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The corrosion behavior of nanocrystalline nickel based thin films
Materials Chemistry and Physics xxx (2016) 1e5
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
The corrosion behavior of nanocrystalline nickel based thin films Murat Danıs¸man Gedik University, Faculty of Engineering, Electric-Electronic Engineering Department, Yakacik-Kartal, Istanbul, Turkey
h i g h l i g h t s Thin film NieCr samples were deposited on glass substrate. Effect of Cr addition on corrosion behavior of Ni thin films were investigated. Potentiodynamic tests and electrochemical impedance spectroscopy methods were used. Cr content in Ni thin films plays and important role on corrosion. Up to a certain Cr content, Cr addition reduces corrosion rate.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 July 2015 Received in revised form 19 November 2015 Accepted 3 January 2016 Available online xxx
In this study, the effect of Cr addition on corrosion behavior of Ni thin films were investigated. Ni thin films and Ni films with three different Cr content were deposited on glass substrates by magnetron sputtering. After deposition process, thin films with different Cr content were thermally treated in a rapid thermal process system. Phase analysis and grain size calculations of the samples were carried out by X-ray diffraction analysis. In order to reveal corrosion properties, potentiodynamic tests were conducted on samples. Analysis revealed that, although Cr addition to pure-Ni thin films improved their corrosion resistance, occurrence of s-Cr3Ni2 phase at higher Cr contents increased corrosion rate. The corrosion properties of the samples were also investigated by electrochemical impedance spectroscopy and surface related parameters caused by corrosion reactions were calculated. The analysis revealed that at 55% wt. Cr, rapid ion exchange occurred and highest corrosion current, 23.4 nA cm2 was observed. © 2016 Elsevier B.V. All rights reserved.
Keywords: Sputtering Thin films Electrochemical techniques Corrosion
1. Introduction Nanocrystalline materials have been on the scope of scientist as they exhibit very different properties than conventional, coarse grained materials. Over the years, many researches have been conducted on these materials for tuning their production and characterization techniques. Previous studies have shown that mechanical, magnetic, thermal, electrical and corrosion properties of most materials are superior in nanocrystalline form [1e3]. To improve corrosion properties of materials, it is important to understand the exact nature of their behavior as nano grained solids. A material in nanocrystalline form has more grain boundaries than conventional materials and this higher density of grain boundaries leads to more favorable nucleation sites for building up a passivation film [4]. There are many techniques for producing nano
E-mail address: [email protected].
materials including both direct ones such as pulse jet electrodeposition [5,6], magnetron sputtering [4,7], thermal evaporation [8] and indirect ones like grain refinement [2,9]. The choice of production method is generally made according to the application area that the material and the condition of the substrate that would be used. Most of the structural materials used in industrial applications are metallic. Unless the material itself has the ability to form a passivation film, depositing a protective coating on structural material is crucial for corrosion protection. Using Cr films for this purpose, have some issues as Cr thin films are highly brittle and porous. Corrosive compounds are known to penetrate through these cracks and corrode the substrate [10]. On the other hand, despite having higher ratio of defects and grain boundaries, nanocrystalline Ni based alloys also have good corrosion properties. Recent studies have shown that nanocrystalline Ni thin films can successfully be used as a corrosion protective coating and the protective properties of Ni can be further improved by Cr addition
http://dx.doi.org/10.1016/j.matchemphys.2016.01.018 0254-0584/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Danıs¸man, The corrosion behavior of nanocrystalline nickel based thin films, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.01.018
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M. Danıs¸man / Materials Chemistry and Physics xxx (2016) 1e5
as Cr forms a stable Cr2O3 [10e13]. However, for evaluating these materials and understanding the true nature of the film, it is important to evaluate them as free standing films without the effect of the substrate. The aim of this study is to investigate the corrosion behavior of nanocrystalline Ni thin films and the effect of Cr addition on improving these properties. It is known that thin films are highly effected by their substrate and it is difficult to investigate their electrical properties on conductive ones. In this perspective, in order to eliminate substrate effects and prepare films with conditions similar to free standing films, all the samples were produced on glass substrates. Pure-Ni and Ni films with three different Cr content were prepared and their corrosion parameters were investigated using potentiodynamic tests and electrochemical impedance spectroscopy (EIS) techniques. 2. Experimental In order to investigate the influence of Cr content on corrosion behavior of nanocrystalline Ni thin films, samples of pure-Ni, pureCr films and Ni films with three different Cr content were prepared on glass substrates (composed of Si 18.60%, O 27.51%, Ca 2.41% by wt.) by using DC magnetron sputtering. Cr content in final films were controlled by changing the total Cr thickness as explained in detail elsewhere [7]. After deposition, the samples with different Ni and Cr ratios were thermally treated at 600 C for 180 s in a rapid thermal process system under protective N2 atmosphere, while other samples were used as deposited. Cr and Ni content of the samples were measured by using energy dispersive spectroscopy (EDS). As summarized in Table 1, resulting wt. % of Cr in Sample 1, 2 and 3 were 30, 25, 55, respectively. The grain size and crystallographic phases of the samples were analyzed with X-ray diffraction analysis (XRD) using low angle CuKa radiation with an incident angle of 1. The grain size of each sample was calculated using Scherrer's equation by using line broadening (full width at half maximum) for relevant peaks in XRD spectrums [14]. Corrosion properties of thin film samples were measured both using potentiodynamic tests and EIS analysis. Additionally, potentiodynamic analysis were also carried out on a bulk Ni sample in order to compare the results with nanocrystalline samples. The electrochemical measurements were performed in a three electrode cell where the sample was immersed in a 3.5 wt. % NaCl solution and connected to Gamry Electrochemical Potentiostat (Gamry Interface 1000/USA) as a working electrode. During the tests, two graphite electrodes and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively [15]. EIS is a unique and a non-destructive tool for understanding the true nature of electrochemical reactions. The steps of electrochemical reactions are observed by proposing electrical circuits and calculating numerical results which are then related with the condition of the surface after the reaction [16]. EIS measurements were conducted by applying an alternating sine signal to the
samples with an amplitude of 5 mV in same cell setup. The frequency range was chosen between 50 mHz and 100 kHz with 10 points per decade. The Nyquist diagram of each sample was plotted according to these settings. The charge transfer resistance (Rct) and other corrosion related parameters (constant phase element (Q-n), double layer capacitance (Cdl), double layer passivation film resistance (Rdl)) were derived by fitting equivalent electrical circuits that represented film/metal and film/solution interface in Nyquist diagrams using nonlinear least squares fitting technique [17e19]. 3. Results and discussion Cr content and grain size of each sample were also listed in Table 1. XRD analysis of the samples were given in Fig. 1. As revealed by the XRD analysis and calculated by the Scherrer's formula all samples had nanocrystalline nature with similar grain sizes. Highest grain size was recorded as 12.5 nm on Sample 3. The corrosion potential (Ecorr) and corrosion current (Icorr) were calculated by using Tafel extrapolation method on anodic polarization curves that were derived from potentiodynamic measurement. The corrosion rate of the samples was calculated according to Icorr values [15]. The change in current vs. electrochemical potential was plotted at the vicinity of open circuit potential (EOCP) for calculating the corrosion resistance of the samples, and the slope of the linear regression line of the plot was calculated as the corrosion resistance (Rcorr) [17]. Furthermore, the polarization resistance (Rpol) of the samples was calculated according to SterneGeary equation using anodic and cathodic Tafel slopes acquired by potentiodynamic tests
Fig. 1. XRD patterns of thin film samples.
