Transcript METL 2441 - HCC Learning Web

Cathodic Protection Theory
Lecture 2
Summer 8A 2016
METL 2441
Cathodic Protection
Cathodic protection
• We have learned that wherever direct current flows from an
underground or submersed structure into an electrolyte, the metal in
the structure will be consumed by corrosion.
• If an insulating barrier were to be placed between the metal and the
electrolyte, current could not flow.
Cathodic protection
• In theory this is sound, but not practical in the real world.
• Such insulating barriers, or coatings, must be perfect when applied
and must remain perfect throughout the life of the structure.
Cathodic protection
• It is not practical to apply a coating that will meet this stringent
criterion.
• In fact, a coated structure often will suffer leaks before a structure
without coatings.
• Because current density is higher at the discharge points, coating
pinholes, of a coated structure than it is with uniform corrosion on a
bare structure.
Cathodic protection
• Metal loss from corrosion is less on a coated structure, but the
effects are more detrimental.
• Cathodic protection is a practical corrosion prevention method for
most external surfaces of structures submersed in an electrolyte.
• With coated structures the cost for cathodic protection is
considerably less than on bare structures.
Cathodic protection
• We learned that corrosion on the surface of a metal is due to voltage
difference between two points on the metal surface.
• The more active, or anodic point will discharge current from the
metal surface to the electrolyte, resulting in corrosion.
• If this current is reduced or eliminated, then corrosion is reduced or
eliminated.
Cathodic protection
• The theory of cathodic protection can be viewed from different
perspectives.
• Cathodic protection is actually a polarization phenomena.
• However, polarization is a complex subject and outside of the scope
of this course.
• It will be mentioned again when discussing criteria for adequate
cathodic protection.
Cathodic protection
• In this course we will take a simplistic view of cathodic protection as
a reduction in the voltage difference, or driving force of a corrosion
cell.
• We have learned from Ohm’s Law that voltage and current are
directly proportional to one another.
• If we reduce the voltage, then current will also be reduced and so will
corrosion.
Cathodic protection
• By electrically connecting an external electrode that is more active,
or negative, to a structure, current is impressed onto the structure.
• Impressing current onto the structure will result in a negative
potential shift on the structure.
Cathodic protection
• The first points on the structure to be affected by the current are the
more noble points.
• Because of the greater voltage difference between these points and
the external electrode.
• A cathodic protection system is an intentional corrosion cell.
Cathodic protection
• The four parts of the intentional corrosion cell are:
• Anode – External electrode
• Cathode – The structure
• Metallic Path – Electrical wire connection between the external electrode
and structure
• Electrolyte – Conductive medium between the external electrode and
structure
Cathodic protection
• In a cathodic protection system, direct current flows from the
cathodic protection anode into the electrolyte.
• Through the electrolyte onto the surface of the structure.
• Down the structure to the return wire.
Cathodic protection
• Through the return wire back to the cathodic protection anode.
• Which completes the electrical circuit.
• Once the entire exposed metal surface of the structure is collecting
current, the entire structure becomes a cathode, and corrosion
ceases.
Cathodic protection
• There are primarily two methods of providing cathodic protection
current to a structure.
• Galvanic anode system
• Impressed current system
CP
GALVANIC ANODE SYSTEMS
• Galvanic (or sacrificial) cathodic protection makes practical use of dissimilar
metal corrosion.
• It is important to note that there must be a substantial potential difference,
or driving voltage.
• Between a galvanic anode and the structure to be protected.
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GALVANIC ANODE SYSTEMS
• The galvanic anode is connected directly to the structure it is protecting.
• There are several metals commonly used as galvanic anodes and are as
follows.
• Aluminum
• Magnesium
• Zinc
Sacrificial Anode Installation (1)
Sacrificial Anode Installation (2)
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GALVANIC ANODE SYSTEMS
• ALUMINUM
• Aluminum anodes are used primarily in seawater applications
• They are produced in a variety of alloys, of which the mercury and
indium alloys are the most common.
FPSO
Ships Propeller
Offshore Anode Installation
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GALVANIC ANODE SYSTEMS
• Indium alloy has a slightly higher corrosion potential, but is less
efficient.
• Aluminum is preferred for seawater applications, where anode
volume is not a constraint.
• Aluminum has a low consumption rate.
