Electrochemical corrosion behavior of a novel antibacterial stainless steel

Corrosion Science 51 (2009) 1083–1086

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Electrochemical corrosion behavior of a novel antibacterial stainless steel Yongqian Liu a, Jibiao Li b, Emeka E. Oguzie b,c, Ying Li b, Demin Chen a, Ke Yang a, Fuhui Wang b,* a b c

Center for Engineering Materials, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Rd., Shenyang 110016, China State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Rd., Shenyang 110015, China Electrochemistry and Materials Science Research Laboratory, Department of Chemistry, Federal University of Technology Owerri, PMB 1526, Owerri, Nigeria

a r t i c l e

i n f o

Article history: Received 6 October 2008 Accepted 4 March 2009 Available online 19 March 2009 Keywords: A. Stainless steel A. Copper B. EIS B. Polarization C. Corrosion

a b s t r a c t A novel antibacterial stainless steel (ASS) with martenstic microstructure has been recently developed, by controlled copper ion implantation, as a new functional material having broad-spectrum antibacterial properties. The electrochemical corrosion behavior of the ASS in 0.05 mol/L NaCl was assessed using linear polarization and electrochemical impedance spectroscopy (EIS) and compared with that of a conventional stainless steel (SS) without copper ion implantation. The ASS exhibited higher corrosion susceptibility in the chloride medium; with a more negative (active) corrosion potential, higher anodic current density and lower charge transfer and polarization resistance. This has been attributed to the occurrence of copper-catalyzed interfacial reactions. A functional tool, 3-D presentation of EIS data, has been employed in analyzing the electrochemical corrosion processes as well as probing complex interfacial phenomena. Ó 2009 Published by Elsevier Ltd.

1. Introduction Copper, incorporated in some stainless steels, has been reported to impart antimicrobial activity by releasing copper ions, which have a devastating effect on bacterial cells. This has formed the basis for design of some copper-containing antibacterial stainless steels (ASS) [1,2]. Such antimicrobial stainless steels find widespread utility in medical appliances and food processing to ensure improved safety and hygiene. Baena et al. [3] demonstrated that an ASS containing copper (3.8%) and niobium (0.1%) exhibited excellent antimicrobial properties due to niobium-stimulated copper precipitation. Yang et al. [4] reported that a ferritic ASS (1.5 wt% Cu) was about 99% potent in eliminating Staphylococcus and Coliform bacteria. The effectiveness of any antibacterial treatment depends strongly on factors such as the nature and conditions of treatment as well as the resulting ASS microstructure, amount of antibacterial additive and the amount of antimicrobial additive that precipitates on the protective oxide film. Copper as an alloying element has been used to improve the uniform corrosion resistance of some stainless steels in a number of environments [5,6], although its effect on localized corrosion in chloride media is neither uniform nor definite. For instance, Cu has been reported to exert both harmful [7] and beneficial [7,8] effects on the corrosion rate and pitting potential of austenitic stainless steels in chloride environments. Ujiro et al. [7] observed that alloying Cu had a harmful effect on the localized corrosion

* Corresponding author. E-mail address: [email protected] (F. Wang). 0010-938X/$ - see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.corsci.2009.03.007

resistance of both austenitic and ferritic stainless steels in the noble potential range. Such detrimental effect of Cu poses significant challenges in the design and fabrication of copper ion-containing ASS. Long-lasting, broad-spectrum antibacterial efficacy must necessarily be balanced by good corrosion resistance. In any situation, an optimal Cu content exists beyond which corrosion resistance is compromised. According to Hong and Koo [9] the Cu content of SUS 304 austenitic stainless steel should not exceed 3.5% to ensure balance between corrosion resistance and antimicrobial activity. In previous reports [10,11] we had produced ASS with martenistic microstructure, maintaining effective, long-lasting and broad-spectrum antibacterial properties. Nonetheless, electrochemical behavior of such martenistic ASS was not explored and to our knowledge has not yet been reported. Accordingly, this paper presents a comparative study of electrochemical properties of the martenistic ASS and those of a contemporary SS, to provide insights into effect of copper on the electrochemical corrosion behavior. The applicability of a new tool; 3D presentation of EIS data, for such studies is also highlighted. 2. Experimental Two martenistic stainless steels, with (ASS) and without (SS) copper addition, were used in this study. The nominal chemical compositions (wt%) are listed in Table 1. Before experimental tests, all the materials were annealed for 6 h at 700 °C, followed by rapid cooling. Details of the Cu ion implantation and microstructural characterization of ASS including details of the heat treatment reg-

