Corrosion and tribological behaviour of electroless Ni–P/nano-SiC composite coating on aluminium 6061

Abstract

Ni–P/nano-SiC composite coating was successfully deposited on 6061 aluminium alloy by electroless coating process. Scanning electron microscopy equipped with energy dispersive spectroscopy as well as Vickers hardness tester was utilised to examine the effect of nano-SiC particle incorporation on the morphological and hardness properties. Pin on disc method was applied to investigate the tribological properties of the coatings. Moreover, the corrosion behaviour of coatings for the protection of 6061 aluminium alloy in 3·5 wt-%NaCl solutions was studied using potentiodynamic polarisation and electrochemical impedance spectroscopy. Results showed that incorporation of silicon carbide nanoparticles in the coatings enhances the corrosion and tribological properties of the composite coatings.

Keywords

SiC nanoparticles Electroless Tribological properties Corrosion behaviour

&SetFont Typeface="3" Bold="1" Italic="1";This paper is part of a special issue on Electrodeposition

Introduction

Al6061 is among the most commonly used aluminium alloys in the automotive and aerospace industries considering its unique properties, such as low density, good formability and high specific strength.^1,2 However, despite the excellent performance of aluminium in anticorrosion conditions, there are still some problems for the protection of aluminium in corrosive and abrasive environments.² According to the literature, in corroding environments such as saline solution or polluted air containing carbon dioxide (CO₂), thin porous natural oxide might be formed on the surface of aluminium and cause localisation of corrosion reactions in these sites.^3,4 Thus, to deal with surface treatment as a serious action, it is required to tailor aluminium. Recently, surface treatment technology has received a great deal of attention for improving the mechanical and electrochemical properties of alloys.⁵ Different methods, for example electroless coatings, are applied to improve the tribological and corrosion behaviours of aluminium alloys.⁵ Because of its excellent corrosion and wear resistance properties, electroless nickel coating has been broadly applied as an effective surface treatment method in a variety of environments. Through this technique, autocatalytic metal reduction occurs in the bath with no external current applied.⁶ Because of their unique properties, such as uniform thickness, high corrosion and abrasion resistance, non-magnetism and improved microhardness, electroless coatings have been widely used in many industrial applications for more than three decades.⁷ The mechanical and electrochemical properties of coatings can be further improved by the addition of secondary particles, such as the embedding SiC (Ref. 8), SiO₂ (Refs. 3 and 9), TiO₂ (Ref. 10),¹⁰ Al₂O₃ (Refs. 11–13), Si₃N₄ (Ref. 14) and PTFE (Ref. 15), within the metal matrix and into the coating to prepare the composite coating. A literature review of numerous publications on the tribological and corrosion behaviour of Ni–P composite coatings embedded with microsized particles indicates that these coatings are not able to satisfy the current high technology's necessities. Therefore, in the recent decade, use of nanoparticles for enhancing mechanical, electrochemical and optical properties has received higher attention.^16–20

The superior properties of SiC nanoparticles, such as economical cost, unique chemical stability, excellent abrasion, erosion strength and adequate mechanical properties,¹⁸ have led to the use of SiC as an apposite abrasive material in grinding wheels and abrasive products, refractories, ceramics and manufacturing composite materials.^21–23 Nevertheless, by now, few investigations have been reported on electroless Ni–P coating containing nanoparticles on the aluminium substrate. The present investigation aims to study the effects of SiC nanoparticle incorporation on the hardness, tribological and anticorrosion behaviour of Ni–P–SiC nanocomposite coating on aluminium 6061 alloy.

