Self-employed consulting corrosion engineer retired from U.S. Steel Corporation
Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN. (9 printed pages)
The following article is a follow-up to "Industry Study Indicates Stress Corrosion Cracking in Anhydrous Ammonia Storage Vessels," which appeared in the October 1988 BULLETIN. This recapitulation of experiences with stress-corrosion cracking of steel in liquefied ammonia summarizes the subject over the last three decades. From the many reports reviewed, an effort was made to concentrate on those that would lead to a better understanding of the problem facing the industry today and suggest a solution to the problem. The review is aimed toward a summary of the conclusions from various sources; specific details are available in the referenced reports.
Stress-corrosion cracking of steel in liquefied ammonia is traced from its early recognition in the 1950s, from experiences in agricultural and industrial use from NACE (National Association of Corrosion Engineers) recommended DOT (Department of Transportation) regulations to prevent damage, and from laboratory studies to explain the factors involved. Control measures are discussed; including control of air contamination, inhibition by water addition, postweld heat treatments, influence of the strength of the steels used and the applied stress, and periodic inspection techniques.
Soon after World War II the Mississippi State College proposed a method of injecting liquefied ammonia directly into the soil as a chief source of nitrogen fertilization (ammonia contains about 82 percent nitrogen). Because excellent results were achieved, very soon a large network of ammonia distribution and application facilities developed, which used many pressure vessels constructed primarily of carbon steels. Although liquefied ammonia had been used without significant difficulties for several decades in the refrigeration, chemical and steel heat-treating industries, inexplicable ruptures of ammonia containers in the agricultural service occurred soon after the introduction of ammonia into this service. A survey indicated that about 3 percent of the pressure vessels failed within 3 years of service life **. Thereupon a research committee was formed by the Agricultural Ammonia Institute to determine the causes of equipment failure and to recommend palliative measures. By evaluating the storage and handling facilities and practices for liquefied ammonia and by conducting controlled tests, it was determined that stress-corrosion cracking (SCC) was the most likely cause of the failures . The probability of SCC increased with increased stress level as well as with increased yield strength of the steel. Of the many contaminants that could have been present in the ammonia, air was identified as the specific agent which caused SCC. (Although at that time the normal carbon dioxide content of air was believed to be essential to the cracking process, later studies modified that concept.) It was also shown repeatedly that at least 0.1 percent of water in the liquefied ammonia inhibits SCC of constructional steels used in the fabrication of the pressure vessels for agricultural service. As the result of the studies the research committee recommended  that:
- Pressure vessels should be either fully stress relieved or fabricated with heads that are hot-formed or stress relieved.
- Extreme care should be used to eliminate air from the ammonia systems; new vessels must be thoroughly purged to eliminate air contamination.
- Ammonia should contain at least 0.2 percent water to inhibit SCC.
The report  also remarked that, inasmuch as each of these recommendations has limitations when considered individually, all three should be followed to avoid SCC. The adoption of these recommendations was very effective in practically eliminating SCC in carbon-steel vessels used in the agricultural industry .
Outside the agricultural industry SCC began to occur in the mid-1960s, primarily involving DOT Specification truck-transport cargo vessels. This onset of SCC may have been due to improvements in ammonia production technology which resulted in a gradual increase of the ammonia purity, mostly by a reduction of the water content which inhibits SCC . An examination of 19 ammonia vessels, that was performed at that time, included several steels representing a wide range of strength levels. Sectioning eight of these vessels that had been in ammonia service for 8 to 23 years and conducting a detailed inspection by a wet fluorescent magnetic particle technique of the entire interior surface did not reveal any indication of cracks. However, two additional DOT Specification 3AA vessels contained stress-corrosion cracks. The hardness of the steels, obtained in lieu of tensile strength, which did not contain SCC ranged from 134 to 272 HV, and the hardness of the cracked vessels was 207 HV and 241 HV. The reason for this lack of specificity with respect to the hardness (or strength) level is not known. Perhaps the applied stress of the cracked vessels was high in relation to their strength, or the contamination of the ammonia was at a detrimental level. The specific grade of ammonia that was contained in any of these vessels is not known, but it can be assumed that it was metallurgical or refrigeration grade ammonia, which do not contain appreciable amounts of inhibiting water (33 ppm max or 150 ppm max , respectively). SCC was also found in three refrigeration system ammonia condensers fabricated of ASTM A515 Grade 70 steel . The condensers had not been postweld heat treated and the cracks were located in the base metal near welds. Two refrigeration system ammonia storage vessels of this steel also showed SCC in cold formed vessel heads near welds that were not postweld heat treated.
