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Stress corrosion cracking (SCC) is the growth of crack formation in a
corrosive Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
environment. It can lead to unexpected and sudden failure of normally ductile metal
alloy An alloy is a mixture of chemical elements of which in most cases at least one is a metal, metallic element, although it is also sometimes used for mixtures of elements; herein only metallic alloys are described. Metallic alloys often have prop ...
s subjected to a tensile stress, especially at elevated temperature. SCC is highly chemically specific in that certain alloys are likely to undergo SCC only when exposed to a small number of chemical environments. The chemical environment that causes SCC for a given alloy is often one which is only mildly
corrosive Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
to the metal. Hence, metal parts with severe SCC can appear bright and shiny, while being filled with microscopic cracks. This factor makes it common for SCC to go undetected prior to failure. SCC often progresses rapidly, and is more common among alloys than pure metals. The specific environment is of crucial importance, and only very small concentrations of certain highly active chemicals are needed to produce catastrophic cracking, often leading to devastating and unexpected failure. The stresses can be the result of the crevice loads due to stress concentration, or can be caused by the type of assembly or residual stresses from fabrication (e.g. cold working); the residual stresses can be relieved by annealing or other surface treatments. Unexpected and premature failure of chemical process equipment, for example, due to stress corrosion cracking constitutes a serious hazard in terms of safety of personnel, operating facilities and the environment. By weakening the reliability of these types of equipment, such failures also adversely affect productivity and profitability.


Mechanisms

Stress corrosion cracking mainly affects
metal A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...
s and metallic alloys. A comparable effect also known as environmental stress cracking also affects other materials such as
polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...
s,
ceramic A ceramic is any of the various hard, brittle, heat-resistant, and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcela ...
s and
glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
.


Metals

Lower pH and lower applied redox potential facilitate the evolution and the enrichment of hydrogen during the process of SCC, thus increasing the SCC intensity. * Certain austenitic
stainless steel Stainless steel, also known as inox, corrosion-resistant steel (CRES), or rustless steel, is an iron-based alloy that contains chromium, making it resistant to rust and corrosion. Stainless steel's resistance to corrosion comes from its chromi ...
s and
aluminium Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...
alloy An alloy is a mixture of chemical elements of which in most cases at least one is a metal, metallic element, although it is also sometimes used for mixtures of elements; herein only metallic alloys are described. Metallic alloys often have prop ...
s crack in the presence of
chloride The term chloride refers to a compound or molecule that contains either a chlorine anion (), which is a negatively charged chlorine atom, or a non-charged chlorine atom covalently bonded to the rest of the molecule by a single bond (). The pr ...
s. This limits the usefulness of austenitic stainless steel for containing water with higher than a few parts per million content of chlorides at temperatures above ; * mild
steel Steel is an alloy of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to steel's high Young's modulus, elastic modulus, Yield (engineering), yield strength, Fracture, fracture strength a ...
cracks in the presence of alkali (e.g. boiler cracking and caustic stress corrosion cracking) and
nitrate Nitrate is a polyatomic ion with the chemical formula . salt (chemistry), Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are solubility, soluble in wa ...
s; *
copper alloys Copper alloys are metal Alloy, alloys that have copper as their principal component. They have high resistance against corrosion. Of the large number of different types, the best known traditional types are bronze, where tin is a significant addi ...
crack in
ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...
cal solutions ( season cracking); * high-tensile steels have been known to crack in an unexpectedly brittle manner in a whole variety of aqueous environments, especially when chlorides are present. 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 critical stress intensity factor (K_\mathrm). The subcritical value of the stress intensity, designated as K_\mathrm, may be less than 1% of K_\mathrm.


