Improving the intergranular corrosion resistance of austenitic stainless steel by high density twinned structure

Improving the intergranular corrosion resistance of austenitic stainless steel by high density twinned structure

Scripta Materialia 130 (2017) 264–268 Contents lists available at ScienceDirect Scripta Materialia journal homepage: www.elsevier.com/locate/scripta...

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Scripta Materialia 130 (2017) 264–268

Contents lists available at ScienceDirect

Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat

Improving the intergranular corrosion resistance of austenitic stainless steel by high density twinned structure A.Y. Chen a,⁎, W.F. Hu a, D. Wang a, Y.K. Zhu a, P. Wang a, J.H. Yang a, X.Y. Wang a,⁎, J.F. Gu b, J. Lu c a b c

School of Material Science & Engineering, University of Shanghai for Science and Technology, Shanghai, 200093, China School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200030, China Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Hong Kong, China

a r t i c l e

i n f o

Article history: Received 17 October 2016 Received in revised form 24 November 2016 Accepted 25 November 2016 Available online xxxx Keywords: Stainless steel Intergranular corrosion Twinned structure Passivation

a b s t r a c t We investigated the effect of high density twins on the intergranular corrosion (IGC) of austenitic stainless steel after sensitization. The twinned samples, especially the one with a twin density over 86% area fraction, exhibited a remarkable IGC resistance, characterized by a higher corrosion potential, a broader passivation zone, and a lower corrosion rate. The chromium depletion was inhibited by the twin boundaries emitted at grain boundaries, while the passivation was enhanced by the chromium enrichment at nanotwins inside the coarse grains. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Austenitic stainless steels (SS) of type 304 and 316 L are widely used as a structural material due to the good mechanical properties and excellent corrosion resistance [1,2]. However, intergranular corrosion (IGC) is a serious problem for austenitic SS exposed to aggressive temperature of 450–850 °C [3]. The major reason of IGC is the so-called sensitization, i.e., chromium depletion at grain boundary (GB) due to the precipitation of chromium carbide [4]. Twin boundary (TB), as a special low-energy boundary, can not only significantly improve the strength and ductility of the face-centered cubic metals [5–8], but also effectively suppress carbide precipitation at the GBs [9–12]. Thus, grain boundary engineering (GBE) concept is proposed to improve the IGC by creating discontinuous low-energy segments free of chromium carbide through the incident TBs at the GBs [13–15]. Limited by the small quantity of twins prepared by the conventional methods [16,17], the kernel thought of GBE is to manipulate the distribution of the low-energy TBs emitted at GBs [18–21]. It is expected that the high density twins should totally evade the chromium depletion at GBs, but no evidence has yet been obtained. More importantly, with the occurrence of high density twins, especially those in much refined scale, the function of TBs located inside the coarse grains (CG) should also be taken into account. Because TB networks occupy a larger area fraction compared with the GBs and they also are active interfaces relative to the regular crystal structure. Nevertheless, the effects of these TBs on the corrosion performance are missing in the GBE design. In this work, we examined IGC ⁎ Corresponding authors. E-mail addresses: [email protected] (A.Y. Chen), [email protected] (X.Y. Wang).

http://dx.doi.org/10.1016/j.scriptamat.2016.11.032 1359-6462/© 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

performance of 304 SSs with different twin densities by using the potentiodynamic polarization and mass loss tests. The effects of TB networks on the chromium depletion at the GBs and chromium enrichment at the TBs are investigated. The chemical compositions of AISI 304 SS are 0.04 C, 0.49 Si, 1.65 Mn, 7.8 Ni, 16.8 Cr, 0.37 Mo and the balance Fe (all in mass%). The high density twins in nanoscale and submicroscale were produced by surface mechanical attrition treatment (SMAT) at a high impacting frequency. The detailed high-speed SMAT experimental setup and procedures were presented in Ref. [22]. The SMATed 304 SS was sensitized at 650 °C for 2 h and furnace cooling. Two types of twinned specimens after sensitization were prepared by polishing 100 and 250 μm depth from the surface, referred to as TW-100 and TW-250, respectively. Potentiodynamic polarization behaviors of the as-received 304 SS, the sensitized 304 SS and the twinned 304 SS samples were tested in 3.5% NaCl solution at a scan rate of 0.2 mV s−1 after immersing for 10 min. Electrochemical impedance spectroscopy (EIS) measurements were performed under 5 mV amplitude of sinusoidal potential signals with respect to the open-circuit potential over a frequency range from 100 kHz to 10 mHz. The mass lose tests were carried out in 65% (wt%) HNO3 solution at boiling temperature. The microstructure of the sensitized 304 SS, TW-100 and TW-250 samples are given in Fig. 1. The grain size of the sensitized 304 SS (Fig. 1a) is about 15 μm. Deep grooves are clearly observed along the GBs. The magnification shows that rod-shaped precipitates with a length approx. 300 nm and a width 50 nm are formed inside the grooves, as indicated by the arrows in Fig. 1b. The line distribution of Fe, Cr, and Ni elements across the GB (dotted line in Fig. 1b) are shown in the lower

