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Effect of selective dissolution on fatigue crack initiation in 2205 duplex stainless steel
Corrosion Science 49 (2007) 1847–1861 www.elsevier.com/locate/corsci
EVect of selective dissolution on fatigue crack initiation in 2205 duplex stainless steel I-Hsuang Lo, Wen-Ta Tsai
¤
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan Received 15 December 2005; accepted 9 October 2006 Available online 26 December 2006
Abstract The eVect of selective dissolution on fatigue crack initiation of 2205 duplex stainless steel (DSS) was investigated in this study. In mixed sulfuric and hydrochloric acid aqueous solution, there existed two distinctly separated anodic peaks in the active-to-passive transition region of the polarization curve. Either ferritic or austenitic phase was selectively dissolved at each characteristic anodic peak potential. Under sinusoidal cyclic loading condition, however, selective dissolution did not assist fatigue crack initiation instead resulting in the elimination of stress concentration site in the selectively dissolving phase. As a consequence, under selective dissolution condition, fatigue crack initiated in the phase while its dissolution rate was lower with respect to the other constituent phase in the duplex stainless steel. The microstructural evolution of the corrosion fatigue crack initiation in 2205 DSS in the mixed sulfuric and hydrochloric acid solution is highlighted in this investigation. © 2006 Elsevier Ltd. All rights reserved. Keywords: A. Duplex stainless steel; C. Selective dissolution; C. Corrosion fatigue crack initiation
1. Introduction It is known that fatigue cracks initiate preferentially in sites where stress concentration occurs or microstructure inhomogeneity exists. For instance, Rios et al. [1] indicated that fatigue crack would initiate in the concave surface rather than the convex part of *
Corresponding author. Tel.: +886 6 2757575x62927; fax: +886 6 2754395. E-mail address: [email protected] (W.-T. Tsai).
0010-938X/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2006.10.013
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shot-pinned A316 stainless steel (SS). Rokhlin et al. [2] reported that fatigue crack initiation strongly depends on the depth of artiWcial pits. In 304 SS, the precipitation of -ferrite in the / grain boundary would lead to the formation of an incoherent interface. Rho et al. [3–5] pointed out that this incohent / interface became the vulnerable site for fatigue crack initiation. In duplex stainless steels (DSSs), fatigue crack initiation was rather complicated. For example, Llanes et al. [6,7] indicated that fatigue crack would initiate at -phase or at the / interfaces of unaged 2205 DSS. On the contrary, fatigue crack could initiate at -phase if 2205 DSS was aged at 475 °C for 200 h. Beside the microstructural eVect mentioned above, the environmental eVect is also very important in aVecting fatigue crack initiation. Localized corrosion, especially pitting corrosion, is known to enhance the fatigue crack initiation susceptibility in 316L SS [8], 403 SS [9] and X-52 steel [10], etc. Though fatigue behavior in 2205 DSS in air has been investigated by some researchers [6,7], the corrosion fatigue crack initiation is less explored. Previous investigations [11,12] have found that selective dissolution could occur in 2205 DSS in H2SO4/HCl solution. The eVect of selective dissolution on the fatigue crack initiation is thus of interested and investigated in this study. 2. Experimental 2.1. Material preparation A 2205 DSS with the form of square bar, 30 mm £ 30 mm, was used. The chemical composition is listed in Table 1. This materials was heat treated at 1100 °C for 30 min then water quenched before the subsequent tests. The metallographs of three diVerent surfaces, after etching in boiling Murakami reagent, are demonstrated in Fig. 1a. As shown in Fig. 1a, the dark phase is ferrite () while the white phase is austenite (). The volume ratio of / is 53/47. The major alloying elements, namely, Fe, Cr, Ni, Mo and Mn, were analyzed by an energy dispersive spectrometer (EDS) to identify the individual phase and also listed in Table 1. 2.2. Electrochemical tests Potentiodynamic polarization test was conducted at a scan rate of 1 mV/s in the range of ¡500 mVSCE to 0 mVSCE from cathodic towards anodic direction. Potentiostatic etching at the anodic peak potential corresponding to selective dissolution of either or was also performed. All potentials were measured with respect to a saturated calomel electrode
Table 1 Chemical compositions (in wt.%) of 2205 DSS and the constituent ferritic and authentic phases Element 2205 DSS -phaseb -phaseb a b
a
Fe
Cr
Ni
Mo
Mn
Si
C
Cu
P
S
N
Balance Balance Balance
22.40 23.5 20.5
5.42 4.8 7.2
3.24 3.5 1.0
1.43 1.0 2.0
0.41 – –
0.014 – –
0.21 – –
0.025 – –
0.004 – –
0.198 – –
Glow discharge spectrometer (GDS). Energy dispersive spectrometer (EDS).