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Table 1 The nominal chemical composition (wt%) of the two stainless steels (ASS and SS). C

Cr

Si

Mn

Cu

Others

ASS SS

0.29 0.30

12.99 12.44

0.64 0.50

0.27 0.15

3.05 –

Bal. Bal.

imen are as previously reported [12]. Specimens for corrosion tests were coupons of dimensions; 9  9  9 mm polished to # 800 finish with SiC paper and degreased with acetone. Electrochemical experiments were performed using a PARC Parstat-2273 Advanced Electrochemical System operated with Powercorr and Powersine software. The test solution was 0.05 mol/L NaCl, prepared from analytical reagent grade sodium chloride and distilled water. A conventional three-electrode glass cell was used for the experiments. Test coupons of with 0.8 cm2 exposed surface area were used as working electrode and a platinum foil as counter electrode. The reference electrode was a saturated calomel electrode (SCE), which was connected via a Luggin’s capillary. The working electrode was immersed in a test solution for 30 min to attain a stable open circuit potential. Potentiodynamic polarization studies were carried out in the potential range 1100 to 600 mV at a scan rate of 0.333 mV s1. Electrochemical impedance spectroscopy (EIS) measurements were obtained by holding the samples at a series of specified potentials in the frequency range 100 kHz–0.1 Hz, with a signal amplitude perturbation of 5 mV. Spectra analyses were performed using Zsimpwin software, also supplied by PARC. All experiments were undertaken in stagnant aerated solutions at 30 ± 1 °C. All experiments were run at least three times and the data always showed good reproducibility. 3. Results and discussion 3.1. Open circuit potential measurements The variation of the open-circuit potential (Eocp) with time for ASS and SS in 0.05 M NaCl solution is shown in Fig. 1. An initial continuous decrease in Eocp is observed for both ASS and SS due to the dissolution of the electrode oxide films, after which Eocp maintained almost steady values. It is clearly seen from the plots that addition of Cu shifts the Eocp of the stainless steel specimen in the negative direction, implying that the ASS surface is more active. This is an indication of a negative effect of Cu addition on the corrosion resistance of martenistic stainless steel.

-0.32 -0.33

SS

-0.34

E vs. SCE (V)

0.2 0.0 -0.2 -0.4 -0.6 -0.8 -7

10

-6

10

-5

10

-4

-3

10

10

-2

10

-1

10

2

i (A/cm ) Fig. 2. Potentiodynamic polarization curves of the ASS and SS in 0.05 M NaCl solution. The potential scan rate was set at 0.5 mV/s. The curve with the solid (empty) square characterizes linear polarization behavior of the ASS (SS) samples.

3.2. Potentiodynamic polarization behavior Typical anodic and cathodic polarization curves of ASS and SS in 0.05 M NaCl solution are illustrated in Fig. 2. Again there is a pronounced difference in the anodic behavior of both specimens in the active region. A higher current density for AAS is observed in the anodic region up to 0.3 V (vs. SCE), above which identical polarization response was obtained for the two materials. The higher current density of ASS can be attributed to the uneven distribution of Cu on the SS surface [12] leading to modification of the thermodynamic properties. According to Ujiro et al. [7] during anodic dissolution of Cu bearing stainless steels, dissolved Cu can form stable metal deposits which suppress the anodic dissolution of steels. Cl ions however diminish the stability of the deposited Cu and hence the ability to cover the anodic surface uniformly. Additional contributions of copper-catalyzed reactions further enhance the anodic dissolution rate. To obtain a quantitative understanding of the copper-assisted anodic reaction, electrochemical parameters such as polarization resistance, corrosion potential and corrosion current density, were derived from the polarization curves. The reduced value of polarization resistance (2698 X-cm2) for ASS in comparison with that of SS (6020 X-cm2) confirms an influence of copper ions on the kinetics of the reactions taking place at the interface. 3.3. Impedance measurements

