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Failure of high pressure ammonia line in a fertilizer plant – A case study
Engineering Failure Analysis 13 (2006) 867–875 www.elsevier.com/locate/engfailanal
Failure of high pressure ammonia line in a fertilizer plant – A case study S. Sivaprasad *, S.K. Narang, R. Singh National Metallurgical Laboratory, Jamshedpur 831 007, India Received 24 June 2005; accepted 14 July 2005 Available online 1 September 2005
Abstract In this investigation the failure of a high-pressure ammonia pipe line made of SA 106 Gr. B carbon–manganese steel has been discussed. The failure in the pipe had occurred in the form of a through thickness pin-hole without any loss of wall thickness or bulging. Fine longitudinal cracks of 4–5 mm in length had also occurred on either side of the pin-hole. The inner surface of the pipe was corrosion free, however, corrosion attack in the form of pitting was observed on the outer surface of the pipe. Laboratory investigations revealed that such pits had formed due to chloride attack at locations where the Fe3O4 mill scale had not been completely removed. As one would expect hydrogen attack concurrently occur, evidences of hydrogen assisted cracking was also found on the fractured surfaces. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Failure analysis; High pressure ammonia line; SA 106 Gr. B Steel; Pitting; Chloride
1. Introduction The SA 106 Gr. B carbon manganese steel is normally used in the fertiliser plants to transport the liquid ammonia from the ammonia heater to the urea reactor. In a fertilizer plant, leakage was detected in the horizontal portion of this ammonia feed line at 108° elevation. Upon examination, a 2–3 mm diameter pin-hole was detected in the 5/7 OÕ clock position through which the chemical had been leaking. Similar leakage had been detected on earlier occasions at identical location, and the pin hole was either repaired by weld deposition or section of the defected pipe was replaced to provide continued service. The pipe line was originally installed in the year 1997, and subsequent to leakage problems it was replaced in the years
*
Corresponding author. Tel.: +91 657 2271709; fax: +91 657 2270527. E-mail address: [email protected] (S. Sivaprasad).
1350-6307/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2005.07.003
868
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1998 and 1999. In a span of 3 years from 1999 to 2001, the pin-hole formation and leakage problem was encountered thrice in the same horizontal segment. Recurrence of such failure increased the frequency of unplanned shut downs and upset the plant operation. This ammonia feed line operates at a pressure of 233 kg/cm2 and 149 °C temperature. Normally, high pressure air is added to pump discharge pulsation dampener at 50 to 60 NM3/h. Presence of moisture (oxygen) in the air could have changed the composition of the ammonia. There is also a possibility of water together with air remaining in the ammonia line after flushing the line with condensate water. However, interaction with the plant engineers ruled out such possibilities, as high pressure air is added to the pump pulsation dampener in the later stage and there was no appreciable change in the composition of the ammonia. No sagging was reported in the horizontal portion where the failure is stated to have occurred. In fact, this horizontal portion was adequately supported on either side. The cause of the failure must therefore be due to some other factor. Keeping these factors in mind, a failed portion of the pipe was brought to the laboratory to study the cause of the failure. The results presented in this paper are based on the investigation made on material properties and analysis of fracture surface.