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Fig. 1. (a) Optical micrograph of 2205 duplex stainless steel, solution heat treated at 1100 °C for 30 min followed by water quench; (b) geometry and dimension of fatigue specimen.
(SCE) and a platinum sheet was used as the counter electrode. After potentiostatic etching, the surface morphology was examined with a scanning electron microscope (SEM), while major alloying elements were analyzed by EDS. 2.3. Corrosion fatigue tests Rectangular shape of specimen with the geometric orientation and dimension shown in Fig. 1b was used for fatigue test. A sinusoidal waveform of load with a frequency of 10 Hz was used. The initial maximum stress of 1.2 ys (yield strength) and a load ratio (R) of 0.5 was applied for fatigue test. The loading was parallel to the short transverse (S) direction, as demonstrated in Fig. 1b. Fatigue tests were conducted in air and in 2 M H2SO4 + 0.7 M HCl aqueous solution at open circuit potential (OCP) and at ¡300 and ¡240 mVSCE, respectively, all at ambient temperature. After determining the fatigue life (Nf), some interrupted fatigue tests at certain loading cycles were also performed. The cross section and fracture surface were examined microscopically. 3. Results and discussion Fig. 2(a) presents the potentiodynamic polarization curve of 2205 DSS determined in the air-exposed 2 M H2SO4 + 0.7 M HCl aqueous solution. The active-to-passive transition peak consisted of two distinct peaks at ¡300 and ¡240 mVSCE, respectively. This characteristic feature of polarization curve has been reported previously [11,12]. Potentiostatic etching at OCP (¡350 mVSCE), ¡300 or ¡240 mVSCE for 3 h was performed. The respective surface morphologies are shown in Fig. 2b–d. At OCP and ¡300 mVSCE, preferential dissolution of was observed, while that of was found at ¡240 mVSCE. The average fatigue life of 2205 DSS in air was 4.05 £ 105 cycles. Interrupted fatigue test after loading for about 0.6 Nf was followed by surface micrographical examination. Fig. 3 is the SEM micrograph showing the surface morphology of S–T plane after interrupted test. As revealed in this Wgure, slip bands were formed in both phases. However, fatigue cracks were observed in -phase rather than in -phase. The result indicated that -phase was more prone to crack initiation, consistent with that reported by Llandes et al. [6,7].
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Current density, logi (A/cm2)
1E-002
2205 Duplex stainless steel in air-exposed 2 M H2SO4 + 0.7 M HCl
1E-003 -240 mV
SCE
-300 mVSCE 1E-004
1E-005 -600
OCP
-400 -200 Potential (mV vs. SCE)
0
Fig. 2. (a) Potentiodynamic polarization curve of 2205 duplex stainless steel in air-exposed 2 M H2SO4 + 0.7 M HCl aqueous solution. Micrographs of 2205 DSS after potentiostatic etching at (b) OCP, (c) ¡300, and (d) ¡240 mVSCE for 3 h, respectively.
It has been reported that -phase could be more easily cyclic-hardened than -phase [13]. Furthermore, the high nitrogen content, as the DSS used in this study, would lead to solid solution strengthening [14,15] and lower the stacking fault energy [16] of -phase. These factors were all responsible for the increase of resistance to fatigue crack initiation in -phase. Therefore, dislocation would glide and condense mostly in the weaker -phase. In order to release the stress, a new surface would be created, and a fatigue crack would initiate in -phase while 2205 DSS fatigued in air. The average fatigue life obtained in 2 M H2SO4 + 0.7 M HCl aqueous solution at OCP and at a frequency of 10 Hz was 1.27 £ 105. The substantial reduction in fatigue life as compared with that obtained in air indicated the importance of environmental eVect on fatigue behavior of 2205 DSS. Fig. 4a shows the surface morphology of the specimen
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Fig. 3. SEM micrograph showing the surface morphology of 2205 duplex stainless steel after fatigue in air for 0.6 Nf cycles.