An ASTM A202 Grade B hot-rolled steel truck-transport vessel contained SCC in and adjacent to repair welds . Although the vessel had been postweld heat treated after fabrication, subsequent repair welds, however, were not heat treated and the residual stresses caused stress corrosion to occur. Note that Compressed Gas Association guidelines for the repair of truck-transport cargo vessels  permit "minor repairs" without subsequent heat treatment; this aspect will be discussed in a later section. SCC in ASTM A202 Grade B steel ammonia transport vessels that were heat treated after welding, however, was conspicuous by being essentially absent.
In 1967 DOT reported about 8000 truck-transport cargo vessels in existence, a half of this number being fabricated of quenched-and- tempered (Q & T) alloy steels (ASTM A517), which represented the upper end of the strength range examined. At that time about 2000 vessels were examined by vessel manufacturers for evidence of cracking, and about 400 vessels contained some defects, but the DOT commented that most of the defects were associated with poor workmanship, the balance probably being caused by SCC. In the period from 1965 to 1977 the writer examined eight cargo vessels that were fabricated of ASTM A517 Q & T steels with a postweld heat treatment and that carried a variety of ammonia grades, some of them without water inhibition . The observed cracks, in all in-stances identified by metallographic examination as being caused by SCC, occurred on the inside surface, mostly in the lower quadrant of the vessel, that is exposed primarily to liquid ammonia at or within a few inches of internal welds as well as in areas opposite external welds, especially opposite those welds that attach load-bearing pads. Cracks were also associated with repair welds that were not heat treated after the repairs.
Until the mid-1960s the service performance of high strength quenched-and-tempered steel (ASTM A517) truck-transport cargo vessels was excellent . However, as SCC started to occur in such vessels, NACE recommended a set of preventive measures that were adopted in the U.S. as DOT regulations in 1968 and 1975 . These regulations required that cargo vessels of Q & T steels in liquefied ammonia service be used only for ammonia containing at least 0.2 percent water. (An initially permitted ammonia of at least 99.995 percent purity was later eliminated from the requirements, because this purity definition considered only water and oil as impurities and did not include air as a contaminant). Cargo vessels of other than Q & T steels were permitted for use with any grade of ammonia. The regulations also stated that all new cargo vessels for ammonia service be postweld heat treated. A wet fluorescent magnetic particle inspection  of the internal surfaces was also required of all vessels in conjunction with the hydrostatic retest every 5 years. A major vessel fabricator reported that subsequent to these requirements SCC damage to Q & T cargo vessels has decreased substantially.
Beneficial and Detrimental Factors
The experience of SCC with liquefied ammonia containers and the resulting regulatory activity in the U.S. prompted several investigations to be conducted in laboratories to further determine the factors involved. Radd and Oertle  suggested that the cracking was due to hydrogen embrittlement, that water additions were acting to accelerate rather than inhibit cracking, and that the observed inhibition was due to dissolved oxygen in water rather than intrinsically to the water. Based on accelerated tests with C02-saturated ammonia, Naito et al.  proposed that SCC in the test environment was initiated by pitting and propagated by hydrogen embrittlement. Addition of about 0.75 percent water to this environment reduced the cracking. Also, the addition of 0.01 percent NH4 C1 suppressed the SCC. Furukimi et al.  proposed to cover up the SCC problem by decreasing the hardness of the surface of high-strength Q & T steel to less than 190 HV by decarburizing the steel surface and controlling the cooling rate during the Q & T treatment.
At U.S. Steel Research many investigations were conducted by the use of the slow strain-rate technique . This technique, which is effective in producing SCC in relatively short time periods, employed dynamic straining to stress a specimen at a rate that is much slower than that used in a conventional tension test. Wilde  developed special sampling and gas chromatography analysis techniques that overcome the problem of selective evaporation frequently encountered in determining the small concentrations of gaseous contaminants in liquefied ammonia. With respect to SCC by liquefied ammonia most of the slow strain rate tests were conducted on the high-strength ASTM A517 steel. Deegan et al. [14, 15] have demonstrated by the use of anodic and cathodic polarization, in conjunction with the slow strain method, that SCC in air-contaminated ammonia is anodically controlled and that cathodic polarization prevents cracking.
