Characterization of Ni–P–SiO2 nano-composite coating on magnesium

Applied Surface Science 324 (2015) 393–398

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Characterization of Ni–P–SiO2 nano-composite coating on magnesium S. Sadreddini a,∗ , Z. Salehi b , H. Rassaie a a b

Department of Materials Science and Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran Department of Chemical Science and Technology, Tor Vergata University, Rome, Italy

a r t i c l e

i n f o

Article history: Received 7 August 2014 Received in revised form 25 October 2014 Accepted 25 October 2014 Available online 4 November 2014 Keywords: Magnesium FESEM XRD Polarization EIS

a b s t r a c t In this study, the effects of SiO2 nanoparticles added to the electroless Ni–P coating were studied. The surface morphology, corrosion behavior, hardness and porosity of Ni–P–SiO2 composite were investigated. The related microstructure was investigated through field emission scanning electron microscopy (FESEM) and the amount of SiO2 was examined by Energy Dispersive Analysis of X-ray (EDX). The corrosion behavior was evaluated through electrochemical impedance spectroscopy (EIS) and polarization techniques. The results illustrated that with increasing the quantity of the SiO2 nanoparticles, the corrosion rate decreased and the hardness increased. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The need of weight reduction, particularly in portable microelectronics, telecommunication, aerospace and automobile sectors, have stimulated engineers to being more creative in their choice of materials [1]. Magnesium is the eighth most abundant element on the earth which is a fairly strong and silvery-white metal that is lighter than aluminum [2]. However, the major disadvantages are: very poor corrosion, hardness and wear resistance, which often significantly restrict its application [3]. One of suitable method to obtain wear and corrosion resistant is coatings on alloys [4]. Wide range of methods and types of coatings has been proposed, such as physical vapor deposition (PVD coatings) [5] or various plasma techniques [6]. Another suitable method is electroless nickel plating [7]. Electroless Ni–P coatings are widely used in chemical, mechanical and electronic industries. It has been thoroughly described and many methods were designed to coat large scale metallic materials [8]. Apart from other excellent properties, including wear and corrosion resistance or high hardness [9], the significant advantage of electroless coatings is that they are suitable even for plating of the complex-shaped components [10]. On the other hand, it is known that the incorporation of particles onto the Ni–P matrix improves surface properties and corrosion resistance, depending on the nature of the incorporated particle

∗ Corresponding author. Tel.: +98 9350717653; fax: +98 3518271002. E-mail address: [email protected] (S. Sadreddini). http://dx.doi.org/10.1016/j.apsusc.2014.10.144 0169-4332/© 2014 Elsevier B.V. All rights reserved.

[11]. In the field of tribology, according to the types of the doped inorganic and/or organic particulates, electroless Ni–P based composite coatings can be divided into two categories, i.e., lubricating composite coatings and wear-resistant composite coatings [12]. The wear-resistant composite coatings usually have co-deposited hard particles such as WC, SiC, Al2 O3 , B4 C, and they usually have increased hardness and wear resistance as compared with electroless Ni–P coating [13–16]. SiO2 nanoparticles embedded in the composite coating are used to elevate the toughness, hardness and wear resistance and improve the lubricant effect, resulting in decreasing the friction coefficient [17]. In this study, SiO2 nanoparticles were added to Ni–P bath as a second phase and Ni–P–SiO2 nano-composite was coated on AZ91HP magnesium alloy. Furthermore, the influence of different concentrations of nanoparticles existing in the bath on the thickness, hardness and corrosion resistance of the coating was investigated. 2. Experimental The AZ91HP magnesium alloy specimens with chemical composition which is shown in Table 1, and dimensions of 15 × 15 × 2 mm3 was used as a substrate for the composite coating. The surface of the samples was polished with a series of emery papers up to #1500 and after grinding; they were degreased in acetone for 15 min (in ultrasonic bath). Two-step of pretreatment consisting of successive etching in a chromium etching solution, including 125 g/l CrO3 , 100 ml/l HNO3 (60 s) and 350 ml/l HF (aq.) fluoride solution (10 min), were applied [18], and after each step,

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Table 1 The chemical composition of AZ91HP magnesium used as the substrate. Element

Mg

Si

Fe

Cu

Mn

Al

Ni

Zn

wt%

Base

0.05

0.01

0.02

0.23

8.49

0.005

0.82

Table 2 The composition of Ni–P–SiO2 composite bath. Chemical composition

Concentration

Nickel sulfate Sodium hypophosphite Sodium acetate Lactic acid Thiourea Nano-SiO2

