INTRODUCTION
Periodically, due to corrosion and/or erosion leaks and cracks occur
which must be corrected immediately.
PART A - CORROSION
Corrosion is the process of any refined metal returning to its natural
form. What happens during corrosion is similar to the chemical actions
that takes place in a battery. Negatively charged atomic particles
called electrons are given up by a metal called an anode. These
particles form an electrical current which passes through an electrical
conductor called an electrolyte. The electrons are attracted to another
metal called a cathode. The circuit is completed when current returns to
the anode by way of a metal connection. During this process the metal
acting as the anode breaks down and deteriorates. The metal acting as a
cathode is protected and suffers no damage.
Different and Same Metals.
Corrosion can involve two different metals.
Also, it can occur when the anode and cathode are the same metal. For
example, a newly cast metal will act as an anode to an older similar
metal. A metal in acidic surroundings will act as an anode to the same
metal in less acidic surroundings. A metal in an area lacking in oxygen
will act as an anode to the same metal in an oxygen rich atmosphere.
Causes of Pipeline Corrosion.
For corrosion to take place, there must
be an anode, a cathode, an electrolyte, and some metallic connection
between the anode and the cathode. Corrosion can occur in a pipeline
under the following conditions:
Different metals come in contact. This occurs where sections of pipe
are joined by couplings and where fittings and valves are used.
Moisture and chemicals in the surrounding ground and air act as the
electrolyte. The pipeline provides the necessary metallic connection
between the anode and cathode.
A new section of pipe is put in the place of an old one. The new pipe
acts as an anode to the neighboring old pipe.
The pipeline passes through different kinds of soil. Electrons will
move from pipe sections laid in high acidic soil to sections in low
acidic soil and from pipe sections in high alkaline areas to sections
in low alkaline areas. (A swamp is an example of an acidic area.
Ground made up of moist clay is alkaline.)
The pipeline moves from under ground to above ground, and it moves
from under water to above water.
The pipeline passes through fine soil. Fine soil holds more water and
dissolved minerals and acts as a better electrolyte than coarse sand
and earth.
Stray electrical currents from outside power sources flow into the
pipeline. They are carried along and then leave the pipeline to
reenter the soil. The area of the pipe where the stray current enters
will act as a cathode, and the area where it leaves will act as an
anode. An example of a possible source of stray currents is the
12-106
QM 5099
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