Page 269 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

CORROSION OF UNDERGROUND GAS AND LIQUID TRANSMISSION PIPELINES 247

4.11.3 Mitigation of External Corrosion
Corrosion is an electrochemical phenomenon and hence can be controlled by altering
the electrochemical condition of the corroding interface. For external wall surfaces
altering the nature of the corroding surface is simply done by altering the voltage field
around the pipe. By applying a negative potential and making the pipe a cathode, the
rate of corrosion (oxidation) is reduced and the reduction process accelerated. This
means of mitigating corrosion is known as CP. CP may be achieved by one of two
primary systems such as sacrificial anode (galvanic anode) CP and impressed-current
CP. Sacrificial anode CP uses an anode material that is electronegative to the pipe
steel. When connected to the pipe, the pipe becomes the cathode in the circuit and
corrosion is mitigated. Typical anode materials for underground pipelines are zinc
and magnesium.
Impressed-current CP uses an outside power supply such as a rectifier to control
the voltage between the pipe and an anode (cast iron, graphite, platinum clad, mixed
metal oxide) in such a manner that the pipe becomes the cathode in the circuit and
corrosion is mitigated.
CP is often used in conjunction with a coating. There are always flaws in the coat-
ings, because of application inconsistencies, construction damage, or a combination
of natural aging and soil stresses. If left unprotected, corrosion will occur at these
coating flaws (holidays). Often, the rate of attack through the wall is much higher at
the holiday than the general attack of a bare metal surface. The use of a coating greatly
reduces the total amount of current required to achieve protection of the pipeline sys-
tem; therefore, CP and external coatings are used together wherever possible.
CP can be used to mitigate all forms of corrosion such as general, stray current,
MIC, and SCC. Sometimes it is difficult to determine the level of CP necessary to
mitigate the different corrosion mechanisms and to identify which type of corrosion
mechanism is present. Stress corrosion cracking (SCC) presents additional problems.
The high pH form of SCC is found only on pipelines protected with CP. The products
that result from cathodic reactions occurring on the pipe surface during CP in con-
junction with soil chemistry produce the environment necessary for high-pH SCC.
As high-pH SCC propagates only in a very limited potential range, maintaining the
potential of the pipe surface outside of this range by proper CP control will prevent
the growth of the high-pH SCC cracks. In addition, it has been established that proper
CP control can inhibit the growth of near-neutral SCC cracks.
Internal corrosion is also an electrochemical process; however, CP is not a viable
option for mitigating internal corrosion in a pipeline. One of the first defense sys-
tems against corrosion of transmission pipelines is to ensure that the product being
transported is free of moisture. Dry, deaerated natural gas and moisture-free oil and
petroleum products are not corrosive. In order for corrosion to occur, there must
be moisture, CO , oxygen, or some other reductant, such as the one produced by
2
microbes. Operators generally control moisture, oxygen, and CO contents of the
2
transported product, but these constituents can enter the pipeline through compressor
or pump stations, metering stations, storage facilities, or other means. Gathering lines