- Email: [email protected]
Failure analysis: Chloride stress corrosion cracking of AISI 316 stainless steel downhole pressure memory gauge cover
Accepted Manuscript Failure analysis: Chloride stress corrosion cracking of AISI 316 stainless steel downhole pressure memory gauge cover S.M.R. Ziaei, J. Mostowfi, M. Golestani pour, S.A.R. Ziaei PII: DOI: Reference:
S1350-6307(13)00222-7 http://dx.doi.org/10.1016/j.engfailanal.2013.06.022 EFA 2089
To appear in:
Engineering Failure Analysis
Received Date: Revised Date: Accepted Date:
1 November 2012 20 June 2013 21 June 2013
Please cite this article as: Ziaei, S.M.R., Mostowfi, J., Golestani pour, M., Ziaei, S.A.R., Failure analysis: Chloride stress corrosion cracking of AISI 316 stainless steel downhole pressure memory gauge cover, Engineering Failure Analysis (2013), doi: http://dx.doi.org/10.1016/j.engfailanal.2013.06.022
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Failure analysis: Chloride stress corrosion cracking of AISI 316 stainless steel downhole pressure memory gauge cover
S. M. R. Ziaei1*, J. Mostowfi2, M. Golestani pour1, S. A. R. Ziaei1
1) Senior technical inspector, East Oil and Gas Production Company, Khangiran, Iran 2) Head of technical inspection department, East Oil and Gas Production Company, Khangiran, Iran
* Corresponding author: [email protected], [email protected]
Abstract The downhole pressure memory gauge cover which is the focal point of this failure case study was hung in the production tubing of a sour gas well at a depth of 3200 meters during a multiple rate productivity index test. Approximately 140 hours after start up, the tool cover failed. The memory gauge cover experienced repeated premature failures. One failed sample was investigated by chemical and microstructural analytical techniques to find out the failure cause and provide preventive measures. The gauge cover alloy was type AISI 316 stainless steel. During investigation many branched cracks were observed on the external surface of the gauge cover grown from the pits perpendicular to the cylinder axis. The results indicate that pressure memory gauge cover failed due to chloride stress corrosion cracking (CLSCC). Also according to slow strain rate test, it was evident that the best alternative for the downhole pressure memory gauge cover alloy is Nitronic 50 and the second option is 17-4 PH stainless steel.
Key words: 316 SS, pressure memory gauge, SCC, Scanning electron microscopy.
1. Introduction The downhole pressure memory gauge cover which is the focal point of this failure case study was hung in the production tubing of a sour gas well at a depth of 3200 meters during a multiple rate productivity index test. Table 1 shows working condition of pressure memory gauge cover in one of the deep sour gas wells in khangiran and indicates that the chloride and sulfur contents are in the level of 15000 ppm and 24000 ppm, respectively. The gauge cover was cylindrical with 415 mm length, 32 mm outside diameter and 3 mm wall thickness. Table 2 shows the exposure period of the gauge cover in the production tubing of the sour gas well. Approximately 140 hours after start up, the tool cover failed. The chemical composition and mechanical properties of the gauge cover alloy are shown in Table 3 and 4, indicating 316 SS stainless steel. The memory gauge cover experienced repeated premature failures.
2. Visual observation Fig. 1a shows photograph of memory gauge cover before exposure period however visual inspection of the external surface of the gauge cover after exposure period (140 hours) indicated accordance severe pitting corrosion and branched cracks as shown in Fig. 1b. 3. Experimental procedure In order to find out the corrosion extent along the thickness direction of the wall, the memory gauge tube was cut along the direction perpendicular to the crack and the metallographic section plane of the fracture was obtained. SEM observations were performed on the surface and the cross-sectional plane of the fracture to detect the failure mode. Also the gauge cover surface deposit was analyzed by energy dispersive spectroscopy (EDS).
