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Home , Corrosion

Corrosion Glossary

calcareous or deposit clad A layer consisting of a mixture of A composite metal containing, two calcium and or more layers that have been deposited on surfaces bonded together. The bonding may being cathodically protected because have been accomplished by co- of the increased pH adjacent to the rolling, co-extrusion, , protected surface. bonding, casting, heavy calomel electrode chemical deposition, heavy An electrode widely used as a , or explosive cladding. reference electrode of known cleavage potential in electrometric Splitting () of a crystal on a measurement of acidity and crystallographic plane of low index. alkalinity, corrosion studies, cleavage fracture voltammetry, and measurement of A fracture, usually of polycrystalline the potentials of other electrodes. metal, in which most of the grains See also , have failed by cleavage, resulting in reference electrode, and saturated bright reflecting facets. It is calomel electrode. associated with low-energy brittle calorizing fracture. Imparting resistance to oxidation to cold cracking an or surface by heating in A type of weld cracking that usually aluminum powder at 800 to 1000 ºC occurs below 203 ºC (400 "F). (1470 to 1830 ºF). Cracking may occur during or after carbonitriding cooling to room , A case hardening process in which a sometimes with a considerable time suitable ferrous material is heated delay. Three factors combine to above the lower transformation produce cold cracks: stress (for temperature in a gaseous example, from thermal expansion atmosphere of such composition as and contraction). (from to cause simultaneous absorption of hydrogen-containing welding carbon and nitrogen by the surface consumables), and a susceptible and, by diffusion, create a micro.structure (plate martensite is concentration gradient. The process most susceptible to cracking, ferritic is completed by cooling at a rate and bainitic structures least that produces the desired properties susceptible). See also hot cracking, in the workpiece. lamellar tearing and stress-relief cracking. The absorption of carbon atoms by a cold working metal at high ; it may Deforming metal plastically under remain dissolved, or form metal conditions of temperature and strain carbides; Absorption and diffusion of rote that induce strain hardening. carbon into solid ferrous alloys by Usually, hut not necessarily, heating, to a temperature usually conducted at room temperature. above Ac in contact with a suitable Contrast with hot working. carbonaceous material. A form of combined carbon case hardening that produces a The part of the total carbon in steel carbon gradient extending inward or that is present as other from the surface, enabling the than free carbon. surface layer to be hardened either complexation by quenching directly from the The formation of complex chemical carburizing temperature or by species by the coordination of cooling to room temperature then groups of atoms termed ligands to a reaustenitizing and quenching. central , commonly a metal ion. Carburization Generally, the ligand coordinates by The absorption of carbon into a providing a pair of electrons that metal surface; may or may not be forms an ionic or covalent bond to desirable. the central ion. See also chelate, case hardening coordination compound, and ligand. A generic term covering several compressive processes applicable to steel that Pertaining to forces on a body or change the chemical composition of part of a body that tend to crush or the surface layer by absorption of compress the body. carbon, nitrogen, or a mixture of the compressive strength two and, by diffusion, create a The maximum compressive stress a concentration gradient. The outer material is capable of developing. portion, or case, is made With a brittle material that fails in substantially harder than the inner compression by fracturing, the portion, or core. The processes compressive strength has a definite commonly used are carburizing and value. In the case of ductile, quench hardening; cyaniding: malleable, or semiviscous materials nitrifying; and carbonitriding. The (which do not fail in compression by use of the applicable specific process a shattering fracture), the value name is preferred. obtained for compressive strength is CASS test an arbitrary value dependent on the See -accelerated -spray degree of distortion that is regarded test. as effective failure of the material. compressive stress The electrode of an electrolytic cell A stress that causes an elastic body at which reduction is the principal to deform (shorten) in the direction reaction. (Electrons How toward the of the applied load. Contrast with cathode in the external circuit.) tensile stress. Typical cathodic processes are taking up electrons and being An electrolytic cell, the discharged, being reduced. electromotive force of which is and the reduction of an element or caused by a difference in group of elements from a high Cl a concentration of some component in lower valence state. Contrast with the . This difference . to the formation of discrete cathode cathode efficiency and anode regions. Current efficiency at the cathode. concentration polarization cathode film That portion of the polarization of a The portion of solution in immediate cell produced by concentration contact with the cathode during changes resulting from passage of electrolysis. current through the electrolyte. cathodic cleaning conductivity Electrolytic cleaning in which the The ratio of the electric current work is the cathode. density to the electric field in a cathodic corrosion material. Also called electrical Corrosion resulting from a cathodic conductivity or specific conductance. condition of a structure usually contact corrosion caused by the reaction of an A term primarily used in Europe to amphoteric metal with the alkaline describe between products of electrolysis. dissimilar . cathodic disbondment contact The destruction of adhesion between A metal plating process wherein the a coating and its substrate by plating current is provided by products of a cathodic reaction. galvanic action between the work metal and a second metal, without cathodic inhibitor the use of an external