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Preparation of Cu–Ni–Fe alloy coating and its evaluation on corrosion behavior in 3.5% NaCl solution
Journal of Alloys and Compounds 563 (2013) 171–175
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Preparation of Cu–Ni–Fe alloy coating and its evaluation on corrosion behavior in 3.5% NaCl solution Qiongyu Zhou, Jibo Jiang, Qingdong Zhong ⇑, Yi Wang, Ke Li, Huijuan Liu Shanghai Key Laboratory of Modern Metallurgy and Material Processing, Shanghai University, Shanghai 200072, PR China
a r t i c l e
i n f o
Article history: Received 13 September 2012 Received in revised form 16 January 2013 Accepted 18 January 2013 Available online 27 February 2013 Keywords: Cu–Ni–Fe alloy Coating Low-carbon steel Homogeneous Corrosion resistance
a b s t r a c t In this paper, an attempt had been made to prepare a Cu–Ni–Fe alloy coating for improving the corrosion resistance of the low-carbon steel. The surface heat treatment of coated low-carbon steel was performed at 1000 °C for 3 h under hydrogen atmosphere. The structure and microstructure of coatings was separately analyzed by X-ray diffraction (XRD) and scanning electron microscope (SEM). The corrosion resistance of the samples was evaluated by potentiodynamic polarization (Tafel) and electrochemical impedance spectroscopy (EIS). Results indicated that a compact alloy coating was formed on the surface of low-carbon steel and the Ni content had a prodigious impact to the microstructure, composition and structure of Cu–Ni–Fe alloy coating. Apart from that, significant improvements in corrosion resistance were achieved by using the Cu–Ni–Fe alloy coating, which constituting of homogeneous c-phases. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Given the well-documented global corrosion challenge, the development of universal coating systems for metals that provide both passive and active protection is desirable [1,2]. To meet the demands of specific properties where they are most needed, there has been an extensive research on the possibilities for coating material design and their protection mechanisms. Recently, Cu– Ni alloy coatings and Fe–Ni alloy has been widely used due to their exciting mechanical, corrosion properties and reasonable price [3–10]. At the same time, the literatures concerning the ternary and quaternary alloy coatings are very numerous because of their more preeminent properties. As previous literatures indicated, the protective action of Cu–Ni–Fe alloy seems to be dependent on a good adherence to the metal and high resistivity towards electronic and ionic conductance [11–14]. Iron can provide added resistance to corrosion in Cu–Ni based alloys, nickel can enter the defective structure of Cu2O, decreasing its conductivity. Hence, the corrosion resistance can be improved with increasing iron or nickel content as long as it remains in solid solution [15]. Generally, the addition of iron to Cu–Ni improves the corrosion resistance in a sea water and polluted water environment. Cu– Ni–Fe alloys also show good resistance to stress corrosion and high-temperature corrosion. Because of these interesting properties, Cu–Ni–Fe alloys are widely used in industrial applications
⇑ Corresponding author. Tel.: +86 13 391312191; fax: +86 21 56338244. E-mail address: [email protected] (Q. Zhong).
such as inert anodes in aluminium electrolysis, heat exchange equipment, hydraulic pipelines, oil rigs and platforms [11,15–17]. However, the iron (Fe)–copper (Cu) system does not form intermetallic compounds and has negligible mutual solid solubility. Cu–Ni–Fe alloys present a two-phased microstructure (Cu-rich and Fe–Ni-rich phases) due to their spinodal decomposition [16–18]. This chemical inhomogeneity may have a negative influence on the corrosion resistance of the material [19]. Therefore, conventional surface alloying processes such as salt bath treatment or electrodeposition technique are unsuitable for preparation of homogeneous Cu–Ni–Fe alloy coating. Recently, many new modified treatment processes are applied for preparation of homogeneous ternary Cu–Ni–Fe alloy. Baricco et al. declare that rapid solidification is a viable process and the addition of Ni to binary Fe–Cu alloys promotes the solubility of Fe in the fcc phase [20]. Mondal et al. [21] and Helle et al. [6,19] both have demonstrated the high efficiency of the ball milling technique for producing monophased Cu–Ni–Fe alloys over a large stoichiometric range. Consequently, design of monophased Cu–Ni– Fe alloy coatings for improving corrosion behavior is considerable. With the above aspects as the backdrop, an attempt is made to develop a more economical and simple possibilities process for improving the corrosion resistance of low-carbon steel. We adopted the nano-hybrid particles (nano-sized CuO–NiO) and applied the surface treatment of low-carbon steel to develop the homogeneous Cu–Ni–Fe alloy coatings. The corrosion behaviors of Cu–Ni–Fe alloy coatings were investigated by using the potentiodynamic polarization curve and electrochemical impedance spectroscopy in conjunction with impedance fitting.
