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anodic metals such as magnesium and aluminum, are occa-
sionally used as anodes. Zinc, however, offers the best trade
-off between cost, reliability and ease of manufacture. It's
the standard marine-anode material for use in seawater.)
Self Destructing Metals
It's not always necessary to have two different metals in
contact to get corrosion. Some alloys actually have corrosion
potential built in. The ever-popular manganese bronze is a
case in point. It's 58% copper and 38% zinc—not really a
bronze at all but a brass! Brasses are bad news for use be-
low the waterline because the zinc in all brass is eaten away
by the more noble copper. This process is called
dezincifica-
tion.
Manganese bronze's popularity is due to its ease of
manufacture, relatively low cost, and moderately good
strength. The dezincification (really internal galvanic corro-
sion) can be controlled with anodes. Many other alloys suffer
from similar problems. Aircraft-grade aluminum alloys,
some high-strength steels and all the brasses have alloy
combinations which add up to trouble in saltwater.
Checking Potential (Voltage) to Detect Corrosion
This brings us to the second critical use of the galvanic se-
ries. As long as zinc electrons are flooding the other metals,
it effectively stuffs them with excess electrons. Rather than
loose their own electrons and corrode, the excess zinc elec-
trons are lost first, to be replaced with still more zinc elec-
trons, for as long as there's zinc available. These extra elec-
trons change the charge or potential of the protected met-
als. And—since electrons have negative charge—the
tected metals become more negative. (Metals that are
odic have more excess or easily freed electrons than
thodic metals. Anodic metal thus read more negative t
cathodic metals on the galvanic series.)
This process is called
polarization.
It's a change in poten
that can be measured using a portable voltmeter. As lon
the measured voltage increase is between -200 to -400
( -0.20 to -0.40 volts) above the indicated "normal" volt
or potential shown on the galvanic series, the metal
been polarized enough to be protected from corrosion. U
this information, you can spend an afternoon going 'ro
your
Dry Roller
testing potentials (polarization or voltages
various fittings and components to see if they're protec
(potential raised -200 to -400 mV) or freely corro
(potential less that -200 mV above the voltage indicated
the galvanic series). It's a good idea to schedule such a
once or twice a year as part of
Dry Roller
's routine mai
nance, along with engine overhauls and bottom cleaning.
Using a Voltmeter (Multimeter) to Test for Polarization
The procedure is as follows: You need is a quality voltmet
(really a multimeter) with a with at least one scale that c
be set to read with a maximum of 1200 to 1500 millivolt
(mV, or 1.2 to 1.5 volts); a silver:silver-chloride electrode;
and long wire to connect it to the voltmeter. Aboard
Dry
Passive & Active
The Mysterious Behavior of Stainless Steel
Stainless steel appears in two locations on the galvanic
scale—one quite noble, and the other fairly anodic. The
key to this mystery is oxygen. Most corrosion-resistant
marine alloys protect themselves—to varying degrees—
by forming a thin surface-layer oxide film. This is pro-
duced by interaction with the oxygen dissolved in sea-
water. Bronze, Monel, copper and copper-nickel will cor-
rode slightly faster if they loose this film, but will still
remain highly corrosion resistant.
Stainless steels, on the other hand, rely chiefly on this
oxide film to protect them from corrosion. When
stainless is in clean flowing water containing plenty of
oxygen it has no difficulty generating and retaining the
oxide film. In this condition it's highly cathodic (noble)—a
state that's also called
passive.
Should stainless be de-
prived of a regular supply of oxygen, however—for in-
stance, pressed for a long time, immobile, against a cut-
lass bearing; smothered by barnacles; or enclosed in a
stern tube—it can loose its protective oxide film. In these
conditions stainless becomes nearly as anodic as mild
steel. Stainless is called
active
in this state, and can suf-
fer severely from pitting corrosion.