Corrosion Glossary - A

Abatement: either removal of the painted surface, covering the painted surface with an impermeable surface, or covering surface with heavy-duty coating or encapsulant.
Abradable coatings: coatings which are designed to be abraded by a mating surface to form a tight gas or air seal, while retaining good erosion resistance
Abrasive: material such as sand, crushed chilled cast iron, crushed steel grit, aluminum oxide, silicon carbide, flint, garnet, of crushed slag used for cleaning or surface roughening.
Abrasive blasting: see preferred term blasting.
Abrasive wear: wear due to hard particles or hard protuberances forced against and moving along a solid surface.
Absolute pressure: pressure above zero pressure; the sum of the gauge and atmospheric pressures.
Absorb: to take in and engulf wholly.
Absorbent: A material, usually a porous solid, which takes another material into its interior. When rain soaks into soil, the soil is an absorbent.
Absorption: a process in which molecules are taken up by a liquid or solid and distributed throughout the body of that liquid or solid. Compare with adsorption.
Absorption: The process in which one substance is taken into the body of an absorbent.
Acetylene: unsaturated aliphatic hydrocarbon gas. Used as a fuel gas in combustion thermal spray processes, welding and cutting. Acetylene has the highest flame temperature and requires the smallest amount of oxygen to form a neutral flame.
AC impedance: See electrochemical impedance spectroscopy (EIS).
Acicular ferrite: a highly substructured non-equiaxed ferrite formed upon continuous cooling by a mixed diffusion and shear mode of transformation that begins at a temperature slightly higher than the transformation temperature range for upper bainite. It is distinguished from bainite in that it has a limited amount of carbon available thus, there is only a small amount of carbide present.
Acid: a substance which releases hydrogen ions when dissolved in water. Most acids will dissolve the common metals and will react with a base to form a neutral salt and water. (see pH)
Acid cleaning: the process of cleaning the interior surfaces of steam generating units by filling the unit with dilute acid accompanied by an inhibitor to prevent corrosion, and subsequently draining, washing and neutralizing the acid by a further wash of alkaline water.
Acid embrittlement: a form of hydrogen embrittlement that may be induced in some metals by acid.
Acid mine drainage: drainage of water from areas that have been mined for coal or other mineral ores; the water has low pH, sometimes less than 2.0, because of its contact with sulfur-bearing material.
Acid rain: atmospheric precipitation with a pH below 3.6 to 5.7. Burning of fossil fuels for heat and power is the major factor in the generation of oxides of nitrogen and sulfur, which are converted into nitric and sulfuric acids washed down in the rain. See also atmospheric corrosion.
Acidity: The quantitative capacity of water or a water solution to neutralize an alkali or base. It is usually measured by titration with a standard solution of sodium hydroxide and expressed in terms of its calcium carbonate equivalent. (See mineral acidity, total acidity, carbon dioxide, pH.)
Acoustic emission: acoustic emissions are sound or ultrasound pulses generated during crack initiation or propagation in materials and coatings as a result of being subjected to stress. Acoustic emissions can be detected by transducers.
Acrylated rubber: resin binder produced from the copolymerisation of styrene and acrylic monomers.
Acrylic: a synthetic resin used in high-performance water-based coatings and produced by polymerizing various acrylic or acrylate monomers. This resin is used in single pack coatings which have good color and gloss retention.
Activated carbon: a water treatment medium, found in block, granulated, or powdered form, which is produced by heating carbonaceous substances (bituminous coal or cellulose-based substances such as wood or coconut shell) to 700^oC or less in the absence of air to form a carbonized char, and then activating (oxidizing) at 800 to 1000^oC with oxidizing gases such as steam and carbon dioxide (oxygen is never used as the oxidizing gas because its reaction with the carbon surface is too rapid and violent) to form pores, thus creating a highly porous adsorbent material.
Activated silica: A material usually formed from the reaction of a dilute silicate solution with a dilute acid. It is used as a coagulant aid.
Activation: the changing of a passive surface of a metal to a chemically active state. Contrast with passivation.
Activation overpotential: the overpotential associated with the charge-transfer reaction elementary step in the overall electrode reaction.
Active: the negative direction of electrode potential (Also used to describe corrosion and its associated potential range when an electrode potential is more negative than an adjacent depressed corrosion rate [passive] range.
