KNOWLEDGE LIBRARY
GLOSSARY OF TERMS
AC: Alternating current.
Ag/AgCl: Silver/silver chloride reference electrode. Potential dependent on concentration of chloride ions within the electrode system. Silver/silver chloride reference electrodes can be either open (sea water electrodes) or sealed. When sealed, chloride concentration is declared in terms of molar concentration typically ranging from 0.5M to 3.8M (saturated).
Ag/AgCl/KCl(xM): Silver/silver chloride/potassium chloride junction reference electrode, with chloride at known concentration x M (x mol/dm-3), typically ranging from 0.5M to 3.8M (saturated); a difference of 1M in concentration changes the potential by approximately 55mV. A porous plug acts as a junction between the fixed chloride solution and the outside electrolyte.
Aerated electrolyte: An electrolyte (in this case the concrete) which is freely exposed to air.
Anode: The part of an electrolytic cell at which the anodic (oxidation) reaction takes place.
Anode overlay: The normally cementitious cover applied to embedded anodes to allow the anodes to function. Anode overlay may be ordinary cast in-situ concrete, sprayed concrete, sprayed mortar, pumped concrete grout or hand applied mortar. In order to function as an anode overlay the resistivity of the material must allow ionic current to flow; typically maximum workable resistivity of anode overlays is 50,000 Ωcm (minimum conductivity of 20 S/cm).
Anodic polarisation: The polarisation by which the electrochemical state of the metallic component is moved in an anodic (corroding) direction.
Anodic reaction: The electrochemical (oxidation) reaction in which electrons are freed into the electric circuit and oxide species formed, typically as:
| M → M+ + ne- | |
| e.g. | Fe → Fe2+ + 2e– |
| 4OH– → O2 + 2H20 + 4e– | |
| 2Cl– → Cl2 + 2e– |
Automatic potential control: Whereby the output current and voltage of the transformer-rectifier is automatically adjusted so that the potential of the reinforcing steel as measured by a fixed (embedded) reference electrode is constant. Because of the non-homogeneity of concrete systems this control mode is not normally used in reinforced concrete structures. Also referred to as ‘Potentiostatic control’.
Cathode: The part of an electrolytic cell at which a cathodic (reduction) reaction occurs.
Cathodic polarisation: The polarisation produced by which the electrochemical state of the metallic component is shifted in a negative (non-corroding) direction.
Cathodic protection: A process of applying direct current (DC) from an external anode to a reinforcement system so that corrosion is progressively inhibited by polarising the steel cathodically. This anode may generate its own current (sacrificial protection) or be externally driven (impressed current).
Cathodic reaction: The electrochemical (reduction) reaction by which electrons are consumed. This reaction is typically either the hydrogen evolution reaction or the oxygen reduction reaction.
| Xn+ + n e– → X | |
| e.g. | 2H+ + 2 e– → H2 |
| O2 + 2H20 + 4 e– → 4OH– |
Chloride extraction: A temporary electrochemical treatment for chloride contaminated concrete based on the removal of negative chloride ions from the concrete to a temporary anode by a direct current. Also known as ‘Electrochemical chloride extraction’ or ‘Chloride removal’.
Conductive coating: A paint or mastic which is heavily loaded with an electrically conducting compound.
Conductive concrete: Concrete containing sufficient high conductance com¬pounds to permit current flow.
Conductivity: The reciprocal of resistivity, normally measured as microsiemens per centimetre (μS/cm).
Constant current control: Whereby the output of a transformer-rectifier is set at constant current with the output voltage of the transformer-rectifier automatically adjusting to provide the same.
Constant voltage control: Whereby the current output of a transformer-rectifier is automatically adjusted so that the output voltage of the transformer-rectifier is constant as set. Often a current override facility would also be incorporated in this control mode.
Continuity bond: A method of metallically joining two or more normally adjacent pieces of reinforcement so that current flow between sections is not restricted.
Corrosion: The spontaneous process whereby an anodic reaction and a corresponding cathodic reaction take place on the same metallic component in an electrolyte leading to production of metal ionic species. For corrosion to proceed, both the anodic reaction and the cathodic reaction must be thermo-dynamically capable of proceeding and must balance (i.e. an equal number of electrons must be produced and consumed).
Current: A measure of the rate of flow of electricity in a conductor and usually expressed in amperes (A) or milliamps (mA).
Current density: Current passing across unit area of surface, usually expressed as milliamps per square metre (mA/m2).
DC: Direct current.
DC power supply: See ‘Transformer-rectifier’.
DVM: Digital voltmeter; for reinforced concrete systems a high input impedance (greater than 20 MΩ) is used to measure the potential of the reinforcing steel with respect to a suitable reference electrode. Also used as a test meter for determining output characteristics of power supplies.
Electrode: In this context refers to either the positive or negative conductor in an electrochemical cell.
Electrode potential: The potential of an electrode within an electrolyte measured with respect to a reference electrode. Measurement indicates whether the electrochemical reactions taking place at the electrode surface are predominantly anodic or cathodic and thus can give an indication of the condition of the electrode material. Also referred to as ‘Half-cell potential’.
