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Corrosion Source - Corrosion glossary
Category: General technical and industrial > Corrosion
Date & country: 11/12/2007, UK
Words: 619

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.

residual stress
Stresses that remain within a body as a result of plastic deformation.

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.

See electrical resistivity.

rest potential
See corrosion potential and open-circuit potential.

ringworm corrosion
Localized corrosion frequently observed in oilwell tubing in which a circumfrential attack is observed near a region of metal 'upset'.

(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.

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

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

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.

salt spray test
See salt fog test.

saturated calomel electrode
A reference electrode composed of mercury, mercurous chloride (calomel), and a saturated aqueous chloride solution.

(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.

selective leaching
Corrosion in which one element is preferentially removed from an alloy, leaving a residue (often porous) of the elements that are more resistant to the particular environment. Also called dealloying or parting. See also decarburization, decobbaltification, denickelification, dezincification, and graphitic corrosion.

In austenitic stainless steels the precipitation of chromium carbides, usually at grain boundaries, on exposure to temperatures of about 550 to 850 ºC (about 1000 to 1550 ºF), leaving the grain boundaries depleted of chromium and therefore susceptible to preferential attack by a corroding (oxidizing) medium.

sensitizing 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 intergranular corrosion or intergranular stress-corrosion cracking. See also sensitization.

That 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.

sigma phase
A hard, brittle, nonmagnetic intermediate phase with a tetragonal crystal structure, containing 30 atoms per unit cell, space group P42mnm, 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…

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 ºC ( 1050 and 1800 ºF). Sigma-phase embrittlement results in severe loss in toughness and ductil…

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.

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.

soft water
Water that is free of magnesium or calcium salts.

solder embrittlement
Reduction in mechanical properties of a metal as a result of local penetration of solder along grain boundaries.

solid solution
A single, solid, homogeneous crystalline phase containing two or more chemical species.

solid-metal embrittlement
The occurrence of embrittlement in a material below the melting point of the embrittling species. See also liquid-metal embrittlement.

The component of either a liquid or solid solution that is present to a lesser or minor extent: the component that is dissolved in thesolution.

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.

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.

Space Outgassing
Release of contained gas in the vaccum of outer space. eg. rapid destructive expansion of plastics and similar materials.

Space Pitting
Pitting resulting from ablation, outgassing or meteor contact.

The spontaneous chipping, fragmentation, or separation of a surtace or surface coating.

An aggregate of iron or alloy carbides of essentially spherical shape dispersed throughout a matrix of ferrite.

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…

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.

The unit of change in the size or shape of a body due to force. Also known as nominal strain.

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.

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 ºC (400 ºF), but requiring a few hours to a year a…

stray current
Current flowing through paths other than the intended circuit.

stray-current corrosion
Corrosion resulting from direct current flow through paths other than the intended circuit. For example, by an extraneous current in the earth.

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 …

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 relieving
Heat treatment carried out in steel to reduce internal stresses.

stress-corrosion cracking (SCC)
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. Stress-corrosion cracking may occur in combina…

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 toughne…

stress-intensity factor range, DK
In fatigue, the variation in the stress-intensity factor in cycle, that is, Kmax-Kmin.

stress-relief cracking
Also called postweld heat treatment cracking, stress-relief cracking occurs when susceptible alloys are suhjected 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 …

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 prop…

subsurface corrosion
Formation of isolated particles of corrosion products beneath a metal surface. This results from the prcferential reactions of certain alloy constituents to inward diffusion of oxygen, nitrogen, or sulfur.

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.

A surface-active agent; usually an organic compound whose moleculei contain a hydrophilic group at one end and a lipophilic group at the other.

Surface discoloration of a metal caused by formation of a thin film of corrosion product.

(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 all…

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.

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 temper…

tempered martensite embrittlement
Embrittlement of ultrahigh-strength steels caused by tempering in the temperature range of 205 to 400 ºC (400 to 750 ºF); also called 350 ºC or 500 ºF 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 segregat…

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.

The force or load that produces elongation.

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.

therdermal 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 ºC (2000 ºF ) and then slow cooling through the temperature range of 815 to 980 ºC (1300 to l800 ºF), and has been attributed to precipitation of titanium carbides and titanium…

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.

thermally induced embrittlement
See embrittlement.

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 be…

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 unifo…

Coating metal with a very thin layer of molten solder or brazing filler metal.

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.

The ability of a metal to absorb energy and deform plastically before fracturing.

See transgranular.

transcrystalline cracking
See transgranular cracking.

The movement of ions through the electrolyte associated with the passage of the electric current. Also called transport or migration.

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 rang…

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.

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.