Glossary of Terms
Acceleration corrosion test Method designed to approximate, in a short time, the deteriorating effect under normal long-term service condition.
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.
Acid embrittlement A form of hydrogen embrittlement that may be induced in some metals by acid.
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.
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.
Aerobic Environment containing oxygen, for instance, normal seawater.
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.
Alclad Composite wrought product comprising 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.
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 code position of two or more metallic elements.
Alloy steel Is one that contains either silicon or manganese in amounts in excess of those quoted in plain carbon steel or that contains any other element, or elements, as the result of deliberately made alloying additions.
Alternate-immersion test A corrosion test in which the specimens are intermittently exposed to a liquid medium at definite time intervals.
Aluminizing Process for impregnating the surface of a metal with aluminum in order to obtain protection from oxidation and corrosion.
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.
Anaerobic Lack of oxygen in the electrolyte adjacent to metallic structure.
Anchor pattern/surface profile Shape and amplitude of profile of blast cleaned or grooved steel, which influences the bond between metallic or paint of films and the substrate.
Anchorite A zinc–iron phosphate coating for iron and steel.
Anaerobic In the absence of air or unreacted or free oxygen.
Anion Negatively charged ion, which migrates to the anode of a galvanic or voltaic cell.
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 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 An electrode at which oxidation of the surface or some component of the solution is occurring.
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 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 Difference between the potential of an anode passing current and equilibrium potential (or steady state potential) of the electrode having the same electrode reaction.
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, by physical, physiochemical, or chemical action, the rate of the anodic or oxidation reaction.
Anodic metallic coating A coating, composed wholly or partially of an anodic metal (in sufficient quantity to set off electrochemical reaction), which is electrically positive to the substrate to which it is applied.
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 anodic current to it to cause its electrode potential to enter and remain in the passive region.
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.
Anodizing Forming a conversion coating on a metal surface by anodic oxidation; most frequently applied to aluminum.
Anti-fouling Intended to prevent fouling of underwater 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 a water solution.
Artificial aging Aging above room temperature.
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.
Attenuation The progressive decrease in potential and current density along buried or immersed pipeline in relation to distance from the point of injection.
Attenuation constant “a” The magnitude of the attenuation constant is directly affected by the longitudinal resistance of the pipe and inversely affected by the resistance across the coating. The attenuation constant describes how much the potential change or current flow decreases with increasing distance from the drain point.
Austenitic stainless steel Steel containing sufficient amount of nickel, nickel and chromium, or manganese to retain austenite at atmospheric temperature.
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.
Automatic welding Welding in which the welding variables and the mean of making the weld are controlled by machine.
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 The electrode of an electrochemical cell that is used to transfer current to or from a test electrode. An electrode commonly used in polarization studies to pass current to or from a test electrode. It is usually made of noncorroding material:
CUI: Corrosion under insulation
DLA: Double layer activation
ER: Electrical resistance
HAZ: Heat affected zone
LPR: Linear polarization resistance
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 but above the martensite start temperature.
Banded structure A segregated structure consisting of alternating nearly parallel bands of different composition, typically aligned in the direction of primary hot working.
Bandwidth (Ultrasonics) Range of frequencies around a mean value that constitutes the nominal frequency.
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-based 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 found on service fractures where the part is loaded randomly, intermittently, or with periodic variations in mean stress or alternating stress. See also striation.
Bead A single run of weld metal on a surface.
Bimetallic corrosion (Galvanic corrosion) Corrosion resulting from dissimilar metal contact.
Biocide A chemical that kills other forms of life in addition to bacteria.
Biological corrosion Biological corrosion is not a type of corrosion; it is the deterioration of a metal by corrosion processes that occur directly or indirectly as a result of the activity of living organisms. These organisms include micro forms such as bacteria and macro types such as algae and barnacles. Microscopic and macroscopic organisms have been observed to live and reproduce in mediums with pH values between 0 and 11, temperatures between −1.1 °C (30 °F) and 82.2 °C (180 °F), and under pressures up to 1020 bar (15,000 lb/in2). Thus, biological activity may influence corrosion in a variety of environments including soil, natural water and seawater, natural petroleum products, and oil emulsion-cutting fluids.
Biostat A chemical that retards growth of other forms of life in addition to bacteria.
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.
Blast cleaning Cleaning and roughening of a surface (particularly steel) by the use of metallic grit or nonmetallic grit or metal shot, which is projected against a surface by compressed air, centrifugal force, or water.
Blast peening Treatment for relieving tensile stress by inducing beneficial compressive stress in the surface by kinetic energy of rounded abrasive particles.
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.
Blistering The formation of swellings on the surface of an unbroken paint film by moisture, gases, or the development of corrosion products between the metal and the paint film.
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 within the range of about 200–370 °C (400–700 °F), 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.
Bond A piece of metal conductor, either solid or flexible, usually of copper, connecting two points on the same or on different structures, to prevent any appreciable change in the potential of the one point with respect to the other.
Bond resistance The ohmic resistance of a bond including the contact resistance at the points of attachment of its extremities.
Bonderizing A proprietary custom process for phosphatizing.
Braze welding The joining of metals using a technique similar to fusion welding and a filler metal with a lower melting point than the parent metal but neither using capillary action as in brazing nor intentionally melting the parent metal.
Brazing A process of joining metals in which, during or after heating, molten filler metal is drawn by capillary action into the space between closed adjacent surfaces of the parts to be joined. In general, the melting point of the filler metal is above 500 °C, but always below the melting temperature of the parent metal.
Brazing alloy Filler metal used in brazing.
Breakdown potential The least noble potential where pitting or crevice corrosion, or both, will initiate and propagate.
Breakway corrosion A sudden increase in corrosion rate, especially in high-temperature “dry” oxidation, etc.
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.
Butt joint A connection between the ends or edges of two parts making an angle to one another of 135–180° inclusive in the region of the joint.
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.
Calorizing Imparting resistance to oxidation to an iron or steel surface by heating in aluminum powder at 800–1000 °C (1470–1830 °F).
Carbon dioxide welding Metal-arc welding in which a bare wire electrode is used, the arc and molten pool being shielded with carbon dioxide gas.
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.
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 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.
Cathode The electrode of an electrolytic cell at which reduction occurs. In corrosion processes, usually the area at which metal ions do not enter the solution. Typical cathodic processes are cations taking up electrons and being discharged, oxygen being reduced, and the reduction from a higher to a lower state of valency.
Cathode depolarization Removal of protective layers and complexing ions, often caused by high acidity, high oxygen content, accidental shorts to other structures, results is increased corrosion current and corrosion rate. Cathode depolarization is indicated by a positive (+) shift in cathode-to-electrolyte potential.
Cathode film The portion of solution in immediate contact with the cathode during electrolysis.
Cathode polarization Build up of protective layers and complexing ions on the cathode electrode. Polarization of the cathode results in decreased corrosion and negative (−) shift in cathode-to-electrolyte potential.
Cathodic area That part of the metal surface which acts as a 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 combination of substances that prevent or reduce the rate of cathodic reaction by a physical, physiochemical, or chemical action.
Cathodic pickling Electrolytic pickling in which the work is the cathode.
Cathodic polarization Polarization of the cathode; a reduction from the initial potential resulting from current flow effects at or near the cathode surface. Potential becomes more active (negative) because of cathodic polarization.
Cathodic protection The process to reduce or prevent corrosion of metal structures in contact with an electrolyte by the flow of direct current from the electrolyte into the structure surface.
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 protection station A combination of equipment installed to provide cathodic protection to the structure(s).
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 s Red.
Catholyte The electrolyte adjacent to the cathode of an electrolytic cell.
Cation Positively charged ion that migrates to the cathode in a galvanic or voltaic cell.
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 200–250 °C (400–480 °F).
Cavitation erosion Cavitation is a particular kind of erosion–corrosion caused by the formation and collapse of vapor bubbles in a liquid contacting a metal surface. The resultant shock forces reach high levels in local areas and can tear out jagged chunks of brittle materials or deform soft metals. Where the environment is corrosive, severity of cavitation damage increases.
Cell A circuit consisting of an anode and a cathode in electrical contact in a solid or liquid electrolyte. Corrosion generally occurs only at anodic areas.
Cementation coating A coating developed on a metal surface by a high-temperature diffusion process (as carburization, calorizing, or chromizing).
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.
Characteristic resistance The characteristic resistance of the pipe is the electrical resistance between the pipe and remote earth in one direction only from the drain point in ohms.
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 cleaning Method of surface preparation or cleaning involving the use of chemicals, with or without electrical force, for removal of mill scale, rust, sediments, and paint. These chemicals can also be introduced into some systems on-stream while the system is operating.
Chemical conversion coating A protective or decorative coating that is produced deliberately on a metal surface by reaction of the surface with a chosen chemical environment. The thin layer formed by this reaction may perform several or all of the following functions: protect against corrosion; provide a base for organic coatings; improve retention of lubricants or compounds; improve abrasion resistance; provide an absorbent layer for rust-preventive oils and waxes.
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 corolling, coextrusion, welding, diffusion bonding, casting, heavy chemical deposition, or heavy electroplating.
Clad metals Two metals rolled together so that the cheaper one forms a backing for the more costly one.
Cleavage Splitting (fracture) of a crystal on a crystallographic plane of low index.
Cleavage fracture A fracture, usually of polycrystalline metal, in which most of the grains have failed by cleavage, resulting in bright reflecting facets. It is associated with low-energy brittle fracture.
Cold 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.
Compatibility The ability of a given material to exist unchanged under certain conditions and environments in the presence of some other material.
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.
Concentration cell A cell involving an electrolyte and two identical electrodes, with potential resulting from differences in the chemistry of the environments adjacent to the two electrodes.
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 potential The potential difference at the junction of two dissimilar substances.
Continuity bond A metallic connection that provides electrical continuity between metal structures. A bond designed and installed specifically to ensure the electrical continuity of a structure.
Controlled galvanic system Cathodic protection system using sacrificial anodes controlled by means of resistors, fixed or variable.
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.
Copper-accelerated salt-spray (CASS) test An accelerated corrosion test for some electrodeposits for anodic coatings on aluminum.
Copper/copper sulfate reference electrode A reference electrode consisting of copper in a saturated copper sulfate solution.
Copper ferrule A ring or cap of copper put around a slender shaft (as a cane or tool handle) to strengthen it or to prevent splitting.
Corrosion The destruction of a substance; usually a metal, or its properties because of a reaction with its surroundings (environment), i.e., physiochemical interaction between a metal and its environment that results in changes in the properties of the metal and that may often lead to impairment of the function of the metal, the environment, or the technical system, of which these form a part.
Corrosion damage Corrosion effect that is considered detrimental to the function of the metal, the environment, or the technical system, of which these form a part.
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 engineer The person responsible for carrying out the corrosion monitoring and interpretation of data produced.
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 limit The maximum cyclic stress value that a metal can withstand for a specified number of cycles or length of time in a given corrosive environment. See corrosion fatigue strength.
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 An inhibitor is a substance that retards or slows down a chemical reaction. Thus, a corrosion inhibitor is a substance that, when added to an environment, decreases the rate of attack by the environment on a metal. Corrosion inhibitors are commonly added in small amounts to acids, cooling water, oil wells and other environments, either continuously or intermittently to prevent serious corrosion.
