Pdf caustic soda is used in several refinery processes, for example. Stress corrosion cracking of 304l stainless steel in. Caustic cracking of pressure vessels mainly occurs in steam and other places prone to repeated evaporation and concentration or in hightemperature vessels in contact with caustic soda. Crack propagation can increase dramatically with temperature and can grow through the wall in a matter of hours during temperature excursions especially in conditions that promote caustic concentration. Stress corrosion cracking behavior of nickel base alloys. The influence of thermal treatment on the scc behavior of. Carryover of caustic soda naoh in the steam path caused. Nace has published recommendations for handling sodium hydroxide naoh in the form of a caustic soda service chart since at least the mid1960s. Caustic stress corrosion cracking caustic embrittlement.
Caustic stress corrosion cracking inspectioneering. So, lack of pwht will increase susceptibility, mostly from residual tensile stresses, and not just hardness. The stress corrosion cracking scc of 304 stainless steel in caustic solutions can be prevented by the use of proper inhibitors. This study aims to investigate and explain the magnetiteaccelerated stress corrosion cracking phenomenon of alloy 600 under caustic conditions, based on the electrochemical behavior. Citeseerx document details isaac councill, lee giles, pradeep teregowda. The important general point is that those environments that cause stress corrosion are frequently highly specific to the particular. As a general rule, the resistance to caustic solutions increases with. Discussion caustic stress corrosion cracking of steels is one of the oldest known forms of scc, dating back to the days of the early steam locomotives, when it was responsible for countless explosions of riveted boilers trethewey et al. Carbon steel in caustic service corrosion engineering eng. Selection stainless steel for handling sodium hydroxide naoh. Stainless steels types 304 and 316 can be considered resistant below 80 degc, up to the limit of solubility. After the scc test that lasted for 300 h, no cracks were observed in any of the magnetitefree specimens, whereas cracks with a depth of 150 to 280. Caustic is found to be a primary cause of stresscorrosion cracking.
Nace sp0403 avoiding caustic stress corrosion cracking of. As a general rule, the resistance to caustic solutions increases with increasing nickel content. Specialized corrosion doehdbk1015193 corrosion ch02 rev. Despite the extensive qualification of inconel for specific applications, a number of corrosion problems have arisen with inconel tubing. Stress corrosion cracking caustic embrittlement of super. There is caustic stress corrosion cracking when carbon steel is exposed to concentrated caustic solution. Avoiding caustic stress corrosion cracking of carbon steel refinery equipment and piping. Caustic stress corrosion cracking, also referred to as caustic embrittlement, is a form of degradation that is caused when a component operates in a caustic environment it is one of the most prevalent afflictions in the industry and one of many environmental cracking mechanisms that typically occur in carbon steel equipment, but it can also afflict low alloy and austenitic stainless steel. Caustic stress corrosion cracking scc of carbon steel cs equipment has been reported in industry since the 1930s, e. It was later known as stress corrosion cracking which is. Mild steels steels with low carbon and low alloy content and stainless steels will. Stress corrosion cracking scc is the cracking induced from the combined influence of tensile stress and a corrosive environment. Nace international standard practice avoiding caustic stress.
Caustic cracking is a form of stress corrosion cracking most frequently encountered in carbon steels or ironchromiumnickel alloys that are exposed to concentrated hydroxide solutions at temperatures of 482f 250c. Caustic stress corrosion cracking of nickelrich, chromium. Stress corrosion cracking cases in refinery equipment g. Stress corrosion cracking scc is the formation and growth of crack through. Caustic stress corrosion cracking scc of cs equipment has been reported in industry since the 1930s riveted steam boilers. The stress corrosion cracking scc behaviour of medium carbon steel mcs in caustic soda, potash and a mixture of these reagents was.
It may occur in equipment of carbon steel, low alloy steel, ferritic steel and austenitic stainless steel in. Standard recommended practice avoiding caustic stress. As with other forms of stress corrosion cracking, caustic cracking will occur only when. Note that figure 2, based on copsons work 5 on scc. It is used in metal degreasing and cleaning processes in a wide range of industry applications. The impact of scc on a material usually falls between dry cracking and the fatigue threshold of that material.
The mechanism is similar to that of chloride stress corrosion. Caustic corrosion and stress corrosion cracking has been a major concern in chemical process industries such as hydroxide manufacturing, pulp and paper industries, oil refineries, alumina processing and other processes that use or produce caustic environments at high temperatures 1. Nace has published guidance for handling sodium hydroxide naoh in the form of a caustic service chart since at least the mid1960s. The useful safe limit of carbon steel is approximately 150 o f65 o c, both with regard to caustic stress corrosion cracking cscc and corrosion. In such cases, caustic concentrations of 50 to 100 ppm are sufficient to cause cracking 4. Stress corrosion cracking scc or called as environmentally assisted cracking eac may take place at the electrochemical potential range. The side and fracture surfaces of the alloy were then examined using scanning electron microscopy after the ssrt test. Api 571 recommends the use of carbon steel materials with caustic soda naoh solution according to the concentration and temperature. Sp04032015, avoiding caustic stress corrosion cracking.
