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active metal - A metal ready to corrode, or being corroded.

annealing - A generic term denoting a treatment, consisting of heating a component to a specific temperature, holding it at that temperature for a specified period of time, followed by cooling at a suitable rate. Used primarily to soften metallic materials, but also to simultaneously produce desired changes in other properties or in microstructure. The purpose of such changes may be, but is not confined to, improvement of machinability, facilitation of cold work, improvement of mechanical or electrical properties, and/or increase in stability of dimensions. When the term is used by itself, full annealing is implied. When applied only for the relief of stress, the process is properly called stress relieving or stress-relief annealing.

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.

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

basecoat - The first coat applied to a surface prior to any subsequent coatings being applied.

barrier coat - Coating used to isolate a coating system from the surface to which it is applied in order to prevent chemical or physical interaction between them. (Similar to plastic wrap on a newspaper which prevents rain from wetting the newspaper.)

binder - Nonvolatile portion of the liquid vehicle of a coating. It binds a coating's components together as a whole to the material to which it is applied. (The "glue" that holds the coating together.)

blast cleaning - Cleaning and roughening of a surface (particularly steel) by the use of natural or artificial grit or fine metal shot (usually steel), which is projected against a surface by compressed air.

blending - Mixing two or more materials to obtain a new material or particular quality.

bondcoat - Coating used to improve the adherence of succeeding coats, and/or to enhance the performance of a topcoat.

bond strength - Amount of adhesion between bonded surfaces; a measure of the stress required to separate a layer of material from the base to which is is bonded.

brazing - A welding process that produces coalescence (uniting or merging into a single mass) of metals by heating them to brazing temperature in the presence of a filler metal having a liquidus above 450 degrees Celsius (840 degrees Fahrenheit). The brazing temperature must be below the solidus of the base metals being joined.

cadmium plating - Electrodeposition of cadmium on steel articles to make them resistant to oxidation and corrosion.

cavitation - The formation and instantaneous collapse of innumerable tiny voids or cavities within a liquid subjected to rapid and intense pressure changes. Cavitation produced by ultrasonic radiation is sometimes used to effect violent localized agitation. Cavitation caused by severe turbulent flow often leads to cavitation damage.

cavitation damage - The degradation of a solid body resulting from its exposure to cavitation. This may include loss of material, surface deformation, or changes in properties or appearance.

cavitation-erosion - Progressive loss of original material from a solid surface due to continuing exposure to cavitation.

corrosion - The chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties.

corrosion effect - A change in any part of the corrosion system caused by corrosion.

corrosion embrittlement - The severe loss of ductility of a metal resulting from corrosive attack, usually intergranular and often not visually apparent.

corrosion 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 that would be required in the absence of the corrosive environment.

corrosion fatigue strength - The maximum repeated stress that can be endured by a metal without failure under defined conditions of corrosion and fatigue and for a specific number of stress cylces and a specified period of time.

corrosion inhibitor - A substance or combination of substances that, when present in the environment, prevents or reduces corrosion by changing the driving method of the corrosion.

corrosion product - Substance formed as a result of corrosion.

corrosion protection - Modification of a corrosion system so that corrosion damage is mitigated.

corrosion resistance - Ability of a metal to withstand corrosion in a given corrosion system.

corrosion system - System consisting of one or more metals and all parts of the environment that influence corrosion.

corrosivity - Tendency of an environment to cause corrosion in a given corrosion system.

cure - To change the properties of a polymeric coating system into a final, more stable, usable condition through the use of heat.

diffusion - (1) Spreading of a constituent in a gas, liquid, or solid, tending to make the composition of all parts uniform. (2) The spontaneous movement of atoms or molecules to new sites within a material. (3) The simultaneous atomic migration of coating atoms or molecules into a substrate and vice versa until equilibrium is reached.

Eddy Current Inspection - An inspection method based on electromagnetic induction that is used to identify or differentiate among a wide variety of physical, structural, and metallurgical conditions in electrically conductive ferromagnetic and nonferromagnetic metals and metal parts.

environment - The surroundings or conditions (physical, chemical, mechanical) in which a material exists.

erosion - Destruction of metals or other materials by the abrasive action of moving fluids, usually accelerated by the presence of solid particles or matter in suspension.

exfoliation - The delamination and loss of corrosion by-products from the surface. Corrosion that proceeds laterally from the sites of initiation along planes paralled to the surface, generally at grain boundaries, forming corrosion products that force metal away from the body of the material, giving rise to a layered appearance.

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 perfoming its intended function, or (3) has deteriorated seriously, to the point that it has become unreliable or unsafe for continued use.

flame spraying - Thermal spraying in which a coating material is fed into an oxy fuel gas flame, where it is melted. Compressed gas may or may not be used to atomize the coating material and propel it into the substrate.

Fluorescent Particle Inspection (FPI) - An inspection technique that utilizes penetrants that are usually green in color and fluoresce brilliantly under ultraviolet light. The sensitivity of a fluorescent penetrant depends on its ability to form indications that appear as small sources of light in an otherwise dark area.

fouling - An accumulation of deposits which restricts fluid flow. Includes accumulation of deposits (usually inorganic) on gas and steam turbines, centrifugal compressors, heat exchanger tubing.

galvanic corrosion - Accelerated corrosion of a metal because of an electrical contact with a more noble metal or nonmetallic conductor in a corrosive electrolyte.

galvanic series - A list of metals and alloys arranged according to their relative corrosion potentials in a given environment.

gas cracking - The process of breaking down certain hydrocarbons into simpler ones of lower boiling points, by means of excess heat, distillation under pressure, etc., in order to give a greater yield of low boiling point products than could be obtained by simple distillation.