2. Experimental 2.1. Visual examination A section of the failed pipe received for investigation is shown in Fig. 1. The pipe had a through thickness pin-hole. Fine longitudinal cracks of about 4–5 mm length were also found to have propagated from either side of the pin-hole. These cracks were visible by the naked eye on the outer surface, however, on the inside surface of the pipe the cracks could be seen only after polishing. On the outer surface in the vicinity of the pin-hole, a number of shallow pits were present (Fig. 1(b)). Fig. 2 shows the crack in both the longitudinal and transverse directions of the pipe. Outer surface of the pipe had mill scale, which appeared black in colour. However, the inner surface of the pipe was corrosion free and no pits were seen on the inside of the pipe. There was no reduction in the wall thickness of the pipe or any change in the circumference. 2.2. Chemical composition and microstructure A small piece of the pipe material was cut and the chemical analysis was carried out using direct reading spectrometer (DRS). The results of the chemical analysis are shown in Table 1. It may be noted that the chemical composition of the material conforms to the permissible limits of SA 106 Gr. B steel [1]. The optical microstructure of the pipe material, both in the longitudinal and transverse directions of the pipe was examined. Small pieces of the material cut from both the directions were polished and etched with 4% Nital. The microstructure of the pipe material in both the directions are shown in Fig. 3. This consists of ferrite and pearlite with mild degree of banding. 2.3. Mechanical properties The mechanical properties of the pipe material were evaluated in terms of hardness, tensile and Charpy impact studies. The bulk hardness on Vickers scale was measured at an applied load of 30 g. For tensile tests, round tensile specimens of 5 mm gauge diameter were prepared from the pipe material such that the loading axis is parallel to the longitudinal direction of the pipe. Tensile tests were carried out in a 100 kN servo-electric test system at a nominal constant displacement rate of 0.003 mm/s following the
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Fig. 1. Sections of pipe showing pits on the outer surface and through wall pin-hole. Photograph (b) is a close view of (a) where the pits growing from it may be seen.
Fig. 2. Cross-section of the pipe showing (a) longitudinal crack from pin-hole as seen on the outer surface (marked by arrow) and (b) hairline crack in the transverse direction.
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Table 1 Chemical composition of SA 106 Gr. B steel (wt%) C
S
P
Cr
Cu
Mn
Si
0.17
0.011
0.03
0.07
0.043
0.58
0.37
Fig. 3. Microstructure of the SA 106 Gr. B Steel in (a) transverse and (b) longitudinal directions.
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Fig. 4. Stress–strain behaviour of SA 106 Gr. B steel.
ASTM standard E 8M [2]. A 25 mm gauge length extensometer was fitted to the uniform gauge length portion of the specimen for strain measurements. Typical engineering stress–strain curve is shown in Fig. 4 and the mechanical properties are shown in Table 2. 2.4. Microscopic examination of fracture surface The damaged part of the pipe was cut and broke-opened in the liquid nitrogen. The matching fracture surfaces after break opening is shown in Fig. 5. These fracture surfaces were examined in scanning electron microscope (SEM). A typical SEM microphotograph of the pin-hole portion along with the propagated crack is shown in Fig. 6, small pits around the pin-hole are shown with arrows. The right-hand side of this figure corresponds to the outer surface of the pipe. The chemical composition of the corrosion products in Table 2 Mechanical properties of SA 106 Gr. B Steel Specimen No.
YS (MPa)
UTS (MPa)
% El
% RA
Average hardness (Hv30)
Average at 0 °C, CVN (J)
1 2
278.5 277.3
473 475.5
29.24 28.97
64.68 64.87
173
32
Fig. 5. Mating fracture surface of the failed pipe; the pin hole AB through the thickness may be noted.
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Fig. 6. Fractography of pit as seen in SEM.
and around the pin-hole were analysed under SEM using energy dispersion analysis of X-rays (EDAX). A typical X-ray spectrum of the chemical analysis at the mouth of the shallow pit is shown in Fig. 7 and the results of this microchemical analysis are listed in Table 3.
60
K Cl
50
Ca
Si S 40
Fe Na
30
20
Al
10
Ca Ca S
Cl
K
Ca
Fe keV
0 0
5
10
Fig. 7. EDAX analysis of corrosion products around the pit showing the presence of chloride.
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Table 3 EDAX analysis of various elements of corrosion product at the pit mouth Elt
X-ray
Int
Error
K
K ratio
W%
A%
Na Al Si S Cl K Ca Fe
Ka Ka Ka Ka Ka Ka Ka Ka
2.9 2.3 5.7 5.1 8.1 7.0 6.4 5.1
0.1859 0.1657 0.2586 0.2456 0.3089 0.2871 0.2753 0.2444
0.0400 0.0314 0.0796 0.0905 0.1572 0.1624 0.1643 0.2745
0.0323 0.0253 0.0642 0.0731 0.1269 0.1311 0.1326 0.2215
8.37 4.08 8.75 8.49 15.20 14.92 15.15 25.05
13.35 5.54 11.42 9.71 15.71 13.99 13.85 16.44
W%, weight percent; A%, atomic percent.