24 g/l 27 g/l 35 g/l 28 ml 0.001 g/l 5–15 g/l

all samples were washed with distilled water. The chemical composition of electroless bath is tabulated in Table 2. Different concentrations of amorphous spherical SiO2 nanoparticles with the size range of 15–20 nm and the purity of 99.5% were added to the plating bath. Then, the pH was adjusted by ammoniac solution and the final volume of the bath was extended to 250 ml by distilled water. Afterwards, the solution was mixed by ultrasonic mixer and magnet with speed of 500 rpm for 40 min and 2 h, respectively. The final electroless process was performed in a solution with a pH of 4.6 ± 0.1 at 90 ± 1 ◦ C for 90 min under mixing speed of 300 rpm. The morphology and composition of coating were evaluated by field emission scanning electron microscopy (FESEM, Hitachi S-4160) and energy dispersive X-ray spectroscopy (EDX, Philips XL30) at a voltage of 20 V, respectively. The atomic mass and fractions of nickel, phosphorus and silicon were examined as well. The crystalline structure of coating was characterized by X-ray diffractometer (XRD, STOE STADI MP). The XRD spectra were obtained from Cu K␣ radiation within a 2 range from 10◦ to 90◦ and ˚  = 1.54 A. The corrosion behavior of the coatings was evaluated in room temperature using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The set up was consisting of a saturated calomel reference electrode (SCE), a platinum auxiliary electrode and a working electrode (sample). The polarization curve of coating was obtained by means of a EG&G 273A apparatus with scanning rate of 0.5 mV/s in the range of −0.5 to 0.5 V in respect to OCP and then, the results were assessed by Corrview software. Moreover, the Nyquist curve of the coating was evaluated using an EG&G 1025A within frequency range of 10 kHz to 0.01 Hz. The charge transfer resistance (Rct ) and double layer capacity (Cdl ) was evaluated by Zview software. Micro-hardness of the samples was measured at a load of 10 g and a dwell time of 10 s (Struers Duramin Model micro-hardness Tester) and it was averaged randomly at 7 different points. 3. Results 3.1. Surface morphology The FESEM images of coating including SiO2 (12.5 g/l) nanoparticles are shown with different magnifications (2000, 3000, 30,000 and 60,000) in Fig. 1, respectively. As it could be observed from these images, there is a uniform distribution of nanoparticles in the coating area and the bubble structure was formed for the Ni particles. Fig 1(d) shows that as the concentration of nanoparticles reached the desire amount the grains size decrease to their minimum value.

Table 3 Variation in thickness as well as P and SiO2 amounts of the coating relative to theSiO2 concentration of Ni–P–SiO2 plating bath. SiO2 concentration in bath (g/l)

SiO2 in coating (wt%)

P in coating (wt%)

Coating thickness (␮m)

Microhardness (VH)

0 7.5 12.5 17.5

0 2.74 4.62 3.97

4.62 4.21 4.55 4.39

20 18.9 25.3 24.5

342 386 429 403

According to Fig. 2, when the concentration of nanoparticles reached to 17.5 g/l, the agglomeration of nanoparticles could be revealed and more agglomeration leads to higher porosity in the coating surface, which is undesired. From Fig. 2(b) clearly it could be observed that the size of the grains is slightly bigger around the agglomerated nanoparticles. Fig. 3 shows cross sectional view of FESEM images coated with SiO2 (12.5 g/l) nanoparticles with different magnification (coating thickness = 25.3 ␮m). Nanoparticles in coating could block the dislocation movements and consequently micro-hardness will increase. On the other hand, the grain refinement is a significant result of uniformity distribution of nanoparticles in the coating which could be discerned by new growth zones [19]. Fig. 4 shows the X-ray diffraction patterns of Ni–P–SiO2 nanocomposite coating with a certain concentration of SiO2 (12.5 g/l) in the bath. The peaks of SiO2 and Ni clearly could be observed. The broad peak at 2 equal to 45◦ is due to amorphous structure. 3.2. The elemental composition of composite coating Table 3 represents the related alternation between the amount of SiO2 and P in the coating andSiO2 concentration in the plating bath. As it could be observed, by increasing SiO2 nanoparticles in the bath, the amount of these particles increased in the coating. Increasing the SiO2 nanoparticles in the bath up to 12.5 g/l, enhanced the amount of coating particles to 4.62 wt%, which is the maximum amount of SiO2 nanoparticles in the coating. On the other hand, by increasing the SiO2 bath concentration into 17.5 g/l, the amount of coating particles decreased to 3.97 wt%. Therefore, it is shows that, at higher concentrations of 12.5 g/l, by the increment of nanoparticles, due to their agglomeration and increasing of solution viscosity, the deposition rate and the amount of coating SiO2 will decrease [19]. In addition, there is a relation between the amount of SiO2 in the bath and phosphorus concentration. It means that increasing SiO2 bath concentration to 12.5 g/l, increases the amount of phosphorus in coating linearly up to 4.55 wt%, but more than this amount causes decreasing the phosphorus weight percentages. Table 3 shows the coating thickness of nano-composite coating in terms of the SiO2 concentration during 90 min plating. At higher concentration of SiO2 nanoparticles, particles have enough opportunity to be entrapped into coating [20] as well as particles start to be agglomerated and higher solution viscosity and lower deposition rate are the results. 3.3. Electrochemical corrosion performance of Ni–P/nano SiO2 composite coating Fig. 5 shows polarization potentiodynamic curves of Ni–P–SiO2 nano-composite coating containing different amounts of SiO2 nanoparticles on magnesium substrate and Table 4 represents the results obtained from the drawing of linear Tafel curves. Surface is protected by sacrificing properties of coating formed on it. The shift in the corrosion potential of Ni–P–SiO2 coating on