fatigued after 0.6 Nf cycles. The SEM micrograph revealed that -phase was slightly and preferentially dissolved, and the slip bands were concentrated in this phase, which resulted in crack initiation. As revealed in Fig. 4a, cracks were also formed at the / interfaces. The cross section of the specimen after loading for 0.6 Nf cycles was also examined. As shown in Fig. 4b, cracks were certainly initiated in -phase and at / boundaries. This Wgure also shows that the cracks propagated in -phase and along / boundaries. As mentioned above, slip bands would be formed favorable -phase at OCP, though preferential dissolution occurred in -phase, the slight corrosion did not cause the elimination of these slip bands. Thus, fatigue crack still initiated in -phase, as demonstrated in Fig. 4. Fig. 5a shows the surface appearance of the specimen after fatigue loading in 2 M H2SO4 + 0.7 M HCl aqueous solution at ¡300 mVSCE for 0.4 Nf cycles, where Nf t 1.06 £ 105 cycles. Similar to that tested at OCP, selective dissolution occurred in -phase. However, unlike that found at OCP, the cracks were found initiated in -phase. The cross section of the specimen fatigued for 0.6 Nf cycles is depicted in Fig. 5b. Cracks mainly initiated and propagated in -phase. Though the slip bands could be generated in -phase, fast selective dissolution could smooth out these bands easily. Thus, the stress raisers could not be developed in -phase, and crack initiation in this selectively dissolved phase was retarded. At ¡240 mVSCE, selective dissolution in -phase took place during fatigue test in 2 M H2SO4 + 0.7 M HCl aqueous solution. Fig. 6a shows the surface appearance after fatigue loading for 0.4 Nf cycles (Nf t 1.04 £ 105 cycles). Again, selective dissolution did not favor crack initiation in -phase. Instead, fatigue initiated in -phase and propagated through it, as demonstrated in Fig. 6b. Though it is generally found that localized corrosion, such as pitting corrosion [8–10], always assists fatigue crack initiation, it is not the case for selective
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Fig. 4. SEM micrographs showing: (a) the surface morphology, and (b) the cross section of 2205 duplex stainless steel after fatigue loading for 0.6 Nf cycles in 2 M H2SO4 + 0.7 M HCl aqueous solution at OCP.
dissolution in 2205 DSS. It is clear that environmentally-assisted fatigue crack initiation may not be prevailed unless an active stress raiser can be developed by the assistance of localized corrosion. As revealed in Figs. 5 and 6, the elimination of slip bands and stress concentration sites in one of the constituent phase in 2205 DSS could occur owing to the selective dissolution. The eVect of selective dissolution on fatigue crack initiation in 2205 DSS could be explained by the schematic diagram demonstrated in Fig. 7. At the early stage of fatigue test, the slip bands could be formed on both phases as illustrated in Fig. 7a. When selective dissolution occurred, particularly at a relative high dissolution rate, the slip bands and the
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Fig. 5. SEM micrographs showing the surface and cross section morphologies of 2205 duplex stainless steel after interrupted fatigue test at ¡300 mVSCE for (a) 0.4 Nf cycles, (b) 0.6 Nf cycles in 2 M H2SO4 + 0.7 M HCl aqueous solution.
associated stress concentration sites could be eliminated in this speciWc phase. Thus, the stress raisers resulting from repeated loading only remained on the other phase, as demonstrated in Fig. 7b. Consequently, as shown in Fig. 7c, a stable fatigue crack could be developed in this phase though with a lower dissolution rate. As fatigue process continued, the crack could extend to the adjacent phase in which selective dissolution prevailed, as depicted in Fig. 7d. The above mechanism could be applied to either - or -phase when a signiWcant diVerence in dissolution rate between them existed during fatigue in aggressive environment. As mentioned above, however, when the stress raiser could not be eliminated
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Fig. 6. SEM micrographs showing the surface and cross section morphologies of 2205 duplex stainless steel after interrupted fatigue test at ¡240 mVSCE for (a) 0.4 Nf cycles, and (b) 0.6 Nf cycles in 2 M H2SO4 + 0.7 M HCl aqueous solution.
by selective dissolution (as at OCP), the phase less resistant to dislocation slip was prone to initiate fatigue crack. The fracture surfaces of 2205 DSS fatigued in air and in 2 M H2SO4 + 0.7 M HCl aqueous solution at various potentials were examined under SEM. Fig. 8a shows the macro image of the fracture surface failed in air. The fatigue crack propagated in transgranular mode, see Fig. 8b, through region I as marked in Fig. 8a. Region II was the area where Wnal overload fracture occurred, as characterized by the dimple mode of fractograph shown in Fig. 8c.
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Fig. 7. Schematic diagram of selective dissolution eVect on corrosion fatigue crack initiation in 2205 DSS.
