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Electrochemical anticorrosion behaviors of the electroless deposited Ni–P and Ni–P–PTFE coatings in sterilized and unsterilized seawater
Materials Chemistry and Physics 124 (2010) 751–759
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Electrochemical anticorrosion behaviors of the electroless deposited Ni–P and Ni–P–PTFE coatings in sterilized and unsterilized seawater Jintao Tian ∗ , Xuezhong Liu, Jianfei Wang, Xin Wang, Yansheng Yin Institute of Materials Science and Engineering, Ocean University of China, Songling Road 238, Qingdao 266100, PR China
a r t i c l e
i n f o
Article history: Received 3 March 2010 Received in revised form 2 June 2010 Accepted 22 July 2010 Keywords: Composite materials Coatings Electrochemical techniques Electrochemical properties
a b s t r a c t The Ni–P and Ni–P–PTFE coatings were deposited on mild carbon steel surface via electroless deposition process. The as-deposited coatings were characterized through XRD, SEM, and EDS. The electrochemical anticorrosion behaviors of the coatings in natural seawater were investigated by measurements of potentiodynamic polarization curves, open circuit potential, and electrochemical impedance spectra. The results showed that by immersion in natural seawater for 3–7 days the Ni–P and Ni–P–PTFE coatings could provide maximum protection for the substrate. With regard to the PTFE incorporation, the coating with large amount of PTFE possessed a porous microstructure and hence poor electrochemical anticorrosion performance. Concerning the effect of the ocean microorganisms noticeable variations on electrochemical parameters have been observed though reliable consequence was yet to draw via cultivation and breeding of the ocean microorganisms in subsequent work. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Electroless deposited nickel–phosphorus (Ni–P) binary coating has been widely used in many industrial applications due to its unique properties such as high corrosion resistance, high wear resistance, good lubricity, high hardness, and acceptable ductility [1,2]. By combing nano-sized particles as a reinforcing phase into Ni–P matrix to form functional nanometer composite coating via electroless co-deposition process, the properties of Ni–P coating can be greatly improved and some new features are entirely added to the coating performance. As a result, the applicability of the coating was enhanced in different industries such as electronic components and computers, general mechanics, automobile, paper mills, textile and food [3,4]. In these cases the mostly employed nano-particles consist of hard particles of SiC, WC, Al2 O3 , ZnO, TiO2 , etc., metals of W, Cr, and Cu, etc., and lubricating particles of graphite and polytetrafluoroethylene (PTFE) [5–14]. Among these particles, PTFE has attracted tremendous interest due to the fact that the incorporation of PTFE nano-particles into Ni–P matrix can improve the properties of the coating such as non-stick, non-fouling, dry lubricity, low friction, and good wear/corrosion resistance [15–17]. Concerning the anticorrosion behaviors of Ni–P and Ni–P–PTFE coatings, a lot of work has been conducted and some achieve-
∗ Corresponding author. Tel.: +86 532 66781690; fax: +86 532 66781320. E-mail address: [email protected] (J. Tian). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.07.053
ments are quite noteworthy [18–23]. It has been reported that Ni–P coating exhibits good anticorrosion behavior in several media [3,19,20]. In the case of Ni–P–PTFE composite coating, it was found that the corrosion resistance of Ni–P coating could be improved to some extent by the incorporation of PTFE as the PTFE particles blocked the pores in the Ni–P coating [24]. In addition, recent investigation indicated that electroless deposited Ni–P–PTFE coating could dramatically reduce biofouling that has been recognized as a widespread problem in design and operation of processing equipment such as heat exchangers, cooling water systems, and food processing equipment [25]. The primary experimental results in this case showed that the stainless steel surface deposited by Ni–P–PTFE coating with small amount of PTFE reduced E. coli attachment by 87.7–92.8% than the uncoated stainless steel 304 [26]. Although these progresses on Ni–P and Ni–P–PTFE coatings as well as their anticorrosion behaviors in several media have been achieved in the past decade, some unexploration still remains. For instance, it is known that marine is one of the most severe environments. Compared to some artificial laboratory saline water, natural seawater was chemically quite complicated. It consists of cations (Na+ , Mg2+ , Ca2+ , K+ , etc.), anions (Cl− , SO4 2− , HCO4 − , etc.), dissolved oxygen and carbon dioxide, and dissolved and suspended solids [27]. The immersed metal such as carbon steel can be attacked and corroded seriously by seawater. Moreover, the ocean microorganisms in seawater can attach to metal surface and breed quickly, causing well-known microbiologically influenced corrosion (MIC) damage while in some few cases they acted in a beneficial way to influence corrosion reactions, i.e. MIC inhibition [28]. It is clear that electrochemical study is one of the most
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Table 1 Employed chemicals and their concentrations in the plating bath for the Ni–P and Ni–P–PTFE depositions via electroless plating process. Chemical
Concentration
NiSO4 ·6H2 O NaH2 PO2 ·H2 O CH3 ·COONa PTFE emulsion Surfactant (FC-4) pH Temperature Time
Ni–P coating
Ni–P–PTFE coating
30 g L−1 36 g L−1 15 g L−1 – – 4.8 89 ± 2 ◦ C 2h
30 g L−1 36 g L−1 15 g L−1 0.2 and 10 mL L−1 0–1 g L−1 4.8 92 ± 2 ◦ C 2h
important and feasible ways to assess its anticorrosion behavior of a material in a corrosive media. The electrochemical studies concerning electroless deposited Ni–P and Ni–P–PTFE coatings in natural seawater were not available in the literature up to now. So it was intended in this study. The Ni–P and Ni–P–PTFE coatings were electroless deposited on carbon steel and then immersed in natural seawater for various days. The electrochemical parameters were measured and discussed and the electrochemical anticorrosion behaviors of the coatings were finally assessed. 2. Experimental procedure 2.1. Coating deposition In this study the Ni–P and Ni–P–PTFE coatings were deposited on mild carbon steel surface via electroless deposition process. Prior to deposition, the steel substrate (Q235) with dimensions of 10 mm × 10 mm × 2 mm was pretreated at room temperature as below. The substrate was ground using SiC abrasive papers (400#, 800#, and 1500#) and then degreased with acetone, followed by alkaline rinsing with a 10 mass% NaOH solution for 5 min. After that, the substrate
Fig. 1. XRD patterns of the uncoated carbon steel, Ni–P, Ni–P–PTFE (0.2), Ni–P–PTFE (10) coatings.
was pickled with a 10 vol% HCl solution for 1 min and then activated with a 5 vol% HCl solution for 1 min. The as-pretreated substrate was electroless deposited in a thermostatically controlled bath. Several analytic reagents were employed for the Ni–P and Ni–P–PTFE depositions and their concentrations in the plating solutions are given in Table 1. The deposition process for the Ni–P coating was carried out at constant temperature for 2 h under a mechanical stirring. In the case of the Ni–P–PTFE co-deposition, a PTFE emulsion with a volume fraction of PTFE nano-particles of 60 vol% and a cationic surfactant of C20 H20 F23 N2 O4 I (FC-4) were employed. A mixture of the PTFE emulsion and the surfactant were diluted with some amount of deionized water and stirred for 0.5–1 h with a mechanical agitation and then incorporated into the plating bath to achieve a uniform dispersion of the PTFE nano-particles. For 1 L bath solution the amounts of PTFE emulsion of 0.2 and 10 mL, corresponding to small and large amounts of PTFE, were intended for comparison. The resulted composite coatings were hereafter referred to be as Ni–P–PTFE (0.2) and Ni–P–PTFE (10). The deposition
Fig. 2. SEM surface morphologies: (A) PTFE nano-particles; (B) Ni–P coating; (C) Ni–P–PTFE (0.2) coating; and (D) Ni–P–PTFE (10) coating.
