Galvanized Steel Stress Corrosion Cracking

• • • • • • • • • • • • • • Stress corrosion cracking ( SCC) is the growth of crack formation in a environment. It can lead to unexpected sudden failure of normally metals subjected to a, 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 Slow Strain Rate Stress Corrosion Cracking. Atmospheric Corrosion Model for Galvanized. McDonald, Atmospheric Corrosion Model for Galvanized Steel.

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. The stresses can be the result of the crevice loads due to, or can be caused by the type of assembly or from fabrication (e.g. Cold working); the residual stresses can be relieved by or other surface treatments.

Contents • • • • • • • • • • • • • Metals attacked [ ] Certain and crack in the presence of, mild cracks in the presence of ( boiler cracking) and, crack in solutions (). 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, all the others display the phenomenon of subcritical growth, i.e. Small surface flaws propagate (usually smoothly) under conditions where predicts that failure should not occur. That is, in the presence of a corrodent, cracks develop and propagate well below K Ic.

In fact, the subcritical value of the stress intensity, designated as K Iscc, may be less than 1% of K Ic, as the following table shows: Alloy K Ic MN/m 3/2 SCC environment K Iscc MN/m 3/2 13Cr steel 60 3% NaCl 12 18Cr-8Ni 200 42% MgCl 2 10 Cu-30Zn 200 NH 4OH, pH7 1 Al-3Mg-7Zn 25 Aqueous halides 5 Ti-6Al-1V 60 0.6M KCl 20 Polymers attacked [ ] A similar process () occurs in polymers, when products are exposed to specific solvents or aggressive chemicals such as and. As with metals, attack is confined to specific polymers and particular chemicals. Thus is sensitive to attack by alkalis, but not by acids.

On the other hand, are readily degraded by acids, and SCC is a likely mechanism. Polymers are susceptible to where attacking agents do not necessarily degrade the materials chemically. Is sensitive to degradation by acids, a process known as, and nylon mouldings will crack when attacked by strong acids. In tubing Cracks can be formed in many different by attack, another form of SCC in polymers.

Tiny traces of the gas in the air will attack double bonds in rubber chains, with, rubber, and 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 can be prevented by adding anti-ozonants to the rubber before.

Ozone cracks were commonly seen in automobile 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 [ ] 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 ). 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. Crack growth [ ] The subcritical nature of propagation may be attributed to the released as the crack propagates. That is, elastic energy released + chemical energy = surface energy + deformation energy The crack initiates at K Iscc 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 K Ic, whereupon fast fracture ensues and the component fails. One of the practical difficulties with SCC is its unexpected nature., 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 [ ] 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. In this context, it should be noted that 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 [ ] 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 [ ] 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 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, 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. Laser peening is widely used in the aerospace and power generation industries in gas fired turbine engines. Control of environment [ ] 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 [ ] 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 [ ]. The collapsed Silver Bridge, as seen from the Ohio side A classic example of SCC is of brass cartridge cases, a problem experienced by the British army in in the early 19th century. It was initiated by from dung and decomposing at the higher temperatures of the spring and summer. There was substantial in the cartridge shells as a result of. The problem was solved by the shells to ameliorate the stress.

A 32-inch diameter gas transmission pipeline, north of, 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 in December 1967, when an across the Ohio river at,, 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 in the eyebar. The disaster led to a nationwide reappraisal of bridges. Suspended ceilings in indoor swimming pools are safety-relevant components. As was demonstrated by the collapses of the ceiling of the () indoor (1985), and again at (, 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. The active chemical was added to the water as a.

See also [ ] • • • • • • • • • • References [ ] Notes. • ASM International, Metals Handbook (Desk Edition) Chapter 32 (Failure Analysis), American Society for Metals • Munnings, C.; Badwal, S. (20 February 2014). 'Spontaneous stress-induced oxidation of Ce ions in Gd-doped ceria at room temperature'.

20 (8): 1117–1126.. • • • • • Lewis, Peter Rhys, Reynolds, K, and Gagg, C, Forensic Materials Engineering: Case studies, CRC Press (2004). Faller and P.

Richner: Material selection of safety-relevant components in indoor swimming pools, Materials and Corrosion 54 (2003) S. 331 - 338.() () 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 External links [ ] • • • • •.

Android Game Gratis Untuk Unduh Game here. Abstract Types and causes of underlying corrosion processes must be determined precisely for each construction element exhibiting signs of deterioration, so that the selected repair works and protection methods are adequate. Only then will the protection and repairs be appropriately durable; this, in turn, will reduce the costs of building maintenance. The corrosion cause analysis of selected construction elements hereby presented is based on research conducted on the buildings as well as on laboratory and structural (microscopy and x-ray microanalysis) research.

■ ■ ■ ■ Stress Corrosion Cracking of Unused Copper Tube from Air Conditioner Coil ENVIRONMENT: EQUIPMENT: Air Conditioner Unit MATERIAL: Copper SERVICE TIME: Never Used FAILURE MODE: Stress Corrosion Cracking Background The received section was approximately 5-1/2-inches long by 2-3/4-inches wide by 4-inches in height (Figure 1). The section consisted of U-shaped copper coils, presumably fabricated of UNS C12200 DHP copper, that passed through a galvanized steel tube sheet and numerous thin corrugated aluminum radiator fins. The copper tubing had an outer diameter (OD) of approximately 0.386-inches, a wall thickness of approximately 0.011-inches, and possessed internal rifling that ran parallel to the longitudinal axis of the tubing. According to information provided by the manufacture, this coil, from which the supplied section was taken, had been shipped to the Middle East. The coil had never been used but was found to have leaks in the copper tubes at the tube sheet. The coil was returned to the US, tested in a water dip tank, and the leak locations marked.

