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Effect of copper addition on mechanical properties, corrosion resistance and antibacterial property of 316L stainless steel
Materials Science and Engineering C 71 (2017) 1079–1085
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Effect of copper addition on mechanical properties, corrosion resistance and antibacterial property of 316L stainless steel Tong Xi a,b,1, M. Babar Shahzad b,1, Dake Xu a, Ziqing Sun b, Jinlong Zhao b, Chunguang Yang b,⁎, Min Qi a, Ke Yang b,⁎ a b
School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
a r t i c l e
i n f o
Article history: Received 2 September 2016 Received in revised form 10 October 2016 Accepted 7 November 2016 Available online 9 November 2016 Keywords: Cu-bearing stainless steel Mechanical properties Corrosion resistance Antibacterial performance
a b s t r a c t The effects of addition of different Cu content (0, 2.5 and 3.5 wt%) on mechanical properties, corrosion resistance and antibacterial performance of 316L austenitic stainless steel (SS) after solution and aging treatment were investigated by mechanical test, transmission electron microscope (TEM), X-ray diffraction (XRD), electrochemical corrosion, X-ray photoelectron spectroscopy (XPS) and antibacterial test. The results showed that the Cu addition and heat treatment had no obvious influence on the microstructure with complete austenite features. The yield strength (YS) after solution treatment was almost similar, whereas the aging treatment obviously increased the YS due to formation of tiny Cu-rich precipitates. The pitting and protective potential of the solution treated Cubearing 316L SS in 0.9 wt% NaCl solution increased with increasing Cu content, while gradually declined after aging, owing to the high density Cu-rich precipitation. The antibacterial test proved that higher Cu content and aging were two compulsory processes to exert good antibacterial performance. The XPS results further indicated that aging enhanced the Cu enrichment in passive film, which could effectively stimulate the Cu ions release from the surface of passive film. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Austenitic stainless steels (SS) have been widely used in many fields such as surgical implant material, food industry, transportation and daily appliances, owing to their superior mechanical properties, corrosion resistance and workability [1]. As a strengthening element copper (Cu) is often added into steel to improve the corrosion resistance and creep resistant strength [1]. In recent years, Cu-bearing antibacterial austenitic SS, developed as a new kind of structural/functional integrated materials, have attracted great attention as a potential biomaterial for implant devices that could help in reduction of the risk of implant related bacterial infections [2,3]. The addition of proper amount of Cu into the steel and an appropriate aging treatment results in homogeneous distribution of high number density of Cu-rich precipitates within the steel matrix. These nano-sized precipitates offer certain attractive properties to the steel that have promising applications in various fields. During the past decade, a variety of studies have been focused on the mechanical properties, hot deformation behavior, antibacterial performance and corrosion resistance of the Cu-bearing austenitic SS [4–8]. As well known, Cu can stabilize the austenitic structure and make it ⁎ Corresponding authors. E-mail addresses: [email protected] (C. Yang), [email protected] (K. Yang). 1 Tong Xi and M. Babar Shahzad equally contributed to this work.
http://dx.doi.org/10.1016/j.msec.2016.11.022 0928-4931/© 2016 Elsevier B.V. All rights reserved.
possible to reduce the Ni content in the steel, which leads to a significant economic saving since the price of Ni is relatively high [9–11]. Besides, the Cu addition can cause an increase in the stacking fault energy (SFE) and subsequently improve the deep drawing quality of steel. However, Cu addition also has an adverse effect on the hot forging property due to the formation of low melting point Cu-rich eutectic phases that preferentially segregate at the grain boundaries and steels surface [12,13]. In the case of antibacterial property of Cu-bearing SS, some researchers [8,14–16] reported that the dissolved Cu ions released from the steel matrix play a dominant role for the antibacterial effects of SS. The released Cu ions could effectively kill the bacteria by collapsing their outer cell membranes. For instance, Dan et al. [17] proposed that Cu ions could adhere to the surface of the bacterial cells and damage the proteins structure in the bacterial cells. In addition, Cu has been added into steels to improve the corrosion resistance. It is well accepted that the addition of Cu can suppress the anodic dissolution when immersed in the acid solutions [18]. As regards the influence of Cu on the corrosion behavior of SS, there are some controversial conclusions in different literatures. Some researchers [19–22] pointed out that Cu addition inhibits the dissolution of soluble sulfide inclusions in SS, due to the re-deposition of previously dissolved Cu on the steel surface and later forming of CuCl2 film, which is beneficial to decrease the corrosion rate and improve the crevice corrosion potential
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Table 1 Chemical composition (wt%) of the experimental steels. No.
Cr
Mo
Ni
Cu
C
Si
S
P
Fe
316L 316L-2.5Cu 316L-3.5Cu
17.61 17.76 17.77
3.14 3.17 3.11
14.4 14.5 14.6
0.02 2.46 3.35
0.004 0.006 0.004
0.24 0.39 0.56
0.004 0.003 0.004
0.007 0.008 0.008
Bal. Bal. Bal.
of SS in acid chloride media. However, other researchers also reported that alloying Cu has a detrimental effect on the passivation [23] and localized corrosion, such as pitting potential [23] and intergranular corrosion [24]. They believed that Cu delays the chromium enrichment on the passive film at the first stage of passivation, and thus declines the stability of passive film and pitting resistance of the SS. Despite the numerous studies related to the influence of Cu addition on microstructure, mechanical, corrosion resistance and antibacterial performance, studies on comparison between the solution and aging treatment for different Cu additions and its effect on various properties are still lack systematic understanding. Hence, in this work, the effect of Cu content and different heat treatments on the mechanical properties, corrosion resistance and antibacterial property of Cu-bearing 316L stainless steel (316L-Cu SS) were investigated. In order to address these problems, hardness and tensile test were performed to measure the mechanical properties of the 316L-Cu SS. Meanwhile, the electrochemical experiments were used to characterize the corrosion properties. Antibacterial tests were employed to evaluate the antibacterial performance of 316L-Cu SS, the passivation film structure of 316L-Cu SS were also analyzed by XPS techniques to examine the difference in antibacterial performance. It is believed that this work could be helpful to provide a profound research base for further development of this novel class of antibacterial stainless steel. 2. Experimental method
Fig. 2. Variation of hardness of 316L SS with different Cu addition after solution and aging treatment.
2.2. Microstructure characterization and XRD analysis The microstructure observation was performed by a MEF4A optical microscopy (OM). Before OM observation, samples were ground with wet SiC paper up to 2000 grit, and then sequentially mechanically polished with 2.5 and 0.5 μm diamond pastes. Samples were electrolytically etched in a solution of 30% nitric acid for 10 s and washed immediately using alcohol to clearly reveal the austenite microstructure. The Cu-rich precipitates were characterized by transmission electron microscopy (TEM, JEM 2100). Thin foil samples for TEM were mechanically ground to a thickness of 50 μm, followed by electro-polishing with a twin-jet Struers Tenupol-3 in a polishing solution of 10 vol% HClO4 ethanol electrolyte maintained at −20 °C. The variation in lattice constant was analyzed by the X-ray diffraction (XRD) (D/Max-2500PC X-ray diffractometer) with Cu-Kα radiation, tube voltage of 50 kV, tube current of 300 mA and scanning rate of 1.2°/min.
2.1. Material preparation The chemical compositions of the investigated 316L SS and 316L-Cu SS are listed in Table 1. The steels were melted in a 25 kg vacuum induction-melting furnace and then forged into bars with diameter of 25 mm. Specimens for testing and observation were solution treated at 1100 °C for 30 min, followed by water quenching, and then aged isothermally at 700 °C for 3, 6, and 15 h, respectively. All the specimens for mechanical, microstructure, electrochemical, and antibacterial tests were cut from the extrude bars.
