Galvanic CP
Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making that
surface the cathode of an electrochemical cell.
It is a method used to protect metal structures from corrosion. Cathodic protection systems are
most commonly used to protect steel, water and fuel pipelines and storage tanks; steel pier piles,
ships, offshore oil platforms and onshore oil well casings.
A side effect of improperly performed cathodic protection may be production of molecular
hydrogen, leading to its absorption in the protected metal and subsequent hydrogen
embrittlement.
Cathodic protection is an effective method of preventing stress corrosion cracking.
Origins
The first use of CP was in 1824, when Sir Humphry Davy, of the British Navy,
attached chunks of iron to the external, below water line, hull of a copper clad
ship. Iron has a stronger tendency to corrode (rust) than copper and when
connected to the hull, the corrosion rate of the copper was dramatically reduced.
Today, galvanic or sacrificial anodes are made in various shapes using alloys of
zinc, magnesium and aluminium. The electrochemical potential, current capacity,
and consumption rate of these alloys are superior for CP than iron.
Galvanic anodes are designed and selected to have a more "active" voltage
(technically a more negative electrochemical potential) than the metal of the
structure (typically steel). For effective CP, the potential of the steel surface is
polarized (pushed) more negative until the surface has a uniform potential. At
that stage, the driving force for the corrosion reaction is halted. The galvanic
anode continues to corrode, consuming the anode material until eventually it
must be replaced. The polarization is caused by the current flow from the anode
to the cathode. The driving force for the CP current flow is the difference in
electrochemical potential between the anode and the cathode.
Impressed Current CP
For larger structures, galvanic anodes cannot economically deliver enough
current to provide complete protection. Impressed Current Cathodic
Protection (ICCP) systems use anodes connected to a DC power source (a
cathodic protection rectifier). Anodes for ICCP systems are tubular and solid rod
shapes or continuous ribbons of various specialized materials. These include
high silicon cast iron, graphite, mixed metal oxide, platinum and niobium coated
wire and others.
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A cathodic protection rectifier connected to a pipeline
A typical ICCP system for a pipeline would include an AC powered rectifier with a
maximum rated DC output of between 10 and 50 amperes and 50 volts. The
positive DC output terminal is connected via cables to the array of anodes buried
in the ground (the anode groundbed). For many applications the anodes are
installed in a 60 m (200 foot) deep, 25 cm (10-inch) diameter vertical hole and
backfilled with conductive coke (a material that improves the performance and
life of the anodes). A cable rated for the expected current output connects the
negative terminal of the rectifier to the pipeline. The operating output of the
rectifier is adjusted to the optimum level by a CP expert after conducting various
tests including measurements of electrochemical potential.
Telephone wiring uses a form of cathodic protection. A circuit consists of a pair of
wires, with forty-eight volts across them when the line is idle. The more positive
wire is grounded, so that the wires are at 0 V and -48 V with respect to earth
ground. The 0 V wire is at the same potential as the surrounding earth, so it
corrodes no faster or slower than if it were not connected electrically. The -48 V
wire is cathodically protected. This means that in the event of minor damage to
the insulation on a buried cable, both copper conductors will be unaffected, and
unless the two wires short together, service will not be interrupted.
If instead the polarity were switched, so that the wires were at 0 V and +48 V with
respect to the surrounding earth, then the 0 V wire would be unaffected as
before, but the +48 V would quickly be destroyed if it came into contact with wet
earth. The electrochemical action would plate metal off the +48 V wire, reducing
its thickness to the point that it would eventually break, interrupting telephone
service.
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Testing
Electrochemical potential is measured with reference electrodes. Coppercopper(II) sulfate electrodes are used for structures in contact with soil or fresh
water. Silver chloride electrodes are used for seawater applications.
Galvanized Steel
Galvanizing (or galvanising, outside of the USA) generally refers to hot-dip
galvanizing which is a way of coating steel with a layer of metallic zinc.
