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Microstructure and corrosion resistance of Ni-Al2O3-SiC nanocomposite coatings produced by electrodeposition technique
Accepted Manuscript Microstructure and corrosion resistance of Ni-Al2O3-SiC nanocomposite coatings produced by electrodeposition technique Shirin Dehgahi, Rasool Amini, Morteza Alizadeh PII:
S0925-8388(16)32642-1
DOI:
10.1016/j.jallcom.2016.08.244
Reference:
JALCOM 38746
To appear in:
Journal of Alloys and Compounds
Received Date: 29 April 2016 Revised Date:
22 July 2016
Accepted Date: 24 August 2016
Please cite this article as: S. Dehgahi, R. Amini, M. Alizadeh, Microstructure and corrosion resistance of Ni-Al2O3-SiC nanocomposite coatings produced by electrodeposition technique, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.08.244. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Microstructure and Corrosion Resistance of Ni-Al2O3-SiC
Shirin Dehgahi a, Rasool Amini a,* , Morteza Alizadeha 1
Department of Materials Science and Engineering, Shiraz University of Technology, 71555-313 Shiraz, Iran
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a
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Nanocomposite Coatings Produced by Electrodeposition Technique
Abstract
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Ni–Al2O3–SiC nanocomposite coatings were electro-deposited on steel in a typical Watt’s bath containing Al2O3 and SiC nanoparticles. Afterwards, the effect of the nanoparticle concentration in the electrolyte bath (ranging from 0 g/L to 40 g/L) on the microstructure and corrosion performance was evaluated. To investigate the microstructural changes and surface morphology
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of the coatings, as well as the nanoparticle distribution in the deposits, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy was utilized. Also, transmission electron microscopy was performed to further investigate the microstructure and to confirm the
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production of the nanocomposite coating. The electrochemical corrosion behavior of the prepared coatings was investigated in a 3.5 wt. % NaCl solution by a potentiostat-galvanostat
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device. It was found that desirable structure of the protruding crystallite morphology with no detectable cracks and pores can be achieved at the medium concentrations of the reinforcement (e.g. 20 g/L). It was also found that the corrosion resistance of the coatings is considerably improved by adding the Al2O3+SiC nanoparticles to the Ni matrix. The optimum concentration
*
Corresponding author. Tel.: +98 917 811 1858; fax: +98 711 735 4520. E-mail addresses: [email protected]
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of the nanoparticles in the electrolyte bath to attain the nanocomposite coating with a desirable microstructure and consequently a desirable corrosion resistance was 20 g/L.
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Keywords: Ni-matrix Nanocomposite Coatings; Electrodeposition; Microstructure; Corrosion Resistance
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1. Introduction
Composite coatings are a new generation of coatings considered in academic and industrial
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societies, due to their superior properties such as good wear resistance, low corrosion tendency, and high hardness [1, 2]. Ni-based coatings reinforced by ceramic particles like Al2O3,TiO2, SiC and WC are typical composite coatings which are predominantly used in industrial applications, especially as a replacement for hard chromium coatings [3-5]. It should be mentioned that the role of reinforcing particles on the coating’s properties can be altered by the reinforcement type,
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size, and content. For instance, the corrosion resistance of Ni-matrix coatings can be improved significantly by incorporation of Al2O3, TiN, or CeO2 [3, 6, 7]; in addition, particles such as SiC,
resistance [4, 5, 8].
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TiO2, or B4C play a crucial role in the enhancement of the coating’s micro-hardness and wear
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Composite coatings can be made by several methods like physical vapor deposition [9], chemical vapor deposition [10], ion implantation [11], and electrodeposition [12], among which the last one has many advantages, such as low production cost, industrial capability, easy implementation, size and shape flexibility, and high production rates [13-15]. During the electrodeposition of Ni-matrix composites, insoluble particles are trapped in the growing Ni layer. Indeed, cations in the electrolyte are adsorbed on the dispersed particles, moving them to the cathode surface. Then, the positive particles are adsorbed on the cathode surface and trapped 2
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by the growing Ni layer, leading to the formation of a composite coating [16]. Based on previous studies [16-18], by decreasing the reinforcement particles size, the mechanical properties of the composite coatings, especially hardness, can be improved significantly, provided that the particle
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agglomeration is hindered by the utilization of suitable surfactants during deposition [2, 19]. This results from the enhancement of nucleation sites and the texture modification of the growing Ni crystals, being responsible for an increase in the particles fraction in the composite coating and a
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considerable decrease in the matrix grain size.
