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Left to themselves, bronze fittings rarely degrade. But when bonded together, these noble alloys speed up the corrosion of the
sacrificial zinc anode. Once that''s gone, the entire system is unprotected. Top, left—Detail of deterioration at a bonded rudder
stuffing box. Note the neon green-blue at the bolt heads indicating electrochemical behavior. Above, right—Severe deterioration
at a bronze seacock. Critics often cite bonding as a solution looking for a problem.
Fungal Decay
Fungi—parasitic microflora that
derive sustenance from wood and
other living organisms—are microscopic
airborne spores spread by
winds (or other carriers such as
insects) to potential hosts, and are
broadly categorized as mold, stain, or
decay. Generally speaking, molds and
stains primarily attack sapwoods, and
cause a nondestructive discoloration
of the cell walls. Early stages of fungal
attack can only be detected by microscopic
examination or through culturing
in a laboratory. Further colonization
and growth leads to advanced
decay; wood in this condition is
generically referred to as rotten.
Good, sound wood has a long
fibrous structure. Green (unseasoned)
wood can be peeled into strands or
slivers, while dry wood will splinter
when it''s pulled with or across the
grain. During the early stages of
decay, some fungi will discolor the
wood or decrease the fiber toughness.
As decay progresses, wood
A
bonding system on a boat links
non-current-carrying metal fittings
that are immersed in salt water. The
procedure for recreational craft is
decribed in detail as Standard E-l in
the American Boat and Yacht
Council''s Standards and Recommended
Practices for Small Craft.
(This volume is available from ABYC
in both print and CD-ROM formats;
call 410-956-1050.) The basic intent
of installing a bonding system is to
provide uniform protection against
galvanic corrosion.
Modern boats use a mix of different
metals for underwater fittings. The
ABYC standard shows how each one
of these alloys has its own particular
galvanic "signature" in millivolts signifying
the level at which that particular
metal freely corrodes in saltwater.
These readings determine the relative
42 PROFESSIONAL BOATBUILDER
ranking, or nobility, of different metals.
If two of these dissimilar metals are
connected, then the difference in their
relative nobility will create enough
potential for current to flow between
the two fittings. The fitting that
receives the positive electrical charge
will blow metal ions off its surface—in
other words, galvanic corrosion.
The argument for bonding throughhull
fittings and equipment is based
on the assumption that there is no
realistic way to electrically isolate
every separate item that sits in the
electrolyte (salt water). For that matter,
there are many alloys such as manganese
bronze that contain highly corrodible
elements such as iron and
zinc. Therefore, the action of merely
leaving some metals immersed without
the protection of a zinc anode could
lead to the alloying elements themselves
battling it out in a process that
has been termed selective corrosion.
Linking all of these underwater fittings
with a bonding system essentially
creates a single body of metals to be
protected. So when zinc anodes are
added to the system, they can provide
reliable protection for the lowest alloy
on the system''s galvanic totem pole.
Bonding systems offer an orderly
way to assure that the more vulnerable
alloys will receive adequate
cathodic protection against their more
noble brethren. But this protection
lasts only as long as there is still zinc
left on the propeller shaft. In other
words, the system is only meant to
ward off the relatively slow and predictable
progress of normal galvanic
action; actual stray current leaking into
the bilge is another matter altogether.
"Bonding" continues on page 44
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