0925-8388/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.01.136
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Q. Zhou et al. / Journal of Alloys and Compounds 563 (2013) 171–175
2. Experimental process 2.1. Preparation of nano-sized particles and samples For the preparation of the coating, nanoCuO was used as the copper source and nanoNiO as the nickel source. CuO and NiO were placed in an agate jar with absolute ethanol used as the dispersing agent and then ball-milled for 24 h. Molar ratio of CuO:NiO was 1:x (x = 1, 2, 4) and liquid: solid ratio was 80:15. All the sizes of CuO–NiO particles were controlled at 100 nmmax. The low-carbon steel (10 10 1.2 mm3) was used as substrate, which was mechanically polished using 240–1200 grit papers in sequence. Then the samples were rinsed in acetone, ethanol and distilled water for 5 min respectively by an ultrasonic cleaner in order to degrease and clean the surfaces to improve the adhesion of the coating. After the cleaning process, the low-carbon steel was placed horizontally. The coating solution was vertically sprayed onto the substrates from a distance of approximately 15 cm using a detail spray gun (IT-powered Manufacturing, HD-470). The spray-dry cycle consisting of spraying for 15 s followed by airdrying the sample for 45 s was repeatedly applied up to 20 times to achieve the desired coating uniformity and thickness. It is noteworthy to underline that the weight of nano-hybrid particles layer should be controlled at 62 ± 0.50 g/dm2. Then the heat treatment was carried out at temperatures of 1000 °C for 3 h under hydrogen atmosphere with the heating rate of 10 °C/min. At the last step, a mass of hydrogen was continually used on the sample surface to accelerate cooling rate for prevention of the spinodal phase separation. Three samples were prepared by different particles in this paper and they are denoted by Cu–Ni–Fe I, Cu–Ni–Fe II, Cu–Ni–Fe III (as shown in Table 1).
2.2. Characterization of the Cu–Ni–Fe alloy coatings After the heat treatment, the structural characters of the coatings were investigated by X-ray diffraction (XRD) with a diffractometer D/max-2200 V and Cu Ka radiation. The surface morphology and cross-section of Cu–Ni–Fe alloy coatings was evaluated by scanning electron microscope (SEM) using the JEOL JSM-6700F microscope, at the same time, the chemical composition of the coatings was determined by an energy dispersive spectroscopy (EDS). The corrosion behavior of the coating was studied using polarization techniques (Tafel) and electrochemical impedance spectroscopy (EIS) with an electrochemistry station (CHI660C). Tafel and EIS experiments for the substrate and the coatings were performed in 3.5 wt.% NaCl solution with a conventional three-electrode cell. A saturated calomel reference electrode (SCE) and a platinum wire as counter-electrode were used in the tests. The surface area of the test coupons (as the working electrode) exposed to the electrolyte was 1 cm2. The impedance data were obtained at the open circuit potential, when the corrosion potential remained stable, a sinusoidal AC signal of 5 mV amplitude at the open circuit potential (OCP) was applied to the electrode over the frequency which ranged from 100 kHz to 0.01 Hz. Each type of electrochemical measurement was repeated at least 3 times until good reproducibility of the data was obtained, and the average results were presented here.
3. Results
the ratio of Cu/Ni in the coating is much higher than that in particles, suggesting that the Ni is diffused more easily than Cu. 3.2. Morphological analysis The SEM micrographs of Cu–Ni–Fe coatings are shown in Fig. 1. In all cases, the coatings are uniform and crack-free. However, the surface morphology of the coatings is strongly influenced by the Cu content and Ni content in the coating. As shown in Fig. 1a, the small dendrite phase exists in the sample Cu–Ni–Fe I, and the coating surface becomes generally smoother as the Cu content decreases and Ni relatively increases (Fig. 1b and c). It shows that the increase of Ni content in the coating has a significant impact to produce a uniform and homogenous Cu–Ni–Fe alloy coating. The cross-section morphology of the sample Cu–Ni–Fe III is displayed in Fig. 1d. As shown, a compact and homogenous alloy layer is formed on the surface. The average thickness of the coating is about 30 lm and satisfactory adhesion between coating and substrate is observed. In addition, a transition zone which contained two phase, is clearly observed between the alloy coating and the substrate. 3.3. XRD analysis Fig. 2 shows the XRD patterns of the three coatings. As shown in Fig. 2, Cu–Ni–Fe I coating exhibits diffraction peaks of both a-phases and c-phases, while there is an obvious increase in the intensity of c-phases together with a sharp decrease in that of a-phases, of which Cu–Ni–Fe II coating was nearly disappeared. Moreover, diffraction peaks of a-phases disappear completely for Cu–Ni–Fe III. 3.4. Potentiodynamic polarization The corrosion behavior of coating can be studied by polarization curves in corrosive media that yield specific data on the behavior of the coating system [22]. Fig. 3 shows the polarization curves of coated low-carbon steel in 3.5 wt.% NaCl solution at ambient temperature, and the data for corrosion current density (Icorr) and corrosion potential (Ecorr) obtained from the polarization curves by Tafel extrapolation method are tabulated in Table 2. The protective efficiency (Pi) of the coatings is also listed in Table 2 and it was estimated by the following equation [23]:
1
Icorr
! 100
3.1. EDS analysis
Pi ð%Þ ¼
Compositions of different Cu–Ni–Fe alloy coatings were analyzed by use of EDS analysis and the results are shown in Table 1. It reveals that all coatings are composed of Cu, Ni and Fe. Although presence of little oxygen at surface of the coating layer is quite probable, however EDS analysis is not accurate enough in detection of such microscale elements. The appearance of Fe in the coatings is caused by the solid diffusion of new generated metal atoms or fusion of nanoparticles with the low-carbon steel surfaces in the coating process. The elements composition of the Cu–Ni–Fe alloy coatings varies with the composite particles. The Cu content decreases and Ni content increases on the surface coating. Moreover,
where I0corr represents the corrosion rate of the low-carbon steel (substrate). Icorr represents the corrosion rate of the Cu–Ni–Fe coatings. It is obvious from Fig. 3 that the bare low-carbon steel reacted acutely in the sodium chloride solution. In contrast with bare carbon steel, all the coated samples have more noble corrosion potential and much lower corrosion current density ranging from 15.30 to 5.20 lA cm2, indicating that the Cu–Ni–Fe alloy coating improved the corrosion resistance of carbon steel distinctly. In comparison with the inhomogeneous coating (Cu–Ni–Fe I), the homogeneous coatings (Cu–Ni–Fe II and Cu–Ni–Fe III) show
I0corr
Table 1 The composition of Cu–Ni–Fe alloy coating in this study. Samples
Composite particles CuO/NiO (at.)