Table 1 Grain size, Cr content, film thickness and test data calculated by corrosion tests. Sample
Wt. % Cr
Grain size (nm)
Icorr (nA.cm2)
Ecorr (mV)
Corrosion rate (mpy) x 103
Rpol (kU.cm2)
Rcorr (kU.cm2)
Film thickness (nm)
Sample 1 Sample 2 Sample 3 Pure-Ni film Bulk Ni Pure-Cr film
30 25 55 0 0 100
10.5 12 12.5 8
18.4 8.12 23.4 39 375 144
125 28.5 111 239 263 148
2.86 1.26 3.63 18 169.8 22.3
1680 5761 2325 1735 33 721
4707 7230 3734 2448 38 1302
322 184 336 500 e 500
8.5
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M. Danıs¸man / Materials Chemistry and Physics xxx (2016) 1e5
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s-Cr3Ni2 phase and de-alloying of Cr at the grain boundaries [26].
[18,20]. Potentiodynamic polarization curves (Fig. 2) of Sample 1, 2, 3 revealed that Ecorr values shifted to more noble values compared to Ecorr of pure-Ni thin films. Even though these results indicated that Cr addition to pure-Ni thin films improved their corrosion resistance, samples with different Cr content had different corrosion properties according to their Ecorr values. In general, corrosion properties of materials are both influenced by the alloying elements and the grain size of the material. According to classical corrosion theory, materials with nanocrystalline nature have poorer corrosion properties than bulk materials due to their higher number of electrochemical corrosion cells [21,22]. However, it was reported in several other studies that samples with grain sizes in nanometer range had acted different than their bulk counterparts [1,22e24]. The difference is mainly related to the grain size and volume fraction ratio. In a nanomaterial, 10e30% of total volume fraction is grain boundary. As it can be followed in Table 1, the grain sizes of all samples exhibited similar values and the change in corrosion behavior was purely affected by the amount of Cr in thin films. Potentiodynamic polarization curves of bulk Ni, nanocrystalline pure-Ni and pure-Cr thin films on glass substrate were also presented for comparison. According to Ecorr values, the bulk Ni is more active than nanocrystalline pure-Ni as expected. Hence, the corrosion rate on bulk Ni is about ten times greater than the corrosion rate of pure-Ni. These results were consistent with previous studies [12,25]. For samples with different Cr content, the highest Ecorr, 28.50 mV, was observed on sample with 25 wt. % Cr. As seen on Fig. 2, with increasing Cr content, Ecorr values of samples shifted to noble values which indicated an improvement in corrosion resistance. However, Ecorr values were slightly lower on samples having 30 and 55 wt. % Cr content. This critical dependency on Cr content was also discussed in a study carried out by Aljohani et al. [26]. In their study, it was reported that 45 and 50% Cr content had significant effect on corrosion behavior of Ni based alloys. Similar to our findings, the authors also reported a small decrease in Rpol value and explained this behavior by the occurrence of metastable
This phenomena might also be related with both Cr content and the nanocrystalline nature of the film. In our study, all thin film samples had nanocrystalline nature with similar grain sizes, which indicated higher grain boundary ratios. Beside grain sizes, as can be followed in Fig. 1, the samples with 30 and 55% Cr had s-Cr3Ni2 phase. The formation of Cr rich s-Cr3Ni2 phase and de-alloying of Cr, could have a negative impact on formation of a passivation layer on Cr rich sample. These regions were believed to act as suitable sites for corrosion. In a nanocrystalline material with high grain boundary ratio, the number of these regions could reach very high numbers. Evidently the occurrence of s-Cr3Ni2 phase, especially in a nanocrystalline material, played in important role on decreasing corrosion resistance. This behavior was also revealed by EIS results especially on Sample 3 (55 wt. % Cr) which had the highest Cr content in our experimental set. As it could be followed on Fig. 4, the charge transfer between materials and solution was rapid as a result of lower resistance on the electrical equivalents circuits. Moreover, Sample 1 and 3 which both had s-Cr3Ni2 phase had also lower Nyquist profile than Sample 2. To reveal the exact corrosion resistance of the samples and to investigate the reason behind Cr dependency, Rcorr of the samples were calculated and presented in Fig. 3. In Fig. 3, the slope of regression lines, the Rcorr, increased with Cr content. Rcorr value of all samples were clearly higher than the Rpol values of both nanocrystalline pure-Ni and pure-Cr samples. The highest Rpol was calculated as 5761 kU cm2 for the sample with 25 wt. % Cr content. This finding was also consistent with the lowest Icorr, 8.12 nA cm2, which was also observed for this sample. In addition to potentiodynamic analysis, the corrosion properties of the samples were also investigated by Nyquist plots acquired by EIS analysis. Rct, Q, n, Cdl, Rdl, Rsol values were calculated by fitting equivalent electrical circuits (Fig. 4aeb) to these data [17,19,27]. In general, the electrical circuits used for fitting electrochemical data consists of circuit elements such as resistors and capacitors. Due to unique nature of electrochemistry, instead of using an ideal
Fig. 2. Potentiodynamic curves of bulk Ni, pure-Ni, pure-Cr thin films and samples with different Cr ratios.
