Galvanic corrosion is an
electrochemical action of two dissimilar metals in the presence of an
electrolyte and an electron conductive path. It occurs when dissimilar
metals are in contact.
It is recognizable by the
presence of a buildup of corrosion at the joint between the dissimilar
metals. For example, when aluminum alloys or magnesium alloys are in
contact with steel (carbon steel or stainless steel), galvanic corrosion
can occur and accelerate the corrosion of the aluminum or magnesium.
This can be seen on the photo above where the aluminum helicopter blade
has corroded near where it was in contact with a steel counterbalance.
Galvanic Series In Sea
18-8-3 Stainless steel, type 316 (passive)
18-8 Stainless steel, type 304 (passive)
13 percent chromium stainless steel, type 410 (passive)
75NI-16Cr-7Fe alloy (passive)
76NI-16Cr-7Fe alloy (active)
18-8-3 Stainless steel, type 316 (active)
18-8 Stainless steel, type 304 (active)
13 percent chromium stainless steel, type 410 (active)
The natural differences in
metal potentials produce galvanic differences, such as the galvanic
series in sea water. If electrical contact is made between any two of
these materials in the presence of an electrolyte, current must flow
between them. The farther apart the metals are in the galvanic series,
the greater the galvanic corrosion effect or rate will be. Metals or
alloys at the upper end are noble while those at the lower end are
active. The more active metal is the anode or the one that will corrode.
Control of galvanic corrosion
is achieved by using metals closer to each other in the galvanic series
or by electrically isolating metals from each other. Cathodic protection
can also be used to control galvanic corrosion effects.
The scuba tank above suffered
galvanic corrosion when the brass valve and the steel tank were wetted
by condensation. Electrical isolation flanges like those shown on the
right are used to prevent galvanic corrosion. Insulating gaskets,
usually polymers, are inserted between the flanges, and insulating
sleeves and washers isolate the bolted connections.
KSC conducts research on the
effects of galvanic corrosion. The photo below shows the corrosion
caused by a stainless steel screw causing galvanic corrosion of
aluminum. The picture shows the corrosion resulting from only six months
exposure at the Atmospheric Test Site.