One of the separate copper tube hairpins was similarly marked with an arrow indicating a leak at the same location as the tubes on the larger section. Findings The coil section and two hairpins were examined in the as-received condition at the indicated leak sites with an optical microscope at up to 40X magnification.

As previously mentioned, the marked leak sites were on the copper tube as it emerged from the tube sheet, at the start of the U-bend. Black and white deposits were noted on all the tubes in the general area of the leak sites, but no obvious leak sites were observed. As-received copper coil section. Arrows indicate leak sites found.

One of the copper U-bends marked as possessing two leak sites was carefully removed from the coil section. One end of the U-bend was crimped and soldered shut, and a valve was clamped to the other end. The U-bend was leak tested by pressurizing it to 10 psig with air and holding it under water. Two very small leak sites, revealed by air bubble streams from the tube surface, were observed approximately 180 ° apart around the tube outer diameter (OD) surface on one leg of the U-bend, one site being in one of the indicated areas.

No leaks were observed at the other marked leak location. The pressurized tube was again examined at up to 40X magnification with an optical microscope. Residual water on the tube surface continued to bubble during the examination, which aided in pinpointing the leak sites. A very faint crack in the copper tube was observed at one leak site.

The other leak site, 180 ° around the tube, appeared to be associated with the black deposit on the copper tubing mentioned earlier, but no obvious hole or crack was observed. An approximately 1-inch length containing the two leak sites was cut from the examined tube. This length was then split longitudinally. An iridescent blue discoloration was observed on the tube inner diameter (ID) surface at both leak sites, as well as an obvious crack at one leak site (Figure 2). Blue discoloration on tube ID surface at leak site. Yellow arrow indicates crack.

In order to determine if failure modes other than cracking were producing the tube leaks; the leak site without the obvious crack was chosen for a metallographic examination (Figure 3). The chosen tube section was mounted in a cold-curing epoxy and metallographically ground and polished in accordance with standard procedures.

The prepared metallographic specimen presented the leak site in the transverse cross-sectional direction. Examination of the specimen revealed four branching cracks, indicative of stress corrosion cracking (SCC), originating at the tube OD surface and proceeding either nearly or completely through the tube wall. Negligible corrosion on the OD surface was noted except for shallow pits (approximately 0.4 mils deep) at a through wall crack.

The specimen was etched, revealing that the cracking was intergranular. The intergranular attack was severe enough to produce grain dropping. Etching revealed an apparently normal annealed copper microstructure (Figure 4). As-polished metallographic specimen showing through-wall crack. Note shallow pits at crack origin at bottom of photomicrograph.

(250X original magnification) Figure 4. Etched metallographic specimen showing intergranular nature of cracking. (Potassium Dichromate Etch, 1250X original magnification) The black and white deposits on another tube from the coil section were examined using a scanning electron microscope (SEM) fitted with an energy dispersive spectroscopy (EDS) microprobe. EDS revealed that the deposits were mostly zinc (approximately 30 weight percent; from the galvanized tube sheet) with magnesium, aluminum, silicon, phosphorus, sulfur, chloride, potassium, and calcium (i.e., “dirt”) each present in quantities from approximately 1 to 3 weight percent. (Copper made up the balance.) EDS analysis of matter present in the branching cracks indicated that it was copper oxide.

Discussion Stress corrosion cracking (SCC) refers to cracking of an alloy that occurs under the simultaneous action of corrosion and sustained tensile stress. Cracking can be either intergranular (as in the present case) or transgranular, depending on the alloy and/or the corrodent.

The classic example of SCC is intergranular SCC of stressed austenitic stainless steels (e.g. Type 304L stainless steel) exposed to hot, aqueous, chloride-containing environments. Branching of the cracking occurs in the direction of crack propagation. High-purity coppers, such as phosphorized copper (C12200), are generally considered almost immune to SCC; however, intergranular SCC of this alloy has been observed. The usual culprit in SCC of copper and copper alloys is ammonia (or other amines capable of reacting with copper to form complex ions) acting in tandem with stresses in the alloy from forming and/or service conditions. The source of ammonia can be enormously diverse, from the decomposition of organic matter, to flooring adhesives, to cleaning products.

In the present case, cracking of the copper tubing initiated at the OD surface and propagated intergranularly through the tube wall in a direction normal to the tube surface. Stress in the copper tubing was due to the forming of the hairpin bends (where the failures occurred); away from the hairpin bends, the copper tubing was in a stress-free annealed condition. EDS analysis did not reveal a definitive “bad actor” chemical species in the cracking failures; but, if ammonia was the culprit, very little or no evidence of its presence would be expected to be found by EDS or other methods. The presence of the blue discoloration on the tube ID surface at the failure sites was indicative of an ammonia-copper complex forming species.