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.
Actuarial analysis: statistical analysis of failure data to determine the age-reliability characteristics of an item.
Asset management: the systematic planning and control of a physical resource throughout its life. This may include the specification, design, and construction of the asset, its operation, maintenance and modification while in use, and its disposal when no longer required.
Adhesion: a binding force that holds together molecules of substances whose surfaces are in contact or near proximity. (Paint) The ability of dry paint to attach to and remain fixed on the surface without blistering, flaking, cracking or being removed by tape.
Adhesive strength: the magnitude of attractive forces, generally physical in character, between a coating and substrate. Two principle interactions that contribute to the adhesion are van der Waals forces and permanent dipole bonds.
Adhesive Wear: wear due to localized bonding between contacting solid surfaces leading to material transfer between the two surfaces or the loss from either surface.
Adsorb: to take in on the surface.
Adsorbent: A material, usually solid, capable of holding gases, liquids and/or suspended matter at its surface and in exposed pores. Activated carbon is a common adsorbent used in water treatment.
Adsorption: the physical process occurring when liquids, gases, or suspended matter adhere to the surfaces of, or in the pores of, an adsorbent medium. Adsorption is a physical process which occurs without chemical reaction.
Aeration: the process whereby water is brought into intimate contact with air by spraying or cascading, or air is brought into intimate contact with water by an air aspirator or by bubbling compressed air through the body of water.
Aeration cell: an oxygen concentration cell; an electrolytic cell resulting from differences in dissolved oxygen at two points. Also see differential aeration cell.
Aerobic: an action or process conducted in the presence of air, such as aerobic digestion of organic matter by bacteria.
Aerosol: aerosols are tiny particles suspended in the air. Some occur naturally, originating from volcanoes, dust storms, forest and grassland fires, living vegetation, and sea spray. Human activities, such as the burning of fossil fuels and the alteration of natural surface cover, also generate aerosols. (Paint) A product that uses compressed gas to spray the coating from its container.
Age hardening: hardening by aging, usually after rapid cooling or cold working.
Agglomerate: several particles adhering together.
Agglomerated powder: a powder made up of agglomerates.
Air Cure: One method by which liquid coatings cure to a dry film. Oxygen from the air enters the film and cross-links the resin molecules. Also called "Air Dry" and "Oxidizing."
Air spray: a system of applying paint in the form of tiny droplets in the air. The paint is broken down into droplets (atomized) by a spray gun as a result of being forced into a high velocity air stream.
Air-assisted airless: an airless system that uses small amounts of air to help atomize the paint in conjunction with airless spray. Used to get rid of the nagging problem of “Rabbit Ears”.
Airless spray: a system of applying paint in which the paint, under high pressure, is passed through a nozzle and is broken into droplets (atomized) when the paint enters the lower pressure atmosphere.
Alclad: composite wrought product comprised of an aluminum alloy core having on one or both surfaces a metallurgically bonded aluminum or aluminum alloy coating that is anodic to the core and thus electrochemically protects the core against corrosion.
Algae: Small primitive plants containing chlorophyll, commonly found in surface water. Excessive growths may create taste and odor problems, and consume dissolved oxygen during decay.
Aliphatic: organic compounds (hydrocarbon) in which carbon atoms are arranged in an open or straight chain more commonly called naphtha.
Alkali: A group of water soluble mineral compounds, usually considered to have moderate strengths as bases (as opposed to the caustic or strongly basic hydroxides, although this differentiation is not always made). In general, the term is applied to bicarbonate and carbonate compounds when they are present in the water or solution. (See alkali, base.)
Alkaline: a) Having properties of an alkali and, b) Having a pH greater than 7.
Alkaline cleaner: a material blended from alkali hydroxides and such alkaline salts as borates, carbonates, phosphates, or silicates. The cleaning action may be enhanced by the addition of surface-active agents and special solvents.
Alkalinity: The quantitative capacity of a water or water solution to neutralize an acid. It is usually measured by titration with a standard acid solution of sulfuric acid and is expressed in terms of its calcium carbonate equivalent. (See alkali, base, pH.)
