accelerated corrosion test. Method designed to approximate, in a short time, the deteriorating effect under normal long-term service conditions.
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 5.6 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.
acicular ferrite. A highly substructured nonequiaxed 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 amount of carbide present.
acrylic. Resin polymerized from acrylic acid, methacrylic acid, esters of these acids, or acrylonitrile.
activation. The changing of a passive surface of a metal to a chemically active state. Contrast with passivation.
active. The negative direction of electrode potential. Also used to describe corrosion and its associated potential range when an electrode potential is more negative than an adjacent depressed corrosion rate (passive) range.
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.
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 (oxygen cell). 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. due to a phase change (precipitation), but never involves a change in chemical composition of the metal or alloy. See also age hardening, artificial aging, natural aging, overaging, precipitation hardening, precipitation heat treatment, quench aging, and strain aging.
alclad. Composite wrought product comprised of an aluminum alloy core having on one or both surfaces a metallurgical 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 IA 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 910oC (1670oF).
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.
anchorite. A zinc-iron phosphate coating for iron and steel.
anaerobic. Free of air or uncombined oxygen.
anion. A negatively charged ion that migrates through the electrolyte toward the anode under the influence of a potential gradient. 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 of machinability. facilitation of cold work, improvement of mechanical or electrical properties, and/or increase in stability of dimensions. When the term is used by itself, full annealing is implied. When applied only for the relief of stress, the process is properly called stress relieving or stress-relief annealing.
anode. The electrode of an electrolyte cell at which oxidation occurs. Electrons flow away from the anode in the external circuit. It is usually at the electrode that corrosion occurs and metal ions enter solution. Contrast with cathode.
anode corrosion. The dissolution of a metal acting as an anode.
anode corrosion efficiency. The ratio of the actual corrosion (weight loss) of an anode to the theoretical corrosion (weight loss) 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 at 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.
anode polarization. See polarization.
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 mixture that prevents or reduces the rate of the anodic or oxidation reaction. See also inhibitor.
anodic polarization. The change of the electrode potential in the noble (positive) direction due to current flow. See also polarization.
anodic protection. (1) A technique to reduce the corrosion rate of a metal by polarizing it into its passive region, where dissolution rates are low. (2) Imposing an external electrical potential to protect a metal from corrosive attack. (Applicable only to metals that show active-passive behavior.) Contrast with cathodic protection.
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 = Men+ + 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.
anti-fouling. Intended to prevent fouling of underwater structures, such as the bottoms of ships.
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.
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. It is usually made from a noncorroding material.
bainite. A metastable aggregate of ferrite and cementite resulting from the transformation of austenite at temperatures below the pearlite range but above Ms, 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 resembling that of tempered martensite.
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 semi-elliptical rings, radiating outward from one or more origins. Beach marks (also known as clamshell marks or arrest marks) are typically found on service fractures where the part is loaded randomly, intermittently, or with periodic variations in mean stress or alternating stress. See also striation.
biaxial stress. See principal stress (normal).
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 salts, deposits, and other impurities.
blue brittleness. Brittleness exhibited by some steels after being heated to a temperature with in the range of about 200 to 370oC (400 to 700oF), particularly if the steel is worked at the elevated temperature.
blushing. Whitening and loss of gloss of a usually an organic coating caused by moisture. Also called blooming.
brackish water. (1) Water having salinity values ranging from approximately 0.5 to 17 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.
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 1000oC (1470 to 1830oF).
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 desired properties in the workpiece.
carburizing. Absorption and diffusion of carbon into solid ferrous alloys by heating. to a temperature usually above Ac3. in contact with a suitable carbonaceous material. A form of case hardening that produces a carbon gradient extending inward from the surface, enabling the surface layer to be hardened either by quenching directly from the carburizing temperature or by cooling to room temperature, then reaustenitizing and quenching.
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 are carburizing and quench hardening; cyaniding; nitriding; and carbonitriding. The use of the applicable specific process name is preferred.