Experimental

Aluminium 6061 alloy is used extensively as a construction material, most commonly in the manufacture of marine structures and automotive components.²⁴ The disc form specimens of this alloy with 20 mm diameter and 0·5 mm thickness were used in this work as substrates. Before applying the coatings, the aluminium substrates were pretreated according to the following procedure:

(i)

Grounding the bare samples using emery papers starting from 120 to 3000 grit size

(ii)

Polishing the samples with 0·3 mm Al₂O₃ powder

(iii)

Rinsing the polished samples with deionised water

(iv)

Dipping the samples in a mixture of nitric acid (HNO₃) and hydrofluoric acid (HF) (volumetric1∶3) for 10 s;

(v)

Zincating pretreatment by immersion of the samples in stirring zincate bath for 30 s; Zincating pretreatment is carried out in order to activate the surface of aluminium for electrochemical reaction between the working surface and the electroless solution.¹ Zincate solution composition is shown in Table 1

(vi)

Stripping the first zincate layer by soaking in HNO₃ solution (5%) for 10 s

(vii)

Second zincating treatment of the samples in the aforementioned zincate solution for 30 s and cleaning the surface by implanting the samples into deionised water.

Table 1

Zincate treatment bath composition

Chemical composition	Concentration/g L⁻¹
NaOH	106
Ni(SO₄)₂	30
CuSO₄	5
KHC₄H₄O₆	40
FeCl₂	2
ZnSO₄	40
KCN	10

After this procedure, the pretreated samples were immersed in an electroless bath. For this purpose, commercial SH490L5 Ni–P electroless bath (Scholter, Germany) containing 7 g L⁻¹ nickel and 30 g L⁻¹ hypophosphite ion (H₂PO₂−) was used. Bath conditions and compositions for the fabrication of the Ni–P–SiC electroless composite coating are shown in Table 2. The powder used in this study was 99·8%SiC powder with particle size of 40 nm. The immersion of specimens in the electroless bath took 2 h. Composite coatings containing SiC nanoparticles and ordinary Ni–P electroless coatings were successfully deposited on the surface of the aluminium samples. Hardness measurements were carried out using a Vickers microhardness tester (Shimadzu) at a 99 load of 50 g for 10 s. To study the effect of SiC nanoparticle incorporation on the surface roughness of the composite coating and its subsequent effects on the wear performance behaviour, the surface roughness was measured by a stylus surface analyser (TR200), with effective measure length of 3 mm, cutoff length of 0·8 mm, measuring range of 400 μm and speed of 0·25 mm s⁻¹. Three measurements were made on each sample, and the average of the roughness profiles for three measurements (Ra) was recorded for each sample. The tribological properties of the coatings were investigated using the pin on disc method at distance of 200 m under the load and speed of 8 N and 0·05 m s⁻¹ respectively. Morphological study was conducted using scanning electron microscopy (SEM, Philips XL30) equipped with energy dispersive spectroscopy (EDS). The distributions of nanoparticles in the nickel matrix were evaluated by an Olympus optical microscope (Bh2). Moreover, the corrosion behaviour of the coatings was examined at room temperature using potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) by an Autolab instrument model PGstat 302 N. To perform the corrosion tests, 1 cm² of the surface was exposed to 3·5 wt-%NaCl solution in a conventional three-electrode cell. The set-up consists of a saturated Ag/AgCl reference electrode, a platinum counter electrode and a working electrode (sample). To ensure accuracy, each test was repeated three times. The EIS was carried out with an amplitude of 10 mV at (OCP) and frequency range of 10⁵ to 10⁻² Hz. Potentiodynamic polarisation curves were provided at a scan rate of 0·2 mV s⁻¹ from −100 to +100 mV OCP. The spectrum of EIS was analysed in terms of an equivalent circuit using Zview2 software.

Table 2

Bath conditions for fabrication of Ni–P–SiC composite coating

Parameter	Condition
Temperature	88±2°C
pH	4·7
Magnetic agitation	160 rev min⁻¹
Surfactant (CTAB)	50 mg L⁻¹
SiC	15 g L⁻¹

