{{Short description|Growth of cracks in a corrosive environment}} {{More citations needed|date=December 2007}} <!-- taken from Fracture mechanics --> thumb|upright=1.2|Stress corrosion cracking caused by tension developed in an unsuitably welded reinforcement collar [[Image:Stress corrosion cracking revealed by magnetic particles.JPG|thumb|upright=1.2|A close-up of the surface of a steel pipeline showing stress corrosion cracking (two clusters of small black lines) revealed by magnetic particle inspection. Cracks which would normally have been invisible are detectable due to the magnetic particles clustering at the crack openings. The scale at the bottom is in centimeters (each division indicates a millimeter).]] {{Mechanical failure modes}}
'''Stress corrosion cracking''' ('''SCC''') is the growth of crack formation in a corrosive environment. It can lead to unexpected and sudden failure of normally ductile metal alloys 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 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.<ref name=":1">{{cite book |title=Metals Handbook |edition=Desk |chapter=Chapter 32: Failure Analysis |publisher=American Society for Metals}}</ref>
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 metals and metallic alloys. A comparable effect also known as environmental stress cracking also affects other materials such as polymers, ceramics and glass.
=== 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.<ref>{{Cite journal |last1=Gu |first1=B. |last2=Luo |first2=J. |last3=Mao |first3=X. |date=January 1999 |title=Hydrogen-facilitated anodic dissolution-type stress corrosion cracking of pipeline steels in near-neutral pH solution |journal=Corrosion |language=en |volume=55 |issue=1 |pages=96–106 |doi=10.5006/1.3283971 |issn=0010-9312 |access-date=2023-02-21 |archive-date=2023-02-21 |archive-url=https://web.archive.org/web/20230221132447/https://meridian.allenpress.com/corrosion/article-abstract/55/1/96/161532/Hydrogen-Facilitated-Anodic-Dissolution-Type?redirectedFrom=fulltext |url-status=live |url=https://meridian.allenpress.com/corrosion/article-abstract/55/1/96/161532/Hydrogen-Facilitated-Anodic-Dissolution-Type?redirectedFrom=fulltext |url-access=subscription }}</ref> {{table alignment}} {| class="wikitable floatright col2right col4right" |- !Alloy !''K''<sub>Ic</sub> MN/m<sup>3/2</sup> !SCC environment !''K''<sub>Iscc</sub> MN/m<sup>3/2</sup> |- |13Cr steel ||60 ||3% NaCl ||12 |- |18Cr-8Ni ||200 ||42% MgCl<sub>2</sub> ||10 |- |Cu-30Zn ||200 ||NH<sub>4</sub>OH (pH 7) ||1 |- |Al-3Mg-7Zn ||25 ||Aqueous halides ||5 |- |Ti-6Al-1V ||60 ||0.6 M KCl ||20 |}
* Certain austenitic stainless steels and aluminium alloys crack in the presence of chlorides. 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 {{Convert|50|C||abbr=}}; * mild steel cracks in the presence of alkali (e.g. '''boiler cracking''' and '''caustic stress corrosion cracking''') and nitrates; * copper alloys crack in ammoniacal 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 (<math>K_\mathrm{Ic}</math>). The subcritical value of the stress intensity, designated as <math>K_\mathrm{Iscc}</math>, may be less than 1% of <math>K_\mathrm{Ic}</math>.
=== Polymers === A similar process (environmental stress cracking) occurs in polymers, 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 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 mechanism. Polymers are susceptible to environmental stress cracking where attacking agents do not necessarily degrade the materials chemically. Nylon is sensitive to degradation by acids, a process known as hydrolysis, and nylon mouldings will crack when attacked by strong acids. thumb|Close-up of broken nylon fuel pipe connector caused by SCC
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 of the polymer by contact with sulfuric acid leaking from a car battery. The degradation reaction is the reverse of the synthesis reaction of the polymer: :500px{{Clear left}} :image:amide hydrolysis.svg
[[Image:Ozone cracks in tube1.jpg|thumb|right|Ozone cracking in natural rubber tubing]] Cracks can be formed in many different elastomers by ozone 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, 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 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.<ref>{{cite journal|last1=Munnings|first1=C.|last2=Badwal|first2=S. P. S.|last3=Fini|first3=D.|title=Spontaneous stress-induced oxidation of Ce ions in Gd-doped ceria at room temperature|journal=Ionics|date=20 February 2014|volume=20|issue=8|pages=1117–1126|doi=10.1007/s11581-014-1079-2|s2cid=95469920}}</ref>
=== Glass === thumb|right| Illustrated are regions of different crack propagation under stress corrosion cracking. In region I, crack propagation is dominated by chemical attack of strained bonds in the crack. In region II, propagation is controlled by diffusion of chemical into the crack. In region III, the stress intensity reaches its critical value and propagates independent of its environment.
