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SKFM observation of SCC on SUS304 stainless steel
Corrosion Science 49 (2007) 120–129 www.elsevier.com/locate/corsci
SKFM observation of SCC on SUS304 stainless steel Hiroyuki Masuda
¤
Corrosion Analysis Group, National Institute for Material Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Available online 3 July 2006
Abstract We have developed a new SCC test device that enables the SKFM (super Kelvin force microscope) observation. By using this test device, SCC test was done at 343 K and 28% RH with attaching MgCl2 droplets. The crack tip deformation was observed by both KFM and SKFM. The results show that cracks initiated from the pit were usually found within half day. The negative potential parts were observed on both cracked parts and some of slip deformation parts and disappeared with time when the specimen was kept in the air. Discontinuous cracks were often found near the tip of the slip deformation part. These discontinuous cracks joined together and the amount of slip deformation increased with the progress of the crack. The negative potential part is considered to be the part where hydrogen exists. The movement of hydrogen generated by the cathodic reaction plays the important role to create the crack. © 2006 Elsevier Ltd. All rights reserved. Keywords: A. SUS304 stainless steel; B. KFM; A. MgCl2; C. Slip deformation; C. SCC
1. Introduction Stainless steel is often used as the structural material near seashore environment because of high corrosion resistance. However, the stress corrosion cracking (SCC) caused by sea salt particles has been found under the conditions that the residual stress existed in the structure. Many excellent research works [1–8] have been done from the aspects of
*
Tel.: +81 29 859 2542; fax: +81 29 859 2501. E-mail address: [email protected]
0010-938X/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2006.05.014
H. Masuda / Corrosion Science 49 (2007) 120–129
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mechanical factors (applied stress, residual strain), environmental factors (temperature, pH, DO) and material factors (additional element, structure) to clarify the mechanism of SCC on stainless steel. Three basic mechanisms of SCC have been proposed, such as active path dissolution, hydrogen embrittlement and Wlm induced cleavage. However, no clear mechanism of SCC has been found in the above environment. We have developed a new SCC test device that enables the SKFM [9] (super Kelvin force microscope) observation. By using this test device, the crack tip deformation is observed by both KFM and SKFM and the mechanism of SCC is discussed. 2. Experimental work 2.1. SKFM equipment The Kelvin force method used in this experiment was originally developed by Yasutake et al. [10]. In this method, topography and surface potential can be obtained at the same time with non-contact mode. The scanning device for X–Y direction is used an accurate X–Y stage. The accuracy of the X–Y stage is less than 0.1 m for repeated positioning. The X–Y stage can move up to 10 £ 10 cm2, but the maximum scanning area is limited to 1 £ 1 cm2. The minimum step of the X–Y stage is 0.1 m, so the scan area of 25.6 £ 25.6 m2 is the minimum scan area for the stage scan mode when the acquired data points are 256 £ 256. The specimen size of 20 £ 20 £ 2.5 cm3 can be observed. Three types of piezo scanner can be used. One is 100 £ 100 £ 15 m3, another is 0 £ 0 £ 40 m3 and the other is 0 £ 0 £ 200 m3 of working distance. After setting the observing position, the scan mode can be chosen either the stage scan mode or the piezo scan mode. If we use the scanner of 100 £ 100 £ 15 m3, the scan range can be chosen from 10 nm to 1 cm. The tip used for SKFM measurement was the conductive gold-coated Si tip with the resonant frequency of around 25 kHz. The SKFM image was taken with a scanning speed of between 0.03 and 0.06 Hz with data points of 256 £ 256. 2.2. SCC test Test specimen used was SUS304 stainless steel with the size of 95 mm in length, 25 mm in width and 0.2 mm in thickness. The test part of specimen was mechanically polished up to 300 nm in roughness. Stress was applied by bending the specimen with a jig of 60 mm in length as shown in Fig. 1. The curvature of radius near the center of the test specimen was about 24 mm. More than 10 pieces of 25% MgCl2 droplet of 5 mm3 in volume were attached on the specimen. Test was done at 343 K and 28% RH. The initiation and the propagation of cracks were monitored by the stereomicroscope. SCC test was carried out several times with the same specimen to observe the progress of the crack. 3. Results and discussion 3.1. Crack morphology Fig. 2 shows the stereomicroscope image of cracks on the surface of specimen after the test. Usually only one pit initiated in one droplet and cracks always initiated from the pit and propagated perpendicular to the stressed direction. Fig. 3 shows the optical microscope images of cracks with diVerent test time. Since the amount of dissolved oxygen was
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H. Masuda / Corrosion Science 49 (2007) 120–129
Fig. 1. New SCC test device and method.