Cu–Ni–Fe I Cu–Ni–Fe II Cu–Ni–Fe III
1/1 1/2 1/4
Surface composite (wt.%) Cu
Ni
Fe
Cu/Ni (at.)
46.3 30.3 20.6
22.8 25.0 46.1
30.9 44.7 33.4
1/0.53 1/0.89 1/2.42
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Fig. 1. SEM micrographs of the Cu–Ni–Fe coatings (a) Cu–Ni–Fe I; (b) Cu–Ni–Fe II; (c) Cu–Ni–Fe III; and (d) cross-section of Cu–Ni–Fe III.
Table 2 Potentiodynamic polarization parameters for the substrate and the alloy coatings. Samples
Ecorr (mV)
Icorr (lA cm2)
Pi (%)
Substrate Cu–Ni–Fe I Cu–Ni–Fe II Cu–Ni–Fe III
764.4 615.2 568.8 554.9
41.88 15.03 8.32 5.20
– 64.11 80.13 87.58
higher protective efficiency and lower corrosion current density (see Table 2). Among these three samples, the Cu–Ni–Fe III coating shows the highest protective efficiency (87.58%). This result indicates that uniform Cu–Ni–Fe alloy coating constituting of homogeneity c-phases can effectively protect the surface of the samples from the effect of Cl ions. Fig. 2. X-ray diffraction patterns of Cu–Ni–Fe coatings.
3.5. EIS
Fig. 3. The potentiodynamic polarization curves of substrate and Cu–Ni–Fe coatings in 3.5 wt.% NaCl solution.
EIS is a powerful and non-destructive electrochemical technique to affirm electrochemical reactions and investigating corrosion behaviors at the electrode/electrolyte interface [24]. EIS spectra is usually displayed either in the form of a Bode plot or a Nyquist plot. Nyquist plots of the alloy coating and the low-carbon steel are presented in Fig. 4. A simplified equivalent circuit (Fig. 5) was also used to model the low-carbon steel electrode and the electrolyte surface. Taking the nature of the coating into consideration, the impedance response cannot be simply represented by the combinations of capacitances and resistances due to its capacitive character. The use of a constant phase element (CPE) as a substitution for Cdl in the equivalent circuit of the impedance not only minimizes the error but also provides more detailed information about the non-ideal dielectric properties of the coating [25,26]. The values of solution resistance (Rs), charge-transfer resistance (Rct), double-layer pseudo-capacitance formed in the substrate/electrolyte interface (Cdl) or total capacitance of the constant phase angle element (CPE) and n for coatings were calculated by simulating
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Q. Zhou et al. / Journal of Alloys and Compounds 563 (2013) 171–175 Table 3 Equivalent circuit parameters for the substrate and the alloy coatings.
Fig. 4. Nyquist plots of the substrate and the Cu–Ni–Fe coatings in 3.5 wt.% NaCl solution.
Samples
Rs (ohm cm2)
Y0-CPE (F cm2)
nc
Rct (ohm cm2)
Substrate Cu–Ni–Fe I Cu–Ni–Fe II Cu–Ni–Fe III
10.95 9.87 15.04 15.14
1.59 6.86 2.36 1.04
– 0.896 0.895 0.913
1.90 7.32 1.07 1.31
E3 (Cdl) E4 E4 E4
E3 E3 E4 E4
Therefore, the values of Rct are strongly dependent on the coating characteristics and indicate the corrosion resistance of the materials. The Rct values of the Cu–Ni–Fe coating were about 10times higher than that of the low-carbon steel, confirming that the coating had higher corrosion resistance than low-carbon steel. Also, the Cu–Ni–Fe III coating has the highest values of Rct, which implies the best anti-corrosion ability. These results are highly in correspondence with results which are extracted from Tafel polarization. 4. Discussions
Fig. 5. An equivalent circuit modelling the electrode (Rs: solution resistance; Cdl: double-layer pseudo-capacitance for substrate; CPE: constant phase element for coatings; Rct: charge-transfer resistance).
the experimental data with Zview software. The results are listed in Table 3. The Nyquist plots presented an unfinished semi-circle arc (shown in Fig. 4). The formation of these semi-circle arcs is attributed to the charge transfer process in the electrode/electrolyte interface, related to changes in the coating property [27].