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Fig. 1. FESEM micrograph of sample coated with Ni–P–SiO2 nano-composite at 12.5 g/l SiO2 at four different magnifications; (a) 2 KX (b) 3 KX (c) 30 KX (d) 60 KX.

Fig. 2. FESEM micrograph of Ni–P–SiO2 nano-composite coating produced at 17.5 g/l SiO2 at different magnifications.

Table 4 The influence of changes in the SiO2 content of the coating on polarization results of nano-composite Ni–P–SiO2 coating. Type

SiO2 in bath concentration (g/l)

SiO2 in the coating (wt%)

ˇa (mv/decade)

ˇc (mv/decade)

Icorr (␮A/cm2 )

Ecorr (V vs. SCE)

Substrate without coating Substrate with coating

– 0 7.5 12.5 17.5

– 0 2.74 4.62 3.97

28 47 44 45 46

128 64 53 54 56

20.6 4.0 2.2 1.3 1.9

−0.79 −0.44 −0.40 −0.29 −0.36

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Fig. 3. Coating cross section of Ni–P–SiO2 coatings plated from bath at 12.5 g/l with different magnifications; (a) 10 KX (b) 20 KX.

Fig. 4. XRD pattern of Ni–P–SiO2 nano-composite coating produced at 12.5 g/l SiO2 .

magnesium to more positive potentials in respect with substrate clearly can be observed. It is also denote that presence of P as well as other nanoparticles could improve the corrosion behavior [21].

Fig. 6. Variation in the Nyquist curve as a function of the amount of SiO2 in the coating in 3.5 wt% NaCl solution.

Fig. 5. Polarization curves for the deposition of Ni–P–SiO2 at different concentration of SiO2 nanopariclesin the coating in 3.5 wt% NaCl solution.

By the increment of nanoparticles in coating, thickness is increased and iCorr is decreased. By increasing nanoparticles amount in coating to 4.62 wt%, not only the coated thickness is enhanced, but coating corrosion resistance could reach its minimum amount (1.3 ␮A/cm2 ). When the concentration of the nanoparticles reached to 17.5 g/l, formed agglomeration and higher solution viscosity, decreasing the amount of both phosphorus and SiO2 in the coating and as a result, the corrosion current increased. On the other hand, better corrosion resistance could be obtained expanding prevention of corrosion cavity with deposition of nanoparticles in the coating [22]. Fig. 6 shows the Nyquist plots of Ni–P–SiO2 nano-composite coating on magnesium substrate by electroless method and it illustrated that changing of SiO2 amount in the coating could influence the Nyquist plot. All curves have been drawn in singular semicircular and in frequency range of 10 kHz to 0.01 Hz which represents charge control reaction and corrosion process of these coatings in a constant time ( = cdl Rct ) [23]. Although, these curves are similar in shape, they are different in terms of size. The difference in the area under curve shows their different corrosion resistance.

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Fig. 9. The equivalent electric circuit Ni–P–SiO2 coating [22]. Table 5 CPEdl and Rct values of Nyquist plots per 1 cm2 of Ni–P–SiO2 coating.

Fig. 7. Bode plots obtained for various compositions of Ni–P–SiO2 deposits.

