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Premature corrosion failure of a 316L stainless steel plate due to the presence of sigma phase
Engineering Failure Analysis 16 (2009) 699–704
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Premature corrosion failure of a 316L stainless steel plate due to the presence of sigma phase O. Conejero *, M. Palacios, S. Rivera Fundacion ITMA-Centro del Acero, P.E.P.A. C/ Calafates, s/n, 33417 Aviles, Asturias, Spain
a r t i c l e
i n f o
Article history: Received 19 March 2008 Accepted 4 June 2008 Available online 1 July 2008 Keywords: Sigma phase Corrosion
a b s t r a c t A 316L austenitic stainless steel plate, exposed to a sulphuric acid environment, has suffered an abnormal corrosion attack. The metallographic examination demonstrated a large amount of sigma phase that was related to the failure. Sigma phase, which is very common in duplex stainless steels, removes chromium and molybdenum from the metal matrix, damaging the corrosion performance of the steel. In this case, the sigma phase was found in an austenitic stainless steel, producing a significant corrosion attack. The sigma phase was formed during the fabrication process of the steel. Ó 2009 Published by Elsevier Ltd.
1. Introduction Several parts of a stainless steel plate, located in one of the electrolysis buildings of a zinc producer company, showed an intensive corrosion attack after one year of service. The plate was made of 316L type austenitic stainless steel. The environment in where the plate was situated was rich in sulphuric acid, from the electrolytic process. The plate was partially immersed in the acid bath (10% sulphuric acid), at a temperature of about 40 °C. Fig. 1 shows one part of the plate where the severe attack can be seen. The plate was electrically insulated and no electric current circulates inside it. 2. Chemical analysis In order to check the correct chemical composition of the stainless steel, a sample of the plate was extracted and analyzed by atomic emission spectrocopy. The results obtained are shown in Table 1. The results obtained show that the chemical composition of the plate match the AISI 316L specification of the material. 3. Metallographic and microscopic examinations Some samples were cut from the cross section of some parts of the plate. The samples were mounted in a resin block and studied in the as-polished and etched condition by means of an optical microscope. Some other samples were extracted from the corroded surface to be studied directly at high magnification by means of a scanning electron microscope (SEM). The samples were also chemically analyzed by means of electron dispersive spectroscopy (EDS) in the SEM. The outer aspect of the corroded surfaces had a sponge-like feature, as it can be seen in Fig. 2. The same feature is shown at higher magnification in Fig. 3, where a kind of dendritic structure can be seen. A chemical analysis of that ‘‘dendrites” is * Corresponding author. Tel.: +34 985129120; fax: +34 985129008. E-mail address: [email protected] (O. Conejero). 1350-6307/$ - see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.engfailanal.2008.06.022
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O. Conejero et al. / Engineering Failure Analysis 16 (2009) 699–704
Fig. 1. Plate plug.
Table 1 Chemical composition (wt%)
AISI 316L Sample
C
Mn
Si
P
S
Cr
Ni
Mo
0.03 max 0.02
2.0 max 1.7
1.0 max 0.5
0.045 max 0.03
0.03 max 0.03
16–18 17.1
10–14 11.6
2–3 2.1
Fig. 2. Corroded surface.
also presented in Fig. 3 and Table 2, displaying a chromium- and molybdenum-rich and nickel-poor area, with respect to the whole metal composition (AISI 316L). The metallographic examination displayed a big amount of intermetallic phases, as shown in Figs. 4 and 5. The last one reveals the corrosion progress in lines along the rolling direction, following the intermetallic phases, leaving a thread-like structure. The intermetallic phase can be seen at higher magnification in Fig. 6, where it was identified as d-ferrite partially transformed into sigma phase. This fact was demonstrated by etching with potassium ferricyanide that reveals the d-ferrite in a brownish colour, whereas the sigma phase appears in a greenish colour. Fig. 7 shows the result of this examination, where sigma phase and delta ferrite appeared in all the microstructure. The chemical composition of the intermetallic phase was analyzed by means of EDS in a scanning electron microscope. Fig. 8 and Table 3 show some of the results obtained, where the Mo and Cr enrichment can be observed in the intermetallic phase (Spectrum 1), as compared to the metal composition (Spectrum 2).