Electrolyte: A solution or solid which conducts electric current by the transport of ionic species. In this case the concrete, contaminated or otherwise, through which the corrosion and cathodic protection currents flow.
Embedded reference electrode: A Reference electrode that is permanently fixed within the concrete structure close to, but not touching, the reinforcing steel.
Half-cell: See ‘Reference electrode’.
Half-cell potential: See ‘Electrode potential’.
Impressed current: The current produced in the process whereby a direct current circuit is created between the reinforcement and an external anode made of material which is relatively non-reactive. Current can only flow onto the reinforcement if a voltage is externally applied between the anode and the steel reinforcement system, as distinct from a sacrificial anode where this voltage is self generative.
Instant Off potential: The potential of the reinforcing steel at the location of measurement which is recorded not less than 0.1 and not more than 1.0 seconds following complete disruption of DC power to the system. It is used as an estimate for the IR (ohmic) free potential of the reinforcing steel.
IR drop: The voltage drop due (in this case) to the concrete resistance between the reinforcement surface and the location of the reference electrode; also referred to as Ohmic drop.
Monitoring system: A system of test items fitted where concrete structures are exposed to varying physical conditions. Examples are wind and water lines on piling or means of checking long-term effects in more static environments, such as a series of permanent reference electrodes being installed either to plot any variation in steel potential or to sense conditions for an automatic current adjustment system.
Ohmic drop: See ‘IR drop’.
Permanent electrochemical treatment: An electrochemical treatment for concrete that is based on a protective effect (e.g. negative potential shift) that must be sustained to sustain the protective effect.
pH: The negative logarithmic value (to the base 10) of the hydrogen ion concentration used to express the acidity/alkalinity of a solution. Values below 7 are progressively more acidic, 7 is neutral and values above 7 are progressively more alkaline.
Polarisation: An application of current causes the potential of an electrode to increase from its static or theoretical decomposition potential as the current is increased. This potential change, termed polarisation, is the result of several changes which occur due to the passage of the current. The most influential of these changes are the build up of thin films of relatively higher resistance at electrochemical interfaces and a change in the concentration of reacting species at the surfaces of the electrode.
Portable reference electrode: Suitable reference electrode normally incorporating a sponge or soft wooden plug used on the outside surface of the reinforced concrete structure to obtain the potential of the reinforcing steel; also referred to as ‘Surface electrode’.
Potential: In this context, the same as ‘Electrode potential’.
Potential decay: In this context the extent of depolarisation from the polarised (IR free) condition following complete disruption of DC current. This will normally be associated with an anodic polarisation (q.v.) and will lead to an increase in the potential (i.e. move more positive).
Potential shift: In this context the extent of polarisation from the natural (as found) potential to the operating potential. This will normally be associated with a cathodic polarisation (q.v.) and will be a reduction in potential (i.e. move more negative).
Primary anode: Used with conductive anode materials (for example conductive asphalt, conductive paint, sprayed metal anodes) to deliver current to the conductive anode; also referred to as ‘Dissipator’.
Probe: See ‘Reference electrode’. More correctly, refers to an inserted coupon or half-cell.
Protection current density: The current density required to be applied by cathodic protection to the reinforcing steel, so that the corrosion of the reinforcing steel is controlled.
PVC: Polyvinyl chloride: cable insulator, typical material for junction box construction.
Realkalisation: A temporary electrochemical treatment for carbonated concrete based on restoring the pH of the concrete.
Redox: Oxidation reduction reaction.
Reference electrode: The standard test cell against which potential movement on a surface is measured. In concrete testing, one half of the electrochemical cell is the reference electrode, usually silver/silver chloride, with the other half of the cell being that between the steel of the reinforcement and the surrounding concrete. Also referred to as a ‘Half-cell’, and see also ‘Probe’.
Resistivity: Value of the electrical resistance of an electrolyte per unit length across a unit cross-sectional area, usually expressed in ohm centimetre (Ωcm). Resistivity is the reciprocal of the conductivity.
Secondary anode: The active portion of a conductive anode system (e.g. conductive asphalt, conductive paint, sprayed metal anodes). The portion of the conductive anode that delivers current to the concrete surface. See also ‘Primary anode’.
Stray current: In concrete structures, usually a form of induced electrical pick up which affects the natural potentials of the reinforcement, possibly increasing the existing corrosion rate.
Stray current corrosion: Corrosion induced from stray current pick up flowing through uncontrolled paths.
Structure to electrode potential: The formal way of defining the potential of the steel at the point of reference electrode application in conjunction with the type of reference electrode used.
Surface reference electrode: See ‘Portable reference electrode’.
Transformer-rectifier: An electrical power unit which transforms and rectifies AC line voltages to the required DC output voltages, also referred to as DC power supplies. Transformer-rectifiers can be manufactured to operate under any conditions of AC voltage and frequency and to any required output current and voltage (within reason) Transformer-rectifiers can be operated in either constant current, constant voltage, or constant potential mode.
Voltage: A potential difference expressed in volts. In the context of cathodic protection is normally used to refer to the output of the power supply.