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Fig. 3. Polarization curves of the Ni–P (A and B) and Ni–P–PTFE (10) (C and D) coatings measured under immersion in sterilized (A and C) and unsterilized (B and D) seawater for various days. The plots of icorr and Ecorr (corrosion current density and potential derived from Tafel equation) against immersion time were also shown in this figure (E and F). process was then conducted at constant temperature for 2 h without any stirring since the PTFE particles have been uniformly dispersed in the bath solution. After deposition, the coatings were rinsed with deionized water and dried below 80 ◦ C.
2.2. Coating characterization and electrochemical measurements The as-deposited Ni–P and Ni–P–PTFE coatings were characterized in detail as follows. The phase compositions of the coatings were identified through X-ray
Table 2 Elemental compositions of the Ni–P and Ni–P–PTFE coatings from EDS analysis. Element (K␣ )
C O F P Fe Ni Total
Ni–P
Ni–P–PTFE (0.2)
Ni–P–PTFE (10)
Weight%
Atom%
Weight%
Atom%
Weight%
Atom%
6.71 0.58 0.00 8.62 0.79 83.30
24.26 1.56 0.00 12.06 0.61 61.51
3.94 0.55 1.42 10.91 0.20 82.98
14.90 1.55 3.38 15.96 0.16 64.05
5.04 1.37 4.72 9.21 0.09 79.57
17.44 3.52 10.31 12.36 0.07 56.30
100.00
100.00
100.00
100.00
100.00
100.00
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Table 3 Kinetic parameters from polarization curves for the Ni–P coating. The coated samples were immersed in sterilized and unsterilized seawater for 1–28 days and the potentiodynamic polarization curves were measured. icorr and Ecorr : corrosion current density and potential derived from Tafel equation within potential ranges of ±(80–140) mV away from the peaks in Fig. 3A–D (linear segments); ba : anodic Tafel slop; bc : cathodic Tafel slop. Fitted parameters In sterilized seawater bc , mV dec−1 ba , mV dec−1 icorr , A cm−2 Ecorr , mV In unsterilized seawater bc , mV dec−1 ba , mV dec−1 icorr , A cm−2 Ecorr , mV
1
3
5
7
14
28
−190 200 2.51 −556.3
−273 233 7.39 −581.4
−203 168 13.8 −769.5
−209 296 5.64 −652.3
−255 201 11 −732.5
−200 201 14.7 −809.1
−238 402 3.51 −677.1
−251 237 2.58 −452.0
−279 247 1.97 −433.5
−256 227 1.77 −443.5
−825 200 2.4 −473
−373 203 2.31 −469.4
diffraction (XRD, Model D8 Advance, BRUKER/AXS, Germany) method. The surface morphologies of the coatings were observed using scanning electron microscopy (SEM, Model JSM-6700F, JEOL, Japan). The elemental compositions were analyzed by means of energy dispersive X-ray Spectroscopy (EDS). The electrochemical anticorrosion behaviors of the Ni–P and Ni–P–PTFE coatings in natural seawater were assessed via measurements of potentiodynamic polarization curves, open circuit potential (OCP), and electrochemical impedance spectroscopy (EIS) through an electrochemical workstation (IM6e, ZAHNER, Germany). The natural seawater was scooped from Qindao offshore and sterilized in a pressure cooker under a pressure of 0.1–0.15 MPa for 20 min. Besides the sterilized seawater, the unsterilized seawater was employed as corrosive media to explore the MIC effect. The coating samples were immersed in sterilized and unsterilized seawater for 1–28 days. The electrochemical parameters were measured with a conventional three electrode cell. The cell was composed of a working electrode (WE) with an exposure surface of 10 mm × 10 mm, a counter platinum electrode (CE) with a surface of 11 mm × 11 mm, and a saturated calomel reference electrode (SCE). The potentiodynamic polarization measurements were carried out from −1.2 V (SCE) to 0.4 V (SCE) with a scanning rate of 2 mV s−1 . The EIS data were measured in the frequency range of 0.01 Hz to 100 kHz in the single sine mode with an AC amplitude of 10 mV peak-to-peak with respect to OCP. The obtained EIS data were interpreted on the basis of equivalent electrical analogs using the program Zview 2.0 to obtain the fitting parameters. Fig. 4. Plots of EOCP (open circuit potential) against immersion time for the Ni–P and Ni–P–PTFE composite coatings in sterilized and unsterilized seawater.
3. Results and discussion
tially similar to those of the Ni–P coating regardless of the PTFE concentration in the plating bath. This suggested that the incorporation of PTFE nano-particles into the Ni–P matrix scarcely affected the crystallization behavior of the coating. It should be noted that there had a small diffraction peak appeared at 2 of 18.1◦ for the Ni–P–PTFE (10) composite coating. By combining several possible elements of Ni, P, C, F, O, Fe, Na, S, etc., the phase related to this peak was most likely P4 S3 (JCPDS No. 65-0463). The surface morphologies of the Ni–P and Ni–P–PTFE coatings through SEM observations are shown in Fig. 2. As seen from Fig. 2A, the as-received PTFE particles were sphere-shaped in a nano-scale with a mean diameter of ∼100 nm. The electroless deposited Ni–P coating had a smooth surface with some few nano-pores indi-
3.1. Phase composition and surface morphology The phase compositions of the as-deposited Ni–P and Ni–P–PTFE coatings were identified through XRD method and the results were shown in Fig. 1. As seen, the employed steel substrate is composed of crystals with a strong X-ray diffraction peak at 2 of 44.9◦ , which accurately agrees to the iron plane of (1 1 0) (2 of 45.1◦ , JCPDS No. 01-1267). The electroless deposited Ni–P coating was in an amorphous state. A rather broad hump around 2 of 42–52◦ was observed. It has been reported that the electroless deposited Ni–P coating with P content larger than 17% was generally in an amorphous state and had a better anticorrosive performance [9,18]. The XRD patterns for the Ni–P–PTFE composite coatings were essen-
Table 4 Kinetic parameters from polarization curves for the Ni–P–PTFE (10) composite coating. The coated samples were immersed in sterilized and unsterilized seawater for 1–28 days and the potentiodynamic polarization curves were measured. icorr and Ecorr : corrosion current density and potential derived from Tafel equation within potential ranges of ±(80–140) mV away from the peaks in Fig. 3A–D (linear segments); ba : anodic Tafel slop; bc : cathodic Tafel slop. Fitted parameters In sterilized seawater bc , mV dec−1 ba , mV dec−1 icorr , A cm−2 Ecorr , mV In unsterilized seawater bc , mV dec−1 ba , mV dec−1 icorr , A cm−2 Ecorr , mV
1
3
5
7
14
28
−151 240 13.2 −823.3
−175 298 9.1 −834.6
−136 264 16.3 −888.4
−97 314 26.6 −878.4
−194 243 15.2 −808.5
−180 258 28.4 −819.2
−235 152 4.21 −568.4
−146 247 5.08 −593.2
−244 186 3.69 −530.8
−235 190 3.12 −532.7
−85 127 1.45 −663.1
−70 249 3.4 −716.3
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Fig. 5. EIS studies of the electroless deposited Ni–P coating measured under immersion in sterilized (A, C, E) and unsterilized (B, D, F) seawater for various days: (A, B) Nyquist diagrams; (C, D) Bode modulus diagrams; (E, F) Bode phase angle diagrams.
cated by arrows (Fig. 2B). The Ni–P–PTFE (0.2) composite coating was quite smooth and dense and only very few nano-pores were observed (Fig. 2C). The surface of the Ni–P–PTFE (10) was essentially rough. Many black spots were observed in Fig. 2D. By observation at high magnification these black spots consisted of nano-particles and nano-voids both particularly in a diameter size of ∼100 nm. This implied that the PTFE nano-particles have been successfully embedded inside of the Ni–P matrix while some of them were uncovered. The porous microstructure in Fig. 2D, as expected, would go against anticorrosive behavior of the coating. During the SEM observations the EDS analysis on element compositions of the Ni–P and Ni–P–PTFE coatings were conducted and the results are shown in Table 2. Elements of Ni and P were detected in the Ni–P coating. The derived atomic ratio of Ni to P was ∼5.1, giving a P
content of ∼20%. For the Ni–P–PTFE (0.2) and Ni–P–PTFE (10), the atomic ratios of Ni to P were ∼4.0 and 4.6, slightly lower than that of the Ni–P coating. Element of F was detected in the two composite coatings. The derived atomic contents of F were 3.38% and 10.31%, respectively. This suggested again that the PTFE has been successfully incorporated into the Ni–P matrix during the co-deposition process. 3.2. Potentiodynamic polarization behavior The electrochemical anticorrosion behaviors of the Ni–P and Ni–P–PTFE coatings were firstly assessed by measurements of potentiodynamic polarization curves of the coated samples in sterilized and unsterilized seawater for an period time of 1–28 days.