Galvanized coatings are quite durable in most environments because they
combine the barrier properties of a coating with some of the benefits of cathodic
protection. If the zinc coating is scratched or otherwise locally damaged and steel
is exposed, the surrounding areas of zinc coating form a galvanic cell with the
exposed steel and protect it from corrosion. This is a form of localised cathodic
protection - the zinc acts as a sacrificial anode.
References:
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NACE International (formerly the National Association of Corrosion
Engineers) - largest professional association of CP experts
US Army Corps of Engineers, "Engineering and Design - Cathodic
Protection Systems for Civil Works Structures", Engineering manual 11102-2704, 12 July 2004
Technical Idioms
Corrosion is deterioration of essential properties in a material due to reactions with its
environment..
A cathode is the electrode at which electrons go into a cell, tube or diode, whether driven
externally or internally..
An electrochemical cell is a setup used for creating an electromotive force (voltage) in a
conductor separating two reactions.
Steel is a metal alloy whose major component is iron, with carbon content between 0.02% and
1.7% by weight.
Pipeline transport is a transportation of goods through a pipe. Most commonly, liquid and gases
are sent, but pneumatic tubes that transport solid capsules using
A tank is a container, usually for liquids, sometimes for gases. Tanks can be many shapes, but
large tanks tend to be cylindrical, or to have rounded corners, to
Piles can mean:



A common name for hemorrhoids.
A town in Valencia
Deep foundations which connect a deep bearing strata to a structure.
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An oil platform is a large structure used to house workers and machinery needed to drill and
then produce oil and natural gas in the ocean.
oil well is a term for any perforation through the Earth's surface designed to find and release both
petroleum oil and gas hydrocarbons.
Hydrogen embrittlement is the process by which various metals, most importantly high-strength
steel, become brittle and crack following exposure to hydrogen.
Stress corrosion cracking (SCC) is the unexpected sudden failure of normally ductile metals
subjected to a constant tensile stress in a corrosive
Sir Humphry Davy, 1st Baronet, FRS (17 December 1778 – 29 May 1829), often incorrectly
spelled Humphrey, was an esteemed English chemist and physicist.
A hull is the body of a ship or boat. It is a central concept in floating vessels as it provides the
buoyancy that keeps the vessel from sinking.
Corrosion is deterioration of essential properties in a material due to reactions with its
environment.
Rust is the chemical produced when iron compounds corrode in the presence of oxygen and
water. It is a mixture of iron oxides and hydroxides.
A galvanic anode, a type of sacrificial anode, is one of the main components of a galvanic
cathodic protection system used to protect metals from corrosion.
A sacrificial anode, or sacrificial rod, is a metallic anode used in an cathodic protection where it
is intended to be dissolved to protect other metallic
An alloy is a combination, either in solution or compound, of two or more elements, at least one
of which is a metal, and where the resulting material has
In electrochemistry, the electrochemical potential is a thermodynamic measure that combines
the concepts of energy stored in the form of chemical potential and
Steel is a metal alloy whose major component is iron, with carbon content between 0.02% and
1.7% by weight.
An anode (from the Greek Üíïäïò = 'going up') is the electrode in a device that electrons flow out
of to return to the circuit.
Direct current (DC or "continuous current") is considered as the constant flow of electrons in the
single direction from low to high potential.
Reference electrode is an electrode which has a stable and well-known electrode potential. The
high stability of the electrode potential is usually reached by employing a redox system with
constant (buffered or saturated) concentrations of each participants of the redox reaction.
Reference electrodes are used to measure electrochemical potential.
Common reference electrodes and potential with respect to the standard hydrogen electrode:
Standard hydrogen electrode (E=0.000V)
Saturated calomel electrode (SCE) (E=-0.247V saturated)
Copper-copper(II) sulfate electrode (E=-0.318V)
Silver Chloride Electrode (E=-0.225V saturated)
pH-electrode (in case of pH buffered solutions, see buffer solution)
Palladium-Hydrogen electrode
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