Page 265 - Corrosion Engineering Principles and Practice

238 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 239

(c)
FIGURE 7.30 (continued)

Impressed current cells in which the soil is the electrolyte are of
two kinds: accidental and deliberate. An example of a deliberate cell
is an impressed current system which supplies cathodic protection to
increase the life of a structure by forcing an anodic current to anodes
in a remote location. Accidental systems, however, may exist under a
variety of circumstances. Any direct current flowing in the soil from
any source whatsoever can, if it finds a pipeline or other metal object
in its path, collect on the pipe in one area and discharge from it in
another. The area where it collects becomes a cathode while the area
where it discharges is an anode, and thus corrosion occurs.
7.4.6 Stress Cells
If an ordinary nail is dropped into a vessel of salt water, it will be
attacked, and, after some time, rust will be observed discoloring the
water. The initial points of attack will be, in almost every case, the
point and the head. This is an example of a type of cell known as a
stress cell. Figure 7.31 shows how dramatic this effect can be on the
heads and nuts used to hold fire hydrants on their base in the soil. In
these cases the two electrodes are of the same metal and the electrolyte
is uniform. The difference lies in that one electrode is more mechanically
stressed than the other. The area of high stress is always the anode of
the cell, made that way by the extra energy supplied by the stress
itself, and the corrosion reaction is the favored stress relieving path.