2.3. Mechanical test M10 threaded tensile specimens of 316L-Cu SS were cut from the extruded bars with gauge of 30 mm and diameter of 5 mm. The tensile tests at room temperature were conducted on a MTS tensile testing machine at a constant cross-head speed of 0.2 mm/min. Three specimens were tested to ensure the reproducibility. Vickers hardness measurements were performed under a 500 g load and testing time of 15 s.
Fig. 1. Microstructures of 316L-Cu SS with different Cu addition: (a) 316L SS, (b) 316L-2.5Cu SS and (c) 316L-3.5Cu SS. The subscript 1 and 2 represents the solid solution treatment at 1100 °C for 30 min and aging treatment at 700 °C for 6 h, respectively.
T. Xi et al. / Materials Science and Engineering C 71 (2017) 1079–1085 Table 2 Mechanical properties of the experimental steels. Conditions Solution
Aging 6 h
Samples
YS/MPa
UTS/MPa
Elongation/%
Z/%
316L 316L-2.5Cu 316L-3.5Cu 316L 316L-2.5Cu 316L-3.5Cu
233 232 229 234 254 257
535 515 507 529 542 543
69 55 54 52 58 60
81.2 84.3 83.1 74.8 75.8 80.2
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Jinghong Laboratory Instrument Co., Ltd., Shanghai, China) at 37 ± 2 °C under a humidity of 95% for 24 h. After the incubation, the samples and bacterial suspension were separated by phosphate buffer saline (PBS). 100 μL of bacterial suspension was first collected and diluted for 40 times, afterwards, it was uniformly spread on the Petri dish filled with LB medium, finally the Petri dishes were incubated at 37 ± 2 °C under a humidity of 95% again for 24 h, to clearly count the number of bacteria. The antibacterial rate was calculated as follows: N cont −Nanti 100% Ncont
The micro-hardness values were derived from 8 measurements on each sample.
K¼
2.4. Electrochemical corrosion test
where K is the antibacterial rate, Ncont stands for the number of bacterial colony for the 316L SS, and Nanti accounts for the number of bacterial colony for the 316L-Cu SS. Each experiment was repeated for three times.
Electrochemical measurements were performed on a three-electrode electrochemical system where the sample was taken as the working electrode, a saturated calomel electro (SCE) and platinum electrode were selected as reference electrode and counter electrode respectively. Open circuit potential (OCP) and electrochemical cyclic polarization were carried out on a Autolab potentiostat/galvanostat (Reference 600 ™, Gamry Instruments, Inc., USA) in 0.9 wt% NaCl solution at 37 ± 2 °C. All the samples were embedded in epoxy resin with an exposure area of 1 cm2. After that, the samples were mechanically ground using 1200 grit SiC papers, followed by washing with ethanol and dried in warm flowing air. The OCP test was conducted for 1 h to obtain a fresh sample surface before the measurement was started. Electrochemical cyclic polarization was conducted at a scan rate of 0.5 mV/s, from the corrosion potential (Ecorr) to 1100 mV (vs. SCE), and then scan was immediately reversed to the starting potential when the current density reached a value of 10−4A cm−2. Correspondingly, the passive film breakdown potential (Eb) and protective potential (Ep) were obtained from the cyclic polarization curves.
ð1Þ
2.6. X-ray photoelectron spectroscopy (XPS) test To further explain the antibacterial mechanism of 316L-Cu SS, XPS was used to analyze the chemical composition and surface structure of the passive film. XPS-measurements were performed by an ESCALAB250 spectrometer (Vacuum Generator Instruments) using non-mono-chromatised Al Kα radiation (1486.6 eV) from a twin Mg/ Al anode operating at 300 W. In this study the C1s peak at 284.6 eV was used as the reference to correct the charging shifts. Before the XPS measurements, all the samples experienced a pre-passivation treatment in atmospheric environment with same surface state. During the experiment, the angle between sample surface and the analyzer kept at 90°, and etching speed was set as 0.2 nm/s. 3. Results and discussions
2.5. Antibacterial test
3.1. Microstructure and mechanical properties
The plate count method was adopted for the antibacterial test in this study [25]. The chosen bacterium Escherichia coli (E. coli) ATCC 25922 was cultured in the Luria-Bertani (LB) medium with following compositions, 5.0 g flesh extract, 5.0 g NaCl, 10.0 g peptone, 20.0 g agar and 1000 mL distilled water with pH 7.2 ± 0.1 [26]. All the experimental instruments were sterilized by autoclaving at 121 ± 2 °C for 20 min before the antibacterial test. After that, 50 μL bacterial suspensions with a concentration of 105 CFU/mL (colony forming unit per milliliter) were homogeneously added onto the surface of the samples. Now samples with the bacterial solution were incubated in an incubator (DNP-9272,
Fig. 1 shows the microstructure of the 316L-Cu SS with different Cu addition after solution treatment at 1100 °C for 30 min and aging treatment at 700 °C for 6 h. It reveals that the microstructure was composed of complete austenite features, and it was found that both solution and aging treatment were resulted in similar microstructure, implying that the aging treatment had no obvious influence on the microstructure. Besides, the grain sizes of the 316L-Cu SS with different content of Cu addition were also similar to each other, and the average grain sizes were about 60 μm, indicating that the Cu addition played no apparent role in the grain refinement.
Fig. 3. TEM bright-field micrographs of 316L-3.5Cu SS after aging at 700 °C for 6 h.
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Fig. 4. Variation of lattice constant and XRD patterns for the 316L SS and 316L-Cu SS after solution and aging treatment.