There are several studies on the individual utilization of Al2O3 and SiC particles as the
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reinforcement in Ni-matrix composite coatings due to their chemical stability, low cost and high hardness [1-4, 12-14, and 17-20]. In these reports, the effect of electrodeposition parameters (e.g. current density, bath conditions, and reinforcement content and size) on the physical and chemical properties of the composite coatings has been widely studied. However, a limited
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number of reports are available on the combination effect of Al2O3 and SiC particles on the coating’s properties. For instance, Masoudi and his co-workers [21] studied the structure, microhardness, wear resistance, and high-temperature oxidation behavior of Ni–Al2O3–SiC composite
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coatings electrodeposited onto pure copper. Also, the effect of the particle content on the structure, microstructure and chemical composition of the coatings has been recently reported by
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Alizadeh et al. [22]. In the present work, Ni-Al2O3-SiC composite coatings were deposited on steel substrates by a co-electrodeposition process using a Watt’s bath containing the various amounts of Al2O3–SiC nanoparticles. Afterwards, the effect of the nanoparticles concentration in the bath on the microstructure and corrosion behavior of the produced composite coatings was investigated.
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2. Experimental Procedures 2.1. Fabrication of nanocomposite coatings Ni–Al2O3–SiC nanocomposite coatings were prepared by a constant-current electrodeposition
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method from a typical Watt’s bath. The bath composition and plating conditions are listed in Table 1. Steel plates were used as cathodes and a pure Ni plate was selected as the anode. Prior to the co-deposition process, the steel plates were mechanically polished to 1000-grit finish,
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floated in an HCl solution for 1 min and degreased in acetone for 5 min. The surface area of the anode was approximately chosen three times greater than that of the cathode. Analytical reagents
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and de-ionized water were used to prepare the plating solution. Each experiment was carried out in a fresh solution and conducted at a current density of 1 A/dm2, wherein the distance between the electrodes was fixed around 3 cm. Prior to electrodeposition, the solution was stirred for 24 hours at 1200 rpm to obtain de-agglomerated nanoparticles. The average size of the as-received
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Al2O3 and SiC particles (purity >99.9%) was 40 nm and 45 nm, respectively. The concentration of the particles in the bath was varied from 0 to 40 g/L, wherein the Al2O3:SiC ratio was 1:1. To reduce agglomeration of the nanoparticles during electrodeposition, 0.1 g/L sodium dodecyl
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sulfate was added to the electrolyte and the solution was continuously stirred during the process.
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2.2. Characterization of nanocomposite coatings To investigate the variation of the surface morphology, as well as the distribution and quantity of the nanoparticles in the nanocomposite coatings, a scanning electron microscope (SEM, TESCAN- VEGA3 SB) coupled with energy dispersive X-ray spectroscopy (EDS) was utilized. Furthermore, the microstructural analysis of the 20 g/l composite coating was conducted by a
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high resolution transmission electron microscope (TEM, JEOL- JEM 2010) in the bright-field
2.3. Aqueous corrosion behavior of nanocomposite coatings
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mode.
The corrosion behavior of the nanocomposite coatings was evaluated in a conventional threeelectrode cell with an Ag/AgCl as a reference electrode and a platinum plate as an auxiliary
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electrode. The samples were embedded in an epoxy resin, leaving an area of 1 cm2 exposed as
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the working electrodes. The measurements were conducted in a 3.5 wt. % NaCl solution at 25 ◦C using a potentiostat/ galvanostat device (Vertex, Ivium Technologies). Linear polarization curves were established from −100 mV to +100 mV vs. open circuit potential (OCP) at a scan rate of 0.05 mV/s; subsequently, Tafel and potentiodynamic polarization curves were set up at the sweep rate of 0.01 mV/s and 0.05 mV/s, respectively.
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The corrosion current density (icorr) of the nanocomposite coatings was calculated from the intersection of the cathodic and anodic Tafel curves using the Tafel extrapolation method (Fig. 1). To confirm the results, icorr was also estimated from the polarization curves based on the
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Stern–Geary equation (Eq. 1) [23]:
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(1)
where ba and bc are the anodic and cathodic Tafel slopes and Rp is the linear polarization resistance. The linear polarization resistance was calculated on the basis of the following equation: (2)
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where E is the potential in volt and i is the current density in A/cm2. The corrosion rate (RM) of the deposits was calculated by the Faraday’s law [24]:
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(3)
where M is the atomic weight of the metal, ρ is the density, n is the number of electrons
3. Results and discussion 3.1. Microstructural studies
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M/n is also sometimes referred to as the equivalent weight.
