INTRODUCTION
Periodically, due to corrosion and/or erosion, leaks and cracks occur which must be corrected immediately
after they happen. This lesson will explain the procedures involved.
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 take 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 ground wire of an electrical device such as an electrically powered pump.
Prevention of Internal Corrosion. A pipeline can corrode on the inside as well as the outside.
Internal corrosion is caused by the accumulation of moisture, wax, and mill scale in the pipeline. Friction in
the pipeline caused by the rapid movement of fuel products produces heat which speeds up the corrosion
process. Internal corrosion can be cut down in two ways.
Scrapers. One way is to run scrapers through the pipeline to prevent material from building up in the
line.
Corrosion inhibitors. The other method involves the use of corrosion inhibitors. Corrosion inhibitors are
chemicals added to fuels to prevent the formation of rust inside pipelines and storage tanks.
Prevention of External Corrosion. Several techniques are used to prevent external corrosion.
None is perfect, so a combination of several methods is usually used to cut down on corrosion as much as
possible.
Design. Corrosion control is considered before the pipeline is built. Soil studies are made, and the
planners try to avoid high acidic areas such as swamps and high alkaline areas such as clay deposits.
8-2
QM 5096
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