Hennecken  also found that SCC of carbon steel in liquid ammonia is anodic in nature, and expanded further from polarization tests that oxidizers in ammonia increase the danger of SCC. The anodic nature of SCC in ammonia was also verified by Wilde and Kim  who demonstrated by the slow strain-rate test that aluminum, which is anodic to steel in air-contaminated ammonia , when coupled to the straining steel specimen can prevent SCC, apparently by cathodic protection. However, the relatively high electrical resistivity of liquefied ammonia and the resulting poor throwing power militate against a successful use of cathodic protection by sacrificial aluminum anodes in combating SCC in industrial applications. This reservation is supported by observations that SCC occurred in ammonia truck-transport cargo vessels, which were equipped with aluminum baffles electrically coupled to the steel vessels .
The studies of the ASTM A517 steel also showed that uncontaminated metallurgical grade ammonia does not cause SCC , but that an 0.38 psig over-pressure of air can result in SCC . The latter authors also showed that separate additions of either 9 ppm oxygen or 3 ppm nitrogen to the ammonia did not cause SCC in A517 steel, but that SCC occurred when both of these contaminants were added together; a smaller addition of 0.9 ppm oxygen together with 3 ppm nitrogen also resulted in SCC . Later, Wilde  showed that in the absence of nitrogen greater amounts of oxygen (200 ppm) can cause slight SCC in plastically deformed steel. He also determined that 0.02 ppm oxygen (added as air to ammonia containing 18 ppm water) did not cause SCC in A517 steel slow strain-rate specimens. As the air contamination level was increased, marked SCC occurred between 1 and 200 ppm oxygen, and as the air contamination was increased further the SCC severity became less pronounced until at an oxygen concentration of 920 ppm and above SCC was not observed. The role of oxygen was also confirmed by Lyle  who concluded from slow strain-rate tests that oxygen contamination is the primary cause of ammonia SCC of steel, as well as by Farrow et al.  who concluded that oxygen contamination is necessary for SCC of mild steel at a level of more than 5 ppm in commercial grade ammonia, but as little as 1 ppm may be sufficient when water is absent.
The early conclusion by Loginow and Phelps  that a small addition of water inhibits SCC of steel in air-contaminated ammonia was later confirmed in slow strain rate tests on A517 steel [14, 15, 21]. Deegan and Wilde  also showed that for the degree of air contamination used (approximately 2 atmospheres partial pressure of air at 25°C) a limit for the effectiveness of water as an inhibitor was observed at 0.08 percent, which is in very close agreement with the lowest amount of 0.1 percent reported earlier . Hennecken  states that high oxygen contents in the ammonia require increased water contents to provide inhibition against SCC in carbon steel, and conversely, that low oxygen contents could be counteracted by low water contents. He also concludes from polarization measurements that the presence of oxidizers in liquid ammonia impede the inhibiting properties of water. Examination of some failures of agricultural and industrial ammonia vessels, that were discussed earlier, suggested that the inhibition with water was not practiced continuously and thus resulted in SCC. In 1975, Poulson and Arup  showed experimentally by the use of the slow strain-rate test on mild steel that when water-inhibited ammonia is replaced with a water-free air contaminated ammonia, cracking started without significant delay, thereby confirming the suspicion that an intermittent exposure to uninhibited ammonia can result in cracking.