4. Results and Discussion Fig. 2 shows the cracks that initiated at the outer surface of the gauge cover and penetrated up to about 2.5 mm deep in the thickness direction. The branches were normal to the main cracks (i.e.
axial to the cover tube). The random cracks, to which the description "craze cracks" could be applied, penetrated almost the full thickness of the cover alloy. Moreover, many branched cracks were observed on the external surface of the gauge cover grown from the pits perpendicular to the cylinder axis (Fig. 3). Scanning electron microscopy show the major crack and branched cracks on the transverse cross-section of the memory gauge cover in Fig. 3a and also indicates micrograph of pits on microstructure at the cross section of the gauge cover in Fig. 3b. Fig. 4 shows the surface EDS (Energy Dispersive X-Ray Spectrometry) spectrum inside the crack. Appreciable amount of chloride and sulfur were detected and high peaks of oxygen and carbon, etc. were also found. Scanning electron microscopy conducted on the transverse cross-section of the cracked portion (Figs. 5) shows that the cracks are apparently transgranular and indicates that the cross-section morphology showing branching of typical SCC. It appears that the cracks were initiated at the outer surface of the tube, and subsequently propagated inwardly. No sensitization has been observed in the microstructural examination. The absence of sensitization in the microstructures indicates the failure is not attributed to polythionic acid corrosion cracking due to sulfur [1]. Both general corrosion and pitting for stress concentration play an important role in SCC [2-5]. The fractographic examination made, which showed a great number of long branched cracks with mainly transgranular run. A big part of microcracks either initiated apparently on inclusions or corrosion pits connected with inclusions were present in their paths (Fig. 3). Cracks initiated on pits were mostly more branched. Surface structural inhomogeneities, either different microstructure zones in the 316 SS or pits and inclusions were priority initiation sites of microcracks [6]. From Fig. 3a, it can be seen that there are some small pits on the inner wall surface near the fracture surface. The partial enlarged view in mourning border of Fig. 3a is expressed in Fig. 3b. Fantechi [7] suggested that CLSCC propagation occurs when cracks grow more quickly from the pit than the rate of corrosion. The high corrosion resistance of stainless steels in most aqueous environments is due to passivation by a thin (~2nm) layer of chromium oxide [8,9]. Wet and humid environments containing chloride ions can cause pitting corrosion of stainless steel components. Components under an applied or residual stress can deteriorate further by stress corrosion cracking in these conditions. It is commonly accepted that CLSCC initiates from sites of active pits and therefore, cracks are considered to grow in the high chloride that develops at sites of localized corrosion [10]. Garcia
[11] suggested that CLSCC only occurs when a crack grows more quickly than the rate of metal removal by localized corrosion from the base of a pit. From the results above, the failure of the gauge cover can be considered to undergo three processes. First, some local pits were formed on the cover surface and became the stress concentration sources. Then, with the coactions between the external stresses (gas pressure of 3200 psi) and aggressive medium of the sour gas well, the chloride SCC was initiated from these small pits. Last, once this kind of transgranular cracks penetrated the full thickness of gauge cover, failure of the cover has occurred.
5. Conclusions The pressure memory gauge cover failed due to the phenomenon of chloride stress corrosion cracking (CLSCC). 6. Alternative alloys Upon distinguishing the reason for tool’s cover failure, i.e., chloride stress corrosion cracking, an attempt is made to propose a suitable material for the gauge cover. AISI 316 SS proved not to be a suitable material for sour gas service with high percentage of chloride present. According to NACE MR0175 [12], the supposed materials with higher stress cracking resistance are the 17-4 PH (UNS 17400) and Nitronic 50 (UNS S20910) and also solid-solution nickelbased alloys or precipitation-hardened nickel-based alloys and also cobalt-based alloys for any combination of temperature, hydrogen sulfide, chloride concentration and in situ PH in production environments which, of course, the last two options are much more expensive. Therefore to find an alternative, with regard to cost of materials, samples of type 17-4 PH and Nitronic 50 stainless steel were tested according to ASTM-129 to determine their CLSCC resistance. The chemical composition of 17-4 PH and Nitronic 50 stainless steel are shown in Table 5.