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.<ref>{{cite book|last1=Wachtman|first1=John B.|last2=Cannon|first2=W. Roger|last3=Matthewson|first3=M. John|title=Mechanical Properties of Ceramics|date=11 September 2009|publisher=John Wiley and Sons|isbn=9780471735816|edition=2nd|doi=10.1002/9780470451519}}</ref>
== 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.<ref>{{Cite web |url=http://www.epri.com/search/Pages/results.aspx?k=Compressor%20Dependability:%20Laser%20Shock%20Peening%20Surface%20Treatment |title=EPRI {{!}} Search Results: Compressor Dependability: Laser Shock Peening Surface Treatment<!-- Bot generated title --> |access-date=2023-02-21 |archive-date=2022-12-06 |archive-url=https://web.archive.org/web/20221206230950/https://www.epri.com/search/Pages/results.aspx?k=Compressor%20Dependability:%20Laser%20Shock%20Peening%20Surface%20Treatment |url-status=live }}</ref> Laser peening is widely used in the aerospace and power generation industries in gas fired turbine engines.<ref name="Crooker_2011">{{Cite web |title=Peening for mitigation of PWSCC in alloy 600 |first1=Paul |last1=Crooker |first2=William |last2=Sims |work=nrc.gov |date=2011-06-09 |access-date=2022-06-01 |url=https://www.nrc.gov/docs/ML1116/ML11167A243.pdf |language=English |archive-date=2022-10-06 |archive-url=https://web.archive.org/web/20221006095242/https://www.nrc.gov/docs/ML1116/ML11167A243.pdf |url-status=live }}</ref> * 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.<ref name=":0">{{Citation |title=Irradiation-Assisted Stress-Corrosion Cracking |date=2017-01-01 |url=http://dx.doi.org/10.31399/asm.tb.sccmpe2.t55090191 |work=Stress-Corrosion Cracking |pages=191–220 |access-date=2023-04-26 |publisher=ASM International|doi=10.31399/asm.tb.sccmpe2.t55090191 |osti=7010172 |isbn=978-1-62708-266-2 |url-access=subscription }}</ref> * 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.<ref name=":0" /> * 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.<ref name=":0" /> * 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.<ref name=":0" /> * 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.<ref name=":0" />
== Notable failures == [[Image:Silver Bridge collapsed, Ohio side.jpg|thumb|right|The collapsed Silver Bridge, as seen from the Ohio side]] * A 32-inch diameter gas transmission pipeline, north of Natchitoches, Louisiana, 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.<ref>{{cite report |title=Corrective Action Order Regarding the TGP 100 Pipeline |date=Dec 3, 2010 |publisher= US Department of Transportation Pipeline and Hazardous Materials Safety Administration |url=http://primis.phmsa.dot.gov/comm/reports/enforce/documents/420101007H/420101007H_CAO_12032010.pdf |archive-url=https://web.archive.org/web/20161226084530/http://primis.phmsa.dot.gov/comm/reports/enforce/documents/420101007H/420101007H_CAO_12032010.pdf |archive-date=2016-12-26 }}</ref><ref>{{Cite news |title= 17 Killed As Gas Line Explodes|url=https://news.google.com/newspapers?id=e95dAAAAIBAJ&sjid=x14NAAAAIBAJ&pg=6377,556265&dq=pipeline+fire&hl=en |newspaper=The Washington Observer |date=Mar 5, 1965 |access-date=2023-02-21 |archive-date=2021-11-02 |archive-url=https://web.archive.org/web/20211102042602/https://news.google.com/newspapers?id=e95dAAAAIBAJ&sjid=x14NAAAAIBAJ&pg=6377%2C556265&dq=pipeline+fire&hl=en |url-status=live }}</ref> * SCC caused the catastrophic collapse of the Silver Bridge in December 1967, when an eyebar suspension bridge across the Ohio River at Point Pleasant, West Virginia, 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.