Page 263 - Corrosion Engineering Principles and Practice

236 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 237

• The soil is the electrolyte
• The pipe itself is the connecting circuit

This is a particularly unfortunate type of cell because the high
temperature just outside the compressor tends to damage the coating,
so that the part of the line which is the anode is also likely to have the
poorest coating.
External corrosion of copper hot water tubing underslab has
resulted in a great deal of construction defect litigations due to this
type of corrosion cell [17]. Oil and gas well casings also experience
similar cell attack. The pipe at some depth below the surface is at a
higher temperature since temperature increases with depth and thus
becomes the anode. The cooler pipe near the surface and the surface
piping make up the cathode, the soil is the electrolyte, and the pipe is
the electrical connection.

7.4.5 Stray Current Cells
In all the cells described so far, the source of the energy which makes
a cell active has been within the cell. However, in stray current cells
the energy comes from an electrical current external to the corrosion
site per se. The source of energy for such cells may be a distant
generator, a direct-current transmission line, a cathodic protection
rectifier on some other line, a street car system, or an electric railway
(Fig. 7.29).
The corrosion effects of stray current can be easily demonstrated
with a simple laboratory setup such as shown in Fig. 7.30(a). After
only a few minutes of passing a small DC current in the cell
containing a dilute saline solution, the formation of hydrogen
bubbles is readily visible on the steel nail connected to the negative
post of the DC power supply [Fig. 7.30(b)], while the nail connected
to the positive post shows signs of rapid corrosion a few minutes
later [Fig. 7.30(c)].

+

Substation
Rail current return path
–
Ground
Stray current Soil current return path
enters Stray current leaves
Buried pipeline

Stray current corrosion
FIGURE 7.29 Stray current corrosion due to the return path of an electric
railway.