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Fig. 6. EIS studies of the electroless deposited Ni–P–PTFE (0.2) composite coating measured under immersion in sterilized (A, C, E) and unsterilized (B, D, F) seawater for various days: (A, B) Nyquist diagrams; (C, D) Bode modulus diagrams; (E, F) Bode phase angle diagrams.
Here the Ni–P and Ni–P–PTFE (10) were selected for measurements. The derived polarization curves are shown in Fig. 3A–D. In order to determine the corrosion current density (icorr ) and corrosion potential (Ecorr ), the well-known Tafel equation was employed [29]: E = a + b log i
(1)
where E and i were polarization potential and current density and b the Tafel slop. By fitting the linear segments of the curves (potential ranges of ±(80–140) mV away from the peaks in Fig. 3A–D) with Eq. (1), the icorr and Ecorr as well as bc (cathodic Tafel slop) and ba (anodic Tafel slop) were obtained and shown in Tables 3 and 4 for the Ni–P and Ni–P–PTFE (10), respectively. The plots of icorr and Ecorr against immersion time were shown in Fig. 3E and F. As seen from Fig. 3A–D, all the polarization curves were essentially comparable to each other. This suggested similar polarization mechanism for the two coatings in sterilized and unsterilized seawater. The variations of the curve positions in Fig. 3A–D, however, were evidently observed. Compared to those immersed in sterilized seawater, the
derived icorr for the Ni–P and Ni–P–PTFE (10) coatings immersed in unsterilized seawater was rather low. Accordingly, the Ecorr for the coatings in unsterilized seawater was more positive than those in sterilized seawater. Generally, a material with a lower icorr and a higher Ecorr has a lower tendency to be oxidized and corroded and therefore higher anticorrosion performance [30]. The results of icorr and Ecorr for the two coatings in sterilized and unsterilized seawater suggested that not MIC damage but MIC inhibition has occurred in our case, as has been observed by some investigators in the literature [31,32]. On the other hand, the polarization behavior of the Ni–P was visibly different from that of the Ni–P–PTFE. With immersion days of 1–28, the Ni–P coating showed notably lower icorr and higher Ecorr than the Ni–P–PTFE (10) composite coating both in sterilized and unsterilized seawater (see Fig. 3E and F). Such results can be well understood by considering the microstructural morphologies of the two coatings, where the Ni–P had smoother and denser surface and consequently better anticorrosion performance than the Ni–P–PTFE (10).
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Fig. 7. EIS studies of the electroless deposited Ni–P–PTFE (10) composite coating measured under immersion in sterilized (A, C, E) and unsterilized (B, D, F) seawater for various days: (A, B) Nyquist diagrams; (C, D) Bode modulus diagrams; (E, F) Bode phase angle diagrams.
3.3. OCP and EIS investigations
Fig. 8. Proposed equivalent circuit model. Rs : solution resistance; CPEdl : double layer capacitance; Rct : charge transfer resistance; Zw : Warburg resistance; L: inductance.
In this section the OCP and EIS investigations were conducted with the Ni–P and Ni–P–PTFE coated samples in sterilized and unsterilized seawater. The measured EOCP was shown in Fig. 4. The EOCP varied visibly with the three coatings. The maximum EOCP was roughly achieved with 3–7 days immersion followed by a declining tendency. In particular, the Ni–P–PTFE (10) had a lower EOCP than the Ni–P and Ni–P–PTFE (0.2). Concerning the effect MIC on EOCP , visible variation of EOCP was not observed. The EIS investigations of the Ni–P coating were conducted at OCP and the results were shown in Fig. 5. The plots of the Nyquist spectra in Fig. 5A and B gave typical semi-circles. The diameters of these semi-circles clearly varied with the immersion days. The largest semi-circle diameters of ∼100 and ∼40 k cm2 were
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Fig. 9. Plots of resistances of Rct and Zw against immersion time for the Ni–P and Ni–P–PTFE coatings in sterilized and unsterilized seawater. The coatings were electroless deposited on carbon steel surface with various concentrations of PTFE emulsion in the plating bath.
achieved for the coated sample with 3 days immersion in sterilized and unsterilized seawater, respectively. The derived curves in the Bode modulus diagrams in Fig. 5C and D agreed well with these results. The largest impendence was therefore achieved for the Ni–P coating with 3 days immersion. The curves in Fig. 5E and F acted essentially in a comparable way, demonstrating similar anticorrosion mechanism of the coating in this study. Concerning the MIC effect, the diameters of semi-circles in Fig. 5B were evidently smaller than those shown in Fig. 5A, a sign of substantial drop of the impedance due to the ocean MIC. The EIS results for the Ni–P–PTFE (0.2) were similar to those of the Ni–P, as shown in Fig. 6. With 5 days immersion in sterilized seawater and 1 day immersion in unsterilized seawater the largest semicircle diameters of ∼25 and ∼30 k cm2 were achieved. In the case of the Ni–P–PTFE (10), the largest semi-circle diameters as small as ∼4 and ∼7 k cm2 were obtained after immersion of 1 day in sterilized seawater and 7 days in unsterilized seawater (Fig. 7). The above EIS results demonstrated that the incorporation of the PTFE nano-particles and its amount had an essential effect on the impedance of the Ni–P matrix. The anticorrosion mechanism of the coating, however, did not varied with the PTFE incorporation. In order to well understand the EIS results given above, the EIS data were interpreted on the basis of equivalent electrical analogs. Several equivalent circuit models were tried and the optimal one was shown in Fig. 8. With this model the derived parameters of resistive impedances of Rct (charge transfer resistance) and Zw (Warburg resistance) were plotted against immersion time in Fig. 9. In the case of the capacitive impedance, the below equation was
Fig. 10. Plots of capacitance parameters of V and ˛ of the double layer (see Eq. (2)) against immersion time for the Ni–P and Ni–P–PTFE coatings in sterilized and unsterilized seawater. The coatings were electroless deposited on carbon steel surface with various concentrations of PTFE emulsion in the plating bath.
