Crazes (surface cracks) produced by ESC in PMMA drinking beaker
![Corrosion Corrosion](/uploads/1/2/5/0/125089627/873243418.jpg)
Tests for Stress-Corrosion Stress-corrosion cracking is a time-dependent process in which a metallurgically susceptible material fractures prematurely due to the synergistic interaction of a corrosive environment and sustained tensile stress at the metal surface. The tensile stress may be residual stress resulting from heat treatment. The history of dynamic straining in stress corrosion cracking studies and the evolution of the slow strain rate test (SSRT) are reviewed. Smooth and notched specimens; the importance of strain rate, electrode potential, and other environmental factors; the evaluation of test results; and comparisons to other techniques are addressed. 7 Stress-corrosion crack initiating from a corrosion pit in a quenched-and-tempered high-strength turbine disk. Steel (339Ni-1.56Cr-0.63Mo-0.11V) test coupon exposed oxygenated, to demineralized water for 800 h under a. Bending stress of 90% of the yield stress, (a) 275x.
3 - Testing and evaluation methods for stress corrosion cracking (SCC) in metals 3.1. It is important to prevent stress corrosion cracking (SCC). General aspects of stress corrosion cracking (SCC) testing. Smooth specimens. In conducting a test programme to select a. Stress corrosion cracking (SCC) is the cracking induced from the combined influence of tensile stress and a corrosive environment. The impact of SCC on a material usually falls between dry cracking and the fatigue threshold of that material. Stress Corrosion Cracking (SCC) Inspection and Consulting Services. When stress corrosion cracking (SCC) is suspected, our metallurgical analysis team can quickly provide you with testing, inspection, and analysis services to help determine the size and scope of the problems.
Environmental Stress Cracking (ESC) is one of the most common causes of unexpected brittle failure of thermoplastic (especially amorphous) polymers known at present. According to ASTM D883, stress cracking is defined as ' an external or internal crack in a plastic caused by tensile stresses less than its short-term mechanical strength.' This type of cracking typically involves brittle cracking, with little or no ductile drawing of the material from its adjacent failure surfaces.[1] Environmental stress cracking may account for around 15-30% of all plastic component failures in service.[2] This behavior is especially prevalent in glassy, amorphous thermoplastics.[3] Amorphous polymers exhibit ESC because of their loose structure which makes it easier for the fluid to permeate into the polymer. Amorphous polymers are more prone to ESC at temperature higher than their glass transition temperature (Tg) due to the increased free volume. When Tg is approached, more fluid can permeate permeation into the polymer chains.[4]
ESC and polymer resistance to ESC (ESCR) have been studied for several decades.[5] Research shows that the exposure of polymers to liquid chemicals tends to accelerate the crazing process, initiating crazes at stresses that are much lower than the stress causing crazing in air.[5][6] The action of either a tensile stress or a corrosive liquid alone would not be enough to cause failure, but in ESC the initiation and growth of a crack is caused by the combined action of the stress and a corrosive environmental liquid. These corrosive environmental liquids are called 'secondary chemical agents', are often organic, and are defined as solvents not anticipated to come into contact with the plastic during its lifetime of use. Failure is rarely associated with primary chemical agents, as these materials are anticipated to come into contact with the polymer during its lifetime, and thus compatibility is ensured prior to use. In air, failure due to creep is known as creep rupture, as the air acts as a plasticizer, and this acts in parallel to environmental stress cracking.[7]
It is somewhat different from polymer degradation in that stress cracking does not break polymer bonds. Instead, it breaks the secondary linkages between polymers. These are broken when the mechanical stresses cause minute cracks in the polymer and they propagate rapidly under the harsh environmental conditions.[8] It has also been seen that catastrophic failure under stress can occur due to the attack of a reagent that would not attack the polymer in an unstressed state. Environmental stress cracking is accelerated due to higher temperatures, cyclic loading, increased stress concentrations, and fatigue.[7]
Metallurgists typically use the term Stress corrosion cracking or Environmental stress fracture to describe this type of failure in metals.
- 2Mechanisms of ESC
- 5Examples
Predicting ESC[edit]
Although the phenomenon of ESC has been known for a number of decades, research has not yet enabled prediction of this type of failure for all environments and for every type of polymer. Some scenarios are well known, documented or are able to be predicted, but there is no complete reference for all combinations of stress, polymer and environment. The rate of ESC is dependent on many factors including the polymer’s chemical makeup, bonding, crystallinity, surface roughness, molecular weight and residual stress. It also depends on the liquid reagent's chemical nature and concentration, the temperature of the system and the strain rate.