7. Slow strain rate test (SSRT) Slow strain rate testing [13] rigs used for the present study were equipped with autoclaves for conducting tests using chloride solutions at high temperatures and pressures per ASTM-129. The
strain was applied through the application of a slow constant extension rate in the range of 10−7 s−1. SSRT specimens (316 SS, 17-4 PH and Nitronic 50) were machined. Uniaxial cylindrical tensile specimens of 8.0 ± 0.2 mm outer diameter (OD) with a gauge section of 3.0 ± 0.2 mm OD and 20.0 ± 0.2 mm length were used. The gauge section was polished in the longitudinal direction progressively to a 1500 grit finished. A layer of Teflon tape was applied on the specimens except for their gauge sections. The specimen was immediately mounted in the autoclave which was then filled with test solution at 155.0 6 1.0°C. While 35 wt.% MgCl2 (~2.60 × 10-5 ppm Cl–) constituted the primary test solution. While immersed in the test solution and subjected to straining at a constant rate, tensile stress was measured using a load cell and recorded every 10 min by a data-taker. All tests were conducted under open circuit potential conditions and the tensile specimen insulated from the grips and autoclave body by using appropriate ceramic insulators. Fractographic examinations of specimens after completion of SSRT tests were carried out using scanning electron microscopy (SEM) in order to characterize the presence of features of CLSCC.
8. Results Fig. 6 presents the force vs. time curves of 316 SS, 17-4 PH and Nitronic 50 tensile specimens tested at 155 °C at 30% of the full motor speeds, which correspond to the strain rates of 3.7×10–7. It is evident from Fig. 6 that time to failure (tf) is considerably lower for 316 SS in comparison to that 17-4 PH and Nitronic 50. In the case of 17-4 PH and Nitronic 50 specimens, failure time increased considerably to approximately 64h and 67h, respectively. Also, max load increased to 5800N and 5300N that is considerably higher than max load for 316 SS specimen (4600N). The SSR test is an evaluative method and can provide information that is useful in selecting the best alloy to apply in lethal corrosion environments.
Comparison of the anodic polarization curves (Fig. 7) clearly shows a considerable improvement in pitting resistance as a result of substituent of cover alloy with 17-4 PH or Nitronic 50 (as evident from the shift in the pitting potential). The most pitting resistance is seen to be for the Nitronic 50 stainless steel. Also, in the case of 17-4 PH in the chloride solution, the passivation characteristics appear to begin to be disturbed, whereas for the 316 SS, the passivation characteristics are considerably disturbed (Fig. 7). It is evident from Fig. 6 and Fig. 7 that the best alternative for the downhole pressure memory gauge cover alloy is Nitronic 50 and the second option is 17-4 PH stainless steel.
Acknowledgements The authors wish to thank East Oil and Gas Production Company (EOGPC) and Shahid Hasheminezhad Gas Processing Company (SGPC) for financial support of this research. We would also like to express our sincere thanks to the Research Council of Ferdowsi University of Mashhad for supporting this work.
References [1] A. A. Yazgi, D. Hardie, "Stress corrosion cracking of super duplex stainless steel in sour environments", corrosion science 40 (1998) 909-930. [2] M. C. Zhao, M. Liu, A. Atrens, "Effect of applied stress and microstructure on stress corrosion cracking resistance of pipeline steels subject to hydrogen sulfide", Materials Science and Engineering A 478 (2008) 43–47. [3] A. Fragiel, S. Serna, R. Perez, "Electrochemical study of two microalloyed pipeline steels in H2S environments", International Journal of Hydrogen Energy 30 (2005) 1303 – 1309. [4] X. Cheng, S. C. Niu, "Electrochemical behaviour of chromium in acid solutions with H2S", Corrosion Science 41 (1999) 773-788. [5] J. Wang, A. Atrens, "Analysis of service stress corrosion cracking in a natural gas transmission pipeline, active or dormant? ", Engineering failure analysis 11 (2004) 3–18. [6] S. Ghosh, V. Kain, "Microstructural changes in AISI 316L stainless steel due to surface machining: Effect on its susceptibility to chloride stress corrosion cracking", Journal of Nuclear Materials 403 (2010) 62–67.
