State of Technology Report for Force Main Rehabilitation, Final ...

in currents or potentials indicate a problem. There are two principal types of cathodic protection system:
impressed current and sacrificial anode as follows:
• Impressed Current Cathodic Protection. An impressed current system uses a rectifier to
convert alternating current to direct current. This current is sent through an insulated wire to
the anodes, which are special metal bars buried in the soil near the pipeline. The current then
flows through the soil to the pipeline and returns to the rectifier through an insulated wire
attached to the pipeline. The pipeline system is protected because the current going to it
overcomes the corrosion-causing current normally flowing away from it.
• Sacrificial Anode Cathodic Protection. Sacrificial or galvanic anode systems employ reactive
metals as auxiliary anodes that are directly electrically connected to the pipeline to be
protected. The difference in natural potentials between the anode and the pipeline metal, as
indicated by their relative positions in the electro-chemical series, causes a positive current to
flow in the electrolyte, from the anode to the pipeline. Thus, the whole surface of the
pipeline becomes more negatively charged and becomes the cathode. The metals commonly
used as sacrificial anodes are aluminum, zinc, and magnesium. These metals are alloyed to
improve their long-term performance and dissolution characteristics.
Maintenance and monitoring of cathodic protection systems is seen as very important by the operating
utilities. System performance can be monitored by measuring the supplied current, by measuring the
potential of the structure, or (preferably) by a combination of the two methods. Scheduled maintenance
may include inspection and adjustment of equipment items, such as current rectifiers or anodes;
unscheduled maintenance may include troubleshooting and repair of items identified as defective during
scheduled inspections, such as anode beds or electrical conductors.
Cathodic protection systems need to be checked at least once every two to four years to make sure they
are functioning. Unfortunately, all too often, the systems are ignored until there is a failure in the
assumed to be protected pipeline. Testing the system is relatively straightforward, but special equipment
is necessary to perform the test. Each cathodic protection system has a test box(es) installed to facilitate
checking the system. Transformer rectifier outputs may be displayed by telemetry at central control
stations. Many cathodic protection systems are increasingly being controlled and monitored by remote
computers and modem links. An important aspect of good maintenance techniques is record keeping.
Without proper record keeping, a maintenance program is essentially useless. Proper record keeping not
only provides historical data for future cathodic protection design, but also often provides clues as to the
source of a detected deficiency. The required recordkeeping for proper maintenance is relatively simple.
6.1.3 Continuous Corrosion Monitoring. Knowing where conditions exist that can give rise to
corrosion is an important element in buried pipeline operation, including force mains. Corrosion of
ferrous pipes may be external or internal.
Likely locations of internal corrosion in force mains are at high points where air may be present and at
downstream discharges. Well-designed pressure pipe systems have air valves at high points and records
showing frequent air valve operation and significant volumes of air being bled can indicate potentially
corrosive conditions. Most force mains discharge to gravity lines and at the discharge point the pipe
seldom runs full. The presence of air can cause corrosion at these locations.
The risk of external corrosion can be identified from knowledge of the soil and groundwater conditions.
Procedures for establishing the corrosivity of soils are well established. Environmental factors that
determine the likelihood of corrosion on a buried ferrous force main are: soil resistivity; soil moisture
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