Fig. 9 shows the macro and micrographs of the fracture surface fatigued in 2 M H2SO4 + 0.7 M HCl aqueous solution at OCP. The fatigue crack also propagated transgranularly as depicted in Fig. 9b, followed by dimple mode of fracture (Fig. 9c). Figs. 10 and 11 demonstrate the fractographs of 2205 DSS fatigue fractured in the acid solution at ¡300 and ¡240 mVSCE, respectively. At ¡300 mVSCE, fatigue crack propagated through region I in Fig. 10a. Under high magniWcation, as revealed in Fig. 10b, the crack surface exhibited a feature showing the extruded -phase with -phase at concave site. At ¡240 mVSCE, on the contrary, the fractograph in region II (Fig. 11a) revealed that -phase was at the convex site while -phase grooves existed on the fractured surface as shown in Fig. 11b. The fractographs revealed in Figs. 10 and 11 suggested that selective dissolution participated in the fatigue propagation process. Close examination of the surface of the fatigued specimen was performed. Fig. 12a shows the SEM micrograph of the S–T plane of 2205 DSS fatigued at ¡300 mVSCE. As revealed in this micrograph, trnsgranular cracking occurred in -phase (with extensive slip bands still remaining on its surface) while selective dissolution of -phase (leaving holes between two adjacent grains)
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Fig. 8. SEM fractographs of 2205 DSS fatigue fractured in air: (a) Macrograph, (b) micrograph of region I, (c) micrograph of region II as marked in (a).
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Fig. 9. SEM fractographs of 2205 DSS fatigue fractured in 2 M H2SO4 + 0.7 M HCl aqueous solution at OCP: (a) Macrograph, (b) micrograph of region III, (c) micrograph of region IV as marked in (a).
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Fig. 10. SEM fractographs of 2205 DSS fatigue fractured in 2 M H2SO4 + 0.7 M HCl aqueous solution at ¡300 mV SCE: (a) Macrograph (b) micrograph of region I as marked in (a).
along the cracking path was observed. The characteristic feature of the fatigue cracking path under selective dissolution of -phase is illustrated in the schematic diagram shown in Fig. 12b. The joining of the fatigue crack in -phase and the selective dissolution of -phase resulted in the convex/concave feature of the fracture surface as demonstrated in Fig. 10. Similar process was found when 2205 DSS was fatigued at ¡240 mVSCE, giving rise to a characteristic fracture surface appearance as revealed in Fig. 11.
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Fig. 11. SEM fractographs of 2205 DSS fatigue fractured in 2 M H2SO4 + 0.7 M HCl aqueous solution at ¡240 mVSCE: (a) Macrograph, (b) micrograph of region II as marked in (a).
4. Conclusions Selective dissolution aVected fatigue crack initiation in 2205 DSS in 2 M H2SO4 + 0.7 M HCl aqueous solution in a complicated manner. At OCP, fatigue crack initiated in -phase as well at /-phase boundaries. At ¡300 mVSCE, selective dissolution occurred in -phase, however, fatigue crack initiated in -phase. On the contrary, phase acted as the fatigue crack initiation site when preferential dissolution of -phase
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Selectively dissolved α
α
α
α γ
γ
γ
γ
γ
γ
γ
γ
α
α
Crack
α
Fig. 12. (a) SEM micrograph showing the surface crack in 2205 DSS fatigued in 2 M H2SO4 + 0.7 M HCl solution at ¡ 300 mVSCE, and (b) schematic diagram showing the involvement of selective dissolution of -phase in fatigue crack growth process in 2205 DSS.
occurred at an applied potential of ¡240 mVSCE. Instead of assisting fatigue crack initiation, selective dissolution could cause the elimination of stress raiser and consequently retard fatigue crack initiation. The participation of selective dissolution was also found in the fatigue crack growth stage at which the crack mainly propagated transgranularly. References [1] [2] [3] [4] [5] [6]
E.R. de los Rios, A. Walley, M.T. Milan, G. Hammersley, Int. J. Fatigue 17 (1995) 493–499. S.I. Rokhlin, J.-Y. Kim, H. Nagy, B. Zoofan, Eng. Fract. Mech. 62 (1999) 425–444. Byung Sup Rho, Hyun Uk Hong, Soo Woo Nam, Scripta Materialia 39 (1998) 1407–1412. Byung Sup Rho, Hyun Uk Hong, Soo Woo Nam, Int. J. Fatigue 22 (2000) 683–690. Je Min Lee, Soo Woo Nm, Mater. Lett. 10 (1990) 223–230. L. Llanes, A. Mateo, P. Violan, J. Méndez, M. Anglada, Mater. Sci. Eng. A 234–236 (1997) 850–852.
