1、Item No. 24061 NACE International Publication 5A192 (2004 Edition) This Technical Committee Report has been prepared by NACE International Specific Technology Group 36*on Process IndustryChemicals. Integrity of Equipment in Anhydrous Ammonia Storage and Handling April 2004, NACE International This N
2、ACE International technical committee report represents a consensus of those individual members who have reviewed this document, its scope, and provisions. Its acceptance does not in any respect preclude anyone from manufacturing, marketing, purchasing, or using products, processes, or procedures no
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9、re than 10 years old. Purchasers of NACE International reports may receive current information on all NACE International publications by contacting the NACE International Membership Services Department, 1440 South Creek Drive, Houston, Texas 77084-4906 (telephone +1281228-6200). Foreword The purpose
10、 of this document is to provide a state-of-the-art report on stress corrosion cracking (SCC) of carbon and low-alloy steels in anhydrous ammonia service. The intent is to provide the corrosion engineer with the information needed to make informed decisions in individual applica-tions. This technical
11、 committee report was originally prepared in 1992 by Task Group T-5A-33, a subcommittee of Unit Com-mittee T-5A on Corrosion in Chemical Processes. It was reaffirmed in 2004 by Specific Technology Group (STG) 36 on Process IndustryChemicals. The report is published by NACE under the auspices of STG
12、36. _ *Chairman Michael H W Renner, FNACE, Bayer Technology Services GmbH, Leverkusen, Germany. NACE International 2 Background Ammonia stress corrosion cracking (SCC) in carbon steel vessels was first reported in the mid-1950s in agricultural service tanks.1Cracking occurred in areas of high residu
13、al stress such as welds and cold-formed dished heads. Hot forming or stress relieving the heads considerably reduced the occurrence of cracking as did the addition of a slight amount of water to the ammonia.2Throughout the l960s and early l970s, cracking problems appeared to be mainly associated wit
14、h high-strength quenched and tempered steels. The late 1970s brought reports of cracking occur-ring in spheres containing anhydrous ammonia with water additions and also in spheres that had been stress relieved after cracks were found and repaired.3Research in the 1980s complemented previous researc
15、h and provided further insight into the causes of cracking and the means of prevention. Causes of Cracking Mechanism Ammonia SCC is an anodic dissolution process that most often progresses via a film rupture mechanism.4,5The nat-ure of the film has been speculated to be either an oxide corrosion pro
16、duct of iron6or a thin nitride layer.7Hydrogen embrittlement apparently does not play a role in the propa-gation of ammonia SCC.4The cracking occurs within a specific electrochemical potential range and can be pre-vented via cathodic polarization.4,5,8Cracking can be either intergranular or transgra
17、nular and cracks are typically filled with an oxide corrosion product. Pure anhydrous ammonia does not cause cracking.9,10The most important contaminant that leads to cracking is oxy-gen.2,10Oxygen concentrations as low as 1 ppm, as measured in the liquid phase, can cause cracking. The role that oxy
18、gen plays in the cracking mechanism is not clearit may only act to change the corrosion potential to the crack-ing range. Water additions of 0.10% by mass or greater have been shown to completely inhibit ammonia SCC in steels,2but only in the liquid phase.9Pressurized storage vessels that have a vap
19、or zone above the liquid level may be susceptible to cracking in the vapor space, even if the liquid contains water. This cracking is caused by the con-densation of ammonia without water on the walls of the ves-sel above the liquid line. Materials Ammonia SCC severity is affected by steel strength.1
20、1When materials have been selected, lower strength steels have been preferred, although a strength threshold value below which cracking does not occur has not been identi-fied. Nickel-alloy steels and carbon-molybdenum steels are more susceptible to ammonia SCC than carbon steels.2,10Material select