Alkyd: synthetic polyester resin modified with oil. Coating that contains alkyd resins in the binder. The original alkyd resins were made by co-polymerizing phtalic anhydride with glycerol, to give a brittle cross. Linked polymer.
Alkyl benzene sulfonate: A term applied to a family of branched chain chemical compounds, formerly used as detergents,. Sometimes called "hard" detergents, because of their resistance to biological degradation, these compounds have been largely replaced with linear alkyl sulfonate (LAS) which are more readily degraded to simpler substances.
Alloy steel: steel with modified properties made by combining iron with one or more elements in addition to carbon. Alloys change the properties of the steel making it, for example, harder, more formable, or more corrosion resistant, depending on the combination and amounts of alloys used.
Alodine (or Irridite): a dichromate conversion coating used with aluminum. Clear, yellow, or gold in appearance, alodine's main purpose is to prepare aluminum for painting. (see hexavalent chromium)
Alpha iron: the body-centered cubic form of pure iron, stable below 910ºC.
Alternate-immersion test: a corrosion test in which the specimens are intermittently exposed to a liquid medium at definite time intervals.
Alum: A common name for aluminum sulfate, used as a coagulant.
Aluminizing: forming of an aluminum or aluminum alloy coating on a metal by hot dipping, hot spraying, or diffusion.
Aluminizing (gas): high temperature (~ 900°C) pack or gaseous diffusion of aluminum into the surface of a component to enhance high temperature corrosion and oxidation resistance.
Aluminizing (hot dip): an aluminum coating process based on submersion in liquid metal, usually with a strip steel product being continuously fed through the bath. Provide galvanic corrosion protection.
Aluminizing (thermal spray): thermal sprayed coatings of aluminum usually on substrates of steel or nickel chromium alloys which are subsequently heat treated to aluminize the surface.
Aluminum ion plating: the deposition of aluminum by a vacuum evaporative process that provides galvanic corrosion resistance. Normally given a passivation treatment.
Amide: a functional group which can act as an epoxy resin curing agent.
Amoeba: A small, single-celled animal or protozoan.
Amorphous: non-crystalline, or devoid of regular structure.
Amphoteric: a term applied to oxides and hydroxides which can act basic toward strong acids and acidic toward strong alkalis. Substances which can dissociate electrolytically to produce hydrogen or hydroxyl ions according to conditions.
Anaerobic: in the absence of air or unreacted or free oxygen.
Angstrom unit: A unit of length equal to one ten-billionth of a meter.
Anion: a negatively charged ion.
Anion exchange: An ion exchange process in which anions in solution are exchanged for other anions from an ion exchanger. In demineralization, for example, bicarbonate, chloride and sulfate anions are removed from solution in exchange for a chemically equivalent number of hydroxide anions from the anion exchange resin.
Annealing: a process involving heating and cooling designed to effect: (1) softening of a cold-worked structure by re-crystallization of grain growth or both; (2) softening of an age-hardened alloy by causing a nearly complete precipitation of the second phase in a relatively coarse form; (3) softening of certain age-hardenable alloys by dissolving the second phase and cooling rapidly enough to obtain a supersaturated solution; (4) relief of residual stress.
Anode: the electrode of an electrolytic cell at which oxidation is the principal reaction. (Electrons flow away from the anode in the external circuit. It is usually the electrode where corrosion occurs and metal ions enter solution.
Anode corrosion efficiency: the ratio of the actual corrosion (weight loss) of an anode to the theoretical corrosion (weight loss) calculated by Faraday's law from the quantity of electricity that has passed.
Anodic cleaning: electrolytic cleaning in which the work is the anode. Also called reverse-current cleaning.
Anodic coating: a coating that becomes the anode in an electrochemical cell with the substrate (cathode). The only metals in common use for thermal spraying which are anodic to iron and steel are zinc and aluminum.
Anodic inhibitor: an inhibitor that reduces the corrosion rate by acting on the anodic (oxidation) reaction.
Anodic polarization: the change of the electrode potential in the noble (positive) direction due to current flow (See polarization).
Anodic protection: a technique to reduce corrosion of a metal surface under some conditions by passing sufficient to it to cause its electrode potential to enter and remain in the passive region; imposing an external electrical potential to protect a metal from corrosive attack. (Applicable only to metals that show active-passive behavior.) Contrast with cathodic protection.
