Stress corrosion cracking , 1320

This is normally found only in alloys such as stainless steel and in specific environments. This type of corrosion is a result of the combined effects of mechanical, electrochemical, and metallurgical properties of the system. 

The residual stress in a metal, or more commonly an alloy, wiU, in certain corrosive environments, result in mechanical failure by cracking. It first became apparent at the end of the nineteenth century in brass (but not copper) condenser tubing used in the electric power generating industry. It was then called season cracking. It is usually prevalent in cold-drawn or cold-roUed alloys which have residual stress. Heat treatments to relieve this stress were developed to solve the problem. It was soon realized that there were three important elements of the phenomenon the mechanical, electrochemical, and metallurgical aspects. 

An essential feature is the presence of tensile stress which may be introduced by loads (compression), cold work, or heat treatment. The first stage involves the initiation of the crack from a pit which forms after the passive oxide film is broken by CF ions the anodic dissolution reaction of metal 

The electrochemical aspect of the process is associated with anodic dissolution, accounting for high cracking velocities. The crack tip is free of the oxide protective coating in the alloy, and crack propagation proceeds as the alloy dissolves. Chloride ions present in solution tend to destroy this passivity in the crevice, which is depleted in oxygen. In stainless steels, the dissolution of chromium in the crevice occurs by the reactions  

Stress corrosiorr cracking (SCC) occurs at points of stress. Usually, the metal or alloy is virtually free of corrosion over most of its surface, yet fine cracks penetrate through the surface at points of stress. The conditions necessary for SCC are  

A suitable environment (chemicals capable of causing SCC in carbon steel and low-alloy carbon steels) 

One advantage of carbon steel is that SCC can be prevented by relieving stress after fabrication. 

Chemical species that induce SCC in carbon and low-alloy carbon steels, even at low concentrations include hydroxides, gaseous hydrogen, gaseous chlorine, hydrogen chloride, hydrogen bromide, aqueous nitrate solutions, hydrogen sulfide gas, MnS and MnSe inclusions in the alloy. As, Sb, and Bi ions in aqueous solution, carbon monoxide-carbon dioxide-water gas mixtures. Many of these chemical systems will crack steel at room temperatures. 

Corrosion fatigue is crack formation due to varying stresses combined with corrosion. - This is different from stress corrosion cracking because stress corrosion cracking develops under static stress while corrosion fatigue develops under varying stresses.  

When an alloy fails by a distinct crack, you might suspect stress-corrosion cracking as the cause. Cracking will occur when there is a combination of corrosion and stress (either externally applied or internally applied by residual stress). It may be either intergranular or trans-granular, depending on the alloy and the type of corrosion. 

Austenitic stainless steels (the 300 series) are particularly susceptible to stress-corrosion cracking. Frequently, chlorides in the process stream are the cause of this t5q)e of attack. Remove the chlorides and you will probably eliminate stress-corrosion cracking where it has been a problem. 

Many austenitic stainless steels have failed during downtime because the piping or tubes were not protected from chlorides. A good precaution is to blanket austenitic stainless steel piping and tubing during downtime with an inert gas (nitrogen). 

If furnace tubes become sensitized and fail by stress corrosion cracking, the remaining tubes can be stabilized by a heat treatment of 24 hours at 1,600°F. In other words, if you can t remove the corrosive condition, remove the stress. 

Chemically stabilized steels, such as T q)e 304L have been successfully used in a sulfidic corrodent environment but actual installation tests have not been consistent. 

Corrosion by hydrogen sulfide in partial oxidation plants can be controlled by the use of austenitic steels, but special care to ensure proper stress relief of welds is advisable to avoid stress corrosion cracking in these plants caused by traces of chlorine sometimes present in the feed oil. 

if any, failure mechanisms have received as much attention as stress-corrosion cracking (SCC). Yet despite an enormous research effort over many years, an acceptable, generalized theory that satisfactorily explains all elements of the phenomenon has not been produced. SCC is a complex failure mechanism. Nevertheless, its basic characteristics are well known, and a wealth of practical experience permits at least a moderately comfortable working knowledge of the phenomenon. 