Mechanisms of ESC[edit]
There are a number of opinions on how certain reagents act on polymers under stress. Because ESC is often seen in amorphous polymers rather than in semicrystalline polymers, theories regarding the mechanism of ESC often revolve around liquid interactions with the amorphous regions of polymers. One such theory is that the liquid can diffuse into the polymer, causing swelling which increases the polymer’s chain mobility. The result is a decrease in the yield stress and glass transition temperature (Tg), as well as a plasticisation of the material which leads to crazing at lower stresses and strains.[2][6] A second view is that the liquid can reduce the energy required to create new surfaces in the polymer by wetting the polymer’s surface and hence aid the formation of voids, which is thought to be very important in the early stages of craze formation.[2] ESC may occur continuously, or a piece-wise start and stop mechanism
There is an array of experimentally derived evidence to support the above theories:
- Once a craze is formed in a polymer this creates an easy diffusion path so that the environmental attack can continue and the crazing process can accelerate.
- Chemical compatibility between the environment and the polymer govern the amount in which the environment can swell and plasticise the polymer.[2]
- The effects of ESC are reduced when crack growth rate is high. This is primarily due to the inability of the liquid to keep up with the growth of the crack.[2]
- Once separated from the other chains, the polymers align, thus allowing embrittlement.
ESC generally occurs at the surface of a plastic and doesn't require the secondary chemical agent to penetrate the material significantly, which leaves the bulk properties unmodified. [7]
Fluid mask 3 review. Another theory for the mechanism of craze propagation in amorphous polymers is proposed by Kramer. According to his theory, the formation of internal surfaces in polymers is facilitated by polymeric surface tension that is determined by both secondary interactions and the contribution of load-bearing chains that must undergo fracture or slippage to form a surface. This theory provides and explanation for the decrease in the stress needed to propagate the craze in the presence of surface-active reagents such as detergents and high temperature.[9]
ESC mechanism in polyethylene[edit]
Stress Corrosion Cracking Testing Methods
Semi-crystalline polymers such as polyethylene show brittle fracture under stress if exposed to stress cracking agents. In such polymers, the crystallites are connected by the tie molecules through the amorphous phase. The tie molecules play an important role in the mechanical properties of the polymer through the transferring of load. Stress cracking agents, such as detergents, act to lower the cohesive forces which maintain the tie molecules in the crystallites, thus facilitating their “pull-out” and disentanglement from the lamellae.[10] As a result, cracking is initiated at stress values lower than the critical stress level of the material.
In general, the mechanism of environmental stress cracking in polyethylene involves the disentanglement of the tie molecules from the crystals. The number of tie molecules and the strength of the crystals that anchor them are considered the controlling factors in determining the polymer resistance to ESC.[11]
Characterizing ESC[edit]
A number of different methods are used to evaluate a polymer’s resistance to environmental stress cracking. A common method in the polymer industry is use of the Bergen jig, which subjects the sample to variable strain during a single test. The results of this test indicate the critical strain to cracking, using only one sample.[5] Another widely used test is the Bell Telephone test where bent strips are exposed to fluids of interest under controlled conditions. Further, new tests have been developed where the time for crack initiation under transverse loading and an aggressive solvent (10% Igepal CO-630 solution) is evaluated. These methods rely on an indentor to stress the material biaxially, while prevent radial stress concentration. The stressed polymer sits in the aggressive agent and the stressed plastic around the indentor is watched to evaluate the time to crack formation, which is the way that ESC resistance is quantified. A testing apparatus for this method is known as the Telecom and is commercially available; initial experiments have shown that this testing gives equivalent results to ASTM D1693, but at a much shorter time scale.[12] Current research deals with the application of fracture mechanics to the study of ESC phenomena.[13][14] In summary, though, there is not a singular descriptor that is applicable to ESC -- rather, the specific fracture is dependent on the material, conditions, and secondary chemical agents present .