[7] F. Fantechi, M. Innocenti, "Chloride stress corrosion cracking of precipitation hardening S.S. impellers in centrifugal compressor. Laboratory investing ations and corrective actions", Engineering Failure Analysis 8 (2001) 477-492. [8] M. Rogante, P. Battistella, F. Cesari, "Hydrogen interaction and stress-corrosion in hydrocarbon storage vessel and pipeline weldings", International Journal of Hydrogen Energy 31 (2006) 597 – 601. [9] R. K. Singh Raman, W. H. Siew, "Role of nitrite addition in chloride stress corrosion cracking of a super duplex stainless steel", Corrosion Science 52 (2010) 113–117. [10] R. C. Yin, A. H. Al-Shawaf, W. Al-Harbi, "Chloride-induced stress corrosion cracking of furnace burner tubes", Engineering Failure Analysis 14 (2007) 36–40. [11] C. Garcia, F. Martin, "Effects of prior cold work and sensitization heat treatment on chloride stress corrosion cracking in type 304 stainless steels", Corrosion Science 43 (2001) 1519-1539. [12] NACE International Standard MR0175, Houston, 2010. [13] ASTM-129, Standard Practice for Slow Strain Rate Testing to Evaluate the Susceptibility of Metallic Materials to Environmentally Assisted Cracking, 2008.
Table 1 Working condition of pressure memory gauge. Setting depth Maximum Downhole pressure Maximum Downhole temperature Natural gas H2S content Natural gas CO2 content Chloride content of product water Duration of the test
3200 meters from surface 5170 psi 130 oC 24000 ppm 2.4 % 15000 ppm 140 Hours
Table 2 Exposure period of pressure memory gauge. a. Well in closed condition:
Holding gauge in final depth of well for 18 hours.
b. Well in open condition:
0.38 million m3 per day flow for 16 hours. 0.70 million m3 per day flow for 16 hours. 0.98 million m3 per day flow for 16 hours.
c. Well in closed condition:
Holding gauge in final depth of well for 74 hours.
Table 3 Chemical composition of memory gauge cover alloy (wt%).
C
Si
S
P
Mn
Ni
Cr
Mo
Fe
0.08
0.55
0.03
0.04
1.91
13.56
17.45
2.21
Bal.
Table 4 Mechanical properties of cover alloy. UTS (MPa)
YS0.2% (MPa)
E(%)
Hardness (HB)
515
205
31
201
Table 5 Chemical composition of 17-4 PH and Nitronic 50.
17-4 PH Nitronic 50
C
Si
S
P
Mn
Ni
Cr
Mo
Cu
Nb
V
Fe
0.04
0.10
0.04
0.03
0.7
4.55
16.5
-
3.30
0.33
-
Bal.
0.035
0.47
0.02
0.24
3.69
13.45
21.45
2.29
0.46
0.21
0.22
Bal.
Fig. 1. Photograph of 316 SS memory gauge cover (a) before exposure period and (b) branched cracks and pits on the external surface, 140 hours after exposure period to lethal environment in a sour gas well.
Fig. 2. SEM micrographs showing the branched cracks on the transverse cross-section of the memory gauge cover, originated from the outer surface.
Fig. 3. SEM micrograph showing (a) branching of typical chloride stress corrosion cracking near the outer surface and (b) the initiation of CSCC cracks from pits.
Fig. 4. EDS microanalysis of inside the CSSC cracks.
Fig. 5. SEM fractograph of 316 SS memory gauge cover surface showing (a) cracks on external surface and (b) transgranular chloride stress corrosion cracking.
Fig. 6. Force vs. time SSRT curves for 316 SS, 17-4 PH, Nitronic 50, in 30 wt. % MgCl2 at 155.0 6 1.0°C.
Fig. 7. Anodic polarization curves of 316 SS, 17-4 PH and Nitronic 50 stainless steel in 0.1 M NaCl at 100 °C.
SEM observations were performed on the surface and the cross-sectional plane of the fracture to detect the failure mode.
Pressure memory gauge cover failed due to chloride stress corrosion cracking (CLSCC).