SCC has been defined as failure by cracking under the combined action of corrosion and stress (Fig. 9.1). The stress and corrosion components interact S3mergistically to produce cracks, which initiate on the surface exposed to the corrodent and propagate in response to the stress state. They may run in any direction but are always perpendicular to the principal stress. Longitudinal or transverse crack orientations in tubes are common (Figs. 9.2 and 9.3). Occasionally, both longitudinal and transverse cracks are present on the same tube (Fig. 9.4). Less frequently, SCC is a secondary result of another primary corrosion mode. In such cases, the cracking, rather than the primary corrosion, may be the actual cause of failure (Fig. 9.5). 

The surface from which the cracks originate may not be apparent without a microstructural examination. Stress-corrosion cracks invariably produce brittle (thick-walled) fractures regardless of the ductility of the metal. 

It is important to realize that the conditions causing See may not only occur during normal operation of equipment but also during start- 

Due to the wide variety of environments to which cooling water components are exposed on the cooling water and process sides, it is difficult to specify favored locations for SCC. However, a few general observations may be permitted  

The overall SCC response is illustrated diagrammatically in Fig. 7.6, with the rate-limited stage of crack growth represented by stage II (left-hand figure) and a schematic representation of the influence of incubation (on the right). From a 

The functional relationship between Ki and a depends on geometry and loading, and is assumed to be known. As such  

The greatest problems with austenitic stainless steel piping usually arise when the unit is off stream rather than when it is operating. Such problems must be anticipated. The use of stainless steels requires that the necessary steps be taken to avoid them. Chlorides and caustics can cause any austenitic stainless steel pipe to crack trans-granularly under some conditions. Plain chromium stainless steels do not crack in chloride solutions, but they usually pit badly enough to be only moderately satisfactory. 

Strictly speaking, chloride stress-corrosion cracking will not occur unless there is contact with an aqueous solution of suitable chloride concentration, a favorable temperature and strain or residual stress. These requirements may, however, be met rather unpredictably. 

For example, the small amounts of chlorides in most extern pipe insulations can be leached out by exposure to weather and become concentrated at the pipe wall. Temperature may be difficult to measure, let alone control, especially during startup or shutdown when gradients exist. Residual stresses usually are present in a relatively low yield strength material like annealed stainless steel pipe. A pipe bumped in shipment or sprung or cold bent in fitting can have all the stress needed. 

In fact, circumferential weld shrinkage alone, particularly in heavy-wall pipe, may create complex bending stresses at the joint. Although post-weld heat treatments should relieve many of these stresses, the subsequent cooling can reintroduce harmful stresses if there is much restraint. The fact that the much higher thermal expansion and contraction of the austenitic stainless steels may introduce unexpected restraint stresses, as well as being troublesome in piping layouts, should not be overlooked. 

When the normal carbon (0.08% maximum) grades of austenitic stainless steel pipe are used in 

The presence of extensive SCC may qualify a pipeline for replacement or rehabilitation. As SCC depends on unique environmental conditions, a large-scale recoating 

Replacement/rehabilitation decisions involve several considerations. These considerations are terrain conditions, expected or required life, excess capacity and throughput requirements, internal versus external corrosion, and so on. A comprehensive list of considerations for pipeline rehabilitation is given below (13). 

The location of the pipeline is important for repair considerations. A pipe in swampy clay would exclude recoating versus a repair option. On the contrary, prairies are conducive to recoating, with firm footing for the equipment and good accessibility. 

If the expected life of a section of pipeline is relatively short, the pipeline operator must decide whether recoating and repair would extend the life of the pipe section to match the rest of the pipeline. If not, replacing the pipe section may be the best solution. 

In order to assess SCC in details, there must exist a lower and an upper bound of strain rate at a constant lied potential and a potential range at 

There are similarities in epe and See with respect to the brittle fracture surface mode in a corrosive medium and mecharucaUy, both have a tensile stiess component that influences crack opening. The cyclic stiess range for epe is a dynamic process that induces crack initiation on the surface of the metallic component. If the component is exposed to a corrosive solution, then the combination of the cyclic stress and environment accelerate cracking and reduce fatigue life. 

In addition, failure analysis is normally conducted on tested specimens for further characterization of the effects of EIC. 

Perhaps the most critical factor concerning IGSCC is that three conditions necessary for producing IGSCC must be simultaneously present. The elimination of any one of these three factors or the reduction of one of these three factors below some threshold level eliminates IGSCC. The three necessary conditions for IGSCC are  

In the BWR environment, two major parameters influence IGSCC aggressiveness. These are water conductivity and electrochemical corrosion potential (ECP). The benefits with respect to preventing IGSCC are attained when both water conductivity and ECP are controlled. 