source of A or mixture current. that prevents or reduces the rate of contact potential the cathodic or reduction reaction by The potential difference at the physical, physico-chemical or junction of two dissimilar chemical action. substances. cathodic pickling continuity bond Electrolytic pickling in which the A metallic connection that provides work is the cathode. electrical continuity between metal cathodic polarization structures. Polarization of the cathode; change of the electrode potential in the A coating consisting of a compound active (negative) direction due to of the surface metal, produced by current flow; a reduction from the chemical or electrochemical initial potential resulting from treatments of the metal. Examples current flow effects at or near the include chromate on , cathode surface. Potential becomes , magnesium, and more active (negative) because of aluminum and and cathodic polarization. See also coatings on steel. See also chromate polarization. treatment and phosphating. coordination compound (1) Reduction of corrosion rate by A compound with a central atom or shifting the corrosion potential of ion bound to a group of or the electrode toward a less oxidizing molecules surrounding it. Also called potential by applying an external coordination complex. See also electromotive force. (2) Partial or chelate, complexation, and ligand. complete protection of a metal from copper-accelerated salt-spray (CASS) corrosion by making it a cathode, test using either a galvanic or an An accelerated corrosion test for impressed current. Contrast with some electrodeposits for anodic anodic protection. coatings on aluminum. cathodic reaction corrodkote test Electrode reaction equivalent to a An accelerated corrosion test for transfer of negative charge from the electrodeposits. electronic to the ionic conductor. A corrosion cathodic reaction is a reduction The chemical or electrochemical process. An example common in reaction between a material, usually corrosion is: Ox + ne s Red. a metal, and its environment that catholyte produces a deterioration of the The electrolyte adjacent to the material and its properties. cathode of an electrolytic cell. corrosion effect cation A change in any part of the A positively charged ion that corrosion system caused by migrates through the electrolyte corrosion. toward the cathode under the corrosion embrittlement influence of a potential gradient. See The severe loss of ductility of a also anion and ion. metal resulting from corrosive caustic attack, usually inter,granular and (1) Burning or corrosive. (2) A often not visually apparent. hydroxide of a light metal, such as corrosion-erosion sodium hydroxide or potassium Corrosion which is increased hydroxide. because of the abrasive action of a caustic dip moving stream; the presence of A strongly alkaline solution into suspended particles greatly which metal is immersed for accelerates abrasive action.See etching. for neutralizing , or for erosion-corrosion. removing organic materials such as corrosion greases or . The process in which a metal caustic embrittlement prematurely under An obsolete historical term denoting conditions of simultaneous corrosion a form of stress-corrosion cracking and repeated cyclic loading at lower most frequently encountered in stress levels or fewer cycles than carbon or iron-- would be required in the absence of nickel alloys that are exposed to the corrosive environment. concentrated hydroxide solutions at Limit temperatures of 200 to 250 ºC (400 The maximum cyclic stress value to 480 ºF). that a metal can with stand for a cavitation specified number of cycles or length The formation and instantaneous of time in a given corrosive collapse of innumerable tiny voids or environment. See corrosion fatigue cavities within a liquid subjected to strength rapid and intense pressure changes. corrosion fatigue strength Cavitation produced by ultrasonic The maximum repeated stress that radiation is sometimes used to effect can he endured by a metal without violent localized agitation. Cavitation failure under definite conditions of caused by severe turbulent flow corrosion and fatigue and for a often leads to cavitation damage. specific number of stress cycles and cavitation corrosion a specified period of time. A process involving conjoint corrosion and cavitation. See inhibitor. cavitation damage corrosion potential (Ecorr) The degradation of a solid body The potential of a corroding surface resulting from its exposure to in an electrolyte, relative to a cavitation. This may include loss of reference electrode. Also called rest material, surface deformation, or potential, open circuit potential, or changes in properties or freely corroding potential. appearance. corrosion product cavitation-erosion Substance formed as a result of Progressive loss of original material corrosion. from a solid surface due to corrosion protection continuing exposure to cavitation. Modification of a corrosion system cell so that corrosion damage is Electrochemical system consisting of mitigated. an anode and a cathode immersed corrosion rate in an electrolyte. The anode and Corrosion effect on a metal per unit cathode may be separate metals or of time. The type of corrosion rate dissimilar areas on the same metal. used depends on the technical The cell includes the external circuit, .system and on the type of corrosion which permits the flow of electrons effect. Thus, corrosion rate may be from the anode toward the cathode. expressed as an increase in See also . corrosion depth per unit of time Cementation Coating (penetration rate, for example, A coating developed on a metal mils/yr.) or the mass of metal surface by a high temperature turned into corrosion products per diffusion process (as carburization, unit area of surface per unit of time calorizing, or chromizing). (weight loss, for example, g/m- cementite /yr.). The corrosion effect may vary A compound of iron and carbon, with time and may not be the same known chemically as iron carbide at all points of the corroding surface. and having the approximate Therefore. reports of corrosion rates chemical formula Fe3C. It is should be accompanied by characterized by an orthorhombic information on the type, time crystal structure. When it occurs