Although cracks in truck-transport cargo vessels and storage spheres appear mostly in the lower part that is continuously exposed to liquid ammonia [2, 6, 16], various reports from Europe were concerned with SCC in the vapor space of storage spheres (mostly fabricated of low-strength carbon steel without postweld heat treatment of the entire sphere) that contained water-inhibited ammonia. These observations suggested that the inhibitive properties of water in liquid ammonia are not transferred to the vapor phase. Studies by Ludwigsen and Arup  showed that ammonia which condenses on a cool steel surface at nonequilibrium conditions, such as on a cool night, is capable of causing SCC. They concluded that such a condensed ammonia contains much less inhibiting water and much more detrimental oxygen than the liquid ammonia from which it evaporated. Later, Brown confirmed by calculation the paucity of water in the vapor over liquid ammonia , and Wilde confirmed the enrichment of air in the vapor by experimentally determining the partition coefficients between liquid and vapor phases of ammonia to be 587 for oxygen and 603 for nitrogen . Ludwigsen and Arup  suggest that the liquid ammonia be maintained at a temperature that is lower than that of the steel in contact with the ammonia vapor to prevent condensation and consequent SCC. This approach apparently does not avoid SCC in the meniscus between vapor and liquid. Lunde  reports that SCC occurred in the vapor phase when the steel was at ambient temperature, whereas cracking was observed only in the meniscus when the top of the container was warmer by 5°C than the water-inhibited liquid ammonia and condensation did not occur.
The environmental effects, that are summarized above, together with electrochemical studies led to the formulation of a mechanism of SCC Gf constructional steels in liquefied ammonia [15, 21].
"In oxygen contaminated ammonia an adsorbed film exists on the steel surface which maintains the corrosion potential at very noble values. During straining after yield, breaks in the film expose film free metal at slip steps, and this metal dissolves rapidly under the noble potential imposed by direct galvanic coupling to the filmed metal surface.
In the absence of any other impurity, oxygen has the tendency to refilm the bare metal and to some degree inhibits the severity of SCC depending on the concentration and the strain rate. Nitrogen dissolved in the ammonia inhibits the filming of the bare slip steps by a process of 'competitive adsorption' with the oxygen and consequently increases the severity of SCC through a process of adsorption rather than through a process of electrochemical origin. Water inhibits SCC by acting as an additional film former which hastens passivation and healing of film breaks at slip steps".
In this proposed model no mention is made about the contribution of carbon dioxide to the SCC process. Wilde  has shown that, contrary to the early conclusion that carbon dioxide is essential to the cracking process , this normal constituent of air when added to ammonia resulted in severe general corrosion, but not SCC; for SCC to occur, air must also be present. In this study Wilde  also examined the effects of various other contaminants and concluded that CO, CO+O2, CH4, SO2, S02+02, NH4C03, (NH4)2CO3 NH4N03, NaN03, N0, N02, N20, NaC1, NH4C1, NaN02, and NaNH2+air did not cause SCC when added to an air-free test ammonia. Some of the test additives (S02, S02+02, NH4N03, and NH4 C1) caused general corrosion to varying degrees.
Although it has been shown convincingly that additions of water on the order of 0.1 percent and more can effectively inhibit SCC in air contaminated ammonia, there are end uses of ammonia in which even such small amounts of water cannot be tolerated, such as in metal heat treating or refrigeration applications. Therefore, some studies were made with additives that would be compatible with the end use of ammonia, primarily amine compounds . The tests showed that some inorganic amines and ethylenediamide inhibited cracking in air-contaminated ammonia. Because the molecular constitution of hydrazine (N2H4) is similar to that of ammonia (NH3), this compound was subjected to further tests at concentrations ranging from 0.002 to 1.0 percent. The inhibitive properties of hydrazine in air-contaminated ammonia were observed at concentrations of 0.025 percent N2;H4 and above. Apparently hydrazine acts as an oxygen scavenger inasmuch as in a hydrazine-treated ammonia, that was initially contaminated with air and which subsequent to the hydrazine addition did not cause SCC, the oxygen content had decreased to zero. The inhibitory effects of hydrazine and water were also reported in relation to mild steel as well as those of ammonium carbonate, ammonium bicarbonate, and carbamate . Lyle  reported that water, methane, and nitrogen exhibit inhibiting properties, the latter being at variance with the findings discussed earlier [15, 21].
The preceding discussion of practical experience and the many investigative studies of SCC of steels in liquefied ammonia service suggest several measures that could prevent SCC or reduce the probability of cracking. However, it should be recognized that each measure has limitations when considered separately. For maximum benefit as many procedures as possible should be employed in the attempt to combat SCC.