Fig. 2 shows the hardness variation of the 316L-Cu SS with different Cu content after solution and different aging treatment. It can be seen that the hardness decreased slightly with increase of the Cu content after solution treatment, whereas the hardness increased rapidly after aging treatment, attributing to the precipitation hardening of the Curich precipitates. It is worth to mention that the hardness remained almost steady with further increase of the aging time even prolonging to 15 h. Besides, the tendency of hardness variation with different Cu content at different aging treatment were almost same, and it can be clearly found that the hardness of 316L-2.5Cu SS is almost same with that of 316L-3.5Cu SS, indicating that the lower Cu addition (2.5 wt%) was enough to precipitate sufficient Cu-rich precipitates for precipitation hardening. The detailed mechanical properties including yield strength (YS), ultimate tensile strength (UTS), elongation (EL) and reduction in area (RA) of the experimental steels under different heat treatment are shown in Table 2. As expected, the aging treatment played an obvious role in the mechanical properties. For the 316L-3.5Cu SS, the steel demonstrated YS of 257 MPa, UTS of 543 MPa and EL of 60% after aging at 700 °C for 6 h, which is obviously higher than those of 229 MPa, 507 MPa and 54% at solution treatment state, respectively. The increment of YS and UTS was both around 30 MPa, compared to those of solution treatment. Moreover, it should be noted that the increase of Cu content had no obvious influence on the YS and UTS of 316L–Cu SS, and no matter the heat treatment was solution or aging, the values were almost unchanged, indicating that the strengthening effect of Cu addition was limited. Based on the above results, there are two possible reasons to explain the difference of mechanical properties. First, it is well known that the Cu-rich precipitate is one of the most effective intermetallic strengthening precipitates [27,28]. Comparing with other strengthening
precipitates, previous studies [29–32] have proved that the steel would separate out large amounts of nanoscale size Cu-rich precipitates from the steel matrix when aged at appropriate aging temperature. These randomly distributed Cu-rich precipitates would accumulate and pin the dislocations during deformation process, and thus effectively increase the strength of the experimental steels. Fig. 3 shows the TEM images of the Cu-rich precipitates in 316L-3.5Cu SS after aging at 700 °C for 6 h. It can be inferred that the high number density of spherical Curich precipitates, with mean diameter size of approximately 10 nm, were homogeneously distributed within the steel matrix. So the aging treatment helped the steel to precipitate the Cu-rich precipitates and led to an increase of the strength. Secondly, Cu is also one of the elements that effectively raise the SFE of austenitic steel. Generally, the effect of Cu for the increase of SFE is about two times higher than that of Ni [11]. The increase of SFE due to the Cu addition can decrease the width of the partial dislocations in the steel, and make the partial dislocations easy to combine and form the perfect dislocations. Therefore, the formation of perfect dislocations due to the SFE increase would prevent the pile up of partial dislocations on one slip plane and make it easy to cross-slip to another slip plane. Obviously, this process would decrease the strength of the steel [33]. Nevertheless, the solution strengthening of the Cu addition can be estimated by an empirical relationship; [34] Δσ s ¼ 33 Ni% þ 40 Cu%
ð2Þ
where Δσs is the increment of yield strength, and Ni% and Cu% are the weight percent of Ni and Cu, respectively. As depicted in relation, the addition of Cu would substantially increase the strength of 316L SS. However, for 316L-Cu SS with different Cu content, solution treatment had tiny change on the mechanical properties. So it is reasonable to believe that the above results were mainly attributed to the synergistic effect of the softening effect by increase of SFE and strengthening effect by the solid solution strengthening. 3.2. XRD analysis Fig. 4 shows the variation of lattice constant of both 316L SS and 316L-Cu SS after solution and aging treatment, and the corresponding XRD curves are displayed in the insets. The Braggs law is employed to achieve the lattice constants of different steels by calculating the high angle (3 1 1)γ peaks in the XRD patterns. As displayed in Fig. 4, the lattice constant of steels increased slightly with the increase of Cu content after both solution and aging treatments. The values of lattice constant for 316L SS, 316L-2.5Cu SS and 316L-3.5Cu SS after solution treatment were 0.3594 nm, 0.3598 nm and 0.3601 nm, respectively, showing a slight increase with increase of the Cu content. It is well known that the Cu addition would form a substitutional solid solution in the steel. Since the atomic radius of Cu atom (128 pm) is almost equal to that of Fe atom (124 pm), so the lattice distortion energy due to the substitution of Cu for Fe was
Fig. 5. Electrochemical cyclic polarization curves of 316L–Cu SS with different Cu content soaked in 0.9 wt% NaCl solution at 37 °C in (a) solution and (b) aging treatment, respectively.
T. Xi et al. / Materials Science and Engineering C 71 (2017) 1079–1085 Table 3 Electrochemical parameters of the experimental steels determined from the cyclic polarization curves. Conditions
Samples
Eb /mV
Ep /mV
ipass /A cm−2
Solution
316L 316L-2.5Cu 316L-3.5Cu 316L 316L-2.5Cu 316L-3.5Cu
502.4 504.9 546.3 492.7 429.3 310.5
114.6 163.4 191.5 126.8 −21.9 15.6
1.95 2.14 2.24 1.69 2.08 1.19
Aging 6 h
× × × × × ×
10−6 10−6 10−6 10−6 10−6 10−6
predictably small and thus resulted in slight increase in the lattice constant. Meanwhile, it should also be noted that the lattice constant of the steels after solution treatment were larger than that of the aging treatment. This is because the large amount of Cu atoms should be dissolved in the lattice of the steel matrix forming an over saturated solution after solution treatment, while the amount of Cu atoms in the steel matrix should be significantly decreased because of the formation of Cu-rich precipitation after aging treatment. 3.3. Corrosion behavior To study the influence of Cu addition and different heat treatments on the corrosion resistance of 316L-Cu SS, the electrochemical cyclic polarization curves of 316L SS and 316L-Cu SS under solution and aging treatment in 0.9 wt% NaCl solution at 37 °C in solution were obtained
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and shown in Fig. 5. The corresponding detailed electrochemical data are quantitatively listed in Table 3. As well known, the passive current density (ipass), pitting corrosion potential (Eb) and protective potential (Ep) are the main factors to evaluate the corrosion resistance for the inactive stainless steel. Higher Eb and Ep and lower ipass would contribute to an excellent corrosion resistance performance for steels. As shown in Fig. 5, the Eb of solution treated 316L-Cu SS had a minor increase with increasing the amount of Cu, and the values of Eb for 316L SS, 316L-2.5Cu SS and 316L-3.5Cu SS were 502.4 mV, 504.9 mV and 546.3 mV, respectively. However, after aging at 700 °C for 6 h, the Eb fell sharply with increasing the amount of Cu, and the corresponding values were reduced down to 492.7 mV, 429.3 mV and 310.5 mV. Meantime, the measured Ep also increased as the Cu increased for the solution treated steel and decreased for the aging steel. And it should be noted that both Eb and Ep of the solution treated 316L-Cu SS were much higher than those of with the aging treatment. More importantly, the ipass remained almost the same for the three investigated 316L-Cu SS no matter solid solution and aging treatment. Above results implied that the Cu addition to the 316L SS could promoted the corrosion resistance in 0.9 wt% NaCl in the solution state, whereas it was detrimental after aging treatment. Generally, it is well accepted that Cu addition is beneficial to the corrosion resistance due to the suppression of anodic dissolution by elemental Cu deposition on the surface of steel [18]. After solution treatment, the previously dissolved Cu ions from the surface would form the protective CuCl2 film in the chloride media, and thus increase
Fig. 6. Photos of bacterial colonies for the experimental steels in E. coli suspension at 37 °C for 24 h: (a) 316L SS, (b) 316L-2.5Cu SS and (c) 316L-3.5Cu SS. The subscript 1, 2 and 3 represents the solution treatment, aged at 700 °C for 3 h and aged at 700 °C for 6 h, respectively.