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.

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.

High Temperature Hot Corrosion - An aggressive attack of metal by molten salts, particularly sodium sulfates, that occurs between 870 and 980 degrees Celsius (1600 and 1800 degrees Fahrenheit).

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 fluxes, destroys, or disrupts the normal protective oxide.

inhibitor - A chemical substance or combination of substances that, when present in the environment, prevents or reduces corrosion without significant reaction with the components of the environment.

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.

intergranular - Between crystals or grains.

intergranular stress corrosion cracking - Stress-corrosion cracking in which the cracking occurs along grain boundaries.

lignite - A brownish-black coal in which decomposition of vegetable matter has proceeded farther than in peat but not so far as in bituminous coal. Also called brown coal.

Liquid Penetrant Inspection - A nondestructive method for revealing discontinuities that are open to the surfaces of solid and essentially nonporous materials. Liquid penetrants seep into various types of minute surface openings by capillary action.

localized corrosion - Corrosion at discrete sites, for example, pitting, and stress corrosion cracking.

Low Temperature Hot Corrosion - Localized pitting of metal by molten salts, particularly those containing calcium, that occurs between 595 and 705 degrees Celsius (1100 and 1300 degrees Fahrenheit).

macroscopic - Visible at magnification to 25x.

macrostructure - The structure of metals as revealed by macroscopic examination of the surface of a polished specimen.

Magnetic Particle Inspection (MPI) - An inspection method for locating surface and subsurface discontinuities in ferromagnetic materials. The material or part being tested is magnetized. Magnetic discontinuities that lie in a direction generally transverse to the direction of the magnetic field will cause a leakage field to be formed at and above the surface of the part. The leakage field and discontinuities are detected by the use of finely divided ferromagnetic particles applied over the surface, with some of the particles being gathered and held by the leakage field. The magnetically held collection of particles forms an outline of the discontinuity and generally indicates its location, size, shape and extent.

MCrAlY - An acronym for Metal (Ni, Co, or Fe base or mixtures thereof) + Chromium+Aluminum+Yttrium. These are coating compositions primarily suited for gas turbine hot section components.

metallizing - Applying a thin coating of metal to a nonmetallic surface.

microstructure - The structure of a prepared surface of a metal as revealed by a microscope at a magnification exceeding 25x.

noble - The positive direction of electrode potential, thus resembling noble metals such as gold and platinum.

noble metal - (1) A metal whose potential is highly positive relative to the hydrogen electrode. (2) A metal with marked resistance to chemical reaction, particularly oxidation and to solution by inorganic acids.

OEM - Original Equipment Manufacturer. In the context of Sermatech product literature and this web site, OEM refers to turbomachinery (gas turbine, steam turbine, compressor) manufacturers.

organic - Being or composed of hydrocarbons or their derivatives, or matter of plant or animal origin. Contrast with inorganic.

overlay - A surface modification (e.g. MCrAlY) applied to a component directly over the surface, primarily to effect certain properties.

oxidation - A corrosion reaction in which the corroded metal forms an oxide; usually applied to reaction with a gas containing elemental oxygen, such as air.

pH - The negative logarithm of the hydrogen activity; it denotes the degree of acidity or basicity of a solution. At 25 degrees Celsius (77 degrees Fahrenheit), 7.0 is the neutral value. Decreasing values below 7.0 indicate increasing acidity; increasing values above 7.0, increasing alkalinity.

pitting - Localized corrosion of a metal surface, confined to a point or small area, that takes the form of cavities.

rainbow test - Side-by-side test of different materials (coatings or alloys) on the same components in the same stage of the same engine.

rust - A visible corrosion product consisting of hydrated oxides of iron. Applied only to ferrous alloys.

sacrificial protection - Reduction of corrosion of a metal in an electrolyte by galvanically coupling it to a more anodic metal.

Solid Particle Erosion (SPE) - Exfoliation from boiler tubing which is a leading cause of erosion of turbine buckets (blades), nozzles, control valves and other steam path components.

sour gas - A gaseous environment containing hydrogen sulfide and carbon dioxide in hydrocarbon resovoirs. Prolonged exposure to sour gas can lead to hydrogen damage, sulfide-stress cracking, and/or stress corrosion cracking in ferrous alloys.

spray coating - The process in which a substrate is sprayed with the coating material.

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.

stress-corrosion cracking - A cracking process that requires the simultaneous action of a corrodent and sustained tensile stress. This excludes corrosion-reduced sections that fail by fast fracture. It also excludes intercrystalline or transcrystalline corrosion, which can disintegrate an alloy without applied or residual stress. Typically stress corrosion fractures are accelerated by the combined actions of corrosion and stress on the alloy.

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 of the metal or alloy.

Thermal Barrier Coating (TBC) - A coating applied to components (primarily those found in a gas turbine's hot section) to reduce the temperature of the underlying substrate.

tie coat - Intermediate coat used to bond different types of coatings. Also known as barrier coat.

topcoat - The coating intended to be the last coat applied in a coating system.

uniform corrosion - (1) A type of corrrosion attack (deterioration) uniformly distributed over a metal surface. (2) Corrosion that proceeds at approximately the same rate over a metal surface. Also called general corrosion.