Comparing with the ratio of Cu/Ni in the coating, it is much higher than that in particles (see Table 1), showing that Ni diffuses more easily than Cu. This phenomenon is probably caused by the high speed diffusion of between Fe substrate and the new Ni atoms, which generated by hydrogen reduction of oxides particles, on the contrary, the diffusivity of Cu is very slow due to the low solid solubility between Cu and Fe. This consequence matches the Cu–Fe binary phase diagram, as c-Fe (fcc) is formed only above 900 °C and that the maximum amount of Cu that can be dissolved in that phase is 13 wt.%, while Ni and Fe are totally miscible (fcc phase) between 912 and 1394 °C [28]. From the XRD results (Fig. 2), it reveals that the increase of Ni help to stabilize the c phases since Fe–Ni system is totally miscible and Ni can promotes the solubility of Fe in the fcc phase [16]. In the present study, The a-phases are bcc [Fe, Ni] phases, while the cphases cannot be totally excluded on the basis of the XRD data of the consolidated samples because the formation of c-[Cu]-rich
Fig. 6. SEM micrograph and X-ray elemental maps of Cu–Ni–Fe III.
Q. Zhou et al. / Journal of Alloys and Compounds 563 (2013) 171–175
fcc phases and c-[Fe, Ni]-rich fcc phases are hardly differentiable by XRD analysis due to the small difference (only 0.3° for 2h) for their peak position [29]. Although Cu–Ni–Fe III coating exhibits only c-phase, the spinodal decomposition of the Cu–Ni–Fe alloys cannot be excluded based on the XRD analyses. However, as illustrated in Fig. 6, the element Cu, Ni, Fe are all distributed homogeneously on the sample surface and no dendritic structures characteristic have been observed. This suggests all metallic atom exist in a single-phases without a spinodal decomposition, if any, spinodal decomposition would have occurred on a very minute scale. This means, homogenous cCu–Ni–Fe alloy coating would be formed on the surface of low-carbon steel, which is expected to be beneficial for the material corrosion resistance properties [15]. In addition, it can be seen from Table 3 that the n value for CPE of the Cu–Ni–Fe coating is equal or close to 1. The CPE is a special element which reflects the non-ideal dielectric properties of the coating. For n = 1, the CPE represents an ideal capacitor; for n = 0, the CPE is an ideal resistor; for n = 0.5, the CPE describes a Warburg impedance with diffusion character; for 0.5 < n < 1, the CPE represents a frequency dispersion of time constants because of local inhomogeneity, roughness or porosity of the electrode surface [26]. The n value close to 1 implies the presence of an approximate ideal double-layer pseudo-capacitance formed in the coating-electrolyte interface, that is, the coating surface is smooth and compact. The value of n come close to 1 with the increase of Ni content in the coatings, suggesting that a possible increase in surface homogeneity or compactness. This result is consistent with the above results drawn out of the SEM micrographs and X-ray diffraction. Polarization and EIS analysis (Figs. 3 and 4) showed that Cu–Ni–Fe alloy III coating has the maximum corrosion resistance. This means, homogenous cCu–Ni–Fe alloy coating resulted from high Ni content can effectively improve the corrosion resistance of lowcarbon steel. 5. Conclusions This study has demonstrated the feasibility of producing Cu– Ni–Fe coating by using nano-particles composed of the CuO–NiO. The increase of Ni content in the coating has a significant impact to produce a uniform Cu–Ni–Fe alloy coating, which is constituted of homogeneous c-phases without spinodal decomposition. Polarization and EIS test results indicated that Cu–Ni–Fe alloy coating provided corrosion protection and barrier properties on the low-carbon steel. The protective efficiency (Pi) and chargetransfer resistance (Rct) enhanced with the increase of Ni content in the coating in relation to its characteristic favorable for corrosion resistance. Consequently, preparation of homogeneous
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Cu–Ni–Fe alloy coating may be an effective method to improve the corrosion resistance of low-carbon steel. Acknowledgment This paper is financially supported by the Natural Science Foundation of China 50571059 and 50615024, the program for New Century Excellent Talents in University NCET-07-0536, and the State Key Laboratory of Development and Application Technology of Automotive Steel (Bao. Steel). References [1] M.J. Hollamby, D. Fix, I. Dönch, D. Borisova, H. Möhwald, D. Shchukin, Adv. Mater. 23 (2011) 1361–1365. [2] S.H. Cho, S.R. White, P.V. Braun, Adv. Mater. 21 (2009) 645–649. [3] E. Chassaing, K. Vu Quang, R. Wiart, J. Appl. Electrochem. 