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Controlling Galvanic Corrosion
By Dave Gerr, © Dave Gerr, 1991 & 2010
It's a sad but inescapable fact that metals corrode in sea-
water. Corrosion problems lead to repairs. This adds up to
down time and expense. Of course, some metals are more
resistant to corrosion than others; but those that corrode
least almost invariably cost most. Worse still, even highly
corrosion resistant metals, like silicon bronze, 316 stainless
steel, Monel, and Aquamet alloys, can corrode or cause
other metals to corrode if they're all attached to the same
hull underwater. Accordingly, it pays dividends—literally—to
spend the time and money needed to understand and con-
trol corrosion before it starts controlling you.
Vagabond Electrons are at the Root of Corrosion
The cause of the vast majority of corrosion is electrons that
won't stay put. They leave their home metal to travel
through any convenient conductor to other metals that keep
a tighter hold on their own electrons. To make matters
worse, the missing electrons leave positively-charged parti-
cles (ions) on the surface of their home metal. These ions
react with negatively charged
ions in seawater to dissolve
away. Such metals—metals
that hold loosely to their elec-
trons—are generally quick to
corrode. They are referred to
as
anodic
,
less noble,
or
active
metals. (Aluminum, mild steel
and zinc are anodic metals.)
Metals that retain their elec-
trons tightly are called
ca-
thodic
,
noble,
or
passive
. They
are generally slow to corrode.
(Silicon bronze, stainless
steel, titanium and gold are
cathodic metals.)
The Potential for Corrosion
Now, when you connect two different metals together elec-
trically and through an electrolyte (in our case seawater), the
electrons from the less-noble (anodic) metal will try to tum-
ble towards the more-noble (cathodic) metal. This flow of
electrons generates a real measurable force, exactly as you
could measure the force in a stream of water flowing
through a pipe, from, say, a tank with a high water level (the
anode) to one with a low water level (the cathode). Where
electrons are concerned, the force of flow is measured not in
pounds, but in
volts,
and is often referred to as
potential.
(It's called potential because it measures how great a poten-
tial there is for a flow to occur—flow or current in
amps
.)
The Galvanic Series
Of course, potential is relative. Mild steel holds onto its elec-
trons only somewhat more strongly than does marine alu
num. If these two materials were in contact in seawater,
aluminum would corrode (too fast by half), but not nearl
fast as if the aluminum were in direct contact with, say,
con bronze. Silicon bronze—relatively speaking—holds m
tightly to its electrons than does mild steel, and much m
tightly still than does marine aluminum.
The key word here is "relative," and the best way to k
tabs on these relative potentials is by listing the voltage
all materials with reference to a single test m
(electrode), in the electrolyte that you're concerned abo
seawater for our purposes. (The most stable and sensi
electrode material for this use is silver/silver-chloride—
AgCl). The list of relative potentials generated this
makes up
the galvanic series
, see next page. (Electrical
tivity increases with temperature, so this is specified as
Standard galvanic tables usually give voltages at 77 F°.)
Selecting Fittings Using
Galvanic Series
There are two critically im
tant uses for the galvanic
ries. First, you should refe
it when installing hardw
Try to make sure the volt
difference between any
metals, in direct contact
seawater, is less than 0
volts or 200 millivolts (
Metals that are less that
mV apart corrode each ot
fairly slowly and need li
additional protection. If
must use two metals in c
tact, further apart than
mV, you need to take step
protect them, either by insulation (so they're not reall
contact) or by using anodes.
Anodes Protect by Flooding with Electrons
There's a nice feature about anodic metals—if you use th
wisely—as long as they're losing electrons, all the other m
-noble (more-cathodic) metals they're connected to are
tected from corrosion. Say, for instance, your
Dry Roller
a stainless-steel shaft, with a manganese-bronze propel
Its bronze prop's electrons would be tumbling toward
more-noble stainless.
Roller
’s prop would waste away.
zinc anode were attached to the shaft, its electrons wou
roughly speaking—tumble toward both the stainless and
bronze (it's far more anodic than both). It would flood
system with zinc electrons—again, roughly speaking. (Ot
Corrosion Misnomers
Or
What’s in a Name
There's considerable confusion about the proper name
for
galvanic corrosion
—the only correct term. Frequently
it’s called
electrolysis,
or
electrolytic
corrosion. Both are
misnomers. Electrolysis is the corrosion or chemical
breakdown of the electrolyte—the fluid medium that
transfers ions between metals. Obviously, in our case,
electrolysis would be breakdown of seawater itself—not
much of a concern. Electrolytic corrosion is the corrosion
produced by externally generated electric currents. It's
also known as
stray-current corrosion.
Stray-current cor-
rosion can be very serious. It's a vast subject in itself,
however, and will have to wait for a future article.