Fig. 2. Stereomicroscope image of cracks.
very low because of high Cl¡ ion concentration of the test solution, no rust was formed near the pit. The specimen was washed with water and then with acetone for the observation. This process did not stop the growth of the cracks. Discontinuous cracks were often found near the crack tip and connected with the progress of the crack as indicated by arrows in Fig. 4. 3.2. Surface potential distribution Fig. 5 shows the optical microscope and SKFM images near the crack tip. On the surface potential image, bright part corresponds to the negative potential part. A small crack initiated near the tip of slip deformation as indicated by an arrow in the optical microscope image. The surface potential near the crack tip was most negative and about 300 mV more negative than other part on 2 days after the test started in this crack. The potential distribution changed with time when the specimen was kept in the air. The surface potential near the crack tip was still most negative and about 200 mV more negative than other part on 18 days after the test started. Fig. 6 also shows the optical microscope and SKFM images near
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Fig. 3. Optical microscope images of cracks. Length of scale corresponds to 0.2 mm.
Fig. 4. Connection of discontinuous cracks and increase of crack width with progress of crack.
the crack tip. The discontinuous crack as indicated by the arrow was also observed near the crack tip. The surface potential changed with time as well as it is shown in Fig. 5. Fig. 7 shows the optical microscope, SEM and KFM images near the small crack as indicated by the arrow in Fig. 5. The surface potential around the small crack was more negative than
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Fig. 5. Optical microscope and SKFM images near crack tip. Image size is 0.4 mm £ 0.4 mm. Arrow indicates a small crack at the tip of slip deformation.
other part. Fig. 8 shows the optical microscope, SEM and KFM images near the crack tip as indicated by the arrow in Fig. 6. Slip steps were clearly observed on the optical microscope, SEM and KFM topographical images. It is apparent that there is no clear relationship between the negative potential part and the slip deformation part. Fig. 9 shows the optical microscope, SEM and KFM images of discontinuous crack. There is a very good correlation between the negative potential parts and the cracks observed by both the optical microscope and SEM image. The step height of more than 500 nm was produced by the slip deformation in the middle of the image. However, this large amount of deformation did not produce both continuous crack and negative potential part. 3.3. Pit growth Fig. 10 shows the color laser microscope images near the pit. Amount of anodic dissolution (DV) increased with time. The crack width also increased with time as shown in Fig. 4. The evolution of bubbles from the crack was often observed by the stereomicroscope. Since the dissolved oxygen is very low because of high Cl¡ ion concentration of the test solution, the bubbles caused by the cathodic reaction is considered to be hydrogen. This indicates that hydrogen was continuously supplied at the slip deformation part.
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Fig. 6. Optical microscope and SKFM images near crack tip. Image size is 0.4 mm £ 0.4 mm. Arrow indicates a discontinuous small crack.
3.4. Crack deformation Fig. 11 shows the change of surface proWle near the pit. The slip deformation of shear type was observed. The amount of slip deformation increased with time and the discontinuous cracks joined together at the slip deformation part. Fig. 12 shows the wide range of surface proWle. It is clear that the rise of surface occurred near the pit. 3.5. Mechanism of SCC The feature of SCC on this environment can be summarized as follows: (1) (2) (3) (4) (5)
Cracks always initiate near the pit. Discontinuous cracks are often found near the crack tip. The most negative potential part is located near the crack tip. The distribution of negative potential part changes with time. There is no clear relationship between the negative potential part and both slip deformation part and corroding part.
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Fig. 7. Optical microscope, SEM and KFM images (26 days after test started) near small crack. Image size is 0.06 mm £ 0.06 mm.
(6) The surface potential of crack is more negative than other part. (7) The pit depth and the crack width increase with time by anodic dissolution. (8) The possible cathodic reaction in this environment is hydrogen generation. From these, the crack is considered to be caused by hydrogen embrittlement. The pit acts both generating site of hydrogen and stress concentration site. The existence of discontinuous cracks near the tip of slip deformation in Figs. 7–9 indicates the crack does not propagate by the anodic dissolution of newly created surface. The negative potential part seems to correspond to the highly anodic precipitate by hydrogen proposed by Vaughan et al. [7]. Hydrogen produced by the cathodic reaction in this test environment and is continuously supplied at the high-tensile-stress region through the slip deformation (Figs. 11 and 12) by anodic dissolution of this precipitate (Fig. 4). The highly anodic precipitate is considered to be produced when the amount of hydrogen exceeds a certain level. The anodic dissolution of this precipitate is not the mechanism of crack propagation, because the area of the negative potential part near the crack tip is usually much wider than the crack width as shown in Figs. 6 and 7. The cracking of this precipitate is natural to think. This highly anodic precipitate formed by hydrogen is unstable, so the negative potential part disappeared with time.