17 (1987) 1267–1280. [4] S.K. Ghosh, G.K. Dey, R.O. Dusane, A.K. Grover, J. Alloys Comp. 426 (2006) 235–243. [5] A. Mürsel, K. Hakan, S. Mürside, B.M. Celalettin, J. Alloys Comp. 453 (2008) 15–19. [6] I. Milošev, M. Metikoš-Hukovic´, Electrochim. Acta 42 (1997) 1537–1548. [7] V. Chapman, B.J. Welch, M. Skyllas-Kazacos, Electrochim. Acta 56 (2011) 1227–1238. [8] A.B. Waheed, M.I. Khaled, M.F. Ahlam, J. Alloys Comp. 484 (2009) 365–370. [9] P. Druska, H.H. Strehblow, S. Golledge, Corros. Sci. 38 (1996) 835–851. [10] P. Druska, H.H. Strehblow, Corros. Sci. 38 (1996) 1369–1383. [11] A. Barbucci, G. Farne, P. Matteazzi, R. Riccieri, G. Cerisola, Corros. Sci. 41 (1998) 463–475. [12] J. Zhang, Q. Wang, Y. Wang, L. Wen, C. Dong, J. Alloys Comp. 505 (2010) 179–182. [13] L. Vrsalovic, E. Oguzie, M. Klisskic, S. Gudic, Chem. Eng. Commun. 198 (2011) 1380–1393. [14] V. Chapman, B.J. Welch, M. Skyllas-Kazacos, Corros. Sci. 53 (2011) 2815–2825. [15] S. Helle, B. Brodu, B. Davis, D. Guay, L. Roué, Corros. Sci. 53 (2011) 3248–3253. [16] W.A. Badawy, K.M. Ismaila, A.M. Fathi, Electrochim. Acta 50 (2005) 3603–3608. [17] R. Haugsrud, T. Norby, P. Kofstad, Corros. Sci. 43 (2001) 283–299. [18] V.M. López-Hirata, K.I. Hirano, J. Mater. Sci. 31 (1996) 1703–1706. [19] S. Helle, M. Pedron, B. Assouli, B. Davis, D. Guay, L. Roué, Corros. Sci. 52 (2010) 3348–3355. [20] M. Baricco, E. Bosco, G. Acconciaioco, P. Rizzi, M. Coisson, Mater. Sci. Eng. A 375 (2004) 1019–1023. [21] B.N. Mondal, A. Basumallick, P.P. Chattopadhyay, J. Magn. Magn. Mater. 309 (2007) 290–294. [22] E.P. Banczek, P.R.P. Rodrigues, I. Costa, Surf. Coat. Technol. 202 (2008) 2008–2014. [23] N.D. Nama, D.S. Jo, J.G. Kim, D.H. Yoon, Thin Solid Films 519 (2011) 6787–6791. [24] F. Mansfeld, Electrochim. Acta 35 (1990) 1533–1544. [25] J. Jiang, Y. Wang, Q. Zhong, Q. Zhou, L. Zhang, Surf. Coat. Technol. 206 (2011) 473–478. [26] G. Vázquez, I. González, Electrochim. Acta 52 (2007) 6771–6777. [27] W. Ye, Y. Li, F. Wang, Electrochim. Acta 51 (2006) 4426–4432. [28] K.P. Gupta, S.B. Rajendraprasad, A.K. Jena, J. Alloys Phase Diagrams 3 (1987) 116–127. [29] H.X. Li, X.J. Hao, G. Zhao, S.M. Hao, J. Mater. Sci. 36 (2001) 779–784.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Preparation of Cu–Ni–Fe alloy coating and its evaluation on corrosion behavior in 3.5% NaCl solution Qiongyu Zhou, Jibo Jiang, Qingdong Zhong ⇑, Yi Wang, Ke Li, Huijuan Liu Shanghai Key Laboratory of Modern Metallurgy and Material Processing, Shanghai University, Shanghai 200072, PR China
a r t i c l e
i n f o
Article history: Received 13 September 2012 Received in revised form 16 January 2013 Accepted 18 January 2013 Available online 27 February 2013 Keywords: Cu–Ni–Fe alloy Coating Low-carbon steel Homogeneous Corrosion resistance
a b s t r a c t In this paper, an attempt had been made to prepare a Cu–Ni–Fe alloy coating for improving the corrosion resistance of the low-carbon steel. The surface heat treatment of coated low-carbon steel was performed at 1000 °C for 3 h under hydrogen atmosphere. The structure and microstructure of coatings was separately analyzed by X-ray diffraction (XRD) and scanning electron microscope (SEM). The corrosion resistance of the samples was evaluated by potentiodynamic polarization (Tafel) and electrochemical impedance spectroscopy (EIS). Results indicated that a compact alloy coating was formed on the surface of low-carbon steel and the Ni content had a prodigious impact to the microstructure, composition and structure of Cu–Ni–Fe alloy coating. Apart from that, significant improvements in corrosion resistance were achieved by using the Cu–Ni–Fe alloy coating, which constituting of homogeneous c-phases. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Given the well-documented global corrosion challenge, the development of universal coating systems for metals that provide both passive and active protection is desirable [1,2]. To meet the demands of specific properties where they are most needed, there has been an extensive research on the possibilities for coating material design and their protection mechanisms. Recently, Cu– Ni alloy coatings and Fe–Ni alloy has been widely used due to their exciting mechanical, corrosion properties and reasonable price [3–10]. At the same time, the literatures concerning the ternary and quaternary alloy coatings are very numerous because of their more preeminent properties. As previous literatures indicated, the protective action of Cu–Ni–Fe alloy seems to be dependent on a good adherence to the metal and high resistivity towards electronic and ionic conductance [11–14]. Iron can provide added resistance to corrosion in Cu–Ni based alloys, nickel can enter the defective structure of Cu2O, decreasing its conductivity. Hence, the corrosion resistance can be improved with increasing iron or nickel content as long as it remains in solid solution [15]. Generally, the addition of iron to Cu–Ni improves the corrosion resistance in a sea water and polluted water environment. Cu– Ni–Fe alloys also show good resistance to stress corrosion and high-temperature corrosion. Because of these interesting properties, Cu–Ni–Fe alloys are widely used in industrial applications
⇑ Corresponding author. Tel.: +86 13 391312191; fax: +86 21 56338244. E-mail address: [email protected] (Q. Zhong).