Results and discussions

Characterisation of Ni–P and Ni–P–SiC coatings

The SEM images of Ni–P and Ni–P/nano-SiC composite coatings are shown in Fig. 1a and c. The figure clearly shows that there is an apparent morphological difference between Ni–P and Ni–P/nano-SiC composite coatings. In the presence of SiC nanoparticles, the coating encounters some nodular structures leading to the formation of a rougher surface. This phenomenon is consistent with the literature and might be a reflection of the fact that adsorption of SiC nanoparticles on the surface could play an indispensable role as nucleating centre during the deposition process and contribute to film growth.^25,26 Therefore, it could be deduced that these nanoparticles cause an increase in the roughness of the coating. For a deeper insight into surface analysis, optical micrograph cross-section images of Ni–P and Ni–P/nano-SiC composite coatings (Fig. 1b and d) were taken in this study. As indicated in the figure, the coatings are dense, homogeneous and crack free, with good adhesion to the substrate and thickness of 25 μm. Figure 1e illustrates the EDS map of silicon by blue dots, which could be assigned to the homogeneous distribution of SiC nanoparticles on the Ni–P matrix.

Figure 1

Surface images (SEM), optical micrographs of coatings cross-section and EDS analysis: a, b Ni–P coating; c, d Ni–P/nano-SiC composite coating; e EDS map from dispersion of SiC nanoparticles

Microhardness of coatings

Microhardness is among the most important properties of electroless coatings. The results of Ni–P–SiC and Ni–P coatings and bare aluminium are shown in Fig. 2. It is evident that the microhardness value of the composite coating significantly goes up as a result of introduction and good dispersion of SiC nanoparticles into the coatings. Note that the uniform dispersion of SiC nanoparticles with good adhesion to the Ni–P matrix might result in the secondary barrier role of nanoparticles against propagation of dislocation throughout the Ni–P matrix, which subsequently contributes to the hindered plastic deformation of coating.^4,14

Figure 2

Results of microhardness test of bare metal, electroless Ni–P and Ni–P/nano-SiC coatings

Tribological behaviour of coatings

The wear and friction behaviour of the Ni–P/nano-SiC composite coating vigorously depend on the microstructure of the coating. In this regard, several parameters, such as volume, size and incorporation of precipitated particles, significantly affect the tribological behaviour of electroless coatings.^23,27 Figure 3 illustrates the weight loss vs. sliding distance diagram of Ni–P–SiC and Ni–P coatings and bare aluminium. The weight loss could be calculated according to the following equation 1 where W_r represents wear rate (mm³ Nm⁻¹), Δm is the weight loss (mg), ρ is the density (g cm⁻³) and l and F are the distance (m) and force (N) respectively. The calculation of weight loss based on equation (1) showed that weight loss rate for Ni–P–SiC coating and Ni–P are 4·9×10⁻⁴ and 1·5×10⁻³ respectively. The lower weight loss rate of Ni–P–SiC could be attributed to the incorporation of SiC nanoparticles in the composite coatings. The friction coefficient recorded simultaneously during the wear test clearly reveals the characteristics of the wear process and serves as a measure of the wear resistance. Figure 4 presents the friction coefficient vs. sliding distance diagrams. According to the figure, introduction of SiC nanoparticles results in an increase in the average friction coefficient of the composite coating up to 0·81, with the average friction coefficient of the ordinary Ni–P composite coating being ∼0·65. The rise in friction coefficient might be attributed to the high strength and hardness of SiC nanoparticles and their effect on the morphology of the coating. In this work, the average roughness (Ra) of the Ni–P–SiC and Ni–P coatings was computed as 0·046 and 0·02 μm respectively. Figure 5a–c depicts the SEM images of the worn surface morphologies of the electroless Ni–P and Ni–P–SiC coated samples. A wide wear track could be observed clearly on the worn surface of the Ni–P coating (Fig. 5a–c). Apparently, this surface is mainly composed of linear grooves and partial irregular pits along the sliding direction. Here, the higher magnification reveals that not only scuffing and peeling off occurred but also several microcracks emerged on the worn surface and a serious plastic deformation occurred. These observations indicate that the wear mechanism in the Ni–P coating is featured as severe adhesive and abrasive wear, which are well established for electroless Ni–P sliding against steels.⁵ The worn surface of the Ni–P–SiC composite coating exhibits a different appearance. As demonstrated in Fig. 5d–f, for this coating, there is a narrow and smooth wear track on the worn surface compared to that of the Ni–P coating. A deeper insight to the figure reveals that neither crack and peeling nor plastic deformation occur on the worn surface of the Ni–P–SiC composite coating. Additionally, the wear scars present a polished appearance on the saliency parts of the surface, indicating that the abrasive and adhesive wears are not the predominant mechanisms.