CASS test. See copper-accelerated salt-spray test.
cathode. The electrode of an electrolytic cell at which reduction is the principal reaction. (Electrons flow toward the cathode in the external circuit.) Typical cathodic processes are cations taking up electrons and being discharged, oxygen being reduced, and the reduction of an element or group of elements from a higher to 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.
cathodic corrosion. Corrosion resulting from a cathodic condition of a structure usually caused by the reaction of an amphoteric metal with the alkaline products of electrolysis.
cathodic disbondment. The destruction of adhesion between a coating and its substrate by products of a cathodic reaction.
cathodic inhibitor. A chemical substance or mixture that prevents or reduces the rate of the cathodic or reduction reaction.
cathodic pickling. Electrolytic pickling in which ,the work is the cathode.
cathodic polarization. The change of the electrode potential in the active (negative) direction due to current flow. See also polarization.
cathodic protection. (1) Reduction of corrosion rate by shifting the corrosion potential of the electrode toward a less oxidizing potential by applying an external electromotive force. (2) Partial or complete protection of a metal from corrosion by making it a cathode, using either a galvanic or an impressed current Contrast with anodic protection.
cathodic reaction. Electrode reaction equivalent to a transfer of negative charge from the electronic to the ionic conductor A cathodic reaction is a reduction process An example common in corrosion is: Ox + ne= Red.
catholyte. The electrolyte adjacent to the cathode of an electrolytic cell cation. A positively charged ion that migrates through the electrolyte toward the (cathode under the influence of a potential gradient See also anion and ion.
caustic. ( 1) Burning or corrosive. (2) A hydroxide of a light metal, such as sodium hydroxide or potassium hydroxide.
caustic dip. A strongly alkaline solution into which metal is immersed for etching, for neutralizing acid, or for removing organic materials such as greases or paints.
caustic embrittlement. An obsolete historical term denoting a form of stress corrosion cracking most frequently encountered in carbon steels or iron-chromium-nickel alloys that are exposed to concentrated hydroxide solutions at temperatures of 200 to 250oC (400 to 480oF)
cavitation. The formation and instantaneous collapse of innumerable tiny voids or cavities within a liquid subjected to rapid and intense pressure changes Cavitation produced by ultrasonic radiation is sometimes used to effect violent localized agitation. Cavitation caused by severe turbulent flow often leads to cavitation damage
cavitation corrosion. A process involving conjoint corrosion and cavitation.
cavitation damage. The degradation of a solid body resulting from its exposure to cavitation. This may include loss of material, surface deformation, or changes in properties or appearance.
cavitation erosion. Progressive loss of original material from a solid surface due to continuing exposure to cavitation.
cell. Electrochemical system consisting of an anode and a cathode immersed in an electrolyte. The anode and cathode may be separate metals or dissimilar areas on the same metal. The cell includes the external circuit, which permits the flow of electrons from the anode toward the cathode. See also electrochemical cell.
cementite. A compound of iron and carbon, known chemically as iron carbide and having the approximate chemical formula Fe3C. It is characterized by an orthorhombic crystal structure. When it occurs as a phase in steel, the chemical composition will be altered by the presence of manganese and other carbide-forming elements.
chalking. The development of loose removable powder at the surface of an organic coating usually caused by weathering.
checking. The development of slight breaks in a coating that do not penetrate to the underlying surface.
checks. 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.
chelate. ( 1 ) A molecular structure in which a heterocyclic ring can be 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.
chelation. A chemical process involving formation of a heterocyclic ring compound that contains at least one metal cation or hydrogen ion in the ring.
chemical conversion coating. A protective or decorative nonmetallic coating produced in situ by chemical reaction of a metal with a chosen environment. It is often used to prepare the surface prior to the application of an organic coating.
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 chamber, along with excess atmosphere gas.
chemisorption. 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.
chromadizing. 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.
chromating. 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.
chromizing. 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, or heavy electroplating.
cleavage. Splitting (fracture) of a crystal on a crystallographic plane of low index.
cleavage fracture. A fracture, usually of a 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 205oC (400oF). 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 susceptible microstructure (plate martensite is most susceptible to cracking, ferritic and bainitic structures least susceptible). See also hot cracking, lamellar tearing, and stress-relief cracking.
cold working. Deforming metal plastically under conditions of temperature and strain rate that induce strain hardening. Usually, but 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.
complexation. 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.
compressive. 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 arbitrary value dependent on the degree of distortion that is regarded as effective failure of the material.