Fig. 3. Rcorr values of pure Ni, Cr thin films and samples with different Cr ratios.
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Fig. 4. Equivalent electrical circuit models used for fitting Nyquist diagrams.
capacitor for representing the double layer, it is sometimes more convenient to use a special capacitor known as constant phase element (CPE). By definition, CPE is different from an ideal capacitor with some amount of leakage current [12]. Mathematical relation for CPE is given as:
Z ¼ Q ðjuÞn
1
(1)
In the equation Q, n and u represented the numerical value of admittance of CPE, a factor related with ideality of the capacitive behavior (n < 1; n ¼ 1 for ideal capacitor) and the angular frequency of the applied voltage, respectively. Generally in electrochemical tests, the value for the solution resistance, Rsol, is effected by the position of the sample and the Luggin-Haber probe [19]. Because of the physical conditions of the samples some variations among Rsol values were observed. The value of CPE is related with the depression of Nyquist semicircle and it is associated with the condition and the surface properties of the film [17,19,21]. In presence of a fully capacitive and protective passive film, the phase angle in Bode diagrams is close to 90 [13]. On the other hand, CPE is generally used as a substitute for an ideal capacitor for representing samples which have rough surfaces and phase values below 90 [17,28,29]. For Samples 1 and 2, values for Rct and CPE were calculated by fitting an equivalent electrical circuit (Fig. 4a). For Sample 1, the value of Q was about three times higher than of Sample 2. This result was also confirmed by Fig. 5. The samples with better corrosion properties had higher radius of Nyquist semi-circle and lower CPE value which indicated that the passivation film formed on the surface was more protective [1,21]. For Samples 1 and 2, occurrence of incomplete Nyquist semi-circle revealed the active charge transfer between metal and NaCl solution [21]. The calculated Rct value of Sample 2 was much higher than Sample 1. According to potentiodynamic tests, Sample 2 had more positive Ecorr value than sample 1 which indicated that Sample 2 had better corrosion resistance. This result also confirmed about three times higher Rpol values between these two samples. According to ASTM G106-89, the Rct was equal to the Rpol when the calculations were carried out using Tafel slopes. In our case, although the Rpol and Rct values were not equal, their values were comparable. In order to show their relation, Rct/Rpol ratio was calculated as about 1.3 for Sample 1 and 2. The difference between Rct and Rpol values and Rct/ Rpol ratio was also discussed by Wang et al. [1]. According to Fig. 5, Sample 3 featured a complete and lower profile Nyquist semicircle, an indication of different characteristics
Fig. 5. Nyquist plots of Sample 1, 2, 3.
than Sample 1 and 2. In order to reveal the corrosion parameters for this sample, a different, two ladder ReC electrical equivalent circuit was proposed (Fig. 4b). In the circuit model, while Rdl-Cdl group represented the passivation filmesolution interface, CPE-Rct represented the properties of the passivation film itself. As it could be followed on Table 2, Rct value which showed the transfer of Niþ2 ions from sample to solution was easier for this sample as the value is relatively smaller than of Sample 1 and 2. Evidently, it was believed that due to higher Cr content and occurrence of metastable, Cr rich s-Cr3Ni2 phase, the passivation film on the surface of the sample was less protective. When Sample 1 and 2 were compared according to their corrosion resistance, Sample 2 was more corrosion resistant than Sample 1 which also had s-Cr3Ni2 phase. This finding emphasized the correlation between corrosion resistance and this significant Cr phase. In addition to these, as it could be followed in the higher frequency region of Nyquist plot and by Cdl-Rdl group in the equivalent circuit, the passivation film on Sample 3 could be composed of two layers. Mareci et al. reported that the capacitive behavior of the passivation film, which was represented by the parameters Q and n, could be used as an indication of passive film thickness [13]. In our case, both Q and n values was lower for this sample and this result could be evaluated as a thinner and unstable passivation film. As a matter of fact for Sample 3, Ecorr and Icorr also revealed a drop in corrosion resistance when compared to Sample 1 and 2.