- Air contamination (even a few ppm) is considered the primary cause of SCC of steels in liquefied ammonia. It is therefore good engineering practice to purge new vessels of air and to eliminate any conceivable possibility of air ingress into the ammonia handling system, such as at detachable ammonia transfer points. Because air tends to concentrate in the vapor space above a liquid ammonia pool (there is about 600 times more air in the vapor phase than in the liquid in equilibrium with it) it is advisable in the case of large storage spheres to continuously condense the ammonia vapor with a simultaneous discharge of the noncondensables, which is mostly air, and to return the condensed liquid to the vessel. For relatively small units it was demonstrated that a bleeding or boil-off method can effectively remove air from ammonia to avoid SCC [2, 13]. Good results in reducing SCC damage to storage spheres have been achieved by a careful control of the oxygen content in ammonia having a low water content .
- Similar to the detrimental effect of oxygen, the inhibitory effect of water content in liquid ammonia has been well documented, the optimum dose apparently being 0.2 percent water in those applications where this amount of water can be tolerated although it was suggested that a lower water concentration may be adequate when the oxygen content is low [16, 20]. Note that for effective inhibition of SCC the required water content must constantly be present in ammonia. Should the water content drop below the critical concentration even for a relatively short time, as by influx of water-free ammonia into a vessel, the irreversible-SCC damage can occur. Thus it is important to monitor the water content on a frequent and regular basis and to replenish the water if it starts to fall below the critical concentration. In metal heat treating and refrigeration applications where this low concentration of water in ammonia is undesirable, laboratory tests have shown that 0.025 percent hydrazine addition is adequate in inhibiting SCC. Although industrial utilization of hydrazine in ammonia is still forthcoming, such use of hydrazine should verify the laboratory findings.
- The inhibitory effect of water may not be realized in the vapor space of liquid ammonia storage tanks, and calculations and laboratory studies indicate that measures, which prevent condensation of ammonia on the steel surface, would tend to avoid SCC. Simultaneous reduction of the oxygen content of the ammonia vapor should also be beneficial.
- Postweld heat treatments are effective in substantially reducing the probability of SCC of steel in contact with air-contaminated ammonia. Although postweld heat treatments are frequently considered only from the standpoint of reducing the residual stresses, it should be recognized that such thermal treatments result in two consequences, both of which are beneficial in reducing SCC. First, properly conceived and executed thermal treatments can substantially lower the residual stresses that result from fabrication (welding and cold forming), erection, and restraint of the steel structure, such as storage sphere. Secondly, an appropriate postweld heat treatment can significantly reduce the peak hardness in the heat affected zone (HAZ) of the weldment . Thus, a hardness peak of 443 HV in the HAZ of an ammonia tank of ASTM A202 Grade B hot rolled steel was reduced to 280 HV by a postweld heat treatment, and similarly a 447 HV peak hardness was reduced to 290 HV in an ASTM A517 Q & T steel weldment . Such reduction in HAZ hardness benefits the resistance to SCC, especially in conjunction with other measures. In this context note also that guidelines promulgated by the Compressed Gas Association for repairs of vessels permit minor welding repairs to be performed without a subsequent heat treatment . However, the size of the area repaired by welding or the extent of welding do not affect the hardness of the HAZ and its susceptibility to SCC. Therefore from the standpoint of avoiding SCC, it is good engineering practice, that steel equipment for liquefied ammonia service be heat treated after any welding operation regardless of size, which is a requirement in the United Kingdom . Although it is generally considered to be difficult and costly, a successful full-scale heat treatment of a large fabricated and erected storage sphere was reported from Japan . Shot peening has been proposed to reduce residual stresses in areas of weld joints or even to induce compressive surface stresses. However, experience has shown that this approach only reduced temporarily the incidence of ammonia SCC, but did not prevent it [16, 27, 31].
- It is recognized that, in general, the probability of SCC of steel in liquefied ammonia service is the greater, the higher the strength of the steel. The formulation of a lower strength limit, below which SCC would not occur, is not practical because it depends on the level and nature of contamination, the degree and nature of inhibition, the magnitude of residual and operating stresses and the operating temperature. Nevertheless, a lower-strength steel appears to provide a higher degree of resistance to SCC than a higher-strength variety. With respect to the stress that exists in a steel component other aspects pertain; the upper limit is governed by applicable statutory regulations, and the lower end, which is desirable from the standpoint of combating SCC, is governed by the operating pressure, the technologically achievable reduction of residual fabricating stresses, and economic and safety considerations. It is good engineering practice to avoid stress concentrating features. Note also that if SCC has initiated, the effective stress at the tip of the crack can be very high indeed. Novel design aspects, such as external reinforcements that would place the interior surface of a tank in compression, could reduce the operating stress level and benefit in decreasing the likelihood of SCC.
