The Equipment Design Process
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surface of the pipe and that, as a result, corrosion would be eliminated.
This would be true if an electrically insulating coating could be installed
that was 100% perfect at the time of installation and that could be maintained in a 100% perfect condition for the lifetime of the pipeline. This is
not a practical possibility under the usual pipeline construction conditions. As a result, there will be a certain number of coating defects where
the steel will be exposed to the pipeline environment. Although corrosion
may still be stopped on better than 99% of the pipeline surface, any corrosion current that does flow will be concentrated at coating defects, and the
rate of corrosion at these points may be such that pipeline penetration will
occur at an earlier date than would have been the case had the pipeline
been left bare (assuming coating to be the only method used).
11.3.6.2 Cathodic Protection
Once it was discovered that the use of coatings would not provide the
total answer to pipeline corrosion, the technique of cathodic protection
was introduced. This is an electrical method of combating corrosion in
that any corrosive currents caused by contact between the pipeline and
an electrically conductive ionic environment are prevented from discharging from the pipeline to the environment (with attendant corrosion) by nullifying them with a superimposed direct current flow from
an external source. When accomplished, the entire metallic structure
being protected is collecting direct current from its environment. By so
doing, the entire structure is forced to become a cathode in the electrical
circuit (and hence the name of this corrosion control method); and if the
condition is fully satisfied, the corrosion will stop. Although cathodic
protection can be applied to bare pipelines, the amount of current may
be so great in the case of large-diameter long pipelines that in addition
to the number and complexity of current sources necessarily installed
along the pipeline, there is a potential problem with stray current from
the high-capacity cathodic protection systems causing corrosion on adjacent underground metallic structures of other ownership. As a result
of this, time has proven that the best practical combination (for use in
the pipeline industry under normal conditions) is that of pipeline coating and supporting cathodic protection. As indicated earlier, a reasonably well-applied coating can be expected to protect better than 99% of
the pipeline surface. With cathodic protection, current from the cathodic
protection system flows only to less than 1% of pipe surface that involves
bare metal contacting the earth. As a result, a single cathodic protection
installation can, under normal circumstances, protect many miles of biginch-diameter coated pipeline with a minimum amount of current by
combining the two methods (i.e., coating and cathodic protection).
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surface of the pipe and that, as a result, corrosion would be eliminated.
This would be true if an electrically insulating coating could be installed
that was 100% perfect at the time of installation and that could be maintained in a 100% perfect condition for the lifetime of the pipeline. This is
not a practical possibility under the usual pipeline construction conditions. As a result, there will be a certain number of coating defects where
the steel will be exposed to the pipeline environment. Although corrosion
may still be stopped on better than 99% of the pipeline surface, any corrosion current that does flow will be concentrated at coating defects, and the
rate of corrosion at these points may be such that pipeline penetration will
occur at an earlier date than would have been the case had the pipeline
been left bare (assuming coating to be the only method used).
11.3.6.2 Cathodic Protection
Once it was discovered that the use of coatings would not provide the
total answer to pipeline corrosion, the technique of cathodic protection
was introduced. This is an electrical method of combating corrosion in
that any corrosive currents caused by contact between the pipeline and
an electrically conductive ionic environment are prevented from discharging from the pipeline to the environment (with attendant corrosion) by nullifying them with a superimposed direct current flow from
an external source. When accomplished, the entire metallic structure
being protected is collecting direct current from its environment. By so
doing, the entire structure is forced to become a cathode in the electrical
circuit (and hence the name of this corrosion control method); and if the
condition is fully satisfied, the corrosion will stop. Although cathodic
protection can be applied to bare pipelines, the amount of current may
be so great in the case of large-diameter long pipelines that in addition
to the number and complexity of current sources necessarily installed
along the pipeline, there is a potential problem with stray current from
the high-capacity cathodic protection systems causing corrosion on adjacent underground metallic structures of other ownership. As a result
of this, time has proven that the best practical combination (for use in
the pipeline industry under normal conditions) is that of pipeline coating and supporting cathodic protection. As indicated earlier, a reasonably well-applied coating can be expected to protect better than 99% of
the pipeline surface. With cathodic protection, current from the cathodic
protection system flows only to less than 1% of pipe surface that involves
bare metal contacting the earth. As a result, a single cathodic protection
installation can, under normal circumstances, protect many miles of biginch-diameter coated pipeline with a minimum amount of current by
combining the two methods (i.e., coating and cathodic protection).