Figure 3

Weight loss vs. sliding distance diagrams of electroless Ni–P and Ni–P/nano-SiC coatings

Figure 4

Friction coefficient vs. sliding distance diagrams of electroless Ni–P and Ni–P/nano-SiC coatings

Figure 5

Worn surface morphologies of a, b, c Ni–P composite coatings and d, e, f Ni–P/nano-SiC composite coatings

Electrochemical corrosion property of coatings

Potentiodynamic polarisation

Potentiodynamic polarisation curves of Ni–P and Ni–P–SiC composite coatings and bare aluminium in 3·5 wt-%NaCl solutions were plotted in Fig. 6. From the plots, it is clear that both cathodic and anodic branches follow the typical Tafel behaviour throughout a wide current and potential range, meaning that Tafel slopes could be calculated accurately. Consequently, the values of anodic Tafel slopes (β_a and β_c) and, above that, the corrosion current densities (i_corr) can be determined from the extrapolation method. The obtained results are shown in Table 3. Through the reaction process, the coating could also interfere in the reaction mechanism, affecting the Tafel slopes. As shown in Table 3, incorporation of SiC nanoparticles decreases and increases the cathodic and anodic Tafel slopes respectively. The reduction in the cathodic Tafel slope of Ni–P–SiC composite coatings can be attributed to the higher rate of cathodic half reaction (most possibly hydrogen evolution) for SiC containing coatings. The low hydrogen overvoltage on SiC nanoparticles could be stated as the main reason of decreasing the cathodic Tafel slope.^28,29 Note that the reduction in cathodic Tafel slope of Ni–P–SiC coatings results in the rise in anodic dissolution of the Ni–P matrix. This behaviour might be attributed to the slow kinetics of the anodic reaction in the presence of SiC nanoparticles. The efficacy of SiC nanoparticles on the corrosion performance of the coating can be investigated from several points of view. The first is that the SiC nanoparticles provide barrier properties that might be connected to their suitable size, which allows their easy infill by the aggressive ions distributed through the film to the interface. Moreover, much higher electrochemical resistance of SiC nanoparticles in comparison with the matrix of Ni–P may be another reason for the enhanced corrosion performance of the coating.^5,28,29

Figure 6

Potentiodynamic polarisation curves of coatings in 3·5 wt-%NaCl solution

Table 3

Electrochemical parameters of Ni–P and Ni–P–SiC coated aluminium obtained using Tafel extrapolation in NaCl solution

Type of coating	E_corr (mV vs. Ag/AgCl)	i_corr/μA cm⁻²	Corrosion rate/mm/year	β_c/mV/decade	β_a/mV/decade
Ni–P	−296·66	1·1	0·01	−343·460	170·74
Ni–P–SiC	−226·51	0·43	0·006	−93·045	180·14