Scc is highly chemically specific in that certain alloys are likely to undergo scc only when exposed to a small number of chemical environments. This environmental cracking occurs well below the critical corrosion parameters. Dec 17, 2019 usability of cs in caustic soda service to prevent scc. Environmental cracking or stress corrosion cracking scc is a major. Stress corrosion cracking scc or environmentally induced cracking. Standard practice avoiding caustic stress corrosion cracking. Alloy 400 n04400 and alloy 600 n06600 also have excellent stress corrosion resistance. This due to high concentration of the caustic soda added to the boiler water to control ph ph 9. This damage mechanism primarily affects standard and high carbon stainless steels, but low carbon and chemically stabilized stainless steels may also be. Stress corrosion cracking, scc, refers to cracking caused by the. Mar 04, 2019 caustic alkali generally refers to caustic soda and caustic potassium, namely sodium hydroxide and potassium hydroxide. Corrosionpedia what is stress corrosion cracking scc.
It can lead to unexpected sudden failure of normally ductile metals. Stress corrosion cracking scc is the formation and growth of crack through materials subjected to tensile stress and a specific corrosive medium. Industrial experience on the caustic cracking of stainless. Other, non published data, suggests that alloy 800 may be resistance to scc at 500 o f260 o c in 10% caustic but not in a 50% solution. Nace sp0403 avoiding caustic stress corrosion cracking. Stainless steels are more resistant to general corrosion compared with carbon steel.
Steels can be subject to a form of stresscorrosion cracking scc to which the misnomer of caustic embrittlement has been applied. Usability of cs in caustic soda service to prevent scc. Lobley consulting services department, saudi arabian oil company, dhahran, saudi arabia 311 abstract cases of stress corrosion cracking scc failures in refinery equipment are presented to highlight the need to prevent recurrence. Structural failure due to scc can be very unpredictablefailure could occur after as little as a few hours of exposure, or the equipment could continue to function normally for. It can lead to unexpected sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature. Discussion caustic stress corrosion cracking of steels is one of the oldest known forms of scc, dating back to the days of the early steam locomotives, when it was responsible for countless explosions of riveted boilers trethewey et. Avoiding caustic stress corrosion cracking of carbon steel refinery equipment and piping this standard practice establishes guidelines to avoid caustic scc of cs refinery equipment and piping. The surface films on the scc specimens were analyzed by auger electron spectroscopy aes.
Caustic scc is driven by tensile stress, susceptible material and concentration of caustic. Sp04032015, avoiding caustic stress corrosion cracking of. The stress corrosion cracking scc behaviour of medium carbon steel mcs in caustic soda, potash and a mixture of these reagents was studied by fracturing this material on a tensometer after every seven days of immersion in these media. Stress corrosion cracking scc is the growth of crack formation in a corrosive environment. Stress corrosion cracking scc tests were conducted by the slow strain rate. Caustic stress corrosion cracking how is caustic stress. It is one of the most prevalent afflictions in the industry and one of many environmental cracking mechanisms that typically occur in carbon steel equipment, but it can also afflict low alloy and austenitic stainless steel equipment. Cracking can occur even at low caustic levels if a concentrating mechanism is present. These alloys can however suffer from caustic stress corrosion cracking at caustic concentrations above 70% at temperatures above 290crmo c 550f. The area of risk in sodium hydroxide is shown on the isocorrosion diagram by the area bounded by a green line. Stress corrosion cracking susceptibility of medium carbon steel in. Percentage elongation and reduction in crosssectional area were used to.
Caustic alkali generally refers to caustic soda and caustic potassium, namely sodium hydroxide and potassium hydroxide. It is a form of stress corrosion cracking characterized by surfaceinitiated cracks that propagate dramatically. Risk of stress corrosion cracking attack caustic stress corrosion cracking occurs at higher temperatures than chloride stress corrosion cracking which can occur at temperatures as low as 60 degc. This standard establishes recommended practices to avoid caustic scc of cs refinery equipment and piping. So, lack of pwht will increase susceptibility, mostly. It can lead to unexpected sudden failure of normally ductile metal. The duplex stainless steels were developed to exhibit comparable general corrosion resistance as 316ss, but with reduced susceptibility to chloride stress corrosion cracking. Caustic soda naoh was added in small amounts to boiler water to prevent scaling but the presence of caustics alkalis, usually concentrated in crevices around rivet heads and at hot spots, combined with the considerable fabrication stresses around rivet holes to caused cracking of the steel boiler shells and tube plates. Thus, the addition of caustic soda to boiler feed waters to reduce the corrosiveness of the latter towards mild steel can result in the form of stress corrosion frequently referred to as caustic cracking. Improved resistance to caustic stress corrosion cracking can be given to inconel by heat treating it at 620 o c to 705 o c, depending upon prior solution treating temperature. The results of this study are combined with other information on amounts of elements and anions in deposits and on steam. How to avoid caustic cracking of carbon steel and low alloy. Other higher nickel stainless alloys like alloy 20, 800 and 825 also resist caustic corrosion and caustic cracking better than the 300 series stainless steels.
