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CorrosionSource - Corrosion terms
Category: Earth and Environment > Corrosion
Date & country: 24/09/2008, US
Words: 336

The development of loose removable powder at the surface of an organic coating usually caused by weathering

The development of slight breaks in a coating that do not penetrate to the underlying surface

Numerous, very fine cracks in a coating or at the surface of a metal part. Checks may appear during processing or during service and are most often associated with thermal treatment or thermal cycling. Also called check marks. checking, or heat checks

(1) A molecular structure in which a heterocyclic ring can he formed by the unshared electrons of neighboring atoms. (2) A coordination compound in which a heterocyclic ring is formed by a metal bound to two atoms of the associated ligand. See also complexation

chelating agent
(1) An organic compound in which atoms form more than one coordinate bond with metals in solution. (2) A substance used in metal finishing to control or eliminate certain metallic ions present in undesirable quantities

A chemical process involving formation of a heterocyclic ring compound that contains at least one metal cation or hydrogen ion in the ring

chemical potential
In a thermodynamic system of several constituents, the rate of change of the Gibbs function of the system with respect to the change in the number of moles of a particular constituent

chemical vapor deposition
A coating process, similar to gas carburizing and carbonitriding, whereby a reactant atmosphere gas is fed into a processing chamber where it decomposes at the surface of the workpiece, liberating one material for either absorption by, or accumulation on the workpiece. A second material is liberated in gas form and is removed from the processing ch..

The binding of an adsorbate to the surface of a solid by forces whose energy levels approximate those of a chemical bond. Contrast with physisorption

chevron pattern
A fractographic pattern of radial marks (shear ledges) that look like nested letters 'V'; sometimes called a herringbone pattern. Chevron patterns are typically found on brittle fracture surfaces in parts whose widths are considerably greater than their thicknesses. The points of the chevrons can be traced back to the fracture origin

Improving paint adhesion on aluminum or aluminum alloys, mainly aircraft skins, by treatment with a solution of' chromic acid. Also called chromodizing or chromatizing. Not to be confused with chromating or chromizing

chromate treatment
A treatment of metal in a solution of a hexavalent chromium compound to produce a conversion coating consisting of trivalent and hexavalent chromium compounds

Performing a chromate treatment

chrome pickle
(1) Producing a chromate conversion coating on magnesium for temporary protection or for a paint base. (2) The solution that produces the conversion coating

A surface treatment at elevated temperature, generally carried out in pack, vapor, or salt bath, in which an alloy is formed by the inward diffusion of chromium into the base metal

clad metal
A composite metal containing, two or more layers that have been bonded together. The bonding may have been accomplished by co-rolling, co-extrusion, welding, diffusion bonding, casting, heavy chemical deposition, heavy electroplating, or explosive cladding

Splitting (fracture) of a crystal on a crystallographic plane of' low index

cleavage fracture
A fracture, usually of' polycrystalline metal, in which most of the grains have failed by cleavage, resulting in bright reflecting facets. It is associated with low-energy brittle fracture

cold cracking
A type of weld cracking that usually occurs below 203 ºC (400 'F). Cracking may occur during or after cooling to room temperature, sometimes with a considerable time delay. Three factors combine to produce cold cracks: stress (for example, from thermal expansion and contraction). hydrogen (from hydrogen-containing welding consumables), and a s..

cold working
Deforming metal plastically under conditions of temperature and strain rote that induce strain hardening. Usually, hut not necessarily, conducted at room temperature. Contrast with hot working

combined carbon
The part of the total carbon in steel or cast iron that is present as other than free carbon

The formation of complex chemical species by the coordination of groups of atoms termed ligands to a central ion, commonly a metal ion. Generally, the ligand coordinates by providing a pair of electrons that forms an ionic or covalent bond to the central ion. See also chelate, coordination compound, and ligand

Pertaining to forces on a body or part of a body that tend to crush or compress the body

compressive strength
The maximum compressive stress a material is capable of developing. With a brittle material that fails in compression by fracturing, the compressive strength has a definite value. In the case of ductile, malleable, or semiviscous materials (which do not fail in compression by a shattering fracture), the value obtained for compressive strength is an..

compressive stress
A stress that causes an elastic body to deform (shorten) in the direction of the applied load. Contrast with tensile stress

concentration cell
An electrolytic cell, the electromotive force of which is caused by a difference in concentration of some component in the electrolyte. This difference leads to the formation of discrete cathode and anode regions

concentration polarization
That portion of the polarization of a cell produced by concentration changes resulting from passage of' current through the electrolyte