21、ion is discussed further under Prevention. Stress and Strain A study of crack growth rates showed that crack propaga-tion varies approximately as the square of the stress intens-ity factor Kl, at least for shallow cracks.8This investigation and others also revealed that cracking strongly depends on
22、the strain rate applied to the material, an effect that is pre-dicted by theory of film rupture crack mechanisms.12,13The importance of strain rate to cracking tendency indicates that large cyclic variations of stress in service may result in much more severe cracking than would occur in steady-stat
23、e operation. Temperature For years it was believed that ammonia SCC would not occur in fully refrigerated storage tanks that are operated at -33C, where the vapor pressure of ammonia is at atmos-pheric pressure. Although there have been recent reports of cracking in this type of storage,14-16in most
24、 refrigerated stor-age tanks inspected no cracks have been reported.17Lab-oratory studies show that cracking is possible at -33C, but crack initiation is less probable than at 18C.18SCC initia-tion at -33C has only been obtained in laboratory experi-ments with high plastic deformation in the test sp
25、ecimen.19Prevention Prevention of ammonia SCC is accomplished by controlling aspects of environment and vessel design that affect crack-ing, detailed as follows. Cathodic Protection by Zinc Spray Electrochemical studies have shown that ammonia SCC can be prevented by cathodic polarization.4,5,8An ef
26、fective way to achieve this is to flame spray zinc onto susceptible areas, such as welds. Cracking cannot be prevented by conventional cathodic protection using anodes (galvanic or impressed current) because of the high resistivity of anhy-drous ammonia. Ammonia Purity Ammonia SCC can be prevented,
27、in theory, by maintaining sufficient purity of the anhydrous ammonia, particularly with respect to oxygen content. A British code of practice for storage of anhydrous ammonia under pressure outlines a commissioning procedure to reduce oxygen in a vessel via a nitrogen purge before addition of ammoni
28、a.20Addition of NACE International 3 water to the ammonia at a level of 0.2% by mass is a pro-ven method to inhibit against cracking in the liquid phase. Water addition is not a panacea since it does not prevent cracking in areas of condensation, and some users of ammonia cannot tolerate water conta
29、mination. Regulatory agencies have required water addition for transport vessels; however, a proposed requirement for water addition to the anhydrous ammonia transported in pipelines was dropped in 1981.21Vessel Design The Compressed Gas Association(1)has issued a standard as ANSI(2)K 61.1, “Safety
30、Requirements for the Storage and Handling of Anhydrous Ammonia,” (or CGA G-2.1) for use during construction of anhydrous ammonia storage vessels.22This standard includes design and construction details regarding joint efficiency, postweld heat treatment, and type of steel. That standard states, “An
31、exception to the ASME(3)Code requirements is that construction under Table UW 12 at a basic joint efficiency of under 80% is not authorized.” It also states, “The entire container shall be postweld heat treated . . . as prescribed in the ASME code.” “Steels used in fabricating pressure containing pa
32、rts of a container shall have tensile strength no greater than a nom-inal 70,000 psi (480 MPa).” A British code of practice also recommends restrictions on the yield strength of materials used for anhydrous ammonia storage.20Fabrication defects can act as sites for initiation of ammonia SCC because
33、of stress concentration. Shot peening has been suggested as a technique for stress modification but no practical experience has been reported. An isolated failure in a piping system has been reported.26These systems are not generally stress relieved. Coping with Ammonia Stress Corrosion Cracking Alt
34、hough ammonia SCC can be prevented or reduced in severity by several means, the presence of cracking has been investigated in existing vessels, especially when fail-ure would result in significant personnel exposure. Probability of Cracking Experience to date indicates that the probability of ammoni
35、a SCC is the highest for carbon or low-alloy steel vessels that meet one or more of the following criteria. The vessels: Contain anhydrous ammonia with oxygen content greater than 1 ppm or the vessel is frequently exposed to air internally, experience cyclic pressure operation, or are fabricated wit
