2.10. Minimizing galvanic corrosion in design.-
Galvanic corrosion can be minimized in design.
Corrosion engineers have found the following
practical rules invaluable in this respect:
Galvanic corrosion can be minimized in design.
Corrosion engineers have found the following
practical rules invaluable in this respect:
(a) Select combinations of metals which will
be in electrical contact from groups as close
together as possible In the galvanic series.
be in electrical contact from groups as close
together as possible In the galvanic series.
(b) Electrically insulate from each other
metals from different groups, wherever
practical. If complete insulation cannot be
achieved, paint or plastic coating at joints will
help.
metals from different groups, wherever
practical. If complete insulation cannot be
achieved, paint or plastic coating at joints will
help.
(c) If you must use dissimilar materials well
apart in the series, avoid joining them by
threaded connections as the threads will
probably deteriorate excessively. Brazed or
thermit joints are preferred, using a brazing
alloy more noble than at least one of the
metals to be joined.
apart in the series, avoid joining them by
threaded connections as the threads will
probably deteriorate excessively. Brazed or
thermit joints are preferred, using a brazing
alloy more noble than at least one of the
metals to be joined.
(d) Avoid making combinations where the
area of the less noble, anodic metal is rela-
tively small compared with the area of the
more noble metal.
area of the less noble, anodic metal is rela-
tively small compared with the area of the
more noble metal.
(e) Apply coatings with judgment. Example:
Do not paint the less noble metal without also
painting the more noble; otherwise, greatly
accelerated attack may be concentrated at
imperfections in coatings on the less noble
metal. Keep such coatings in good repair.
Do not paint the less noble metal without also
painting the more noble; otherwise, greatly
accelerated attack may be concentrated at
imperfections in coatings on the less noble
metal. Keep such coatings in good repair.
(f) Consider use of cathodic protection.
3. Stray current corrosion
3.1. Description.- Stray currents which cause
corrosion may originate from direct-current
distribution lines, substations, or street railway
systems, etc., and flow into a pipe system or
other steel structure. Alternating currents very
rarely cause corrosion. The corrosion resulting
from stray currents (external sources) is similar
to that from galvanic cells (which generate their
own current) but different remedial measures
may be indicated. In the electrolyte and at the
metal-electrolyte interfaces, chemical and elec-
trical reactions occur and are the same as those
in the galvanic cell; specifically, the corroding
metal is again considered to be the anode from
which current leaves to flow to the cathode. Soil
and water characteristics affect the corrosion
corrosion may originate from direct-current
distribution lines, substations, or street railway
systems, etc., and flow into a pipe system or
other steel structure. Alternating currents very
rarely cause corrosion. The corrosion resulting
from stray currents (external sources) is similar
to that from galvanic cells (which generate their
own current) but different remedial measures
may be indicated. In the electrolyte and at the
metal-electrolyte interfaces, chemical and elec-
trical reactions occur and are the same as those
in the galvanic cell; specifically, the corroding
metal is again considered to be the anode from
which current leaves to flow to the cathode. Soil
and water characteristics affect the corrosion
rate in the same manner as with galvanic-type
corrosion. However, stray current strengths may
be much higher than those produced by
galvanic cells and, as a consequence, corrosion
may be much more rapid. Another difference
between galvanic-type currents and stray
currents is that the latter are more likely to
operate over long distances since the anode
and cathode are more likely to be remotely
separated from one another. Seeking the path
of least resistance, the stray current from a
foreign installation may travel along a pipeline
causing severe corrosion where it leaves the
line. Knowing when stray currents are present
becomes highly important when remedial
measures are undertaken since a simple
sacrificial anode system is likely to be inef-
fectual in preventing corrosion under such cir-
cumstances.
corrosion. However, stray current strengths may
be much higher than those produced by
galvanic cells and, as a consequence, corrosion
may be much more rapid. Another difference
between galvanic-type currents and stray
currents is that the latter are more likely to
operate over long distances since the anode
and cathode are more likely to be remotely
separated from one another. Seeking the path
of least resistance, the stray current from a
foreign installation may travel along a pipeline
causing severe corrosion where it leaves the
line. Knowing when stray currents are present
becomes highly important when remedial
measures are undertaken since a simple
sacrificial anode system is likely to be inef-
fectual in preventing corrosion under such cir-
cumstances.
3.2. Detection of stray currents.- Detection of
stray currents which may be causing corrosion
is somewhat involved and involves technical
operations for which field staffs are usually not
equipped. Their presence may be suspected
when large direct-current installations are in the
vicinity of the structure experiencing corrosion
and especially when very rapid corrosion oc-
curs. The services of a corrosion specialist
should then be requested.
stray currents which may be causing corrosion
is somewhat involved and involves technical
operations for which field staffs are usually not
equipped. Their presence may be suspected
when large direct-current installations are in the
vicinity of the structure experiencing corrosion
and especially when very rapid corrosion oc-
curs. The services of a corrosion specialist
should then be requested.
4. Protective coatings
4.1. Coatings and corrosion cells.- Protective
coatings are widely used to prevent corrosion,
and they serve this function by interposing a
mechanical and often electrical barrier between
the metal surface being protected and the
corrosive environment. As long as the barrier
remains intact, corrosion usually will not
progress. Viewed from the standpoint of the
corrosion cell such as a battery or corroding
pipeline, an organic coating acts rather as an
envelope insulating an electrode away from the
electrolyte, thus ideally removing that electrode
from contact and braking the electrical circuit of
the cell. However, coatings may be damaged
mechanically during installation, they de-
teriorate at varying rates with time, and high
cathodic or stray currents may destroy their
bond and continuity. Further, some coatings
offer little or no electrical resistance. In
practice, then, the
coatings are widely used to prevent corrosion,
and they serve this function by interposing a
mechanical and often electrical barrier between
the metal surface being protected and the
corrosive environment. As long as the barrier
remains intact, corrosion usually will not
progress. Viewed from the standpoint of the
corrosion cell such as a battery or corroding
pipeline, an organic coating acts rather as an
envelope insulating an electrode away from the
electrolyte, thus ideally removing that electrode
from contact and braking the electrical circuit of
the cell. However, coatings may be damaged
mechanically during installation, they de-
teriorate at varying rates with time, and high
cathodic or stray currents may destroy their
bond and continuity. Further, some coatings
offer little or no electrical resistance. In
practice, then, the
5 (FIST4- 5)