Corrosion potential The potential of a corroding surface in an electrolyte, relative to a reference electrode.
Corrosion product The chemical compound or compounds produced by the reaction of a corroding metal with its environment.
Corrosion protection Modification of a corrosion system so that corrosion damage is mitigated.
Corrosion rate The rate at which corrosion proceeds, expressed by inches of penetration per year (ipy); mils penetration per year (mpy); milligrams weight loss per square decimeter per day (mdd); microns per year (μm/year) or millimeters per year (mmpy). One micron is equal to 0.0395 mils.
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.
Corrosive agent Substance that when in contact with a given metal will react with it.
Corrosive environment Environment that contains one or more corrosive agents.
Corrosivity Tendency of an environment to cause corrosion in a given corrosion system.
Couple A cell developed in an electrolyte resulting from electrical contact between two dissimilar metals. See galvanic corrosion.
Coupons Specimens of materials exposed to tests or real environments to assess the effect of degradation on the material.
Covered filler rod A filler rod having a covering of flux.
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 When a metal part fails by cracking, it is generally obvious that it cracked, but the exact type of cracking and the cause are less obvious. To determine the type of cracking, microscopic examination is necessary. In some instances, the environment plays a minor role, while in others its role is major.
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 Crevice corrosion is a special type of pitting. The anode of a corrosion cell is fixed by the geometry in a crevice or under a deposit. To function as a corrosion site, a crevice must be wide enough to permit entry of the liquid, but narrow enough to maintain a stagnant zone. Metals or alloys that depend on oxide films or passive layers for corrosion resistance are particularly susceptible to crevice corrosion.
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 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.
Current density The direct current per unit area, generally expressed as milliamperes per square meter. Current density required to achieve cathodic protection varies depending on environment and metal being protected.
Current efficiency The ratio of the electrochemical equivalent current density for a specific reaction to the total applied current density.
Deactivation The process of prior removal of the active corrosive constituents, usually oxygen, from a corrosive liquid by controlled corrosion of expendable metal or by other chemical means, thereby making the liquid less corrosive.
Dealloying The selective corrosion of one or more components of a solid solution alloy, usually in the form of ions. Also called parting or selective leaching.
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.
Decobaltification Corrosion in which cobalt is selectively leached from cobalt-based 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/substance thereof.
Deep anode bed Type of ground bed using a drilled vertical hole to contain impressed current anodes. Construction techniques vary widely but objectives are low ground bed resistance, uniform current distribution, less interference, and permanent moisture. Presently, current output is up to 150 A per hole. Typical depth range from 30 to 400 m to reach moist low-resistivity strata.
Deep ground bed 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 ground bed.
Demineralization Removal of dissolved mineral matter, generally from water.
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.
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 The elimination or reduction of polarization by physical or chemical means; depolarization results in increased corrosion.
Depolarizer A substance that produces depolarization.
Deposit A foreign substance that comes from the environment, adhering to a surface of a material.
Deposit attack Pitting corrosion resulting from deposits on a metal surface that cause concentration cells.
Deposit corrosion Corrosion occurring under or around a discontinuous deposit on a metallic surface. Also called poultice corrosion.
Deposited metal Filler metal after it becomes part of a weld or joint.
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 83% copper after extended service in water containing dissolved oxygen; the parting of zinc from an alloy (in some brasses, zinc is lost leaving a weak, brittle, porous, copper-rich residue behind). See also dealloying and selective leaching.
Dichromate treatment A chromate conversion coating produced on magnesium alloys in a boiling solution of sodium dichromate.
Dielectric strength Degree of electrical nonconductance of a material; the maximum electric field a material can withstand without breakdown.
Diffusion coating Application of metallic coating, the chemical composition of which was modified by diffusing this at melting temperature into the substrate. 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.
Disbandment 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 (charge separation) 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 condenser of 2 × 10−8 cm in thickness. In general, the electrode will be positively charged with respect to the solution.
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.
Drainage (current requirement) tests Tests with current applied for a short period, usually with temporary anodes and power sources in order to determine the current needed to achieve cathodic protection.
Driving EMF (galvanic anode system) The difference between the structure/electrolyte potential and the anode/electrolyte potential.
Dry corrosion Gaseous corrosion.
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.
Dynamic equilibrium The condition of an electrode when the rate of anodic dissolution just balances the rate of cathodic plating.
Embrittlement 885F (475C) Embrittlement of stainless steels upon extended exposure to temperatures between 400 and 510 °C (730 and 930 °F). This type of embrittlement is caused by fine, chromium-rich precipitates that segregate at grain boundaries: time at temperature directly influences the amount of segregation. Grain-boundary segregation of the chromium-rich precipitates increases strength and hardness, decreases ductility and toughness, and changes corrosion resistance. This type of embrittlement can be reversed by heating above the precipitation range.
Edge preparation Squaring, grooving, chamfering, or beveling an edge in preparation for welding.
Elastic deformation A change in dimensions directly proportional to and in phase with an increase or decrease in applied force.
Elasticity The property of a material by virtue of which deformation caused by stress disappears upon removal of the stress. A perfectly elastic body completely recovers its original shape and dimensions after release of stress.
Elastic limit The maximum stress that a material is capable of sustaining without any permanent strain (deformation) remaining upon complete release of the stress.
Elastomer A natural or synthetic polymer, which at room temperature can be stretched repeatedly to at least twice its original length, and which after removal of the tensile load will immediately and forcibly return to approximately its original length.
Electrical grounding Provides a low-resistance path to ground for fault currents in electrical equipment and distribution networks. Since bare copper is commonly used, current requirement calculations must include the copper as a substantial sink for cathodic protection current to adequately size for cathodic protection system.
Electrical isolation The condition of being electrically separated from other metallic structures or the environment.
Electrical resistivity The electrical resistance offered by a material to the flow of current, times the cross-sectional area of current flow and per unit length of current path; the reciprocal of the conductivity. Also called resistivity or specific resistance.
Electrochemical admittance The inverse of electrochemical impedance.
Electrochemical cell An electrochemical system consisting of an anode and a cathode in metallic contact and immersed in an electrolyte. (The anode and cathode may be different metals or dissimilar areas on the same metal surface.)
Electrochemical corrosion Corrosion that is accompanied by a flow of electrons between cathodic and anodic areas on metallic surfaces.
Electrochemical equivalent The weight of an element or group of elements oxidized or reduced at 100%, efficiency by the passage of a unit quantity of electricity. Usually expressed as grams per coulomb (1 amp/s).
Electrochemical impedance The frequency-dependent complex-valued proportionality factor (SE/6i) between the applied potential or current and the response signal. This factor is the total opposition of an electrochemical system to the passage of charge. The value is related to the corrosion rate under certain circumstances.
Electrochemical impedance spectroscopy (AC impedance)—A method to study the impedance of a metal/fluid interface by electrochemical properties by applying a sinusoidal polarization potential to the interface through a range of frequencies.
Electrochemical potential The partial derivative of the total electrochemical free energy at a constituent with respect to the number of moles of this constituent where all factors are kept constant. It is analogous to the chemical potential of a constituent except that it includes the electric as well as chemical contributions to the free energy. The potential of an electrode in an electrolyte relative to a reference electrode measured under open circuit conditions.
Electrochemical series Same as electromotive force series.
Electrode (1) An electronic conductor used to establish electrical contact with an electrolytic part of a circuit. (2) An electronic conductor in contact with an ionic conductor.
Electrode polarization Change of electrode potential with respect to a reference value. Often the free corrosion potential is used as the reference value. The change may be caused, for example, by the application of an external electrical current or by the addition of on oxidant or reductant.
Electrodeposition The deposition of a substance on an electrode by passing electric current through an electrolyte.
Electrode potential The potential of an electrode in an electrolyte as measured against a reference electrode. The electrode potential does not include any resistance losses in potential in either the solution or external circuit. It represents the reversible work to move a unit charge from the electrode surface through the solution to the reference electrode. The potential of an electrode as measured against a reference electrode. The electrode potential does not include any resistance loss in potential in solution due to the current passing to or from the electrode.
Electrode reaction Interfacial reaction equivalent to a transfer of charge between electronic and ionic conductors. See also anodic reaction and cathodic reaction.
Electrogalvanising Galvanized by electroplating.
Electrokinetic potential This potential, sometimes called zeta potential, is a potential difference in the solution caused by residual, unbalanced charge distribution in the adjoining solution, producing a double layer. The electrokinetic potential is different from the electrode potential in that it occurs exclusively in the solution phase, that is, it represents the reversible work necessary to bring a unit charge from infinity in the solution up to the interface in question but not through the interface.
Electroless plating A process in which metal ions in a dilute aqueous solution are plated out on a substrate by means of autocatalytic chemical reduction.
Electrolysis Production of chemical changes of the electrolyte by the passage of current through an electrochemical cell.
Electrolyte A chemical substance or mixture, usually liquid, containing ions that migrate in an electric field. An ionic conductor (usually in aqueous solution). (1) A chemical substance or mixture, usually liquid, containing ions that migrate in an electric field. (2) A chemical compound or mixture of compounds that when molten or in solution will conduct an electric current.
Electrolytic cell An assembly, consisting of a vessel, electrodes, and an electrolyte, in which electrolysis can be carried out.
Electrolytic cleaning A process of removing soil, scale, or corrosion products from a metal surface by subjecting it as an electrode to an electric current in an electrolytic bath; process of cleaning, degreasing, of a metal by making it an electrode in a suitable bath.
Electrolytic protection cathodic protection.
Electromotive force Electrical potential; voltage.
Electromotive force series (EMF series)—The potential of an electrode in an electrolytic solution when the forward rate of a given reaction is exactly equal to the reverse rate. (The equilibrium potential can only be defined with respect to a specific electrochemical reaction.).
Electromotive force series (emf series) A list of elements arranged according to their standard electrode potentials (hydrogen electrode is a reference point and given the value zero), with “noble” metals such as gold being positive and “active” metals such as zinc being negative.
Electron flow A movement of electrons in an external circuit connecting an anode and cathode in a corrosion cell; the current flow is arbitrarily considered to be in an opposite direction to the electron flow.
Electronegative A qualification applied to a metallic electrode to indicate that its potential is negative with respect to another metallic electrode in the system.
Electroplating Electrodeposition of a thin adherent layer of a metal or alloy of desirable chemical, physical, and mechanical properties on metallic or nonmetallic substrate.
Electropolishing A technique commonly used to prepare metallographic specimens, in which a high polish is produced by making the specimen the anode in an electrolytic cell, where preferential dissolution at high points smooths the surface.
Electropositive A qualification applied to a metallic electrode to indicate that its potential is positive with respect to another metallic electrode in the system.
Electroslag welding A welding process in which consumable electrodes are fed into a joint containing flux; the current melts the flux, and the flux in turn melts the faces of the joint and the electrodes, allowing the weld metal to form a continuous cast ingot between the joint faces.
Electron-beam welding Fusion welding in which the joint is made by fusing the parent metal by the impact of a focused beam of electrons.