3. Results and discussion The results of the chemical analysis showed that the chemistry of the material conforms to that of the SA 106 Gr. B steel and within the permissible limits as specified in the standard [1]. The microstructure of the material is ferrite–pearlite which is a typical of this class of steels [3] and the material contained only fewer amounts of oxide/sulphide type of inclusions. There was no significant difference in the microstructures of pipe material in the longitudinal and transverse directions, although the longitudinal microstructure showed mild degree of banding. The mechanical properties of the material are also found to qualify the minimum requirement as mentioned in the specification for this variety of steels. Thus, it is clear that the pipe material is not deficient both in terms of chemistry and mechanical properties. The failure is therefore, not due to the material problem, but could be due to operational or environmental factors. However, from the SEM microphotograph shown in Fig. 6, it may be noted that the width of the pinhole on the outer surface of the pipe is more than at the interior. Moreover, a number of small shallow pits were found around this pin-hole (as shown by arrows in Fig. 6). Such shallow pits were also seen elsewhere on the outer surface of the pipe. The inside of the pipe, however, was free from any pits not withstanding some evidence of rusting. The failure therefore has initiated from the outside of the pipe. The outer surface of the pipe has suffered severe pitting with single pit taking a leading role to form a pin-hole. Pitting attack usually reported to take place due to the break down of the protective film on the metal surface [4]. In the present case, it appears that the pit has formed due to the break down of protective Fe2O3 mill scale that formed during hot rolling of the steel pipe. Small anodic sites are expected to have developed at these mill scale areas leading to nucleation of the pits. Once the pits have nucleated, it will propagate through anodic dissolution of the metal by its autocatalytic nature [4]. The pitting reaction is expected to have aggravated by the operating temperature of around 150 °C. There are various factors, such as inclusions, second phase particles, flaws, heat treatment/cold work, temperature and chemical species that influence the formation and growth of pits [5]. Particularly, the presence of chloride or chlorine containing ions are reported to be responsible for most pitting failures [4]. The results of the EDAX analysis on the fracture surfaces (Fig. 7 and Table 3) showed the presence of chloride ions at the mouth of the main and shallow pits. This provides strong evidence to the fact that the pit formation is due to the presence of chloride ions. The source of the chloride ion was first thought to have come from the condensate water used to wash the line after depressurisation. However, the condensate chemical composition used for washing the line did not contain such detrimental elements. Moreover, the pit formation had occurred on the outer surface of the pipe and the inner surface was corrosion free. It was, therefore, contended that chloride ions had come from the atmosphere. It is worth mentioning at this point that the concerned plant was located very near to the seacoast, and the environment around the plant is
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Fig. 8. Intergranular fracture showing the evidence of hydrogen assisted cracking.
expected to have high moisture and chloride ions in air. Although the mechanism by which the chloride promotes pitting is not yet clear, perhaps the best explanation is the acid forming tendency of chloride salts and the high strength of its free acid [5]. It is shown that the presence of FeCl2 which is supposed to be present in corrosion pits on iron based alloys exhibits a pH of 3.8 in oxygen free condition while an excess of oxygen in the solution decreases its pH to 0.6 [5]. Such low pH of electrolyte containing chloride inside the pit is liable to accelerate the pit growth. It is believed that the presence of an adherent salt layer inside the pit covering the pit bottom provides a protective effect against pit growth [6]. In fact, indirect evidences for existence of such a protective layer inside the pits were provided by the results of a study on the effect of specimen position on the shape of corrosion pits [6]. It is demonstrated that in specimens with exposed surfaces facing upward, the in-depth growth of pit was slower compared to those with exposed surfaces facing downward. In the first case, salt layer was thicker and more protective than in the latter case. A similar analogy can be drawn to the present investigation, and this may be the reason for the formation of pin-hole always in the 5/7Õ OÕ clock position in the present case. The fracture surface examination of the crack adjacent to the main pit exhibited an intergranular fracture as depicted in Fig. 8. As discussed above, under the low pH condition inside the anodic pit due to the presence of chloride ions, the compensating cathodic reaction will be hydrogen reduction. The nascent hydrogen thus generated caused the brittle intergranular cracking as shown in Fig. 8. The presence of hoop stress and the notch effect produced by the pit are likely to aggravate the cracking tendency in the longitudinal direction which eventually had propagated through the thickness of the pipe.