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Table 4 Standard plate count bacteria (CFU) after the samples incubated with E. coli suspension at 37 °C for 24 h. Heat treatment conditions Solution treatment at 1100 °C for 30 min
Aging at 700 °C for 3 h
Aging at 700 °C for 6 h
Samples
CFU mL−1
Antibacterial rate %
316L 316L-2.5Cu 316L-3.5Cu 316L 316L-2.5Cu 316L-3.5Cu 316L 316L-2.5Cu 316L-3.5Cu Black control
2000 ± 200 1500 ± 100 1500 ± 100 2000 ± 150 130 ± 12 37 ± 12 2000 ± 150 110 ± 23 21 ± 6 2000 ± 200
0 25% 25% 0 93.5% 98.2% 0 94.5% 98.9% 0
the corrosion resistance of 316L-Cu SS. The different role of Cu in solution and aging treatments about on Eb and Ep is probably attributed to the Cu-rich precipitates which precipitated out after aging treatment. During the aging process, once the Cu-rich precipitates precipitated out from the steel matrix, the uniform continuity and compactness of the passive film would be destroyed. The Cu-rich precipitates (Fig. 3) would act as the “weak points” on the steel surface which are susceptible to the local attack [35]. Therefore, the discontinuity of passive film caused by Cu-rich precipitates would reduce the resistance to pitting corrosion and ability of self-passivation and self-repairing. Besides, because of the corrosion potential difference of Cu and Fe in steel, the galvanic corrosion between the Cu-rich precipitates and the steel matrix could also accelerate the corrosion rate of 316L-Cu SS. 3.4. Antibacterial and XPS tests The plate count method was adopted for evaluating antibacterial properties of the 316L-Cu SS. Fig. 6 shows the bacterial colony images of the 316L-Cu SS with different Cu content after solution and aging treatments, evaluated by co-culturing experimental samples with E. coli bacterial suspensions at 37 °C for 24 h. It can be clearly seen that the Petri dishes of all the experimental steels under solution treatment were mostly covered by countless bacterial colonies (Fig. 6a1), while on the contrary, only a small number of bacterial colonies were observed for the 316L-Cu SS after aging treatment, as shown in Fig. 6c3. Therefore, it can be reasonably concluded that the addition of Cu and aging treatment were two crucial factors to endow the steel with the antibacterial property. Table 4 presents the detailed antibacterial rates of the experimental steels. As shown, the antibacterial rates of all experimental steels against E. coli under solution treatment were much lower than those under aging treatment. Besides, the standard plate count bacteria for control were almost same with those of 316L SS both under solution and aging treatment, and the antibacterial rates were zero, exhibiting that it has no antibacterial property against E. coli. For 316L-2.5Cu SS, the standard plate count bacteria after solution treatment was 1500 ± 100, showing an approximately 25% antibacterial rate, whereas
it was only 130 ± 12 after aging at 700 °C for 3 h, showing that the antibacterial rate rapidly increased to 93.5%. After prolonging the aging time up to 6 h, the standard plate count bacteria and antibacterial rate were 37 ± 12 and 98.2%, respectively. Same tendency could be observed for 316L-3.5Cu SS. These results indicated that the aging treatment could ensured Cu-bearing 316L SS a strong antibacterial ability. It should be noted that the antibacterial rate of 316L-3.5Cu SS was higher than that of 316L-2.5Cu SS. This is because higher Cu addition did favor to release enough Cu ions for killing bacteria. As it is known, the released Cu ions from the Cu-bearing steels can destroy the bacterial cell walls and inhibit the normal growth of the bacteria [17,36], so the antibacterial property is controlled by the amount of released Cu ions, which is closely bounded up to the corrosion resistance. Generally, the higher the corrosion resistance is, the lower the antibacterial property becomes, and vice versa. To well understand the relation of corrosion resistance and antibacterial property, surface structure of the passive film on the 316L-Cu SS in state of solution and aging treatments were further analyzed by the XPS analysis. Fig. 7 presents the XPS spectra of Cu 2p for 316L-3.5Cu SS in solution and aging treatments at etching time of 15 s, and the corresponding parameters for decomposition of Cu 2p spectra are displayed in Table 5. As indicated in Fig. 7, the existence of Cu 2p2/3 lines confirmed the presence of Cu on these samples surface both in solution and aging states. The Cu 2p spectra were mainly composed of two peaks: Cu and CuO peaks, and the peak of metallic Cu was significantly higher than that of CuO. Besides, the overall Cu content in the passive film of aged 316L-3.5Cu SS was a little bit higher than that of solution treated 316L-3.5Cu SS, with values of 2.63 at.% and 2.32 at.%, respectively. On the same time, compared with solution treatment, both Cu and CuO contents in Cu 2p spectra increased slightly after aging treatment. Combining the results of antibacterial property and XPS analysis, possible reasons are deduced that the presence of metallic Cu in the passive film of the aged 316L-Cu SS could easily lose two electrons, and then transfer to the bivalent Cu ions, which are the key antibacterial agent. Obviously, the enrichment of Cu content in the passive film would be harmful to the stability of oxide film, due to the reaction of Cu + 2e → Cu2 +, and correspondingly increasing the flux of charge
Fig. 7. Surface XPS spectra of Cu 2p detected in the passive films of 316L-3.5Cu SS at 15 s etching time, (a) solution, (b) aging at 700 °C for 6 h.
T. Xi et al. / Materials Science and Engineering C 71 (2017) 1079–1085 Table 5 Parameters for decomposition of Cu 2p spectra for 316L-Cu SS at 15 s etching time by XPS analysis. Conditions
Cu Area,%
Solution 1.91 Aging at 700 °C for 6 h 2.29
CuO Binding energy, eV
Area,%
Binding energy, eV
932.5 932.4
0.41 0.34
933.5 933.6
transfer. As a consequence, the increment of Cu content in the passive film significantly decreased the corrosion resistance of 316L-Cu SS, while the release of more Cu ions prominently improved the antibacterial property. The XPS results were in good agreement with the results of corrosion resistance and antibacterial properties. 4. Conclusion (1) The addition of Cu and different heat treatment in the present study had no influence on the microstructure of 316L SS. (2) The difference in mechanical properties of 316L-Cu SS, the increase of the YS and UTS, under different heat treatment was mainly caused by the Cu-rich precipitates hardening effect in aging treatment, and synergistic effect of SFE and solution strengthening in solution treatment. (3) The lattice constant of 316L-Cu SS after aging treatment was smaller than that after solution treatment, due to the precipitation of Cu-rich precipitates from the steel matrix. (4) The pitting and protective potential of 316L-Cu SS increased with increasing Cu content in the solution state, while those decreased after aging treatment, owing to the generated Cu-rich precipitates.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 51371168 and 51501188), the National Key Research and Development Program of China (No. 2016YFB0300205) and the State Key Program of National Natural Science of China (Grant No. 51631009). References [1] K.H. Lo, C.H. Shek, J.K.L. Lai, Recent developments in stainless steels, Mater. Sci. Eng. R. Rep. 65 (2009) 39–104. [2] L. Ren, K. Yang, L. Guo, H.-w. Chai, Preliminary study of anti-infective function of a copper-bearing stainless steel, Mater. Sci. Eng. C 32 (2012) 1204–1209. [3] H. Chai, L. Guo, X. Wang, Y. Fu, J. Guan, L. Tan, L. Ren, K. Yang, Antibacterial effect of 317L stainless steel contained copper in prevention of implant-related infection in vitro and in vivo, J. Mater. Sci. Mater. Med. 22 (2011) 2525–2535. [4] T. Xi, C. Yang, M. Babar Shahzad, K. Yang, Study of the processing map and hot deformation behavior of a Cu-bearing 317LN austenitic stainless steel, Mater. Des. 87 (2015) 303–312. [5] K.K. Alaneme, S.M. Hong, I. Sen, E. Fleury, U. Ramamurty, Effect of copper addition on the fracture and fatigue crack growth behavior of solution heat-treated SUS 304H austenitic steel, Mater. Sci. Eng. A 527 (2010) 4600–4604. [6] I. Sen, E. Amankwah, N.S. Kumar, E. Fleury, K. Oh-ishi, K. Hono, U. Ramamurty, Microstructure and mechanical properties of annealed SUS 304H austenitic stainless steel with copper, Mater. Sci. Eng. A 528 (2011) 4491–4499. [7] L. Nan, Y. Liu, M. Lu, K. Yang, Study on antibacterial mechanism of copper-bearing austenitic antibacterial stainless steel by atomic force microscopy, J. Mater. Sci. Mater. Med. 19 (2008) 3057–3062.