Electrosmosis The passage of a liquid through a porous medium under the influence of a potential difference.
Embrittlement Severe loss of ductility of a metal (or alloy). Loss of load carrying capacity of a metal or alloy; the severe loss of ductility or toughness or both, of a material, usually a metal or alloy. Many forms of embrittlement can lead to brittle fracture. Many forms can occur during thermal treatment or elevated-temperature service (thermally induced embrittlement). Some of these forms of embrittlement, which affect steels, include blue brittleness, 885 °F (475 °C) embrittlement, quench-age embrittlement, sigma-phase embrittlement, strain-age embrittlement, temper embrittlement, tempered martensite embrittlement, and thermal embrittlement. In addition, steels and other metals and alloys can be embrittled by environmental conditions (environmentally assisted embrittlement). The forms of environmental embrittlement include acid embrittlement, caustic embrittlement, corrosion embrittlement, creep-rupture embrittlement, hydrogen embrittlement, liquid metal embrittlement, neutron embrittlement, solder embrittlement, solid metal embrittlement, and stress-corrosion cracking.
Encapsulation To protect the assembly by inhibited organic sealant, plastic caps, or cast potting compound.
Endurance limit The maximum stress that a material can withstand for an infinitely large number of fatigue cycles; maximum cyclic stress level a metal can withstand without fatigue failure. See also fatigue strength.
Environment The surroundings or conditions (physical, chemical, mechanical) in which a material exists.
Environmental cracking Brittle fracture of a normally ductile material in which the corrosive effect of the environment is a causative factor. Environmental cracking is a general term that includes corrosion fatigue, high-temperature hydrogen attack, hydrogen blistering, hydrogen embrittlement, liquid metal embrittlement, solid metal embrittlement, stress-corrosion cracking, and sulfide stress cracking. The following terms have been used in the past in connection with environmental cracking, but are becoming obsolete: caustic embrittlement, delayed fracture, season cracking, static fatigue, stepwise cracking, sulfide corrosion cracking, and sulfide stress-corrosion cracking. See also embrittlement.
Epoxy Resin formed by the reaction of bisphenol and epichlorohydrin.
Equilibrium (reversible) potential The potential of an electrode in an electrolytic solution when the forward rate of a given reaction is exactly equal to the reverse rate. The equilibrium potential can only be defined with respect to a specific electrochemical reaction.
Erosion Destruction of metals by the abrasive action of moving fluids accelerated by the presence of solid particles in suspension. When corrosion occurs simultaneously, the term “erosion–corrosion” is often used.
Erosion and erosion–corrosion Erosion is a strictly mechanical phenomenon, while erosion–corrosion is a combination of mechanical action and chemical or electrochemical reaction. Pure erosion seldom occurs in aqueous systems. Erosion–corrosion is characterized by grooves, gullies, waves, rounded holes, and valleys, and usually exhibits a directional pattern. In copper alloy heat exchanger tubes, the attack frequently results in the formation of horseshoe-shaped depressions. Erosion–corrosion is the acceleration of metal loss because of the relative movement between a fluid and a metal surface. Generally, the movement is rapid, and the effects of mechanical wear are involved. Metal is removed as dissolved ions or as solid corrosion products that are swept from the surfaces.
Erosion–corrosion Corrosion that is increased because of the abrasive action of a moving stream; the presence of suspended particles greatly accelerates abrasive action.
Etch To corrode the surface of a metal in order to reveal its composition and structure.
Eutectic (1) An isothermal reversible reaction in which a liquid solution is converted into two or more intimately mixed solids on cooling, the number of solids formed being the same as the number of components in the system. (2) An alloy having the composition indicated by the eutectic point on an equilibrium diagram. (3) An alloy structure of intermixed solid constituents formed by a eutectic reaction.
Eutectoid (1) An isothermal reversible reaction in which a solid solution is converted into two or more intimately mixed solids on cooling, the number of solids formed being the same as the number of components in the system. (2) An alloy having the composition indicated by the eutectoid point on an equilibrium diagram. (3) An alloy structure of intermixed solid constituents formed by a eutectoid reaction.
Exchange current When an electrode reaches dynamic equilibrium in a solution, the rate of anodic dissolution balances the rate of cathodic plating. The rate at which either positive or negative charges are entering or leaving the surface at this point is known as the exchange current.
Exchange current density The rate of charge transfer per unit area when an electrode reaches dynamic equilibrium (at its reversible potential) in a solution; that is, the rate of anodic charge transfer (oxidation) balances the rate of cathodic charge transfer (reduction).
Exfoliation Exfoliation is a type of subsurface corrosion that occurs and propagates as cracks approximately parallel to the surface. It leaves the metal in a laminated, flaky, or blistered condition, and appears most frequently in aluminum alloys or cupronickels.
Exothermal (cadweld/thermit) weld Process of insuring electrical connection of cable to piping or steel structures incendiary mixture of fine aluminum powder with a metallic oxide of iron or chromium, which when ignited yields an intense heat.
External circuit The wires, connectors, measuring devices, current sources, etc. that are used to bring about or measure the desired electrical conditions within the test cell. It is this portion of the cell through which electrons travel.
Failure A general term used to imply that a part in service (1) has become completely inoperable, (2) is still operable but is incapable of satisfactorily performing its intended function, or (3) has deteriorated seriously, to the point that it has become unreliable or unsafe for continued use.
Faraday's law (1) The amount of any substance dissolved or deposited in electrolysis is proportional to the total electric charge passed. (2) The amounts of different substances dissolved or deposited by the passage of the same electric charge are proportional to their equivalent weights.
Fatigue Subjecting a material to repeated stresses ultimately results in cracking. The environment may have an effect on the fatigue limit of a metal, although this is usually a minor factor. Generally, a fatigue failure is a single fracture, which is transgranular in most common metals. There is normally only a single fracture because stresses on other regions are relieved when the fracture occurs. Characteristic chevron patterns or beach marks can appear on the fracture face.
Fatigue crack growth rate The rate of crack extension caused by constant-amplitude fatigue loading, expressed in terms of crack extension per cycle of load application.
Fatigue life The number of cycles of stress that can be sustained prior to failure under a stated test condition.
Fatigue limit The maximum stress that presumably leads to fatigue fracture in a specified number of stress cycles. If the stress is not completely reversed, the value of the mean stress, the minimum stress, or the stress ratio should also be stated. Compare with endurance limit.
Fatigue strength The maximum stress that can be sustained for a specified number of cycles without failure, the stress being completely reversed within each cycle unless otherwise stated.
Ferritic Pertaining to the body-centered cubic crystal structure (BCC) of many ferrous (iron-based) metals.
Filiform corrosion Corrosion that occurs under film in the form of randomly distributed hairlines.
Filler rod Filler metal in the form of a rod. It may also take the form of filler wire.
Filler metal Metal added during welding, braze welding, brazing, or surfacing.
Film A thin, not necessarily visible, layer of material.
Finite line If the line terminates in an insulated flange or dead ends, it is called a finite line.
Flakes Short, discontinuous internal fissures in wrought metals attributed to stresses produced by localized transformation and decreased solubility of hydrogen during cooling after hot working. In a fracture surface, flakes appear as bright silvery areas; on an etched surface, they appear as short, discontinuous cracks. Also called shatter cracks or snowflakes.
Flame hardening Hardening of metal surface by heating with oxyacetylene torch, followed by rapid cooling with water or air jet.
Flame spraying Thermal spraying in which coating material is fed into an oxyfuel gas flame, where it is melted. Compressed gas may or may not be used to atomize the coating material and propel it onto the substrate.
Flow line Pipeline carrying product from wellhead to gas–oil separator plant (GOSP), typically 100–250 DN, coated or uncoated usually above ground on pipe supports with periodic road crossings.
Flux Material used during welding, brazing, or braze welding to clean the surfaces of joint, prevent atmospheric oxidation, and reduce impurities.
Fogged metal A metal whose luster has been reduced because of a surface film, usually a corrosion product layer.
Foreign structure Any metallic structure that is not intended as part of a cathodic protection system of interest.
Fouling An accumulation of deposits. This term includes accumulation and growth of marine organisms on a submerged metal surface and also includes the accumulation of deposits (usually inorganic) on heat exchanger tubing.
Fouling organism Any aquatic organism with a sessile adult stage that attaches to and fouls underwater structures of ships.
Fractography Descriptive treatment of fracture, especially in metals, with specific reference to photographs of the fracture surface. Macrofractography involves photographs at low magnification (<25×); microfractography, photographs at high magnification (>25×).
Fracture mechanics A quantitative analysis for evaluating structural behavior in terms of applied stress, crack length, and specimen or machine component geometry.
Fracture toughness A generic term for measures of resistance to extension of a crack. The term is sometimes restricted to results of fracture mechanics tests, which are directly applicable in fracture control. However, the term commonly includes results from simple tests of notched or precracked specimens not based on fracture mechanics analysis. Results from test of the latter type are often useful for fracture control, based on either service experience or empirical correlations with fracture mechanics tests.
Free carbon The part of the total carbon in steel or cast iron that is present in elemental form as graphite or temper carbon. Contrast with combined carbon.
Free corrosion potential Corrosion potential in the absence of net electrical current flowing to or from the metal surface.
Free ferrite Ferrite that is formed directly from the decomposition of hypoeutectoid austenite during cooling, without the simultaneous formation of cementite. Also called proeutectoid ferrite.
Free machining Pertains to the machining characteristics of an alloy to which one or more ingredients have been introduced to give small broken chips, lower power consumption, better surface finish, and longer tool life; among such additions are sulfur or lead to steel, lead to brass, lead and bismuth to aluminum, and sulfur or selenium to stainless steel.
Fretting A type of wear that occurs between tight-fitting surfaces subjected to cyclic relative motion of extremely small amplitude. Usually, fretting is accompanied by corrosion, especially of the very fine wear debris. Fretting refers to metal deterioration caused by repetitive slip at the interface between two surfaces. When metal loss is increased by corrosion, the term “fretting corrosion” should be used.
Fretting corrosion Another special case of erosion–corrosion, fretting corrosion, occurs when two heavily loaded metals rub rapidly together, causing damage to one or both metals. Vibration is usually responsible for the damage, but corrosion is also a factor because the frictional heat increases oxidation. In addition, mechanical removal of protective corrosion products continually exposes fresh metal. Fretting corrosion occurs more frequently in air than in water.
Furan Resin formed from reactions involving furfuryl alcohol alone or in combination with other constituents.
Fusion penetration Depth to which the parent metal has been fused.
Fusion welding Welding in which the weld is made between metals in a molten state without the application of pressure.
Fusion zone The part of the parent metal that is melted into the weld metal.
Galvanic Pertaining to the current resulting from the coupling of dissimilar electrodes in an electrolyte.
Galvanic anode or sacrificial A metal, which because of its relative position in the electromotive force (emf) series, provides sacrificial protection to metals that are less negative (lower) in the series, when the two are electrically coupled in an electrolyte. The voltage difference between the anode and the structure causes a current flow in the structure that opposes the corrosion current. The common types of galvanic anodes are rod, bracelet, and ribbon.