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exchanged in the dissolution reaction, and F is the Faraday constant (96.485 C/mol). The ratio
To determine the deposition effectiveness and to estimate the fraction of the nanoparticles deposited during electrodeposition, the SEM images of the cross sections were analyzed by a
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suitable image analyzer and the EDS analyses were conducted on the coating’s cross sections at different locations. Fig. 2 (a) displays the cross-sectional SEM image of the 20 g/L nanocomposite coating, wherein the microstructure is compact and the distribution of the
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reinforcing particles (dark points) in the Ni matrix is almost uniform. As it is clear, a desirable interface with no detectable porosity or delamination between the coating and substrate was
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obtained. The EDS point analyses of the coating cross section at three different locations were shown in Figs. 2 (b), 2 (c), and 2 (d). As can be seen in the EDS patterns, as well as the Ni peaks, peaks corresponding to Al (kα), Si(kα), C(kα), and O(kα) are visible even at locations with no detectable reinforcement at the present SEM resolution. That is, the Al2O3 and SiC particles were properly incorporated to the Ni matrix during deposition. Fig. 3 indicates the approximate quantity of the Al2O3 and SiC reinforcements in the deposited coatings at the different
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concentrations of the nanoparticles in the electrodeposition bath. Concerning the figure, by increasing the particle content in the electrolyte bath, their incorporation rate is enhanced and the quantity of the co-deposited Al2O3 and SiC nanoparticles initially increases and reaches the
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maximum value at 20 g/L (10 g/L Al2O3 + 10 g/L SiC). At this moment, the reinforcement
particles are saturated in the composite coating; that is, the optimum nanoparticle concentration in the electrodeposition bath is 20 g/L. At the higher concentrations of the nanoparticles, a
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reduction in the deposited particles is evident, which can be attributed to the increase of the nanoparticle agglomeration [25] and the enhancement of elastic collisions between the particles
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[14, 26]. It should be noticed that by increasing the particle contents in the electrolyte, a longer period is required for the nanoparticles to be embedded into the Ni matrix since the capturing capacity of the growing metal matrix remains virtually constant [27]. A more focus on Fig. 1 indicates that the quantity of the deposited Al2O3 nanoparticles is more than that of SiC, which
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can be due to the higher activity of Al2O3 in respect to SiC to attach the Ni matrix [19]. Fig. 4 demonstrates a bright-field transmission electron microscopy image of the 20 g/L nanocomposite coating. As it is evident, Ni crystals are in the nanometric scale, and dark
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nanoparticles are visible in the Ni matrix. That is, the Ni–Al2O3–SiC nanocomposite coating with the nanometric matrix and reinforcements is successfully deposited on the steel substrate by the
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electrodeposition technique. Moreover, the selected area diffraction (SAD) pattern of the figure indicates visible Ni-crystal diffraction rings and arrow-pointed feeble diffraction rings corresponding to the reinforcements, confirming the deposition of a nanocomposite Ni- matrix coating. It can also be seen that the nanoparticles are strongly embedded in the matrix with no sign of cracks and holes.
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Fig. 5 shows the surface morphology of the coatings as a function of the particle concentration in the deposition bath. Obviously, the surface morphology of the Ni-matrix changes considerably by adding the nanoparticles. By embedding the Al2O3-SiC particles into
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the Ni matrix, the grain morphology of polyhedral Ni crystallites would change into the
protruding crystallite morphology and the surface heterogeneity increases considerably, which are compatible with the literature [28]. Moreover, it is apparent that in the various nanoparticle
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quantities, the coating’s morphology is quite different. Regarding Fig. 5 (b), the surface
morphology of the 5 g/L composite coating contains a high density of micro-cracks, which can
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significantly deteriorate its properties. This can be due to the texture modification from the soft [1 0 0] mode to the preferred [2 1 1] mode [29, 30], as well as a considerable reduction in the Ni matrix ductility at a very low crystallite size (less than 25 nm) [31]. At the sufficiently high amounts of the reinforcements (e.g. 20 g/L, Fig. 5 (c)), epitaxial growth prevails over the
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polynomial growth of the Ni crystals, and because of an increase in nucleation sites [32], the growing rate of Ni crystals is reduced considerably. In other words, the amount of residual stresses is reduced remarkably and micro-cracks are therefore diminished. Fig. 5 (d) reveals that
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at the high particles concentrations (40 g/L), due to the adsorption of highly agglomerated particles into the surface, the composite coating contains a high fraction of porosity which is
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rarely filled by the deposited Ni ions during electrodeposition. The hydrogen generation and high level of particle collision can also assist the porosity generation at the high particle concentrations [33].