Electrochemical impedance spectroscopy

For further investigation of the Ni–P and Ni–P–SiC corrosion behaviours of composite coatings, the coated samples were examined by EIS. Figure 7 shows the Nyquist plots of the Ni–P and Ni–P–SiC composite coatings. From Fig. 7, it could be concluded the polarisation resistance of the Ni–P coated aluminium is lower than that of the Ni–P–SiC coated aluminium, which is in accordance with the potentiodynamic polarisation results stating that the incorporation of SiC nanoparticles considerably improves the corrosion resistance of the Ni–P–SiC coatings. The fitted results of EIS spectra for Ni–P and Ni–P–SiC composite coatings are listed in Table 4. Moreover, Fig. 8 shows the Bode plots of the EIS evolution of the coatings in 3·5 wt-%NaCl. At first glance, a qualitative comparison among the corrosion resistance of the coatings can be made using a simple visual analysis of the time dependence |Z| and phase angle (Theta) values. Bode phase angle diagrams of Ni–P and Ni–P–SiC coatings show a maximum single phase angle of 90°, indicating a pure capacitive behaviour. Such behaviour implies that these processes involve only a single time constant. On the basis of published data,³⁰ the phase angle (θ) at high frequency provides better understanding of the performance of these kinds of systems. Since it is believed that when current passes through the resistor the phase angle is lower than the phase angle of the capacitor, a drop in the phase angle might correspond to the tendency of AC current to pass through the resistor in the circuit. Consequently, the system indicating higher resistance can be characterised by the higher phase angle. Superior performance of Ni–P–nano-SiC is evident from Fig. 8. The values of phase angle at high frequencies for Ni–P–SiC and Ni–P coatings are 22·77 and 13·08° respectively. At low frequencies, the |Z| values of Ni–P and Ni–P–SiC coatings are 14708·4 and 19863 Ω cm² respectively. Higher phase angle and |Z| values of Ni–P–SiC compared with Ni–P coatings indicate a better anticorrosion property in the corrosive environment.

Figure 7

Nyquist plots of coatings in 3·5 wt-%NaCl solution

Figure 8

Bode plots of Ni–P and Ni–P–SiC coatings in 3·5 wt-%NaCl solution

Table 4

Electrochemical parameters from EIS data of Ni–P and Ni–P–SiC composite coatings in 3·5%NaCl solution

Type of coating	R_s/Ω cm²	R_ct/kΩ cm²	CPE/Ω⁻¹ sⁿ cm⁻²	n _dl	Error/%
Ni–P	118	13	1·9×10⁻⁵	0·85	1·6
Ni–P/nano-SiC	123	21	1·5×10⁻⁵	0·9	1·5

The corresponding equivalent circuits for EIS tests are shown in Fig. 9, where R_s represents the solution resistance, R_ct is the charge transfer resistance and CPE is the constant phase element. The CPE was used instead of ideal capacitors, to which is attached a phenomenological constant n often used to fit the impedance behaviour at the electrode/electrolyte interface, which shows deviation from the real capacitor behaviour. In accordance with Ref. 5, the CPE and n_dl values can be attributed to the porosity and surface inhomogeneity of the coating. It should be noted that incorporation of SiC nanoparticles in the matrix of Ni–P coatings reduces the surface and structural defects by filling the crevices, gaps and micrometre holes of the coating⁵ and causes the increasing n_dl values in the Ni–P–SiC coatings. Thus, incorporating the SiC nanoparticles into the Ni–P matrix would result in the formation of a denser and more homogenous coating.

Figure 9

Equivalent electrical circuit model used to analyse EIS data

Conclusion

The incorporation of SiC nanoparticles, as an important factor affecting the microstructure, tribological and corrosion performance of electroless Ni–P/nano-SiC coating, was evaluated in the present work using surface analysis as well as electrochemical measurements. The main conclusions of this work can be summarised as follows:

Evidence provided from surface analysis showed that regardless of SiC incorporation, the coatings were dense, homogeneous and crack free, with good adhesion to the substrate. Moreover, it was confirmed that the homogenous incorporation of SiC nanoparticles into the lattice of Ni–P enhances the microhardness of the coatings.

As SiC nanoparticles introduced into the electroless Ni–P coating, an increasing trend in friction coefficient was observed while occurring a significant drop in weight loss. This behaviour could be attributed to the higher wear resistance and hardness of the SiC nanoparticles, which might result in a secondary barrier role of nanoparticles against propagation of dislocation throughout the Ni–P matrix hindering plastic deformation of composite coating.

Through electrochemical measurements, the effectiveness of SiC nanoparticle incorporation on the corrosion performance of the coating (which enhanced barrier properties of the coating) was confirmed.

Footnotes

Acknowledgement

This study was financially supported by the Iran Initiative Council of Nanoscience and Nanotechnology.

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