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4. Conclusions On the basis of the laboratory investigations of the failed pipe, it is concluded that the failure of the high pressure ammonia line is due to the pitting corrosion initiated at the partially removed Fe2O3 scale on the outer surface of the pipe. The formation of the pit and its rapid growth had occurred due to the presence of chloride ions in the atmosphere. The nascent hydrogen produced at the local electrochemical cell, eventually caused the through thickness cracking of the pipe with the aid of hoop stress and the notch effect produced by the leading pit.
References [1] Metals hand book. Properties and selection-irons, steels and high temperature performance alloys, 10th ed., Materials Park (OH, USA): ASM International; 1990. p. 617. [2] ASTM Standard E-8M. Test methods for tension testing of metallic materials. Annual Book of ASTM Standards, vol. 03.01. PA: USA; 1994. p. 81. [3] Metals hand book. Atlas of microstructures of industrial alloys, 8th ed., Materials Park (OH, USA): ASM International; 1972. p. 27. [4] Fontana MG, Greene ND. Corrosion engineering, materials science & engineering series. McGraw Hill; 1978. p. 51. [5] Szklarska-Smialowska Z. Pitting corrosion of metals. Houston, (TX, USA): NACE Publications; 1986. p. 532. [6] Mankowski J, Szklarska-Smialowska Z. Corr Sci 1977;17:725.
Failure of high pressure ammonia line in a fertilizer plant – A case study S. Sivaprasad *, S.K. Narang, R. Singh National Metallurgical Laboratory, Jamshedpur 831 007, India Received 24 June 2005; accepted 14 July 2005 Available online 1 September 2005
Abstract In this investigation the failure of a high-pressure ammonia pipe line made of SA 106 Gr. B carbon–manganese steel has been discussed. The failure in the pipe had occurred in the form of a through thickness pin-hole without any loss of wall thickness or bulging. Fine longitudinal cracks of 4–5 mm in length had also occurred on either side of the pin-hole. The inner surface of the pipe was corrosion free, however, corrosion attack in the form of pitting was observed on the outer surface of the pipe. Laboratory investigations revealed that such pits had formed due to chloride attack at locations where the Fe3O4 mill scale had not been completely removed. As one would expect hydrogen attack concurrently occur, evidences of hydrogen assisted cracking was also found on the fractured surfaces. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Failure analysis; High pressure ammonia line; SA 106 Gr. B Steel; Pitting; Chloride
1. Introduction The SA 106 Gr. B carbon manganese steel is normally used in the fertiliser plants to transport the liquid ammonia from the ammonia heater to the urea reactor. In a fertilizer plant, leakage was detected in the horizontal portion of this ammonia feed line at 108° elevation. Upon examination, a 2–3 mm diameter pin-hole was detected in the 5/7 OÕ clock position through which the chemical had been leaking. Similar leakage had been detected on earlier occasions at identical location, and the pin hole was either repaired by weld deposition or section of the defected pipe was replaced to provide continued service. The pipe line was originally installed in the year 1997, and subsequent to leakage problems it was replaced in the years
*
Corresponding author. Tel.: +91 657 2271709; fax: +91 657 2270527. E-mail address: [email protected] (S. Sivaprasad).