-0.31

-0.35 -0.36 -0.37 -0.38 -0.39

ASS

-0.40 -0.41

ASS SS

0.4

E vs. SCE (V)

Martensitic steels

0.6

0

200

400

600

800

1000

Time (s) Fig. 1. Open circuit potential–time variation of ASS and SS in 0.05 M NaCl solution.

To investigate the influence of applied potential on the character of the electrochemical reactions in the active anodic region,, impedance measurements were performed at potentials from just below Ecorr to +0.4 V (vs. SCE). Fig. 3 shows that the magnitude of the impedance response was very sensitive to the applied potential. At each specific potential, the Nyquist plots for both steel specimens exhibit single capacitive character. A smaller arc for ASS was observed at all potentials without exception, confirming our deduction that the presence of copper ions at the interface adversely affected the electrochemical corrosion resistance. The CPE index (n) increased steadily in the range of (0.75–0.90) and (0.84–0.90) with increasing potential for ASS and SS, respectively. This indicates that variation of potential plays a vital role in tuning surface charge density, and that the character of the interfacial capacitance deviates from ideal capacitive behavior. The lower n values observed for ASS is another indication that the presence of copper ions altered the potential distribution and charge density

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Fig. 3. Nyquist spectra of the ASS and SS at a series of potential in 0.05 M NaCl. The curve with the solid (empty) square is of the ASS (SS) sample. The inset shows the interfacial resistance which is obtained by fitting the data based on an equivalent circuit Rs(QRct). All given potentials are with reference to SCE.

at the interface. The largest capacitive loop was found around 0.25 and 0.4 V (vs. SCE) for SS and ASS, respectively. This phenomenon is more clearly represented in the resistance–potential plots shown in the inset of Fig. 3, where a parabolic shape of R vs E can be seen for both samples. The existence of the resistance peak can be attributed to electron transfer controlled interfacial reactions, where ion transfer inevitably leads to a decrease in interfacial resistance. Again, the peak potential (0.38 V vs. SCE) in the case of ASS lies in a cathodic direction with respect to that of SS, which is in agreement with the polarization results. Bode representation of impedance data is truly a powerful tool in characterizing electrochemical processes with more than one time constant. Even so, a global view of the splitting of phase-angle peaks is beyond the effective scope conventional 2-D EIS spectra. In this study we propose a rarely used tool; 3-D representation of impedance data, for efficient analysis of such processes. The simplest frame of reference for analyzing these data is the case of an ideal polarized electrode with the equivalent circuit Rs(QRct). The impedance (Z) of this circuit is related to the frequency of the signal (f), the polarization resistance (Rp), the double layer capacity of the interface (C) and the resistance of the electrolyte (Rs) by [13]:

Z ¼ Rs þ

Rp 1 þ 2pfCRp

ð1Þ

At the high frequency limit Eq. (1) predicts that the impedance of the interface (Z) approaches Rs. This much is obvious from Fig. 4a, where, in the wide frequency range 10–10000 Hz, the impedance is reduced to a low and flat plateau, characterizing the solution resistance in the electrochemical response. Similar features with solution resistance can also be observed in the case of SS (Fig. 4b). In addition to the observation of greatly reduced impedance modulus which is in line with Nyquist data, our results facilitate frequency-specific analysis of interfacial processes. For any frequency within the low-frequency range (101 to 10 Hz), Eq. (1) predicts that Z approaches the sum of Rs + Rp, in which case Z attains values which are orders of magnitude higher than Rs and hence is taken to be virtually equal to Rp. Accordingly, the modulus–potential relationship exhibits parabolic character with a resistance peak. However, the peak shape exhibits pronounced broadening with increasing frequency and as the potential is made more positive. Consistent with our Nyquist analysis, both the peak height and peak potential can serve as useful parameters in evaluating corrosion resistance. The decrease in impedance modulus observed with increasing potential indicates a decrease in the resistance of the protective corrosion product films on the specimens’ surfaces. Fig. 4c and d show the phase angle plots. There are several common features for both the materials. A highly capacitive behavior, typical of passive materials, is indicated at medium to low