<ref>{{cite book | last=Lewis | first=Peter Rhys | last2=Reynolds | first2=Ken | last3=Gagg | first3=Colin | title=Forensic Materials Engineering | publisher=CRC Press | date=2003-09-29 | isbn=978-0-203-48453-1 | doi=10.1201/9780203484531 | page=}}</ref> * 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.<ref>{{Cite book |last1=Hong-bing |first1=Du |last2=Qing-qing |first2=Zhang |title=2015 International Conference on Transportation Information and Safety (ICTIS) |chapter=Simulation of the effect of safety investment on flight safety level in the airlines |date=June 2015 |chapter-url=http://dx.doi.org/10.1109/ictis.2015.7232149 |pages=780–786 |publisher=IEEE |doi=10.1109/ictis.2015.7232149|isbn=978-1-4799-8694-1 |s2cid=2908608 }}</ref> * 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.<ref>{{Cite journal |last=Busenberg |first=George J. |date=September 2011 |title=The Policy Dynamics of the Trans-Alaska Pipeline System |url=http://dx.doi.org/10.1111/j.1541-1338.2011.00508.x |journal=Review of Policy Research |volume=28 |issue=5 |pages=401–422 |doi=10.1111/j.1541-1338.2011.00508.x |issn=1541-132X|url-access=subscription }}</ref> * 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.<ref>{{Cite magazine |magazine=Live Science |date=March 23, 2009 |title=USS Hartford Periscope Snaps, Falls Into Submarine |first= Jeremy |last=Hsu }}{{Failed verification|reason=Cannot find this article anywhere in the Live Science archives|date=March 2024}}</ref><ref>{{Cite web|url=https://www.theday.com/local-news/20091117/report-sub-crew-caused-hartford-collision/ |website=The Day |date=November 17, 2009 |first=Jennifer |last=Grogan |title=Report: Sub crew caused Hartford collision }}</ref> * NDK Crystal: In 2009, a 50-foot-tall quartz-growing autoclave in a plant in Belvidere, Illinois violently ruptured due to SCC, causing one fatality and one injury in nearby businesses. The autoclave was filled with hot, highly pressurized sodium hydroxide solution, and the thick steel pressure vessel 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.<ref>{{cite report |title=NDK Crystal Case Study: Final Investigation Report |date=Nov 14, 2013 |publisher= US Chemical Safety and Hazard Investigation Board |url=https://www.csb.gov/file.aspx?DocumentId=5746 }}</ref>
==See also== * {{annotated link|Forensic chemistry}} * {{annotated link|Forensic engineering}} * {{annotated link|Forensic materials engineering}} * {{annotated link|Forensic polymer engineering}} * {{annotated link|Fracture mechanics}} * {{annotated link|Environmental stress cracking}} * {{annotated link|Environmental stress fracture}} * {{annotated link|Hydrogen embrittlement}} * {{annotated link|Ozone cracking}} * {{annotated link|Polymer degradation}} * {{annotated link|Season cracking}} * {{annotated link|Sulfide stress cracking}}
==References== === Citations === {{reflist|30em}}
=== General and cited references === {{Refbegin}} * 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. * {{Cite web |title=ASTM G36-94 (2018) Standard practice for evaluating stress-corrosion-cracking resistance of metals and alloys in a boiling magnesium chloride solution |author=ASTM |work=astm.org |date=5 November 2018 |access-date=1 June 2022 |url=https://www.astm.org/g0036-94r18.html |archive-date=3 December 2022 |archive-url=https://web.archive.org/web/20221203193503/https://www.astm.org/g0036-94r18.html |url-status=live }} * {{cite book |title=Mechanical Properties of Ceramics |first1=John B.|last1= Wachtman |first2=W. Roger |last2=Cannon |first3= M. John |last3=Matthewson |chapter= Chapter 8}} {{Refend}}
== External links == * {{Commons inline}}
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{{DEFAULTSORT:Stress Corrosion Cracking}} Category:Corrosion Category:Engineering failures Category:Fracture mechanics Category:Materials degradation