July 1959 Popular Electronics
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No, the electrolysis and corrosion of boat propellers is not
really in line with the theme of RF Cafe; however, it presents
the same sort of problems that grounding and anchoring systems
for radio antennas and equipment shacks have. If you bury a
piece of metal in the Earth, it will, over time, magically disappear.
Much effort has been expended on the part of both amateurs and
professionals to mitigate the anodic action that occurs when
dissimilar conductors come into intimate contact. Because each
metal - be it a base or an alloy - has an electric potential
relative to other metals. What happens when there is a difference
of potentials and a conduction path is present? Yep, current
flows. Through that action, material is physically transferred
from the more positive metal to the less positive metal. A relatively
simple solution was discovered more than a hundred years ago
- a sacrificial element whose only purpose is to supply the
electrons, and hence material loss, in lieu of the important
structure. It really works. There was an article in the ARRL's
QST magazine that did an excellent job covering this topic.
Electrolysis and Corrosion
How to save your boat from the ravages of electrical corrosion
By Elbert Robberson
To a jet pilot, it's a flameout. To a parkway pilot, it's
a flat tire in the Holland Tunnel. For the owner of a boat,
it's electrolysis - a boating problem ever since metal parts
have found their way aboard.
Talk to old-time boatmen, dealers, or shipyard mechanics,
and you'll probably hear plenty about this sea-water scourge
... stories of boat and engine parts falling off, propellers
turned to lace, boats sinking. It's true that these things can
happen because of electrolysis, but no conscientious boatman
need suffer - because electrolysis is easily prevented. First,
however, let's find out exactly what it is.
What Is Electrolysis? The term "electrolysis" has come to
be very loosely applied in the small-boat field. What many boatmen
call "electrolysis" is often really some kind of corrosion.
Depending upon your dictionary, you will get all kinds of
definitions from the simple "The decomposition of a chemical
by an electric current," to a fat paragraph in Webster's New
International Dictionary dealing with electrolytes, ions, and
In the strict sense of the term, electrolysis concerns chemical
changes in the solution (in this case, the salt water) due to
the passage of current. Since we need a term to work with, however,
let's call the corrosion of metal parts involved in electrolysis
When Does It Occur? Electrolysis corrosion occurs when direct
current, as from a battery or generator, passes through the
water from one conductor to another. You can get a good example
of this activity very quickly and easily.
Severe galvanic corrosion in a steel rudder
connected to copper-bottom sheathing, bronze shaft and propeller.
The boat almost became a casualty.
Dunk a pair of copper strips in a cup of salt water (table
salt will do), and connect them to a 6-volt battery. Bubbles
boil from the negative electrode and nothing much happens at
the positive one - except that you may observe a green cloud
creeping out from it into the water. What is happening is that
the metal of the positive strip is going into the solution.
In a short while, the weight loss of this electrode can be measured
with a sensitive scale. Carryon your experiment long enough
and there won't be any positive-end copper left.
Electrolysis corrosion can be studied by
immersing copper strips in brine, and connecting a battery.
In a short time, the current flow will cause the positive electrode
to corrode - eventually be entirely eaten.
This is what boatmen commonly call electrolysis - loss of metal
due to battery current between underwater parts. Run battery
current between a boat's underwater metal parts, and the positive
part corrodes rapidly away. Stop the current flow, and the process
Since no one would reasonably connect a battery between underwater
metals on his boat, how can this possibly happen? Very easily,
due to the fact that one side of the battery circuit for motors
and many electrical boat accessories and lights, etc., is connected
to the frame, shell, or "ground."
"Self-devouring" propeller at right suffered
from faulty alloy, so zinc block, mounted on wood, above, gave
If you cross-ground one gadget by using opposite ground polarity
from that used on any other fixture, you've set up your boat
for possible electrolysis corrosion. The damaging current flow
can take place through bilge water as well as through the water
under the boat; and it has also been known to take place through
wet wood. So, when installing fixtures, taking batteries ashore
for charging, or doing anything with the electrical system of
the boat - keep the polarity of the ground connections the same.
Lately, manufacturers have begun to standardize to a large degree
on negative grounding; but don't count on it - investigate and
Zinc blocks must be connected to metal that
they are to protect. This propeller nut has a zinc overcoat.
Blocks on deadwood and rudder are connected by copper straps
to rudder post and stern bearing.
Another way to invite electrolysis corrosion is to use insufficient
heavy wire to supply power to heavy-drain items, such as a radiotelephone,
which have ground connections to the water. Voltage drop occurring
in the ground leg of the power supply circuit is just the same
as with battery voltage: if there is a difference of potential
between points A and B underwater, no matter if the voltage
is from a generator, battery, or a voltage drop in a wire, trouble
can result. Use large wire for power leads to keep voltage drop
close to zero. In addition, it is a good idea to connect grounded
objects together inside the boat with a "bonding" wire of at
least No. 10 gauge, and also bond in the engine frame and radio
ground plate, if any.