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O. Conejero et al. / Engineering Failure Analysis 16 (2009) 699–704
Fig. 3. Surface examination.
Table 2 Chemical composition (wt%) in Fig. 3
Spectrum 1
Si
Cr
Fe
Ni
Mo
1.64
23.39
63.80
5.88
5.29
Fig. 4. Intermetallic precipitates.
4. Discussion Austenitic stainless steels have good combination of mechanical strength, fabricability and general corrosion resistance, and hence are extensively used as construction material in many highly demanding environments. The effective use of these alloys in a corrosive environment depends on the proper behaviour of the material. Nevertheless, there are several factors affecting the anti-corrosive behaviour of the stainless steels such as microsegregation, secondary phases precipitation and recrystallization. Among the different phases that can occur in the microstructure of a stainless steel, intermetallic phases (sigma, chi and Laves), and carbides are the two most frequently encountered. Sigma phase is probably the most studied and undesirable intermetallic phase in a stainless steel. Precipitation of sigma phase can occur in the austenitic, ferritic and duplex stainless steels. The precipitation of that phase not only causes losses in
702
O. Conejero et al. / Engineering Failure Analysis 16 (2009) 699–704
Fig. 5. Intermetallic precipitates.
Fig. 6. Sigma phase and d-ferrite.
Fig. 7. d-Ferrite partially transformed into sigma phase.
ductility and toughness of the steel, but also reduces its corrosion resistance by removing chromium and molybdenum from the austenitic matrix [1]. Typically, the sigma phase precipitates between 600 and 1000 °C [2], although this range varies somewhat with composition and processing. The shortest time for formation generally occurs between about 700 and 810 °C, and the temperature that produces the greatest amount of sigma phase with time is usually somewhat lower [3].
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O. Conejero et al. / Engineering Failure Analysis 16 (2009) 699–704
Fig. 8. Chemical analysis of the intermetallic phase.
Table 3 Chemical composition (wt%) in Fig. 8
Spectrum 1 Spectrum 2
Si
Cr
Mn
Fe
Ni
Mo
0.72 0.65
25.59 18.64
1.30 1.31
63.98 67.63
3.88 9.27
4.53 2.50
The sigma phase formed in austenitic stainless steels is a hard brittle intermetallic phase of nominal composition FeCr, although the chemical composition of sigma phase extracted from type 316 steel has been determined as Fe 44%, Cr 29.2%, Mo 8.3% and C < 0.1% all by weight [4]. Since the chromium content of sigma phase is about twice the content of the matrix, formation of this phase requires redistribution of alloying elements by substitutional diffusion [5] from the adjacent areas [6]. Molybdenum and silicon promote the formation of sigma phase, and the chromium content also favours its formation. Small amounts of nickel and manganese increase the rate of sigma phase formation, but large amounts, which stabilize austenite, retard its formation. Carbon additions decrease sigma phase formation by forming chromium carbides, thereby reducing the amount of chromium in solid solution. Additions of tungsten, vanadium, titanium and niobium also promote sigma phase formation [3,4]. The tendency to sigma phase formation of an austenitic stainless steel can be known from the equivalent chromium content formula [7]:
ECC ¼ %Cr þ 0:31% Mn þ 1:76% Mo þ 0:97% W þ 2:02% V þ 1:58% Si þ 2:44% Ti þ 1:7% Nb þ 1:22% Ta 0:266% Ni 0:177% Co:
If the equivalent Cr content (ECC) is greater than 17–18 wt%, the steel is susceptible to sigma phase formation. In this case, the steel has an ECC of 19, being thus susceptible to form that phase. Sigma phase nucleates mainly at grain boundaries, and is found in 316L type stainless steels approximately in 100 h at 800 °C [8]. In fully austenitic alloys, sigma phase forms from the austenite along grain boundaries. If d-ferrite is present in the austenitic alloy, sigma formation is more rapid and occurs in the d-ferrite [3]. The d-ferrite forms in austenitic stainless steels above 1100 °C [9]. Stainless steels need to be solution annealed and water quenched in order to put in solution all the different phases that they can contain and to homogenize their structure. A bad production or fabrication process can lead to a deleterious microstructure due to thermal effects that result in formation of deleterious phases such as sigma phase. In this case, the plate was not subjected to any welding process or to high service temperatures, so the sigma phase had to be formed during the production process of the steel.