3.2. Corrosion resistance
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Fig. 6 illustrates the potentiodynamic polarization curves of the nickel and Ni–Al2O3–SiC nanocomposite coatings in the 3.5 wt. % NaCl solution. As can be seen, the potential of the pure Ni coating electrode is -0.25 V vs. the reference Ag/AgCl electrode. With the addition of
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nanoparticles to the Ni matrix, due to the creation of micro-cracks in the coating surface and consequently the exposure of the under-layer steel surface, the measured corrosion potential initially moves to the active directions; then, because of the coating microstructure improvement
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and the defects reduction, it returns to the noble directions and reaches -0.22 V vs. Ag/AgCl at the concentration of 20 g/L. At the sufficiently high concentrations of the nanoparticles in the
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electrodeposition bath (e.g. 40 g/L), the corrosion tendency of the coating is re-increased and the corrosion potentials shift to the negative direction since the quantity of micro-pores and -holes is increased particularly due to the particle agglomeration. A further focus on the potentiodynamic curves discloses that by increasing the reinforcement content in the electrolyte, the passive
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potential range is initially reduced, then enhanced, and afterward decreased again. On the other hand, the inverse behaviors occur for the current density variations in the passive region. At the moderate concentrations of the nanoparticles (e.g. 20 g/L), the polarization curve shows the
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broadest passive region and the smallest passive current density. This designates that the corrosion resistance of the Ni–Al2O3–SiC nanocomposite coating deposited at the 20 g/L
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concentration is the best amongst the other coatings. In fact, at a sufficient quantity of the nanoparticles, owing to the formation of several micro-galvanic cells, the passivation process of the metal matrix is accelerated and the development of corrosion pits is prevented [3, 18], improving the corrosion resistance of the coating. At the low and high concentrations of the nanoparticles in the bath, due to the existence of microstructural defects (e.g. micro-cracks and micro-holes) in the deposited coatings, the electrolyte containing corrosive ions (e.g. chloride)
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can be easily diffused to the substrate surface through the coating’s defects, thereby reducing the passivation stability and increasing the current density in the passivation region. The linear polarization curves of the deposited coatings, in which the polarization resistances
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are estimated from their slopes, are illustrated in Fig. 7. Apparently, by incorporation of the nanoparticles, the polarization resistance increases considerably from 44.5 kΩ (pure Ni (0 g/L)) to 99.4 kΩ (20 g/L) (i.e. more than twice). At the sufficiently high concentrations of the
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nanoparticles in the electrolyte bath (> 20 g/L), the polarization resistance indicates a decreasing tend, which can be attributed to the increase in the coating porosity, as previously explained.