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 When two dissimilar metals are in contact with each other and exposed to a conductive environment, a potential exists between them, and a current flows. The less resistant metal becomes anodic, and the more resistant, cathodic. Attack on the less resistant metal increases, while on the more resistant one, it decreases.
Galvanic couple A pair of dissimilar conductors, commonly metals, in electrical contact.
Galvanic couple potential Mixed potential.
Galvanic current The electric current that flows between metals or conductive nonmetal in a galvanic couple.
Galvanic series A list of metals arranged according to their relative corrosion potentials in some specific environment; seawater is often used.
Galvanize To coat a metal surface with zinc using any of various processes.
Galvanizing The accepted term for the coating of iron or steel with zinc by the immersion of the metal in a bath of molten zinc. Galvanizing comes from “GALVANO.”
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.
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.
Gamma iron The face-centered cubic form of pure iron, stable from 910 to l400 °C (1670–2550 °F).
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.
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 of gray cast iron in which the metallic constituents are selectively leached or converted to corrosion products leaving the graphite intact.
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 iron from the kraft recovery furnace in water.
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.
Ground (anode) bed Commonly a group of manufactured electrodes or scrap steel that serve as the anode for the cathodic protection of pipelines, tanks, or other buried metallic structures. Types of ground beds are surface and deep anode configurations.
Half-cell A pure metal in contact with a solution of known concentration of its own ion, at a specific temperature, develops a potential that is characteristic and reproducible; when coupled with another half-cell, an overall potential develops, which is the sum of both half-cells.
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 applications.
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 surface at which the metal exhibits a specific hardness (50 HRC, for example) or a specific percentage of martensite in the microstructure.
Hardfacing Depositing filler metal on a surface by welding, spraying, or braze welding to increase resistance to abrasion, erosion, wear, galling, impact, or cavitation damage.
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.
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 caused microstructural changes that generally affect corrosion behavior.
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.
Hematite (1) An iron mineral crystallizing in the rhombohedral system; the most important are of iron. (2) An iron oxide, FeO, corresponding to an iron content of approximately 70%.
Hermetic seal An impervious seal made by the fusion of metals or ceramic materials (as by brazing, soldering, welding, and fusing glass or ceramic), which prevents the passage of gas or moisture.
High-temperature hydrogen attack A loss of strength and ductility of steel by high-temperature reaction of absorbed hydrogen with carbides in the steel resulting in decarburization and internal fissuring.
Holidays Discontinuities in a coating (such as porosity, cracks, etc.) 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 effect of oxidation and reactions with sulfur compounds and other contaminants, such as chlorides, to form a molten salt on a metal surface that f1uxes, destroys, or disrupts the normal protective oxide. See also gaseous erosion.
Hot cracking Also called solidification cracking, hot cracking of weldments is caused by the segregation at grain boundaries of low-melting constituents in the weld metal. This can result in grain-boundary tearing under thermal contraction stresses. Hot cracking can be minimized by the use of low-impurity welding materials and proper joint design. See also cold cracking, lamellar tearing, and stress-relief cracking.
Hot working Deforming metal plastically at such a temperature and strain rate that recrystallization takes place simultaneously with the deformation, thus avoiding any strain hardening. Contrast with cold working.
Hot dip coating A metallic coating obtained by dipping the base metal into a molten metal.
Hot shortness A tendency for some alloys to separate along grain boundaries when stressed or deformed 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.
Huey test Corrosion testing in a boiling solution of nitric acid. This test is mainly used to detect the susceptibility to intergranular corrosion of stainless steel.
Humidity test A corrosion test involving exposure of specimens at controlled levels of humidity and temperature. Contrast with salt fog test.
Hydrogen-assisted cracking (HAC) Hydrogen embrittlement.
Hydrogen-assisted stress-corrosion cracking (HSCC) Hydrogen embrittlement.
Hydrogen blistering The formation of blisters on or below a metal surface from excessive internal hydrogen pressure; formation of blisterlike bulges on a ductile metal surface caused by internal hydrogen pressures. Hydrogen may be formed during cleaning, plating, corrosion, and so forth.
Hydrogen controlled electrode A covered electrode that, when used correctly, produces less than a specified amount of diffusible hydrogen in the weld deposit.
Hydrogen damage At moderate temperatures, hydrogen damage can occur as a result of a corrosion reaction on a surface or cathodic protection. Atomic hydrogen diffuses into the metal and collects at internal voids or laminations where it combines to form more voluminous molecular hydrogen. In steels, blisters sometimes occur. At higher temperatures and pressures, atomic hydrogen can diffuse into steel and collect at grain boundaries. Either molecular hydrogen is then formed, or the hydrogen reacts with iron carbides to form methane, resulting in cracking and decarburization. Hydrogen cracking is intergranular and highly branched, but not continuous.
Hydrogen disintegration Deep internal cracks caused by hydrogen.
Hydrogen embrittlement A process resulting in a decrease in 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 solidification. The second type, environmental hydrogen embrittlement, results from hydrogen being absorbed by solid metals. This can occur during elevated-temperature thermal treatments and in service during electroplating, contact with maintenance chemicals, corrosion reactions, cathodic protection, and operating in high-pressure hydrogen. In the absence of residual stress or external loading, environmental hydrogen embrittlement is manifested in various forms, such as blistering, internal cracking, hydride formation, and reduced ductility. With a tensile stress or stress-intensity factor exceeding a specific threshold, the atomic hydrogen interacts with the metal to induce subcritical crack growth leading to fracture. In the absence of a corrosion reaction (polarized cathodically), the usual term used is hydrogen-assisted cracking (HAC) or hydrogen stress cracking (HSC). In the presence of active corrosion, usually as pits or crevices (polarized anodically), the cracking is generally called stress-corrosion cracking (SCC), but should more properly be called hydrogen-assisted stress-corrosion cracking (HSCC). Thus, HSC and electrochemically anodic SCC can operate separately or in combination (HSCC). In some metals, such as high-strength steels, the mechanism is believed to be all, or nearly all, HSC. The participating mechanism of HSC is not always recognized and may be evaluated under the generic heading of SCC. Embrittlement of a metal caused by hydrogen; sometimes observed in cathodically protected steel, electroplated parts, pickled steel, etc. Hydrogen embrittlement process results decrease of toughness or ductility of a metal due to absorption of hydrogen.
Hydrogen probes Probes designed to measure the permeation rate of atomic hydrogen H+ (measured as hydrogen gas H2) associated with hydrogen-induced cracking.
Hydrogen stress cracking (HSC) Hydrogen embrittlement.
Hydrogen sulfide Without H2S: environment containing less than 1 ppm H2S. With H2S: environment containing more than 1 ppm H2S. Low H2S: greater than 3 ppm but less than 6 ppm H2S. High H2S: more than 6 ppm.
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, or failing 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 effects of water vapor. (3) Pertaining to water absorbed by dry soil minerals from the atmosphere; the amounts depend on the physiochemical character of the surfaces, and increase with rising relative humidity.
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 power source external to the anode system. Typical power sources are rectifiers solar modules and engine generators.
Impressed current protection Cathodic protection of structures, where the cathodic polarization of metal is secured by electric currents emitted from an independent source.
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.
Infinite line It is a relatively long finite line or a line directly connected to a structure of quite different electrical characteristics such as bare line or a tank farm.
Inhibitor A chemical substance or combination of substances that, when present in the proper concentration and forms in the environment, prevents or reduces corrosion.
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.
Insulated flange A flanged joint in which flange faces and securing bolts are electrically insulated from each other by insulating sleeves, washers and gaskets. Pipe size and pressure rating must be specified. Insulated flanges are used to electrically isolate pipelines and systems.
Insulated joint Performs similar function to an insulating but does not incorporate flange gaskets or bolts. Provides electrical isolation via epoxy-sealed gap in a section of pipe.
Insulated spool It is similar to insulated joint but incorporates internal nonconductive lining where electrically conductive pipeline products are involved.
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.
Interference bond A welded metallic connection between two sections of a structure or two or more different structures to carry electrical current. Often used to bypass current in known locations to avoid corrosive current discharge (interference) in unknown or undesirable locations. Then resistance bonds are used, the resistance value must be less than the parallel structure-to- structure soil resistance.
Interference test A structure-to-electrolyte test to determine the existence of corrosion interaction between two buried or immersed structures where one or both are cathodically protected.
Intergranular Between crystals or grains. Also called intercrystalline. Contrast with transgranular.
Intergranular corrosion Metals are composed of grains or crystals that form as solidification occurs. A crystal grows until it meets another advancing crystal. The regions of disarray between crystals are called grain boundaries, which differ in composition from the crystal center. Intergranular corrosion is the selective attack of the grain boundary or an adjacent zone. The most common example of intergranular corrosion is that of sensitized austenitic stainless steels in heat-affected zones at welds. Intergranular corrosion usually leaves the surface roughened, but definite diagnosis must be made by microscopic examination. Corrosion that occurs preferentially at grain boundaries.
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.
Intergranular stress-corrosion cracking (IGSCC) Stress-corrosion cracking in which the cracking occurs along grain boundaries.
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 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.
Killed steel Thoroughly deoxidized steel, for example, by addition of aluminum or silicon, in which the reduction between carbon and oxygen during solidification is suppressed.
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 Wood-pulping process in which sodium sulfate is used in the caustic soda pulp-digestion liquor. Also called kraft pulping or sulfate pulping.
Laminar scale Rust formation in heavy layers.
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 minimized by designing joints to minimize weld shrinkage stresses and joint restraint. See also cold cracking, hot cracking, and stress-relief cracking.
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.
Leakage or coating resistance Leakage resistance of pipe or resistance of pipe radially to remote earth. This resistance includes the resistance of the coating (if any) and is affected by the resistivity of the environment. Unit is ohm-kilometer.
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.
Linear pipe resistance The longitudinal pipe resistance of the pipe is the pipe resistance in ohms per unit length, which can be calculated from the specific resistivity of steel or iron and by regarding the pipeline as an annular cylinder. The specific resistivity of steel pipe will normally vary from 15 to 23 microhm-cm depending on its chemistry. In the absence of specific test results it is normal to use a value of 18 microhm-cm.
Linear polarization resistance (LPR) (See polarization resistance.) At small applied polarization potentials, the relationship between applied potential approximates the polarization resistance.
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.
Longline 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.
Luggin probe (luggin haber capillary) A small tube or capillary filled with electrolyte, terminating close to the metal surface under study, and used to provide an ionically conducting path without diffusion between an electrode under study and a reference electrode.
Macroscopic Visible at magnifications to 25×.
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).
Manual welding Welding in which the means of making the weld are held in the hand.
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 martensitic lattice (such as carbon in iron), the structure is hard and highly strained; but where the solute atoms occupy substitutional positions (such as nickel in iron), the martensite is soft and ductile. The amount of high-temperature phase that transforms to martensite on cooling depends to a large extent on the lowest temperature attained, there being a rather distinct beginning temperature (Ms) and a temperature at which the transformation is essentially complete (Mf).
Mechanical plating Plating wherein fine metal powders are peened onto the work by tumbling or other means.
Metal-arc welding Arc welding using a consumable electrode.
