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Corrosion Source - Corrosion glossary
Category: General technical and industrial > Corrosion
Date & country: 11/12/2007, UK
Words: 620


galvanic cell
A cell in which chemical change is the source of electrical energy. It usually consists of two dissimilar conductors in contact with each other and with an electrolyte. or of two similar conductors in contact with each other and with dissimilar electrolytes.

galvanic corrosion
Accelerated corrosion of a metal because of an electrical contact with a more noble metal or nonmetallic conductor in a corrosive electrolyte.

galvanic couple
A pair of dissimilar conductors, commonly metals, in electrical contact. See also galvanic corrosion.

galvanic couple potential
See mixed potential.

galvanic current
The electric current that flows between metals or conductive nonmetal in a galvanic couple.

galvanic series
A list of metals and alloys arranged according to their relative corrosion potentials in a given environment. Compare with electromotive series.

galvanize
To coat a metal surface with zinc using any of various processes.

galvanneal
To produce a zinc-iron alloy coating on iron or steel by keeping the coating molten after hot dip galvanizing until the zinc alloys completely with the base metal.

galvanometer
An instrument for indicating or measuring a small electric current by means of a mechanical motion derived from electromagnetic or electrodynamic forces produced by the current.

galvanostatic
An experimental technique where by an electrode is maintained at a constant current in an electrolyte.

gamma iron
The face-centered cubic form of pure iron, stable from 910 to l400 ºC (1670 to 2550 ºF).

gaseous corrosion
Corrosion with gas as the only corrosive agent and without any aqueous phase on the surface of the metal. Also called dry corrosion.

General corrosion
A form of deterioration that is distributed more or less uniformly over a surface; See uniform corrosion.

Gibbs free energy
The thermodynamic function 3G = 5H - TSS, where H is enthalpy, T is absolute temperature. and S is entropy. Also called free energy, free enthalpy, or Gibbs function.

glass electrode
A glass membrane electrode used to measure pH or hydrogen-ion activity.

grain
An individual crystal in a polycrystalline metal or alloy; it may or may not contain twinned regions and subgrains; a portion of a solid metal (usually a fraction of an inch in size), in which the atoms are arranged in an orderly pattern.

grain boundary
A narrow zone in a metal corresponding to the transition from one crystallographic orientation to another, thus separating one grain from another; the atoms in each grain are arranged in an orderly pattern; the irregular junction of two adjacent grains is known as a grain boundary.

grain-boundary corrosion
Same as intergranular corrosion. See also interdendritic corrosion.

graphitic corrosion
Deterioration of gray cast iron in which the metallic constituents are selectively leached or converted to corrosion products leaving the graphite intact. The term graphic quotation is commonly used to identify this form of corrosion, but is not recommended because of its use in metallurgy for the decomposition of carbide to graphite; deterioration…

graphitization
A metallurgical term describing the formation of graphite in iron or steel, usually from decomposition of iron carbide at elevated temperatures. Not recommended as a term to describe graphitic corrosion.

green liquor
The liquor resulting from dissolving molten melt irom the kraft recovery furnace in water. See also kraft process and smelt.

Green Rot
A form of high-temperature corrosion of chromium-bearing alloys in which green chromium oxide (Cr2O3) forms, but certain other alloy constituents remain metallic; some simultaneous carburization is sometimes observed.

groundbed
A buried item, such as junk steel or graphite rods, that serves as the anode for the cathodic protection of pipelines or other buried structures. See also deep groundbed.

half cell
An electrode immersed in a suitable electrolyte, designed for measurements of electrode potential; A pure metal in contact with a solution of known concentration of its own ion, at a specific temperature develops a potential which is characteristic and reproducible; when coupled with another half cell, an overall potential develops which is the sum…

halogen
Any of the elements of the halogen family, consisting of fluorine, chlorine, bromine, iodine, and astatine.

hard chromium
Chromium plated for engineering rather than decorative applicactions.

hard water
Water that contains certain salts, such as those of calcium or magnesium, which form insoluble deposits in boilers and form precipitates with soap.

hardenability
The relative ability of a ferrous alloy to form martensite when quenched from a temperature above the upper critical temperature. Hardenability is commonly measured as the distance below a quenched surfsce at which the metal exhibits a specific hardness (50 HRC, for example) or a specific percentange of martensite in the microstructure.

hardfacing
Depositing filler metal on a surfsae by welding, spraying, or braze welding to increase resistance to abrasion, erosion, wear, galling. impact, or cavitation damage.

heat check
A pattern of parallel surface cracks that are formed by alternate rapid heating and cooling of the extreme surface metal, sometimes found on forging dies and piercing punches. There may be two sets of parallel cracks one set perpendicular to the other.

heat-affected zone
That portion of the base metal that was not melted during brazing, cutting, or welding, but whose microstructure and mechanical properties were altered by the heat; Refers to area adjacent to a weld where the thermal cycle has coused microstructural changes which generally affect corrosion behavior.