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Solioz, Metallic copper as an antimicrobial surface, Appl. Environ. Microbiol. 77 (2011) 1541–1547. [15] L. Nan, D. Xu, T. Gu, X. Song, K. Yang, Microbiological influenced corrosion resistance characteristics of a 304L-Cu stainless steel against Escherichia coli, Mater. Sci. Eng. C 48 (2015) 228–234. [16] I.T. Hong, C.H. Koo, Antibacterial properties, corrosion resistance and mechanical properties of Cu-modified SUS 304 stainless steel, Mater. Sci. Eng. A 393 (2005) 213–222. [17] Z.G. Dan, H.W. Ni, J. Xiong, B.F. Xu, P.Y. Xiong, Antibacterial properties of AISI 420 stainless steel implanted by Ag/Cu ions, Nanoelectronics Conference (INEC), 2010 3rd International 2010, pp. 388–389. [18] A. Yamamoto, T. Ashiura, E. Kamisaka, Mechanism of improvement on corrosion resistance by copper addition to ferritic stainless steels, Zairyo-to-Kankyo 35 (1986) 448–454. [19] A.A. Hermas, K. Ogura, S. Takagi, T. 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Sawada, Effects of Cu, Mo and C on the corrosion of deformed 18Cr8Ni stainless steels in H2SO4/NaCl solutions, Corros. Sci. 16 (1976) 323–330. [25] J. Bartram, J. Cotruvo, M. Exner, C. Fricker, A. Glasmacher, Heterotrophic plate count measurement in drinking water safety management: report of an expert meeting Geneva, 24–25 April 2002, Int. J. Food Microbiol. 92 (2004) 241–247. [26] L. Nan, K. Yang, Cu ions dissolution from Cu-bearing antibacterial stainless steel, J. Mater. Sci. Technol. 26 (2010) 941–944. [27] M.D. Mulholland, D.N. Seidman, Nanoscale co-precipitation and mechanical properties of a high-strength low-carbon steel, Acta Mater. 59 (2011) 1881–1897. [28] Z. Jiao, J. Luan, M. Miller, C. Liu, Precipitation mechanism and mechanical properties of an ultra-high strength steel hardened by nanoscale NiAl and Cu particles, Acta Mater. 97 (2015) 58–67. [29] M.E. Fine, D. Isheim, Origin of copper precipitation strengthening in steel revisited, Scr. Mater. 53 (2005) 115–118. [30] D. Isheim, M.S. Gagliano, M.E. Fine, D.N. Seidman, Interfacial segregation at Cu-rich precipitates in a high-strength low-carbon steel studied on a sub-nanometer scale, Acta Mater. 54 (2006) 841–849. [31] R.P. Kolli, D.N. Seidman, The temporal evolution of the decomposition of a concentrated multicomponent Fe–Cu-based steel, Acta Mater. 56 (2008) 2073–2088. [32] T. Xi, M. Babar Shahzad, D. Xu, J. Zhao, C. Yang, M. Qi, K. Yang, Copper precipitation behavior and mechanical properties of Cu-bearing 316L austenitic stainless steel: a comprehensive cross-correlation study, Mater. Sci. Eng. A 675 (2016) 243–252. [33] I.T. Hong, C.H. Koo, Antibacterial properties, corrosion resistance and mechanical properties of Cu-modified SUS 304 stainless steel, Mater. Sci. Eng. A 393 (2005) 213–222. [34] F.B. Pickering, Physical Metallurgy and the Design of Steels, 1978. [35] J. Banas, A. 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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Effect of copper addition on mechanical properties, corrosion resistance and antibacterial property of 316L stainless steel Tong Xi a,b,1, M. Babar Shahzad b,1, Dake Xu a, Ziqing Sun b, Jinlong Zhao b, Chunguang Yang b,⁎, Min Qi a, Ke Yang b,⁎ a b
School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
a r t i c l e
i n f o
Article history: Received 2 September 2016 Received in revised form 10 October 2016 Accepted 7 November 2016 Available online 9 November 2016 Keywords: Cu-bearing stainless steel Mechanical properties Corrosion resistance Antibacterial performance
a b s t r a c t The effects of addition of different Cu content (0, 2.5 and 3.5 wt%) on mechanical properties, corrosion resistance and antibacterial performance of 316L austenitic stainless steel (SS) after solution and aging treatment were investigated by mechanical test, transmission electron microscope (TEM), X-ray diffraction (XRD), electrochemical corrosion, X-ray photoelectron spectroscopy (XPS) and antibacterial test. The results showed that the Cu addition and heat treatment had no obvious influence on the microstructure with complete austenite features. The yield strength (YS) after solution treatment was almost similar, whereas the aging treatment obviously increased the YS due to formation of tiny Cu-rich precipitates. The pitting and protective potential of the solution treated Cubearing 316L SS in 0.9 wt% NaCl solution increased with increasing Cu content, while gradually declined after aging, owing to the high density Cu-rich precipitation. The antibacterial test proved that higher Cu content and aging were two compulsory processes to exert good antibacterial performance. The XPS results further indicated that aging enhanced the Cu enrichment in passive film, which could effectively stimulate the Cu ions release from the surface of passive film. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Austenitic stainless steels (SS) have been widely used in many fields such as surgical implant material, food industry, transportation and daily appliances, owing to their superior mechanical properties, corrosion resistance and workability [1]. As a strengthening element copper (Cu) is often added into steel to improve the corrosion resistance and creep resistant strength [1]. In recent years, Cu-bearing antibacterial austenitic SS, developed as a new kind of structural/functional integrated materials, have attracted great attention as a potential biomaterial for implant devices that could help in reduction of the risk of implant related bacterial infections [2,3]. The addition of proper amount of Cu into the steel and an appropriate aging treatment results in homogeneous distribution of high number density of Cu-rich precipitates within the steel matrix. These nano-sized precipitates offer certain attractive properties to the steel that have promising applications in various fields. During the past decade, a variety of studies have been focused on the mechanical properties, hot deformation behavior, antibacterial performance and corrosion resistance of the Cu-bearing austenitic SS [4–8]. As well known, Cu can stabilize the austenitic structure and make it ⁎ Corresponding authors. E-mail addresses: [email protected] (C. Yang), [email protected] (K. Yang). 1 Tong Xi and M. Babar Shahzad equally contributed to this work.
http://dx.doi.org/10.1016/j.msec.2016.11.022 0928-4931/© 2016 Elsevier B.V. All rights reserved.
possible to reduce the Ni content in the steel, which leads to a significant economic saving since the price of Ni is relatively high [9–11]. Besides, the Cu addition can cause an increase in the stacking fault energy (SFE) and subsequently improve the deep drawing quality of steel. However, Cu addition also has an adverse effect on the hot forging property due to the formation of low melting point Cu-rich eutectic phases that preferentially segregate at the grain boundaries and steels surface [12,13]. In the case of antibacterial property of Cu-bearing SS, some researchers [8,14–16] reported that the dissolved Cu ions released from the steel matrix play a dominant role for the antibacterial effects of SS. The released Cu ions could effectively kill the bacteria by collapsing their outer cell membranes. For instance, Dan et al. [17] proposed that Cu ions could adhere to the surface of the bacterial cells and damage the proteins structure in the bacterial cells. In addition, Cu has been added into steels to improve the corrosion resistance. It is well accepted that the addition of Cu can suppress the anodic dissolution when immersed in the acid solutions [18]. As regards the influence of Cu on the corrosion behavior of SS, there are some controversial conclusions in different literatures. Some researchers [19–22] pointed out that Cu addition inhibits the dissolution of soluble sulfide inclusions in SS, due to the re-deposition of previously dissolved Cu on the steel surface and later forming of CuCl2 film, which is beneficial to decrease the corrosion rate and improve the crevice corrosion potential
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Table 1 Chemical composition (wt%) of the experimental steels. No.
