Anodic protection
Encyclopedia
Anodic protection is a technique to control the corrosion
of a metal surface by making it the anode
of an electrochemical cell
and controlling the electrode potential
in a zone where the metal is passive
.
AP is used to protect metals that exhibit passivation in environments whereby the current density
in the freely corroding state is significantly higher than the current density in the passive state over a wide range of potentials.
Anodic protection is used for carbon steel storage tank
s containing extreme pH
environments including concentrated sulfuric acid
and 50 percent caustic soda where cathodic protection
is not suitable due to very high current
requirements.
An anodic protection system includes an external power supply
connected to auxiliary cathode
s and controlled by a feedback from reference electrode
. Careful design and control is required when using anodic protection for several reasons, including excessive current when passivation is lost or unstable, leading to possible accelerated corrosion.
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
of a metal surface by making it the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
of an electrochemical cell
Electrochemical cell
An electrochemical cell is a device capable of either deriving electrical energy from chemical reactions, or facilitating chemical reactions through the introduction of electrical energy. A common example of an electrochemical cell is a standard 1.5-volt "battery"...
and controlling the electrode potential
Electrode potential
Electrode potential, E, in electrochemistry, according to an IUPAC definition, is the electromotive force of a cell built of two electrodes:* on the left-hand side is the standard hydrogen electrode, and...
in a zone where the metal is passive
Passivation
Passivation is the process of making a material "passive", and thus less reactive with surrounding air, water, or other gases or liquids. The goal is to inhibit corrosion, whether for structural or cosmetic reasons. Passivation of metals is usually achieved by the deposition of a layer of oxide...
.
AP is used to protect metals that exhibit passivation in environments whereby the current density
Current density
Current density is a measure of the density of flow of a conserved charge. Usually the charge is the electric charge, in which case the associated current density is the electric current per unit area of cross section, but the term current density can also be applied to other conserved...
in the freely corroding state is significantly higher than the current density in the passive state over a wide range of potentials.
Anodic protection is used for carbon steel storage tank
Storage tank
A storage tank is a container, usually for holding liquids, sometimes for compressed gases . The term can be used for reservoirs , and for manufactured containers. The usage of the word tank for reservoirs is common or universal in Indian English, American English and moderately common in British...
s containing extreme pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
environments including concentrated sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
and 50 percent caustic soda where cathodic protection
Cathodic protection
Cathodic protection is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. The simplest method to apply CP is by connecting the metal to be protected with another more easily corroded "sacrificial metal" to act as the anode of the...
is not suitable due to very high current
Current density
Current density is a measure of the density of flow of a conserved charge. Usually the charge is the electric charge, in which case the associated current density is the electric current per unit area of cross section, but the term current density can also be applied to other conserved...
requirements.
An anodic protection system includes an external power supply
Power supply
A power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy to electrical energy...
connected to auxiliary cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
s and controlled by a feedback from reference electrode
Reference electrode
A 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 concentrations of each participants of the redox reaction.There are many ways reference...
. Careful design and control is required when using anodic protection for several reasons, including excessive current when passivation is lost or unstable, leading to possible accelerated corrosion.