Polymers

A similar process ( environmental stress cracking) occurs in
polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...
s, when products are exposed to specific solvents or aggressive chemicals such as acids and alkalis. As with metals, attack is confined to specific polymers and particular chemicals. Thus
polycarbonate Polycarbonates (PC) are a group of thermoplastic polymers containing carbonate ester, carbonate groups in their chemical structures. Polycarbonates used in engineering are strong, toughness, tough materials, and some grades are optically transp ...
is sensitive to attack by alkalis, but not by acids. On the other hand, polyesters are readily degraded by acids, and SCC is a likely
failure Failure is the social concept of not meeting a desirable or intended objective, and is usually viewed as the opposite of success. The criteria for failure depends on context, and may be relative to a particular observer or belief system. On ...
mechanism. Polymers are susceptible to environmental stress cracking where attacking agents do not necessarily degrade the materials chemically.
Nylon Nylon is a family of synthetic polymers characterised by amide linkages, typically connecting aliphatic or Polyamide#Classification, semi-aromatic groups. Nylons are generally brownish in color and can possess a soft texture, with some varieti ...
is sensitive to degradation by acids, a process known as
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...
, and nylon mouldings will crack when attacked by strong acids. For example, the fracture surface of a fuel connector showed the progressive growth of the crack from acid attack (Ch) to the final cusp (C) of polymer. In this case the failure was caused by
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...
of the polymer by contact with
sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...
leaking from a
car battery An automotive battery, or car battery, is a usually 12 Volt lead-acid rechargeable battery that is used to start a motor vehicle, and to power lights, screen wiper etc. while the engine is off. Its main purpose is to provide an electric current ...
. The degradation reaction is the reverse of the synthesis reaction of the polymer: : : image:amide hydrolysis.svg Cracks can be formed in many different
elastomers An elastomer is a polymer with viscoelasticity (i.e. both viscosity and Elasticity (physics), elasticity) and with weak intermolecular forces, generally low Young's modulus (E) and high failure strain compared with other materials. The term, a ...
by
ozone Ozone () (or trioxygen) is an Inorganic compound, inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , break ...
attack, another form of SCC in polymers. Tiny traces of the gas in the air will attack double bonds in rubber chains, with
natural rubber Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Types of polyisoprene ...
, styrene-butadiene rubber, and nitrile butadiene rubber being most sensitive to degradation. Ozone cracks form in products under tension, but the critical strain is very small. The cracks are always oriented at right angles to the strain axis, so will form around the circumference in a rubber tube bent over. Such cracks are dangerous when they occur in fuel pipes because the cracks will grow from the outside exposed surfaces into the bore of the pipe, so fuel leakage and fire may follow. Ozone cracking can be prevented by adding anti-ozonants to the rubber before vulcanization. Ozone cracks were commonly seen in automobile
tire A tire (North American English) or tyre (Commonwealth English) is a ring-shaped component that surrounds a Rim (wheel), wheel's rim to transfer a vehicle's load from the axle through the wheel to the ground and to provide Traction (engineeri ...
sidewalls, but are now seen rarely thanks to the use of these additives. On the other hand, the problem does recur in unprotected products such as rubber tubing and seals.


Ceramics

This effect is significantly less common in ceramics which are typically more resilient to chemical attack. Although phase changes are common in ceramics under stress these usually result in toughening rather than failure (see Zirconium dioxide). Recent studies have shown that the same driving force for this toughening mechanism can also enhance oxidation of reduced cerium oxide, resulting in slow crack growth and spontaneous failure of dense ceramic bodies.


Glass

Subcritical crack propagation in glasses falls into three regions. In region I, the velocity of crack propagation increases with ambient humidity due to stress-enhanced chemical reaction between the glass and water. In region II, crack propagation velocity is diffusion controlled and dependent on the rate at which chemical reactants can be transported to the tip of the crack. In region III, crack propagation is independent of its environment, having reached a critical stress intensity. Chemicals other than water, like ammonia, can induce subcritical crack propagation in silica glass, but they must have an electron donor site and a proton donor site.


Prevention

* The compressive residual stresses imparted by laser peening are precisely controlled both in location and intensity and can be applied to mitigate sharp transitions into tensile regions. Laser peening imparts deep compressive residual stresses on the order of 10 to 20 times deeper than conventional shot peening, making them significantly more beneficial at preventing SCC. Laser peening is widely used in the aerospace and power generation industries in gas fired turbine engines. * Material Selection: Choosing the right material for a specific environment can help prevent SCC. Materials with higher resistance to corrosion and stress corrosion cracking should be used in corrosive environments. For example, using stainless steel instead of carbon steel in a marine environment can reduce the likelihood of SCC. * Protective Coatings: Applying a protective coating or barrier can help prevent corrosive substances from coming into contact with the metal surface, thus reducing the likelihood of SCC. For example, using an epoxy coating on the interior surface of a pipeline can reduce the likelihood of SCC. * Cathodic Protection: Cathodic protection is a technique used to protect metals from corrosion by applying a small electrical current to the metal surface. This technique can also help prevent SCC by reducing the corrosion potential of the metal. * Environmental Controls: Controlling the environment around the metal can help prevent SCC. For example, reducing the temperature or acidity of the environment can help prevent SCC. * Inspection and Maintenance: Regular inspections and maintenance can help detect SCC before it causes a failure. This includes visual inspections, non-destructive testing, and monitoring of environmental factors.