Fig. 4. Bode spectra of the ASS (a and c) and SS (b and d) at a series of potential in 0.05 M NaCl. (a and b) Frequency dependence of impedance modulus. (c and d) Frequency dependence of phase angle.

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frequencies by phase angles approaching 90°, suggesting that a highly stable film is formed on both ASS and SS. The phase angle seems to hardly change with frequency and with potential in the high frequency limit but particularly sensitive to both potential and frequency in the medium to low frequency range. As the potential is made more positive, the phase angles measured at different frequencies shift slowly but steadily towards lower values. The phase angle reduction rate is more pronounced for ASS, indicating an inhomogeneous surface compared to SS. The overall region presents no obvious features of peak splitting and symmetric shapes of phase angle and frequency were observed, indicating that single time constant processes within the oxide/solution interface play the dominant role in electrochemical responses. The absence of a second time constant in the Bode plot confirms that the resistance of the deposited Cu during the dissolution of ASS is much lower than the charge transfer resistance. 4. Conclusions The influence of Cu addition on the electrochemical behavior of the stainless steel (SS) was investigated. Cu addition was found to diminish the corrosion resistance by shifting the open circuit potential to more negative values, increasing the corrosion current density and lowering the polarization resistance. Impedance parameters were found to be sensitive to the electrode potential with characteristic peak potentials for the charge transfer resis-

tance and the impedance modulus, while the phase angles decreased as potential was made more positive. The reduced corrosion resistance of the Cu containing antibacterial stainless steel was induced by copper catalyzed interfacial reactions. Acknowledgements E.E. Oguzie is grateful to TWAS and UNESCO for the award of a TWAS-UNESCO Associateship Appointment. Prof. D.G. Lees is acknowledged for useful discussions. References [1] Z.G. Dan, H.W. Ni, B.F. Xu, J. Xiong, P.Y. Xiong, Thin Solid Films 492 (2005) 93. [2] P. Audebert, P. Hapiot, J. Electroanal. Chem. 361 (1993) 177. [3] M.I. Baena, M.C. Marquez, V. Matres, J. Botella, A. Ventosa, Curr. Microbiol. 53 (2006) 491. [4] J. Yang, D.N. Zou, X.M. Li, J. Zhu, Mater Sci Forum 510–511 (2006) 970. [5] Y. Jiangnan, W. Lichang, S. Wenhao, Corros. Sci. 33 (1992) 851. [6] M. Seo, G. Hultquist, C. Leygraf, N. Sato, Corros. Sci. 26 (1986) 949. [7] T. Ujiro, S. Satoh, R.W. Staehle, W.H. Smyrl, Corros. Sci. 43 (2001) 2185. [8] H.T. Lin, W.T. Tsai, J.T. Lees, C.S. Huang, Corros. Sci. 33 (1992) 691. [9] I.T. Hong, C.H. Koo, Mater. Sci. Eng. A 393 (2005) 213. [10] K. Yang, M.Q. Lu, J. Mater. Sci. Tech. 23 (2007) 333. [11] S.H. Chen, M.Q. Lu, J.D. Zhang, J.S. Dong, K. Yang, Acta Metall. Sin. 40 (2004) 314. [12] S.H. Chen, M.Q. Lu, K. Yang, J.D. Zhang, Chinese Patent, 0214568.0 [13] S.Y. Sayed, M.S. El-Deab, B.E. El-Anadouli, B.G. Ateya, J. Phys. Chem. B 107 (2003) 5575.