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Fig. 1. The microstructure of the 304 SS and the twinned SS after sensitization at 650 °C for 2 h. (a) and (b) SEM images of the sensitized 304 SS; (c) and (d) are the SEM and TEM images of the TW-100 sample, respectively; (e) and (f) are the SEM and TEM images of the TW-250 sample, respectively; (b) is the magnification of the rectangular zone in (a) and the lower inset in (b) is the element distribution along the dotted line. The first upper inset in (d) is the magnification of the rectangular zone in (d), these lower insets are the mappings of Cr, Ni, and Fe elements from upper to lower. The upper inset in (f) is the SAED pattern of the grain A, and the relative lower inset is the magnification of the rectangular zone.

inset of Fig. 1b, where the rod-shaped precipitate exhibits a higher Cr content. The SEM images of TW-100 (Fig. 1c) and TW-250 (Fig. 1e) samples exhibit that most of the CGs contain high density twins with twin spacing in submicroscale and twin length in microscale, which separate the CGs into twin/matrix lamellar networks. The twin density is characterized by an area fraction of the grains containing twins to the total selected zone. The statistic data estimated from 10 pieces of SEM images show that the twin density is 86% for the TW-100 and 65% for the TW-250 specimens, respectively. Furthermore, the TEM image (Fig. 1d) of the TW-100 specimen displays that lots of nanotwins exist inside the grains. The insets in Fig. 1d are the mappings of the Cr, Ni, and Fe, elements in a nanotwin with a twin spacing of 30 nm, where the Cr atoms are enriched, and the Fe and Ni atoms correspondingly decrease. From the left to the right marked by A, B, and C in the second upper inset of Fig. 1d, the Cr content is 14.4%, 18.7%, and 14.0% (wt%), respectively. In order to observe the detailed interface structure of the GBs intersected with TBs, the TEM image of the TW-250 specimen is given in Fig. 1f. The GB between the two grains A and B is clear, no

precipitation is observed along the whole GB. The grain A contains lots of nanotwins, which are emitted from the GB, as shown in the inset of Fig. 1f. The corresponding SAED pattern of grain A is given in the upper inset, which verifies the twinned structure. Since the nanotwins are not stable thermally at higher temperature, therefore, the deformed nanotwins produced by the SMAT were used as the starting structure without further annealing at high temperature. The SEM and TEM observations both confirm that the high density nanotwins retain after sensitization at 650 °C (in Fig. 1c-f). The other research work found that the deformed nanotwins can endure the heat treatment up to 800 °C [23]. It should be mentioned that the deformed twins in the two twinned samples still contain high density dislocations after sensitization, as shown in Fig. 1d and f. Compared with the sensitized 304 SS, the two twinned samples have few grooves at the GBs with incident TBs. Nevertheless, a few GBs without incident TBs show deeper cracks, especially in the TW-250 sample, as indicated by dotted arrows in Fig. 1c and e. Moreover, no precipitation are observed in the SEM and TEM images of the twinned samples (Fig. 1c–f), indicating

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that the high density nanotwins can inhibit the formation of chromium carbides at GBs. Generally, the corrosion potential (Ecorr) and the corrosion current density (Icorr) are used to characterize the active dissolution ability of materials, while the passive current density (Ipassive) and the passivation potential are used to evaluate the chemical stability and corrosion resistance of passive films [24–26]. Fig. 2a gives the potentiodynamic polarization curves of the four types of 304 SS. The Ecorr of the sensitized 304 SS is −0.31 V (vs SCE), lower than −0.25 V of the as-received 304 SS, and no passivation is observed. However, the Ecorr of both TW-100 and TW-250 specimens are significantly higher than that of the as-received 304 SS, shifting toward the positive potential of − 0.12 and −0.20 V, respectively. Meanwhile, the Icorr of TW-100 sample decreases one order of magnitude compared with the as-received 304 SS. Furthermore, an obvious passivation is observed in the twinned samples, where the passive potential zone is from −0.11 to 0.19 V for the TW-100 and from −0.18 to 0.18 V for the TW-250, wider than that of the as-received sample (−0.22 to −0.11 V). The Ipassive of the as-received 304 SS, TW100, and TW-250 specimens are 5.2 × 10−7, 5.1 × 10− 8, and 1.2 × 10− 7 A/cm− 2, respectively. The TW-100 specimen has the smallest Ipassive among all the samples, one order of magnitude smaller than the as-received 304 SS. The above results verify that the twinned