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Fig. 8. Optical microscope, SEM and KFM images (19 days after test started) near the crack tip. Image size is 0.06 mm £ 0.06 mm.
Fig. 9. Optical microscope, SEM and KFM images of discontinuous crack (26 days after test started).
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Fig. 10. Color laser microscope images of pit with time.
Fig. 11. SKFM images near pit with time.
Fig. 12. SKFM images near pit with time.
4. Conclusion The SCC behavior of SUS304 stainless steel was studied from the aspect of slip deformation, pit growth and surface potential distribution. The results show that the negative potential parts were observed on both cracked parts and some of slip deformation parts and disappeared with time when the specimen was kept in the air. Discontinuous cracks were often found near the tip of the slip deformation part. These discontinuous cracks join together and the amount of slip deformation increases with the progress of the crack. The negative potential part is considered to be the part where hydrogen precipitate exists. The movement of hydrogen generated by the cathodic reaction plays the important role to create the crack.
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Acknowledgement A part of this study was Wnancially supported by the Budget for Nuclear Research of the Ministry of Education, Culture, Sports, Science and Technology, based on the screening and counseling by the Atomic Energy Commission. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
T.P. Hoar, J.G. Hines, J. Iron Steel Inst. 182 (1956) 124–143. T.P. Hoar, J.C. Scully, J. Elecrtochem. Soc. 111 (1964) 348–352. H.H. Uhlig, E.V. Cook, J. Elecrtochem. Soc. 116 (1969) 173–177. N.A. Nielsen, Corrosion 27 (1971) 173–189. M.B. Whiteman, A.R. Troiano, Corrosion 21 (1965) 53–56. M.L. Holzworth, Corrosion 25 (1969) 107–115. D.A. Vaughan, D.I. Phalen, C.L. Peterson, W.K. Boyd, Corrosion 19 (1963) 315t–326t. P. Rhodes, Corrosion 25 (1969) 462–472. H. Masuda, Corrosion 57 (2001) 99–109. M. Yasutake, D. Aoki, M. Fujihira, Thin Solid Films 273 (1996) 279–283.
SKFM observation of SCC on SUS304 stainless steel Hiroyuki Masuda
¤
Corrosion Analysis Group, National Institute for Material Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Available online 3 July 2006
Abstract We have developed a new SCC test device that enables the SKFM (super Kelvin force microscope) observation. By using this test device, SCC test was done at 343 K and 28% RH with attaching MgCl2 droplets. The crack tip deformation was observed by both KFM and SKFM. The results show that cracks initiated from the pit were usually found within half day. The negative potential parts were observed on both cracked parts and some of slip deformation parts and disappeared with time when the specimen was kept in the air. Discontinuous cracks were often found near the tip of the slip deformation part. These discontinuous cracks joined together and the amount of slip deformation increased with the progress of the crack. The negative potential part is considered to be the part where hydrogen exists. The movement of hydrogen generated by the cathodic reaction plays the important role to create the crack. © 2006 Elsevier Ltd. All rights reserved. Keywords: A. SUS304 stainless steel; B. KFM; A. MgCl2; C. Slip deformation; C. SCC
1. Introduction Stainless steel is often used as the structural material near seashore environment because of high corrosion resistance. However, the stress corrosion cracking (SCC) caused by sea salt particles has been found under the conditions that the residual stress existed in the structure. Many excellent research works [1–8] have been done from the aspects of
*
Tel.: +81 29 859 2542; fax: +81 29 859 2501. E-mail address: [email protected]
0010-938X/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2006.05.014
H. Masuda / Corrosion Science 49 (2007) 120–129
121
mechanical factors (applied stress, residual strain), environmental factors (temperature, pH, DO) and material factors (additional element, structure) to clarify the mechanism of SCC on stainless steel. Three basic mechanisms of SCC have been proposed, such as active path dissolution, hydrogen embrittlement and Wlm induced cleavage. However, no clear mechanism of SCC has been found in the above environment. We have developed a new SCC test device that enables the SKFM [9] (super Kelvin force microscope) observation. By using this test device, the crack tip deformation is observed by both KFM and SKFM and the mechanism of SCC is discussed. 