Scanning electron microscopy and fractographic methods have historically been used to analyze the failure mechanism, particularly in high density polyethylene (HDPE). Freeze fracture has proved particularly useful for examining the kinetics of ESC, as they provide a snapshot in time of the crack propagation process.[1]
Strain hardening as a measure of environmental stress cracking resistance (ESCR)[edit]
Many different methods exist for measuring ESCR. However, the long testing time and high costs associated with these methods slow down the R&D activities for designing materials with higher resistance to stress cracking. To overcome these challenges, a new simpler and faster method was developed by SABIC to assess ESCR for high density polyethylene (HDPE) materials. In this method, the resistance of slow crack growth or environmental stress cracking is predicted from simple tensile measurement at a temperature of 80℃.[9] When polyethylene is deformed under a uniaxiial tension, before yield, the stiff crystalline phase of the polymer undergoes small deformation while the amorphous domains deforms significantly. After the yield point but before the material undergoes strain hardening, the crystalline lamellae slips where both the crystalline phase and the amorphous domains contribute to load bearing and straining. At some point, the amorphous domains will stretch fully at which the strain hardening begin. In the strain hardening region, the elongated amorphous domains become the loading bearing phase whereas the crystalline lamellae undergoes fracture and unfold to adjust for the change in strain. The load-bearing chains in the amorphous domains in polyethylene are made of tie-molecules and entangles chains. Because of the key role of tie-molecules and entanglements in resisting environmental stress cracking in polyethylene, it follows that ESCR and strain hardening behaviors can very well be correlated.[15]
In the strain hardening method, the slope of strain hardening region (above the natural draw ratio) in the true stress-strain curves is calculated and used as a measure of ESCR. This slope is called the strain hardening modulus (Gp). The strain hardening modulus is calculated over the entire strain hardening region in the true stress strain curve. The strain hardening region of the stress-strain curve is considered to be the homogeneously deforming part well above the natural draw ratio, which is determined by presence of the neck propagation, and below the maximum elongation.[9] The strain hardening modulus when measured at 80℃ is sensitive to the same molecular factors that govern slow crack resistance in HDPE as measured by an accelerated ESCR test where a surface active agent is used.[9] The strain hardening modulus and ESCR values for polyethylene have been found to be strongly correlated with each others.
Examples[edit]
An obvious example of the need to resist ESC in everyday life is the automotive industry, in which a number of different polymers are subjected to a number of fluids. Some of the chemicals involved in these interactions include petrol, brake fluid and windscreen cleaning solution.[6]Plasticisers leaching from PVC can also cause ESC over an extended period of time, for example.One of the first examples of the problem concerned ESC of LDPE. The material was initially used in insulating electric cables, and cracking occurred due to the interaction of the insulation with oils. The solution to the problem lay in increasing the molecular weight of the polymer. A test of exposure to a strong detergent such as Igepal was developed to give a warning of ESC.
SAN piano key[edit]
A more specific example comes in the form of a piano key made from injection moulded styrene acrylonitrile (SAN). The key has a hook end which connects it to a metal spring, which causes the key to spring back into position after being struck. During assembly of the piano an adhesive was used, and excess adhesive which had spilled onto areas where it was not required was removed using a ketone solvent. Some vapour from this solvent condensed on the internal surface of the piano keys. Some time after this cleaning, fracture occurred at the junction where the hook end meets the spring.[16]
To determine the cause of the fracture, the SAN piano key was heated above its glass transition temperature for a short time. If there is residual stress within the polymer, the piece will shrink when held at such a temperature. Results showed that there was significant shrinkage, particularly at the hook end-spring junction. This indicates stress concentration, possibly the combination of residual stress from forming and the action of the spring. It was concluded that although there was residual stress, the fracture was due to a combination of the tensile stress from the spring action and the presence of the ketone solvent.[16]
See also[edit]
References[edit]
- ^ abChoi, Byoung-Ho; Weinhold, Jeffrey; Reuschle, David; Kapur, Mridula (2009). 'Modeling of the fracture mechanism of HDPE subjected to environmental stress crack resistance test'. Polymer Engineering & Science. 49 (11): 2085–2091. doi:10.1002/pen.21458. ISSN1548-2634.
- ^ abcdeH. F. Mark. Encyclopedia of Polymers Science and Technology – 3rd Ed. Vol 12. John Miley & Sons Inc. 2004
- ^Henry, L. F. (1974). 'Prediction and evaluation of the susceptibilities of glassy thermoplastics to environmental stress cracking'. Polymer Engineering & Science. 14 (3): 167–176. doi:10.1002/pen.760140304. ISSN1548-2634.