applied for a CPEdl (double layer capacitance) element [33] ZCPE =
1 · ω0 · V
jω −˛ ω0
(2)
where ZCPE was the capacitive impedance of the double layer formed on the coating surface, ω0 the normalizaiton factor (setted to be 2·1000 s−1 ), ω the circular frequency, V in a Farad dimension, and ˛ dimensionless parameter in a range of 0–1 describing deviation of the element from behavior of an ideal capacitor (˛ = 0, Ohmic impedance with phase angle of 0◦ ; ˛ = 1, idea capacitor with an absolute phase angle of 90◦ ). With Eq. (2) the derived parameter of V and ˛ were shown in Fig. 10. As seen from Figs. 9 and 10, maximum values of Rct and Zw and minimum values of V were reached for the three coatings after 3–7 days immersion both in sterilized and unsterilized seawater. Note that low value of V was corresponding to high capacitive impedance. Thus, the above results demonstrated that with 3–7 days immersion the largest impedance has been accomplished for the three coatings. With regard to the effect of the PTFE amount in the matrix, the values of Rct and Zw and V for the Ni–P were comparable to those of the Ni–P–PTFE (0.2). In the case of the Ni–P–PTFE (10), the Rct and Zw were rather small while the V was qutie large. Concerning the parameter of ˛, the Ni–P–PTFE (10) possessed low value and therefore large deviation of the element from an ideal capacitor, compared to the Ni–P and Ni–P–PTFE (0.2). Note that the ˛ could also be visually understood from the modulus and phase angle shown in Figs. 5–7. With lower ˛ value the modulus slop in graph C and graph D and maximum
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phase angle in graph E and graph F in Figs. 5–7 were consequently lower. The differences of EIS behaviors between the Ni–P, Ni–P–PTFE (0.2), and Ni–P–PTFE (10) can be well understood by considering the microstructural morphologies of the three coatings. With an incorporation of large amount of PTFE a rough and porous microstructure was observed for the Ni–P–PTFE (10) (see Fig. 2D). With such porous microstructure the coating could be easily penetrated by the corrosive seawater and therefore poor electrochemical anticorrosion behavior. In the cases of the Ni–P and Ni–P–PTFE (0.2) the coatings were rather smooth and dense (see Fig. 2B and C) and hence good prevention from seawater penetration. It should be pointed out that, as reviewed previously in Section 1 [25,26], with small amount of PTFE nano-particles incorporated the Ni–P–PTFE should reduce biofouling to some extent and therefore influences on the electrochemical parameters. Concerning this point it was quite difficult to draw a decisive conclusion at present in our study though noticeable differences have been definitely observed (compare the curves obtained in sterilized and unsterilized seawater in Figs. 3–10). This could most likely be due to the fact that the unsterilized natural seawater was directly scooped from Qingdao offshore and the amount of the ocean microorganisms would be rather small. Thus, the cultivation of the ocean microorganisms and its breeding behavior on the Ni–P–PTFE composite coating with small amount of PTFE nano-particles incorporated were quite essential. The research related to the above content will be conducted in our future work. 4. Conclusion The Ni–P and Ni–P–PTFE coatings were deposited on mild carbon steel surface via electroless deposition process and their electrochemical behaviors in sterilized and unsterilized seawater were assessed. The as-deposited coatings were in an amorphous state with an atomic ratio of Ni to P of 4.0–5.1. The Ni–P and Ni–P–PTFE (0.2) possessed quite smooth and dense surfaces with some few nano-voids while in the case of the Ni–P–PTFE (10) it was rather rough and porous. The electrochemical investigations showed that by immersion in natural seawater for 3–7 days the Ni–P and Ni–P–PTFE coatings reached maximum protection for the substrate. With regard to the incorporation of PTFE nano-particles, both the polarization and EIS results as well as the measured open circuit potential revealed a poor electrochemical anticorrosion behavior for the Ni–P–PTFE (10) composite coating that has been attributed to its porous microstructure. Concerning the effect of the ocean microorganisms although visible variations of electrochemical parameters have been observed between the coated samples immersed in sterilized and unsterilized seawater, definite
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conclusion could not be drawn at present since the amount of the ocean microorganisms was not large enough. Further investigation on the cultivation of the ocean microorganisms and their breeding on the sample with small amount of PTFE were yet to conduct in our future work. Acknowledgements This work was financially supported by Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP, 200804231004) and Shandong Province Middle-aged and Young Scientists Research Incentive Fund (BS2009CL016). References [1] R. Elansezhian, B. Ramamoorthy, P. Kesavan Nair, J. Mater. Process. Technol. 209 (2009) 233. [2] P. Peeters, G. Hoorn, T. Daenen, A. Kurowski, G. Staikov, Electrochim. Acta 47 (2001) 161. [3] Q. Zhao, Y. Liu, Corros. Sci. 47 (2005) 2807. [4] G. Straffelini, D. Colombo, A. Molinari, Wear 236 (1999) 179. [5] S. Zhang, K. Han, L. Cheng, Surf. Coat. Technol. 202 (2008) 2807. [6] Z.A. Hamid, S.A. Badry, A.A. Aal, Surf. Coat. Technol. 201 (2007) 5948. [7] Y. Hazan, D. Werner, M. Zgraggen, M. Groteklaes, T. Graule, J. Colloid Interface Sci. 328 (2008) 103. [8] S.M.A. Shibli, B. Jabeera, R.I. Anupama, Surf. Coat. Technol. 200 (2006) 3903. [9] S.M.A. Shibli, V.S. Dilimon, Int. J. Hydrogen Energy 32 (2007) 1694. [10] J. Novakovic, P. Vassiliou, Kl. Samara, Th. Argyropoulos, Surf. Coat. Technol. 201 (2006) 895. [11] X.Y. Yuan, T. Xie, G.S. Wu, Y. Lin, G.W. Meng, L.D. Zhang, Physica E: Low-dimens. Syst. Nanostruct. 23 (2004) 75. [12] L. Zhang, Y. Jin, B. Peng, Y. Zhang, X. Wang, Q. Yang, J. Yu, Appl. Surf. Sci. 255 (2008) 1686. [13] Y.T. Wu, L. Lei, B. Shen, W.B. Hu, Surf. Coat. Technol. 201 (2006) 441. [14] Q. Zhao, Y. Liu, Appl. Surf. Sci. 229 (2004) 56. [15] Q. Zhao, Y. Liu, H. Muller-Steinhagen, G. Liu, Surf. Coat. Technol. 155 (2002) 279. [16] M.D. Ger, K.H. Hou, L.M. Wang, B.J. Hwang, Mater. Chem. Phys. 77 (2002) 755. [17] Y. Wu, H. Liu, B. Shen, L. Liu, W. Hu, Tribol. Int. 39 (2006) 553. [18] M. Crobu, A. Scorciapino, B. Elsener, A. Rossi, Electrochim. Acta 53 (2008) 3364. [19] C. Gu, J. Lian, G. Li, L. Niu, Z. Jiang, Surf. Coat. Technol. 197 (2005) 61. [20] A. Bai, P. Chuang, C. Hu, Mater. Chem. Phys. 82 (2003) 93. [21] Q. Zhao, Y. Liu, Surf. Coat. Technol. 200 (2005) 2510. [22] Q. Zhao, Y. Liu, E.W. Abel, Mater. Chem. Phys. 87 (2004) 332. [23] R. Xu, Z. Guo, J. Pan, Trans. Nonferrous Met. Soc. China 16 (2006) 666. [24] S. Takashi, Y. Shuji, K. Yuichi, Corrosion preventing structure, EP0737759 (1996). [25] Q. Zhao, Surf. Coat. Technol. 185 (2004) 199. [26] Q. Zhao, Y. Liu, J. Food Eng. 72 (2006) 266. [27] S. Al-Fozan, A. Malik, Desalination 228 (2008) 61. [28] D. Örnek, A. Jayaraman, T. Wood, Z. Sun, C. Hsu, F. Mansfeld, Corros. Sci. 43 (2001) 2121. [29] http://www.zahner.de/pdf/IE.pdf, p. 27. [30] D.A. Jones, Principles and Prevention of Corrosion, Prentice Hall Inc./Simon & Schuster A Viacom Company, Upper Saddle River, NJ, 1996, p. 32. [31] A. Pedersen, M. Hermansson, Biofouling 1 (1989) 313. [32] M. Eashwar, S. Maruthamuthu, S. Sathyanarayanan, K. Balakrishnan, Proc. 12th Int. Corros. Cong., vol. 5b, NACE, Houston, TX, 1993, p. 3708. [33] http://www.zahner.de/pdf/SIM.pdf, p. 27.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Electrochemical anticorrosion behaviors of the electroless deposited Ni–P and Ni–P–PTFE coatings in sterilized and unsterilized seawater Jintao Tian ∗ , Xuezhong Liu, Jianfei Wang, Xin Wang, Yansheng Yin Institute of Materials Science and Engineering, Ocean University of China, Songling Road 238, Qingdao 266100, PR China