I.-H. Lo, W.-T. Tsai / Corrosion Science 49 (2007) 1847–1861 [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
L. Llanes, A. Mateo, P. Villechaise, J. Méndez, M. Anglada, Int. J. Fatigue 21 (1999) S119–S125. Jianhui Xie, Ahmet T. Alpas, Derek O. Northwood, Mater. Charact 48 (2002) 271–277. H.M. Shalaby, J.A. Begley, D.D. Macdonald, Corrosion 52 (1996) 262–274. M. Puiggali, S. Rousserie, M. Touzet, Corrosion 58 (2002) 961–970. W.-T. Tsai, Kuen-Ming Tsai, Chang-Jian Lin, Corrosion/2003 NACE, (2003) paper #03398. I.-H. Lo, Y. Fu, C.-J. Lin, W.-T. Tsai, Corros. Sci. 48 (2006) 696–708. J. Johansson, M. Odén, Metall. Mater. Trans. A 31A (2000) 1557–1570. P.R. Levey, A. van Bennekom, Corrosion 51 (1995) 911–921. J. Rawers, M. Grujicic, Mater. Sci. Eng. A 207 (1996) 188–194. R.E. Schramm, R.P. Reed, Metall. Trans. A 6A (1975) 1345–1351.
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EVect of selective dissolution on fatigue crack initiation in 2205 duplex stainless steel I-Hsuang Lo, Wen-Ta Tsai
¤
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan Received 15 December 2005; accepted 9 October 2006 Available online 26 December 2006
Abstract The eVect of selective dissolution on fatigue crack initiation of 2205 duplex stainless steel (DSS) was investigated in this study. In mixed sulfuric and hydrochloric acid aqueous solution, there existed two distinctly separated anodic peaks in the active-to-passive transition region of the polarization curve. Either ferritic or austenitic phase was selectively dissolved at each characteristic anodic peak potential. Under sinusoidal cyclic loading condition, however, selective dissolution did not assist fatigue crack initiation instead resulting in the elimination of stress concentration site in the selectively dissolving phase. As a consequence, under selective dissolution condition, fatigue crack initiated in the phase while its dissolution rate was lower with respect to the other constituent phase in the duplex stainless steel. The microstructural evolution of the corrosion fatigue crack initiation in 2205 DSS in the mixed sulfuric and hydrochloric acid solution is highlighted in this investigation. © 2006 Elsevier Ltd. All rights reserved. Keywords: A. Duplex stainless steel; C. Selective dissolution; C. Corrosion fatigue crack initiation
1. Introduction It is known that fatigue cracks initiate preferentially in sites where stress concentration occurs or microstructure inhomogeneity exists. For instance, Rios et al. [1] indicated that fatigue crack would initiate in the concave surface rather than the convex part of *
Corresponding author. Tel.: +886 6 2757575x62927; fax: +886 6 2754395. E-mail address: [email protected] (W.-T. Tsai).
0010-938X/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2006.10.013
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I.-H. Lo, W.-T. Tsai / Corrosion Science 49 (2007) 1847–1861
shot-pinned A316 stainless steel (SS). Rokhlin et al. [2] reported that fatigue crack initiation strongly depends on the depth of artiWcial pits. In 304 SS, the precipitation of -ferrite in the / grain boundary would lead to the formation of an incoherent interface. Rho et al. [3–5] pointed out that this incohent / interface became the vulnerable site for fatigue crack initiation. In duplex stainless steels (DSSs), fatigue crack initiation was rather complicated. For example, Llanes et al. [6,7] indicated that fatigue crack would initiate at -phase or at the / interfaces of unaged 2205 DSS. On the contrary, fatigue crack could initiate at -phase if 2205 DSS was aged at 475 °C for 200 h. Beside the microstructural eVect mentioned above, the environmental eVect is also very important in aVecting fatigue crack initiation. Localized corrosion, especially pitting corrosion, is known to enhance the fatigue crack initiation susceptibility in 316L SS [8], 403 SS [9] and X-52 steel [10], etc. Though fatigue behavior in 2205 DSS in air has been investigated by some researchers [6,7], the corrosion fatigue crack initiation is less explored. Previous investigations [11,12] have found that selective dissolution could occur in 2205 DSS in H2SO4/HCl solution. The eVect of selective dissolution on the fatigue crack initiation is thus of interested and investigated in this study. 2. Experimental 2.1. Material preparation A 2205 DSS with the form of square bar, 30 mm £ 30 mm, was used. The chemical composition is listed in Table 1. This materials was heat treated at 1100 °C for 30 min then water quenched before the subsequent tests. The metallographs of three diVerent surfaces, after etching in boiling Murakami reagent, are demonstrated in Fig. 1a. As shown in Fig. 1a, the dark phase is ferrite () while the white phase is austenite (). The volume ratio of / is 53/47. The major alloying elements, namely, Fe, Cr, Ni, Mo and Mn, were analyzed by an energy dispersive spectrometer (EDS) to identify the individual phase and also listed in Table 1. 2.2. Electrochemical tests Potentiodynamic polarization test was conducted at a scan rate of 1 mV/s in the range of ¡500 mVSCE to 0 mVSCE from cathodic towards anodic direction. Potentiostatic etching at the anodic peak potential corresponding to selective dissolution of either or was also performed. All potentials were measured with respect to a saturated calomel electrode
Table 1 Chemical compositions (in wt.%) of 2205 DSS and the constituent ferritic and authentic phases Element 2205 DSS -phaseb -phaseb a b
a
Fe
Cr
Ni
Mo
Mn
Si
C
Cu
P
S
N
Balance Balance Balance
22.40 23.5 20.5
5.42 4.8 7.2
3.24 3.5 1.0
1.43 1.0 2.0
0.41 – –
0.014 – –
0.21 – –
0.025 – –
0.004 – –
0.198 – –
Glow discharge spectrometer (GDS). Energy dispersive spectrometer (EDS).