Slow strain rate testing rigs were equipped with autoclaves for conducting tests at high temperatures and pressures.
Anodic polarization curves clearly shows a considerable improvement in pitting resistance.
The most pitting resistance is seen to be for the Nitronic 50 stainless steel.
S1350-6307(13)00222-7 http://dx.doi.org/10.1016/j.engfailanal.2013.06.022 EFA 2089
To appear in:
Engineering Failure Analysis
Received Date: Revised Date: Accepted Date:
1 November 2012 20 June 2013 21 June 2013
Please cite this article as: Ziaei, S.M.R., Mostowfi, J., Golestani pour, M., Ziaei, S.A.R., Failure analysis: Chloride stress corrosion cracking of AISI 316 stainless steel downhole pressure memory gauge cover, Engineering Failure Analysis (2013), doi: http://dx.doi.org/10.1016/j.engfailanal.2013.06.022
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Failure analysis: Chloride stress corrosion cracking of AISI 316 stainless steel downhole pressure memory gauge cover
S. M. R. Ziaei1*, J. Mostowfi2, M. Golestani pour1, S. A. R. Ziaei1
1) Senior technical inspector, East Oil and Gas Production Company, Khangiran, Iran 2) Head of technical inspection department, East Oil and Gas Production Company, Khangiran, Iran
* Corresponding author: [email protected], [email protected]
Abstract The downhole pressure memory gauge cover which is the focal point of this failure case study was hung in the production tubing of a sour gas well at a depth of 3200 meters during a multiple rate productivity index test. Approximately 140 hours after start up, the tool cover failed. The memory gauge cover experienced repeated premature failures. One failed sample was investigated by chemical and microstructural analytical techniques to find out the failure cause and provide preventive measures. The gauge cover alloy was type AISI 316 stainless steel. During investigation many branched cracks were observed on the external surface of the gauge cover grown from the pits perpendicular to the cylinder axis. The results indicate that pressure memory gauge cover failed due to chloride stress corrosion cracking (CLSCC). Also according to slow strain rate test, it was evident that the best alternative for the downhole pressure memory gauge cover alloy is Nitronic 50 and the second option is 17-4 PH stainless steel.
Key words: 316 SS, pressure memory gauge, SCC, Scanning electron microscopy.
1. Introduction The downhole pressure memory gauge cover which is the focal point of this failure case study was hung in the production tubing of a sour gas well at a depth of 3200 meters during a multiple rate productivity index test. Table 1 shows working condition of pressure memory gauge cover in one of the deep sour gas wells in khangiran and indicates that the chloride and sulfur contents are in the level of 15000 ppm and 24000 ppm, respectively. The gauge cover was cylindrical with 415 mm length, 32 mm outside diameter and 3 mm wall thickness. Table 2 shows the exposure period of the gauge cover in the production tubing of the sour gas well. Approximately 140 hours after start up, the tool cover failed. The chemical composition and mechanical properties of the gauge cover alloy are shown in Table 3 and 4, indicating 316 SS stainless steel. The memory gauge cover experienced repeated premature failures.
2. Visual observation Fig. 1a shows photograph of memory gauge cover before exposure period however visual inspection of the external surface of the gauge cover after exposure period (140 hours) indicated accordance severe pitting corrosion and branched cracks as shown in Fig. 1b. 3. Experimental procedure In order to find out the corrosion extent along the thickness direction of the wall, the memory gauge tube was cut along the direction perpendicular to the crack and the metallographic section plane of the fracture was obtained. SEM observations were performed on the surface and the cross-sectional plane of the fracture to detect the failure mode. Also the gauge cover surface deposit was analyzed by energy dispersive spectroscopy (EDS).