• Aluminum anodes require chloride ions to function, so they are not
used in fresh water.
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GALVANIC ANODE SYSTEMS
• MAGNESIUM
• Magnesium anodes are available in two alloys.
• A high potential alloy having a nominal corrosion potential of -1.75 to
-1.77 volts referenced a copper/copper sulfate electrode.
• A low-potential alloy having a nominal corrosion potential of -1.55
volts referenced a copper/copper sulfate electrode.
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GALVANIC ANODE SYSTEMS
• Magnesium is normally used in soils and fresh waters.
• Although magnesium can be used in seawater, its consumption rate is
high.
• The higher potential has a tendency to cause overprotection on
certain structures (e.g., aluminum ship hulls).
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GALVANIC ANODE SYSTEMS
• At 50%, the efficiency of magnesium anodes is significantly less than
the efficiency of other anodes.
• This is due primarily to the activity of local-action corrosion cells on
the anode surface, which results in self corrosion of the anode.
• Although magnesium is relatively inefficient, its high driving
potential makes it the preferred anode in high-resistivity soil
applications.
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GALVANIC ANODE SYSTEMS
• ZINC
• Zinc anodes also are commercially available in two alloys.
• One for use in soils and fresh waters and the other for seawater
applications.
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GALVANIC ANODE SYSTEMS
• Zinc may undergo rapid intergranular corrosion at temperatures
above 120°F (49°C).
• The “high” amp alloy material generally should not be used in
stagnant aqueous environments.
• Because the primary activating element in the material is cadmium,
which may react badly in such environments.
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APPLICATION OF GALVANIC ANODES
• Principal cathodic protection system when relatively small increments of
current are required and/or low resistivity electrolyte exist.
• Local cathodic protection to provide current to a specific area on a structure.
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APPLICATION OF GALVANIC ANODES
• Some operators install galvanic anodes at each location where a leak
is repaired, rather than installing a complete cathodic protection
system.
• Such practices may be encountered on bare metal or very poorlycoated systems, where complete cathodic protection may not be
feasible due to cost.
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APPLICATION OF GALVANIC ANODES
• Typical applications include:
• Poorly or incompletely coated buried-valve installations.
• Shorted casings that cannot be cleared
• Isolated sections where the coating has been badly damaged.
• Areas where electrical shielding impairs effective current distribution
from remotely located impressed-current systems.
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APPLICATION OF GALVANIC ANODES
• In cases of cathodic interference, if conditions are suitable, galvanic
anodes can be used at discharge points on the foreign line to safely
return interfering currents.
• They can also be used to provide protection to structures located
near many other underground metallic structures.
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APPLICATION OF GALVANIC ANODES
• Galvanic anodes can be used where conditions makes it difficult to
install impressed-current systems without creating stray-current
interference problems.
• Galvanic anodes can be an economical choice for cathodic protection
under the above conditions.
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APPLICATION OF GALVANIC ANODES
• Galvanic anodes can be used where additional current is needed at
problem areas.
• Some structures with overall impressed-current cathodic protection
systems may have isolated points where additional current in
relatively small amounts is needed.
• Those requirements can be met with galvanic anodes.
CP
ADVANTAGES OF GALVANIC ANODES
• No external power source required.
• Low maintenance requirements.
• Small current output resulting in less stray-current interference.
• Easy to install.
• Easy to add anodes in most cases.
• Provide uniform distribution of current.
• Minimum right-of-way/easement costs.
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DISADVANTAGES OF GALVANIC ANODES
• Low driving voltage/current output.
• Many anodes may be required for poorly coated structures.
• May be ineffective in high-resistivity environments.
• Higher cost per unit ampere than impressed current due to lower
efficiency (self-consumption).
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GALVANIC ANODE EFFICIENCY
• The efficiency of a galvanic anode depends on the alloy of the anode
and the environment in which it is installed.
• The consumption of any metal is directly proportional to the amount
of current discharged from its surface.
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GALVANIC ANODE EFFICIENCY
• For galvanic anodes, part of the current discharge is due to the
cathodic protection current provided to the structure and part is
caused by local corrosion cells on it surface.
• Anode efficiency is the ratio of metal consumed producing useful
cathodic protection current to the total metal consumed.
• For magnesium, anode efficiency is as low as 50%.