704
O. Conejero et al. / Engineering Failure Analysis 16 (2009) 699–704
5. Conclusions A 316L stainless steel plate subjected to a sulphuric acid environment has failed due to the presence of sigma phase. The sigma phase has been demonstrated to be deleterious to the corrosion performance of the stainless steels by removing chromium and molybdenum from the austenitic matrix. Sigma phase precipitates at temperatures between 600 and 1000 °C, and it was formed in this case during the production process of the steel, as the plate was not subjected to any welding process or to high service temperatures. Acknowledgement The authors thank to Asturiana de Zinc S.A. for allowing the publication of the data employed in this article. References [1] Villanueva DME, Junior FCP, Plaut RL, Padilha AF. Comparative study on sigma phase precipitation of three types of stainless steels: austenitic, superferritic and duplex. Mater Sci Technol 2006;22(9):1098–104. [2] Pohl M, Storz O, Glogowski T. Effect of intermetallic precipitations on the properties of duplex stainless steel. Mater Charact 2007;58(1):65–71. [3] Davis JR. Stainless steels (ASM Specialty Handbook). ASM International; 1995. [4] Marshall P. Austenitic Stainless Steels: Microstructure and Mechanical Properties. Springer; 1984. [5] Schwind M, Kallqvist J, Nilsson JO, Agren J, Andren HO. Sigma-phase precipitation in stabilized austenitic stainless steels. Acta Mater 2000;48(10):2473–81. [6] Korb LJ. Corrosion. ASM International; 1987. [7] Khatak HS, Raj B. Corrosion of austenitic stainless steels: mechanism mitigation and monitoring. Woodhead publishing; 2002. [8] Aydogdu GH, Aydinol MK. Determination of susceptibility to intergranular corrosion and electrochemical reactivation behaviour of AISI 316L type stainless steel. Corros Sci 2006;48(11):3565–83. [9] Steiner R. Properties and selection: irons, steels and high performance alloys. ASM International; 1990.
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Premature corrosion failure of a 316L stainless steel plate due to the presence of sigma phase O. Conejero *, M. Palacios, S. Rivera Fundacion ITMA-Centro del Acero, P.E.P.A. C/ Calafates, s/n, 33417 Aviles, Asturias, Spain
a r t i c l e
i n f o
Article history: Received 19 March 2008 Accepted 4 June 2008 Available online 1 July 2008 Keywords: Sigma phase Corrosion
a b s t r a c t A 316L austenitic stainless steel plate, exposed to a sulphuric acid environment, has suffered an abnormal corrosion attack. The metallographic examination demonstrated a large amount of sigma phase that was related to the failure. Sigma phase, which is very common in duplex stainless steels, removes chromium and molybdenum from the metal matrix, damaging the corrosion performance of the steel. In this case, the sigma phase was found in an austenitic stainless steel, producing a significant corrosion attack. The sigma phase was formed during the fabrication process of the steel. Ó 2009 Published by Elsevier Ltd.
1. Introduction Several parts of a stainless steel plate, located in one of the electrolysis buildings of a zinc producer company, showed an intensive corrosion attack after one year of service. The plate was made of 316L type austenitic stainless steel. The environment in where the plate was situated was rich in sulphuric acid, from the electrolytic process. The plate was partially immersed in the acid bath (10% sulphuric acid), at a temperature of about 40 °C. Fig. 1 shows one part of the plate where the severe attack can be seen. The plate was electrically insulated and no electric current circulates inside it. 2. Chemical analysis In order to check the correct chemical composition of the stainless steel, a sample of the plate was extracted and analyzed by atomic emission spectrocopy. The results obtained are shown in Table 1. The results obtained show that the chemical composition of the plate match the AISI 316L specification of the material. 3. Metallographic and microscopic examinations Some samples were cut from the cross section of some parts of the plate. The samples were mounted in a resin block and studied in the as-polished and etched condition by means of an optical microscope. Some other samples were extracted from the corroded surface to be studied directly at high magnification by means of a scanning electron microscope (SEM). The samples were also chemically analyzed by means of electron dispersive spectroscopy (EDS) in the SEM. The outer aspect of the corroded surfaces had a sponge-like feature, as it can be seen in Fig. 2. The same feature is shown at higher magnification in Fig. 3, where a kind of dendritic structure can be seen. A chemical analysis of that ‘‘dendrites” is * Corresponding author. Tel.: +34 985129120; fax: +34 985129008. E-mail address: [email protected] (O. Conejero). 1350-6307/$ - see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.engfailanal.2008.06.022
700
O. Conejero et al. / Engineering Failure Analysis 16 (2009) 699–704
Fig. 1. Plate plug.