2. Experimental work 2.1. SKFM equipment The Kelvin force method used in this experiment was originally developed by Yasutake et al. [10]. In this method, topography and surface potential can be obtained at the same time with non-contact mode. The scanning device for X–Y direction is used an accurate X–Y stage. The accuracy of the X–Y stage is less than 0.1 m for repeated positioning. The X–Y stage can move up to 10 £ 10 cm2, but the maximum scanning area is limited to 1 £ 1 cm2. The minimum step of the X–Y stage is 0.1 m, so the scan area of 25.6 £ 25.6 m2 is the minimum scan area for the stage scan mode when the acquired data points are 256 £ 256. The specimen size of 20 £ 20 £ 2.5 cm3 can be observed. Three types of piezo scanner can be used. One is 100 £ 100 £ 15 m3, another is 0 £ 0 £ 40 m3 and the other is 0 £ 0 £ 200 m3 of working distance. After setting the observing position, the scan mode can be chosen either the stage scan mode or the piezo scan mode. If we use the scanner of 100 £ 100 £ 15 m3, the scan range can be chosen from 10 nm to 1 cm. The tip used for SKFM measurement was the conductive gold-coated Si tip with the resonant frequency of around 25 kHz. The SKFM image was taken with a scanning speed of between 0.03 and 0.06 Hz with data points of 256 £ 256. 2.2. SCC test Test specimen used was SUS304 stainless steel with the size of 95 mm in length, 25 mm in width and 0.2 mm in thickness. The test part of specimen was mechanically polished up to 300 nm in roughness. Stress was applied by bending the specimen with a jig of 60 mm in length as shown in Fig. 1. The curvature of radius near the center of the test specimen was about 24 mm. More than 10 pieces of 25% MgCl2 droplet of 5 mm3 in volume were attached on the specimen. Test was done at 343 K and 28% RH. The initiation and the propagation of cracks were monitored by the stereomicroscope. SCC test was carried out several times with the same specimen to observe the progress of the crack. 3. Results and discussion 3.1. Crack morphology Fig. 2 shows the stereomicroscope image of cracks on the surface of specimen after the test. Usually only one pit initiated in one droplet and cracks always initiated from the pit and propagated perpendicular to the stressed direction. Fig. 3 shows the optical microscope images of cracks with diVerent test time. Since the amount of dissolved oxygen was
122
H. Masuda / Corrosion Science 49 (2007) 120–129
Fig. 1. New SCC test device and method.
Fig. 2. Stereomicroscope image of cracks.
very low because of high Cl¡ ion concentration of the test solution, no rust was formed near the pit. The specimen was washed with water and then with acetone for the observation. This process did not stop the growth of the cracks. Discontinuous cracks were often found near the crack tip and connected with the progress of the crack as indicated by arrows in Fig. 4. 3.2. Surface potential distribution Fig. 5 shows the optical microscope and SKFM images near the crack tip. On the surface potential image, bright part corresponds to the negative potential part. A small crack initiated near the tip of slip deformation as indicated by an arrow in the optical microscope image. The surface potential near the crack tip was most negative and about 300 mV more negative than other part on 2 days after the test started in this crack. The potential distribution changed with time when the specimen was kept in the air. The surface potential near the crack tip was still most negative and about 200 mV more negative than other part on 18 days after the test started. Fig. 6 also shows the optical microscope and SKFM images near
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Fig. 3. Optical microscope images of cracks. Length of scale corresponds to 0.2 mm.
Fig. 4. Connection of discontinuous cracks and increase of crack width with progress of crack.
the crack tip. The discontinuous crack as indicated by the arrow was also observed near the crack tip. The surface potential changed with time as well as it is shown in Fig. 5. Fig. 7 shows the optical microscope, SEM and KFM images near the small crack as indicated by the arrow in Fig. 5. The surface potential around the small crack was more negative than
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Fig. 5. Optical microscope and SKFM images near crack tip. Image size is 0.4 mm £ 0.4 mm. Arrow indicates a small crack at the tip of slip deformation.
other part. Fig. 8 shows the optical microscope, SEM and KFM images near the crack tip as indicated by the arrow in Fig. 6. Slip steps were clearly observed on the optical microscope, SEM and KFM topographical images. It is apparent that there is no clear relationship between the negative potential part and the slip deformation part. Fig. 9 shows the optical microscope, SEM and KFM images of discontinuous crack. There is a very good correlation between the negative potential parts and the cracks observed by both the optical microscope and SEM image. The step height of more than 500 nm was produced by the slip deformation in the middle of the image. However, this large amount of deformation did not produce both continuous crack and negative potential part. 3.3. Pit growth Fig. 10 shows the color laser microscope images near the pit. Amount of anodic dissolution (DV) increased with time. The crack width also increased with time as shown in Fig. 4. The evolution of bubbles from the crack was often observed by the stereomicroscope. Since the dissolved oxygen is very low because of high Cl¡ ion concentration of the test solution, the bubbles caused by the cathodic reaction is considered to be hydrogen. This indicates that hydrogen was continuously supplied at the slip deformation part.