hematite
(1) An iron mineral crystallizing in therhombohedral system; the most important oreof iron. (2) An iron oxide, Fe,O,, corrcsponding to an iron content of approximately 70%.

high-temperature hydrogen attack
A loss ofstrength and ductility of .steel by high-temperature reaction of absorhcd hydrogen with carbides in the steel resulting in dec arbwri:.alien and internal fissuring.

holidays
Discontinuities in ci coating (suchasporosity, cracks, gape. and similar Bawd) that allow areas of base metal to be exposed to any corrosive environment that contacts the coated surface.

hot corrosion
An accelerated corrosion of metal surfaces that results from the combined elTect of oxidation and reactions with sulfur compounds and other contaminunts, such us chlorides, to form a molten salt on a metal iurfuce that f1uxes, destroys, or disrupts the normal protective oxide. Seealso gaseous r erosion.

hot cracking
Also called solidification crackinghot cracking of weldments is caused by the segregation at grain boundaries of low-melting constituents in the weld metal. This can resultin grain-boundary tearing under thermal contraction stresses. Hot cracking can be minimized bythe use of low-impurity welding materials and proper joint design. See also cold cra…

hot dip coating
A metallic coating obtained bydipping the base metal into a molten metal.

hot shortness
A tendency for some alloys to separate along grain boundaries when stressed ordeformed at temperatures near the melting point.Hot shortness is caused by a low-melting constituent, often present only in minute amounts,that is segregated at grain boundaries.

hot working
Deforming metal plastically at sucha temperature and strain rate that recrystallization takes place simultaneously with the deformation, thus avoiding any strain hardening.Contrast with

huey test
Corrosion testing in a boiling solution of nitric acid. This test is mainly used to detect the susceptibilty to intergranular corrosion of stainless steel.

humidity test
A corrosion test involving exposureof specimens at controlled levels of humidity and temperature. Contrast with salt-fog test.

hydrogen blistering
The formation of blisters on or below a metal surface from excessive internal hydrogen pressure; Formation of blister-like bulges on a ductile metal surface caused by internal hydrogen pressures. Hydrogen may beformed during cleaning, plating, corrosion, and so forth.

hydrogen damage
A general term for the emhrittlement, cracking, blistering. and hydride formation that can occur when hydrogen is present in some metals.

hydrogen disintegration
Deep internal cracks caused by hydrogen.

hydrogen embrittlement
A process resulting in adecrease of the toughness or ductility of a metal due to the presence of atomic hydrogen. Hydrogen embrittlement has been recognized classically as being of two types. The first known as internal hydrogen embrittlement, occurs when the hydrogen enters molten metal which becomes supersaturated with hydrogen immediately after …

hydrogen overvoltage
Overvoltage associated with the liberation of hydrogen gas.

hydrogen stress cracking (HSC)
See hydrogen embrittlement.

hydrogen-assisted cracking (HAC)
See hydrogenembriltlement.

hydrogen-induced cracking (HIC)
Same as hydrogen embrittlement.

hydrolysis
(1) Decomposition or alteration of a chemical substance by water. (2) In aqueous solutions of electrolytes, the reactions of cations with water to produce a weak base or of anions to produce a weak acid.

hydrophilic
Having an affinity for water. Contrast with hydrophobic.

hydrophobic
Lacking an affinity for, repelling, orfailing to absorb or adsorb water. Contrast with hydrophilic.

hygroscopic
(1) Possessing a marked ability to accelerate the condensation of water vapor; applied to condensation nuclei composed of salts that yield aqueous solutions of a very low equilibrium vapor pressure compared with that of pure water at the same temperature. (2) Pertaining to a substance whose physical characteristics are appreciably altered by effect…

immersion plating
Depositing a metallic coating on a metal immersed in a liquid solution, without the aid of an external electric current. Also called dip plating.

immunity
A state of resistance to corrosion or anodic dissolution of a metal caused by thermodynamic stability of the metal.

impingement corrosion
A form of erosion-corrosion generally associated with the local impingement of a high-velocity. Flowing fluid against a solid surface.

impressed current
Direct current supplied by a device employing a power source external to the electrode system of a cathodic protection installation.

inclusions
Particles of foreign material in a metallic matrix. The particles are usually compounds (such as oxides, sulfides, or silicates), but may be of any substance that is foreign to (and essentially insoluble in) the matrix.

incubation period
A period prior to the detection of corrosion while the metal is in contact with a corrodent.

industrial atmosphere
An atmosphere in an area of heavy industry with soot, fly ash, and sulfur compounds as the principal constituents.

inert anode
An anode that is insoluble in the electrolyte under the conditions prevailing in the electrolysis.

inhibitor
A chemical substance or combination of substances that, when present in the environment, prevents or reduces corrosion without significant reaction with the components of the environment.

inorganic
Being or composed of matter other than hydrocarbons and their derivatives, or matter that is not of plant or animal origin. Contrast with organic.

inorganic zinc-rich paint
Coating containing a zinc powder pigment in an inorganic vehicle.

intensiostatic
See galvanostatic.

intercrystalline corrosion
See intergranular corrosion.

intercrystalline cracking
See intergranular cracking.

interdendritic corrosion
Corrosive attack that progresses preferentially along interdendritic paths. This type of attack results from local differences in composition, such as coring commonly encountered in alloy castings.

intergranular
Between crystals or grains. Also called intercrystalline. Contrast with transgranular.

intergranular corrosion
Corrosion occurring preferentially at grain boundaries, usually with slight or negligible attack on the adjacent grains. Also called intercrystalline corrosion.

intergranular cracking
Cracking or fracturing that occurs between the grains or crystals in a polycrystalline aggregate. Also called intercrystalline cracking. Contrast with transgranular cracking.

intergranular fracture
Brittle fracture of a metal in which the fracture is between the grains, or crystals, that form the metal. Also called intercrystalline fracture. Contrast with transgranular fracture.

intermediate electrode
Same as bipolar electrode.

internal oxidation
The formation of isolated particles of corrosion products beneath the metal surface. This occurs as the result of preferential oxidation of certain alloy constituents by inward diffusion of oxygen, nitrogen, sulfur, and so forth.

intumescence
The swelling or bubbling of a coating usually because of heating (term currently used in space and fire protection applications).

ion
An atom, or group of atoms, that has gained or lost one or more outer electrons and thus carries an electric charge. Positive ions, or cations, are deficient in outer electrons. Negative ions, or anions, have an excess of outer electrons.

Ion Erosion
Deterioration of material caused by ion impact.

ion exchange
The reversible interchange of ions between a liquid and solid, with no substantial structural changes in the solid.

iron rot
Deterioration of wood in contact with iron-based alloys.

isocorrosion diagram
A graph or chart that shows constant corrosion behavior with changing solution (environment) composition and temperature.

Kiscc
KISCC. Abbreviation for the critical value of the plane strain stress-intensity factor that will produce crack propagation by stress-corrosion cracking of a given material in a given environment.

knife-line attack.
Intergranular corrosion of an alloy, usually stabilized stainless steel, along a line adjoining or in contact with a weld after heating into the sensitization temperature range.

kraft process.
A wood-pulping process in which sodium sulfate is used in the caustic soda pulp-digestion liquor. Also called kraft pulping or sulfate pulping.

lamellar corrosion
See exfoliation corrosion.

lamellar tearing
Occurs in the base metal adjacent to weldments due to high through-thickness strains introduced by weld metal shrinkage in highly restrained joints. Tearing occurs by decohesion and linking along the working direction of the base metal; cracks usually run roughly parallel to the fusion line and are steplike in appearance. Lamellar tearing can be mi…

Langelier saturation index
An index calculated from total dissolved solids, calcium concentration, total alkalinity, pH and solution temperature that shows the tendency of a water solution to precipitate or dissolve calcium carbonate.

ledeburite
The eutectic of the iron-carbon system, the constituents of which are austenite and cementite. The austenite decomposes into ferrite and cementite on cooling below the temperature at which transformation of austenite to ferrite or ferrite plus cementite is completed.

ligand
The molecule, ion, or group bound to the central atom in a chelate or a coordination compound.

limiting current density
The maximum current density that can be used to obtain a desired electrode reaction without undue interference such as from polarization.

linear elastic fracture mechanics
A method of fracture analysis that can determine the stress (or load) required to induce fracture instability in a structure containing a cracklike flaw of known size and shape. See also fracture mechanics and stress-intensity factor.

lipophilic
Having an amenity for oil. See also hydrophilic and hydrophobic.

liquid metal embrittlement
Catastrophic brittle failure of a normally ductile metal when in contact with a liquid metal and subsequently stressed in tension.

local action
Corrosion due to the action of 'local cells,' that is, galvanic cells resulting from inhomogeneities between adjacent areas on a metal surface exposed to an electrolyte.

local cell
A galvanic cell resulting from inhomogeneities between areas on a metal surface in an electrolyte. The inhomogeneities may be of physical or chemical nature in either the metal or its environment.

localized corrosion
Corrosion at discrete sites, stress-corrosion cracking.

long-line current
Current that flows through the earth from an anodic to a cathodic area of a continuous metallic structure. Usually used only where the areas are separated by considerable distance and where the current results from concentration-cell action.

macroscopic.
Visible at magnifications to 25x.

macrostructure.
The structure of metals as revealed by macroscopic examination of the etched surface of a polished specimen.

magnetite.
Naturally occurring magnetic oxide of iron (Fe3O4).

martensite.
A generic term for microstructures formed by diffusionless phase transformation in which the parent and product phases have a specific crystallographic relationship. Martensite is characterized by an acicular pattern in the microstructure in both ferrous and nonferrous alloys. In alloys where the solute atoms occupy interstitial positions in the ma…