Table 1 Chemical composition (wt%)
AISI 316L Sample
C
Mn
Si
P
S
Cr
Ni
Mo
0.03 max 0.02
2.0 max 1.7
1.0 max 0.5
0.045 max 0.03
0.03 max 0.03
16–18 17.1
10–14 11.6
2–3 2.1
Fig. 2. Corroded surface.
also presented in Fig. 3 and Table 2, displaying a chromium- and molybdenum-rich and nickel-poor area, with respect to the whole metal composition (AISI 316L). The metallographic examination displayed a big amount of intermetallic phases, as shown in Figs. 4 and 5. The last one reveals the corrosion progress in lines along the rolling direction, following the intermetallic phases, leaving a thread-like structure. The intermetallic phase can be seen at higher magnification in Fig. 6, where it was identified as d-ferrite partially transformed into sigma phase. This fact was demonstrated by etching with potassium ferricyanide that reveals the d-ferrite in a brownish colour, whereas the sigma phase appears in a greenish colour. Fig. 7 shows the result of this examination, where sigma phase and delta ferrite appeared in all the microstructure. The chemical composition of the intermetallic phase was analyzed by means of EDS in a scanning electron microscope. Fig. 8 and Table 3 show some of the results obtained, where the Mo and Cr enrichment can be observed in the intermetallic phase (Spectrum 1), as compared to the metal composition (Spectrum 2).
701
O. Conejero et al. / Engineering Failure Analysis 16 (2009) 699–704
Fig. 3. Surface examination.
Table 2 Chemical composition (wt%) in Fig. 3
Spectrum 1
Si
Cr
Fe
Ni
Mo
1.64
23.39
63.80
5.88
5.29
Fig. 4. Intermetallic precipitates.
4. Discussion Austenitic stainless steels have good combination of mechanical strength, fabricability and general corrosion resistance, and hence are extensively used as construction material in many highly demanding environments. The effective use of these alloys in a corrosive environment depends on the proper behaviour of the material. Nevertheless, there are several factors affecting the anti-corrosive behaviour of the stainless steels such as microsegregation, secondary phases precipitation and recrystallization. Among the different phases that can occur in the microstructure of a stainless steel, intermetallic phases (sigma, chi and Laves), and carbides are the two most frequently encountered. Sigma phase is probably the most studied and undesirable intermetallic phase in a stainless steel. Precipitation of sigma phase can occur in the austenitic, ferritic and duplex stainless steels. The precipitation of that phase not only causes losses in
702
O. Conejero et al. / Engineering Failure Analysis 16 (2009) 699–704
Fig. 5. Intermetallic precipitates.
Fig. 6. Sigma phase and d-ferrite.
Fig. 7. d-Ferrite partially transformed into sigma phase.
ductility and toughness of the steel, but also reduces its corrosion resistance by removing chromium and molybdenum from the austenitic matrix [1]. Typically, the sigma phase precipitates between 600 and 1000 °C [2], although this range varies somewhat with composition and processing. The shortest time for formation generally occurs between about 700 and 810 °C, and the temperature that produces the greatest amount of sigma phase with time is usually somewhat lower [3].
703
O. Conejero et al. / Engineering Failure Analysis 16 (2009) 699–704
Fig. 8. Chemical analysis of the intermetallic phase.