Galvanic Corrosion. Most underwater corrosion has no connection
whatever with electrolysis, contrary to popular thought. This
trouble is simply "galvanic corrosion." It takes place when
three conditions are satisfied: the metals are in contact with
salt water; they are different in composition; and they are
in electrical contact through a metallic path: This forms a
galvanic cell - like a primary battery cell. Current is generated,
flows through the water, and the metal supplying the current
This automatically happens when ordinary hardware-store brass
is used for underwater fittings or fastenings on a boat. Ordinary
brass is an alloy composed of about 30% zinc and 70% copper.
The table below the galvanic activity of boat metals. Note that
zinc is at one end of the scale and copper near the other. Result:
the zinc is corroded, leaving a spongy copper mass in place
of the brass. This has no strength, so holes appear and fastenings
Many alloys are so cannibalistic that they will feed on themselves
in this fashion. So, the first rule which must be followed to
avoid galvanic corrosion is to use only fittings and fastenings
which are made of seaworthy metal: bronze, Everdur, Monel, stainless
steel, and - if you are loaded - titanium, gold and platinum.
All are extremely resistant to corrosion.
Next, make sure that all of the different underwater metals
are of the same family. Avoid galvanized iron or steel if there
is copper or any of the other more noble metals around. Always
try to use fastenings. which are more noble than the metal of
the object they are holding in place. This way, if there is
the least bit of corrosion, it will affect the comparatively
bulky piece of hardware (which can afford to lose a few grams
weight) instead of the smaller fastening which may weaken to
the point of failure after the loss of just a little metal.
An outboard motor is, unfortunately, mostly active metal,
and likely to corrode quite rapidly if given the right conditions.
To be on the safe side, when your motor is not actually in use,
keep it up out of the water ... especially if it has a bronze
The importance of avoiding unlike combinations of metal underwater
cannot be stressed too much. And never install an object made
of a metal about which you are unsure.
The builders of a beautiful yacht, the "Sea Call," learned
this the hard way. Monel is practically proof against corrosion,
so they built the underwater shell of pure Monel. But the frames,
stem and rudder were steel; and while most of the rivets were
Monel, a few steel rivets were accidentally mixed in. Shortly
after the vessel was placed in the water, one of these steel
rivets disappeared, and people began to get suspicious. Upon
dry-docking the yacht, it was found that all of the steel near
the Monel metal was rapidly corroding, while the Monel, of course,
remained unaffected. The yacht was built in 1915 and scrapped
GALVANIC ACTIVITY OF BOAT METALS
different electrically connected metals (listed below) are immersed
in salt water, the metals closest to the base end corrode, and
the ones closest to the noble end of the scale do not corrode.
Cures for Corrosion. Prevention is the best cure for corrosion,
of course, but many times it is not possible to have conditions
as perfect as would be desired. Good underwater metals may pick
up dirt or oxide which changes their galvanic activity; small
impurities in a metal can create galvanic hot spots; and it
is conceivable that somewhat different metals must, of necessity,
sometimes be used. On large ships, you will often find combinations
of aluminum and steel or bronze.
If it is necessary to use different metals, they must be
electrically insulated from each other. Plastic gaskets, Micarta
separators and sleeves, rubber or other waterproof insulating
material can be used. As long as there is "no circuit," corrosion
will stay away.
You can also obtain protection with unbroken coverings of
plastic paint, neoprene compound, or other insulating coverings.
But the most popular method of protecting metals which unavoidably
suffer from galvanic corrosion was cooked up by Sir Humphrey
Davy in 1824. The copper sheathing on warships was corroding.
"Attach blocks of zinc to the copper," said Sir Humphrey. The
idea was that, being more active than any surrounding metal,
the zinc would supply metal for all of the galvanic action,
and the other metal would remain unscathed. This idea was so
good that even today U. S. Navy vessels which must be as light
and maneuverable as possible have tons of zinc anodes attached
to their hull plating.
Special zinc anodes are made for small boats in the form
of plates, propeller-nut caps, and shaft sleeves, and are available
from marine hardware stores. The zinc must be as pure as possible
(less than .0014% iron content), and it must be attached or
electrically connected to the metal which it is supposed to
protect. It must not be painted, however, and when it has corroded
badly (it will corrode, if it is providing any protection),
the zinc anodes must be replaced.
A more modern method of protection is to supply a reverse
current from a platinum anode fitted to the hull, powered by
a battery-operated regulated supply. Although this is highly
effective, it is somewhat more expensive.
The Scapegoat. Underwater metal comes apart for many more
reasons than electrolysis and corrosion. Among these are abrasion
from mud and sand, and cavitation.
At the time when comparatively little was known about the
installation of electrical and electronic equipment on small
boats it was fashionable, and very easy, to blame every bit
of underwater trouble on electrolysis. Radio had a bad name.
Soberfaced "experts" actually claimed that the instant you
installed a radiotelephone on a boat, the underwater hardware
would start to fall off.
But since ghosts have gone out of vogue, don't tremble at
the awful specter of electrolysis. Keep track of your electrical
circuits so that current does not flow through the water; use
sea-going metals and be sure they are properly mated; and use
zinc anodes, if necessary. Take these basic protective measures,
and you should have no trouble.
Posted October 26, 2011