Cr
Mo
Ni
Cu
C
Si
S
P
Fe
316L 316L-2.5Cu 316L-3.5Cu
17.61 17.76 17.77
3.14 3.17 3.11
14.4 14.5 14.6
0.02 2.46 3.35
0.004 0.006 0.004
0.24 0.39 0.56
0.004 0.003 0.004
0.007 0.008 0.008
Bal. Bal. Bal.
of SS in acid chloride media. However, other researchers also reported that alloying Cu has a detrimental effect on the passivation [23] and localized corrosion, such as pitting potential [23] and intergranular corrosion [24]. They believed that Cu delays the chromium enrichment on the passive film at the first stage of passivation, and thus declines the stability of passive film and pitting resistance of the SS. Despite the numerous studies related to the influence of Cu addition on microstructure, mechanical, corrosion resistance and antibacterial performance, studies on comparison between the solution and aging treatment for different Cu additions and its effect on various properties are still lack systematic understanding. Hence, in this work, the effect of Cu content and different heat treatments on the mechanical properties, corrosion resistance and antibacterial property of Cu-bearing 316L stainless steel (316L-Cu SS) were investigated. In order to address these problems, hardness and tensile test were performed to measure the mechanical properties of the 316L-Cu SS. Meanwhile, the electrochemical experiments were used to characterize the corrosion properties. Antibacterial tests were employed to evaluate the antibacterial performance of 316L-Cu SS, the passivation film structure of 316L-Cu SS were also analyzed by XPS techniques to examine the difference in antibacterial performance. It is believed that this work could be helpful to provide a profound research base for further development of this novel class of antibacterial stainless steel. 2. Experimental method
Fig. 2. Variation of hardness of 316L SS with different Cu addition after solution and aging treatment.
2.2. Microstructure characterization and XRD analysis The microstructure observation was performed by a MEF4A optical microscopy (OM). Before OM observation, samples were ground with wet SiC paper up to 2000 grit, and then sequentially mechanically polished with 2.5 and 0.5 μm diamond pastes. Samples were electrolytically etched in a solution of 30% nitric acid for 10 s and washed immediately using alcohol to clearly reveal the austenite microstructure. The Cu-rich precipitates were characterized by transmission electron microscopy (TEM, JEM 2100). Thin foil samples for TEM were mechanically ground to a thickness of 50 μm, followed by electro-polishing with a twin-jet Struers Tenupol-3 in a polishing solution of 10 vol% HClO4 ethanol electrolyte maintained at −20 °C. The variation in lattice constant was analyzed by the X-ray diffraction (XRD) (D/Max-2500PC X-ray diffractometer) with Cu-Kα radiation, tube voltage of 50 kV, tube current of 300 mA and scanning rate of 1.2°/min.
2.1. Material preparation The chemical compositions of the investigated 316L SS and 316L-Cu SS are listed in Table 1. The steels were melted in a 25 kg vacuum induction-melting furnace and then forged into bars with diameter of 25 mm. Specimens for testing and observation were solution treated at 1100 °C for 30 min, followed by water quenching, and then aged isothermally at 700 °C for 3, 6, and 15 h, respectively. All the specimens for mechanical, microstructure, electrochemical, and antibacterial tests were cut from the extrude bars.
2.3. Mechanical test M10 threaded tensile specimens of 316L-Cu SS were cut from the extruded bars with gauge of 30 mm and diameter of 5 mm. The tensile tests at room temperature were conducted on a MTS tensile testing machine at a constant cross-head speed of 0.2 mm/min. Three specimens were tested to ensure the reproducibility. Vickers hardness measurements were performed under a 500 g load and testing time of 15 s.
Fig. 1. Microstructures of 316L-Cu SS with different Cu addition: (a) 316L SS, (b) 316L-2.5Cu SS and (c) 316L-3.5Cu SS. The subscript 1 and 2 represents the solid solution treatment at 1100 °C for 30 min and aging treatment at 700 °C for 6 h, respectively.
T. Xi et al. / Materials Science and Engineering C 71 (2017) 1079–1085 Table 2 Mechanical properties of the experimental steels. Conditions Solution
Aging 6 h
Samples
YS/MPa
UTS/MPa
Elongation/%
Z/%
316L 316L-2.5Cu 316L-3.5Cu 316L 316L-2.5Cu 316L-3.5Cu
233 232 229 234 254 257
535 515 507 529 542 543
69 55 54 52 58 60
81.2 84.3 83.1 74.8 75.8 80.2
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Jinghong Laboratory Instrument Co., Ltd., Shanghai, China) at 37 ± 2 °C under a humidity of 95% for 24 h. After the incubation, the samples and bacterial suspension were separated by phosphate buffer saline (PBS). 100 μL of bacterial suspension was first collected and diluted for 40 times, afterwards, it was uniformly spread on the Petri dish filled with LB medium, finally the Petri dishes were incubated at 37 ± 2 °C under a humidity of 95% again for 24 h, to clearly count the number of bacteria. The antibacterial rate was calculated as follows: N cont −Nanti 100% Ncont
The micro-hardness values were derived from 8 measurements on each sample.
K¼
2.4. Electrochemical corrosion test
where K is the antibacterial rate, Ncont stands for the number of bacterial colony for the 316L SS, and Nanti accounts for the number of bacterial colony for the 316L-Cu SS. Each experiment was repeated for three times.
Electrochemical measurements were performed on a three-electrode electrochemical system where the sample was taken as the working electrode, a saturated calomel electro (SCE) and platinum electrode were selected as reference electrode and counter electrode respectively. Open circuit potential (OCP) and electrochemical cyclic polarization were carried out on a Autolab potentiostat/galvanostat (Reference 600 ™, Gamry Instruments, Inc., USA) in 0.9 wt% NaCl solution at 37 ± 2 °C. All the samples were embedded in epoxy resin with an exposure area of 1 cm2. After that, the samples were mechanically ground using 1200 grit SiC papers, followed by washing with ethanol and dried in warm flowing air. The OCP test was conducted for 1 h to obtain a fresh sample surface before the measurement was started. Electrochemical cyclic polarization was conducted at a scan rate of 0.5 mV/s, from the corrosion potential (Ecorr) to 1100 mV (vs. SCE), and then scan was immediately reversed to the starting potential when the current density reached a value of 10−4A cm−2. Correspondingly, the passive film breakdown potential (Eb) and protective potential (Ep) were obtained from the cyclic polarization curves.
ð1Þ
2.6. X-ray photoelectron spectroscopy (XPS) test To further explain the antibacterial mechanism of 316L-Cu SS, XPS was used to analyze the chemical composition and surface structure of the passive film. XPS-measurements were performed by an ESCALAB250 spectrometer (Vacuum Generator Instruments) using non-mono-chromatised Al Kα radiation (1486.6 eV) from a twin Mg/ Al anode operating at 300 W. In this study the C1s peak at 284.6 eV was used as the reference to correct the charging shifts. Before the XPS measurements, all the samples experienced a pre-passivation treatment in atmospheric environment with same surface state. During the experiment, the angle between sample surface and the analyzer kept at 90°, and etching speed was set as 0.2 nm/s. 3. Results and discussions
2.5. Antibacterial test
3.1. Microstructure and mechanical properties
The plate count method was adopted for the antibacterial test in this study [25]. The chosen bacterium Escherichia coli (E. coli) ATCC 25922 was cultured in the Luria-Bertani (LB) medium with following compositions, 5.0 g flesh extract, 5.0 g NaCl, 10.0 g peptone, 20.0 g agar and 1000 mL distilled water with pH 7.2 ± 0.1 [26]. All the experimental instruments were sterilized by autoclaving at 121 ± 2 °C for 20 min before the antibacterial test. After that, 50 μL bacterial suspensions with a concentration of 105 CFU/mL (colony forming unit per milliliter) were homogeneously added onto the surface of the samples. Now samples with the bacterial solution were incubated in an incubator (DNP-9272,
Fig. 1 shows the microstructure of the 316L-Cu SS with different Cu addition after solution treatment at 1100 °C for 30 min and aging treatment at 700 °C for 6 h. It reveals that the microstructure was composed of complete austenite features, and it was found that both solution and aging treatment were resulted in similar microstructure, implying that the aging treatment had no obvious influence on the microstructure. Besides, the grain sizes of the 316L-Cu SS with different content of Cu addition were also similar to each other, and the average grain sizes were about 60 μm, indicating that the Cu addition played no apparent role in the grain refinement.
