Page 607 - Corrosion Engineering Principles and Practice

570 C h a p t e r 1 3 C a t h o d i c P r o t e c t i o n 571

Depending on the severity of the stray current effect, the
magnitude of the anodic potentials at areas where stray current is
discharging from the affected structure and causing corrosion may be
such that they cannot be counteracted readily with the usual CP
system. This is particularly true where, at areas of current discharge,
the structure may be a matter of several volts positive with respect to
its environment. Such conditions necessitate special stray current
control techniques involving metallic bonds from the affected
structure to the source of the damaging current or other means of
removing the stray current without harm to the affected structure.
The stray current sources described in Chap. 7 can result in very
rapid corrosion which is usually much more severe than the corrosion
caused by other environmental factors (Fig. 13.34). Another type of
stray current which is variable in nature may be observed during
periods of “magnetic storm” activity. Long structures such as
pipelines or cables are most apt to be affected. During magnetic
storms, the intensity of the earth’s magnetic field can vary. When
these variations occur, potentials are induced in the pipe or cable in
much the same manner as potentials are induced in an electric
generator.
Man-made variable stray currents usually demonstrate some
patterns, as revealed by recording instrument records, showing rush
hour peaks on transit systems, shift changes in mining operations,
and so forth. However, variable stray currents resulting from magnetic
disturbances rarely, if ever, demonstrate any pattern and may be
active in a given area for a time without reappearing for a long time
thereafter. Although sometimes intense for short periods, stray
telluric currents resulting from magnetic induction will seldom cause
as much corrosion as uncontrolled man-made stray current because
the currents are of relatively short duration and usually are not
concentrated in any given area for any appreciable lengths of time.

FIGURE 13.34 Pipeline perforated by a large fault current. (Courtesy of
MACAW’s Pipeline Defects, published by Yellow Pencil Marketing Co)