- Many catastrophic failures, with resultant personal injuries and property damage that have their origin in SCC, could be avoided by periodic and frequent inspections of the steel surfaces that are in contact with ammonia, and by making repairs when needed. Cracknell  has proposed an inspection schedule for as-welded carbon steel vessels that is based on oxygen and water contents. Guild  recommends inspections to be conducted both before and after pressure testing of storage spheres, because fine cracks can open up during the pressure test and become visible afterward. In the writer's experience, the most sensitive inspection method for detecting the frequently very fine stress-corrosion cracks is the wet fluorescent magnetic particle method with an alternating current yoke 16, 7, 31]. An acoustic emission technique , which detects actively propagating cracks by sensors applied to the outside of an operating vessel, could furnish a continuous report on the soundness of the vessel if equipped with appropriate monitoring, processing, and warning devices.
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- E. H. Phelps, Ammonia Plant Safety, vol. 16, 1974, p. 32.
- D. R. Pratt, Battelle Pacific Northwest Laboratories, 1976.
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- CGA Technical Bulletin TB-2, 2nd Ed., Compressed Gas Association, New York 1975.
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- F. J. Radd and D. H. Oertle, NACE Annual Conference, Chicago, Illinois, 1971.
- N. Naito, W. Hotta, and H. Okada, "Accelerated Test Method for SCC of High Strength Steels in Liquid Ammonia Environments", Manuscript, 1973.
- O. Furukimi et al., Lecture No. S1371, ISIJ Meeting, 1981.
- C. D. Kim and B. E. Wilde, Stress Corrosion Cracking - The Slow Strain-Rate Technique, ASTM STP 665, G. M. Ugiansky and J. H. Payer, Eds., 1979, p. 97.
- B. E. Wilde, Corrosion, vol. 37, No. 3, 1981, p. 131.
- D. C. Deegan and B. E. Wilde, Corrosion, vol. 29, No. 8, 1973, p. 310.
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- H. Hennecken, Dissertation, Technical University Munich, Germany, 1984.
- B. E. Wilde and C. D. Kim, Corrosion, vol. 38, No. 3, 1982, p. 168.
- D. A. Jones and B. E. Wilde, Corrosion, vol. 33, No.2, 1977, p.46.
- F. F. Lyle, Final Report DOT-FH-BMC-11-8568, 1976.
- K. Farrow, J. Hutchings, and G. Sanderson, Br. Corr. J. vol. 16, No 1,1981, p.11.
- D. A. Jones, C. D. Kim, and B. E. Wilde, Corrosion, vol. 33, No. 2, 1977, p.50.
- B. Poulson and H. Arup, Proceedings - 7th Scandinavian Corrosion Congress,1975, p. 661.
- P. B. Ludwigsen and H. Arup, Corrosion, vol. 32, No. 11, 1976, p. 430.
- R. S. Brown, AIChE Symposium, Safety in Ammonia Plants, 1982.
- L. Lunde, Ammonia Plant Safety. vol. 24, 1982.
- A. Cracknell. AIChE Symposium, Safety in Ammonia Plants, 1982.
- O. L. Towers, Metal Construction, 1984, p. 479.
- T. Kawamoto, T. Kenjo, Y. Imasaka, IHI Engineering Review, vol. 10 No. 4, 1977, p. 17.
- H. Spaehn, AIChE Svmposium, Safety in Ammonia Plants, 1982.
- J. C. Guild, AIChE Symposium, Safety in Ammonia Plants, 1982.
- ASME SE-138, Standard Method for Wet Magnetic Particle Inspection.
- L. Adams, AIChE Symposium. Safety in Ammonia Plants, 1982.
*Reprinted with permission from Materials Performance, Vol. 25, No. 12, December 1986, pp 18-22. ©1986, National Association of Corrosion Engineers Houston, TX. Reprinted text has been edited for clarification and space requirements.
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition of the ASME Boiler and Pressure Vessel Code for current requirements.