Port Credit Marine Survey
& Yacht Delivery
From Ontario, Canada
Society of Accredited Marine Surveyors
American Boat & Yacht Council
Anodes, Zincs, Magnesium, Aluminum and Galvanic
Corrosion on Boats
So it is with "zincs". which have become the "Kleenex" of the boating world. "Anode" is the proper term and zinc, magnesium and aluminum alloys are variants for use in different types of water. Curiously most new (almost all) boats are delivered to dealers in the Great Lakes with totally inappropriate zinc anodes !
On a recent visit to an Ontario chandlery I found shelves of anodes mostly unidentified as to material. Almost half of the anodes on the shelf were zinc which is a completely inappropriate material for use in the fresh water of the Great Lakes, some were aluminum-indium and some were magnesium, the only appropriate material for use in fresh water.
|From the moment a metal is created it starts
to return to it's orginal form. We
corrosion. Some do it very quickly
take a long time. Corrosion can be
or delayed by various environmental
(salt water vs. fresh water etc.),
involvement and various other factors.
metals have a natural voltage potential
these potential voltages are listed
Noble Scale (sometimes called Galvanic
When two metals with different potentials are in contact and submerged in an electrolyte (lake or seawater) current begins to flow from the less noble metal to the more noble metal. The further apart they are on the galvanic scale, the more dramatic the corrosion. This current carries electrons away from the less noble metal and we call this electrolytic corrosion. Electrolytic corrosion simply means corrosion involving electricity whether from natural voltage potentials or external sources.
Galvanic couple (cell)
|If the aluminum (lower nobility) of an i/o
drive or a saildrive is in contact with stainless
steel (higher nobility), the aluminum will
corrode. This applies to any metal with differing
potentials. Anode materials are designed
to be the lowest nobility metals in the system
and sacrifice (corrode) themselves to protect
the other more expensive metals such as drive
housings, propellers, propeller shafts, shaft
struts and rudders.
There must be contact (continuity) between the metals involved in order for current to flow. Because we are dealing with very low voltages (millivolts) resistance between the metals must be very low, preferably a fraction of an ohm or even 0.0ohms In order for the anode to sacrifice itself.
Pure water is not conductive but our fresh
water lakes contain contaminents which are
somewhat conductive and this may vary from
harbour to harbour depending on local pollution
from outflow or drainage etc. Salt water
is much more conductive than our lakes and
this is why we different types of anodes
.... we are always trying to balance the
flow of electrons and this is affected by
the electrolyte (water).
On a boat with bronze, aluminum, galvanized
and stainless steel that are connected either
with direct contact or with bonding wires
and immersed in water...... you are in the
same neighbourhood, you turn all of these
metals into a big battery. The more noble
metal is the "cathode", the less
noble, the "anode". In this process
the less noble metal gives up electrons to
the more noble thus weakening the metal,
otherwise known as "galvanic corrosion". The further apart these metals are on the
galvanic scale (sometimes called the Noble
Scale), the more corrosion occurs.
The "sacrificial" anodes on your shafts, trim tabs etc. are supposed to sacrifice themselves thereby protecting expensive metal parts. This is why it's important to keep your anodes or "zinc's" in good condition and never paint them.
A vessel suffering from galvanic corrosion
is usually the source of it's own problem,
although two vessel's linked by shore power
grounds can create a galvanic cell between
two very close boats, more on this later.
Complicating this picture somewhat is the
fact that DC can be super-imposed on your
AC wiring through the common ground on board
or the ground in the shore power pedestal
we all share on the dock. As all vessels
in the marina are connected through shorepower
grounds there is potential for widespread
damage. Aside from concerns of corrosion
there is also potential for electrocution
if shorepower cords are allowed to lie in
the water let alone the fools that leave
their shorepower cord plugged in at the dock
while they go out for an afternoon cruise.
Metals near each other in the galvanic series have little effect on each other. Generally, as the separation between metals in the series increases, the corroding effect on the metal higher in the series increases as well.
Relative surface areas of contacting dissimilar metals is also relevant in determining which metal exhibits accelerated corrosion. It is undesirable to have a large cathode surface in contact with a relatively small anode surface.
Galvanic corrosion occurs when two different metals are in contact in a corrosive environment: one of the metals experiences an accelerated corrosion rate. The contacting metals form a bimetallic couple because of their different affinities (or attraction) for electrons. These different affinities create an electrical potential between the two metals, allowing current to flow.