- ^J. Scheirs (2000). Compositional and Failure Analysis of Polymers. J. Wiley & Sons.
- ^ abcXiangyang Li. Environmental Stress Cracking Resistance of a New Copolymer of Bisphenol-A. Polymer Degradation and Stability. Volume 90, Issue 1, October 2005, Pages 44-52
- ^ abcJ. C. Arnold. The Effect of Diffusion on Environmental Stress Crack Initiation in PMMA. Journal of Materials Science 33 (1998) p 5193 – 5204
- ^ abc'Plastics Engineering - November/December 2015 - Plastic Failure through Environmental Stress Cracking'. read.nxtbook.com. Retrieved 23 May 2019.
- ^Michigan University – College of Engineering, Properties of PlasticsArchived 6 May 2008 at the Wayback Machine. Accessed 22 April 2008.
- ^ abcdKureleca, L.; Teeuwenb, M.; Schoffeleersb, H.; Deblieckb, R. (2005). 'Strain hardening modulus as a measure of environmental stress crack resistance of high density polyethylene'. Polymer. 46 (17): 6369–6379. doi:10.1016/j.polymer.2005.05.061.CS1 maint: multiple names: authors list (link)
- ^Chen, Yang (2014). 'Investigations of environmental stress cracking resistance of HDPE/EVA and LDPE/EVA blends'. Journal of Applied Polymer Science. 131 (4): n/a. doi:10.1002/app.39880. ISSN1097-4628.
- ^Ward, A. L.; Lu, X.; Huang, Y.; Brown, N. (1 January 1991). 'The mechanism of slow crack growth in polyethylene by an environmental stress cracking agent'. Polymer. 32 (12): 2172–2178. doi:10.1016/0032-3861(91)90043-I. ISSN0032-3861.
- ^Jar, Ben (2017). 'A NEW METHOD TO CHARACTERIZE ENVIRONMENTAL STRESS CRACKING RESISTANCE (ESCR) OF POLYETHYLENE PIPES'(PDF). SPE ANTEC® Anaheim 2017: 1994–1998.
- ^Andena, Luca; Castellani, Leonardo; Castiglioni, Andrea; Mendogni, Andrea; Rink, Marta; Sacchetti, Francisco (1 March 2013). 'Determination of environmental stress cracking resistance of polymers: Effects of loading history and testing configuration'. Engineering Fracture Mechanics. Fracture of Polymers, Composites and Adhesives. 101: 33–46. doi:10.1016/j.engfracmech.2012.09.004.
- ^Kamaludin, M.A.; Patel, Y.; Williams, J.G.; Blackman, B.R.K. (2017). 'A fracture mechanics approach to characterising the environmental stress cracking behaviour of thermoplastics'. Theoretical and Applied Fracture Mechanics. 92: 373–380. doi:10.1016/j.tafmec.2017.06.005. hdl:10044/1/49864.
- ^Cheng, Joy J.; Polak, Maria A.; Penlidis, Alexander (1 June 2008). 'A Tensile Strain Hardening Test Indicator of Environmental Stress Cracking Resistance'. Journal of Macromolecular Science, Part A. 45 (8): 599–611. doi:10.1080/10601320802168728. ISSN1060-1325.
- ^ abEzrin, M & Lavigne, G. Unexpected and Unusual Failures of Polymeric Materials. Engineering Failure Analysis, Volume 14, Pages 1153-1165, January 2007
Further reading[edit]
- Ezrin, Meyer, Plastics Failure Guide: Cause and Prevention, Hanser-SPE (1996).
- Wright, David C., Environmental Stress Cracking of Plastics RAPRA (2001).
- Lewis, Peter Rhys, Reynolds, K and Gagg, C, Forensic Materials Engineering: Case studies, CRC Press (2004)
External links[edit]
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Environmental_stress_cracking&oldid=910030130'
A close-up of the surface of a steel pipeline showing indications of 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 millimetres.
Stress corrosion cracking (SCC) is the growth of crack formation in a corrosive environment. It can lead to unexpected 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.[1]
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.