a r t i c l e
i n f o
Article history: Received 3 March 2010 Received in revised form 2 June 2010 Accepted 22 July 2010 Keywords: Composite materials Coatings Electrochemical techniques Electrochemical properties
a b s t r a c t The Ni–P and Ni–P–PTFE coatings were deposited on mild carbon steel surface via electroless deposition process. The as-deposited coatings were characterized through XRD, SEM, and EDS. The electrochemical anticorrosion behaviors of the coatings in natural seawater were investigated by measurements of potentiodynamic polarization curves, open circuit potential, and electrochemical impedance spectra. The results showed that by immersion in natural seawater for 3–7 days the Ni–P and Ni–P–PTFE coatings could provide maximum protection for the substrate. With regard to the PTFE incorporation, the coating with large amount of PTFE possessed a porous microstructure and hence poor electrochemical anticorrosion performance. Concerning the effect of the ocean microorganisms noticeable variations on electrochemical parameters have been observed though reliable consequence was yet to draw via cultivation and breeding of the ocean microorganisms in subsequent work. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Electroless deposited nickel–phosphorus (Ni–P) binary coating has been widely used in many industrial applications due to its unique properties such as high corrosion resistance, high wear resistance, good lubricity, high hardness, and acceptable ductility [1,2]. By combing nano-sized particles as a reinforcing phase into Ni–P matrix to form functional nanometer composite coating via electroless co-deposition process, the properties of Ni–P coating can be greatly improved and some new features are entirely added to the coating performance. As a result, the applicability of the coating was enhanced in different industries such as electronic components and computers, general mechanics, automobile, paper mills, textile and food [3,4]. In these cases the mostly employed nano-particles consist of hard particles of SiC, WC, Al2 O3 , ZnO, TiO2 , etc., metals of W, Cr, and Cu, etc., and lubricating particles of graphite and polytetrafluoroethylene (PTFE) [5–14]. Among these particles, PTFE has attracted tremendous interest due to the fact that the incorporation of PTFE nano-particles into Ni–P matrix can improve the properties of the coating such as non-stick, non-fouling, dry lubricity, low friction, and good wear/corrosion resistance [15–17]. Concerning the anticorrosion behaviors of Ni–P and Ni–P–PTFE coatings, a lot of work has been conducted and some achieve-
∗ Corresponding author. Tel.: +86 532 66781690; fax: +86 532 66781320. E-mail address: [email protected] (J. Tian). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.07.053
ments are quite noteworthy [18–23]. It has been reported that Ni–P coating exhibits good anticorrosion behavior in several media [3,19,20]. In the case of Ni–P–PTFE composite coating, it was found that the corrosion resistance of Ni–P coating could be improved to some extent by the incorporation of PTFE as the PTFE particles blocked the pores in the Ni–P coating [24]. In addition, recent investigation indicated that electroless deposited Ni–P–PTFE coating could dramatically reduce biofouling that has been recognized as a widespread problem in design and operation of processing equipment such as heat exchangers, cooling water systems, and food processing equipment [25]. The primary experimental results in this case showed that the stainless steel surface deposited by Ni–P–PTFE coating with small amount of PTFE reduced E. coli attachment by 87.7–92.8% than the uncoated stainless steel 304 [26]. Although these progresses on Ni–P and Ni–P–PTFE coatings as well as their anticorrosion behaviors in several media have been achieved in the past decade, some unexploration still remains. For instance, it is known that marine is one of the most severe environments. Compared to some artificial laboratory saline water, natural seawater was chemically quite complicated. It consists of cations (Na+ , Mg2+ , Ca2+ , K+ , etc.), anions (Cl− , SO4 2− , HCO4 − , etc.), dissolved oxygen and carbon dioxide, and dissolved and suspended solids [27]. The immersed metal such as carbon steel can be attacked and corroded seriously by seawater. Moreover, the ocean microorganisms in seawater can attach to metal surface and breed quickly, causing well-known microbiologically influenced corrosion (MIC) damage while in some few cases they acted in a beneficial way to influence corrosion reactions, i.e. MIC inhibition [28]. It is clear that electrochemical study is one of the most
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Table 1 Employed chemicals and their concentrations in the plating bath for the Ni–P and Ni–P–PTFE depositions via electroless plating process. Chemical
Concentration
NiSO4 ·6H2 O NaH2 PO2 ·H2 O CH3 ·COONa PTFE emulsion Surfactant (FC-4) pH Temperature Time
Ni–P coating
Ni–P–PTFE coating
30 g L−1 36 g L−1 15 g L−1 – – 4.8 89 ± 2 ◦ C 2h
30 g L−1 36 g L−1 15 g L−1 0.2 and 10 mL L−1 0–1 g L−1 4.8 92 ± 2 ◦ C 2h
important and feasible ways to assess its anticorrosion behavior of a material in a corrosive media. The electrochemical studies concerning electroless deposited Ni–P and Ni–P–PTFE coatings in natural seawater were not available in the literature up to now. So it was intended in this study. The Ni–P and Ni–P–PTFE coatings were electroless deposited on carbon steel and then immersed in natural seawater for various days. The electrochemical parameters were measured and discussed and the electrochemical anticorrosion behaviors of the coatings were finally assessed. 2. Experimental procedure 2.1. Coating deposition In this study the Ni–P and Ni–P–PTFE coatings were deposited on mild carbon steel surface via electroless deposition process. Prior to deposition, the steel substrate (Q235) with dimensions of 10 mm × 10 mm × 2 mm was pretreated at room temperature as below. The substrate was ground using SiC abrasive papers (400#, 800#, and 1500#) and then degreased with acetone, followed by alkaline rinsing with a 10 mass% NaOH solution for 5 min. After that, the substrate
Fig. 1. XRD patterns of the uncoated carbon steel, Ni–P, Ni–P–PTFE (0.2), Ni–P–PTFE (10) coatings.
was pickled with a 10 vol% HCl solution for 1 min and then activated with a 5 vol% HCl solution for 1 min. The as-pretreated substrate was electroless deposited in a thermostatically controlled bath. Several analytic reagents were employed for the Ni–P and Ni–P–PTFE depositions and their concentrations in the plating solutions are given in Table 1. The deposition process for the Ni–P coating was carried out at constant temperature for 2 h under a mechanical stirring. In the case of the Ni–P–PTFE co-deposition, a PTFE emulsion with a volume fraction of PTFE nano-particles of 60 vol% and a cationic surfactant of C20 H20 F23 N2 O4 I (FC-4) were employed. A mixture of the PTFE emulsion and the surfactant were diluted with some amount of deionized water and stirred for 0.5–1 h with a mechanical agitation and then incorporated into the plating bath to achieve a uniform dispersion of the PTFE nano-particles. For 1 L bath solution the amounts of PTFE emulsion of 0.2 and 10 mL, corresponding to small and large amounts of PTFE, were intended for comparison. The resulted composite coatings were hereafter referred to be as Ni–P–PTFE (0.2) and Ni–P–PTFE (10). The deposition
Fig. 2. SEM surface morphologies: (A) PTFE nano-particles; (B) Ni–P coating; (C) Ni–P–PTFE (0.2) coating; and (D) Ni–P–PTFE (10) coating.
