Accelerated corrosion
which can occur when dissimilar metals are in electrical contact in the
presence of an electrolyte (i.e. conductive solution). An example of this
corrosion phenomenon is increased rate of corrosion of steel in seawater
when in contact with copper alloys. Galvanic attack can be uniform in nature
or localized at the junction between the alloys depending on service conditions.
Galvanic corrosion can be particularly severe under conditions where protective
corrosion films do not form or where they are removed by conditions of
erosion corrosion.
Prevention or
Remedial Action
- selection of alloys
which are similar in electrochemical behavior and/or alloy content.
- area ratio of more
actively corroding material (anode) should be large relative to the more
inert material(cathode).
- use coatings to
limit cathode area.
- insulate dissimilar
metals.
- use of effective
inhibitor.
Standard Test
Methods
- ASTM G-71 - guide
for conducting and evaluating galvanic corrosion tests in electrolytes.
- ASTM G-82 - guide
for development and use of a galvanic series for predicting galvanic corrosion
performance.
- ASTM G-104 - test
method for assessing galvanic corrosion caused by the atmosphere.
Evaluation
for Galvanic Corrosion
Many people utilized
the standard galvanic series of materials in seawater to predict service
performance relative to galvanic corrosion. In fact, this galvanic series
is specific to only seawater at near ambient conditions. Other factors
such as temperature and the presence of other chemical species can greatly
affect the rank ordering of materials. Such differences in environmental
conditions can reverse galvanic couples whereby the material expected to
be the cathode may actually be the anode and experience severe corrosion.
In making galvanic
corrosion measurements, it is good practice to try to separate the effects
if crevices between contacting materials and actual galvanic corrosion.
This is the reason that in many tests, the actual electrical coupling of
the two materials is performed in a region protected from the environment
or externally from the environment. The external coupling is a good idea
since it allows for measurement of the mixed potential of the couple and
the galvanic corrosion current. While the potential serves as a measure
of the thermodynamic driving force for galvanic corrosion, it is the galvanic
corrosion current that indicates the acceleration of corrosion by the influence
of the galvanic couple.
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