as dependency, and location of the a phase in steel, the chemical corrosion effect. composition will be altered by the corrosion resistance presence of manganese and other Ability of a metal to withstand carbide-forming elements. corrosion in a given corrosion chalking system. The development of loose removable corrosion system powder at the surface of an organic System consisting of one or more coating usually caused by metals and all parts of the weathering. environment that influence checking corrosion. The development of slight breaks in corrosivity a coating that do not penetrate to Tendency of an environment to the underlying surface. cause corrosion in a given corrosion checks system. Numerous, very fine cracks in a counter electrode coating or at the surface of a metal See auxiliary electrode . part. Checks may appear during couple processing or during service and are A cell developed in an electrolyte most often associated with thermal resulting from electrical contact treatment or thermal cycling. Also between two dissimilar metals. See called check marks. checking, or galvanic corrosion. heat checks. covering power chelate The ability of a solution to give (1) A molecular structure in which a satisfactory plating at very low heterocyclic ring can he formed by current densities. a condition that the unshared electrons of exists in recesses and pits. This neighboring atoms. (2) A term suggests an ability to cover, coordination compound in which a but not necessarily to build up, a heterocyclic ring is formed by a uniform coating, whereas throwing metal bound to two atoms of the power suggests the ability to obtain associated ligand. See also a coating of uniform thickness of an complexation. irregularly shaped object. chelating agent cracking (of coating) (1) An organic compound in which Breaks in a coating that extend atoms form more than one through to the underlying surface. coordinate bond with metals in crazing solution. (2) A substance used in A network of checks or cracks metal finishing to control or appearing on the surface. eliminate certain metallic ions present in undesirable quantities. Time-dependent strain occurring chelation under stress. The creep strain A chemical process involving occurring at a diminishing rate is formation of a heterocyclic ring called primary creep; that occurring compound that contains at least one at a minimum and almost constant metal cation or hydrogen ion in the rate, secondary creep; and that ring. occurring at an accelerating rate, chemical conversion coating tertiary creep. A protective or decorative creep-rupture embrittlement nonmetallic coating produced in silo Embrittlement under creep by chemical reaction of a metal with conditions of, for example, a chosen environment. It is often aluminum alloys and steels that used to prepare the surface prior to results in abnormally low rupture the application of an organic ductility. In aluminum alloys, iron in coating. amounts above the solubility limit is chemical potential known to cause such embrittlement; In a thermodynamic system of in steels, the phenomenon is related several constituents, the rate of to the amount of impurities (for change of the Gibbs function of the example. , , system with respect to the change in copper, arsenic, antimony, and ) the number of moles of a particular present. In either case, failure constituent. occurs by intergranular cracking of chemical vapor deposition the embrittled material. A coating process, similar to creep-rupture strength carburizing and carbonitriding, The stress that will cause fracture in whereby a reactant atmosphere gas a creep test at a given time in a is fed into a processing chamber specified constant environment. Also where it decomposes at the surface called stress-rupture strength. of the workpiece, liberating one crevice corrosion material for either absorption by, or Localized corrosion of a metal accumulation on the workpiece. A surface at, or immediately adjacent second material is liberated in gas to, an area that is shielded from full form and is removed from the exposure to the environment processing chamber, along with because of close proximity between excess atmosphere gas. the metal and the surface of another chemisorption material. The binding of an adsorbate to the critical anodic current density surface of a solid by forces whose The maximum anodic current energy levels approximate those of density observed in the active region a . Contrast with for a metal or electrode that physisorption. exhibits active-passive behavior in chevron pattern an environment. A fractographic pattern of radial critical flaw size marks (shear ledges) that look like The size of a flaw (defect) in a nested letters "V"; sometimes called structure that will cause failure at a a herringbone pattern. Chevron particular stress level. patterns are typically found on critical brittle fracture surfaces in parts The relative humidity above which whose widths are considerably the atmospheric corrosion rate of greater than their thicknesses. The some metals increases sharply. points of the chevrons can be traced critical pitting potential (Ecp, Ep, Epp) back to the fracture origin. The lowest value of oxidizing chromadizing potential at which pits nucleate and Improving adhesion on grow. It is dependent on the test aluminum or aluminum alloys, method used. mainly aircraft skins, by treatment current with a solution of chromic acid. Also The net transfer of electric charge called chromodizing or chromatizing. per unit time. Also called electric Not to be confused with chromating current. See also current density. or chromizing. current density chromate treatment The current flowing to or from a unit A treatment of metal in a solution of area of an electrode surface, a hexavalent chromium compound generally expressed as amps per sq to produce a conversion coating ft or milliamperes per sq ft (also consisting of trivalent and milliamps per sq cm, etc). hexavalent chromium compounds. current efficiency chromating The ratio of the electrochemical Performing a chromate treatment . equivalent current density for a chrome pickle specific reaction to the total applied (1) Producing a chromate current density. 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 .