Fig. 3. TEM bright-field micrographs of 316L-3.5Cu SS after aging at 700 °C for 6 h.
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Fig. 4. Variation of lattice constant and XRD patterns for the 316L SS and 316L-Cu SS after solution and aging treatment.
Fig. 2 shows the hardness variation of the 316L-Cu SS with different Cu content after solution and different aging treatment. It can be seen that the hardness decreased slightly with increase of the Cu content after solution treatment, whereas the hardness increased rapidly after aging treatment, attributing to the precipitation hardening of the Curich precipitates. It is worth to mention that the hardness remained almost steady with further increase of the aging time even prolonging to 15 h. Besides, the tendency of hardness variation with different Cu content at different aging treatment were almost same, and it can be clearly found that the hardness of 316L-2.5Cu SS is almost same with that of 316L-3.5Cu SS, indicating that the lower Cu addition (2.5 wt%) was enough to precipitate sufficient Cu-rich precipitates for precipitation hardening. The detailed mechanical properties including yield strength (YS), ultimate tensile strength (UTS), elongation (EL) and reduction in area (RA) of the experimental steels under different heat treatment are shown in Table 2. As expected, the aging treatment played an obvious role in the mechanical properties. For the 316L-3.5Cu SS, the steel demonstrated YS of 257 MPa, UTS of 543 MPa and EL of 60% after aging at 700 °C for 6 h, which is obviously higher than those of 229 MPa, 507 MPa and 54% at solution treatment state, respectively. The increment of YS and UTS was both around 30 MPa, compared to those of solution treatment. Moreover, it should be noted that the increase of Cu content had no obvious influence on the YS and UTS of 316L–Cu SS, and no matter the heat treatment was solution or aging, the values were almost unchanged, indicating that the strengthening effect of Cu addition was limited. Based on the above results, there are two possible reasons to explain the difference of mechanical properties. First, it is well known that the Cu-rich precipitate is one of the most effective intermetallic strengthening precipitates [27,28]. Comparing with other strengthening
precipitates, previous studies [29–32] have proved that the steel would separate out large amounts of nanoscale size Cu-rich precipitates from the steel matrix when aged at appropriate aging temperature. These randomly distributed Cu-rich precipitates would accumulate and pin the dislocations during deformation process, and thus effectively increase the strength of the experimental steels. Fig. 3 shows the TEM images of the Cu-rich precipitates in 316L-3.5Cu SS after aging at 700 °C for 6 h. It can be inferred that the high number density of spherical Curich precipitates, with mean diameter size of approximately 10 nm, were homogeneously distributed within the steel matrix. So the aging treatment helped the steel to precipitate the Cu-rich precipitates and led to an increase of the strength. Secondly, Cu is also one of the elements that effectively raise the SFE of austenitic steel. Generally, the effect of Cu for the increase of SFE is about two times higher than that of Ni [11]. The increase of SFE due to the Cu addition can decrease the width of the partial dislocations in the steel, and make the partial dislocations easy to combine and form the perfect dislocations. Therefore, the formation of perfect dislocations due to the SFE increase would prevent the pile up of partial dislocations on one slip plane and make it easy to cross-slip to another slip plane. Obviously, this process would decrease the strength of the steel [33]. Nevertheless, the solution strengthening of the Cu addition can be estimated by an empirical relationship; [34] Δσ s ¼ 33 Ni% þ 40 Cu%
ð2Þ
where Δσs is the increment of yield strength, and Ni% and Cu% are the weight percent of Ni and Cu, respectively. As depicted in relation, the addition of Cu would substantially increase the strength of 316L SS. However, for 316L-Cu SS with different Cu content, solution treatment had tiny change on the mechanical properties. So it is reasonable to believe that the above results were mainly attributed to the synergistic effect of the softening effect by increase of SFE and strengthening effect by the solid solution strengthening. 3.2. XRD analysis Fig. 4 shows the variation of lattice constant of both 316L SS and 316L-Cu SS after solution and aging treatment, and the corresponding XRD curves are displayed in the insets. The Braggs law is employed to achieve the lattice constants of different steels by calculating the high angle (3 1 1)γ peaks in the XRD patterns. As displayed in Fig. 4, the lattice constant of steels increased slightly with the increase of Cu content after both solution and aging treatments. The values of lattice constant for 316L SS, 316L-2.5Cu SS and 316L-3.5Cu SS after solution treatment were 0.3594 nm, 0.3598 nm and 0.3601 nm, respectively, showing a slight increase with increase of the Cu content. It is well known that the Cu addition would form a substitutional solid solution in the steel. Since the atomic radius of Cu atom (128 pm) is almost equal to that of Fe atom (124 pm), so the lattice distortion energy due to the substitution of Cu for Fe was
Fig. 5. Electrochemical cyclic polarization curves of 316L–Cu SS with different Cu content soaked in 0.9 wt% NaCl solution at 37 °C in (a) solution and (b) aging treatment, respectively.
T. Xi et al. / Materials Science and Engineering C 71 (2017) 1079–1085 Table 3 Electrochemical parameters of the experimental steels determined from the cyclic polarization curves. Conditions
Samples
Eb /mV
Ep /mV
ipass /A cm−2
Solution
316L 316L-2.5Cu 316L-3.5Cu 316L 316L-2.5Cu 316L-3.5Cu
502.4 504.9 546.3 492.7 429.3 310.5
114.6 163.4 191.5 126.8 −21.9 15.6
1.95 2.14 2.24 1.69 2.08 1.19
Aging 6 h
× × × × × ×
10−6 10−6 10−6 10−6 10−6 10−6
predictably small and thus resulted in slight increase in the lattice constant. Meanwhile, it should also be noted that the lattice constant of the steels after solution treatment were larger than that of the aging treatment. This is because the large amount of Cu atoms should be dissolved in the lattice of the steel matrix forming an over saturated solution after solution treatment, while the amount of Cu atoms in the steel matrix should be significantly decreased because of the formation of Cu-rich precipitation after aging treatment. 3.3. Corrosion behavior To study the influence of Cu addition and different heat treatments on the corrosion resistance of 316L-Cu SS, the electrochemical cyclic polarization curves of 316L SS and 316L-Cu SS under solution and aging treatment in 0.9 wt% NaCl solution at 37 °C in solution were obtained
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and shown in Fig. 5. The corresponding detailed electrochemical data are quantitatively listed in Table 3. As well known, the passive current density (ipass), pitting corrosion potential (Eb) and protective potential (Ep) are the main factors to evaluate the corrosion resistance for the inactive stainless steel. Higher Eb and Ep and lower ipass would contribute to an excellent corrosion resistance performance for steels. As shown in Fig. 5, the Eb of solution treated 316L-Cu SS had a minor increase with increasing the amount of Cu, and the values of Eb for 316L SS, 316L-2.5Cu SS and 316L-3.5Cu SS were 502.4 mV, 504.9 mV and 546.3 mV, respectively. However, after aging at 700 °C for 6 h, the Eb fell sharply with increasing the amount of Cu, and the corresponding values were reduced down to 492.7 mV, 429.3 mV and 310.5 mV. Meantime, the measured Ep also increased as the Cu increased for the solution treated steel and decreased for the aging steel. And it should be noted that both Eb and Ep of the solution treated 316L-Cu SS were much higher than those of with the aging treatment. More importantly, the ipass remained almost the same for the three investigated 316L-Cu SS no matter solid solution and aging treatment. Above results implied that the Cu addition to the 316L SS could promoted the corrosion resistance in 0.9 wt% NaCl in the solution state, whereas it was detrimental after aging treatment. Generally, it is well accepted that Cu addition is beneficial to the corrosion resistance due to the suppression of anodic dissolution by elemental Cu deposition on the surface of steel [18]. After solution treatment, the previously dissolved Cu ions from the surface would form the protective CuCl2 film in the chloride media, and thus increase
Fig. 6. Photos of bacterial colonies for the experimental steels in E. coli suspension at 37 °C for 24 h: (a) 316L SS, (b) 316L-2.5Cu SS and (c) 316L-3.5Cu SS. The subscript 1, 2 and 3 represents the solution treatment, aged at 700 °C for 3 h and aged at 700 °C for 6 h, respectively.
