83 - Corrosion Protection
Viking Sport Cruiser 61FY
Owner’s Manual
v1.0
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Electrolytic corrosion
Stray currents are present at most marinas, and near other boats.
A particularly strong source is shore power cords lying in the wa-
ter –they cause severe corrosion, and are a safety hazard. At the
docks themselves there are often metal reinforcing rods in con-
crete that lie in the water. These conduct stray currents that can
cause a boat’s zincs to erode quickly. Stray current corrosion is
also caused by DC or AC current leaks from other boats around
you. Aluminum is particularly susceptible to this –just look at
some outdrives at a marina!
In the lazarette the two 15 kVA isolation transformers have a rated
input of 100 amps @ 240 volts. Shore power coming aboard is
isolated through the transformer windings from any direct contact
with outside grounds. ONLY red, black and green are brought into
the transformer. In effect, 240 volts AC ONLY goes into the pri-
mary winding of each transformer. 120 volt power is produced by
the transformer. This isolation protects against stray currents and
electrolytic corrosion that destroys underwater fittings.
In effect, the vessel has its own CLOSED CIRCUIT electrical sys-
tem which also protects crew and passengers from potentially
lethal electricity flowing through them to a shore ground.
The engines and generator have their own internal zinc anodes for
protection of their metal parts, these MUST be maintained ac-
cording to manufacturers’recommendations.
Stainless steel corrosion
All metals except gold, silver, titanium, and platinum corrode –
some much more than others. Despite its name, stainless steel
is also subject to corrosion because it is a ferrous alloy. 18/8
stainless, for example, is 67% iron, with 22% chromium and 11%
of other metals added to make it more resistant to corrosion. The
three types of stainless steels common in the marine industry
are 302 or 18-8, used for screws; 304, used for stamped parts;
and 316 or CF-8M, used for tubing, etc.
Though high up on the noble chart (-30 mV for resistance to cor-
rosion), stainless steel WILL rust quickly in its natural state, par-
ticularly in the warm moist climate of Florida. For stainless to be
corrosion RESISTANT it must be highly polished, plated, or electro-
polished in a number of ways during production. Unpolished stain-
less steel has iron or iron oxide sitting on the surface of the metal,
but polishing brings the chromium to the surface. The more shine,
the more resistance to corrosion.
If stainless is scratched, or cut and not polished at the edges,
corrosion will result. Thus, if a screw driver bit slips on top of a
screw, it will rust. And a hole drilled in stainless steel will corrode
inside the hole unless protected by a watertight sealer.
When washing the vessel, never let water dry on the stainless.
To
avoid unsightly water spotting dry off stainless with a chamois.
Stanchions and rails must be kept clean, use a wax such as
Collinites 850 cleaner/wax to further protect them.
A corrosion and safety hazard
A shore power cord in the water is a hazard to
anyone in the water. The flow of AC current
inside the cable will also make the vessel more
susceptible to corrosion.
Isolation transformers
Isolation transformers effectively give the vessel
a ‘closed circuit”electrical system that is safe,
and will not leak current and cause corrosion.
Onan generator
Internal zinc anodes must be changed on schedule.
(optional model shown)
