the cathodic current to protect the entire
dock could be huge. Fortunately, a shore-power galvanic isolator or an isolation
transformer can be used to block the
galvanic current between boat and dock,
eliminating the problem.
For an investment of about $200,
you can make sure your vessel has
adequate cathodic protection. Go to
BoatUS.com/Corrosion-Test for a complete explanation.
PROBLEM NO. 2:
Electrolytic stray-current corrosion
Stray-current corrosion is usually caused
by a DC voltage source, such as a battery,
accidentally connected between two isolated metals wetted with a common electrolyte, such as seawater. In seawater, the
metal connected to the positive terminal
will suffer from rapid and catastrophic
damage, and white calcareous deposits
will form on the metal connected to the
battery’s negative terminal.
Note that for stray-current corrosion to occur, there must be two isolated
in seawater (with
no bonding system
installed). For example, if a radio ground
plate is connected
to the battery negative and an alternator positive conductor faults to an
block, the prop shaft
and prop can be
destroyed in a matter
of days. However, if
the two wetted metals are interconnected with a bonding
or grounding conductor, then stray-current corrosion is
virtually impossible because the bonding/
grounding conductor would short-circuit
the errant voltage source. Preventing
stray-current corrosion requires boat
owners to periodically check their 12-volt
electrical system for loose wires or poor
or inadequate connections, especially in
bilge areas, which can be full of seawater
that acts as an electrolyte.
PROBLEM NO. 3:
Also known as crevice corrosion, oxygen-depletion attack can catch unsuspecting boaters unaware. It attacks in crevices and other hidden areas, and it most
commonly attacks stainless steel, which
the typical layperson believes should be
impervious to corrosion. However, while
some grades of stainless steel are resistant
to oxygen-depletion attack, others are
quite susceptible. Using 316L stainless
steel under the waterline, for example,
would be akin to using pine for your
wood decks instead of teak.
The characteristic of stainless steel
that makes it “stainless” is a chemical
reaction of the metal at its surface with
the oxygen in its environment. This reac-
tion results in a film that protects the
surface from corrosion. However, if part
of the surface is in contact with low-
oxygen stagnant water, then this part
loses its protective film and becomes less
noble than the remaining metal surfaces;
it then behaves like a sacrificial anode.
For example, if a vessel lies dockside for
months without its prop shafts turning
over, the water under the cutless bearing
becomes stagnant. Pitting may develop,
which causes the shaft to weaken; under
power, it could snap while underway and
potentially flood the boat.
Fortunately, there are grades of stainless steel whose compositions have been
altered to reduce susceptibility to crevice
corrosion; these grades of stainless steel
are resistant to oxygen-depletion attack.
The bottom line is that most stainless
steel, including the typical 316L type,
can corrode underwater, and thus, for the
most part, shouldn’t be used there.
Preventing corrosion damage to submerged metals may seem, in the planning
stages, to be a technically complicated
task, but enacting such a plan is relatively
straightforward. Virtually all corrosion
issues can be resolved in the pre-crisis
stage through proper design, adequate
cathodic protection, and a minimal
amount of monitoring and maintenance.
James Coté, an electrical engineer at Coté
Marine ( cotemarine.net), his marine electric and corrosion-control consulting firm,
is an ABYC Master Technician, a Fire
Investigator, and a Marine Investigator.
about aluminum alloys,
well in a seawater environment but are
Top Left: Stray-current corrosion can
destroy a sterndrive in a matter of weeks.
Bottom Left: This stainless-steel propshaft
is a victim of crevice corrosion; without a
supply of fresh oxygen, even stainless steel
is vulnerable. Above: An errant 12-volt
wire destroyed this bow thruster.