4. Conclusions In current study, the corrosion behavior of nanocrystalline Ni and influence of Cr content on corrosion properties of these films were investigated. Similar to earlier studies, the nanocrystalline Ni on glass substrates showed better corrosion properties than bulk Ni. As all thin film samples in our study had similar grain sizes, the exact relation between corrosion and wt. % of Cr in Ni films could be investigated. In order to understand the nature of the passivation film on samples, EIS analysis were employed. Analysis revealed that both calculated and measured corrosion parameters by potentiodynamic tests and EIS were in good match. For samples with different Cr
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Table 2 Calculated results for EIS analysis by fitting equivalent electrical circuits given as Fig. 4aeb. Sample Sample 1 Sample 2 Sample 3
Cdl (F cm2)
Rsol (U cm2)
Rct (kU cm2)
13.5 1012
200 918 1480
2485 7998 880
content, Rcorr and Rpol values were calculated by Tafel plots and Rct value calculated by EIS analysis gave comparable results. According to Ecorr and Icorr values calculated by potentiodynamic test, up to some degree of increasing Cr content in Ni thin films improves corrosion resistance. The highest corrosion protection was assessed at wt. % 25 Cr which represented lowest Icorr and highest Rpol values. However, when Cr content reached 30% by wt. corrosion rate increased and at 55% wt. reached its maximum value in our set of samples. EIS analysis revealed that for 55% wt. Cr, the passivation film was thinner, less stable and protective. These findings were also consisted with previous reports for this Cr content. Acknowledgments Author would gratefully thanks to Prof. Dr. Nurhan Cansever for the inspiration, her support and discussions and to Mustafa Kocabas¸ for his experimental assistance. References [1] L. Wang, J. Zhang, Y. Gao, Q. Xue, L. Hu, T. Xu, Grain size effect in corrosion behavior of electrodeposited nanocrystalline Ni coatings in alkaline solution, Scr. Mater 55 (2006) 657e660, http://dx.doi.org/10.1016/ j.scriptamat.2006.04.009. [2] R. Mishra, B. Basu, R. Balasubramaniam, Effect of grain size on the tribological behavior of nanocrystalline nickel, Mater. Sci. Eng. A 373 (2004) 370e373, http://dx.doi.org/10.1016/j.msea.2003.09.107. [3] A. Chiolerio, I. Ferrante, A. Ricci, S. Marasso, P. Tiberto, G. Canavese, et al., Toward mechano-spintronics: nanostructured magnetic multilayers for the realization of microcantilever sensors featuring wireless actuation for liquid environments, J. Intell. Mater. Syst. Struct. 24 (2012) 2189e2196, http:// dx.doi.org/10.1177/1045389X12445031. [4] S.S. Viswanathan, R. Cook, Corrosion Protection and Control Using Nanomaterials, 2012. [5] P. Yong, Z. Yi-chun, Z. Zhao-feng, Fabrication, Lattice Strain, Corrosion Resistance and Mechanical Strength of Nanocrystalline Nickel Films, 2007, pp. 2e6. [6] S.H. Kim, K.T. Aust, U. Erb, F. Gonzalez, G. Palumbo, A comparison of the corrosion behaviour of polycrystalline and nanocrystalline cobalt, Scr. Mater 48 (2003) 1379e1384, http://dx.doi.org/10.1016/S1359-6462(02)00651-6. [7] M. Danisman, N. Cansever, Effect of Cr content on mechanical and electrical properties of NieCr thin films, J. Alloy. Compd. 493 (2010) 649e653, http:// dx.doi.org/10.1016/j.jallcom.2009.12.180. [8] A. Chiolerio, P. Allia, A. Chiodoni, F. Pirri, F. Celegato, M. Coïsson, Thermally evaporated CueCo top spin valve with random exchange bias, J. Appl. Phys. 101 (2007), http://dx.doi.org/10.1063/1.2749289. [9] R. Mishra, R. Balasubramaniam, Effect of nanocrystalline grain size on the electrochemical and corrosion behavior of nickel, Corros. Sci. 46 (2004) 3019e3029, http://dx.doi.org/10.1016/j.corsci.2004.04.007. [10] M.S. Marwah, V. Srinivas, A.K. Pandey, S.R. Kumar, K. Biswas, J. Maity, Morphological changes during annealing of electrodeposited Ni-Cr coating on steel and their effect on corrosion in 3% of NaCl solution, J. Iron Steel Res. Int. 18 (2011) 72e78, http://dx.doi.org/10.1016/S1006-706X(11)60040-X.
Rdl (kU cm2)
Q (mF cm2)
n
51
6.1 2.1 0.114
0.7 0.8 0.3
[11] G. Lu, G. Zangari, Corrosion resistance of ternary Ni P based alloys in sulfuric acid solutions, Electrochim. Acta 47 (2002) 2969e2979, http://dx.doi.org/ 10.1016/S0013-4686(02)00198-6. [12] D. Sachdeva, N. Gupta, R. Balasubramaniam, Electrochemical characterisation of nanocrystalline nickel, Def. Sci. J. 58 (2008) 525e530. [13] D. Mareci, A. Cailean, G. Ciurescu, D. Sutiman, Electrochemical determination of the corrosion resistance of NiCr dental casting alloys, Open Corros. J. 3 (2010) 45e53, http://dx.doi.org/10.2174/1876503301003010045. [14] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley Publishing Company, Inc., Massachusetts, 1956. [15] G5-87 standard reference test method for making potentiostatic and potentiodynamic anodic polarization measurements, Annu. B. ASTM Stand (1994) 79e85, http://dx.doi.org/10.1520/G0005-94R11E01.2. [16] D. a. Harrington, P. Van Den Driessche, Mechanism and equivalent circuits in electrochemical impedance spectroscopy, Electrochim. Acta 56 (2011) 8005e8013, http://dx.doi.org/10.1016/j.electacta.2011.01.067. [17] A. Bai, P.Y. Chuang, C.C. Hu, The corrosion behavior of ni-p deposits with high phosphorous contents in brine media, Mater. Chem. Phys. 82 (2003) 