Notable failures

* A 32-inch diameter gas transmission pipeline, north of
Natchitoches, Louisiana Natchitoches ( ; , ), officially the City of Natchitoches, is a small city in, and the parish seat of, Natchitoches Parish, Louisiana, Natchitoches Parish, Louisiana, United States. At the 2020 United States census, the city's population was ...
, belonging to the Tennessee Gas Pipeline exploded and burned from SCC on March 4, 1965, killing 17 people. At least 9 others were injured, and 7 homes 450 feet from the rupture were destroyed. * SCC caused the catastrophic collapse of the Silver Bridge in December 1967, when an eyebar suspension bridge across the
Ohio River The Ohio River () is a river in the United States. It is located at the boundary of the Midwestern and Southern United States, flowing in a southwesterly direction from Pittsburgh, Pennsylvania, to its river mouth, mouth on the Mississippi Riv ...
at Point Pleasant,
West Virginia West Virginia is a mountainous U.S. state, state in the Southern United States, Southern and Mid-Atlantic (United States), Mid-Atlantic regions of the United States.The United States Census Bureau, Census Bureau and the Association of American ...
, suddenly failed. The main chain joint failed and the entire structure fell into the river, killing 46 people who were traveling in vehicles across the bridge. Rust in the eyebar joint had caused a stress corrosion crack, which went critical as a result of high bridge loading and low temperature. The failure was exacerbated by a high level of residual stress in the eyebar. The disaster led to a nationwide reappraisal of bridges. * Aloha Airlines Flight 243: In 1988, Aloha Airlines Flight 243 experienced a partial fuselage failure due to SCC. The Boeing 737-200 was flying from Hilo to Honolulu, Hawaii when a section of the fuselage ruptured, causing a decompression event. The investigation into the failure found that SCC had occurred in the aluminum skin of the fuselage due to the repeated pressurization and depressurization cycles of the aircraft. The incident led to changes in maintenance procedures and inspections for aircraft to prevent similar failures in the future. * Trans-Alaska Pipeline: In 2001, a section of the Trans-Alaska Pipeline failed due to SCC. The pipeline is used to transport crude oil from the North Slope of Alaska to the Valdez Marine Terminal. The failure occurred when a 34-foot section of the pipeline ruptured, causing a spill of over 285,000 gallons of crude oil. The investigation into the failure found that SCC had occurred in the pipeline due to the presence of water and bacteria, which had created a corrosive environment. * USS ''Hartford'' submarine periscope: In 2009, the periscope of the submarine USS ''Hartford'' failed due to SCC. The periscope is used to provide a view of the surface while the submarine is submerged. The failure occurred when the periscope was extended through the hull of the submarine, causing seawater to enter the periscope's seal. The seawater caused SCC to occur in the periscope's steel support structure, which led to the periscope falling back into the submarine. Fortunately, there were no injuries, but the submarine had to be taken out of service for repairs. * NDK Crystal: In 2009, a 50-foot-tall
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...
-growing autoclave in a plant in
Belvidere, Illinois Belvidere is a city in and the county seat of Boone County, Illinois, United States. It is settled on the Kishwaukee River in far northern Illinois. Known as the 'City of Murals', Belvidere is home to several public art installations througho ...
violently ruptured due to SCC, causing one fatality and one injury in nearby businesses. The autoclave was filled with hot, highly pressurized
sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula . It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly corrosive base (chemistry), ...
solution, and the thick
steel Steel is an alloy of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to steel's high Young's modulus, elastic modulus, Yield (engineering), yield strength, Fracture, fracture strength a ...
pressure vessel A pressure vessel is a container designed to hold gases or liquids at a pressure substantially different from the ambient pressure. Construction methods and materials may be chosen to suit the pressure application, and will depend on the size o ...
was not rated to withstand corrosive environments. However, the plant's operators incorrectly believed that the vessel walls would be protected from corrosion by the formation of a self-passivating layer of iron sodium silicate. Despite a previous SCC incident in 2007 and specific warnings from NDK Crystal's insurance provider, none of the plant's eight autoclaves were ever given safety inspections.


See also

* * * * * * * * * * * *


References


Citations


General and cited references

* ASM International, ''Metals Handbook'' (Desk Edition) Chapter 32 ("Failure Analysis"), American Society for Metals, (1997) pp 32–24 to 32–26 * ASM Handbook Volume 11 ''Failure Analysis and Prevention'' (2002) "Stress-Corrosion Cracking" Revised by W.R. Warke, American Society of Metals. Pages 1738-1820. * *


External links

* {{DEFAULTSORT:Stress Corrosion Cracking Corrosion Engineering failures Fracture mechanics Materials degradation