Crevices significantly increase the probability for SCC due to the highly aggressive local environment that may form within the crevice. Creviced Alloy 600 has suffered IGSCC in the BWR (e g., nozzle safe ends, shroud head bolts, access hole covers). Alloy 182 has also experienced IGSCC in nozzle safe end applications where weld residual stresses and fairly high applied stresses were present. It must conservatively be assumed that Alloy 182 exposed to normal coolant conditions is susceptible to IGSCC. 

The BWR coolant is high purity water. Therefore, conductivity is very low. In fact, many BWRs have conductivity that approaches the theoretical limit of 0.055 pS/cm at 25°C. The ability of the BWR coolant to conduct electricity is due to the presence of ions in the solution. Although pure water is a low conducting medium, it conducts electricity due to the 

There are three primary sources of tensile stresses for RPVDs These are (1) fabrication induced stresses, (2) primary stresses, (3) and secondary stresses. Fabrication induced stresses consist of stresses introduced during manufacture and installation (i.e., fit-up and assembly in the shop or field plus those introduced by machining or forming operations and welding). As is the case for weld residual stresses, hard machining, abusive grinding can produce surface residual stresses near or above the yield point of the material. 

It is not uncommon for growing cracks to be considered acceptable (Fig. 6.47) so long as they are substantially less than a critical crack size [Fig. 6.47(a) and Eq. (6.11)] and can be repaired at the next 

SCC is an anodic process, a fact which can be verified by using cathodic protection as an effective remedial measure. SCC may occasionally lead to fatigue corrosion, or the opposite. Usually, the true nature of the cracking can be identified by the morphology of the observed cracks. In a failure by SCC there is usually little metal loss due to general corrosion. Thus, the failure of a stress bolt rusted away until it eventually cannot sustain the applied load is not classified as SCC. 

Aluminum alloys NaCI-HjO, NaCI solutions, seawater, mercury 

Copper alloys Ammonia vapors and solutions, mercury 

Gold alloys FeClg solutions, acetic acid-salt solutions 

Brasses Many waters, especially under stagnant conditions Zinc (dezincification) 

Gray iron Soils, many waters Iron (graphitic corrosion) 

Aluminum bronzes Hydrofluoric acid, acids containing chloride ions Aluminum (dealuminification) 

Silicon bronzes High-temperature steam and acidic species Silicon (desiliconification) 

Copper-gold single crystals Ferric chloride Copper 

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A process involving combined corrosion and straining of the metal due to residual or applied stresses. The occurrence of stress corrosion cracking is highly specific only particular metal/environment systems will crack. 

The appearance of stress corrosion cracking may be either intergranular or transgranular in nature. 

Stainless steel alloys show exceUent corrosion resistance to HCl gas up to a temperature of 400°C. However, these are normally not recommended for process equipment owing to stress corrosion cracking during periods of cooling and shut down. The corrosion rate of Monel is similar to that of mild steel. Pure (99.6%) nickel and high nickel alloys such as Inconel 600 can be used for operation at temperatures up to 525°C where the corrosion rate is reported to be about 0.08 cm/yr (see Nickel and nickel alloys). 

Duplex stainless steels (ca 4% nickel, 23% chrome) have been identified as having potential appHcation to nitric acid service (75). Because they have a lower nickel and higher chromium content than typical austenitic steels, they provide the ductabdity of austenitic SS and the stress—corrosion cracking resistance of ferritic SS. The higher strength and corrosion resistance of duplex steel offer potential cost advantages as a material of constmction for absorption columns (see CORROSION AND CORROSION CONTROL). 

Titanium is resistant to nitric acid from 65 to 90 wt % and ddute acid below 10 wt %. It is subject to stress—corrosion cracking for concentrations above 90 wt % and, because of the potential for a pyrophoric reaction, is not used in red filming acid service. Tantalum exhibits good corrosion resistance to nitric acid over a wide range of concentrations and temperatures. It is expensive and typically not used in conditions where other materials provide acceptable service. Tantalum is most commonly used in appHcations where the nitric acid is close to or above its normal boiling point. 

Stress corrosion cracking, prevalent where boiling occurs, concentrates corrosion products and impurity chemicals, namely in the deep tubesheet crevices on the hot side of the steam generator and under deposits above the tubesheet. The cracking growth rates increase rapidly at both high and low pH. Either of these environments can exist depending on the type of chemical species present. 