93e100, http://dx.doi.org/10.1016/S0254-0584(03)00193-7. [18] A.C. Ciubotariu, L. Benea, M. Lakatos-Varsanyi, V. Dragan, Electrochemical impedance spectroscopy and corrosion behaviour of Al2O3-Ni nano composite coatings, Electrochim. Acta 53 (2008) 4557e4563, http://dx.doi.org/ 10.1016/j.electacta.2008.01.020. [19] P. Anodic, P. Measurements, R. Measurements, Standard practice for verification of algorithm and equipment for electrochemical, Astm 03 (1999) 1e11, http://dx.doi.org/10.1520/G0106-89R10.2. [20] Z.F. Zhou, Y. Pan, W.X. Lei, Ni nanocomposite films formed by Ni nanowires embedded in Ni matrix using electrodeposition (English Ed), Trans. Nonferrous Met. Soc. China 20 (2010) 637e642, http://dx.doi.org/10.1016/S10036326(09)60191-6. [21] L.Y. Qin, J.S. Lian, Q. Jiang, Effect of grain size on corrosion behavior of electrodeposited bulk nanocrystalline Ni (English Ed), Trans. Nonferrous Met. Soc. China 20 (2010) 82e89, http://dx.doi.org/10.1016/S1003-6326(09)60101-1. n, F.C. Walsh, The corrosion behaviour of nano[22] P. Herrasti, C. Ponce de Leo grained metals and alloys, Rev. Metal. 48 (2012) 377e394, http://dx.doi.org/ 10.3989/revmetalm.1243. [23] L. Qin, J. Xu, J. Lian, Z. Jiang, Q. Jiang, A novel electrodeposited nanostructured Ni coating with grain size gradient distribution, Surf. Coatings Technol. 203 (2008) 142e147, http://dx.doi.org/10.1016/j.surfcoat.2008.08.044. [24] M. Aliofkhazraei, Nanocoatings Size Effect in Nanostructured Films, 2011, http://dx.doi.org/10.1007/978-3-642-17966-2. [25] S.V. Carl C. Koch, Ilya A. Ovid’ko, Sudipta Seal, Structural Nanocrystalline Materials Fundamentals and Applications, Cambridge University Press, 2007. [26] T. a Aljohani, B.E. Hayden, A. Anastasopoulos, The high throughput electrochemical screening of the corrosion resistance of NieCr thin film alloys, Electrochim. Acta 76 (2012) 389e393, http://dx.doi.org/10.1016/ j.electacta.2012.05.045. [27] B. Breyer, Electrochemical cells as electrical circuit elements, J. Electroanal. Chem. 12 (1966) 411e415. [28] S. Zelinka, L. Ortiz-Candelaria, Electrochemical impedance spectroscopy (EIS) as a tool for measuring corrosion of polymer-coated fasteners used in treated wood, For. Prod. 59 (2009) 77e82. http://ddr.nal.usda.gov/handle/10113/ 28829. [29] A. Chiolerio, S. Bocchini, F. Scaravaggi, S. Porro, D. Perrone, D. Beretta, et al., Synthesis of polyaniline-based inks for inkjet printed devices: electrical characterization highlighting the effect of primary and secondary doping, Semicond. Sci. Technol. 30 (2015) 104001, http://dx.doi.org/10.1088/02681242/30/10/104001.
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
The corrosion behavior of nanocrystalline nickel based thin films Murat Danıs¸man Gedik University, Faculty of Engineering, Electric-Electronic Engineering Department, Yakacik-Kartal, Istanbul, Turkey
h i g h l i g h t s Thin film NieCr samples were deposited on glass substrate. Effect of Cr addition on corrosion behavior of Ni thin films were investigated. Potentiodynamic tests and electrochemical impedance spectroscopy methods were used. Cr content in Ni thin films plays and important role on corrosion. Up to a certain Cr content, Cr addition reduces corrosion rate.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 July 2015 Received in revised form 19 November 2015 Accepted 3 January 2016 Available online xxx
In this study, the effect of Cr addition on corrosion behavior of Ni thin films were investigated. Ni thin films and Ni films with three different Cr content were deposited on glass substrates by magnetron sputtering. After deposition process, thin films with different Cr content were thermally treated in a rapid thermal process system. Phase analysis and grain size calculations of the samples were carried out by X-ray diffraction analysis. In order to reveal corrosion properties, potentiodynamic tests were conducted on samples. Analysis revealed that, although Cr addition to pure-Ni thin films improved their corrosion resistance, occurrence of s-Cr3Ni2 phase at higher Cr contents increased corrosion rate. The corrosion properties of the samples were also investigated by electrochemical impedance spectroscopy and surface related parameters caused by corrosion reactions were calculated. The analysis revealed that at 55% wt. Cr, rapid ion exchange occurred and highest corrosion current, 23.4 nA cm2 was observed. © 2016 Elsevier B.V. All rights reserved.
Keywords: Sputtering Thin films Electrochemical techniques Corrosion
1. Introduction Nanocrystalline materials have been on the scope of scientist as they exhibit very different properties than conventional, coarse grained materials. Over the years, many researches have been conducted on these materials for tuning their production and characterization techniques. Previous studies have shown that mechanical, magnetic, thermal, electrical and corrosion properties of most materials are superior in nanocrystalline form [1e3]. To improve corrosion properties of materials, it is important to understand the exact nature of their behavior as nano grained solids. A material in nanocrystalline form has more grain boundaries than conventional materials and this higher density of grain boundaries leads to more favorable nucleation sites for building up a passivation film [4]. There are many techniques for producing nano
E-mail address: [email protected].