Fig. 2. Potentiodynamic polarization curves (a), Nyquist plots (b) and mass loss curves (c) of the as-received, sensitized 304 SS, TW-100, and TW-250 samples.

samples have better IGC resistance than the sensitized 304 SS, and even better than the as-received 304 SS. Especially, the TW-100 sample exhibits the higher Ecorr, the lower Icorr and Ipassive than the TW-250 sample, and thus, the higher twin density, the better IGC resistance. The EIS measurements were carried out to reflect the passivation behavior, as shown in Fig. 2b. All Nyquist plots have a common feature of a single capacitive semicircle over the frequency range, which could be fitted by an equivalent circuit (the inset in Fig. 2b). The Rs represents electrolyte solution resistance, Rt is the charge transfer resistance, and Qdl is the double layer capacitance. The Rt of the sensitized 304 SS is approx. 1.6 × 104 Ω, while that of the as-received 304 SS, TW-100 and TW-250 specimens are 3.8 × 104, 5.4 × 105 and 2.0 × 105 Ω, respectively. The TW-100 specimen exhibits the highest charge transfer resistance, which suggests that the passive film on the TW-100 surface are denser than the other samples due to the higher twin density. Mass loss per area of each specimen as a function of the exposure time is shown in Fig. 2c. Compared with the sensitized 304 SS, the two twinned specimens have much lower corrosion rates. Although the two twinned samples have higher corrosion rates than the as-received 304 SS after immersing 5 h, lower corrosion rates at the initial stage are observed (inset in Fig. 2c), suggesting a rapid formation of the passivation film on the twinned surface. The corrosion morphologies of the sensitized and the twinned samples after etching at initial 4 h and final 35 h are observed (in Fig. 3). In the early stage of 4 h, corrosion attacks the whole GBs of the sensitized 304 SS sample (Fig. 3a), whereas only a few TBs occur on the TW-100 surface and some TBs appear on the TW-250 surface, as indicated by arrows in Fig. 3b and c, respectively. Actually, the IGC of the sensitized sample propagates along GBs from the surface into the material interior and causes mass loss due to grain dropping, as indicated by arrow in Fig. 3a. Thereafter, the corrosion rate is accelerated with the drop of the grains, characterized by an inflection point at 5 h in the sensitized 304 SS sample (in Fig. 2c). With the increase of immersing time, significant grain dropping emerges in the sensitized 304 SS, as shown in Fig. 3d, and correspondingly the corrosion rate rapidly increases. Nevertheless, the surface morphologies of the twinned samples exhibit obvious TB and GB corrosions, as indicated in Fig. 3e and f, where the grain dropping is depressed, especially in the TW-100 specimen. Noted that the TW-250 specimen exhibits two types of segments, one is the detached twin lamella from the surface, and the other is the detached grain without twins interior, as indicated by the solid and dotted arrows in Fig. 3f, respectively. The fall of the twin lamella in the TW-250 sample, together with the grains, results in an inflection point at 14 h in the mass loss curve of Fig. 2c. In contrast, the TW100 specimen exhibits a uniform corrosion, characterized by a relatively stable increase of mass loss. The cross-sectional SEM images of the three samples after immersing corrosion for 35 h are shown in Fig. 3g–i. The sensitized 304 SS exhibits an obvious IGC corrosion, and the depth of crack propagation is up to 62 μm. However, the TW-100 sample displays a relative flat surface and no IGC cracks are observed, as shown in Fig. 3h. A few IGC cracks occur in the TW-250 sample, as indicated by arrows in Fig. 3i. Thus, the twinned samples possess much better IGC resistance than the sensitized 304 SS. Compared with annealing twins in GBE, the deformed twins in the twinned samples contain high density dislocations after sensitization, as shown in Fig. 1d and f. Most research works indicate that dislocation multiplication enhances the pit initiation and decreases the passivation ability due to the high stress concentration although the dynamic recovery of dislocations partially alleviates the detrimental effect [9,27]. Hence, the dislocations in the deformed twins might degrade the corrosion properties. However, the effect of nanotwins is predominant because the two twinned samples exhibit an obvious improvement of IGC resistance. The high IGC resistance of the twinned 304 SS can be attributed to two functions of the high density TB networks, as illustrated in Fig. 4. The first is the high density TBs, including those incident TBs (red solid line in Fig. 4a) and those inside the grains (red dotted line in Fig. 4a), which can accelerate the formation of passivation film.