I.-H. Lo, W.-T. Tsai / Corrosion Science 49 (2007) 1847–1861
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Fig. 1. (a) Optical micrograph of 2205 duplex stainless steel, solution heat treated at 1100 °C for 30 min followed by water quench; (b) geometry and dimension of fatigue specimen.
(SCE) and a platinum sheet was used as the counter electrode. After potentiostatic etching, the surface morphology was examined with a scanning electron microscope (SEM), while major alloying elements were analyzed by EDS. 2.3. Corrosion fatigue tests Rectangular shape of specimen with the geometric orientation and dimension shown in Fig. 1b was used for fatigue test. A sinusoidal waveform of load with a frequency of 10 Hz was used. The initial maximum stress of 1.2 ys (yield strength) and a load ratio (R) of 0.5 was applied for fatigue test. The loading was parallel to the short transverse (S) direction, as demonstrated in Fig. 1b. Fatigue tests were conducted in air and in 2 M H2SO4 + 0.7 M HCl aqueous solution at open circuit potential (OCP) and at ¡300 and ¡240 mVSCE, respectively, all at ambient temperature. After determining the fatigue life (Nf), some interrupted fatigue tests at certain loading cycles were also performed. The cross section and fracture surface were examined microscopically. 3. Results and discussion Fig. 2(a) presents the potentiodynamic polarization curve of 2205 DSS determined in the air-exposed 2 M H2SO4 + 0.7 M HCl aqueous solution. The active-to-passive transition peak consisted of two distinct peaks at ¡300 and ¡240 mVSCE, respectively. This characteristic feature of polarization curve has been reported previously [11,12]. Potentiostatic etching at OCP (¡350 mVSCE), ¡300 or ¡240 mVSCE for 3 h was performed. The respective surface morphologies are shown in Fig. 2b–d. At OCP and ¡300 mVSCE, preferential dissolution of was observed, while that of was found at ¡240 mVSCE. The average fatigue life of 2205 DSS in air was 4.05 £ 105 cycles. Interrupted fatigue test after loading for about 0.6 Nf was followed by surface micrographical examination. Fig. 3 is the SEM micrograph showing the surface morphology of S–T plane after interrupted test. As revealed in this Wgure, slip bands were formed in both phases. However, fatigue cracks were observed in -phase rather than in -phase. The result indicated that -phase was more prone to crack initiation, consistent with that reported by Llandes et al. [6,7].
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Current density, logi (A/cm2)
1E-002
2205 Duplex stainless steel in air-exposed 2 M H2SO4 + 0.7 M HCl
1E-003 -240 mV
SCE
-300 mVSCE 1E-004
1E-005 -600
OCP
-400 -200 Potential (mV vs. SCE)
0
Fig. 2. (a) Potentiodynamic polarization curve of 2205 duplex stainless steel in air-exposed 2 M H2SO4 + 0.7 M HCl aqueous solution. Micrographs of 2205 DSS after potentiostatic etching at (b) OCP, (c) ¡300, and (d) ¡240 mVSCE for 3 h, respectively.