Metal dusting Accelerated deterioration of metals in carbonaceous gases at elevated temperatures to form a dustlike corrosion product; a unique form of high-temperature corrosion that forms a dustlike corrosion product and sometimes develops hemispherical pits on a susceptible metal surface; simultaneous carburization is generally observed.
Metal ion concentration cell A galvanic cell caused by a difference in metal ion concentration at two locations on the same metal surface.
Metalizing (1) The application of an electrically conductive metallic layer to the surface of nonconductors. (2) The application of metallic coatings by nonelectrolytic procedures such as spraying of molten metal and deposition from the vapor phase.
Meteor perforation Perforation of material in outer space resulting from meteor strikes.
Microbial corrosion biological corrosion.
Microbiologically influenced corrosion (MIC) Corrosion that is substantially increased as the result of the presence of bacteria (such as sulfate-reducing bacteria, SRB; or acid-producing bacteria, APB).
Microscopic Visible at magnifications above 25×.
Microstructure The structure of a prepared surface of a metal as revealed by a microscope at a magnification exceeding 25×.
MIG-welding Metal inert gas arc welding using a consumable electrode.
Mill scale An oxide layer on metals or alloys produced by metal rolling, hot forming, welding, or heat treatment. Especially applicable to iron and steel.
Mixed potential The potential of a specimen (or specimens in a galvanic couple) when two or more electrochemical reactions are occurring. Also called galvanic couple potential.
Molal solution Concentration of a solution expressed in moles of solute divided by 1000 g of solvent.
Molar solution Aqueous solution that contains 1 mol (gram-molecular weight) of solute in 1 L of the solution.
Mole One mole is the mass numerically equal (in grams) to the relative molecular mass of a substance. It is the amount of substance of a system that contains as many elementary units (6.023 exp23) as there are atoms of carbon in 0.012 kg of the pure nuclide C12; the elementary unit must be specified and may be an atom, molecule, ion, electron, photon, or even a specified group of such units.
Monomer A molecule usually an organic compound, having the ability to join with a number of identical molecules to form a polymer.
Noble metal A metal that is not very reactive (such as silver, gold, or copper) and may be found naturally in metallic form on earth.
Normalizing Heating a ferrous alloy to a suitable temperature above the transformation range and then cooling in air to a temperature substantially below the transformation range.
Open-circuit potential The potential of an electrode measured with respect to a reference electrode or another electrode when no current flows to or from it.
Organic Being or composed of hydrocarbons or their derivatives, or matter of plant or animal origin. Contrast with inorganic.
Organic acid A chemical compound with one or more carboxyl radicals (COOH) in its structure; examples are butyric acid, CH3(CH2)2COOH; maleic acid, HOOCCH-CHCOOH; and benzoic acid, C6H5COOH.
Organic zinc coating A paint containing zinc powder pigment and an organic (containing carbon) resin.
Organic zinc-rich paint Coating containing zinc powder pigment and an organic resin.
Overaging Aging under conditions of time and temperature greater than those required to obtain maximum change in a certain property, so that the property is altered in the direction of the initial value.
Overheating Heating a metal or alloy to such a high temperature that its properties are impaired. When the original properties cannot be restored by further heat treating, by mechanical working, or by a combination of working and heat treating, the overheating is known as burning.
Overload When a metal part has been subjected to a single stress beyond its tensile strength, it can fail by overload. The fracture can be either ductile or brittle, depending on factors such as the metal's hardness and operating temperature. In most cases, a single fracture results.
Overvoltage The difference between the actual electrode potential when appreciable electrolysis begins and the reversible electrode potential.
Oxyacetylene welding Gas welding in which fuel gas is acetylene and which is burnt in an oxygen atmosphere.
Oxidation (1) A reaction in which there is an increase in valence resulting from a loss of electrons. Contrast with reduction. (2) A corrosion reaction in which the corroded metal forms an oxide; usually applied to reaction with a gas containing elemental oxygen, such as air.
Oxidized surface (on steel) Surface having a thin, tightly adhering, oxidized skin (from straw to blue in color), extending in from the edge of a coil or sheet.
Oxidizing agent A compound that causes oxidation, thereby itself being reduced.
Oxygen concentration cell A galvanic cell resulting from difference in oxygen concentration between two locations.
Ozone A powerfully oxidizing allotropic form of the element oxygen. The ozone molecule contains three atoms (O3). Ozone gas is decidedly blue, and both liquid and solid ozone are an opaque blue-black color, similar to that of ink.
Parent metal Metal to be joined, base metal.
Parkerizing Trade name for process for the production of phosphate coating on steel articles by immersion in an aqueous solution of manganese or zinc acid with phosphate.
Partial annealing An imprecise term used to denote a treatment given cold-worked material to reduce its strength to a controlled level or to effect stress relief. To be meaningful, the type of material, the degree of cold work, and the time–temperature schedule must be stated.
Parting The selective attack of one or more components of a solid solution alloy; e.g., dezincification, dealumination, etc. See dealloying.
Parts per million A measure of proportion by weight, equivalent to one unit weight of a material per million (106) unit weights of compound. One part per million is equivalent to l mg/g.
Passivation A reduction of the anodic reaction rate of an electrode involved in electrochemical action such as corrosion.
Passivator A type of inhibitor that appreciably changes the potential of a metal to a more noble (positive) value.
Passive The state of a metal when it is much more resistant to corrosion than its position in the EMF series would predict. Passivation is a surface phenomenon.
Passive–active cell (1) A cell, the emf of which is due to the potential difference between a metal in an active state and the same metal in a passive state. (2) A corrosion cell in which the anode is a metal in the active state and the cathode is the same metal in the passive state.
Passivity A metal or alloy that is thermodynamically unstable in a given electrolytic solution is said to be passive when it remains visibly unchanged for a prolonged period. The following should be noted:
• During passivation the appearance may change if the passivating film is sufficiently thick (e.g., interference films);
• The electrode potential of a passive metal is always appreciably more noble than its potential in the active state;
• Passiity is an anodic phenomenon and thus control of corrosion by decreasing cathodic reactivity (e.g., amalgamated zinc in sulfuric acid) or by cathodic protection is not passivity.
Patina The coating, usually green, that forms on the surface of metals such as copper and copper alloys exposed to the atmosphere. Also used to describe the appearance of a weathered surface of any metal.
Pearlite A metastable lamellar aggregate of ferrite and cementite resulting from the transformation of austenite at temperatures above the bainite range.
Peen plating Deposition of the coating metal, in powder form, on the substrate by a tumbling action in presence of peening shot.
Phosphating Forming an adherent phosphate coating on a metal by immersion in a suitable aqueous phosphate solution. Also called phosphatizing.
Phosphatizing The forming of a thin inert phosphate coating on a surface, usually accomplished by treating with H3PO4 (phosphoric acid).
pH A measure of the acidity or alkalinity of a solution; The negative logarithm of the hydrogen ion activity; it denotes the degree of acidity or basicity of a solution. At 25 °C (77 °F), 7.0 is the neutral value. Decreasing values below 7.0 indicate increasing acidity; increasing values above 7.0, increasing basicity.
Physical vapor deposition A coating process whereby the cleaned and masked component to be coated is heated and rotated on a spindle above the streaming vapor generated by melting and evaporating a coating material source bar with a focused electron beam in an evacuated chamber.
Physisorption The binding of an adsorbate to the surface of a solid by forces whose energy levels approximate those of condensation. Contrast with chemisorption.
Pickle A solution, usually acid, used to remove mill scale or other corrosion products from a metal.
Pickle/Pickling Form of chemical and electrolytic removal or loosening of mill scale and corrosion products from the surface of a metal in a chemical solution (usually acidic). Electrolytic pickling can be anodic or cathodic depending on polarization of metal in the solution.
Pitting Highly localized corrosion resulting in deep penetration at only a few spots.
Pitting factor Ratio of the depth of the deepest pit resulting from corrosion divided by the average penetration as calculated from weight loss.
Plane strain The stress condition in linear elastic fracture mechanics in which there is zero strain in a direction normal to both the axis of applied tensile stress and the direction of crack growth (that is, parallel to the crack front); most nearly achieved in loading thick plates along a direction parallel to the plate surface. Under plane-strain conditions, the plane of fracture instability is normal to the axis of the principal tensile stress.
Plane stress The stress condition in linear elastic fracture mechanics in which the stress in the thickness direction is zero; most nearly achieved in loading very thin sheet along a direction parallel to the surface of the sheet. Under plane-stress conditions, the plane of fracture instability is inclined 45° to the axis of the principal tensile stress.
Plasma plating Deposition on critical areas of metal coatings resistant to wear and abrasion, by means of a high-velocity and high-temperature ionized inert gas jet.
Plasma spraying A thermal spraying process in which the coating material is melted with heat from a plasma torch that generates a nontransferred arc; molten coating material is propelled against the base metal by the hot, ionized gas issuing from the torch.
Plastic deformation The permanent (inelastic) distortion of metals under applied stresses that strain the material beyond its elastic limit.
Plasticity The property that enables a material to undergo permanent deformation without rupture.
Polarization The deviation from the open circuit potential of an electrode resulting from the passage of current.
Polarization admittance The reciprocal of polarization resistance (di/dE).
Polarization curve A plot of r current density versus electrode potential for a specific electrode–electrolyte combination.
Polarization resistance The slope (dE/di) at the corrosion potential of a potential (E) versus current density (i) curve. (It is inversely proportional to the corrosion current density when the polarization resistance technique is applicable.).
Polyester Resin formed by condensation of polybasic and monobasic acids with polyhydric alcohols.
Polymer A chain of organic molecules produced by the joining of primary units called monomers.
Potential Any of various functions from which intensity or velocity at any point in a field may be calculated. The driving influence of an electrochemical reaction. See also active potential, chemical potential, corrosion potential, critical pitting potential, decomposition potential, electrochemical potential, electrode potential, electrokinetic potential, equilibrium (reversible) potential, free corrosion potential, noble potential, open-circuit potential, protective potential, redox potential, and standard electrode potential.
Potential surveys Measurement of potential of a structure or pipeline relative to a reference electrode potential pitting.
Potentiodynamic (potentiokinetic) The technique for varying the potential of an electrode in a continuous manner at a preset rate.
Potentiostat An electronic device that maintains an electrode at a constant potential; used in anodic protection devices.
Poultice corrosion A term used in the automotive industry to describe the corrosion of vehicle body parts due to the collection of road salts and debris on ledges and in pockets that are kept moist by weather and washing. Also called deposit corrosion or attack.
Pourbaix (potential–pH) diagram A plot of the redox potential of a corroding system versus the pH of the system, compiled using thermodynamic data and the Nernst equation. The diagram shows regions within which the metal itself or some of its compounds are stable.
Powder metallurgy The art of producing metal powders and utilizing metal powders for production of massive materials and shaped objects.
Precious metal One of the relatively scarce and valuable metals: gold, silver, and the platinum-group metals. Also called noble metal(s).
Precipitation hardening Hardening caused by the precipitation of a constituent from a supersaturated solution. See also age hardening and aging.
Precipitation hardening Hardening caused by the precipitation of a constituent from a supersaturated solid solution. See also age hardening and aging.
Precipitation heat treatment Artificial aging in which a constituent precipitated from a supersaturated solid solution.
