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Encyclopedia > Stress corrosion cracking

Stress corrosion cracking (SCC) is the unexpected sudden failure of normally ductile metals subjected to a constant tensile stress in a corrosive environment, especially at elevated temperature. This type of corrosion often progresses rapidly. Ductility is the physical property of being capable of sustaining large plastic deformations without fracture (in metals, such as being drawn into a wire). ... Tensile stress (or tension) is the stress state leading to expansion; that is, the length of a material tends to increase in the tensile direction. ... Corrosion is deterioration of intrinsic properties in a material due to reactions with its environment. ... Corrosion is deterioration of intrinsic properties in a material due to reactions with its environment. ...

The stresses can be the result of the service loads, or can be caused by the type of assembly or residual stresses from fabrication (eg. cold working); the residual stresses can be relieved by annealing. Residual stresses are stresses that remain after the original cause of the stresses has been removed. ... Annealing, in metallurgy and materials science, is a heat treatment wherein the microstructure of a material is altered, causing changes in its properties such as strength and hardness. ...

Certain austenitic stainless steels and aluminium alloys crack in the presence of chlorides, mild steel cracks in the present of alkali (boiler cracking) and copper alloys crack in ammoniacal solutions (season cracking). This limits the usefulness of stainless steel for containing water with higher than few ppm content of chlorides at temperatures above 50 °C. Worse still, high-tensile structural steels crack in an unexpectedly brittle manner in a whole variety of aqueous environments, especially chloride. With the possible exception of the latter, which is a special example of hydrogen cracking, all the others display the phenomenon of subcritical crack growth, i.e. small surface flaws propagate (usually smoothly) under conditions where fracture mechanics predicts that failure should not occur. That is, in the presence of a corrodent, cracks develop and propagate well below KIc. In fact, the subcritical value of the stress intensity, designated as KIscc, may be less than 1% of KIc, as the following table shows: The 630 foot high, stainless-clad (type 304) Gateway Arch defines St. ... General Name, Symbol, Number aluminium, Al, 13 Chemical series poor metals Group, Period, Block 13, 3, p Appearance silvery Atomic mass 26. ... An alloy is a combination, either in solution or compound, of two or more elements, at least one of which is a metal, and where the resulting material has metallic properties. ... The chloride ion is formed when the element chlorine picks up one electron to form an anion (negatively-charged ion) Cl−. The salts of hydrochloric acid HCl contain chloride ions and are also called chlorides. ... The old Steel cable of a colliery winding tower Steel is sometimes described as a sea of electrons. ... General Name, Symbol, Number copper, Cu, 29 Chemical series transition metals Group, Period, Block 11, 4, d Appearance metallic pinkish red Atomic mass 63. ... Ammonia is a compound of nitrogen and hydrogen with the formula NH3. ... Hydrogen embrittlement is the process by which various metals, most importantly steel, become brittle and crack following exposure to hydrogen. ... Structural failure refers to loss of the load-carying capacity of a component or member within the structure or of the structure itself. ... To meet Wikipedias quality standards, this article or section may require cleanup. ...

Alloy KIc


SCC environment KIscc


13Cr steel 60 3% NaCl 12
18Cr-8Ni 200 42% MgCl2 10
Cu-30Zn 200 NH4OH, pH7 1
Al-3Mg-7Zn 25 Aqueous halides 5
Ti-6Al-1V 60 0.6M KCl 20

The subcritical nature of propagation may be attributed to the chemical energy released as the crack propagates. That is, In chemistry, a chemical bond is the force which holds together atoms in molecules or crystals. ...

elastic energy released + chemical energy = surface energy + deformation energy

The crack initiates at KIscc and thereafter propagates at a rate governed by the slowest process, which most of the time is the rate at which corrosive ions can diffuse to the crack tip. As the crack advances so K rises (because crack length appears in the calculation of stress intensity). Finally it reaches KIc , whereupon fast fracture ensues and the component fails. One of the practical difficulties with SCC is its unexpected nature. Stainless steels, for example, are employed because under most conditions they are 'passive', i.e. effectively inert. Very often one finds a single crack has propagated while the rest of the metal surface stays apparently unaffected. The 630 foot high, stainless-clad (type 304) Gateway Arch defines St. ...

See also

To meet Wikipedias quality standards, this article or section may require cleanup. ...

External links

  Results from FactBites:
Stress corrosion cracking (SCC) (483 words)
The required tensile stresses may be in the form of directly applied stresses or in the form of residual stresses, see an example of SCC of an aircraft component.
Chloride stress corrosion cracking in austenitic stainless steel is characterized by the multi-branched "lightning bolt" transgranular crack pattern.
Chloride stress corrosion is a type of intergranular corrosion and occurs in austenitic stainless steel under tensile stress in the presence of oxygen, chloride ions, and high temperature.
Intergranular corrosion is proposed to occur by preferential oxidation of the grain boundaries.
Corrosion fatigue is the conjoint action of a cyclic stress and a corrosive environment to decrease the number of cycles to failure in comparison to the life when no corrosion is present.
One mechanism that may be involved is the general corrosion of the crack tip in extending the crack length at all times in addition to the increase in crack length from the mechanical stressing.
  More results at FactBites »



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