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Fig. 3. Polarization curves of the Ni–P (A and B) and Ni–P–PTFE (10) (C and D) coatings measured under immersion in sterilized (A and C) and unsterilized (B and D) seawater for various days. The plots of icorr and Ecorr (corrosion current density and potential derived from Tafel equation) against immersion time were also shown in this figure (E and F). process was then conducted at constant temperature for 2 h without any stirring since the PTFE particles have been uniformly dispersed in the bath solution. After deposition, the coatings were rinsed with deionized water and dried below 80 ◦ C.
2.2. Coating characterization and electrochemical measurements The as-deposited Ni–P and Ni–P–PTFE coatings were characterized in detail as follows. The phase compositions of the coatings were identified through X-ray
Table 2 Elemental compositions of the Ni–P and Ni–P–PTFE coatings from EDS analysis. Element (K␣ )
C O F P Fe Ni Total
Ni–P
Ni–P–PTFE (0.2)
Ni–P–PTFE (10)
Weight%
Atom%
Weight%
Atom%
Weight%
Atom%
6.71 0.58 0.00 8.62 0.79 83.30
24.26 1.56 0.00 12.06 0.61 61.51
3.94 0.55 1.42 10.91 0.20 82.98
14.90 1.55 3.38 15.96 0.16 64.05
5.04 1.37 4.72 9.21 0.09 79.57
17.44 3.52 10.31 12.36 0.07 56.30
100.00
100.00
100.00
100.00
100.00
100.00
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Table 3 Kinetic parameters from polarization curves for the Ni–P coating. The coated samples were immersed in sterilized and unsterilized seawater for 1–28 days and the potentiodynamic polarization curves were measured. icorr and Ecorr : corrosion current density and potential derived from Tafel equation within potential ranges of ±(80–140) mV away from the peaks in Fig. 3A–D (linear segments); ba : anodic Tafel slop; bc : cathodic Tafel slop. Fitted parameters In sterilized seawater bc , mV dec−1 ba , mV dec−1 icorr , A cm−2 Ecorr , mV In unsterilized seawater bc , mV dec−1 ba , mV dec−1 icorr , A cm−2 Ecorr , mV
1
3
5
7
14
28
−190 200 2.51 −556.3
−273 233 7.39 −581.4
−203 168 13.8 −769.5
−209 296 5.64 −652.3
−255 201 11 −732.5
−200 201 14.7 −809.1
−238 402 3.51 −677.1
−251 237 2.58 −452.0
−279 247 1.97 −433.5
−256 227 1.77 −443.5
−825 200 2.4 −473
−373 203 2.31 −469.4
diffraction (XRD, Model D8 Advance, BRUKER/AXS, Germany) method. The surface morphologies of the coatings were observed using scanning electron microscopy (SEM, Model JSM-6700F, JEOL, Japan). The elemental compositions were analyzed by means of energy dispersive X-ray Spectroscopy (EDS). The electrochemical anticorrosion behaviors of the Ni–P and Ni–P–PTFE coatings in natural seawater were assessed via measurements of potentiodynamic polarization curves, open circuit potential (OCP), and electrochemical impedance spectroscopy (EIS) through an electrochemical workstation (IM6e, ZAHNER, Germany). The natural seawater was scooped from Qindao offshore and sterilized in a pressure cooker under a pressure of 0.1–0.15 MPa for 20 min. Besides the sterilized seawater, the unsterilized seawater was employed as corrosive media to explore the MIC effect. The coating samples were immersed in sterilized and unsterilized seawater for 1–28 days. The electrochemical parameters were measured with a conventional three electrode cell. The cell was composed of a working electrode (WE) with an exposure surface of 10 mm × 10 mm, a counter platinum electrode (CE) with a surface of 11 mm × 11 mm, and a saturated calomel reference electrode (SCE). The potentiodynamic polarization measurements were carried out from −1.2 V (SCE) to 0.4 V (SCE) with a scanning rate of 2 mV s−1 . The EIS data were measured in the frequency range of 0.01 Hz to 100 kHz in the single sine mode with an AC amplitude of 10 mV peak-to-peak with respect to OCP. The obtained EIS data were interpreted on the basis of equivalent electrical analogs using the program Zview 2.0 to obtain the fitting parameters. Fig. 4. Plots of EOCP (open circuit potential) against immersion time for the Ni–P and Ni–P–PTFE composite coatings in sterilized and unsterilized seawater.
3. Results and discussion
tially similar to those of the Ni–P coating regardless of the PTFE concentration in the plating bath. This suggested that the incorporation of PTFE nano-particles into the Ni–P matrix scarcely affected the crystallization behavior of the coating. It should be noted that there had a small diffraction peak appeared at 2 of 18.1◦ for the Ni–P–PTFE (10) composite coating. By combining several possible elements of Ni, P, C, F, O, Fe, Na, S, etc., the phase related to this peak was most likely P4 S3 (JCPDS No. 65-0463). The surface morphologies of the Ni–P and Ni–P–PTFE coatings through SEM observations are shown in Fig. 2. As seen from Fig. 2A, the as-received PTFE particles were sphere-shaped in a nano-scale with a mean diameter of ∼100 nm. The electroless deposited Ni–P coating had a smooth surface with some few nano-pores indi-
3.1. Phase composition and surface morphology The phase compositions of the as-deposited Ni–P and Ni–P–PTFE coatings were identified through XRD method and the results were shown in Fig. 1. As seen, the employed steel substrate is composed of crystals with a strong X-ray diffraction peak at 2 of 44.9◦ , which accurately agrees to the iron plane of (1 1 0) (2 of 45.1◦ , JCPDS No. 01-1267). The electroless deposited Ni–P coating was in an amorphous state. A rather broad hump around 2 of 42–52◦ was observed. It has been reported that the electroless deposited Ni–P coating with P content larger than 17% was generally in an amorphous state and had a better anticorrosive performance [9,18]. The XRD patterns for the Ni–P–PTFE composite coatings were essen-
Table 4 Kinetic parameters from polarization curves for the Ni–P–PTFE (10) composite coating. The coated samples were immersed in sterilized and unsterilized seawater for 1–28 days and the potentiodynamic polarization curves were measured. icorr and Ecorr : corrosion current density and potential derived from Tafel equation within potential ranges of ±(80–140) mV away from the peaks in Fig. 3A–D (linear segments); ba : anodic Tafel slop; bc : cathodic Tafel slop. Fitted parameters In sterilized seawater bc , mV dec−1 ba , mV dec−1 icorr , A cm−2 Ecorr , mV In unsterilized seawater bc , mV dec−1 ba , mV dec−1 icorr , A cm−2 Ecorr , mV
1
3
5
7
14
28
−151 240 13.2 −823.3
−175 298 9.1 −834.6
−136 264 16.3 −888.4
−97 314 26.6 −878.4
−194 243 15.2 −808.5
−180 258 28.4 −819.2
−235 152 4.21 −568.4
−146 247 5.08 −593.2
−244 186 3.69 −530.8
−235 190 3.12 −532.7
−85 127 1.45 −663.1
−70 249 3.4 −716.3
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Fig. 5. EIS studies of the electroless deposited Ni–P coating measured under immersion in sterilized (A, C, E) and unsterilized (B, D, F) seawater for various days: (A, B) Nyquist diagrams; (C, D) Bode modulus diagrams; (E, F) Bode phase angle diagrams.
cated by arrows (Fig. 2B). The Ni–P–PTFE (0.2) composite coating was quite smooth and dense and only very few nano-pores were observed (Fig. 2C). The surface of the Ni–P–PTFE (10) was essentially rough. Many black spots were observed in Fig. 2D. By observation at high magnification these black spots consisted of nano-particles and nano-voids both particularly in a diameter size of ∼100 nm. This implied that the PTFE nano-particles have been successfully embedded inside of the Ni–P matrix while some of them were uncovered. The porous microstructure in Fig. 2D, as expected, would go against anticorrosive behavior of the coating. During the SEM observations the EDS analysis on element compositions of the Ni–P and Ni–P–PTFE coatings were conducted and the results are shown in Table 2. Elements of Ni and P were detected in the Ni–P coating. The derived atomic ratio of Ni to P was ∼5.1, giving a P
content of ∼20%. For the Ni–P–PTFE (0.2) and Ni–P–PTFE (10), the atomic ratios of Ni to P were ∼4.0 and 4.6, slightly lower than that of the Ni–P coating. Element of F was detected in the two composite coatings. The derived atomic contents of F were 3.38% and 10.31%, respectively. This suggested again that the PTFE has been successfully incorporated into the Ni–P matrix during the co-deposition process. 3.2. Potentiodynamic polarization behavior The electrochemical anticorrosion behaviors of the Ni–P and Ni–P–PTFE coatings were firstly assessed by measurements of potentiodynamic polarization curves of the coated samples in sterilized and unsterilized seawater for an period time of 1–28 days.