Precracked specimen A specimen that is notched and subjected to alternating stresses until a crack has developed at the root of the notch.
Pressure welding A welding process in which a weld is made by a sufficient pressure to cause plastic flow of the surfaces, which may or may not be heated.
Primary current distribution The current distribution in an electrolytic cell that is free of polarization.
Primary passive potential (passivation potential) The potential corresponding to the maximum active current density (critical anodic current density) of an electrode that exhibits active–passive corrosion behavior.
Principal stress (normal) The maximum or minimum value at the normal stress at a point in a plane considered with respect to all possible orientations of the considered plane. On such principal planes the shear stress is zero. There are three principal stresses on three mutually perpendicular planes. The state of stress at a point may be (1) uniaxial, a state of stress in which two of the three principal stresses are zero, (2) biaxial, a state of stress in which only one of the three principal stresses is zero, and (3) triaxial, a state of stress in which none of the principal stresses is zero. Multiaxial stress refers to either biaxial or triaxial stress.
Profile Anchor pattern on a surface produced by abrasive blasting or acid treatment.
Protection current The current made to flow into a metallic structure from its ectrolytic environment in order to effect cathodic protection of the structure.
Protective potential The threshold value of the corrosion potential that has to be reached to enter a protective potential range. The term used in cathodic protection to refer to the minimum potential required to suppress corrosion.
Protective potential range A range of corrosion potential values in which unacceptable corrosion resistance is achieved for a particular purpose.
Quench-age embrittlement Embrittlement of low-carbon steels resulting from precipitation of solute carbon of existing dislocations and from precipitation hardening of the steel caused by differences in the solid solubility of carbon in ferrite at different temperatures. Quench-age embrittlement usually is caused by rapid cooling of the steel from temperature slightly below AC1 (the temperature at which austenite begins to form), and can be minimized by quenching from lower temperature.
Quench aging Aging induced by rapid cooling after solution heat treatment.
Quench cracking Fracture of a metal during quenching from elevated temperature. Most frequently observed in hardened carbon steel, alloy steel, or tool steel parts of high hardness and low toughness. Cracks often emanate from filets, holes, corners, or other stress raisers and result from high stresses due to the volume changes accompanying transformation to martensite.
Quench hardening (1) In ferrous alloys, hardening by austenitizing and then cooling at a rate such that a substantial amount of austenite transforms to martensite. (2) In copper and titanium alloys, hardening by solution treating and quenching to develop a martensite-like structure.
Quenching Rapid cooling of metals (often steels) from a suitable elevated temperature. This generally is accomplished by immersion in water, oil, polymer solution, or salt, although forced air is sometimes used.
Radiation damage A general term for the alteration of properties of a material arising from exposure to ionizing radiation (penetrating radiation), such as X-rays, gamma rays. neutrons, heavy-particle radiation, or fission fragments in nuclear fuel material.
Radiography (X-ray) Use of X-rays to measure thickness or imperfection within solid materials.
Rare earth metal One of the group of l5 chemically similar metals with atomic numbers 57 through 7l, commonly referred to as the lanthanides.
Reactive metal A metal that readily combines with oxygen at elevated temperatures to form very stable oxides, for example, titanium, zirconium, and beryllium. Reactive metals may also become embrittled by the interstitial absorption of oxygen, hydrogen, and nitrogen.
Recrystallization (1) Formation of a new, strain-free grain structure from that existing in cold worked metal, usually accomplished by heating. (2) The change from one crystal structure to another, as occurs on heating or cooling through a critical temperature.
Redox potential The potential of a reversible oxidation–reduction electrode measured with respect to a reference electrode, corrected to the hydrogen electrode, in a given electrolyte.
Reducing agent A compound that causes reduction, thereby itself becoming oxidized.
Reduction A reaction in which there is a decrease in valence resulting from a gain in electrons. Contrast with oxidation.
Reference electrode A nonpolarizable electrode with a known and highly reproducible potential used for potentiometric and voltammetric analyses.
Refractory metal A metal having an extremely high melting point, for example, tungsten, molybdenum, tantalum, niobium, chromium, vanadium, and rhenium. In the broad sense, this term refers to metals having melting points above the range for iron, cobalt, and nickel.
Relative humidity The ratio, expressed as a percentage, of the amount of water vapor present in a given volume of air at a given temperature to the amount required to saturate the air at that temperature.
Remedial bond A bond installed between a primary and a secondary structure in order to eliminate or reduce corrosion interaction.
Remote earth The areas in which the structure-to-electrolyte potential change is negligible with change in reference electrode position away from the structure.
Residual stress Stresses that remain within a body as a result of plastic deformation.
Resistance The opposition that a device or material offers to the flow of direct current, equal to the voltage drop across the element divided by the current through the element. Also called electrical resistance.
Resistance bond A bond either incorporating resistors or of adequate resistance in itself to limit the flow of current.
Resistance welding Welding in which force is applied to surfaces in contact and in which the heat for welding is produced by the passage of electric current through the electrical resistance at, and adjacent to, these surfaces.
Resolution (Ultrasonics) The ability of a system to give simultaneous separate indications from discontinuities that are close together both in depth and lateral position.
Reverse current switch The reverse current switch is installed in series with interference bonds where stray current is known to reverse direction. Prevents serious corrosion, where reversed current can discharge to the electrolyte, by interrupting the reversed current. A failed switch becomes an open circuit or a solid bond.
Ringworm corrosion Localized corrosion frequently observed in oil well tubing in which a circumferential attack is observed near a region of metal “upset.”
Riser (1) That section of pipeline extending from the ocean floor up the platform. Also, the vertical tube in a steam generator convection bank that circulates water and steam upward. (2) A reservoir of molten metal connected to a casting to provide additional metal to the casting, required as the result of shrinkage before and during solidification.
Run The metal method or deposited during one passage of an electrode, torch, or blow-pipe.
Rust A visible corrosion product consisting of hydrated oxides of iron. Applied only to ferrous alloys. See also white rust.
Rusting Corrosion of iron or an iron-based alloy to form a reddish-brown product, which is primarily hydrated ferric oxide.
Sacrificial anode The use of an alloy electrode less noble than the structure to which it is connected, to induce galvanic corrosion on the electrode (anode) in preference to that of the structure.
Sacrificial protection/sacrificial anodes/sacrificial pieces Pieces of metal that, being anodic to the equipment into which they are introduced, will galvanically corrode and so protect the equipment. Cathodic protection, based on wasting of anodic metal to prevent corrosion of cathodic metal—zinc, aluminum, magnesium, carbon steel, etc.—so protecting steel and other more noble metals.
Safety bond A bond connecting the metallic framework or enclosure of electrical apparatus with earth, in order to limit its rise in potential above earth in the event of a fault, and so reduce the risk of electric shock to anyone touching the framework or enclosure.
Salt fog test An accelerated corrosion test in which specimens are exposed to a fine mist of a solution usually containing sodium chloride, but sometimes modified with other chemicals.
Saturated calomel electrode A reference electrode composed of mercury, mercurous chloride (calomel), and a saturated aqueous chloride solution.
Scaling (1) The formation at high temperatures of thick corrosion product layers on a metal surface. (2) The deposition of water-insoluble constituents on a metal surface.
Season cracking An obsolete historical term usually applied to stress-corrosion crackling of brass. A term usually applied to stress-corrosion cracking of brass.
Selective corrosion The selective corrosion of certain alloying constituents from an alloy (as dezincification), or in an alloy (as internal oxidation).
Selective leaching Selective leaching describes a corrosion process also called “parting” or dealloying. More specifically, it can be called dezincification in the case of brasses, denickelification in cupronickels, etc. Selective leaching may occur in a plug form or in a more evenly distributed layer type. Stagnant conditions and regions under deposits are conducive to selective leaching. In brasses, it can occur at pH extremes in water; high dissolved solids and high temperature also promote selective leaching. The overall dimensions of a part do not change drastically, but appreciable weakening can occur.
Semiautomatic welding Welding in which some of the variables are automatically controlled, but manual guidance is necessary.
Sensitization heat treatment A heat treatment, whether accidental, intentional, or incidental (as during welding), that causes precipitation of constituents at grain boundaries, often causing the alloy to become susceptible to intergranullar corrosion, cracking, or SCC (stress-corrosion cracking).
Sensitization In austenitic stainless steels, the precipitation of chromium carbide usually at grain boundaries, on exposure to temperatures of about 550–850 °C.
Shear The type of force that causes or tends to cause two contiguous parts of the same body to slide relative to each other in a direction parallel to their plane of contact.
Shear strength The stress required to produce fracture in the plane of cross-section, the conditions of loading being such that the directions of force and of resistance are parallel and opposite, although their paths are offset a specified minimum amount. The maximum load divided by the original cross-sectional area of a section separated by shear.
Sherodising The coating of iron or steel with zinc by heating the product to be coated in zinc powder at a temperature below the melting point of zinc.
Sigma phase A hard, brittle, nonmagnetic intermediate phase with a tetragonal crystal structure, containing 30 atoms per unit cell, space group P42 mnm, occurring in many binary and ternary alloys of the transition elements. The composition of this phase in the various systems is not the same and the phase usually exhibits a wide range in homogeneity. Alloying with a third transition element usually enlarges the field of homogeneity and extends it deep into the ternary section.
Sigma-phase embrittlement Embrittlement of iron–chromium alloys (most notably austenitic stainless steels) caused by precipitation at grain boundaries of the hard, brittle intermetallic sigma phase during long periods of exposure to temperatures between approximately 560 and 980 °C (1050 and 1800 °F). Sigma-phase embrittlement results in severe loss in toughness and ductility, and can make the embrittled material susceptible to intergranular corrosion. See also sensitization.
Silver/silver chloride electrode (Ag/AgCl) A reference electrode consisting of silver, coated with silver chloride in an electrolyte containing chloride ions.
Slip Plastic deformation by the irreversible shear displacement (translation) of one part of a crystal relative to another in a definite crystallographic direction and usually on a specific crystallographic plane. Sometimes called glide.
Slow strain rate technique An experimental technique for evaluating susceptibility to stress-corrosion cracking. It involves pulling the specimen to failure in uniaxial tension at a controlled slow strain rate while the specimen is in the test environment and examining the specimen for evidence of stress-corrosion cracking.
Slushing compound An obsolete term describing oil or grease coatings used to provide temporary protection against atmospheric corrosion.
Smelt Molten slag; in the pulp and paper industry, the cooking chemicals tapped from the recovery boiler as molten material and dissolved in the smelt tank as green liquor.
S–N diagram A plot showing the relationship of stress, S, and the number of cycles, N, before fracture in fatigue testing.
Soft water Water that is free of magnesium or calcium salts.
Soil resistivity The electrical resistivity of the soil. This is important in cathodic protection systems as it affects current distribution through the soil and potentials on the protected structure.
Solder embrittlement Reduction in mechanical properties of a metal as a result of local penetration of solder along grain boundaries.
Solid metal embrittlement The occurrence of embrittlement in a material below the melting point of the embrittling species. See also liquid metal embrittlement.
Solid solution A single, solid, homogeneous crystalline phase containing two or more chemical species.