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Table 4 Standard plate count bacteria (CFU) after the samples incubated with E. coli suspension at 37 °C for 24 h. Heat treatment conditions Solution treatment at 1100 °C for 30 min
Aging at 700 °C for 3 h
Aging at 700 °C for 6 h
Samples
CFU mL−1
Antibacterial rate %
316L 316L-2.5Cu 316L-3.5Cu 316L 316L-2.5Cu 316L-3.5Cu 316L 316L-2.5Cu 316L-3.5Cu Black control
2000 ± 200 1500 ± 100 1500 ± 100 2000 ± 150 130 ± 12 37 ± 12 2000 ± 150 110 ± 23 21 ± 6 2000 ± 200
0 25% 25% 0 93.5% 98.2% 0 94.5% 98.9% 0
the corrosion resistance of 316L-Cu SS. The different role of Cu in solution and aging treatments about on Eb and Ep is probably attributed to the Cu-rich precipitates which precipitated out after aging treatment. During the aging process, once the Cu-rich precipitates precipitated out from the steel matrix, the uniform continuity and compactness of the passive film would be destroyed. The Cu-rich precipitates (Fig. 3) would act as the “weak points” on the steel surface which are susceptible to the local attack [35]. Therefore, the discontinuity of passive film caused by Cu-rich precipitates would reduce the resistance to pitting corrosion and ability of self-passivation and self-repairing. Besides, because of the corrosion potential difference of Cu and Fe in steel, the galvanic corrosion between the Cu-rich precipitates and the steel matrix could also accelerate the corrosion rate of 316L-Cu SS. 3.4. Antibacterial and XPS tests The plate count method was adopted for evaluating antibacterial properties of the 316L-Cu SS. Fig. 6 shows the bacterial colony images of the 316L-Cu SS with different Cu content after solution and aging treatments, evaluated by co-culturing experimental samples with E. coli bacterial suspensions at 37 °C for 24 h. It can be clearly seen that the Petri dishes of all the experimental steels under solution treatment were mostly covered by countless bacterial colonies (Fig. 6a1), while on the contrary, only a small number of bacterial colonies were observed for the 316L-Cu SS after aging treatment, as shown in Fig. 6c3. Therefore, it can be reasonably concluded that the addition of Cu and aging treatment were two crucial factors to endow the steel with the antibacterial property. Table 4 presents the detailed antibacterial rates of the experimental steels. As shown, the antibacterial rates of all experimental steels against E. coli under solution treatment were much lower than those under aging treatment. Besides, the standard plate count bacteria for control were almost same with those of 316L SS both under solution and aging treatment, and the antibacterial rates were zero, exhibiting that it has no antibacterial property against E. coli. For 316L-2.5Cu SS, the standard plate count bacteria after solution treatment was 1500 ± 100, showing an approximately 25% antibacterial rate, whereas
it was only 130 ± 12 after aging at 700 °C for 3 h, showing that the antibacterial rate rapidly increased to 93.5%. After prolonging the aging time up to 6 h, the standard plate count bacteria and antibacterial rate were 37 ± 12 and 98.2%, respectively. Same tendency could be observed for 316L-3.5Cu SS. These results indicated that the aging treatment could ensured Cu-bearing 316L SS a strong antibacterial ability. It should be noted that the antibacterial rate of 316L-3.5Cu SS was higher than that of 316L-2.5Cu SS. This is because higher Cu addition did favor to release enough Cu ions for killing bacteria. As it is known, the released Cu ions from the Cu-bearing steels can destroy the bacterial cell walls and inhibit the normal growth of the bacteria [17,36], so the antibacterial property is controlled by the amount of released Cu ions, which is closely bounded up to the corrosion resistance. Generally, the higher the corrosion resistance is, the lower the antibacterial property becomes, and vice versa. To well understand the relation of corrosion resistance and antibacterial property, surface structure of the passive film on the 316L-Cu SS in state of solution and aging treatments were further analyzed by the XPS analysis. Fig. 7 presents the XPS spectra of Cu 2p for 316L-3.5Cu SS in solution and aging treatments at etching time of 15 s, and the corresponding parameters for decomposition of Cu 2p spectra are displayed in Table 5. As indicated in Fig. 7, the existence of Cu 2p2/3 lines confirmed the presence of Cu on these samples surface both in solution and aging states. The Cu 2p spectra were mainly composed of two peaks: Cu and CuO peaks, and the peak of metallic Cu was significantly higher than that of CuO. Besides, the overall Cu content in the passive film of aged 316L-3.5Cu SS was a little bit higher than that of solution treated 316L-3.5Cu SS, with values of 2.63 at.% and 2.32 at.%, respectively. On the same time, compared with solution treatment, both Cu and CuO contents in Cu 2p spectra increased slightly after aging treatment. Combining the results of antibacterial property and XPS analysis, possible reasons are deduced that the presence of metallic Cu in the passive film of the aged 316L-Cu SS could easily lose two electrons, and then transfer to the bivalent Cu ions, which are the key antibacterial agent. Obviously, the enrichment of Cu content in the passive film would be harmful to the stability of oxide film, due to the reaction of Cu + 2e → Cu2 +, and correspondingly increasing the flux of charge
Fig. 7. Surface XPS spectra of Cu 2p detected in the passive films of 316L-3.5Cu SS at 15 s etching time, (a) solution, (b) aging at 700 °C for 6 h.
T. Xi et al. / Materials Science and Engineering C 71 (2017) 1079–1085 Table 5 Parameters for decomposition of Cu 2p spectra for 316L-Cu SS at 15 s etching time by XPS analysis. Conditions
Cu Area,%
Solution 1.91 Aging at 700 °C for 6 h 2.29
CuO Binding energy, eV
Area,%
Binding energy, eV
932.5 932.4
0.41 0.34
933.5 933.6
transfer. As a consequence, the increment of Cu content in the passive film significantly decreased the corrosion resistance of 316L-Cu SS, while the release of more Cu ions prominently improved the antibacterial property. The XPS results were in good agreement with the results of corrosion resistance and antibacterial properties. 4. Conclusion (1) The addition of Cu and different heat treatment in the present study had no influence on the microstructure of 316L SS. (2) The difference in mechanical properties of 316L-Cu SS, the increase of the YS and UTS, under different heat treatment was mainly caused by the Cu-rich precipitates hardening effect in aging treatment, and synergistic effect of SFE and solution strengthening in solution treatment. (3) The lattice constant of 316L-Cu SS after aging treatment was smaller than that after solution treatment, due to the precipitation of Cu-rich precipitates from the steel matrix. (4) The pitting and protective potential of 316L-Cu SS increased with increasing Cu content in the solution state, while those decreased after aging treatment, owing to the generated Cu-rich precipitates.
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