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Fig. 6. Optical microscope and SKFM images near crack tip. Image size is 0.4 mm £ 0.4 mm. Arrow indicates a discontinuous small crack.
3.4. Crack deformation Fig. 11 shows the change of surface proWle near the pit. The slip deformation of shear type was observed. The amount of slip deformation increased with time and the discontinuous cracks joined together at the slip deformation part. Fig. 12 shows the wide range of surface proWle. It is clear that the rise of surface occurred near the pit. 3.5. Mechanism of SCC The feature of SCC on this environment can be summarized as follows: (1) (2) (3) (4) (5)
Cracks always initiate near the pit. Discontinuous cracks are often found near the crack tip. The most negative potential part is located near the crack tip. The distribution of negative potential part changes with time. There is no clear relationship between the negative potential part and both slip deformation part and corroding part.
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Fig. 7. Optical microscope, SEM and KFM images (26 days after test started) near small crack. Image size is 0.06 mm £ 0.06 mm.
(6) The surface potential of crack is more negative than other part. (7) The pit depth and the crack width increase with time by anodic dissolution. (8) The possible cathodic reaction in this environment is hydrogen generation. From these, the crack is considered to be caused by hydrogen embrittlement. The pit acts both generating site of hydrogen and stress concentration site. The existence of discontinuous cracks near the tip of slip deformation in Figs. 7–9 indicates the crack does not propagate by the anodic dissolution of newly created surface. The negative potential part seems to correspond to the highly anodic precipitate by hydrogen proposed by Vaughan et al. [7]. Hydrogen produced by the cathodic reaction in this test environment and is continuously supplied at the high-tensile-stress region through the slip deformation (Figs. 11 and 12) by anodic dissolution of this precipitate (Fig. 4). The highly anodic precipitate is considered to be produced when the amount of hydrogen exceeds a certain level. The anodic dissolution of this precipitate is not the mechanism of crack propagation, because the area of the negative potential part near the crack tip is usually much wider than the crack width as shown in Figs. 6 and 7. The cracking of this precipitate is natural to think. This highly anodic precipitate formed by hydrogen is unstable, so the negative potential part disappeared with time.
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Fig. 8. Optical microscope, SEM and KFM images (19 days after test started) near the crack tip. Image size is 0.06 mm £ 0.06 mm.
Fig. 9. Optical microscope, SEM and KFM images of discontinuous crack (26 days after test started).
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Fig. 10. Color laser microscope images of pit with time.
Fig. 11. SKFM images near pit with time.
Fig. 12. SKFM images near pit with time.
4. Conclusion The SCC behavior of SUS304 stainless steel was studied from the aspect of slip deformation, pit growth and surface potential distribution. The results show that the negative potential parts were observed on both cracked parts and some of slip deformation parts and disappeared with time when the specimen was kept in the air. Discontinuous cracks were often found near the tip of the slip deformation part. These discontinuous cracks join together and the amount of slip deformation increases with the progress of the crack. The negative potential part is considered to be the part where hydrogen precipitate exists. The movement of hydrogen generated by the cathodic reaction plays the important role to create the crack.
H. Masuda / Corrosion Science 49 (2007) 120–129
129
Acknowledgement A part of this study was Wnancially supported by the Budget for Nuclear Research of the Ministry of Education, Culture, Sports, Science and Technology, based on the screening and counseling by the Atomic Energy Commission. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
T.P. Hoar, J.G. Hines, J. Iron Steel Inst. 182 (1956) 124–143. T.P. Hoar, J.C. Scully, J. Elecrtochem. Soc. 111 (1964) 348–352. H.H. Uhlig, E.V. Cook, J. Elecrtochem. Soc. 116 (1969) 173–177. N.A. Nielsen, Corrosion 27 (1971) 173–189. M.B. Whiteman, A.R. Troiano, Corrosion 21 (1965) 53–56. M.L. Holzworth, Corrosion 25 (1969) 107–115. D.A. Vaughan, D.I. Phalen, C.L. Peterson, W.K. Boyd, Corrosion 19 (1963) 315t–326t. P. Rhodes, Corrosion 25 (1969) 462–472. H. Masuda, Corrosion 57 (2001) 99–109. M. Yasutake, D. Aoki, M. Fujihira, Thin Solid Films 273 (1996) 279–283.