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The corrosion current density (icorr) estimated from the intersection of the anodic and cathodic Tafel curves using the Tafel extrapolation method and the one calculated from the linear polarization curves based on the Stern–Geary equation are summarized in Table 2. The results estimated by both the methods are quite similar, confirming the precision of the results. The data
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clearly indicates that despite the existence of defects in some of the prepared nanocomposite coatings, the corrosion rate of the Ni matrix is reduced considerably by the Al2O3–SiC reinforcement. It indicates the effectiveness of the nanoparticle incorporation on the corrosion
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resistance of the prepared coatings. The improvement of corrosion resistance could be simply as a result of limiting surface area of the exposed nickel-base to the corrosive electrolyte. Also, the
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nanoparticles act as inert physical barriers to the initiation and development of corrosion, improving the corrosion resistance of the coating [3]. Moreover, by adding the nanoparticles to the Ni matrix, the structure of Ni crystallites is modified and the crystallite size is considerably refined, improving the corrosion resistance of the coating [3]. According to our results, it can also be specified that among the composite coatings, the coating deposited in the bath containing 20 g/L nanoparticles shows the highest corrosion resistance. This is due to the fact that a deposit
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with a desirable microstructure is achieved at a moderate concentration of the reinforcement in the electrolyte bath. To determine the advantages or disadvantages of the simultaneous over separate incorporation
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of the Al2O3 and SiC nanoparticles into the Ni matrix during electrodeposition in terms of resistance to corrosion, the Ni/Al2O3, and Ni/SiC nanocomposites coatings were also
electrodeposited under the same condition in the electrolyte bath with 20 g/L reinforcement (20
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g/L Al2O3 for Ni/Al2O3 and 20 g/L SiC for Ni/SiC). Subsequently, their polarization behaviors were evaluated and compared with the Ni/Al2O3+SiC coating deposited in the electrolyte bath
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with 20 g/L Al2O3+SiC (10 g/L Al2O3+ 10 g/L SiC). Three important corrosion parameters, i.e. corrosion potential, corrosion rate, and breakdown potential in the 3.5 wt. % NaCl solution were compared (Table 3). According to the results, it is evident that the corrosion resistance of the produced Ni/Al2O3+SiC nanocomposite coating is higher than that of the Ni/Al2O3 or Ni/SiC
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coating. The lower corrosion rate, higher corrosion and breakdown potential, and developed passivation range of the Ni/Al2O3+SiC coating compared to those of Ni/Al2O3 or Ni/SiC are distinctive. Indeed, it can be inferred that the resistance of the Ni coatings to electrochemical
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corrosion in the 3.5 wt. % NaCl solution is improved significantly by the simultaneous incorporation of the Al2O3 and SiC nanoparticles into the Ni matrix with respect to the separate
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addition.
4. Conclusions
In the present work, Ni–Al2O3–SiC nanocomposite coatings were electro-deposited on a steel substrate in the conventional Watt’s bath containing dispersed Al2O3 and SiC nanoparticles.
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Subsequently, their microstructure and corrosion behavior were investigated. The results can be summarized as follows: 1) At the present electrodeposition condition, the successful deposition of the Ni–Al2O3–SiC
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nanocomposite coatings with a compact microstructure, proper distribution of the reinforcements and desirable interface between the coating and substrate was obtained especially at the medium concentration of the nanoparticles (20 g/L) in the electrolyte bath.
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2) The optimum concentration of the nanoparticles in the electrolyte bath to attain the maximum amount of incorporation of the reinforcements into the Ni matrix was 20 g/L. At this
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concentration, a nanocomposite coating with 4.75 vol. % Al2O3 + 2.13 vol. % SiC was deposited. 3) By nanoparticles addition to the electrodeposition bath, the surface morphology of the Nimatrix changed and its heterogeneity increased considerably. The protruding crystallite morphology with no evidence of micro -cracks and -pores were attained at 20 g/L reinforcement
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concentration in the electrolyte bath.
4) Despite the existence of cracks and holes in some of the prepared Ni/Al2O3+SiC nanocomposite coatings, their corrosion rate was considerably less than pure Ni coating.
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5) By incorporation of the nanoparticles, the coating polarization resistance was increased significantly (more than twice). However, at the sufficiently high concentrations of the
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nanoparticles in the electrolyte bath (> 20 g/L), a decreasing trend was appeared in the polarization resistance.
6) At the low concentrations of the reinforcement (e.g. 5 g/L), micro-cracks were developed in the coating and in comparison to the pure Ni coating, the corrosion potential was moved to the active regions, the passive potential range was reduced, and the passive current density was increased.
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7) In the composite coatings deposited at the high reinforcement concentrations in the bath (e.g. 40 g/L), the high levels of porosity and micro-holes were detected. These coatings indicated the lower corrosion resistance and inferior passivation behavior in comparison to the coatings
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deposited at the moderate concentrations of the nanoparticles in the electrolyte bath (e.g. 20 g/L). 8) By the simultaneous incorporation of Al2O3 and SiC into the Ni-matrix, the resistance of the Ni coatings to electrochemical corrosion in the 3.5 wt. % NaCl solution was improved
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significantly with respect to the separate incorporation of Al2O3 or SiC.
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[33] A.K. Pradhan, S. Das, Pulse-reverse electrodeposition of Cu–SiC nanocomposite coating:
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effect of concentration of SiC in the electrolyte, J. Alloy. Compd. 590 (2014) 294–302.
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Table 1
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Electrodeposition bath composition and plating conditions.
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Table 2 Corrosion current density (icorr) estimated from intersection of the anodic and cathodic Tafel curves using the Tafel extrapolation method (a) and linear polarization curves based on the
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Stern–Geary equation (b).