36、h higher-strength steels (minimum specified tensile strength higher than 480 MPa), espe-cially if not stress relieved. Inspection Techniques Wet fluorescent magnetic particle inspection (WFMT) with an AC yoke on a properly prepared surface has been reported to be the most sensitive inspection techni
37、que for detecting ammonia SCC.23This technique also finds linear indications that are not stress corrosion cracks.24Metallo-graphy has been used to identify the nature of the results. Detection of cracks by WFMT followed by removal of crack indications is an approach that has been successfully used
38、to restore the integrity of existing anhydrous ammonia ves-sels. When cracks were of sufficient depth that weld repair was required after crack removal, stress relief of at least the repair area has been employed to help prevent further cracking. Note that reinspection by WFMT is often used after fi
39、eld heat treatment. A fracture mechanics analysis has been used to assess the structural integrity of a vessel when complete removal of ammonia stress corrosion cracks was not practical.25Local jurisdictional requirements may have an impact on the repair plans of a cracked anhy-drous ammonia pressur
40、e vessel. Detection of small ammonia stress corrosion cracks has not always been successful using visual, dye penetrant, or radiographic techniques. Ultrasonic examination has detected some cracks, but is limited in sensitivity. Acoustic emission testing has been used in conjunction with ultra-sonic
41、 examination in order to pinpoint areas for close exam-ination.24,27An advantage of the use of this combined tech-nique is that it can be done from the outside of a vessel without removing it from service. The successful use of this type of examination includes a thorough analysis of the con-sequenc
42、e of cracks that are beneath the detection threshold of the inspection technique. The frequency of inspection may vary according to the ser-vice conditions of the particular vessel. A vessel with a high probability of cracking or one with a history of cracking has been considered for more frequent i
43、nspection than a vessel with a low probability of cracking. Vessels constructed to current guidelines have not experienced significant crack-ing. _ (1)Compressed Gas Association, 4221 Walney Road, 5th Floor, Chantilly, VA 20151-2923. (2)American National Standards Institute (ANSI), 1819 L St. NW, Wa
44、shington, DC 20036. (3)ASME International (ASME), Three Park Ave., New York, NY 10016-5990. NACE International 4 Conclusions Ammonia SCC is not likely to occur in carbon steel vessels that have been stress relieved and are free of serious stress concentrating defects. Reduction of oxygen contaminati
45、on of ammonia would be expected to reduce the probability of ammonia SCC in vessels that are not stress relieved. Water addition of 0.2% by mass to anhydrous ammonia prevents cracking in the liquid phase but not in the vapor phase. WFMT is sufficiently sensitive to detect ammonia SCC cracks that are
46、 likely to be injurious to a pressure ves-sel. The successful use of less sensitive inspection meth-ods includes a thorough analysis of the consequence of cracks that are beneath the detection threshold of the inspection technique. References 1. T.J. Dawson, “Behavior of Welded Pressure Vessels in A
47、gricultural Ammonia Service,” Welding Journal 35 (1956): p. 568. 2. A.W. Loginow, E.H. Phelps, “Stress Corrosion Crack-ing of Steels in Agricultural Ammonia,” Corrosion 18, 8 (1962): p. 299. 3. J.M. Blanken, “Stress Corrosion Cracking of Ammonia Storage Spheres: Survey and Panel Discussion,” Safety
48、in Ammonia Plants Symposium (New York, NY: American Institute of Chemical Engineers AIChE(4), 1983). 4. D.C. Deegan, B.E. Wilde, “Stress Corrosion Cracking Behavior of ASTM(5)A 517 Grade F Steel in Liquid Ammonia Environments,” Corrosion 29, 8 (1973): p. 310. 5. D.C. Deegan, B.E. Wilde, R.W. Staehle
49、, “Some Electrochemical Aspects of the Stress Corrosion Cracking of Steels in Liquid Ammonia Environments,” Corrosion 32, 4 (1976): p. 139. 6. D.A. Jones, B.E. Wilde, “Corrosion Performance of Some Metals and Alloys in Liquid Ammonia,” Corrosion 33, 2 (1977): p. 46. 7. R.C. Newman, W. Zheng, C.R. Tilley, R.P.M. Proctor, “Exploration of a Nitrogen-Induced Cleavage Model for Anhydrous Ammonia Cracking of Steel,” CORRO-SION/89, paper no. 568 (Houston, TX: NACE, 1989). 8. L. Lunde, R. Nyborg, “SCC of Carbon Steels in AmmoniaCr
