Glossary of Corrosion Related Terms

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

T

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/i_o)

where h = change in open circuit potential, i = the current density, B and i_o = 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.

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 205C to 400C (400F to 750F); also called 350C or 500F 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 re tempering above the critical temperature range, then cooling rapidly.

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 to 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.

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 l095C (2000F ) and then slow cooling through the temperature range of 815C to 980C (1300F to l800F), and has been attributed to precipitation of titanium carbides and titanium carbonitrides at austenite grain boundaries during cooling through the critical temperature range.

thermally induced embrittlement: See embrittlement.

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. Compare with covering power.

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 u 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.

transcrystalline: See transgranular.

transcrystalline cracking: See transgranular cracking.

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 away that the d-band contains less than its maximum number of ten 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 of 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.

triaxial stress: See principal stress (normal).

tuberculation: The formation of localized corrosion products scattered over the surface in the form of knoblike mounds called tubercles.

U

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.

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.

uniaxial stress: See principal stress (normal).

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.

V

vacuum deposition: Condensation of thin metal coatings on the cool surface of work in vacuum.

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 deposition: See chemical vapor deposition, physical vapor deposition and sputtering.

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.

W

wash primer: A thin, inhibiting paint, usually chromate pigmented with a polyvinyl butyrate binder.

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-base alloys, that occurs as the result of sensitization in the heat-affected zone during the welding operation.

wetting: A condition in which the interfacial tension between a liquid and a solid is such that the contact angle is 0 to 90 degrees.

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.

X

No X definitions.

Y

yield: Evidence of plastic deformation in structural materials. Also called plastic flow or creep. See also flow.

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.

Z

zeta potential: See electrokinetic potential.

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