1350-6307/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2005.07.003
868
S. Sivaprasad et al. / Engineering Failure Analysis 13 (2006) 867–875
1998 and 1999. In a span of 3 years from 1999 to 2001, the pin-hole formation and leakage problem was encountered thrice in the same horizontal segment. Recurrence of such failure increased the frequency of unplanned shut downs and upset the plant operation. This ammonia feed line operates at a pressure of 233 kg/cm2 and 149 °C temperature. Normally, high pressure air is added to pump discharge pulsation dampener at 50 to 60 NM3/h. Presence of moisture (oxygen) in the air could have changed the composition of the ammonia. There is also a possibility of water together with air remaining in the ammonia line after flushing the line with condensate water. However, interaction with the plant engineers ruled out such possibilities, as high pressure air is added to the pump pulsation dampener in the later stage and there was no appreciable change in the composition of the ammonia. No sagging was reported in the horizontal portion where the failure is stated to have occurred. In fact, this horizontal portion was adequately supported on either side. The cause of the failure must therefore be due to some other factor. Keeping these factors in mind, a failed portion of the pipe was brought to the laboratory to study the cause of the failure. The results presented in this paper are based on the investigation made on material properties and analysis of fracture surface.
2. Experimental 2.1. Visual examination A section of the failed pipe received for investigation is shown in Fig. 1. The pipe had a through thickness pin-hole. Fine longitudinal cracks of about 4–5 mm length were also found to have propagated from either side of the pin-hole. These cracks were visible by the naked eye on the outer surface, however, on the inside surface of the pipe the cracks could be seen only after polishing. On the outer surface in the vicinity of the pin-hole, a number of shallow pits were present (Fig. 1(b)). Fig. 2 shows the crack in both the longitudinal and transverse directions of the pipe. Outer surface of the pipe had mill scale, which appeared black in colour. However, the inner surface of the pipe was corrosion free and no pits were seen on the inside of the pipe. There was no reduction in the wall thickness of the pipe or any change in the circumference. 2.2. Chemical composition and microstructure A small piece of the pipe material was cut and the chemical analysis was carried out using direct reading spectrometer (DRS). The results of the chemical analysis are shown in Table 1. It may be noted that the chemical composition of the material conforms to the permissible limits of SA 106 Gr. B steel [1]. The optical microstructure of the pipe material, both in the longitudinal and transverse directions of the pipe was examined. Small pieces of the material cut from both the directions were polished and etched with 4% Nital. The microstructure of the pipe material in both the directions are shown in Fig. 3. This consists of ferrite and pearlite with mild degree of banding. 2.3. Mechanical properties The mechanical properties of the pipe material were evaluated in terms of hardness, tensile and Charpy impact studies. The bulk hardness on Vickers scale was measured at an applied load of 30 g. For tensile tests, round tensile specimens of 5 mm gauge diameter were prepared from the pipe material such that the loading axis is parallel to the longitudinal direction of the pipe. Tensile tests were carried out in a 100 kN servo-electric test system at a nominal constant displacement rate of 0.003 mm/s following the
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Fig. 1. Sections of pipe showing pits on the outer surface and through wall pin-hole. Photograph (b) is a close view of (a) where the pits growing from it may be seen.
Fig. 2. Cross-section of the pipe showing (a) longitudinal crack from pin-hole as seen on the outer surface (marked by arrow) and (b) hairline crack in the transverse direction.
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S. Sivaprasad et al. / Engineering Failure Analysis 13 (2006) 867–875
Table 1 Chemical composition of SA 106 Gr. B steel (wt%) C
S
P
Cr
Cu
Mn
Si
0.17
0.011
0.03
0.07
0.043
0.58
0.37
Fig. 3. Microstructure of the SA 106 Gr. B Steel in (a) transverse and (b) longitudinal directions.
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Fig. 4. Stress–strain behaviour of SA 106 Gr. B steel.