Anodizing: the production of an oxide layer on aluminum alloys. The process is electrolytic, a typical electrolyte being sulfuric acid. Treatment at room temperature produces thin, decorative layers with some corrosion protection. Treatment at lower temperatures can produce harder, thicker layers (up to 100µ) with wear resistance. They can be post sealed to give improved corrosion resistance.
Anolyte: the electrolyte adjacent to the anode of an electrolytic cell.
Anti-fouling paint: a paint formulated to prevent fouling of under-water structures, such as the bottoms of ships; refers to the prevention of marine organism's attachment or growth on a submerged metal surface, generally through chemical toxicity caused by the composition of the metal or coating layer.
Anti Pitting agent: an addition agent for electroplating solutions to prevent the formation of pits or large pores in the electrodeposits.
Apparent density: the weight of a unit volume of powder or coating.
Apparent hardness: the value obtained by testing a coating or sintered material with standard indentation hardness equipment. Since the reading reflects a composite of pores and solid material, (which may be particles relatively poorly bonded together) it is usually lower than that of an equivalent solid wrought or cast material. Not to be confused with particle hardness. (See also microhardness)
Aqueous: pertaining to water; an aqueous solution is made by using water as a solvent.
Aquifer: A layer or zone below the surface of the earth which is capable of yielding a significant volume of water.
Arc wire spraying: a thermal spray process where two electrically conducting wires are brought together to form an electric arc. Molten material formed in the arc is projected by a compressed gas stream towards the work piece to form a coating.
Argon (Ar): monatomic noble gas, atomic number 18, one of the most inert elements. Commonly used as a plasma gas for plasma spraying and providing inert environments for many processes.
Aromatic: a type of solvent based on Benzene ring molecules (e.g. Benzene, Xylene,
Arrester: a device to impede the flow of large dust particles or sparks from a stack, usually screening at the top.
Artificial aging: aging above room temperature. See also aging. Compare with natural aging.
Ash: the incombustible inorganic matter in the fuel.
ASME: the American Society of Mechanical Engineers.
ASTM: American Society for testing of Materials
Atmospheric air: air under the prevailing atmospheric conditions.
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.
Atmospheric pressure: the barometric reading of pressure exerted by the atmosphere. At sea level 0.101 Mpa or 760 mm of mercury.
Atom: the smallest particle of an element that can exist either alone or in combination.
Atomic-hydrogen welding: arc welding in which molecular hydrogen, passing through an arc between two tungsten or other suitable electrodes, is changed to its atomic form and then re-combines to supply the heat for welding.
Atomization: 1) The dispersion of a molten material into particles by a rapidly moving gas or liquid stream or by mechanical dispersion. 2) The formation of tiny droplets of liquid as in the paint spraying process. Atomization is usually caused by turbulence in an air stream, or a sudden drop in pressure.
Atomized powder: a powder produced by the dispersion of a molten material into particles by a rapidly moving gas or liquid stream or by mechanical dispersion.
Attrition: In water treatment, the process in which solids are worn down or ground down by friction, often between particles of the same material. Filter media and ion exchange materials are subject to attrition during backwashing, regeneration and service.
Austenite: a solid solution of one or more elements in face-centered cubic iron. Unless otherwise designated (such as nickel austenite), the solute is generally assumed to be carbon.
Austenitic: the name given to the face-centered cubic crystal structure (FCC) of ferrous metals. Ordinary iron and steel has this structure at elevated temperatures; also certain stainless steels (300 series) have this structure at room temperature.
Austenitizing: forming austenite by heating a ferrous alloy into the transformation range (partial austenitizing) or above the transformation range (complete austenitizing). When used without qualification, the term implies complete austenitizing.
Autoclaving: the production of a stable, protective oxide on steel parts by treatment in a pressurized, high temperature steam containing atmosphere.
Auxiliary electrode: the electrode in an electrochemical cell that is used to transfer current to or from a test electrode, usually made of non-corroding material.
Availability: the proportion of total time that an item of equipment is capable of performing its specified functions, normally expressed as a percentage. It can be calculated by dividing the equipment available hours by the total number of hours in any given period.
Average life: how long, on average, a component will last before it suffers a failure. Commonly measured by Mean Time Between Failures (MTBF).