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Fig. 3. Corrosion morphologies of the sensitized 304 SS, TW-100 and TW-250 samples after etching 4 (a–c) and 35 h (d–i) in 65% HNO3 solution at boiling temperature. (a)–(f) are the surface morphologies, while (g)–(i) are the cross-sectional morphologies of the (d)–(f), respectively. All images at the horizontal direction have the same scale bar.

Although the TB is a low-energy interface, the deformed twins still contain many defects at TBs [28]. Therefore, the TBs, especially for the nanotwins inside the grains, are active for the micro zone diffusion of C, Cr and other elements, as illustrated in grain A in Fig. 4a–c. Actually, the enrichment of Cr atoms at nanotwin is observed by the elemental mapping (upper second inset in Fig. 1d), which is beneficial for the formation of the dense passivation film as estimated from the EIS fitting. The second is these incident TBs emitted from GBs to inhibit the precipitation of chromium carbides. According to the percolation theory in GBE [29], the resistance to IGC depends on the disconnectivity of the precipitates at GBs interrupted by the TBs since the successive chromium carbides accelerate the drop-off of the CGs, as shown in Fig. 3d and g. Fig. 4b and c illustrate the accumulation and precipitation of

chromium and carbon atoms at the GB1–2, while the interruption of connectivity of chromium carbides by the incident TB is presented in the GB2–3. However, for the GB2–4, the chromium and carbon atoms diffused to the GB2–4 are retarded by the high density incident TBs, and conversely, are enriched by the nanotwins close to the GB2–4. Thus, the concentration of the chromium and carbon atoms at the GB2–4 is lower than that of the GB1–2, and no chromium carbides are formed at the GB2–4, as illustrated in Fig. 4b and c. The homogeneously distributed TB networks, especially for the TW-100 sample with much denser and finer twins, can stimulate the formation of passivation film and inhibit the precipitation of chromium carbide, resulting in a higher corrosion potential, a wider passivation range and a lower corrosion rate.

Fig. 4. Schematic illustration of the element diffusion in the twinned structure during sensitization.

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The AISI 304 SS with a high density twinned structure exhibits an excellent resistance to intergranular corrosion after sensitization at 650 °C for 2 h. The potentiodynamic polarization results show that twinned samples, especially the TW-100, possess higher Ecorro and lower Icorro than the sensitized and the as-received 304 SSs. The corrosion morphologies of the twinned samples indicate that the uniform corrosion of the TBs and GBs occurs, while the grain dropping is significantly inhibited, especially in the TW-100 sample with denser and finer twins. The high frequency of TBs can suppress the chromium depletion by twinemission at the GBs and stimulate the formation of passivation film by the enrichment of Cr atoms at the TB networks inside the grains. Acknowledgement The authors are grateful to the supports from NSFC (nos. 11202134, 51271123, 51402193, 51572173 and 11402149). References [1] C. Hu, S. Xia, H. Li, T. Liu, B. Zhou, W. Chen, N. Wang, Corros. Sci. 53 (2011) 1880. [2] P.M. Ahmedabadi, V. Kain, K.V. Muralidhar, I. Samajdar, J. Nucl. Mater. 432 (2013) 243. [3] D.N. Wasnik, V. Kain, I. Samajdar, B. Verlinden, P.K. De, Acta Mater. 50 (2002) 4587. [4] L.V. Jin-long, L. Hong-yun, Mater. Chem. Phys. 135 (2012) 973. [5] X. Guo, G. Yang, G.J. Weng, L.L. Zhu, Mater. Sci. Eng. A 630 (2015) 13. [6] Y. Wei, Y. Li, L. Zhu, Y. Liu, X. Lei, G. Wang, Y. Wu, Z. Mi, J. Liu, H. Wang, H. Gao, Nat. Commun. 5 (2014) 4580.

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