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.
conductivity. 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.
copper-accelerated salt-spray (CASS) test. An accelerated corrosion test for some electrodeposits and for anodic coatings on aluminum.
corrodkote test. An accelerated corrosion test for electrodeposits .
corrosion. 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 intergranular and often not visually apparent.
corrosion-erosion. See erosion corrosion.
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 be 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 surface per unit of time (weight loss, for example, g/m2/yr). The corrosion effect may vary with time and may not be the same at all points of the corroding surface. Therefore, reports of corrosion rates should be accompanied by information on the type, time dependency, and location of the corrosion effect.
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.
corrosivity. Tendency of an environment to cause corrosion in a given corrosion system.
counter electrode. See auxiliary electrode.
couple. 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.
crazing. A network of checks or cracks appearing on the surface.
creep. 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. antimony, and tin) present. In either case, failure occurs by intergranular cracking of the embrittled material.
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.
critical pitting potential (Ecp, Ep, Epp). The lowest value of oxidizing potential at which pits nucleate and grow. It is dependent on the test method used.
crud A term used in the nuclear power industry. Crud refers to the coolant-borne impurities and the low solubility deposit (due to corrosion of the structural components of the nuclear reactor system). Crud deposits consist mainly of iron oxides with high porosities (65~85%). Magnetite (Fe3O4) in which the iron is partially replaced by the other constituents (such as nickel or chromium) of the alloys exposed to the coolants, and hematite (Fe2O3), are the most common crud constituents in PWRs and BWRs, respectively. Formation of crud deposits on fuel element surfaces is largely dictated by solubility and heat transfer considerations.
current. 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.
current efficiency. The ratio of the electrochemical equivalent current density for a specific reaction to the total applied current density.
dealloying. The selective corrosion of one or more components of a solid solution alloy. Also called parting or selective leaching. See also decarburization. decobaltific ation. denickelification, dezincification, and graphitic corrosion .
decarburization. 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.
decobaltification. Corrosion in which cobalt is selectively leached from cobalt-base alloys. such as Stellite. or from cemented carbides. See also dealloying and selective leaching.
decomposition potential (or voltage). The potential of a metal surface necessary to decompose the electrolyte of a cell or a component thereof.
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.
dendrite. A crystal that has a treelike branching pattern. being most evident in cast metals slowly cooled through the solidification range.
denickelification. 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 leaching.
density (of gases). The mass of a unit volume of gas at a stated temperature and pressure.
density (of solids and liquids). The mass of unit volume of a material at a specified temperature.
deoxidizing. (1) The removal of oxygen from molten metals by use of suitable deoxidizers. (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
depolarization. A decrease in the polarization of an electrode.
depolarizer. A substance that produces depolarization.
deposit corrosion. Corrosion occurring under or around a discontinuous deposit on a metallic surface. Also called poultice corrosion.
descaling. Removing the thick layer of oxides formed on some metals at elevated temperatures.
dezincification. Corrosion in which zinc is selectively leached from zinc-containing alloys. Most commonly found in copper-zinc alloys containing less than 85% copper after extended service in water containing dissolved oxygen. See also dealloying and selective leaching.
dichromate treatment. A chromate conversion coating produced on magnesium alloys in a boiling solution of sodium dichromate.
dielectric shield. In a cathodic protection system, an 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.
differential aeration cell. An electrolytic cell, the electromotive force of which is due to a difference in air (oxygen) concentration at one electrode as compared with that at another electrode of the same material. See also concentration cell.
diffusion. (1) 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 in the composition and properties of its surface.
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.
disbondment. The destruction of adhesion between a coating and the surface coated.
discontinuity. 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.
dislocation. 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 distortion that joins normally parallel planes together to form a continuous helical ramp (with a pitch of one interplanar distance) winding about the dislocation. Most prevalent is the so-called mixed dislocation, which is any combination of an edge dislocation and a screw dislocation.
double layer. The interface between an electrode or a suspended particle and an electrolyte created by charge-charge interaction leading to an alignment of oppositely charged ions at the surface of the electrode or particle. The simplest model is represented by a parallel plate condensor.
drainage. 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 negative bus of a trolley substation.
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.
ductility. 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. usually 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.
dummying, Plating with dummy cathodes.