4. Results and Discussion Fig. 2 shows the cracks that initiated at the outer surface of the gauge cover and penetrated up to about 2.5 mm deep in the thickness direction. The branches were normal to the main cracks (i.e.
axial to the cover tube). The random cracks, to which the description "craze cracks" could be applied, penetrated almost the full thickness of the cover alloy. Moreover, many branched cracks were observed on the external surface of the gauge cover grown from the pits perpendicular to the cylinder axis (Fig. 3). Scanning electron microscopy show the major crack and branched cracks on the transverse cross-section of the memory gauge cover in Fig. 3a and also indicates micrograph of pits on microstructure at the cross section of the gauge cover in Fig. 3b. Fig. 4 shows the surface EDS (Energy Dispersive X-Ray Spectrometry) spectrum inside the crack. Appreciable amount of chloride and sulfur were detected and high peaks of oxygen and carbon, etc. were also found. Scanning electron microscopy conducted on the transverse cross-section of the cracked portion (Figs. 5) shows that the cracks are apparently transgranular and indicates that the cross-section morphology showing branching of typical SCC. It appears that the cracks were initiated at the outer surface of the tube, and subsequently propagated inwardly. No sensitization has been observed in the microstructural examination. The absence of sensitization in the microstructures indicates the failure is not attributed to polythionic acid corrosion cracking due to sulfur [1]. Both general corrosion and pitting for stress concentration play an important role in SCC [2-5]. The fractographic examination made, which showed a great number of long branched cracks with mainly transgranular run. A big part of microcracks either initiated apparently on inclusions or corrosion pits connected with inclusions were present in their paths (Fig. 3). Cracks initiated on pits were mostly more branched. Surface structural inhomogeneities, either different microstructure zones in the 316 SS or pits and inclusions were priority initiation sites of microcracks [6]. From Fig. 3a, it can be seen that there are some small pits on the inner wall surface near the fracture surface. The partial enlarged view in mourning border of Fig. 3a is expressed in Fig. 3b. Fantechi [7] suggested that CLSCC propagation occurs when cracks grow more quickly from the pit than the rate of corrosion. The high corrosion resistance of stainless steels in most aqueous environments is due to passivation by a thin (~2nm) layer of chromium oxide [8,9]. Wet and humid environments containing chloride ions can cause pitting corrosion of stainless steel components. Components under an applied or residual stress can deteriorate further by stress corrosion cracking in these conditions. It is commonly accepted that CLSCC initiates from sites of active pits and therefore, cracks are considered to grow in the high chloride that develops at sites of localized corrosion [10]. Garcia
[11] suggested that CLSCC only occurs when a crack grows more quickly than the rate of metal removal by localized corrosion from the base of a pit. From the results above, the failure of the gauge cover can be considered to undergo three processes. First, some local pits were formed on the cover surface and became the stress concentration sources. Then, with the coactions between the external stresses (gas pressure of 3200 psi) and aggressive medium of the sour gas well, the chloride SCC was initiated from these small pits. Last, once this kind of transgranular cracks penetrated the full thickness of gauge cover, failure of the cover has occurred.
5. Conclusions The pressure memory gauge cover failed due to the phenomenon of chloride stress corrosion cracking (CLSCC). 6. Alternative alloys Upon distinguishing the reason for tool’s cover failure, i.e., chloride stress corrosion cracking, an attempt is made to propose a suitable material for the gauge cover. AISI 316 SS proved not to be a suitable material for sour gas service with high percentage of chloride present. According to NACE MR0175 [12], the supposed materials with higher stress cracking resistance are the 17-4 PH (UNS 17400) and Nitronic 50 (UNS S20910) and also solid-solution nickelbased alloys or precipitation-hardened nickel-based alloys and also cobalt-based alloys for any combination of temperature, hydrogen sulfide, chloride concentration and in situ PH in production environments which, of course, the last two options are much more expensive. Therefore to find an alternative, with regard to cost of materials, samples of type 17-4 PH and Nitronic 50 stainless steel were tested according to ASTM-129 to determine their CLSCC resistance. The chemical composition of 17-4 PH and Nitronic 50 stainless steel are shown in Table 5.