- 6Prevention
Metals attacked[edit]
Certain austeniticstainless steels and aluminiumalloys crack in the presence of chlorides, mild steel cracks in the presence of alkali (boiler cracking) and nitrates, copper alloys crack in ammoniacal solutions (season cracking). This limits the usefulness of austenitic stainless steel for containing water with higher than few ppm content of chlorides at temperatures above 50 °C. Also of concern is the fact that high-tensile structural 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 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:
Alloy | KIc MN/m3/2 | SCC environment | KIscc MN/m3/2 |
---|---|---|---|
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 |
Polymers attacked[edit]
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.
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:
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 very 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. The problem of 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 attacked[edit]
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.[2]
Glass attacked[edit]
Shown here are the 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.
Given that most glasses contain a substantial silica phase, the introduction of water can chemically weaken the bonds preventing subcritical crack propagation. Indeed, the silicon oxygen bonds present at the tip of a crack are strained, and thus more susceptible to chemical attack. In the instance of chemical attack by water, silicon-oxygen bonds bridging the crack are separated into non-connected silicon hydroxide groups. The addition of external stress will serve to further weaken these bonds. 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.[3]
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Crack growth[edit]
The subcritical nature of propagation may be attributed to the chemical energy released as the crack propagates. That is,
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- 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 crack propagates perpendicular to the applied stress.
Prevention[edit]
SCC is the result of a combination of three factors – a susceptible material, exposure to a corrosive environment, and tensile stresses above a threshold. If any one of these factors are eliminated, SCC initiation becomes impossible. However, there are a number of approaches that can be used to prevent or at least delay the onset of SCC. In an ideal world a stress corrosion cracking control strategy will start operating at the design stage, and will focus on the selection of material, the limitation of stress and the control of the environment. The skill of the engineer then lies in selecting the strategy that delivers the required performance at minimum cost. Part of the performance requirements relate to the acceptability of failure. The primary containment pressure vessel in a nuclear reactor obviously requires a very low risk of failure. For the pressed brass decorative trim on a light switch, the occasional stress corrosion crack is not going to be a serious problem, although frequent failures would have an undesirable impact on product returns and the manufacturer's image. The conventional approach to controlling the problem has been to develop new alloys that are more resistant to SCC. This is a costly proposition and can require a massive time investment to achieve only marginal success.
Selection and control of material[edit]
The first line of defence in controlling stress corrosion cracking is to be aware of the possibility at the design and construction stages. By choosing a material that is not susceptible to SCC in the service environment, and by processing and fabricating it correctly, subsequent SCC problems can be avoided. Unfortunately, it is not always quite that simple. Some environments, such as high temperature water, are very aggressive, and will cause SCC of most materials. Mechanical requirements, such as a high yield strength, can be very difficult to reconcile with SCC resistance (especially where hydrogen embrittlement is involved).
Control of stress[edit]
As one of the requirements for stress corrosion cracking is the presence of stress in the components, one method of control is to eliminate that stress, or at least reduce it below the threshold stress for SCC. This is not usually feasible for working stresses (the stress that the component is intended to support), but it may be possible where the stress causing cracking is a residual stress introduced during welding or forming.Residual stresses can be relieved by stress-relief annealing, and this is widely used for carbon steels. These have the advantage of a relatively high threshold stress for most environments, consequently it is relatively easy to reduce the residual stresses to a low enough level. In contrast austenitic stainless steels have a very low threshold stress for chloride SCC. This, combined with the high annealing temperatures that are necessary to avoid other problems, such as sensitization and sigma phase embrittlement, means that stress relief is rarely successful as a method of controlling SCC for this system.For large structures, for which full stress-relief annealing is difficult or impossible, partial stress relief around welds and other critical areas may be of value. However, this must be done in a controlled way to avoid creating new regions of high residual stress, and expert advice is advisable if this approach is adopted.Stresses can also be relieved mechanically. For example, hydrostatic testing beyond yield will tend to ‘even-out’ the stresses and thereby reduce the peak residual stress. Similarly laser peening, shot-peening, or grit-blasting tend to introduce a surface compressive stress, and are beneficial for the control of SCC. The uniformity with which these processes are applied is important. If, for example, only the weld region is shot-peened, damaging tensile stresses may be created at the border of the peened area. 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 it significantly more beneficial at preventing SCC.[4] Laser peening is widely used in the aerospace and power generation industries in gas fired turbine engines.[5]
Control of environment[edit]
The most direct way of controlling SCC through control of the environment is to remove or replace the component of the environment that is responsible for the problem, though this is not usually feasible. Where the species responsible for cracking are required components of the environment, environmental control options consist of adding inhibitors, modifying the electrode potential of the metal, or isolating the metal from the environment with coatings.