The ratio of the electric current density to the electric field in a material. Also called electrical conductivity or specific conductance

contact corrosion
A term primarily used in Europe to describe galvanic corrosion between dissimilar metals

contact plating
A metal plating process wherein the plating current is provided by galvanic action between the work metal and a second metal, without the use of an external source of current

contact potential
The potential difference at the junction of two dissimilar substances

continuity bond
A metallic connection that provides electrical continuity between metal structures

conversion coating
A coating consisting of' a compound of the surface metal, produced by chemical or electrochemical treatments of the metal. Examples include chromate coatings on zinc, cadmium, magnesium, and aluminum and oxide and phosphate coatings on steel. See also chromate treatment and phosphating

coordination compound
A compound with a central atom or ion bound to a group of ions or molecules surrounding it. Also called coordination complex. See also chelate, complexation, and ligand

corrodkote test
An accelerated corrosion test for electrodeposits

The chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties

corrosion effect
A change in any part of the corrosion system caused by corrosion

corrosion embrittlement
The severe loss of ductility of a metal resulting from corrosive attack, usually inter,granular and often not visually apparent

corrosion fatigue
The process in which a metal fractures prematurely under conditions of simultaneous corrosion and repeated cyclic loading at lower stress levels or fewer cycles than would be required in the absence of the corrosive environment

corrosion fatigue strength
The maximum repeated stress that can he endured by a metal without failure under definite conditions of corrosion and fatigue and for a specific number of stress cycles and a specified period of time

corrosion inhibitor
See inhibitor

corrosion potential (Ecorr)
The potential of a corroding surface in an electrolyte, relative to a reference electrode. Also called rest potential, open circuit potential, or freely corroding potential

corrosion product
Substance formed as a result of corrosion

corrosion protection
Modification of a corrosion system so that corrosion damage is mitigated

corrosion rate
Corrosion effect on a metal per unit of time. The type of corrosion rate used depends on the technical .system and on the type of corrosion effect. Thus, corrosion rate may be expressed as an increase in corrosion depth per unit of time (penetration rate, for example, mils/yr.) or the mass of metal turned into corrosion products per unit area of su..

corrosion resistance
Ability of a metal to withstand corrosion in a given corrosion system

corrosion system
System consisting of one or more metals and all parts of the environment that influence corrosion

Corrosion which is increased because of the abrasive action of a moving stream; the presence of suspended particles greatly accelerates abrasive action.See erosion-corrosion

Tendency of an environment to cause corrosion in a given corrosion system

counter electrode
See auxiliary electrode

A cell developed in an electrolyte resulting from electrical contact between two dissimilar metals. See galvanic corrosion

covering power
The ability of a solution to give satisfactory plating at very low current densities. a condition that exists in recesses and pits. This term suggests an ability to cover, but not necessarily to build up, a uniform coating, whereas throwing power suggests the ability to obtain a coating of uniform thickness of an irregularly shaped object

cracking (of coating)
Breaks in a coating that extend through to the underlying surface

A network of checks or cracks appearing on the surface

Time-dependent strain occurring under stress. The creep strain occurring at a diminishing rate is called primary creep; that occurring at a minimum and almost constant rate, secondary creep; and that occurring at an accelerating rate, tertiary creep

creep-rupture embrittlement
Embrittlement under creep conditions of, for example, aluminum alloys and steels that results in abnormally low rupture ductility. In aluminum alloys, iron in amounts above the solubility limit is known to cause such embrittlement; in steels, the phenomenon is related to the amount of impurities (for example. phosphorus, sulfur, copper, arsenic, an..

creep-rupture strength
The stress that will cause fracture in a creep test at a given time in a specified constant environment. Also called stress-rupture strength

crevice corrosion
Localized corrosion of a metal surface at, or immediately adjacent to, an area that is shielded from full exposure to the environment because of close proximity between the metal and the surface of another material

critical anodic current density
The maximum anodic current density observed in the active region for a metal or alloy electrode that exhibits active-passive behavior in an environment

critical flaw size
The size of a flaw (defect) in a structure that will cause failure at a particular stress level

critical humidity
The relative humidity above which the atmospheric corrosion rate of some metals increases sharply

The net transfer of electric charge per unit time. Also called electric current. See also current density

current density
The current flowing to or from a unit area of an electrode surface, generally expressed as amps per sq ft or milliamperes per sq ft (also milliamps per sq cm, etc)

current efficiency
The ratio of the electrochemical equivalent current density for a specific reaction to the total applied current density

Loss of carbon from the surface layer of a carbon-containing alloy due to reaction with one or more chemical substances in a medium that contacts the surface. See also dealloying

Corrosion in which cobalt is selectively leached from cobalt-base alloys, such as Stellite®, or from cemented carbides. See also dealloying and selective leaching

deep groundbed
One or more anodes installed vertically at a nominal depth of 15 m (50 ft) or more below the earth's surface in a drilled hole for the purpose of supplying cathodic protection for an underground or submerged metallic structure. See also groundbed

delta ferrite
See ferrite

A crystal that has a treelike branching pattern, being most evident in cast metals, slowly cooled through the solidification range