Solute The component of either a liquid or solid solution that is present to a lesser or minor extent: the component that is dissolved in the solution.
Solution In chemistry, a homogeneous dispersion of two or more kinds of molecular or ionic species. Solution may be composed of any combination of liquids, solids, or gases, but they always consist of a single phase.
Solution heat treatment Heating an alloy to a suitable temperature, holding at that temperature long enough to cause one or more constituents to enter into solid solution, and then cooling rapidly enough to hold these constituents in solution.
Solution potential Electrode potential where half-cell reaction involves only the metal electrode and its ion.
Solvent The component of either a liquid or solid solution that is present to a greater or major extent; the component that dissolves the solute.
Sour gas A gaseous environment containing hydrogen sulfide and carbon dioxide in hydrocarbon reservoirs. Prolonged exposure to sour gas can lead to hydrogen damage, sulfide-stress cracking, and/or stress-corrosion cracking in ferrous alloys.
Sour water Waste waters containing fetid materials, usually sulfur compounds.
Spalling The spontaneous chipping, fragmentation, or separation of a surface or surface coating.
Spatter Globules of metal expelled during welding onto the surface of parent metal or of a weld.
Spelter A brazing alloy consisting nominally of 50% Cu and 50% Zn.
Spheroidite An aggregate of iron or alloy carbides of essentially spherical shape dispersed throughout a matrix of ferrite.
Splash zone That part of external structure between a level 2 m above mean high water springs (MHWS) and a level 3 m below the lowest astronomical tide (LAT).
Sputtering A coating process whereby thermally emitted electrons collide with inert gas atoms, which accelerate toward and impact a negatively charged electrode that is a target of the coating material. The impacting ions dislodge atoms of the target material, which are in turn projected to and deposited on the substrate to form the coating.
Stabilizing treatment (1) Before finishing to final dimensions, repeatedly heating a ferrous or nonferrous part to or slightly above its normal operating temperature and then cooling to room temperature to ensure dimensional stability in service. (2) Transforming retained austenite in quenched hardenable steels, usually by cold treatment. (3) Heating a solution-treated stabilized grade of austenitic stainless steel to 870–900 °C (1600–1650 °F) to precipitate all carbon, such as TiC, NbC, or TaC, so that sensitization is avoided on subsequent exposure to elevated temperature.
Standard electrode potential The reversible potential for an electrode process when all products and reactions are at unit activity on a scale in which the potential for the standard hydrogen half-cell is zero.
Steel
• Ferrite: Ferrite is the name given to the body-centered cubic allotropes of iron, á and ä iron, and to body-centered cubic solid solutions.
• Austenite: Austenite is the name given to the face-centered cubic, or ã variety of iron, and to the face-centered cubic solid solutions.
• Cementite: Cementite is the name given to the carbide of iron, Fe3C. This is an extremely hard and brittle constituent.
• Pearlite: Pearlite is the eutectoid mixture of ferrite and cementite, and is formed when austenite decomposes during cooling. It consists of alternate thin layers, or lamellae, of ferrite and cementite.
• Martensite: This is the name given to the very hard and brittle constituent that is formed when a steel is very rapidly cooled from the austenitic state. It is a ferrite, highly supersaturated with dissolved carbon.
• Sorbite and troostite: These are names given to the structures produced when martensite or bainite is tempered, that is, heated to same temperature not exceeding 700 °C for the purpose of reducing brittleness and hardness.
• Bainite: This is the term given to the decomposition product that is formed when austenite decomposes by either isotherm transformation, or at a cooling rate intermediate between the very rapid cooling necessary for martensite and the slower rate of cooling at which pearlite is formed.
• Plain carbon steel: This is a steel containing up to 1.5% of carbon together with not more than 0.5% of silicon and not more than 1.5% of manganese, and only traces of other elements.
• Alloy steel: It is one that contains either silicon or manganese in amounts in excess of those quoted above, or that contains any other element, or elements, as the result of deliberately made alloying additions.
Strain The unit of change in the size or shape of a body due to force. Also known as nominal strain.
Strain-age embrittlement A loss in ductility accompanied by an increase in hardness and strength that occurs when low-carbon steel (especially rimmed or capped steel) is aged following plastic deformation. The degree of embrittlement is a function of aging time and temperature, occurring in a matter of minutes at about 200 °C (400 °F), but requiring a few hours to a year at room temperature.
Strain aging Aging induced by cold working.
Strain hardening An increase in hardness and strength caused by plastic deformation at temperatures below the recrystallization range.
Strain rate The time rate of straining for the usual tensile test. Strain as measured directly on the specimen gage length is used for determining strain rate. Because strain is dimensionless, the units of strain rate are reciprocal time.
Stray current Current flowing through paths other than the intended circuit.
Stray-current corrosion Stray-current corrosion differs from other forms in that the source of the current causing the corrosion is external to the affected equipment. This cause of metal deterioration is frequently misdiagnosed. Stray-current corrosion can cause local metal loss in buried or submerged metal structures, but it occurs much less frequently in underwater transporting equipment than in underground structures. Stray-current corrosion is almost always associated with direct current. At the anodic areas, metal goes into solution and the electrolyte tends to become acidic. It is most commonly encountered in soils containing water.
Stress The intensity of the internally distributed forces or components of forces that resist a change in the volume or shape of a material that is or has been subjected to external forces. Stress is expressed in force per unit area and is calculated on the basis of the original dimensions of the cross-section of the specimen. Stress can be either direct (tension or compression) or shear. See also residual stress.
Stress concentration factor (Kt) A multiplying factor for applied stress that allows for the presence of a structural discontinuity such as a notch or hole; Kt equals the ratio of the greatest stress in the region of the discontinuity to the nominal stress for the entire section. Also called theoretical stress concentration factor.
Stress corrosion or stress-accelerated corrosion Corrosion that is accelerated by stress.
Stress-corrosion cracking Stress-corrosion cracking is the result of the combined action of static stresses and corrosion. The static stresses may be residual or applied service stresses. The environment plays an important role in this type of cracking. The resulting cracks are branched, and can propagate either transgranularly or intergranularly, and sometimes both ways. Caustic cracking of steel is a case of stress-corrosion cracking that is sometimes called caustic embrittlement. Generally speaking, it is not necessarily the concentration of the corrodent in the bulk environment that causes the cracking, but the increased concentration occurring in crevices or in alternately wetted and dried regions.
Stress-intensity factor A scaling factor, usually denoted by the symbol K, used in linear-elastic fracture mechanics to describe the intensification of applied stress at the tip of a crack of known size and shape. At the onset of rapid crack propagation in any structure containing a crack, the factor is called the critical stress-intensity factor, or the fracture toughness. Various subscripts are used to denote different loading conditions or fracture toughnesses: Kc. Plane-stress fracture toughness. The value of stress intensity at which crack propagation becomes rapid in sections thinner than those in which plane-strain conditions prevail. KI. Stress-intensity factor for a loading condition that displaced the crack faces in a direction normal to the crack plane (also known as the opening mode of deformation). KIc. Plane-strain fracture toughness. The minimum value of Kc for any given material and condition, which is attained when rapid crack propagation in the opening mode is governed by plane-strain conditions. KId. Dynamic fracture toughness. The fracture toughness determined under dynamic loading conditions; it is used as an approximation of KIc for very tough materials. KISCC. Threshold stress-intensity factor for stress-corrosion cracking. The critical plane-strain stress intensity at the onset of stress-corrosion cracking under specified conditions. KQ. Provisional value for plane-strain fracture toughness. Kth. Threshold stress intensity for stress-corrosion cracking. The critical stress intensity at the onset of stress-corrosion cracking under specified conditions. DK. The range of the stress-intensity factor during a fatigue cycle.
Stress-intensity factor range, DK. In fatigue, the variation in the stress-intensity factor in cycle, that is, Kmax–Kmin.
Stress raisers Changes in contour or discontinuities in structure that cause local increases in stress.
Stress ratio, A or R The algebraic ratio of two specified stress values in a stress cycle. Two commonly used stress ratios are (1) the ratio of the alternating stress amplitude to the mean stress, A = Sa/Sm, and (2) the ratio of the minimum stress to the maximum stress, R = Smin/Smax.
Stress-relief cracking Also called postweld heat treatment cracking, stress-relief cracking occurs when susceptible alloys are subjected to thermal stress relief after welding to reduce residual stresses and improve toughness. Stress-relief cracking occurs only in metals that can precipitation-harden during such elevated-temperature exposure; it usually occurs at stress raisers, is intergranular in nature, and is generally observed in the coarse-grained region of the weld heat-affected zone. See also cold cracking, hot cracking, and lamellar tearing.
Striation A fatigue fracture feature, often observed in electron micrographs, that indicates the position of the crack front after each succeeding cycle of stress. The distance between striations indicates the advance of the crack front across that crystal during one stress cycle, and a line normal to the striation indicates the direction of local crack propagation. See also beach marks.
Structure-to-electrolyte potential The potential difference between a metallic structure and the electrolyte surrounding it, measured with a reference electrolyte. Typical examples are pipe-to-seawater potentials.
Submerged-arc welding Metal-arc welding in which a bare wire electrode is used; the arc is enveloped in flux; some of which fuse to form a removable covering of slag on the weld.
Submerged zone That part of external structure below the splash zone.
Substrate The basic metal or nonmetal whose surface is being protected.
Subsurface corrosion Formation of isolated particles of corrosion products beneath a metal surface. This results from the preferential reactions of certain alloy constituents to inward diffusion of oxygen, nitrogen, or sulfur.
Sulfate-reducing bacteria (SRB) A group of bacteria found in most soils and natural waters, but active only in anaerobic conditions of near neutral pH. They reduce sulfates in their environment, with the production of sulfides. SRB increase cathodic protection current requirements as a result of cathodic depolarization by sulfides.
Sulfidation The reaction of a metal or alloy with a sulfur-containing species to produce a sulfur compound that forms on or beneath the surface on the metal or alloy.
Sulfide stress cracking Brittle failure by cracking under the combined action of tensile stress and corrosion in the presence of water and hydrogen sulfide. See also environmental cracking.
Surface anode bed Type of ground bed using impressed current or galvanic anodes. This type of ground bed is used in relatively low-resistivity soils like marl (low-resistivity limestone and clay) with water-retaining capability.
Surface treatment Any suitable means of cleaning and treating a surface that will result in the desired surface profile and cleanliness and the required coating characteristics.
Surfactant A surface-active agent; usually an organic compound whose molecules contain a hydrophilic group at one end and a lipophilic group at the other.
Tafel line, tafel slope, tafel diagram An electrode when polarized frequently yields a current potential relationship over a region, which can be approximated by: h = ± B log (i/i0) where h = change in open circuit potential, i = the current density, B and i0 = constants. B is known as the Tafel slope. If this behavior is observed, a plot of the semilogarithmic components is known as the Tafel line and the diagram is called the Tafel diagram.
Tafel slope The slope of the linear portion of a plot of potential versus the log of the current density for a particular electrode in a corroding system.
Tarnish Surface discoloration of a metal caused by formation of a thin film of corrosion product.
