are also fas-
tened to the struts themselves
well as to the rudder.
Using this approach, if
Dry Roll
were single-screw, with its shaft emerging from a bearing
a centerline keel, a short square-sectioned ring collar wo
fitted around the shaft between the prop and the bearing.
it had long exposed shafts—most commonly on twin-scre
boats—a single streamlined zinc collar would be installed
around each shaft. Zincs also come in teardrop and strea
lined shapes for installation on the sides of rudders and
struts, and in slabs and blocks (some quite large) for use
metal hulls.
Severely Corroded Bronze Propeller
Photos: Paul Bremer
Continued next p