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Fig. 6. EIS studies of the electroless deposited Ni–P–PTFE (0.2) composite coating measured under immersion in sterilized (A, C, E) and unsterilized (B, D, F) seawater for various days: (A, B) Nyquist diagrams; (C, D) Bode modulus diagrams; (E, F) Bode phase angle diagrams.
Here the Ni–P and Ni–P–PTFE (10) were selected for measurements. The derived polarization curves are shown in Fig. 3A–D. In order to determine the corrosion current density (icorr ) and corrosion potential (Ecorr ), the well-known Tafel equation was employed [29]: E = a + b log i
(1)
where E and i were polarization potential and current density and b the Tafel slop. By fitting the linear segments of the curves (potential ranges of ±(80–140) mV away from the peaks in Fig. 3A–D) with Eq. (1), the icorr and Ecorr as well as bc (cathodic Tafel slop) and ba (anodic Tafel slop) were obtained and shown in Tables 3 and 4 for the Ni–P and Ni–P–PTFE (10), respectively. The plots of icorr and Ecorr against immersion time were shown in Fig. 3E and F. As seen from Fig. 3A–D, all the polarization curves were essentially comparable to each other. This suggested similar polarization mechanism for the two coatings in sterilized and unsterilized seawater. The variations of the curve positions in Fig. 3A–D, however, were evidently observed. Compared to those immersed in sterilized seawater, the
derived icorr for the Ni–P and Ni–P–PTFE (10) coatings immersed in unsterilized seawater was rather low. Accordingly, the Ecorr for the coatings in unsterilized seawater was more positive than those in sterilized seawater. Generally, a material with a lower icorr and a higher Ecorr has a lower tendency to be oxidized and corroded and therefore higher anticorrosion performance [30]. The results of icorr and Ecorr for the two coatings in sterilized and unsterilized seawater suggested that not MIC damage but MIC inhibition has occurred in our case, as has been observed by some investigators in the literature [31,32]. On the other hand, the polarization behavior of the Ni–P was visibly different from that of the Ni–P–PTFE. With immersion days of 1–28, the Ni–P coating showed notably lower icorr and higher Ecorr than the Ni–P–PTFE (10) composite coating both in sterilized and unsterilized seawater (see Fig. 3E and F). Such results can be well understood by considering the microstructural morphologies of the two coatings, where the Ni–P had smoother and denser surface and consequently better anticorrosion performance than the Ni–P–PTFE (10).
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Fig. 7. EIS studies of the electroless deposited Ni–P–PTFE (10) composite coating measured under immersion in sterilized (A, C, E) and unsterilized (B, D, F) seawater for various days: (A, B) Nyquist diagrams; (C, D) Bode modulus diagrams; (E, F) Bode phase angle diagrams.
3.3. OCP and EIS investigations
Fig. 8. Proposed equivalent circuit model. Rs : solution resistance; CPEdl : double layer capacitance; Rct : charge transfer resistance; Zw : Warburg resistance; L: inductance.
In this section the OCP and EIS investigations were conducted with the Ni–P and Ni–P–PTFE coated samples in sterilized and unsterilized seawater. The measured EOCP was shown in Fig. 4. The EOCP varied visibly with the three coatings. The maximum EOCP was roughly achieved with 3–7 days immersion followed by a declining tendency. In particular, the Ni–P–PTFE (10) had a lower EOCP than the Ni–P and Ni–P–PTFE (0.2). Concerning the effect MIC on EOCP , visible variation of EOCP was not observed. The EIS investigations of the Ni–P coating were conducted at OCP and the results were shown in Fig. 5. The plots of the Nyquist spectra in Fig. 5A and B gave typical semi-circles. The diameters of these semi-circles clearly varied with the immersion days. The largest semi-circle diameters of ∼100 and ∼40 k cm2 were
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Fig. 9. Plots of resistances of Rct and Zw against immersion time for the Ni–P and Ni–P–PTFE coatings in sterilized and unsterilized seawater. The coatings were electroless deposited on carbon steel surface with various concentrations of PTFE emulsion in the plating bath.
achieved for the coated sample with 3 days immersion in sterilized and unsterilized seawater, respectively. The derived curves in the Bode modulus diagrams in Fig. 5C and D agreed well with these results. The largest impendence was therefore achieved for the Ni–P coating with 3 days immersion. The curves in Fig. 5E and F acted essentially in a comparable way, demonstrating similar anticorrosion mechanism of the coating in this study. Concerning the MIC effect, the diameters of semi-circles in Fig. 5B were evidently smaller than those shown in Fig. 5A, a sign of substantial drop of the impedance due to the ocean MIC. The EIS results for the Ni–P–PTFE (0.2) were similar to those of the Ni–P, as shown in Fig. 6. With 5 days immersion in sterilized seawater and 1 day immersion in unsterilized seawater the largest semicircle diameters of ∼25 and ∼30 k cm2 were achieved. In the case of the Ni–P–PTFE (10), the largest semi-circle diameters as small as ∼4 and ∼7 k cm2 were obtained after immersion of 1 day in sterilized seawater and 7 days in unsterilized seawater (Fig. 7). The above EIS results demonstrated that the incorporation of the PTFE nano-particles and its amount had an essential effect on the impedance of the Ni–P matrix. The anticorrosion mechanism of the coating, however, did not varied with the PTFE incorporation. In order to well understand the EIS results given above, the EIS data were interpreted on the basis of equivalent electrical analogs. Several equivalent circuit models were tried and the optimal one was shown in Fig. 8. With this model the derived parameters of resistive impedances of Rct (charge transfer resistance) and Zw (Warburg resistance) were plotted against immersion time in Fig. 9. In the case of the capacitive impedance, the below equation was
Fig. 10. Plots of capacitance parameters of V and ˛ of the double layer (see Eq. (2)) against immersion time for the Ni–P and Ni–P–PTFE coatings in sterilized and unsterilized seawater. The coatings were electroless deposited on carbon steel surface with various concentrations of PTFE emulsion in the plating bath.
