Download Lecture 17 Cathodic Protection – Influencing Factors and

Lecture 17: Cathodic Protection – Influencing Factors and Monitoring
NPTEL Web Course
Lecture 17
Cathodic Protection – Influencing Factors and Monitoring
Keywords: Coated Surfaces, Protection Criterion, Anode Materials, Pipeline Protection.
For large structures such as underground pipe lines, impressed current cathodic
protection is used, while for smaller structures such as house-hold water tanks, ship’s
hull etc, sacrificial anodes can be effectively used. Painting of steel pipe lines and
tubes can significantly reduce protection current requirements and thus save cost.
Approximate current requirements for cathodic protection of steel pipes are given
below:
Uncoated in flowing sea water
10-15 mA/ft2
Well-coated in water
0.01-0.003 mA/ft2
Excellently coated and exposed to water
or under soil
0.0003 or less mA/ft 2
As can be seen above, good surface coating significantly reduces protection current
requirements.
Electrochemical basis for protection criterion can be assessed:
Protection of steel is taken as example:
Fe = Fe++ + 2e
E0 = - 0.44 V
When polarized to half –cell potential of above reaction, corrosion rate reduces to 0.
Rate of forward and reverse reaction are same when net reaction rate is zero.
Eh = - 0.44 +
log [Fe++]
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Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore
Lecture 17: Cathodic Protection – Influencing Factors and Monitoring
NPTEL Web Course
Fe++ + 2OH- = Fe (OH)2
Calculated potential (based on solubility product) is -0.59V (SHE) which
corresponds to about -0.90V (vs Cu/CuSO4).
Accepted criterion for protection of steel in water is -0.85V (vs Cu/CuSO4).
Potential of structure to environment is generally measured using Cu/CuSO 4
reference electrode. Test coupons made of same metal and previously weighed can
be electrically connected to protected structures. These coupons are also exposed to
same cathodic current in the corrosive environment. Estimation of weight losses of
such coupons is a better proof of cathodic protection.
Table 17.1 Potentials for Cathodic protection (Cu/CuSO 4 electrode)
Iron and Steel
-0.85 to -0.95 V
Lead
-0.6 V
Copper and alloys
-0.5 to -0.66 V
Aluminium
-0.95 to -1.2 V
2
Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore
Lecture 17: Cathodic Protection – Influencing Factors and Monitoring
NPTEL Web Course
Anode materials that can be used as ground-beds in impressed current cathodic
protection are given Table 17.2
Table 17.2 Anode materials for impressed current cathodic protection
Material
Cast Iron
Average
consumption rate
kg/A-year
5–7
Steel scrap
5-8
Aluminium
4–5
Graphite
0.6 – 1.0
Lead
-----
Platinum
-----
Magnesium, zinc and aluminium and their alloys can be used as sacrificial anodes.
Design considerations for both impressed current and sacrificial anode systems have
some common steps.
a) Area to be protected –
Exposed areas of the structure – in coated system, exposed area at breaks and
deteriorated coatings.
b) Polarised potential – Current density based on area need be estimated.
c) Current demand – Current – density demands depend on the environment and
nature of surface coating.
d) Anode consumption – Required number and weights of anode materials
determined from known consumption rates for the desired current demand.
Anode number and distribution for the protected structure can be thus
estimated.
Anode resistance and design output current can then be estimated.
3
Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore
Lecture 17: Cathodic Protection – Influencing Factors and Monitoring
NPTEL Web Course
Monitoring of effectiveness of pipeline protection
Most widespread method is based on potential measurements of a cathodically
polarized structure with reference to a standard electrode. A potential of -0.85V (Cu
/ CuSO4) is sufficient for protection of steel in soil and natural water environments.
It may however be borne in mind that the above criterion is not optimum and
situations may arise when more negative (upto – 1.0V) may be required or even
lower (-0.7V) potential may suffice for protection. Interference from IR components
can introduce errors in pipeline potential measurements. Elimination of IR drop can
be achieved using ‘switch – off’ method. Potential measurements in chosen control
points in a pipeline are frequently insufficient to ensure effective protection. Close
Interval Potential Survey (CIPS) is an intensive monitoring technique based on
connecting a thin cable to a pipeline to monitor frequent potential readings all the
way. Special computer software together with appropriate instrumentation can be
used for gathering and processing the data. Another technique called Direct Current
Voltage Gradient (DCVG) method enables protection evaluation and also detection
of defects in insulation. Potential gradient is monitored in the soil with a sensitive
potential measurement meter using two reference electrodes kept at both sides of the
pipeline at shorter distances.
Corrosion coupons (probes) are generally used for monitoring of cathodic protection.
A schematic representation of a coupon probe connected to a cathodically protected
pipeline is illustrated in Fig. 17.1 . The arrangement allows measurement of switchoff potential without any interruption of pipeline protection.
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Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore
Lecture 17: Cathodic Protection – Influencing Factors and Monitoring
NPTEL Web Course
Fig 17.1 Circuit for monitoring cathodic protection.
Different types of simulation probes are available for determination of :
a) Level of protection in sections in casing pipes.
b) Polarization resistance and depolarization rate.
c) Insulation coating resistance.
d) Any interference on neighbouring underground installations.
e) Corrosion rate of protected structures.
Such probes need be located in various geological locations through a running
pipeline. Recently kinetic cathodic protection criterion has been proposed to allow
maintenance of metal corrosion rate at a desired level. There are several pipeline
corrosion rate control methods including both physical and electrochemical
techniques, which allow determination of effective protection in chosen regions of
structures.
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Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore
Lecture 17: Cathodic Protection – Influencing Factors and Monitoring
NPTEL Web Course
Table 17.3 Corrosion rate control in pipelines.
Electrochemical
Impedance
spectroscopy
Electrochemical noise
Harmonic synthesis
Polarization curves
Polarization
resistance
Physical
Electrical resistance
Radiography
Ultrasonic
Weight loss determination
There are several developments in cathodic protection instrumentation.
Use of
thyristor – controlled rectifiers will enable automatic control of current output
depending on corrosive environment requirements.
There is also a possibility of controlled potential cathodic protection to suit specific
structures. For example, in sea-going vessels, the hull is subjected to variations in
flow velocities leading to alteration in limiting current density (with respect to
oxygen reduction).
Such limiting current fluctuations significantly influence
cathodic protection current requirements from time to time. In such environments,
controlling the potential (rather than current) would be more beneficial. Controlled
potential protection is extensively used for ship hulls incorporating anode –
reference electrode attachment along with automatically – controlled power supply
unit.
6
Course Title: Advances in Corrosion Engineering
Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore