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


absorption
A process in which Quid molecules are taken up by a liquid or solid and distributed throughout the body of that liquid or solid. Compare with adsorption.

accelerated corrosion test
Method designed to approximate, in a short time, the deteriorating effect under normal long-term service conditions.

acicular ferrite
A highly substructured non-equiaxed ferrite formed upon continuous cooling by a mixed diffusion and shear mode of transformation that begins at a temperature slightly higher than the transformation temperature range for upper bainite. It is distinguished from bainite in that it has a limited amount of carbon available thus, there is only a small am…

acid
A chemical substance that yields hydrogen ions (H+) when dissolved in water. Compare with base..

acid embrittlement
A form of hydrogen embrittlement that may be induced in some metals by acid.

acid rain
Atmospheric precipitation with a pH below 3.6 to 5.7. Burning of fossil fuels for heat and power is the major factor in the generation of oxides of nitrogen and sulfur, which are converted into nitric and sulfuric acids washed down in the rain. See also atmospheric corrosion.

acrylic
Resin polymerized from acrylic acid, methacrylic acid, eaters of these acids, or acrylonitrile.

activation
The changing of a passive surface of a metal to a chemically active state. Contrast with passivation..

active
A state in which a metal tends to corrode; referring to the negative direction of electrode potential (opposite of passive or noble).

active Metal
A metal ready to corrode, or being corroded

active potential
The potential of a corroding material.

activity
A measure of the chemical potential of a substance, where chemical potential is not equal to concentration, that allows mathematical relations equivalent to those for ideal systems to be used to correlate changes in an experimentally measured quantity with changes in chemical potential.

activity (ion)
The ion concentration corrected for deviations from ideal behavior. Concentration multiplied by activity coefficient. activity coefficient. A characteristic of a quantity expressing the deviation of a solution from ideal thermodynamic behavior; often used in connection with electrolytes.

addition agent
A substance added to a solution for the purpose of altering or controlling a process. Examples include wetting agents in acid pickles, brighteners or antipitting agents in plating solutions, and inhibitors.

additive
A substance added in a small amount, usually to a fluid, for a special purpose, such as to reduce friction, corrosion, etc.

adsorption
The surface retention of solid, liquid, or gas molecules, atoms, or ions by a solid or liquid. Compare with absorption..

aeration
(1) Exposing to the action of air. (2) Causing air to bubble through. (3) Introducing air into a solution by spraying, stirring, or a similar method. (4) Supplying or infusing with air, as in sand or soil.

aeration Cell
An oxygen concentration cell; an electrolytic cell resulting from differences in dissolved oxygen at two points. Also see differential aeration cell..

age hardening
Hardening by aging, usually after rapid cooling or cold working.

aging
A change in the properties of certain metals and alloys that occurs at ambient or moderately elevated temperatures after hot working or a heat treatment (quench aging in ferrous alloys, natural or artificial aging in ferrous and nonferrous alloys) or after a cold-working operation (strain aging). The change in properties is often, but not always, d…

alclad
Composite wrought product comprised of an aluminum alloy core having on one or both surfaces a metallurgically bonded aluminum or aluminum alloy coating that is anodic to the core and thus electrochemically protects the core against corrosion.

alkali metal
A metal in group lA of the periodic system ­ namely, lithium, sodium, potassium, rubidium, cesium, and francium. They form strongly alkaline hydroxides, hence the name.

alkaline
(1) Having properties of an alkali. (2) Having a pH greater than 7.

alkaline cleaner
A material blended from alkali hydroxides and such alkaline salts as borates, carbonates, phosphates, or silicates. The cleaning action may be enhanced by the addition of surface-active agents and special solvents.

alkyd
Resin used in coatings. Reaction products of polyhydric alcohols and polybasic acids.

alkylation
(1) A chemical process in which an alkyl radical is introduced into an organic compound by substitution or addition. (2) A refinery process for chemically combining isoparaffin with olefin hydrocarbons.

alligatoring
(1) Pronounced wide cracking over the entire surface of a coating having the appearance of alligator hide. (2) The longitudinal splitting of flat slabs in a plane parallel to the rolled surface. Also called fish-mouthing.

alloy plating
The codeposition of two or more metallic elements.

alpha ferrite
See ferrite..

alpha iron
The body-centered cubic form of pure iron, stable below 910 ºC (l670 ºF).

alternate-immersion test
A corrosion test in which the specimens are intermittently exposed to a liquid medium at definite time intervals.

aluminizing
Forming of an aluminum or aluminum alloy coating on a metal by hot dipping, hot spraying, or diffusion.

amalgam
An alloy of mercury with one or more other metals.

ammeter
An instrument for measuring the magnitude of electric current flow.

amorphous solid
A rigid material whose structure lacks crystalline periodicity; that is, the pattern of its constituent atoms or molecules does not repeat periodically in three dimensions. See also metallic glass..

amphoteric
A term applied to oxides and hydroxides which can act basic toward strong acids and acidic toward strong alkalis. Substances which can dissociate electrolytically to produce hydrogen or hydroxyl ions according to conditions.

anaerobic
In the absence of air or unreacted or free oxygen.

anchorite
A zinc-iron phosphate coating for iron and steel.

anion
An ion or radical which is attracted to the anode because of the negative charge. See also cation and ion.

annealing
A generic term denoting a treatment. consisting of heating to and holding at a suitable temperature, followed by cooling at a suitable rate, used primarily to soften metallic materials, but also to simultaneously produce desired changes in other properties or in microstructure. The purpose of such changes may be. but is not confined to. improvement…

anode
The electrode at which oxidation or corrosion of some component occurs (opposite of cathode). Electrons flow away from the anode in the external circuit.

anode corrosion
The dissolution of a metal acting as an anode.

anode corrosion efficiency
Ratio of actual to theoretical corrosion based on the total current flow calculated by Faraday's law from the quantity of electricity that has passed.

anode effect
The effect produced by polarization of the anode in electrolysis. It is characterized by a sudden increase in voltage and a corresponding decrease in amperage due to the anode becoming virtually separated from the electrolyte by a gas film.

anode efficiency
Current efficiency of the anode..

anode film
(1) The portion of solution in immediate contact with the anode, especially if the concentration gradient is steep. (2) The outer layer of the anode itself.

anodic cleaning
Electrolytic cleaning in which the work is the anode. Also called reverse-current cleaning.

anodic coating
A film on a metal surface resulting from an electrolytic treatment at the anode..

anodic inhibitor
A chemical substance or combination of substances that prevent or reduce the rate of the anodic or oxidation reaction by a physical, physico-chemical or chemical action.

anodic polarization
The change in the initial anode potential resulting from current flow effects at or near the anode surface. Potential becomes mode noble (more positive) because of anodic polarization.

anodic potential
An appreciable reduction in corrosion by making a metal an anode and maintaining this highly polarized condition with very little current flow.

anodic protection
A technique to reduce corrosion of a metal surface under some conditions by passing sufficient to it to cause its electrode potential to enter and remain in the passive region; imposing an external electrical potential to protect a metal from corrosive attack. (Applicable only to metals that show active-passive behavior.) Contrast with cathodic pro…

anodic reaction
Electrode reaction equivalent to a transfer of positive charge from the electronic to the ionic conductor. An anodic reaction is an oxidation process. An example common in corrosion is: Me ~ Me n+ + ne ..

anodizing
Forming a conversion coating on a metal surface by anodic oxidation; most frequently applied to aluminum.

anolyte
The electrolyte adjacent to the anode in an electrolytic cell. ually made of noncorroding material.

anti-fouling
Intended to prevent fouling of under-water structures, such as the bottoms of ships; refers to the prevention of marine organism's attachment or growth on a submerged metal surface, generally through chemical toxicity caused by the composition of the metal or coating layer.

antipitting agent
An addition agent for electroplating solutions to prevent the formation of pits or large pores in the electrodeposit.

aqueous
Pertaining to water; an aqueous solution is made by using water as a solvent.

artificial aging
Aging above room temperature. See also aging. Compare with natural aging..

atmospheric corrosion
The gradual degradation or alteration of a material by contact with substances present in the atmosphere, such as oxygen. carbon dioxide, water vapor, and sulfur and chlorine compounds.

austenite
A solid solution of one or more elements in face-centered cubic iron. Unless otherwise designated (such as nickel austenite), the solute is generally assumed to be carbon.

austenitic
The name given to the face-centered cubic crystal structure (FCC) of ferrous metals. Ordinary iron and steel has this structure at elevated temperatures; also certain stainless steels (300 series) have this structure at room temperature.

austenitizing
Forming austenite by heating a ferrous alloy into the transformation range (partial austenitizing) or above the transformation range (complete austenitizing). When used without qualification, the term implies complete austenitizing.

auxiliary anode
In electroplating, a supplementary anode positioned so as to raise the current density on a certain area of the cathode and thus obtain better distribution of plating.

auxiliary electrode
An electrode commonly used in polarization studies to pass current to or from a test electrode.

backfill
Material placed in a drilled hole to fill space around anodes, vent pipe, and buried components of a cathodic protection system.

bainite
A metastable aggregate of ferrite and cementite resulting from the transformation of austenite at temperatures below the pearlite range but above M the martensite start temperature. Bainite formed in the upper part of the bainite transformation range has a feathery appearance; bainite formed in the lower part of the range has an acicular appearance…

banded structure
A segregated structure consisting of alternating nearly parallel bands of different composition, typically aligned in the direction of primary hot working.

base
A chemical substance that yields hydroxyl ions (OH ) when dissolved in water. Compare with acid.

base metal
(1) The metal present in the largest proportion in an alloy; brass, for example, is a copper-base alloy. (2) An active metal that readily oxidizes, or that dissolves to form ions. (3) The metal to be brazed, cut, soldered, or welded. (4) After welding, that part of the metal which was not melted.

beach marks
Macroscopic progression marks on a fatigue fracture or stress-corrosion cracking surface that indicate successive positions of the advancing crack front. The classic appearance is of irregular elliptical or semielliptical rings, radiating outward from one or more origins. Beach marks (also known as clamshell marks or arrest marks) are typically fou…

biaxial stress
See principal stress (normal).

bimetallic corrosion
(Galvanic Corrosion) Corrosion resulting from dissimilar metal contact.

biological corrosion
Deterioration of metals as a result of the metabolic activity of microorganisms.

bipolar electrode
An electrode in an electrolytic cell that is not mechanically connected to the power supply, but is so placed in the electrolyte, between the anode and cathode, that the part nearer the anode becomes cathodic and the part nearer the cathode becomes anodic. Also called intermediate electrode.

bituminous coating
Coal tar or asphalt-based coating.

black liquor
The liquid material remaining from pulpwood cooking in the soda or sulfate paper-making process.

black oxide
A black finish on a metal produced by immersing it in hot oxidizing salts or salt solutions.

blister
A raised area, often dome shaped, resulting from (1) loss of adhesion between a coating or deposit and the base metal or (2) delamination under the pressure of expanding gas trapped in a metal in a near-subsurface zone. Very small blisters may be called pinhead blisters or pepper blisters.

blow down
(1) Injection of air or water under high pressure through a tube to the anode area for the purpose of purging the annular space and possibly correcting high resistance caused by gas blocking. (2) In connection with boilers or cooling towers, the process of discharging a significant portion of the aqueous solution in order to remove accumulated salt…

blue brittleness
Brittleness exhibited by some steels after being heated to a temperature within the range of about 200 to 370 ºC (400 to 700 ºF), particularly if the steel is worked at the elevated temperature.

blushing
Whitening and loss of gloss of a usually organic coating caused by moisture. Also called blooming.

brackish water
(1) Water having salinity values ranging from approximately 0.5 to l7 parts per thousand. (2) Water having less salt than seawater, but undrinkable.

breakdown potential
The least noble potential where pitting or crevice corrosion, or both, will initiate and propagate.

brightener
An agent or combination of agents added to an electroplating bath to produce a smooth, lustrous deposit.

brine
Seawater containing a higher concentration of dissolved salt than that of the ordinary ocean.

brittle fracture
Separation of a solid accompanied by little or no macroscopic plastic deformation. Typically, brittle fracture occurs by rapid crack propagation with less expenditure of energy than for ductile fracture .

burning
(1) Permanently damaging a metal or alloy by heating to cause either incipient melting or intergranular oxidation. See also over-heating. (2) In grinding, getting the work hot enough to cause discoloration or to change the microstructure by tempering or hardening.

calcareous coating or deposit
A layer consisting of a mixture of calcium carbonate and magnesium hydroxide deposited on surfaces being cathodically protected because of the increased pH adjacent to the protected surface.

calomel electrode
An electrode widely used as a reference electrode of known potential in electrometric measurement of acidity and alkalinity, corrosion studies, voltammetry, and measurement of the potentials of other electrodes. See also electrode potential, reference electrode, and saturated calomel electrode.

calorizing
Imparting resistance to oxidation to an iron or steel surface by heating in aluminum powder at 800 to 1000 ºC (1470 to 1830 ºF).

carbonitriding
A case hardening process in which a suitable ferrous material is heated above the lower transformation temperature in a gaseous atmosphere of such composition as to cause simultaneous absorption of carbon and nitrogen by the surface and, by diffusion, create a concentration gradient. The process is completed by cooling at a rate that produces the d…

Carburization
The absorption of carbon into a metal surface; may or may not be desirable.

carburizing
The absorption of carbon atoms by a metal at high temperatures; it may remain dissolved, or form metal carbides; Absorption and diffusion of carbon into solid ferrous alloys by heating, to a temperature usually above Ac in contact with a suitable carbonaceous material. A form of case hardening that produces a carbon gradient extending inward from t…

case hardening
A generic term covering several processes applicable to steel that change the chemical composition of the surface layer by absorption of carbon, nitrogen, or a mixture of the two and, by diffusion, create a concentration gradient. The outer portion, or case, is made substantially harder than the inner portion, or core. The processes commonly used a…

CASS test
See copper-accelerated salt-spray test.

cathode
The electrode of an electrolytic cell at which reduction is the principal reaction. (Electrons How toward the cathode in the external circuit.) Typical cathodic processes are taking up electrons and being discharged, oxygen being reduced. and the reduction of an element or group of elements from a high Cl a lower valence state. Contrast with anode.

cathode efficiency
Current efficiency at the cathode.

cathode film
The portion of solution in immediate contact with the cathode during electrolysis.

cathodic cleaning
Electrolytic cleaning in which the work is the cathode.