applied for a CPEdl (double layer capacitance) element [33] ZCPE =
1 · ω0 · V
jω −˛ ω0
(2)
where ZCPE was the capacitive impedance of the double layer formed on the coating surface, ω0 the normalizaiton factor (setted to be 2·1000 s−1 ), ω the circular frequency, V in a Farad dimension, and ˛ dimensionless parameter in a range of 0–1 describing deviation of the element from behavior of an ideal capacitor (˛ = 0, Ohmic impedance with phase angle of 0◦ ; ˛ = 1, idea capacitor with an absolute phase angle of 90◦ ). With Eq. (2) the derived parameter of V and ˛ were shown in Fig. 10. As seen from Figs. 9 and 10, maximum values of Rct and Zw and minimum values of V were reached for the three coatings after 3–7 days immersion both in sterilized and unsterilized seawater. Note that low value of V was corresponding to high capacitive impedance. Thus, the above results demonstrated that with 3–7 days immersion the largest impedance has been accomplished for the three coatings. With regard to the effect of the PTFE amount in the matrix, the values of Rct and Zw and V for the Ni–P were comparable to those of the Ni–P–PTFE (0.2). In the case of the Ni–P–PTFE (10), the Rct and Zw were rather small while the V was qutie large. Concerning the parameter of ˛, the Ni–P–PTFE (10) possessed low value and therefore large deviation of the element from an ideal capacitor, compared to the Ni–P and Ni–P–PTFE (0.2). Note that the ˛ could also be visually understood from the modulus and phase angle shown in Figs. 5–7. With lower ˛ value the modulus slop in graph C and graph D and maximum
J. Tian et al. / Materials Chemistry and Physics 124 (2010) 751–759
phase angle in graph E and graph F in Figs. 5–7 were consequently lower. The differences of EIS behaviors between the Ni–P, Ni–P–PTFE (0.2), and Ni–P–PTFE (10) can be well understood by considering the microstructural morphologies of the three coatings. With an incorporation of large amount of PTFE a rough and porous microstructure was observed for the Ni–P–PTFE (10) (see Fig. 2D). With such porous microstructure the coating could be easily penetrated by the corrosive seawater and therefore poor electrochemical anticorrosion behavior. In the cases of the Ni–P and Ni–P–PTFE (0.2) the coatings were rather smooth and dense (see Fig. 2B and C) and hence good prevention from seawater penetration. It should be pointed out that, as reviewed previously in Section 1 [25,26], with small amount of PTFE nano-particles incorporated the Ni–P–PTFE should reduce biofouling to some extent and therefore influences on the electrochemical parameters. Concerning this point it was quite difficult to draw a decisive conclusion at present in our study though noticeable differences have been definitely observed (compare the curves obtained in sterilized and unsterilized seawater in Figs. 3–10). This could most likely be due to the fact that the unsterilized natural seawater was directly scooped from Qingdao offshore and the amount of the ocean microorganisms would be rather small. Thus, the cultivation of the ocean microorganisms and its breeding behavior on the Ni–P–PTFE composite coating with small amount of PTFE nano-particles incorporated were quite essential. The research related to the above content will be conducted in our future work. 4. Conclusion The Ni–P and Ni–P–PTFE coatings were deposited on mild carbon steel surface via electroless deposition process and their electrochemical behaviors in sterilized and unsterilized seawater were assessed. The as-deposited coatings were in an amorphous state with an atomic ratio of Ni to P of 4.0–5.1. The Ni–P and Ni–P–PTFE (0.2) possessed quite smooth and dense surfaces with some few nano-voids while in the case of the Ni–P–PTFE (10) it was rather rough and porous. The electrochemical investigations showed that by immersion in natural seawater for 3–7 days the Ni–P and Ni–P–PTFE coatings reached maximum protection for the substrate. With regard to the incorporation of PTFE nano-particles, both the polarization and EIS results as well as the measured open circuit potential revealed a poor electrochemical anticorrosion behavior for the Ni–P–PTFE (10) composite coating that has been attributed to its porous microstructure. Concerning the effect of the ocean microorganisms although visible variations of electrochemical parameters have been observed between the coated samples immersed in sterilized and unsterilized seawater, definite
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conclusion could not be drawn at present since the amount of the ocean microorganisms was not large enough. Further investigation on the cultivation of the ocean microorganisms and their breeding on the sample with small amount of PTFE were yet to conduct in our future work. Acknowledgements This work was financially supported by Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP, 200804231004) and Shandong Province Middle-aged and Young Scientists Research Incentive Fund (BS2009CL016). References [1] R. Elansezhian, B. Ramamoorthy, P. Kesavan Nair, J. Mater. Process. Technol. 209 (2009) 233. [2] P. Peeters, G. Hoorn, T. Daenen, A. Kurowski, G. Staikov, Electrochim. Acta 47 (2001) 161. [3] Q. Zhao, Y. Liu, Corros. Sci. 47 (2005) 2807. [4] G. Straffelini, D. Colombo, A. Molinari, Wear 236 (1999) 179. [5] S. Zhang, K. Han, L. Cheng, Surf. Coat. Technol. 202 (2008) 2807. [6] Z.A. Hamid, S.A. Badry, A.A. Aal, Surf. Coat. Technol. 201 (2007) 5948. [7] Y. Hazan, D. Werner, M. Zgraggen, M. Groteklaes, T. Graule, J. Colloid Interface Sci. 328 (2008) 103. [8] S.M.A. Shibli, B. Jabeera, R.I. Anupama, Surf. Coat. Technol. 200 (2006) 3903. [9] S.M.A. Shibli, V.S. Dilimon, Int. J. Hydrogen Energy 32 (2007) 1694. [10] J. Novakovic, P. Vassiliou, Kl. Samara, Th. Argyropoulos, Surf. Coat. Technol. 201 (2006) 895. [11] X.Y. Yuan, T. Xie, G.S. Wu, Y. Lin, G.W. Meng, L.D. Zhang, Physica E: Low-dimens. Syst. Nanostruct. 23 (2004) 75. [12] L. Zhang, Y. Jin, B. Peng, Y. Zhang, X. Wang, Q. Yang, J. Yu, Appl. Surf. Sci. 255 (2008) 1686. [13] Y.T. Wu, L. Lei, B. Shen, W.B. Hu, Surf. Coat. Technol. 201 (2006) 441. [14] Q. Zhao, Y. Liu, Appl. Surf. Sci. 229 (2004) 56. [15] Q. Zhao, Y. Liu, H. Muller-Steinhagen, G. Liu, Surf. Coat. Technol. 155 (2002) 279. [16] M.D. Ger, K.H. Hou, L.M. Wang, B.J. Hwang, Mater. Chem. Phys. 77 (2002) 755. [17] Y. Wu, H. Liu, B. Shen, L. Liu, W. Hu, Tribol. Int. 39 (2006) 553. [18] M. Crobu, A. Scorciapino, B. Elsener, A. Rossi, Electrochim. Acta 53 (2008) 3364. [19] C. Gu, J. Lian, G. Li, L. Niu, Z. Jiang, Surf. Coat. Technol. 197 (2005) 61. [20] A. Bai, P. Chuang, C. Hu, Mater. Chem. Phys. 82 (2003) 93. [21] Q. Zhao, Y. Liu, Surf. Coat. Technol. 200 (2005) 2510. [22] Q. Zhao, Y. Liu, E.W. Abel, Mater. Chem. Phys. 87 (2004) 332. [23] R. Xu, Z. Guo, J. Pan, Trans. Nonferrous Met. Soc. China 16 (2006) 666. [24] S. Takashi, Y. Shuji, K. Yuichi, Corrosion preventing structure, EP0737759 (1996). [25] Q. Zhao, Surf. Coat. Technol. 185 (2004) 199. [26] Q. Zhao, Y. Liu, J. Food Eng. 72 (2006) 266. [27] S. Al-Fozan, A. Malik, Desalination 228 (2008) 61. [28] D. Örnek, A. Jayaraman, T. Wood, Z. Sun, C. Hsu, F. Mansfeld, Corros. Sci. 43 (2001) 2121. [29] http://www.zahner.de/pdf/IE.pdf, p. 27. [30] D.A. Jones, Principles and Prevention of Corrosion, Prentice Hall Inc./Simon & Schuster A Viacom Company, Upper Saddle River, NJ, 1996, p. 32. [31] A. Pedersen, M. Hermansson, Biofouling 1 (1989) 313. [32] M. Eashwar, S. Maruthamuthu, S. Sathyanarayanan, K. Balakrishnan, Proc. 12th Int. Corros. Cong., vol. 5b, NACE, Houston, TX, 1993, p. 3708. [33] http://www.zahner.de/pdf/SIM.pdf, p. 27.