It has been reported that -phase could be more easily cyclic-hardened than -phase [13]. Furthermore, the high nitrogen content, as the DSS used in this study, would lead to solid solution strengthening [14,15] and lower the stacking fault energy [16] of -phase. These factors were all responsible for the increase of resistance to fatigue crack initiation in -phase. Therefore, dislocation would glide and condense mostly in the weaker -phase. In order to release the stress, a new surface would be created, and a fatigue crack would initiate in -phase while 2205 DSS fatigued in air. The average fatigue life obtained in 2 M H2SO4 + 0.7 M HCl aqueous solution at OCP and at a frequency of 10 Hz was 1.27 £ 105. The substantial reduction in fatigue life as compared with that obtained in air indicated the importance of environmental eVect on fatigue behavior of 2205 DSS. Fig. 4a shows the surface morphology of the specimen
I.-H. Lo, W.-T. Tsai / Corrosion Science 49 (2007) 1847–1861
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Fig. 3. SEM micrograph showing the surface morphology of 2205 duplex stainless steel after fatigue in air for 0.6 Nf cycles.
fatigued after 0.6 Nf cycles. The SEM micrograph revealed that -phase was slightly and preferentially dissolved, and the slip bands were concentrated in this phase, which resulted in crack initiation. As revealed in Fig. 4a, cracks were also formed at the / interfaces. The cross section of the specimen after loading for 0.6 Nf cycles was also examined. As shown in Fig. 4b, cracks were certainly initiated in -phase and at / boundaries. This Wgure also shows that the cracks propagated in -phase and along / boundaries. As mentioned above, slip bands would be formed favorable -phase at OCP, though preferential dissolution occurred in -phase, the slight corrosion did not cause the elimination of these slip bands. Thus, fatigue crack still initiated in -phase, as demonstrated in Fig. 4. Fig. 5a shows the surface appearance of the specimen after fatigue loading in 2 M H2SO4 + 0.7 M HCl aqueous solution at ¡300 mVSCE for 0.4 Nf cycles, where Nf t 1.06 £ 105 cycles. Similar to that tested at OCP, selective dissolution occurred in -phase. However, unlike that found at OCP, the cracks were found initiated in -phase. The cross section of the specimen fatigued for 0.6 Nf cycles is depicted in Fig. 5b. Cracks mainly initiated and propagated in -phase. Though the slip bands could be generated in -phase, fast selective dissolution could smooth out these bands easily. Thus, the stress raisers could not be developed in -phase, and crack initiation in this selectively dissolved phase was retarded. At ¡240 mVSCE, selective dissolution in -phase took place during fatigue test in 2 M H2SO4 + 0.7 M HCl aqueous solution. Fig. 6a shows the surface appearance after fatigue loading for 0.4 Nf cycles (Nf t 1.04 £ 105 cycles). Again, selective dissolution did not favor crack initiation in -phase. Instead, fatigue initiated in -phase and propagated through it, as demonstrated in Fig. 6b. Though it is generally found that localized corrosion, such as pitting corrosion [8–10], always assists fatigue crack initiation, it is not the case for selective
1852
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Fig. 4. SEM micrographs showing: (a) the surface morphology, and (b) the cross section of 2205 duplex stainless steel after fatigue loading for 0.6 Nf cycles in 2 M H2SO4 + 0.7 M HCl aqueous solution at OCP.
dissolution in 2205 DSS. It is clear that environmentally-assisted fatigue crack initiation may not be prevailed unless an active stress raiser can be developed by the assistance of localized corrosion. As revealed in Figs. 5 and 6, the elimination of slip bands and stress concentration sites in one of the constituent phase in 2205 DSS could occur owing to the selective dissolution. The eVect of selective dissolution on fatigue crack initiation in 2205 DSS could be explained by the schematic diagram demonstrated in Fig. 7. At the early stage of fatigue test, the slip bands could be formed on both phases as illustrated in Fig. 7a. When selective dissolution occurred, particularly at a relative high dissolution rate, the slip bands and the
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Fig. 5. SEM micrographs showing the surface and cross section morphologies of 2205 duplex stainless steel after interrupted fatigue test at ¡300 mVSCE for (a) 0.4 Nf cycles, (b) 0.6 Nf cycles in 2 M H2SO4 + 0.7 M HCl aqueous solution.
associated stress concentration sites could be eliminated in this speciWc phase. Thus, the stress raisers resulting from repeated loading only remained on the other phase, as demonstrated in Fig. 7b. Consequently, as shown in Fig. 7c, a stable fatigue crack could be developed in this phase though with a lower dissolution rate. As fatigue process continued, the crack could extend to the adjacent phase in which selective dissolution prevailed, as depicted in Fig. 7d. The above mechanism could be applied to either - or -phase when a signiWcant diVerence in dissolution rate between them existed during fatigue in aggressive environment. As mentioned above, however, when the stress raiser could not be eliminated
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Fig. 6. SEM micrographs showing the surface and cross section morphologies of 2205 duplex stainless steel after interrupted fatigue test at ¡240 mVSCE for (a) 0.4 Nf cycles, and (b) 0.6 Nf cycles in 2 M H2SO4 + 0.7 M HCl aqueous solution.
by selective dissolution (as at OCP), the phase less resistant to dislocation slip was prone to initiate fatigue crack. The fracture surfaces of 2205 DSS fatigued in air and in 2 M H2SO4 + 0.7 M HCl aqueous solution at various potentials were examined under SEM. Fig. 8a shows the macro image of the fracture surface failed in air. The fatigue crack propagated in transgranular mode, see Fig. 8b, through region I as marked in Fig. 8a. Region II was the area where Wnal overload fracture occurred, as characterized by the dimple mode of fractograph shown in Fig. 8c.
