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Galvanic Corrosion
by Dr. Stephen
C. Dexter
Professor of Applied Science and Marine Biology
Galvanic corrosion, often misnamed "electrolysis," is one
common form of corrosion in marine environments. It occurs
when two (or more) dissimilar metals are brought into electrical
contact under water. When a galvanic couple forms, one of
the metals in the couple becomes the anode and corrodes faster
than it would all by itself, while the other becomes the
cathode and corrodes slower than it would alone. Either (or
both) metal in the couple may or may not corrode by itself
(themselves) in seawater. When contact with a dissimilar
metal is made, however, the self-corrosion rates will change:
corrosion of the anode will accelerate; corrosion of the
cathode will decelerate or even stop. We can use the seawater
Galvanic Series, shown in Table 1, to predict which metal
will become the anode and how rapidly it will corrode.
The seawater Galvanic Series is a list of metals and alloys
ranked in order of their tendency to corrode in marine environments.
If any two metals from the list are coupled together, the
one closer to the anodic (or active) end of the series, the
upper end in this case, will be the anode and thus will corrode
faster, while the one toward the cathodic (or noble) end
will corrode slower.
For example, suppose we have an aluminum alloy with a voltage
range of -0.7 to -0.9 V (an average of -0.8 V) as shown on
the series, coupled to a 300 series stainless steel with
an average voltage of -0.07 V. The Galvanic Series predicts
that aluminum will be the anode, and the voltage difference
between the two alloys will be about 0.73 V (obtained by
subtracting the two average voltages). It is this voltage
difference that drives the current flow to accelerate corrosion
of the anodic metal.
The two major factors affecting the severity of galvanic
corrosion are (1) the voltage difference between the two
metals on the Galvanic Series, and (2) the size of the exposed
area of cathodic metal relative to that of the anodic metal.
Corrosion of the anodic metal is both more rapid and more
damaging as the voltage difference increases and as the cathode
area increases relative to the anode area.
The approximate voltage difference for any two metals can
be taken directly from Table 1. It is worth noting that marine
slime films composed primarily of microscopic bacteria and
diatoms can change the potentials of many of the alloys near
the noble end of the Galvanic Series as indicated. The potentials
of these alloys become more positive in the presence of slime
films, thus increasing the voltage difference when these
metals are placed in contact with more anodic alloys. This
has been found to increase the corrosion rate of copper,
steel, and aluminum anodes by a factor of 2 to 5, but to
have no effect on the corrosion rate of zinc anodes. It may
also change which metal in the couple becomes the anode.
When the two metals in a galvanic couple are close together
on the series, such as manganese bronze and silicon bronze,
their voltage ranges overlap, and either one can be the anode,
depending on the exact exposure conditions. In this case,
more detailed information than is given in this MAS Notes
will be needed to predict the behavior of the couple. Several
sources of additional information are listed in the Recommended
Reading section.
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Figure 1. Effect of cathode-to-anode
(C/A) ratio on galvanic corrosion.
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The effect of the second factor above, the cathode-to-anode
area ratio, C/A, is illustrated in Figure 1 for a rivet in
a plate. In both couples A and B, aluminum is the anode,
and stainless steel is the cathode. In couple A, the aluminum
rivet is comparatively small, and the C/A ratio is large.
In couple B, the situation is reversed: the stainless steel
rivet is small, and the C/A ratio is also small. Corrosion
of the aluminum rivet in couple A will be severe. However,
corrosion of the large aluminum plate in couple B will be
much less, even though the potential difference is the same
in each case.
What can you do to minimize galvanic corrosion? First,
always try to eliminate the cathodic metal by making all
parts of a structure out of the same material. When this
is not possible, use nonmetallic, nonabsorbent insulators
between the dissimilar metals to prevent current flow. For
example, use plastic or ceramic washers and sleeves to isolate
bolts as they pass through a plate of a different alloy instead
of fiber and paper washers, which absorb water.
Another approach is to make small critical parts out of
the more cathodic metal so they will be protected. Always
avoid connecting small anodes to large cathodes as in couple
A of Figure 1.
Painting a galvanic couple can also be helpful in reducing
the corrosion rate, but always be careful to paint both the
anodic and cathodic members of the couple to keep the exposed
C/A ratio small. If only the corroding member (the aluminum
rivet) of couple A were painted, there would still be a large,
bare cathode, which would make corrosion of the rivet even
worse if the paint coating were scratched.
If the cathodic member of the couple is one of those alloys
identified in Table 1 as being influenced by slime films,
and if there is a reason not to paint, periodic cleaning
of the bare metal surface can minimize the effect of the
slime and reduce the corrosion rate of the anode. Periodic
cleaning is not recommended for reducing galvanic corrosion
of other alloys in Table 1 because removal of corrosion product
films from those metals usually increases corrosion rates.
Finally, let’s see how we can use galvanic effects to our
advantage in preventing corrosion. Suppose the steel member
of a structure is being damaged by contact with silicon bronze.
That galvanic corrosion can be stopped by connecting both
metals to a third metal more anodic than either of them.
According to our Galvanic Series, the third metal in this
case could be magnesium, zinc, aluminum, or cadmium. In practice,
and for reasons too complex to cover here, zinc works best.
(Some of the difficulties and dangers inherent in using the
other anode materials are discussed in publications listed
below.) The zinc corrodes preferentially to both of the original
members of the couple. The steel is now protected, and the
zinc is called a sacrificial anode. Such anodes are commonly
used together with coatings to control corrosion on the underwater
portions of boats, ships, and other marine structures. The
same principle can be used to protect steel in marine atmospheres
if the anodic metal is applied to the steel as a coating.
Zinc (called galvanizing) and aluminum coatings are used
extensively to protect steel in marine atmospheres. Under
fully immersed conditions in seawater, the rule of thumb
is that a heavy, hot-dipped zinc coating will protect steel
for about one year per mil thickness of zinc applied. One
mil is equal to 0.001 inch or 0.025 millimeter. In contrast,
electroplated zinc and cadmium coatings are nearly worthless
for protecting steel from corroding in seawater.
Recommended Reading
Amos, Duncan. 1980. Marine Corrosion: An Alternative
Approach to the Prevention of Corrosion in Wooden and Fiberglass
Hull Fishing and Recreational Vessels. RIU-T-80-004.
University of Rhode Island Marine Advisory Service, Marine
Bulletin 38. 12 pp. Dexter, Stephen C. 1979. Handbook
of Oceanographic Engineering Materials. New York: John
Wiley. 314 pp.
Dexter, Stephen C., and J. P. LaFontaine. 1998. Effect of
natural marine biofilms on galvanic corrosion. Corrosion
Journal, 54(11): 851.
Jones, Denny A. 1996. Principles and prevention of corrosion.
In Galvanic Corrosion, 2d ed., Chapter 6, pp. 168-198.
Upper Saddle River, NJ: Prentice Hall.
Mallon, Michael, and Edward Kolbe. 1979. Cathodic Protection
for Boats in Sea Water: A Review of Recommendations. ORESU-T-79-003.
Oregon State University Sea Grant College Program. 53 pp.
Sackinger, William. 1980. Cause and Prevention of Corrosion. AKU-T1-80-001.
University of Alaska Sea Grant Program, Marine Advisory Bulletin
#10. 15 pp.
Warren, Nigel. 1980. Metal Corrosion in Boats. Camden,
Maine: International Marine Publishing Co. 224 pp.
Rev. 11/99:2K 
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