materials including both direct ones such as pulse jet electrodeposition [5,6], magnetron sputtering [4,7], thermal evaporation [8] and indirect ones like grain refinement [2,9]. The choice of production method is generally made according to the application area that the material and the condition of the substrate that would be used. Most of the structural materials used in industrial applications are metallic. Unless the material itself has the ability to form a passivation film, depositing a protective coating on structural material is crucial for corrosion protection. Using Cr films for this purpose, have some issues as Cr thin films are highly brittle and porous. Corrosive compounds are known to penetrate through these cracks and corrode the substrate [10]. On the other hand, despite having higher ratio of defects and grain boundaries, nanocrystalline Ni based alloys also have good corrosion properties. Recent studies have shown that nanocrystalline Ni thin films can successfully be used as a corrosion protective coating and the protective properties of Ni can be further improved by Cr addition
http://dx.doi.org/10.1016/j.matchemphys.2016.01.018 0254-0584/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Danıs¸man, The corrosion behavior of nanocrystalline nickel based thin films, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.01.018
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M. Danıs¸man / Materials Chemistry and Physics xxx (2016) 1e5
as Cr forms a stable Cr2O3 [10e13]. However, for evaluating these materials and understanding the true nature of the film, it is important to evaluate them as free standing films without the effect of the substrate. The aim of this study is to investigate the corrosion behavior of nanocrystalline Ni thin films and the effect of Cr addition on improving these properties. It is known that thin films are highly effected by their substrate and it is difficult to investigate their electrical properties on conductive ones. In this perspective, in order to eliminate substrate effects and prepare films with conditions similar to free standing films, all the samples were produced on glass substrates. Pure-Ni and Ni films with three different Cr content were prepared and their corrosion parameters were investigated using potentiodynamic tests and electrochemical impedance spectroscopy (EIS) techniques. 2. Experimental In order to investigate the influence of Cr content on corrosion behavior of nanocrystalline Ni thin films, samples of pure-Ni, pureCr films and Ni films with three different Cr content were prepared on glass substrates (composed of Si 18.60%, O 27.51%, Ca 2.41% by wt.) by using DC magnetron sputtering. Cr content in final films were controlled by changing the total Cr thickness as explained in detail elsewhere [7]. After deposition, the samples with different Ni and Cr ratios were thermally treated at 600 C for 180 s in a rapid thermal process system under protective N2 atmosphere, while other samples were used as deposited. Cr and Ni content of the samples were measured by using energy dispersive spectroscopy (EDS). As summarized in Table 1, resulting wt. % of Cr in Sample 1, 2 and 3 were 30, 25, 55, respectively. The grain size and crystallographic phases of the samples were analyzed with X-ray diffraction analysis (XRD) using low angle CuKa radiation with an incident angle of 1. The grain size of each sample was calculated using Scherrer's equation by using line broadening (full width at half maximum) for relevant peaks in XRD spectrums [14]. Corrosion properties of thin film samples were measured both using potentiodynamic tests and EIS analysis. Additionally, potentiodynamic analysis were also carried out on a bulk Ni sample in order to compare the results with nanocrystalline samples. The electrochemical measurements were performed in a three electrode cell where the sample was immersed in a 3.5 wt. % NaCl solution and connected to Gamry Electrochemical Potentiostat (Gamry Interface 1000/USA) as a working electrode. During the tests, two graphite electrodes and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively [15]. EIS is a unique and a non-destructive tool for understanding the true nature of electrochemical reactions. The steps of electrochemical reactions are observed by proposing electrical circuits and calculating numerical results which are then related with the condition of the surface after the reaction [16]. EIS measurements were conducted by applying an alternating sine signal to the
samples with an amplitude of 5 mV in same cell setup. The frequency range was chosen between 50 mHz and 100 kHz with 10 points per decade. The Nyquist diagram of each sample was plotted according to these settings. The charge transfer resistance (Rct) and other corrosion related parameters (constant phase element (Q-n), double layer capacitance (Cdl), double layer passivation film resistance (Rdl)) were derived by fitting equivalent electrical circuits that represented film/metal and film/solution interface in Nyquist diagrams using nonlinear least squares fitting technique [17e19]. 3. Results and discussion Cr content and grain size of each sample were also listed in Table 1. XRD analysis of the samples were given in Fig. 1. As revealed by the XRD analysis and calculated by the Scherrer's formula all samples had nanocrystalline nature with similar grain sizes. Highest grain size was recorded as 12.5 nm on Sample 3. The corrosion potential (Ecorr) and corrosion current (Icorr) were calculated by using Tafel extrapolation method on anodic polarization curves that were derived from potentiodynamic measurement. The corrosion rate of the samples was calculated according to Icorr values [15]. The change in current vs. electrochemical potential was plotted at the vicinity of open circuit potential (EOCP) for calculating the corrosion resistance of the samples, and the slope of the linear regression line of the plot was calculated as the corrosion resistance (Rcorr) [17]. Furthermore, the polarization resistance (Rpol) of the samples was calculated according to SterneGeary equation using anodic and cathodic Tafel slopes acquired by potentiodynamic tests
Fig. 1. XRD patterns of thin film samples.
Table 1 Grain size, Cr content, film thickness and test data calculated by corrosion tests. Sample
Wt. % Cr
Grain size (nm)
Icorr (nA.cm2)
Ecorr (mV)
Corrosion rate (mpy) x 103
Rpol (kU.cm2)
Rcorr (kU.cm2)
Film thickness (nm)
Sample 1 Sample 2 Sample 3 Pure-Ni film Bulk Ni Pure-Cr film
30 25 55 0 0 100
10.5 12 12.5 8
18.4 8.12 23.4 39 375 144
125 28.5 111 239 263 148
2.86 1.26 3.63 18 169.8 22.3
1680 5761 2325 1735 33 721
4707 7230 3734 2448 38 1302
322 184 336 500 e 500
8.5
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s-Cr3Ni2 phase and de-alloying of Cr at the grain boundaries [26].