Table 3 Chemical composition (wt%) in Fig. 8
Spectrum 1 Spectrum 2
Si
Cr
Mn
Fe
Ni
Mo
0.72 0.65
25.59 18.64
1.30 1.31
63.98 67.63
3.88 9.27
4.53 2.50
The sigma phase formed in austenitic stainless steels is a hard brittle intermetallic phase of nominal composition FeCr, although the chemical composition of sigma phase extracted from type 316 steel has been determined as Fe 44%, Cr 29.2%, Mo 8.3% and C < 0.1% all by weight [4]. Since the chromium content of sigma phase is about twice the content of the matrix, formation of this phase requires redistribution of alloying elements by substitutional diffusion [5] from the adjacent areas [6]. Molybdenum and silicon promote the formation of sigma phase, and the chromium content also favours its formation. Small amounts of nickel and manganese increase the rate of sigma phase formation, but large amounts, which stabilize austenite, retard its formation. Carbon additions decrease sigma phase formation by forming chromium carbides, thereby reducing the amount of chromium in solid solution. Additions of tungsten, vanadium, titanium and niobium also promote sigma phase formation [3,4]. The tendency to sigma phase formation of an austenitic stainless steel can be known from the equivalent chromium content formula [7]:
ECC ¼ %Cr þ 0:31% Mn þ 1:76% Mo þ 0:97% W þ 2:02% V þ 1:58% Si þ 2:44% Ti þ 1:7% Nb þ 1:22% Ta 0:266% Ni 0:177% Co:
If the equivalent Cr content (ECC) is greater than 17–18 wt%, the steel is susceptible to sigma phase formation. In this case, the steel has an ECC of 19, being thus susceptible to form that phase. Sigma phase nucleates mainly at grain boundaries, and is found in 316L type stainless steels approximately in 100 h at 800 °C [8]. In fully austenitic alloys, sigma phase forms from the austenite along grain boundaries. If d-ferrite is present in the austenitic alloy, sigma formation is more rapid and occurs in the d-ferrite [3]. The d-ferrite forms in austenitic stainless steels above 1100 °C [9]. Stainless steels need to be solution annealed and water quenched in order to put in solution all the different phases that they can contain and to homogenize their structure. A bad production or fabrication process can lead to a deleterious microstructure due to thermal effects that result in formation of deleterious phases such as sigma phase. In this case, the plate was not subjected to any welding process or to high service temperatures, so the sigma phase had to be formed during the production process of the steel.
704
O. Conejero et al. / Engineering Failure Analysis 16 (2009) 699–704
5. Conclusions A 316L stainless steel plate subjected to a sulphuric acid environment has failed due to the presence of sigma phase. The sigma phase has been demonstrated to be deleterious to the corrosion performance of the stainless steels by removing chromium and molybdenum from the austenitic matrix. Sigma phase precipitates at temperatures between 600 and 1000 °C, and it was formed in this case during the production process of the steel, as the plate was not subjected to any welding process or to high service temperatures. Acknowledgement The authors thank to Asturiana de Zinc S.A. for allowing the publication of the data employed in this article. References [1] Villanueva DME, Junior FCP, Plaut RL, Padilha AF. Comparative study on sigma phase precipitation of three types of stainless steels: austenitic, superferritic and duplex. Mater Sci Technol 2006;22(9):1098–104. [2] Pohl M, Storz O, Glogowski T. Effect of intermetallic precipitations on the properties of duplex stainless steel. Mater Charact 2007;58(1):65–71. [3] Davis JR. Stainless steels (ASM Specialty Handbook). ASM International; 1995. [4] Marshall P. Austenitic Stainless Steels: Microstructure and Mechanical Properties. Springer; 1984. [5] Schwind M, Kallqvist J, Nilsson JO, Agren J, Andren HO. Sigma-phase precipitation in stabilized austenitic stainless steels. Acta Mater 2000;48(10):2473–81. [6] Korb LJ. Corrosion. ASM International; 1987. [7] Khatak HS, Raj B. Corrosion of austenitic stainless steels: mechanism mitigation and monitoring. Woodhead publishing; 2002. [8] Aydogdu GH, Aydinol MK. Determination of susceptibility to intergranular corrosion and electrochemical reactivation behaviour of AISI 316L type stainless steel. Corros Sci 2006;48(11):3565–83. [9] Steiner R. Properties and selection: irons, steels and high performance alloys. ASM International; 1990.