7. Slow strain rate test (SSRT) Slow strain rate testing [13] rigs used for the present study were equipped with autoclaves for conducting tests using chloride solutions at high temperatures and pressures per ASTM-129. The
strain was applied through the application of a slow constant extension rate in the range of 10−7 s−1. SSRT specimens (316 SS, 17-4 PH and Nitronic 50) were machined. Uniaxial cylindrical tensile specimens of 8.0 ± 0.2 mm outer diameter (OD) with a gauge section of 3.0 ± 0.2 mm OD and 20.0 ± 0.2 mm length were used. The gauge section was polished in the longitudinal direction progressively to a 1500 grit finished. A layer of Teflon tape was applied on the specimens except for their gauge sections. The specimen was immediately mounted in the autoclave which was then filled with test solution at 155.0 6 1.0°C. While 35 wt.% MgCl2 (~2.60 × 10-5 ppm Cl–) constituted the primary test solution. While immersed in the test solution and subjected to straining at a constant rate, tensile stress was measured using a load cell and recorded every 10 min by a data-taker. All tests were conducted under open circuit potential conditions and the tensile specimen insulated from the grips and autoclave body by using appropriate ceramic insulators. Fractographic examinations of specimens after completion of SSRT tests were carried out using scanning electron microscopy (SEM) in order to characterize the presence of features of CLSCC.
8. Results Fig. 6 presents the force vs. time curves of 316 SS, 17-4 PH and Nitronic 50 tensile specimens tested at 155 °C at 30% of the full motor speeds, which correspond to the strain rates of 3.7×10–7. It is evident from Fig. 6 that time to failure (tf) is considerably lower for 316 SS in comparison to that 17-4 PH and Nitronic 50. In the case of 17-4 PH and Nitronic 50 specimens, failure time increased considerably to approximately 64h and 67h, respectively. Also, max load increased to 5800N and 5300N that is considerably higher than max load for 316 SS specimen (4600N). The SSR test is an evaluative method and can provide information that is useful in selecting the best alloy to apply in lethal corrosion environments.
Comparison of the anodic polarization curves (Fig. 7) clearly shows a considerable improvement in pitting resistance as a result of substituent of cover alloy with 17-4 PH or Nitronic 50 (as evident from the shift in the pitting potential). The most pitting resistance is seen to be for the Nitronic 50 stainless steel. Also, in the case of 17-4 PH in the chloride solution, the passivation characteristics appear to begin to be disturbed, whereas for the 316 SS, the passivation characteristics are considerably disturbed (Fig. 7). It is evident from Fig. 6 and Fig. 7 that the best alternative for the downhole pressure memory gauge cover alloy is Nitronic 50 and the second option is 17-4 PH stainless steel.
Acknowledgements The authors wish to thank East Oil and Gas Production Company (EOGPC) and Shahid Hasheminezhad Gas Processing Company (SGPC) for financial support of this research. We would also like to express our sincere thanks to the Research Council of Ferdowsi University of Mashhad for supporting this work.
References [1] A. A. Yazgi, D. Hardie, "Stress corrosion cracking of super duplex stainless steel in sour environments", corrosion science 40 (1998) 909-930. [2] M. C. Zhao, M. Liu, A. Atrens, "Effect of applied stress and microstructure on stress corrosion cracking resistance of pipeline steels subject to hydrogen sulfide", Materials Science and Engineering A 478 (2008) 43–47. [3] A. Fragiel, S. Serna, R. Perez, "Electrochemical study of two microalloyed pipeline steels in H2S environments", International Journal of Hydrogen Energy 30 (2005) 1303 – 1309. [4] X. Cheng, S. C. Niu, "Electrochemical behaviour of chromium in acid solutions with H2S", Corrosion Science 41 (1999) 773-788. [5] J. Wang, A. Atrens, "Analysis of service stress corrosion cracking in a natural gas transmission pipeline, active or dormant? ", Engineering failure analysis 11 (2004) 3–18. [6] S. Ghosh, V. Kain, "Microstructural changes in AISI 316L stainless steel due to surface machining: Effect on its susceptibility to chloride stress corrosion cracking", Journal of Nuclear Materials 403 (2010) 62–67.