[18,20]. Potentiodynamic polarization curves (Fig. 2) of Sample 1, 2, 3 revealed that Ecorr values shifted to more noble values compared to Ecorr of pure-Ni thin films. Even though these results indicated that Cr addition to pure-Ni thin films improved their corrosion resistance, samples with different Cr content had different corrosion properties according to their Ecorr values. In general, corrosion properties of materials are both influenced by the alloying elements and the grain size of the material. According to classical corrosion theory, materials with nanocrystalline nature have poorer corrosion properties than bulk materials due to their higher number of electrochemical corrosion cells [21,22]. However, it was reported in several other studies that samples with grain sizes in nanometer range had acted different than their bulk counterparts [1,22e24]. The difference is mainly related to the grain size and volume fraction ratio. In a nanomaterial, 10e30% of total volume fraction is grain boundary. As it can be followed in Table 1, the grain sizes of all samples exhibited similar values and the change in corrosion behavior was purely affected by the amount of Cr in thin films. Potentiodynamic polarization curves of bulk Ni, nanocrystalline pure-Ni and pure-Cr thin films on glass substrate were also presented for comparison. According to Ecorr values, the bulk Ni is more active than nanocrystalline pure-Ni as expected. Hence, the corrosion rate on bulk Ni is about ten times greater than the corrosion rate of pure-Ni. These results were consistent with previous studies [12,25]. For samples with different Cr content, the highest Ecorr, 28.50 mV, was observed on sample with 25 wt. % Cr. As seen on Fig. 2, with increasing Cr content, Ecorr values of samples shifted to noble values which indicated an improvement in corrosion resistance. However, Ecorr values were slightly lower on samples having 30 and 55 wt. % Cr content. This critical dependency on Cr content was also discussed in a study carried out by Aljohani et al. [26]. In their study, it was reported that 45 and 50% Cr content had significant effect on corrosion behavior of Ni based alloys. Similar to our findings, the authors also reported a small decrease in Rpol value and explained this behavior by the occurrence of metastable
This phenomena might also be related with both Cr content and the nanocrystalline nature of the film. In our study, all thin film samples had nanocrystalline nature with similar grain sizes, which indicated higher grain boundary ratios. Beside grain sizes, as can be followed in Fig. 1, the samples with 30 and 55% Cr had s-Cr3Ni2 phase. The formation of Cr rich s-Cr3Ni2 phase and de-alloying of Cr, could have a negative impact on formation of a passivation layer on Cr rich sample. These regions were believed to act as suitable sites for corrosion. In a nanocrystalline material with high grain boundary ratio, the number of these regions could reach very high numbers. Evidently the occurrence of s-Cr3Ni2 phase, especially in a nanocrystalline material, played in important role on decreasing corrosion resistance. This behavior was also revealed by EIS results especially on Sample 3 (55 wt. % Cr) which had the highest Cr content in our experimental set. As it could be followed on Fig. 4, the charge transfer between materials and solution was rapid as a result of lower resistance on the electrical equivalents circuits. Moreover, Sample 1 and 3 which both had s-Cr3Ni2 phase had also lower Nyquist profile than Sample 2. To reveal the exact corrosion resistance of the samples and to investigate the reason behind Cr dependency, Rcorr of the samples were calculated and presented in Fig. 3. In Fig. 3, the slope of regression lines, the Rcorr, increased with Cr content. Rcorr value of all samples were clearly higher than the Rpol values of both nanocrystalline pure-Ni and pure-Cr samples. The highest Rpol was calculated as 5761 kU cm2 for the sample with 25 wt. % Cr content. This finding was also consistent with the lowest Icorr, 8.12 nA cm2, which was also observed for this sample. In addition to potentiodynamic analysis, the corrosion properties of the samples were also investigated by Nyquist plots acquired by EIS analysis. Rct, Q, n, Cdl, Rdl, Rsol values were calculated by fitting equivalent electrical circuits (Fig. 4aeb) to these data [17,19,27]. In general, the electrical circuits used for fitting electrochemical data consists of circuit elements such as resistors and capacitors. Due to unique nature of electrochemistry, instead of using an ideal
Fig. 2. Potentiodynamic curves of bulk Ni, pure-Ni, pure-Cr thin films and samples with different Cr ratios.
Fig. 3. Rcorr values of pure Ni, Cr thin films and samples with different Cr ratios.
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M. Danıs¸man / Materials Chemistry and Physics xxx (2016) 1e5
Fig. 4. Equivalent electrical circuit models used for fitting Nyquist diagrams.
capacitor for representing the double layer, it is sometimes more convenient to use a special capacitor known as constant phase element (CPE). By definition, CPE is different from an ideal capacitor with some amount of leakage current [12]. Mathematical relation for CPE is given as:
Z ¼ Q ðjuÞn
1
(1)
In the equation Q, n and u represented the numerical value of admittance of CPE, a factor related with ideality of the capacitive behavior (n < 1; n ¼ 1 for ideal capacitor) and the angular frequency of the applied voltage, respectively. Generally in electrochemical tests, the value for the solution resistance, Rsol, is effected by the position of the sample and the Luggin-Haber probe [19]. Because of the physical conditions of the samples some variations among Rsol values were observed. The value of CPE is related with the depression of Nyquist semicircle and it is associated with the condition and the surface properties of the film [17,19,21]. In presence of a fully capacitive and protective passive film, the phase angle in Bode diagrams is close to 90 [13]. On the other hand, CPE is generally used as a substitute for an ideal capacitor for representing samples which have rough surfaces and phase values below 90 [17,28,29]. For Samples 1 and 2, values for Rct and CPE were calculated by fitting an equivalent electrical circuit (Fig. 4a). For Sample 1, the value of Q was about three times higher than of Sample 2. This result was also confirmed by Fig. 5. The samples with better corrosion properties had higher radius of Nyquist semi-circle and lower CPE value which indicated that the passivation film formed on the surface was more protective [1,21]. For Samples 1 and 2, occurrence of incomplete Nyquist semi-circle revealed the active charge transfer between metal and NaCl solution [21]. The calculated Rct value of Sample 2 was much higher than Sample 1. According to potentiodynamic tests, Sample 2 had more positive Ecorr value than sample 1 which indicated that Sample 2 had better corrosion resistance. This result also confirmed about three times higher Rpol values between these two samples. According to ASTM G106-89, the Rct was equal to the Rpol when the calculations were carried out using Tafel slopes. In our case, although the Rpol and Rct values were not equal, their values were comparable. In order to show their relation, Rct/Rpol ratio was calculated as about 1.3 for Sample 1 and 2. The difference between Rct and Rpol values and Rct/ Rpol ratio was also discussed by Wang et al. [1]. According to Fig. 5, Sample 3 featured a complete and lower profile Nyquist semicircle, an indication of different characteristics
Fig. 5. Nyquist plots of Sample 1, 2, 3.