ASTM standard E 8M [2]. A 25 mm gauge length extensometer was fitted to the uniform gauge length portion of the specimen for strain measurements. Typical engineering stress–strain curve is shown in Fig. 4 and the mechanical properties are shown in Table 2. 2.4. Microscopic examination of fracture surface The damaged part of the pipe was cut and broke-opened in the liquid nitrogen. The matching fracture surfaces after break opening is shown in Fig. 5. These fracture surfaces were examined in scanning electron microscope (SEM). A typical SEM microphotograph of the pin-hole portion along with the propagated crack is shown in Fig. 6, small pits around the pin-hole are shown with arrows. The right-hand side of this figure corresponds to the outer surface of the pipe. The chemical composition of the corrosion products in Table 2 Mechanical properties of SA 106 Gr. B Steel Specimen No.
YS (MPa)
UTS (MPa)
% El
% RA
Average hardness (Hv30)
Average at 0 °C, CVN (J)
1 2
278.5 277.3
473 475.5
29.24 28.97
64.68 64.87
173
32
Fig. 5. Mating fracture surface of the failed pipe; the pin hole AB through the thickness may be noted.
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Fig. 6. Fractography of pit as seen in SEM.
and around the pin-hole were analysed under SEM using energy dispersion analysis of X-rays (EDAX). A typical X-ray spectrum of the chemical analysis at the mouth of the shallow pit is shown in Fig. 7 and the results of this microchemical analysis are listed in Table 3.
60
K Cl
50
Ca
Si S 40
Fe Na
30
20
Al
10
Ca Ca S
Cl
K
Ca
Fe keV
0 0
5
10
Fig. 7. EDAX analysis of corrosion products around the pit showing the presence of chloride.
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Table 3 EDAX analysis of various elements of corrosion product at the pit mouth Elt
X-ray
Int
Error
K
K ratio
W%
A%
Na Al Si S Cl K Ca Fe
Ka Ka Ka Ka Ka Ka Ka Ka
2.9 2.3 5.7 5.1 8.1 7.0 6.4 5.1
0.1859 0.1657 0.2586 0.2456 0.3089 0.2871 0.2753 0.2444
0.0400 0.0314 0.0796 0.0905 0.1572 0.1624 0.1643 0.2745
0.0323 0.0253 0.0642 0.0731 0.1269 0.1311 0.1326 0.2215
8.37 4.08 8.75 8.49 15.20 14.92 15.15 25.05
13.35 5.54 11.42 9.71 15.71 13.99 13.85 16.44
W%, weight percent; A%, atomic percent.
3. Results and discussion The results of the chemical analysis showed that the chemistry of the material conforms to that of the SA 106 Gr. B steel and within the permissible limits as specified in the standard [1]. The microstructure of the material is ferrite–pearlite which is a typical of this class of steels [3] and the material contained only fewer amounts of oxide/sulphide type of inclusions. There was no significant difference in the microstructures of pipe material in the longitudinal and transverse directions, although the longitudinal microstructure showed mild degree of banding. The mechanical properties of the material are also found to qualify the minimum requirement as mentioned in the specification for this variety of steels. Thus, it is clear that the pipe material is not deficient both in terms of chemistry and mechanical properties. The failure is therefore, not due to the material problem, but could be due to operational or environmental factors. However, from the SEM microphotograph shown in Fig. 6, it may be noted that the width of the pinhole on the outer surface of the pipe is more than at the interior. Moreover, a number of small shallow pits were found around this pin-hole (as shown by arrows in Fig. 6). Such shallow pits were also seen elsewhere on the outer surface of the pipe. The inside of the pipe, however, was free from any pits not withstanding some evidence of rusting. The failure therefore has initiated from the outside of the pipe. The outer surface of the pipe has suffered severe pitting with single pit taking a leading role to form a pin-hole. Pitting attack usually reported to take place due to the break down of the protective film on the metal surface [4]. In the present case, it appears that the pit has formed due to the break down of protective Fe2O3 mill scale that formed during hot rolling of the steel pipe. Small anodic sites are expected to have developed at these mill scale areas leading to nucleation of the pits. Once the pits have nucleated, it will propagate through anodic dissolution of the metal by its autocatalytic nature [4]. The pitting reaction is expected to have aggravated by the operating temperature of around 150 °C. There are various factors, such as inclusions, second phase particles, flaws, heat treatment/cold work, temperature and chemical species that influence the formation and growth of pits [5]. Particularly, the presence of chloride or chlorine containing ions are reported to be responsible for most pitting failures [4]. The results of the EDAX analysis on the fracture surfaces (Fig. 7 and Table 3) showed the presence of chloride ions at the mouth of the main and shallow pits. This provides strong evidence to the fact that the pit formation is due to the presence of chloride ions. The source of the chloride ion was first thought to have come from the condensate water used to wash the line after depressurisation. However, the condensate chemical composition used for washing the line did not contain such detrimental elements. Moreover, the pit formation had occurred on the outer surface of the pipe and the inner surface was corrosion free. It was, therefore, contended that chloride ions had come from the atmosphere. It is worth mentioning at this point that the concerned plant was located very near to the seacoast, and the environment around the plant is
874
S. Sivaprasad et al. / Engineering Failure Analysis 13 (2006) 867–875
Fig. 8. Intergranular fracture showing the evidence of hydrogen assisted cracking.
