38
Fundamentals of Corrosion
Sometimes, nonmetallic conductors can act as cathodes in galvanic couples.
Both carbon brick in vessels made of common structural metals and impervious graphite in heat exchanger applications are examples. Conductive films,
such as mill-scale (Fe 2 O 3 ) or iron sulfide on steel, or lead sulfate on lead, are
cathodic to the base metal or to some metallic components in their contact.
The galvanic series is an arrangement of metals and alloys according to
their actual measured corrosion potentials in a given environment.
It should be noted that alloys have been included in the series as has graphite, a nonmetallic of electrochemical interest. Note that different positions in
the series are occupied by active-passive metals and alloys.
It is common practice to predict galvanic behavior in various environments
based on the galvanic series in seawater. However, caution must be exercised.
Changes in temperature and electrolyte composition can change the electrode potential, thereby causing a change in position in the galvanic series.
Ideally, there should be a galvanic series for each environment, which
would require an infinite number of tests, which would not be practicable.
3.3.3 Preventive Measures
Preventive measures that may be taken include:
1. When joining two dissimilar metals together, galvanic corrosion can
be prevented by insulating the two materials from each other. For
example, when bolting flanges of dissimilar metals together, plastic
washers can be used to separate the two metals.
2. Do not couple metals that are far apart in the galvanic series.
3. Avoid small anode–large cathode combinations. Fasteners should be
cathodic to the parts being fastened.
4. Any coating being applied should be applied to the cathodic member and not to the anodic member. This is to prevent an unfavorable
anode/cathode ratio should any discontinuity in the coating occur.
5. Anodic parts may be made thicker to provide a longer life.
6. A sacrificial metal may be installed that would be anodic to both
metals.
3.4 Crevice Corrosion
Crevice corrosion is a localized type of corrosion occurring within or adjacent
to narrow gaps or openings formed by metal-to-metal or metal-to-nonmetal
contact. It results from local differences in oxygen concentrations, associated
Fundamentals of Corrosion
Sometimes, nonmetallic conductors can act as cathodes in galvanic couples.
Both carbon brick in vessels made of common structural metals and impervious graphite in heat exchanger applications are examples. Conductive films,
such as mill-scale (Fe 2 O 3 ) or iron sulfide on steel, or lead sulfate on lead, are
cathodic to the base metal or to some metallic components in their contact.
The galvanic series is an arrangement of metals and alloys according to
their actual measured corrosion potentials in a given environment.
It should be noted that alloys have been included in the series as has graphite, a nonmetallic of electrochemical interest. Note that different positions in
the series are occupied by active-passive metals and alloys.
It is common practice to predict galvanic behavior in various environments
based on the galvanic series in seawater. However, caution must be exercised.
Changes in temperature and electrolyte composition can change the electrode potential, thereby causing a change in position in the galvanic series.
Ideally, there should be a galvanic series for each environment, which
would require an infinite number of tests, which would not be practicable.
3.3.3 Preventive Measures
Preventive measures that may be taken include:
1. When joining two dissimilar metals together, galvanic corrosion can
be prevented by insulating the two materials from each other. For
example, when bolting flanges of dissimilar metals together, plastic
washers can be used to separate the two metals.
2. Do not couple metals that are far apart in the galvanic series.
3. Avoid small anode–large cathode combinations. Fasteners should be
cathodic to the parts being fastened.
4. Any coating being applied should be applied to the cathodic member and not to the anodic member. This is to prevent an unfavorable
anode/cathode ratio should any discontinuity in the coating occur.
5. Anodic parts may be made thicker to provide a longer life.
6. A sacrificial metal may be installed that would be anodic to both
metals.
3.4 Crevice Corrosion
Crevice corrosion is a localized type of corrosion occurring within or adjacent
to narrow gaps or openings formed by metal-to-metal or metal-to-nonmetal
contact. It results from local differences in oxygen concentrations, associated