Commercially pure ni is the most resistant material for caustic applications both from the general corrosion and the stress corrosion cracking scc perspectives. Which alloy material is most resistant to caustic soda. This means that all environments do not cause scc on all of the alloys. Caustic stress corrosion cracking, also referred to as caustic embrittlement, is a form of degradation that is caused when a component operates in a caustic environment. Stress corrosion cracking scc is a progressive fracturing that occurs in metals as a result of the combined influence of tensile stress and a corrosive environment. Stress corrosion cracking scc caustic stress corrosion cracking cscc hydrogen induced cracking hic we also evaluate the performance of nondestructive examination nde methods used to detect and characterize defects in metal components. The limiting parameters of temperature and concentration for scc determined by a. Pdf a case study on caustic corrosion in refinery piping. Polarisation data for alloy 800, alloy 690 and the pure metals cr, ni and fe and stress corrosion cracking scc data from cring specimens at controlled potentials for the nickel base alloys 800 and 690 in boiling solutions of 50% naoh with or without addition of sio 2 and na 2 s 2 o 3, are presented. Caustic embrittlement was first used to describe the cracking of riveted mild steel boiler plates due to local deposition of concentrated hydroxide at temperatures of 200 to 250 o c 400 to 480 o f. Zheng and bogaerts 6 have attributed scc of austenitic stainless steels in hot caustic solutions to the breakdown of the primary passivation and. Polythionic acid stress corrosion cracking pascc is a type of intergranular stress corrosion that affects sensitized stainless steels in refineries processing feedstocks containing sulphur. Electrochemically controlled test methods have been used to determine the influence of thermal treatment on the stress corrosion cracking scc resistance of inconel alloy 600 i600 in deaerated 10% naoh solutions at 315 c 600 f.
Standard recommended practice avoiding caustic stress corrosion cracking of carbon steel refinery equipment and piping. These alloys can be candidates for service in caustic exposure when corrosion from cooling waters is a concern. It is one of the most prevalent afflictions in the industry and one of many environmental cracking mechanisms that typically occur in carbon steel equipment, but it can also afflict low alloy and austenitic stainless steel. The area of risk in sodium hydroxide is shown on the iso corrosion diagram by the area bounded by a green line. Another source suggests that the corrosion rate of alloy 800 in boiling 50% caustic 300 o f148 o c is excessive 39 mpy. Stress corrosion cracking scc behaviors of alloy 690 were studied in leadcontaining aqueous alkaline solutions using the slow strain rate tension ssrt tests in 0. This damage mechanism primarily affects standard and high carbon stainless steels, but low carbon and chemically stabilized stainless steels may also be vulnerable under certain conditions. Stress corrosion cracking of stainless steels in high temperature. The required tensile stresses may be in the form of directly applied stresses or in the form of residual. How to avoid caustic cracking of carbon steel and low. Brown chemistrydepartment theamericanuniversity washington,d. Aug 01, 2018 caustic cracking is a form of stress corrosion cracking most frequently encountered in carbon steels or ironchromiumnickel alloys that are exposed to concentrated hydroxide solutions at temperatures of 482f 250c. Metalenvironment combinations susceptible to cracking are specific. For caustic stress corrosion cracking, what is the effect of temperature and concentration mechanisms.
Sodium chromate was found to be an effective inhibitor for preventing the caustic stress corrosion cracking. Scc, mcs, caustic soda, potash, hydrogen cracking, carbon cracking. Corrosion of duplex stainless steels in high ph caustic. The stress corrosion cracking scc behavior of cold worked mild steel in hot, aqueous, 33 pct naoh solutions was studied with prefatigue cracked double ca. From the chemical composition point of view, corrosion resistant nickel based alloys are categorized as pure nickel, nickelcopper alloys, nickelmolybdenum alloys, nicrmo alloys and nicrfemo alloys. Caustic stress corrosion cracking of mild steel springerlink. Sp04032015, avoiding caustic stress corrosion cracking of refinery equipment and piping available for download designing, fabricating, andor maintaining carbon steel equipment and piping that is exposed to caustic environments. Scc of the 300 series stainless steels is classically transgranular. Standard practice avoiding caustic stress corrosion. Caustic is found to be a primary cause of stress corrosion cracking. Nace international standard practice avoiding caustic. Carbon steel in caustic service corrosion engineering. Corrosion of stainless steel by hot caustic research using solutions of chemically pure caustic sodium hydroxide naoh led to the development of a diagram that attempts to delineate the param eters of concentration and tem perature governing stress corrosion cracking scc of type.
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