Acid
Containing an excess of hydrogen ions over hydroxyl ions.
Alkaline
Containing an excess of hydroxyl ions over hydrogen ions.
Anode
The electrode through which direct current enters an electrolyte.
Anodic Area
The part of the metal surface that acts as an anode.
Bond
A piece of metal conductor, either solid or flexible, usually of copper, connecting two points on the same or on different structures, to prevent any appreciable change in the potential of the one point with respect to the other.
Calomel Reference Electrode
A reference electrode consisting of mercury and mercurous chloride (calomel) in a standard solution of potassium chloride.
Carbonation
The chemical reaction between carbon dioxide and the calcium hydroxide present in Portland cement.
Cathode
The electrode through which direct current leaves an electrolyte.
Cathodic Area
The part of the metal surface that acts as a cathode.
Cathodic Protection
A means of rendering a metal immune from corrosive attack by causing direct current to flow from its electrolytic environment into the entire metal surface.
Cell
A complete electrolytic system comprising of a cathode and an anode in electrical contact with an intervening electrolyte.
Conductor
A substance (mainly a metal or carbon) in which electric current flows by the movement of electrons.
Continuity Bond
A bond designed and installed specifically to ensure the electrical continuity of a structure.
Copper/Copper Sulphate Reference Electrode
A reference electrode consisting of copper in a saturated copper sulphate solution.
Corrosion
The chemical or electrochemical reaction of a metal with its environment, resulting in its progressive degradation or destruction.
Corrosion Interaction
The increase or decrease in the rate of corrosion, or tendency towards corrosion, of a buried or immersed structure caused by the interception of part of the cathodic protection current applied to another structure or current from other source.
Current Density
The current per unit geometrical area of the protected structure, in contact with the electrolyte.
Electrode
A conductor of the metallic class (including carbon) by means of which current passes to or from an electrolyte.
Electrolyte
A liquid, or the liquid component in a composite material such as soil, in which the electric current flows by movement of ions.
Electronegative
The state of a metallic electrode when its potential is negative with respect to another metallic electrode in the system.
Electropositive
The state of a metallic electrode when its potential is positive with respect to another metallic electrode in the system.
Electrolyte
A liquid, or the liquid component in a composite material such as soil, in which the electric current flows by movement of ions.
Electro-osmosis
The passage of a liquid through a porous medium under the influence of a potential difference.
Galvanic action
A spontaneous chemical reaction which occurs in a system comprising a cathode and an anode in electrical contact and with an intervening electrolyte, resulting in corrosion of the anode.
Groundbed
A system of buried or submerged electrodes connected to the positive terminal of an independent source of direct current, in order to lead to earth the current used for the cathodic protection of a buried or immersed metallic structure.
Holiday
A defect in a protective coating at which metal is exposed.
Hydration (of cement)
The chemical and physical reactions between cement and water from which the material derives its strength.
Immunity
Potential shifts on the monitoring reference cells being more negative than -720 mV on interrupting the current. This is the standard used for steel in seawater and is the value at which corrosion is thermodynamically impossible since the anode reaction cannot be supported.
Impressed Current
The current supplied by a rectifier or other direct-current source (specifically excluding a sacrificial anode) to a protected structure in order to attain the necessary protection potential.
Instantaneous-off Potential
The structure/electrolyte potential measured immediately after the synchronous interruption of all sources of applied cathodic protection current.
Interaction test
A test to determine the severity of corrosion interaction between two buried or immersed structures.
Ion
An atom, or group of atoms, carrying a positive or negative electrical charge.
Neutral
Containing equal concentrations of hydrogen ions and hydroxyl ions.
Passivity
The state of the surface of a metal or alloy susceptible to corrosion where its electrochemical behavior becomes that of a less reactive metal and its corrosion rate is reduces.
pH value
A logarithmic index for the concentration of hydrogen ions in an electrolyte.
Pitting
A non-uniform corrosion of a metal whereby a number of cavities, not in the form of cracks, are formed in the surface.
Polarisation
Change in the potential of an electrode as the result of current flow.
Polarisation Cell
A device inserted in the earth connection of a structure that drains only a small current from the source used to provide cathodic protection for the structure, but provides a low resistance path to currents from high d.c. voltages and all a.c. voltages carried by the structure.
Primary Structure
A buried or immersed structure cathodically protected by a system that may constitute a source of corrosion interaction with another (secondary) structure.
Protected Structure
A structure to which cathodic protection is applied.
Protection Current
The current made to flow into a metallic structure, with respect to a specified reference electrode in an electrolytic environment, has to be depressed in order to effect cathodic protection of the structure.
Protection Potential
The more negative level to which the potential of a metallic structure, with respect to a specified reference electrode in an electrolytic environment, has to be depressed in order to effect cathodic protection of the structure.
Protective Coating
A dielectric material adhering to or bonded to a structure to separate it from its environment in order to prevent corrosion.
Reaction (anodic, cathodic)
A process of chemical or electrochemical change, particularly taking place at or near an electrode in a cell.
Redox Potential
The potential taken up by a platinum electrode with respect to a reference electrode.
Reference Electrode
An electrode the potential of which is accurately reproducible and which serves as a basis of comparison in the measurement of other electrode potentials. NOTE: Sometimes called a ‘half cell'.
Saponification
The chemical process of forming a soap; more particularly a deterioration by softening of paint films caused by the action of aqueous alkali on fatty-acid constituents of the film.
Silver/Silver Chloride Reference Electrode
A reference electrode consisting of silver, coated with silver chloride, in an electrolyte containing chloride ions.
Standard Hydrogen Electrode
A reference electrode consisting of an electro-positive metal, such as platinum, in an electrolyte containing hydrogen ions at unit activity and saturated with hydrogen gas at 1 atm.
Stray Current
Current flowing in the soil or water environment of a structure and arising mainly from cathodic protection, electric power or traction installations, and which can pass from the environment into the structure and vice versa.
Secondary Structure
A buried or immersed structure that may be subject to corrosion interaction arising from the cathodic protection of another (the primary) structure.
Structure/Electrolyte Potential
The difference in potential between a structure and a specified reference electrode in contact with the electrolyte at a point sufficiently close to (but without actually touching) the structure to avoid error due to the voltage drop associated with any current flowing in the electrolyte. NOTE: similar terms such as metal/electrolyte potential, pipe/electrolyte potential, pipe/soil (water) potentials etc., as applicable in the particular context are used.
Unprotected Structure
A structure to which cathodic protection is not applied.
Utilization Factor
That proportion of anode material on an anode that may be consumed before the anode ceases to provide a current output as required in the design.