Page 240 - Corrosion Engineering Principles and Practice

214 C h a p t e r 7 C o r r o s i o n F a i l u r e s , F a c t o r s , a n d C e l l s 215

higher than the current produced by ordinary 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, stray currents from
a foreign installation may travel along a pipeline or any other buried
metallic structure causing severe corrosion where it leaves the line
or current carrier.
Stray currents causing corrosion may originate from direct-
current distribution lines, substations, or street railway systems to
damage a metallic structure somewhere in its vicinity as described in
more details later in the present chapter. Alternating stray currents
very rarely cause corrosion.
Investigating the presence of stray currents to prevent or explain
corrosion problems is not a new field in corrosion engineering. In
fact, as mentioned in App. A, such activities were carried out by
probably the first corrosion engineers in North America when the
American Committee on Electrolysis was established at the turn of
the twentieth century to combat the serious effects of railcar stray
currents to underground metal structures (Figs. 7.4 and 7.5).

FIGURE 7.4 An electrolysis surveyor using a McCollum earth current meter
from the early 1920s. (Courtesy of East Bay Municipal Utility District).