As SiO2 content of the coating increased up to 4.62 wt%, the area under curve and consequently, coating resistance in 3.5 wt% NaCl solution increases. When nanoparticles concentration increases up to 17.5 g/l, the nanoparticles content of the coating reduces to 3.97 wt% and thus the area under curve decreases. Changing in Ni–P–SiO2 composition have affected on both Bode plots and plots of  (phase angle) vs. log f. It is exhibited in Figs. 7 and 8 respectively, and as can be seen, Bode plot shows a broad (phase angle vs. frequency) peak at analyzed frequency range and defined the presence of two time constants behavior. Two obtained relaxation process in the desire frequency range from impedance spectra specified that the first relaxation process can be checked at the higher and intermediate frequency range and the second relaxation revealed at lower frequency range [24]. The semicircle at the higher frequency is probably related to the Ni–P–SiO2 nano-composite layer, while polarization semicircle can be observable at a lower frequency where the interface of the substrate/coating is appeared. The equivalent electric circuit of this coating is shown in Fig. 9 and it includes: solution resistance (Rs ), double layer capacity (CPEdl ), coating capacity (CPEcoat ), coating resistance (Rcoat ) and charge transfer resistance (Rct ), which demonstrates kinetic corrosion parameters. This circuit is applied to simulate solution/metal interface as well as Nyquist curve analysis [23]. The values of Rct and CPEdl are tabled in Table 5. As it is clear, the Equivalent

Fig. 8. ␪ vs log f plots for various compositions of Ni–P–SiO2 deposits.

SiO2 bath concentration (g/l)

SiO2 in coating (wt%)

n

CPEdl (␮F cm−2 S−n )

Rct ( cm2 )

0 7.5 12.5 17.5

0 2.74 4.62 3.97

0.78 0.83 0.89 0.85

26.92 24.53 17.12 18.76

4124 4256 5072 4695

circuit obtained for these deposits shows two time constants (two capacitive responses). Increasing the charge transfer resistance to its maximum value (5072  cm2 ), is a function of SiO2 content of the coating, thus, the optimized value for this parameter characterized and it is 4.62 wt%. This approves that, as the SiO2 content of the coating increases, the coating resistance against corrosion circumstance of 3.5 wt% NaCl also improves. The n values for the nano-composite coatings lie in between 0.83 and 0.89, indicating that the surface of the coatings does not exhibit an ideal capacitive behavior. CPE accounts for the deviation from ideal capacitive behavior and is related to surface inhomogeneities which directly correlated to the roughness and surface porosity of the coating. According to CPEdl values, from Table 5, it can be concluded that with increasing nanoparticles, porosity declined to the extent that the CPEdl of lowest porosity coating of nano-composite coating containing 12.5 g/l nanoparticles reaches to 17.12 microfarad per each square centimeter and a denser coating will deposit on magnesium [25]. 3.4. Micro-hardness Fig. 10 and Table 3 show the variations of micro-hardness values of Ni–P–SiO2 coating with SiO2 particles concentration. It is

Fig. 10. Variation in microhardness as a function of SiO2 nanoparticles concentration in the bath.

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evident that, the hardness increases with increasing the SiO2 particles concentration and the grain refinement of the matrix. The maximum value for the peak could be observed at 429 VH which is caused by the dispersive amplification and retentive the mobility of dislocations of the matrix [26]. As mentioned before, with higher amount of SiO2 nanoparticles and consequently higher agglomeration of particles which itself caused by specific surface area, the movement of nickel ions toward the cathode surface could be effected and as a result, decreasing in the deposition rate could be dominant [19], Thus, the lower amount of SiO2 nanoparticles, result in lower dispersion-hardening effect and less hardness of the coating. It is worth noting that enhancement of hardness from 78 to 429 VH could be reach by increasing the incorporation of SiO2 nanoparticles with nickel. 4. Conclusion • The enhancement of SiO2 nanoparticles in the bath up to 12.5 g/l, increased the amount of coating particles to 4.62 wt%, which is the maximum amount of SiO2 nanoparticles in the coating. In higher concentrations than 12.5 g/l, the increment of nanoparticles, due to the agglomeration of particles and increasing of solution viscosity, will decrease the deposition rate and the amount of coating SiO2 concentration. Also, the Size of the grains is slightly bigger around the agglomerated nanoparticles. • Better corrosion resistance and lower porosity could be obtained by expansion prevention of corrosion cavity with deposition of nanoparticles in the coating. • The grain refinement is a significant result of uniformity distribution of nanoparticles in the coating which could be discerned by new growth zones. • Micro-hardness increases with the increasing concentration of SiO2 particles and the grain refinement of the matrix. The maximum value for the peak is observed at 429 VH, which is caused by the dispersive amplification and retentive mobility of matrix dislocations. Acknowledgements This work was financially supported by Islamic Azad University (Tehran, Iran). Mr. Barkhordary, the director of material production and processing laboratory also is gratefully acknowledged. References [1] X. Wan, Y. Sun, F. Xue, J. Bai, W. Tao, Effects of Sr and Ca on the microstructure and properties of Mg–12Zn–4Al–0.3Mn alloy, Mater. Sci. Eng. A 508 (2009) 50–58. [2] A. Tharumarajah, P. Koltun, Is there an environmental advantage of using magnesium, components for light-weighting cars, J. Clean. Prod. 15 (2007) 1007–1013.

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