[7] F. Fantechi, M. Innocenti, "Chloride stress corrosion cracking of precipitation hardening S.S. impellers in centrifugal compressor. Laboratory investing ations and corrective actions", Engineering Failure Analysis 8 (2001) 477-492. [8] M. Rogante, P. Battistella, F. Cesari, "Hydrogen interaction and stress-corrosion in hydrocarbon storage vessel and pipeline weldings", International Journal of Hydrogen Energy 31 (2006) 597 – 601. [9] R. K. Singh Raman, W. H. Siew, "Role of nitrite addition in chloride stress corrosion cracking of a super duplex stainless steel", Corrosion Science 52 (2010) 113–117. [10] R. C. Yin, A. H. Al-Shawaf, W. Al-Harbi, "Chloride-induced stress corrosion cracking of furnace burner tubes", Engineering Failure Analysis 14 (2007) 36–40. [11] C. Garcia, F. Martin, "Effects of prior cold work and sensitization heat treatment on chloride stress corrosion cracking in type 304 stainless steels", Corrosion Science 43 (2001) 1519-1539. [12] NACE International Standard MR0175, Houston, 2010. [13] ASTM-129, Standard Practice for Slow Strain Rate Testing to Evaluate the Susceptibility of Metallic Materials to Environmentally Assisted Cracking, 2008.
Table 1 Working condition of pressure memory gauge. Setting depth Maximum Downhole pressure Maximum Downhole temperature Natural gas H2S content Natural gas CO2 content Chloride content of product water Duration of the test
3200 meters from surface 5170 psi 130 oC 24000 ppm 2.4 % 15000 ppm 140 Hours
Table 2 Exposure period of pressure memory gauge. a. Well in closed condition:
Holding gauge in final depth of well for 18 hours.
b. Well in open condition:
0.38 million m3 per day flow for 16 hours. 0.70 million m3 per day flow for 16 hours. 0.98 million m3 per day flow for 16 hours.
c. Well in closed condition:
Holding gauge in final depth of well for 74 hours.
Table 3 Chemical composition of memory gauge cover alloy (wt%).
C
Si
S
P
Mn
Ni
Cr
Mo
Fe
0.08
0.55
0.03
0.04
1.91
13.56
17.45
2.21
Bal.
Table 4 Mechanical properties of cover alloy. UTS (MPa)
YS0.2% (MPa)
E(%)
Hardness (HB)
515
205
31
201
Table 5 Chemical composition of 17-4 PH and Nitronic 50.
17-4 PH Nitronic 50
C
Si
S
P
Mn
Ni
Cr
Mo
Cu
Nb
V
Fe
0.04
0.10
0.04
0.03
0.7
4.55
16.5
-
3.30
0.33
-
Bal.
0.035
0.47
0.02
0.24
3.69
13.45
21.45
2.29
0.46
0.21
0.22
Bal.
Fig. 1. Photograph of 316 SS memory gauge cover (a) before exposure period and (b) branched cracks and pits on the external surface, 140 hours after exposure period to lethal environment in a sour gas well.
Fig. 2. SEM micrographs showing the branched cracks on the transverse cross-section of the memory gauge cover, originated from the outer surface.
Fig. 3. SEM micrograph showing (a) branching of typical chloride stress corrosion cracking near the outer surface and (b) the initiation of CSCC cracks from pits.
Fig. 4. EDS microanalysis of inside the CSSC cracks.
Fig. 5. SEM fractograph of 316 SS memory gauge cover surface showing (a) cracks on external surface and (b) transgranular chloride stress corrosion cracking.
Fig. 6. Force vs. time SSRT curves for 316 SS, 17-4 PH, Nitronic 50, in 30 wt. % MgCl2 at 155.0 6 1.0°C.
Fig. 7. Anodic polarization curves of 316 SS, 17-4 PH and Nitronic 50 stainless steel in 0.1 M NaCl at 100 °C.
SEM observations were performed on the surface and the cross-sectional plane of the fracture to detect the failure mode.
Pressure memory gauge cover failed due to chloride stress corrosion cracking (CLSCC).
Slow strain rate testing rigs were equipped with autoclaves for conducting tests at high temperatures and pressures.
Anodic polarization curves clearly shows a considerable improvement in pitting resistance.
The most pitting resistance is seen to be for the Nitronic 50 stainless steel.