Temper (1) In heat treatment, to reheat hardened steel or hardened cast iron to some temperature below the eutectoid temperature for the purpose of decreasing hardness and increasing toughness. The process is also sometimes applied to normalized steel. (2) In tool steels, temper is sometimes inadvisably used to denote carbon content. (3) In nonferrous alloys and in some ferrous alloys (steels that cannot be hardened by heat treatment), the hardness and strength produced by mechanical or thermal treatment, or both, and characterized by a certain structure, mechanical properties, or reduction of area during cold working.
Temper color A thin, tightly adhering oxide skin (only a few molecules thick) that forms when steel is tempered at a low temperature, or, for a short time, in air or a mildly oxidizing atmosphere. The color, which ranges from straw to blue depending on the thickness of the oxide skin, varies with both tempering time and temperature.
Tempered martensite embrittlement Embrittlement of ultra high-strength steels caused by tempering in the temperature range of 205–400 °C (400–750 °F); also called 350 °C or 500 °F embrittlement. Tempered martensite embrittlement is thought to result from the combined effects of cementite precipitation on prior-austenite grain boundaries or interlath boundaries and the segregation of impurities at prior-austenite grain boundaries.
Temper embrittlement Embrittlement of alloy steels caused by holding within or cooling slowly through a temperature range just below the transformation range. Embrittlement is the result of the segregation at grain boundaries of impurities such as arsenic, antimony, phosphorus, and tin; it is usually manifested as an upward shift in ductile-to-brittle transition temperature. Temper embrittlement can be reversed by retempering above the critical temperature range, then cooling rapidly.
Tempering To reheat hardened steel or hardened cast iron to some temperature below the eutectoid temperature for the purpose of decreasing hardness and increasing toughness.
Tensile strength In tensile testing, the ratio of maximum load to original cross-sectional area. Also called ultimate tensile strength.
Tensile stress A stress that causes two parts of an elastic body, on either side of a typical stress plane, to pull apart. Contrast with compressive stress.
Tension The force or load that produces elongation.
Terne An alloy of lead containing 3–l5% Sn, used as a hot dip coating for steel sheet or plate. Terne coatings, which are smooth and dull in appearance, give the steel better corrosion resistance and enhance its ability to be formed, soldered, or painted.
Terne plate Deposition of lead–tin alloy on iron or steel sheets by the hot dip process.
Test access hole Provides a means of contacting soil through concrete or asphalt for measuring structure-to-soil potentials. Contains no wires and is usually capped but easily accessible.
Test station Permanent wires attached to the structure and led to a convenient location for electrical measurements. Used at points where the structure or soil is otherwise inaccessible for electrical testing (underground or underwater).
Thermal electromotive force The electromotive force generated in a circuit containing two dissimilar metals when one junction is at a temperature different from that of the other. See also thermocouple.
Thermal embrittlement Intergranular fracture of maraging steels with decreased toughness resulting from improper processing after hot working. Thermal embrittlement occurs upon heating above l095 °C (2000 °F) and then slow cooling through the temperature range of 815–980 °C (1300F–l800 °F), and has been attributed to precipitation of titanium carbides and titanium carbonitrides at austenite grain boundaries during cooling through the critical temperature range.
Thermal cutting The parting or shaping of materials by the application of heat with or without a stream of cutting oxygen.
Thermal spraying A group of coating or welding processes in which finely divided metallic or nonmetallic materials are deposited in a molten or semimolten condition to form a coating. The coating material may be in the form of powder, ceramic rod, wire, or molten materials. See also flame spraying and plasma spraying.
Thermocouple A device for measuring temperatures, consisting of lengths of two dissimilar metals or alloys that are electrically joined at one end and connected to a voltage-measuring instrument at the other end. When one junction is hotter than the other, a thermal electromotive force is produced that is roughly proportional to the difference in temperature between the hot and cold junctions.
Thermogalvanic corrosion Corrosion resulting from an electrochemical cell caused by a thermal gradient.
Threshold stress Threshold stress for stress-corrosion cracking. The critical gross section stress at the onset of stress-corrosion cracking under specified conditions.
Throwing power (1) The relationship between the current density at a point on a surface and its distance from the counter electrode. The greater the ratio of the surface resistivity shown by the electrode reaction to the volume resistivity of the electrolyte, the better is the throwing power of the process. (2) The ability of a plating solution to produce a uniform metal distribution on an irregularly shaped cathode.
TIG-welding Tungsten inert gas arc welding using a nonconsumable electrode of pure or activated tungsten.
Tinning Coating metal with a very thin layer of molten solder or brazing filler metal.
Torsion A twisting deformation of a solid body about an axis in which lines that were initially parallel to the axis become helices.
Torsional stress The shear stress on a transverse cross-section resulting from a twisting action.
Total carbon The sum of the free carbon and combined carbon (including carbon in solution) in a ferrous alloy.
Toughness The ability of a metal to absorb energy and deform plastically before fracturing.
Transference The movement of ions through the electrolyte associated with the passage of the electric current. Also called transport or migration.
Transgranular Through or across crystals or grains. Also called intracrystalline or transcrystalline.
Transgranular cracking Cracking or fracturing that occurs through or across a crystal or grain. Also called transcrystalline cracking. Contrast with intergranular cracking.
Transgranular fracture Fracture through or across the crystals or grains of a metal. Also called transcrystalline fracture or intracrystalline fracture. Contrast with intergranular fracture.
Transition metal A metal in which the available electron energy levels are occupied in such a way that the d-band contains less than its maximum number of 10 electrons per atom, for example, iron, cobalt, nickel, and tungsten. The distinctive properties of the transition metals result from the incompletely filled d-levels.
Transition temperature (1) An arbitrarily defined temperature that lies within the temperature range in which metal fracture characteristics (as usually determined by tests of notched specimens) change rapidly, such as from primarily fibrous (shear) to primarily crystalline (cleavage) fracture. (2) Sometimes used to denote an arbitrarily defined temperature within a range in which the ductility changes rapidly with temperature.
Transpassive region The region of an anodic polarization curve, noble to and above the passive potential range, in which there is a significant increase in current density (increased metal dissolution) as the potential becomes more positive (noble).
Transpassive state (1) State of anodically passivated metal characterized by a considerable increase in the corrosion current, in the absence of pitting, when the potential is increased. (2) The noble region of potential where an electrode exhibits at higher than passive current density.
Tuberculation The formation of localized corrosion products scattered over the surface in the form of knoblike mounds called tubercles.
u-bend specimen Horseshoe-shaped test piece used to detect the susceptibility of a material to stress-corrosion cracking.
Ultimate strength The maximum stress (tensile, compressive, or shear) a material can sustain without fracture, determined by dividing maximum load by the original cross-sectional area of the specimen. Also called nominal strength or maximum strength.
Ultrasonic measurement The timing of the transmission of ultrasonic sound waves through a material to determine the material's thickness.
Underfilm corrosion Corrosion that occurs under organic films in the form of randomly distributed threadlike filaments or spots. In many cases, this is identical to filiform corrosion.
Uniform corrosion (1) A type of corrosion attack (deterioration) uniformly distributed over metal surface. (2) Corrosion that proceeds at approximately the same rate over a metal surface. Also called general corrosion. General corrosion is characterized by a chemical or electrochemical reaction that occurs uniformly over the exposed surface. Anodic and cathodic sites shift constantly so that corrosion spreads over the entire metal surface. Identifying general corrosion is usually simple, but determining its cause is often difficult. Chemical dissolution by acids, bases, or chelants frequently results in general corrosion.
Vacuum deposition/vapor depositions/gas plating Deposition of metal coatings by the precipitation, sometimes in vacuum, or metal vapor on the treated surface. Vapor may be produced by thermal decomposition, cathode sputtering, or by evaporation of the molten metal in air or inert gas.
Valence A positive number that characterizes the combining power of an element for other elements, as measured by the number of bonds to other atoms that one atom of the given element forms upon chemical combination: hydrogen is assigned valence 1, and the valence is the number of hydrogen atoms, or their equivalent, with which an atom of the given element combines.
Vapor plating Deposition of a metal or compound on a heated surface by reduction or decomposition of a volatile compound at a temperature below the melting points of the deposit and the base material. The reduction is usually accomplished by a gaseous reducing agent such as hydrogen. The decomposition process may involve thermal dissociation or reaction with the base material. Occasionally used to designate deposition on cold surfaces by vacuum evaporation. See also vacuum deposition.
Voids A term generally applied to paints to describe holidays, holes, and skips in a film. Also used to describe shrinkage in castings and weld.
Vulcanized rubber Treatment of rubber with sulfur or sulfur compound resulting in a change in physical properties of rubber.
Wash primer A thin, inhibiting paint, usually chromate pigmented with a polyvinyl butyrate binder.
Weather resistance Ability of a material to resist all ambient weather conditions. These include changes of temperature, precipitation, effect of wind and humidity, sunlight, oxygen and other gases and impurities in the atmosphere, ultraviolet rays, radiation, and ozone.
Weight coat Steel mesh-reinforced concrete layer applied over a primary coating system. Provides negative buoyancy for submarine pipelines.
Weld A union between pieces of metal at faces rendered plastic or liquid by heat or by pressure, or by both. A filler metal whose melting temperature is of the same order as that of the parent metal may or may not be used.
Welding The making of a weld.
Weld cracking Cracking that occurs in the weld metal. See also cold cracking, hot cracking, lamellar tearing, and stress-relief cracking.
Weld decay Intergranular corrosion, usually of stainless steels or certain nickel-based alloys, that occurs as the result of sensitization in the heat-affected zone during the welding operation.
Weld metal All metal melted during the making of a weld and retained in the weld.
Weld zone The zone containing the weld metal and the heat-affected zone.
Wetting A condition in which the interfacial tension between a liquid and a solid is such that the contact angle is 0–90°.
Wetting agent A substance that reduces the surface tension of a liquid, thereby causing it to spread more readily on a solid surface.
White liquor Cooking liquor from the kraft pulping process produced by recausticizing green liquor with lime.
White rust Zinc oxide: the powdery product of corrosion of zinc or zinc-coated surfaces.
Work hardening Same as strain hardening.
Working electrode The test or specimen electrode in an electrochemical cell.
Yeld Evidence of plastic deformation in structural materials. Also called plastic flow or creep.
Yield point The first stress in a material, usually less than the maximum attainable stress, at which an increase in strain occurs without an increase in stress. Only certain metals—those that exhibit a localized, heterogeneous type of transition from elastic deformation to plastic deformation—produce a yield point. If there is a decrease in stress after yielding, a distinction may be made between upper and lower yield points. The load at which a sudden drop in the flow curve occurs is called the upper yield point. The constant load shown on the flow curve is the lower yield point.
Yield strength The stress at which a material exhibits a specified deviation from proportionality of stress and strain. An onset of 0.2% is used for many metals.
Yield stress The stress level in a material at or above the yield strength but below the ultimate strength, i.e., a stress in the plastic range.
Zinc reference electrode A 99.9% metallic zinc rod, with an iron content not exceeding 0.0014%, which contacts directly with the electrolyte around the structure for potential measurement purposes. Some of the zinc alloys used for galvanic anodes are also suitable. The metallic electrode can be made in any convenient form.
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