For example, chloride stress corrosion cracking of austenitic stainless steel has been experienced in hot-water jacketed pipes carrying molten chocolate in the food industry. It is difficult to control the temperature, while changing pipe material or eliminating residual stresses associated with welding and forming the pipework is costly and incurs plant downtime. However, this is a rare case where environment may be modified: an ion exchange process may be used to remove chlorides from the heating water.
Testing of susceptible materials[edit]
One of the primary methods used to detect and remove materials that are susceptible to SCC is corrosion testing. A variety of SCC corrosion tests exist for different metal alloy.
Examples[edit]
The collapsed Silver Bridge, as seen from the Ohio side
A classic example of SCC is season cracking of brass cartridge cases, a problem experienced by the British army in India in the early 19th century. It was initiated by ammonia from dung and horse manure decomposing at the higher temperatures of the spring and summer. There was substantial residual stress in the cartridge shells as a result of cold forming. The problem was solved by annealing the shells to ameliorate the stress.
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.[6][7]
SCC caused the catastrophic collapse of the Silver Bridge in December 1967, when an eyebarsuspension 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.[8]
Suspended ceilings in indoor swimming pools are safety-relevant components. As was demonstrated by the collapses of the ceiling of the Uster (Switzerland) indoor swimming pool (1985), and again at Steenwijk (Netherlands, 2001), attention must be paid to selecting suitable materials and inspecting the state of such components. The reason for the failures was stress corrosion cracking of metal fastening components made of stainless steel. Further in 2004 a swimming pool in Moscow collapsed as caused by stress corrosion cracking [ref 1] resulting in 28 fatalities. The same for Chusovoy RU, resulting in 14 fatalities (2005, ref 1). And in 2011 a five month old baby got killed by stress corrosion cracking of the exotic stainless steel SS 1.4539 in Tilburg NL. Scientific research of NACE TG 498 confirmed that 1.4529 is very dangerous.[9] The active chemical was chlorine added to the water as a disinfectant.
See also[edit]
References[edit]
- Notes
- ^ASM International, Metals Handbook (Desk Edition) Chapter 32 (Failure Analysis), American Society for Metals
- ^Munnings, C.; Badwal, S. P. S.; Fini, D. (20 February 2014). 'Spontaneous stress-induced oxidation of Ce ions in Gd-doped ceria at room temperature'. Ionics. 20 (8): 1117–1126. doi:10.1007/s11581-014-1079-2.
- ^Wachtman, John B.; Cannon, W. Roger; Matthewson, M. John (11 September 2009). Mechanical Properties of Ceramics (2nd ed.). John Wiley and Sons. doi:10.1002/9780470451519. ISBN9780471735816.
- ^EPRI | Search Results: Compressor Dependability: Laser Shock Peening Surface Treatment
- ^http://pbadupws.nrc.gov/docs/ML1116/ML11167A243.pdf
- ^http://primis.phmsa.dot.gov/comm/reports/enforce/documents/420101007H/420101007H_CAO_12032010.pdf
- ^The Washington Observer - Google News Archive Search
- ^Lewis, Peter Rhys, Reynolds, K, and Gagg, C, Forensic Materials Engineering: Case studies, CRC Press (2004).
- ^[1] RVS in zwembaden is als een kanarie in een kolenmijn. AluRVS 2017. [2] Province of Noord Brabant: 'Investigation on accident Reeshof Tilburg. 2012. [3] Proceedings of NACE TG 498. NACE International, Houston TX USA. 2015. M. Faller and P. Richner: Material selection of safety-relevant components in indoor swimming pools, Materials and Corrosion 54 (2003) S. 331 - 338.(only online in German (3.6 MB)) (ask for a copy of the English version)
- Sources
- 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
- 'Mechanical Properties of Ceramics' by John B. Wachtman, W. Roger Cannon, and M. John Matthewson. Chapter 8.
External links[edit]
- Decoupling stress and corrosion to predict metal failure: Arizona State University
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