such as inert anodes in aluminium electrolysis, heat exchange equipment, hydraulic pipelines, oil rigs and platforms [11,15–17]. However, the iron (Fe)–copper (Cu) system does not form intermetallic compounds and has negligible mutual solid solubility. Cu–Ni–Fe alloys present a two-phased microstructure (Cu-rich and Fe–Ni-rich phases) due to their spinodal decomposition [16–18]. This chemical inhomogeneity may have a negative influence on the corrosion resistance of the material [19]. Therefore, conventional surface alloying processes such as salt bath treatment or electrodeposition technique are unsuitable for preparation of homogeneous Cu–Ni–Fe alloy coating. Recently, many new modified treatment processes are applied for preparation of homogeneous ternary Cu–Ni–Fe alloy. Baricco et al. declare that rapid solidification is a viable process and the addition of Ni to binary Fe–Cu alloys promotes the solubility of Fe in the fcc phase [20]. Mondal et al. [21] and Helle et al. [6,19] both have demonstrated the high efficiency of the ball milling technique for producing monophased Cu–Ni–Fe alloys over a large stoichiometric range. Consequently, design of monophased Cu–Ni– Fe alloy coatings for improving corrosion behavior is considerable. With the above aspects as the backdrop, an attempt is made to develop a more economical and simple possibilities process for improving the corrosion resistance of low-carbon steel. We adopted the nano-hybrid particles (nano-sized CuO–NiO) and applied the surface treatment of low-carbon steel to develop the homogeneous Cu–Ni–Fe alloy coatings. The corrosion behaviors of Cu–Ni–Fe alloy coatings were investigated by using the potentiodynamic polarization curve and electrochemical impedance spectroscopy in conjunction with impedance fitting.
0925-8388/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.01.136
172
Q. Zhou et al. / Journal of Alloys and Compounds 563 (2013) 171–175
2. Experimental process 2.1. Preparation of nano-sized particles and samples For the preparation of the coating, nanoCuO was used as the copper source and nanoNiO as the nickel source. CuO and NiO were placed in an agate jar with absolute ethanol used as the dispersing agent and then ball-milled for 24 h. Molar ratio of CuO:NiO was 1:x (x = 1, 2, 4) and liquid: solid ratio was 80:15. All the sizes of CuO–NiO particles were controlled at 100 nmmax. The low-carbon steel (10 10 1.2 mm3) was used as substrate, which was mechanically polished using 240–1200 grit papers in sequence. Then the samples were rinsed in acetone, ethanol and distilled water for 5 min respectively by an ultrasonic cleaner in order to degrease and clean the surfaces to improve the adhesion of the coating. After the cleaning process, the low-carbon steel was placed horizontally. The coating solution was vertically sprayed onto the substrates from a distance of approximately 15 cm using a detail spray gun (IT-powered Manufacturing, HD-470). The spray-dry cycle consisting of spraying for 15 s followed by airdrying the sample for 45 s was repeatedly applied up to 20 times to achieve the desired coating uniformity and thickness. It is noteworthy to underline that the weight of nano-hybrid particles layer should be controlled at 62 ± 0.50 g/dm2. Then the heat treatment was carried out at temperatures of 1000 °C for 3 h under hydrogen atmosphere with the heating rate of 10 °C/min. At the last step, a mass of hydrogen was continually used on the sample surface to accelerate cooling rate for prevention of the spinodal phase separation. Three samples were prepared by different particles in this paper and they are denoted by Cu–Ni–Fe I, Cu–Ni–Fe II, Cu–Ni–Fe III (as shown in Table 1).
2.2. Characterization of the Cu–Ni–Fe alloy coatings After the heat treatment, the structural characters of the coatings were investigated by X-ray diffraction (XRD) with a diffractometer D/max-2200 V and Cu Ka radiation. The surface morphology and cross-section of Cu–Ni–Fe alloy coatings was evaluated by scanning electron microscope (SEM) using the JEOL JSM-6700F microscope, at the same time, the chemical composition of the coatings was determined by an energy dispersive spectroscopy (EDS). The corrosion behavior of the coating was studied using polarization techniques (Tafel) and electrochemical impedance spectroscopy (EIS) with an electrochemistry station (CHI660C). Tafel and EIS experiments for the substrate and the coatings were performed in 3.5 wt.% NaCl solution with a conventional three-electrode cell. A saturated calomel reference electrode (SCE) and a platinum wire as counter-electrode were used in the tests. The surface area of the test coupons (as the working electrode) exposed to the electrolyte was 1 cm2. The impedance data were obtained at the open circuit potential, when the corrosion potential remained stable, a sinusoidal AC signal of 5 mV amplitude at the open circuit potential (OCP) was applied to the electrode over the frequency which ranged from 100 kHz to 0.01 Hz. Each type of electrochemical measurement was repeated at least 3 times until good reproducibility of the data was obtained, and the average results were presented here.