than Sample 1 and 2. In order to reveal the corrosion parameters for this sample, a different, two ladder ReC electrical equivalent circuit was proposed (Fig. 4b). In the circuit model, while Rdl-Cdl group represented the passivation filmesolution interface, CPE-Rct represented the properties of the passivation film itself. As it could be followed on Table 2, Rct value which showed the transfer of Niþ2 ions from sample to solution was easier for this sample as the value is relatively smaller than of Sample 1 and 2. Evidently, it was believed that due to higher Cr content and occurrence of metastable, Cr rich s-Cr3Ni2 phase, the passivation film on the surface of the sample was less protective. When Sample 1 and 2 were compared according to their corrosion resistance, Sample 2 was more corrosion resistant than Sample 1 which also had s-Cr3Ni2 phase. This finding emphasized the correlation between corrosion resistance and this significant Cr phase. In addition to these, as it could be followed in the higher frequency region of Nyquist plot and by Cdl-Rdl group in the equivalent circuit, the passivation film on Sample 3 could be composed of two layers. Mareci et al. reported that the capacitive behavior of the passivation film, which was represented by the parameters Q and n, could be used as an indication of passive film thickness [13]. In our case, both Q and n values was lower for this sample and this result could be evaluated as a thinner and unstable passivation film. As a matter of fact for Sample 3, Ecorr and Icorr also revealed a drop in corrosion resistance when compared to Sample 1 and 2.
4. Conclusions In current study, the corrosion behavior of nanocrystalline Ni and influence of Cr content on corrosion properties of these films were investigated. Similar to earlier studies, the nanocrystalline Ni on glass substrates showed better corrosion properties than bulk Ni. As all thin film samples in our study had similar grain sizes, the exact relation between corrosion and wt. % of Cr in Ni films could be investigated. In order to understand the nature of the passivation film on samples, EIS analysis were employed. Analysis revealed that both calculated and measured corrosion parameters by potentiodynamic tests and EIS were in good match. For samples with different Cr
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Table 2 Calculated results for EIS analysis by fitting equivalent electrical circuits given as Fig. 4aeb. Sample Sample 1 Sample 2 Sample 3
Cdl (F cm2)
Rsol (U cm2)
Rct (kU cm2)
13.5 1012
200 918 1480
2485 7998 880
content, Rcorr and Rpol values were calculated by Tafel plots and Rct value calculated by EIS analysis gave comparable results. According to Ecorr and Icorr values calculated by potentiodynamic test, up to some degree of increasing Cr content in Ni thin films improves corrosion resistance. The highest corrosion protection was assessed at wt. % 25 Cr which represented lowest Icorr and highest Rpol values. However, when Cr content reached 30% by wt. corrosion rate increased and at 55% wt. reached its maximum value in our set of samples. EIS analysis revealed that for 55% wt. Cr, the passivation film was thinner, less stable and protective. These findings were also consisted with previous reports for this Cr content. Acknowledgments Author would gratefully thanks to Prof. Dr. Nurhan Cansever for the inspiration, her support and discussions and to Mustafa Kocabas¸ for his experimental assistance. References [1] L. Wang, J. Zhang, Y. Gao, Q. Xue, L. Hu, T. Xu, Grain size effect in corrosion behavior of electrodeposited nanocrystalline Ni coatings in alkaline solution, Scr. Mater 55 (2006) 657e660, http://dx.doi.org/10.1016/ j.scriptamat.2006.04.009. [2] R. Mishra, B. Basu, R. Balasubramaniam, Effect of grain size on the tribological behavior of nanocrystalline nickel, Mater. Sci. Eng. A 373 (2004) 370e373, http://dx.doi.org/10.1016/j.msea.2003.09.107. [3] A. Chiolerio, I. Ferrante, A. Ricci, S. Marasso, P. Tiberto, G. Canavese, et al., Toward mechano-spintronics: nanostructured magnetic multilayers for the realization of microcantilever sensors featuring wireless actuation for liquid environments, J. Intell. Mater. Syst. Struct. 24 (2012) 2189e2196, http:// dx.doi.org/10.1177/1045389X12445031. [4] S.S. Viswanathan, R. Cook, Corrosion Protection and Control Using Nanomaterials, 2012. [5] P. Yong, Z. Yi-chun, Z. Zhao-feng, Fabrication, Lattice Strain, Corrosion Resistance and Mechanical Strength of Nanocrystalline Nickel Films, 2007, pp. 2e6. [6] S.H. Kim, K.T. Aust, U. Erb, F. Gonzalez, G. Palumbo, A comparison of the corrosion behaviour of polycrystalline and nanocrystalline cobalt, Scr. Mater 48 (2003) 1379e1384, http://dx.doi.org/10.1016/S1359-6462(02)00651-6. [7] M. Danisman, N. Cansever, Effect of Cr content on mechanical and electrical properties of NieCr thin films, J. Alloy. Compd. 493 (2010) 649e653, http:// dx.doi.org/10.1016/j.jallcom.2009.12.180. [8] A. Chiolerio, P. Allia, A. Chiodoni, F. Pirri, F. Celegato, M. Coïsson, Thermally evaporated CueCo top spin valve with random exchange bias, J. Appl. Phys. 101 (2007), http://dx.doi.org/10.1063/1.2749289. [9] R. Mishra, R. Balasubramaniam, Effect of nanocrystalline grain size on the electrochemical and corrosion behavior of nickel, Corros. Sci. 46 (2004) 3019e3029, http://dx.doi.org/10.1016/j.corsci.2004.04.007. [10] M.S. Marwah, V. Srinivas, A.K. Pandey, S.R. Kumar, K. Biswas, J. Maity, Morphological changes during annealing of electrodeposited Ni-Cr coating on steel and their effect on corrosion in 3% of NaCl solution, J. Iron Steel Res. Int. 18 (2011) 72e78, http://dx.doi.org/10.1016/S1006-706X(11)60040-X.
Rdl (kU cm2)
Q (mF cm2)
n
51
6.1 2.1 0.114
0.7 0.8 0.3
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Please cite this article in press as: M. Danıs¸man, The corrosion behavior of nanocrystalline nickel based thin films, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.01.018