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Fig. 7. Schematic diagram of selective dissolution eVect on corrosion fatigue crack initiation in 2205 DSS.
Fig. 9 shows the macro and micrographs of the fracture surface fatigued in 2 M H2SO4 + 0.7 M HCl aqueous solution at OCP. The fatigue crack also propagated transgranularly as depicted in Fig. 9b, followed by dimple mode of fracture (Fig. 9c). Figs. 10 and 11 demonstrate the fractographs of 2205 DSS fatigue fractured in the acid solution at ¡300 and ¡240 mVSCE, respectively. At ¡300 mVSCE, fatigue crack propagated through region I in Fig. 10a. Under high magniWcation, as revealed in Fig. 10b, the crack surface exhibited a feature showing the extruded -phase with -phase at concave site. At ¡240 mVSCE, on the contrary, the fractograph in region II (Fig. 11a) revealed that -phase was at the convex site while -phase grooves existed on the fractured surface as shown in Fig. 11b. The fractographs revealed in Figs. 10 and 11 suggested that selective dissolution participated in the fatigue propagation process. Close examination of the surface of the fatigued specimen was performed. Fig. 12a shows the SEM micrograph of the S–T plane of 2205 DSS fatigued at ¡300 mVSCE. As revealed in this micrograph, trnsgranular cracking occurred in -phase (with extensive slip bands still remaining on its surface) while selective dissolution of -phase (leaving holes between two adjacent grains)
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Fig. 8. SEM fractographs of 2205 DSS fatigue fractured in air: (a) Macrograph, (b) micrograph of region I, (c) micrograph of region II as marked in (a).
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Fig. 9. SEM fractographs of 2205 DSS fatigue fractured in 2 M H2SO4 + 0.7 M HCl aqueous solution at OCP: (a) Macrograph, (b) micrograph of region III, (c) micrograph of region IV as marked in (a).
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Fig. 10. SEM fractographs of 2205 DSS fatigue fractured in 2 M H2SO4 + 0.7 M HCl aqueous solution at ¡300 mV SCE: (a) Macrograph (b) micrograph of region I as marked in (a).
along the cracking path was observed. The characteristic feature of the fatigue cracking path under selective dissolution of -phase is illustrated in the schematic diagram shown in Fig. 12b. The joining of the fatigue crack in -phase and the selective dissolution of -phase resulted in the convex/concave feature of the fracture surface as demonstrated in Fig. 10. Similar process was found when 2205 DSS was fatigued at ¡240 mVSCE, giving rise to a characteristic fracture surface appearance as revealed in Fig. 11.
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Fig. 11. SEM fractographs of 2205 DSS fatigue fractured in 2 M H2SO4 + 0.7 M HCl aqueous solution at ¡240 mVSCE: (a) Macrograph, (b) micrograph of region II as marked in (a).
4. Conclusions Selective dissolution aVected fatigue crack initiation in 2205 DSS in 2 M H2SO4 + 0.7 M HCl aqueous solution in a complicated manner. At OCP, fatigue crack initiated in -phase as well at /-phase boundaries. At ¡300 mVSCE, selective dissolution occurred in -phase, however, fatigue crack initiated in -phase. On the contrary, phase acted as the fatigue crack initiation site when preferential dissolution of -phase
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Selectively dissolved α
α
α
α γ
γ
γ
γ
γ
γ
γ
γ
α
α
Crack
α
Fig. 12. (a) SEM micrograph showing the surface crack in 2205 DSS fatigued in 2 M H2SO4 + 0.7 M HCl solution at ¡ 300 mVSCE, and (b) schematic diagram showing the involvement of selective dissolution of -phase in fatigue crack growth process in 2205 DSS.
occurred at an applied potential of ¡240 mVSCE. Instead of assisting fatigue crack initiation, selective dissolution could cause the elimination of stress raiser and consequently retard fatigue crack initiation. The participation of selective dissolution was also found in the fatigue crack growth stage at which the crack mainly propagated transgranularly. References [1] [2] [3] [4] [5] [6]
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