3. Results
the ratio of Cu/Ni in the coating is much higher than that in particles, suggesting that the Ni is diffused more easily than Cu. 3.2. Morphological analysis The SEM micrographs of Cu–Ni–Fe coatings are shown in Fig. 1. In all cases, the coatings are uniform and crack-free. However, the surface morphology of the coatings is strongly influenced by the Cu content and Ni content in the coating. As shown in Fig. 1a, the small dendrite phase exists in the sample Cu–Ni–Fe I, and the coating surface becomes generally smoother as the Cu content decreases and Ni relatively increases (Fig. 1b and c). It shows that the increase of Ni content in the coating has a significant impact to produce a uniform and homogenous Cu–Ni–Fe alloy coating. The cross-section morphology of the sample Cu–Ni–Fe III is displayed in Fig. 1d. As shown, a compact and homogenous alloy layer is formed on the surface. The average thickness of the coating is about 30 lm and satisfactory adhesion between coating and substrate is observed. In addition, a transition zone which contained two phase, is clearly observed between the alloy coating and the substrate. 3.3. XRD analysis Fig. 2 shows the XRD patterns of the three coatings. As shown in Fig. 2, Cu–Ni–Fe I coating exhibits diffraction peaks of both a-phases and c-phases, while there is an obvious increase in the intensity of c-phases together with a sharp decrease in that of a-phases, of which Cu–Ni–Fe II coating was nearly disappeared. Moreover, diffraction peaks of a-phases disappear completely for Cu–Ni–Fe III. 3.4. Potentiodynamic polarization The corrosion behavior of coating can be studied by polarization curves in corrosive media that yield specific data on the behavior of the coating system [22]. Fig. 3 shows the polarization curves of coated low-carbon steel in 3.5 wt.% NaCl solution at ambient temperature, and the data for corrosion current density (Icorr) and corrosion potential (Ecorr) obtained from the polarization curves by Tafel extrapolation method are tabulated in Table 2. The protective efficiency (Pi) of the coatings is also listed in Table 2 and it was estimated by the following equation [23]:
1
Icorr
! 100
3.1. EDS analysis
Pi ð%Þ ¼
Compositions of different Cu–Ni–Fe alloy coatings were analyzed by use of EDS analysis and the results are shown in Table 1. It reveals that all coatings are composed of Cu, Ni and Fe. Although presence of little oxygen at surface of the coating layer is quite probable, however EDS analysis is not accurate enough in detection of such microscale elements. The appearance of Fe in the coatings is caused by the solid diffusion of new generated metal atoms or fusion of nanoparticles with the low-carbon steel surfaces in the coating process. The elements composition of the Cu–Ni–Fe alloy coatings varies with the composite particles. The Cu content decreases and Ni content increases on the surface coating. Moreover,
where I0corr represents the corrosion rate of the low-carbon steel (substrate). Icorr represents the corrosion rate of the Cu–Ni–Fe coatings. It is obvious from Fig. 3 that the bare low-carbon steel reacted acutely in the sodium chloride solution. In contrast with bare carbon steel, all the coated samples have more noble corrosion potential and much lower corrosion current density ranging from 15.30 to 5.20 lA cm2, indicating that the Cu–Ni–Fe alloy coating improved the corrosion resistance of carbon steel distinctly. In comparison with the inhomogeneous coating (Cu–Ni–Fe I), the homogeneous coatings (Cu–Ni–Fe II and Cu–Ni–Fe III) show
I0corr
Table 1 The composition of Cu–Ni–Fe alloy coating in this study. Samples
Composite particles CuO/NiO (at.)
Cu–Ni–Fe I Cu–Ni–Fe II Cu–Ni–Fe III
1/1 1/2 1/4
Surface composite (wt.%) Cu
Ni
Fe
Cu/Ni (at.)
46.3 30.3 20.6
22.8 25.0 46.1
30.9 44.7 33.4
1/0.53 1/0.89 1/2.42
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Fig. 1. SEM micrographs of the Cu–Ni–Fe coatings (a) Cu–Ni–Fe I; (b) Cu–Ni–Fe II; (c) Cu–Ni–Fe III; and (d) cross-section of Cu–Ni–Fe III.
Table 2 Potentiodynamic polarization parameters for the substrate and the alloy coatings. Samples
Ecorr (mV)
Icorr (lA cm2)
Pi (%)
Substrate Cu–Ni–Fe I Cu–Ni–Fe II Cu–Ni–Fe III
764.4 615.2 568.8 554.9
41.88 15.03 8.32 5.20
– 64.11 80.13 87.58
higher protective efficiency and lower corrosion current density (see Table 2). Among these three samples, the Cu–Ni–Fe III coating shows the highest protective efficiency (87.58%). This result indicates that uniform Cu–Ni–Fe alloy coating constituting of homogeneity c-phases can effectively protect the surface of the samples from the effect of Cl ions. Fig. 2. X-ray diffraction patterns of Cu–Ni–Fe coatings.
3.5. EIS
Fig. 3. The potentiodynamic polarization curves of substrate and Cu–Ni–Fe coatings in 3.5 wt.% NaCl solution.
EIS is a powerful and non-destructive electrochemical technique to affirm electrochemical reactions and investigating corrosion behaviors at the electrode/electrolyte interface [24]. EIS spectra is usually displayed either in the form of a Bode plot or a Nyquist plot. Nyquist plots of the alloy coating and the low-carbon steel are presented in Fig. 4. A simplified equivalent circuit (Fig. 5) was also used to model the low-carbon steel electrode and the electrolyte surface. Taking the nature of the coating into consideration, the impedance response cannot be simply represented by the combinations of capacitances and resistances due to its capacitive character. The use of a constant phase element (CPE) as a substitution for Cdl in the equivalent circuit of the impedance not only minimizes the error but also provides more detailed information about the non-ideal dielectric properties of the coating [25,26]. The values of solution resistance (Rs), charge-transfer resistance (Rct), double-layer pseudo-capacitance formed in the substrate/electrolyte interface (Cdl) or total capacitance of the constant phase angle element (CPE) and n for coatings were calculated by simulating
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Q. Zhou et al. / Journal of Alloys and Compounds 563 (2013) 171–175 Table 3 Equivalent circuit parameters for the substrate and the alloy coatings.