expected to have high moisture and chloride ions in air. Although the mechanism by which the chloride promotes pitting is not yet clear, perhaps the best explanation is the acid forming tendency of chloride salts and the high strength of its free acid [5]. It is shown that the presence of FeCl2 which is supposed to be present in corrosion pits on iron based alloys exhibits a pH of 3.8 in oxygen free condition while an excess of oxygen in the solution decreases its pH to 0.6 [5]. Such low pH of electrolyte containing chloride inside the pit is liable to accelerate the pit growth. It is believed that the presence of an adherent salt layer inside the pit covering the pit bottom provides a protective effect against pit growth [6]. In fact, indirect evidences for existence of such a protective layer inside the pits were provided by the results of a study on the effect of specimen position on the shape of corrosion pits [6]. It is demonstrated that in specimens with exposed surfaces facing upward, the in-depth growth of pit was slower compared to those with exposed surfaces facing downward. In the first case, salt layer was thicker and more protective than in the latter case. A similar analogy can be drawn to the present investigation, and this may be the reason for the formation of pin-hole always in the 5/7Õ OÕ clock position in the present case. The fracture surface examination of the crack adjacent to the main pit exhibited an intergranular fracture as depicted in Fig. 8. As discussed above, under the low pH condition inside the anodic pit due to the presence of chloride ions, the compensating cathodic reaction will be hydrogen reduction. The nascent hydrogen thus generated caused the brittle intergranular cracking as shown in Fig. 8. The presence of hoop stress and the notch effect produced by the pit are likely to aggravate the cracking tendency in the longitudinal direction which eventually had propagated through the thickness of the pipe.
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4. Conclusions On the basis of the laboratory investigations of the failed pipe, it is concluded that the failure of the high pressure ammonia line is due to the pitting corrosion initiated at the partially removed Fe2O3 scale on the outer surface of the pipe. The formation of the pit and its rapid growth had occurred due to the presence of chloride ions in the atmosphere. The nascent hydrogen produced at the local electrochemical cell, eventually caused the through thickness cracking of the pipe with the aid of hoop stress and the notch effect produced by the leading pit.
References [1] Metals hand book. Properties and selection-irons, steels and high temperature performance alloys, 10th ed., Materials Park (OH, USA): ASM International; 1990. p. 617. [2] ASTM Standard E-8M. Test methods for tension testing of metallic materials. Annual Book of ASTM Standards, vol. 03.01. PA: USA; 1994. p. 81. [3] Metals hand book. Atlas of microstructures of industrial alloys, 8th ed., Materials Park (OH, USA): ASM International; 1972. p. 27. [4] Fontana MG, Greene ND. Corrosion engineering, materials science & engineering series. McGraw Hill; 1978. p. 51. [5] Szklarska-Smialowska Z. Pitting corrosion of metals. Houston, (TX, USA): NACE Publications; 1986. p. 532. [6] Mankowski J, Szklarska-Smialowska Z. Corr Sci 1977;17:725.