CATHODIC PROTECTION
Cathodic protection is a secondary line of defence against corrosion, the primary defence being
the coating. When damage to the coating occurs e.g. through impact on the coating during back
filling on a pipeline, sling damage during the lowering in operation, or flotsam impact on an
offshore platform leg, the underlying steel can then be in contact with electrolyte and corrosion
can occur. But if these areas can become cathodic i.e. receive current, corrosion can be avoided.
In order for cathodic protection to be applied, an electrolyte must be present. For example the
external surface of a tank cannot have cathodic protection, but internal surfaces can if the tank is
holding an electrolytic medium, but only up to the level of medium, not above. Underground
and subsea pipelines can be protected, but steelwork above ground in an AGI needs painting.
Cathodic protection can be applied in one of two ways.
a) Sacrificial Anodes Systems.
b) Impressed Current Systems.
Sacrificial anode systems
This system sometimes called, Galvanic Anode System, works on the principle of bimetallic
corrosion, the natural potential between metals. Any metal which is more electronegative (less
noble) or below steel on the galvanic list can be used as an anode. The choice of metal used
would depend upon the potential required to protect the prescribed area. Sacrificial systems
/
only protect small areas and the anodes need changing regularly as they corrode away.
Connecting wire of
copper. Minimum
resistance
Aluminium zinc or
magnesIUm or
alloys of these
Figure 11.1 Sacrificial system
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Impressed current system
The impressed current system is used to protect long lengths of pipeline from one installation, a
distance of approximately 10 miles. The current needed to run the system comes from the
national grid and is connected through a transformer rectifier (TR). The national grid is very
high voltage and very high amperage and also AC. Anti-corrosion currents need to be DC. The
TR rectifies the current to DC and transforms it to low voltage and amperage. The positive side
of the TR is connected to a ground bed (anode system) and the negative to the pipe, making the
pipe the cathode.
The current is released into the electrolyte at the ground bed, passes through the electrolyte and
is received at areas of coating damage on the pipe.
A typical ground bed will be approximately 50 m in length, at the same depth as, and running
parallel to the pipe. The cables carrying the current are of a substantial diameter and pure
copper to produce a circuit of Iittle or no resistance at the anode. The resistance encountered
comes in the soil/clay/rock bearing the electrolyte and this will govern the driving voltage ~
required, and the number of anodes required to maintain negative potential on the buried pipe.
The voltage required varies but is usually within the range of 10v to 50v at an amperage of
around 0.15 amps. A CP system does not eliminate corrosion, it controls where corrosion
occurs.
To national grid
supply
TR.
Transformer
rectifier
Current received at
cathode. Protected.
Ground bed
releases current
into electrolyte
Figure 11.2 Impressed current system
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Interference
When a buried steel structure is near to, or in the case of another pipeline, passes over or below a
pipeline which is cathodically protected, problems can occur. This is "interference" but the term
can be misleading. The offending structure does not adversely affect the CP system, but instead
is affected by it.
The "interference" structure picks up current released from the anode bed and conducts the
current through a circuit of minimal resistance and releases the current again into the electrolyte
near to the protected line. The interference therefore becomes a secondary anode and can suffer
severe corrOSIon.
Ifthere is a possibility of a structure becoming interference then precautions need to be taken to
avoid this eventuality. With the permission of the owner of the offending structure, three main
methods can be employed.
Attach isolation joints one pipe length either side of the nearest point of the offending line
to the protected line. Join the two pipe lengths to the protected line with insulated wire
and doubler plates, thus making them the same potential.
2 Attach isolation joints to both lines, one pipe length either side of the nearest point. Join
the two isolated sections together and install a sacrificial anode to protect both sections.
3 Double wrap and contra-wrap the protected line giving four tape thicknesses with Cold
Applied Laminate Tape for one pipe length either side of the nearest point.
The method chosen would be at the discretion of the engineer.
Monitoring CP
It is considered that -850 mv will maintain a pipeline in a passive state but most CP engineers
will require a more negative value, -1 to -2v being typical. To ensure that the required potential
is being maintained, checks need to be carried out at regular intervals. One method of
monitoring is known as half-cell reference electrode. The most commonly used half-cel]
electrode is the copper/copper sulphate half-cell electrode. It is used for measuring the pipe to
earth potential, i.e. cathode to earth, the other half of the circuit being anode to earth.
Periodically along the line, CP monitoring posts are installed, with a direct wire connection to
the pipe, accessed trom a stud on the CP post panel. A voltmeter is connected to the stud and to
the copper/copper sulphate half-cell, which is then pushed into the earth directly above the pipe.
This provides a circuit for electrons from the pipe, into the electrolyte, back to the anode bed.
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D Half cell reference
o 0
electrode filled with
copper sulphate
solution
Voltmeter
CP post
Ground level
Porous plug
f
Pipe
Figure I 1.3 Monitoring CP
Cathodic disbondment
Part of the electrical circuit of the corrosion reaction is the evolvement of Hydrogen gas from
the cathode. Hydrogen is a very powerful gas and can cause cracking in steel, (HICC). If
Hydrogen gas can penetrate underneath a coating it can easily disbond it. This is known as
Cathodic or Hydrogen Disbondment. Over protection of damaged areas on a pipe, results in
over production of Hydrogen and subsequent disbondment of more of the coating, resulting in a
bigger area to protect, needing more current.
All material used on a pipeline have to undergo tests to determine their resistance to cathodic
disbondment. The test is done in the following manner.
A 6 mm diameter hole is drilled into a plate coated with the material to be tested, through the
coating and into but not through the underlying steel. A short length, approximately 50 mm of
plastic tube approximately 50 mm diameter is fixed in position, using typically araldite epoxy or
elastomeric sealant with the drilled hole central to the tube. This is then part filled with 3%
solution of common salt, sodium chloride, and a lid fitted. The lid can be machined from a
block of polyethylene with a suitable diameter hole drilled through. The plate is connected to
the negative pole of a battery, an anode is connected to the positive pole and inserted through the
hole in the lid into the salt solution.
When the circuit is switched on the plate is the cathode and Hydrogen (and Chlorine) will be
evolved from the steel, and also at the interface of steel/coating. This enables Hydrogen to
penetrate under the coating, simulating areas of coating damage.
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The circuit is stopped after 28 days stripped down, dried off, and using a craft knife, two cuts are
made at an inclusive angle of approximately 30° radiating from the centre of the hole, through
the coating to the substrate. Where disbondment has occurred the coating will chip of as the
cuts are being made. The distance from the edge of the hole to the extent of the disbondment is
measured and should not exceed the stated requirements. For example FBE maximum 5mm
after 28 days.
Plastic ring Lid
Elastomeric
sealant
Battery
8
Coating
Plate
6 mm diameter Salt
hole solution
Figure 11.4 Cathodic disbondment
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