Fig. 4. Nyquist plots of the substrate and the Cu–Ni–Fe coatings in 3.5 wt.% NaCl solution.
Samples
Rs (ohm cm2)
Y0-CPE (F cm2)
nc
Rct (ohm cm2)
Substrate Cu–Ni–Fe I Cu–Ni–Fe II Cu–Ni–Fe III
10.95 9.87 15.04 15.14
1.59 6.86 2.36 1.04
– 0.896 0.895 0.913
1.90 7.32 1.07 1.31
E3 (Cdl) E4 E4 E4
E3 E3 E4 E4
Therefore, the values of Rct are strongly dependent on the coating characteristics and indicate the corrosion resistance of the materials. The Rct values of the Cu–Ni–Fe coating were about 10times higher than that of the low-carbon steel, confirming that the coating had higher corrosion resistance than low-carbon steel. Also, the Cu–Ni–Fe III coating has the highest values of Rct, which implies the best anti-corrosion ability. These results are highly in correspondence with results which are extracted from Tafel polarization. 4. Discussions
Fig. 5. An equivalent circuit modelling the electrode (Rs: solution resistance; Cdl: double-layer pseudo-capacitance for substrate; CPE: constant phase element for coatings; Rct: charge-transfer resistance).
the experimental data with Zview software. The results are listed in Table 3. The Nyquist plots presented an unfinished semi-circle arc (shown in Fig. 4). The formation of these semi-circle arcs is attributed to the charge transfer process in the electrode/electrolyte interface, related to changes in the coating property [27].
Comparing with the ratio of Cu/Ni in the coating, it is much higher than that in particles (see Table 1), showing that Ni diffuses more easily than Cu. This phenomenon is probably caused by the high speed diffusion of between Fe substrate and the new Ni atoms, which generated by hydrogen reduction of oxides particles, on the contrary, the diffusivity of Cu is very slow due to the low solid solubility between Cu and Fe. This consequence matches the Cu–Fe binary phase diagram, as c-Fe (fcc) is formed only above 900 °C and that the maximum amount of Cu that can be dissolved in that phase is 13 wt.%, while Ni and Fe are totally miscible (fcc phase) between 912 and 1394 °C [28]. From the XRD results (Fig. 2), it reveals that the increase of Ni help to stabilize the c phases since Fe–Ni system is totally miscible and Ni can promotes the solubility of Fe in the fcc phase [16]. In the present study, The a-phases are bcc [Fe, Ni] phases, while the cphases cannot be totally excluded on the basis of the XRD data of the consolidated samples because the formation of c-[Cu]-rich
Fig. 6. SEM micrograph and X-ray elemental maps of Cu–Ni–Fe III.
Q. Zhou et al. / Journal of Alloys and Compounds 563 (2013) 171–175
fcc phases and c-[Fe, Ni]-rich fcc phases are hardly differentiable by XRD analysis due to the small difference (only 0.3° for 2h) for their peak position [29]. Although Cu–Ni–Fe III coating exhibits only c-phase, the spinodal decomposition of the Cu–Ni–Fe alloys cannot be excluded based on the XRD analyses. However, as illustrated in Fig. 6, the element Cu, Ni, Fe are all distributed homogeneously on the sample surface and no dendritic structures characteristic have been observed. This suggests all metallic atom exist in a single-phases without a spinodal decomposition, if any, spinodal decomposition would have occurred on a very minute scale. This means, homogenous cCu–Ni–Fe alloy coating would be formed on the surface of low-carbon steel, which is expected to be beneficial for the material corrosion resistance properties [15]. In addition, it can be seen from Table 3 that the n value for CPE of the Cu–Ni–Fe coating is equal or close to 1. The CPE is a special element which reflects the non-ideal dielectric properties of the coating. For n = 1, the CPE represents an ideal capacitor; for n = 0, the CPE is an ideal resistor; for n = 0.5, the CPE describes a Warburg impedance with diffusion character; for 0.5 < n < 1, the CPE represents a frequency dispersion of time constants because of local inhomogeneity, roughness or porosity of the electrode surface [26]. The n value close to 1 implies the presence of an approximate ideal double-layer pseudo-capacitance formed in the coating-electrolyte interface, that is, the coating surface is smooth and compact. The value of n come close to 1 with the increase of Ni content in the coatings, suggesting that a possible increase in surface homogeneity or compactness. This result is consistent with the above results drawn out of the SEM micrographs and X-ray diffraction. Polarization and EIS analysis (Figs. 3 and 4) showed that Cu–Ni–Fe alloy III coating has the maximum corrosion resistance. This means, homogenous cCu–Ni–Fe alloy coating resulted from high Ni content can effectively improve the corrosion resistance of lowcarbon steel. 5. Conclusions This study has demonstrated the feasibility of producing Cu– Ni–Fe coating by using nano-particles composed of the CuO–NiO. The increase of Ni content in the coating has a significant impact to produce a uniform Cu–Ni–Fe alloy coating, which is constituted of homogeneous c-phases without spinodal decomposition. Polarization and EIS test results indicated that Cu–Ni–Fe alloy coating provided corrosion protection and barrier properties on the low-carbon steel. The protective efficiency (Pi) and chargetransfer resistance (Rct) enhanced with the increase of Ni content in the coating in relation to its characteristic favorable for corrosion resistance. Consequently, preparation of homogeneous
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