Corrosion in which nickel is selectively leached from nickel-containing alloys. Most commonly observed in copper-nickel alloys after extended service in fresh water. See also dealloying, and selective Ieaching

density (of gases)
The mass of a unit volume of a gas at a stated temperature and pressure

density (of solids and liquids)
The mass of unit volume of a material at a specified temperature

(1) The removal of oxygen from molten metals by use of suitable deoxidixers. (2) Sometimes refers to the removal of undesirable elements other than oxygen by the introduction of elements or compounds that readily react with them. (3) In metal finishing, the removal of oxide films from metal surfaces by chemical or electrochemical reaction

A substance that produces depolarization

deposit corrosion
Corrosion occurring under or around a discontinuous deposit on a metallic surface. Also called poultice corrosion

Removing the thick layer of oxides formed on some metals at elevated temperatures

dichromate treatment
A chromate conversion coating produced on magnesium alloys in a boiling solution of sodium dichromate

dielectric shield
In a cathodic protection system, in electrically nonconductive material, such as a coating, plastic sheet or pipe that is placed between an anode and an adjacent cathode to avoid current wastage and to improve current distribution, usually on the cathode

(l) Spreading of a constituent in a gas, liquid, or solid, tending to make the composition of all parts uniform. (2) The spontaneous movement of atoms or molecules to new sites within a material

diffusion coating
Any process whereby a base metal or alloy is either (1) coated with another metal or alloy and heated to a sufficient temperature in a suitable environment or (2) exposed to a gaseous or liquid medium containing the other metal or alloy, thus causing diffusion of the coating or of the other metal or alloy into the base metal with resultant changes ..

diffusion coefficient
A factor of proportionality representing the amount of substance diffusing across a unit area through a unit concentration gradient in unit time

diffusion-limited current density
The current density, often referred to as limiting current density, that corresponds to the maximum transfer rate that a particular species can sustain because of the limitation of diffusion

dimple rupture
A fractographic term describing ductile fracture that occurs through the formation and coalescence of microvoids along the fracture path. The fracture surface of such a ductile fracture appears dimpled when observed at high magnification and usually is most clearly resolved when viewed in a scanning electron microscope

The destruction of adhesion between a coating and the surface coated

Any interruption in the normal physical structure or configuration of a part, such as cracks, laps, seams, inclusions, or porosity. A discontinuity may or may not affect the usefulness of the part

A linear imperfection in a crystalline array of atoms. Two basic types are recognized: (1) an edge dislocation corresponds to the row of mismatched atoms along the edge formed by an extra, partial plane of atoms within the body of a crystal; (2) a screw dislocation corresponds to the axis of a spiral structure in a crystal, characterized by a disto..

Conduction of electric current from an underground metallic structure by means of a metallic conductor. Forced drainage is that applied to underground metallic structures by means of an applied electromotive force or sacrificial anode. Natural drainage is that from an underground structure to a more negative (more anodic) structure, such as the neg..

dry corrosion
See gaseous corrosion

drying oil
An oil capable of conversion from a liquid to a solid by slow reaction with oxygen in the air

ductile fracture
Fracture characterized by tearing of metal accompanied by appreciable gross plastic deformation and expenditure of considerable energy. Contrast with brittle fracture

The ability of a material to deform plastically without fracturing, measured by elongation or reduction of area in a tensile test, by height of cupping in an Erichsen test, or by other means

dummy cathode
(1) A cathode, usuully corrugated to give variable current densities, that is plated at low current densities to preferentially remove impurities from a plating solution. (2) A substitute cathode that is used during adjustment of operating conditions

Plating with dummy cathodes

A general term used to imply that a part in service (1) has become completely inoperable, (2) is still operable but is incapable of satisfactorily performing its intended function, or (3) has deteriorated seriously, to the point that it has become unreliable or unsafe for continued use

Faraday's law
(1) The amount of any substance dissolved or deposited in electrolysis is proportional to the total electric charge passed. (2) The amounts of different substances dissolved or deposited by the passage of the same electric charge are proportional to their equivalent weights

The phenomenon leading to fracture under repeated or fluctuating stresses having a maximum value less than the tensile strength of the material. Fatigue fractures are progressive and grow under the action of the fluctuating stress

fatigue crack growth rate
The rate of crack extension caused by constant-amplitude fatigue loading, expressed in terms of crack extension per cycle of load application

fatigue life
The number of cycles of stress that can be sustained prior to failure under a stated test condition

fatigue limit
The maximum stress that presumably leads to fatigue fracture in a specified number of stress cycles. If the stress is not completely reversed. the value of the mean stress. the minimum stress, or the stress ratio should also be stated. Compare with endurance limit

fatigue strength
The maximum stress that can be sustained for a specified number of cycles without failure, the stress being completely reversed within each cycle unless otherwise staled