Many instances of intergranular stress corrosion cracking (IGSCC) of stainless steel and nickel-based alloys have occurred in the reactor water systems of BWRs. IGSCC, first observed in the recirculation piping systems (21) and later in reactor vessel internal components, has been observed primarily in the weld heat-affected zone of Type 304 stainless steel. 

Stress Corrosion Cracking of Ahoy 600," Report NP-2114-SR, Electric Power Research Institute, Palo Alto, Calif., Nov. 1981. 

Final Purification. Oxygen containing compounds (CO, CO2, H2O) poison the ammonia synthesis catalyst and must be effectively removed or converted to inert species before entering the synthesis loop. Additionally, the presence of carbon dioxide in the synthesis gas can lead to the formation of ammonium carbamate, which can cause fouHng and stress-corrosion cracking in the compressor. Most plants use methanation to convert carbon oxides to methane. Cryogenic processes that are suitable for purification of synthesis gas have also been developed. 

R. J. I In dinger and R. M. Curran, Experience with Stress Corrosion Cracking in Earge Steam Turbines, Corrosion 81, National Association of Corrosion Engineers, Toronto, Ontario, Canada, 1981. 

Stress Corrosion Crocking. Stress corrosion cracking occurs from the combined action of corrosion and stress. The corrosion may be initiated by improper chemical cleaning, high dissolved oxygen levels, pH excursions in the boiler water, the presence of free hydroxide, and high levels of chlorides. Stresses are either residual in the metal or caused by thermal excursions. Rapid startup or shutdown can cause or further aggravate stresses. Tube failures occur near stressed areas such as welds, supports, or cold worked areas. 

Localized corrosion, which occurs when the anodic sites remain stationary, is a more serious industrial problem. Forms of localized corrosion include pitting, selective leaching (eg, dezincification), galvanic corrosion, crevice or underdeposit corrosion, intergranular corrosion, stress corrosion cracking, and microbiologicaHy influenced corrosion. Another form of corrosion, which caimot be accurately categorized as either uniform or localized, is erosion corrosion. 

Corrosion control requires a change in either the metal or the environment. The first approach, changing the metal, is expensive. Also, highly alloyed materials, which are resistant to general corrosion, are more prone to failure by localized corrosion mechanisms such as stress corrosion cracking. 

Zirconium resists attack by nitric acid at concentrations up to 70 wt % and up to 250°C. Above concentrations of 70 wt %, zirconium is susceptible to stress-corrosion cracking in welds and points of high sustained tensile stress (29). Otherwise, zirconium is resistant to nitric acid concentrations of 70—98 wt % up to the boiling point. 

Materials of Construction. GeneraHy, carbon steel is satisfactory as a material of construction when handling propylene, chlorine, HCl, and chlorinated hydrocarbons at low temperatures (below 100°C) in the absence of water. Nickel-based aHoys are chiefly used in the reaction area where resistance to chlorine and HCl at elevated temperatures is required (39). Elastomer-lined equipment, usuaHy PTFE or Kynar, is typicaHy used when water and HCl or chlorine are present together, such as adsorption of HCl in water, since corrosion of most metals is excessive. Stainless steels are to be avoided in locations exposed to inorganic chlorides, as stainless steels can be subject to chloride stress-corrosion cracking. Contact with aluminum should be avoided under aH circumstances because of potential undesirable reactivity problems. 

Pitting corrosion may occur generaHy over an entire aHoy surface or be localized in a specific area. The latter is the more serious circumstance. Such attack occurs usuaHy at surfaces on which incomplete protective films exist or at external surface contaminants such as dirt. PotentiaHy serious types of corrosion that have clearly defined causes include stress—corrosion cracking, deaHoying, and corrosion fatigue (27—34). 

Excellent resistance to saltwater corrosion and biofouling are notable attributes of copper and its dilute alloys. High resistance to atmospheric corrosion and stress corrosion cracking, combined with high conductivity, favor use in electrical/electronic appHcations. 

Brasses are susceptible to dezincification in aqueous solutions when they contain >15 wt% zinc. Stress corrosion cracking susceptibiUty is also significant above 15 wt % zinc. Over the years, other elements have been added to the Cu—Zn base alloys to improve corrosion resistance. For example, a small addition of arsenic or phosphoms helps prevent dezincification to make brasses more usefiil in tubing appHcations. 

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