Section 13
Cathodic Protection
13 Cathodic Protection
Cathodic protection is a secondary line of defence against corrosion, the
primary defence being the coating. When damage to the coating occurs eg
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 ie 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:
Sacrificial anodes systems.
Impressed current systems.
13.1 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.
Approximately
50m maximum Connecting wire of
copper. Minimum
resistance
Aluminium zinc or
magnesium or
+ alloys of these
Figure 13.1 Sacrificial system.
13.2 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.
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A typical ground bed will be approximately 50m 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 little 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 10-50v at an
amperage of around 0.15 amps. A CP system does not eliminate corrosion, it
controls where corrosion occurs.
To national grid
TR. supply
Transformer
rectifier
Current received
at cathode.
Protected.
Ground bed
releases
current into
Figure 13.2 Impressed current system.
13.3 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.
If there 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.
1 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.
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13.4 Monitoring CP
It is considered that -850mv 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-cell electrode is the
copper/copper sulphate half-cell electrode. It is used for measuring the pipe to
earth potential, ie 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 from 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.
Half cell
reference
electrode
filled with
Voltmeter
copper
CP post sulphate
solution
Ground
level
Porous plug
Pipe
Figure 13.3 Monitoring CP.
13.5 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 6mm 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 50mm of plastic tube approximately 50mm
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.
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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.
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 off 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.
Figure 13.4 Cathodic disbondment.
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