Page 331 - Advanced Mine Ventilation

308 Advanced Mine Ventilation

will serve as the cathode for the electrochemical cell. If resistivity is high, then not as
much current will flow. If the resistivity is higher in one area than in another, more
corrosion protection will be indicated in the area of low resistive soil.
Measurements for stray currents can also be made. If the magnitude of the currents
are known and the source of the currents are identified, then action can be taken to
eliminate the current at the source, reverse the polarity of the pipeline so that the stray
currents cannot enter the line, and/or provide the currents with a free path back to the
source or to ground.
The problem with dependence on protective coating is the determination of what
coating is suitable because it is not always possible to define the long-term corrosive
nature of the mine environment.
For example, the corrosivity of the gas transport atmosphere can change rapidly and
unexpectedly as will happen should hydrogen sulfide be introduced into the gas. The
corrosivity of water in the mine changes, too; it increases with increase in the velocity
of flow, with the amount of dissolved oxygen in the water, and with decrease in pH.
Soil corrosion is highly localized and is usually caused by electrochemical action
resulting from variations in the soil and the moisture therein; thus, the corrosivity of
the mine floor material may range considerably from point to point and from time
to time.
Other problems affecting dependence on coatings are that
1. The chipped or uncoated area of a coated pipe may become an area of concentrated corrosion.
That area becomes an electrochemical cell. Dissimilar materials in electrical contact accel-
erate corrosion of the one that happens to be anodic (in this case the chipped area). The
smaller the anodic area (or chip) in relation to cathodic areas (soil, supports, and other por-
tions of the pipe), the greater is the rate of penetration at the anodic points.
2. In the restricted confines of a mine, it is reasonable to expect that a pipe weighing many hun-
dreds of pounds will be banged and scraped; thus, it is also reasonable to expect that a coating
applied before the pipe was brought to its place in the mine will be chipped.
3. It is not reasonable to expect that after a pipeline is installed in a mine adequate anticorrosion
treatments and coatings can be applied on all surfaces.
4. Unless pipe is cleaned properly a coating provides little protection against corrosion and may
in fact cause accelerated, localized corrosion.
5. Some externally applied coatings, particularly the bitumen, degrade rapidly and unexpect-
edly. Many of these are combustible and would, if ignited, be the source of copious, toxic
smoke. Some are subject to soil stress. That is, soil may cling to the coating and pull it
from the pipe as the soil dries and shrinks; or, irregular soil pressures, imposed by both small
and large bearing forces, may distort the bitumen and cause thin spots. Some coatings are
quickly oxidized and become brittle when protective coatings are under water or are in con-
tact with highly alkaline or acid soils. Most protective coatings have a life expectancy less
than 10 years.

18.5.2 Cathodic Protection

The “best” solution to the corrosion problem that is likely to occur in coal mine degas-
ification pipelines is a combination of protective coatings and cathodic protection and
means to prevent stray electric currents from affecting the line.