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Table 3 Corrosion resistance comparison of the produced Ni/Al2O3+SiC, Ni/Al2O3, and Ni/SiC coatings
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in the 3.5 wt. % NaCl solution
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Figure Captions Fig. 1. Estimation of corrosion current density from Tafel extrapolation method
(b), 2 (c), and 3 (d) of 20 g/L nanocomposite coating.
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Fig. 2. Cross-sectional SEM image (a) and the corresponding EDS point analyses at locations 1
Fig. 3. Approximant quantity of deposited nanoparticles as a function of their concentration in the electrodeposition bath.
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Fig. 4. Bright-field TEM image of the coating electro-deposited in the electrolyte bath containing 20 g/L nanoparticles.
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Fig.5. Surface morphology of the prepared coatings as a function of particles concentration in the bath: (a) pure Ni (0 g/L); (b) 5 g/L; (c) 20 g/L; (d) 40 g/L.
Fig. 6. Potentiodynamic polarization curves of the pure nickel and Ni-Al2O3-SiC nanocomposite coatings in the 3.5 wt. % NaCl solution.
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Fig. 7. Linear polarization curves of the pure nickel and Ni-Al2O3-SiC nanocomposite coatings
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in the 3.5 wt. % NaCl solution.
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Highlights Synergism effect of Al2O3+SiC on corrosion resistance enhancement in 3.5 % wt. NaCl Corrosion rate reduction by nanocomposite formation in comparison to pure Ni coating
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Maximum nanoparticles incorporation (4.75 % Vol. Al2O3 + 2.13 % Vol SiC) at 20 g/L Proper microstructure and good corrosion behavior at moderate concentrations (20 g/L)
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Development of micro-cracks and -holes at low and high concentrations, respectively
S0925-8388(16)32642-1
DOI:
10.1016/j.jallcom.2016.08.244
Reference:
JALCOM 38746
To appear in:
Journal of Alloys and Compounds
Received Date: 29 April 2016 Revised Date:
22 July 2016
Accepted Date: 24 August 2016
Please cite this article as: S. Dehgahi, R. Amini, M. Alizadeh, Microstructure and corrosion resistance of Ni-Al2O3-SiC nanocomposite coatings produced by electrodeposition technique, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.08.244. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Microstructure and Corrosion Resistance of Ni-Al2O3-SiC
Shirin Dehgahi a, Rasool Amini a,* , Morteza Alizadeha 1
Department of Materials Science and Engineering, Shiraz University of Technology, 71555-313 Shiraz, Iran
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Nanocomposite Coatings Produced by Electrodeposition Technique
Abstract
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Ni–Al2O3–SiC nanocomposite coatings were electro-deposited on steel in a typical Watt’s bath containing Al2O3 and SiC nanoparticles. Afterwards, the effect of the nanoparticle concentration in the electrolyte bath (ranging from 0 g/L to 40 g/L) on the microstructure and corrosion performance was evaluated. To investigate the microstructural changes and surface morphology
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of the coatings, as well as the nanoparticle distribution in the deposits, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy was utilized. Also, transmission electron microscopy was performed to further investigate the microstructure and to confirm the
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production of the nanocomposite coating. The electrochemical corrosion behavior of the prepared coatings was investigated in a 3.5 wt. % NaCl solution by a potentiostat-galvanostat
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device. It was found that desirable structure of the protruding crystallite morphology with no detectable cracks and pores can be achieved at the medium concentrations of the reinforcement (e.g. 20 g/L). It was also found that the corrosion resistance of the coatings is considerably improved by adding the Al2O3+SiC nanoparticles to the Ni matrix. The optimum concentration
*
Corresponding author. Tel.: +98 917 811 1858; fax: +98 711 735 4520. E-mail addresses: [email protected]
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of the nanoparticles in the electrolyte bath to attain the nanocomposite coating with a desirable microstructure and consequently a desirable corrosion resistance was 20 g/L.
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Keywords: Ni-matrix Nanocomposite Coatings; Electrodeposition; Microstructure; Corrosion Resistance
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1. Introduction
Composite coatings are a new generation of coatings considered in academic and industrial
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societies, due to their superior properties such as good wear resistance, low corrosion tendency, and high hardness [1, 2]. Ni-based coatings reinforced by ceramic particles like Al2O3,TiO2, SiC and WC are typical composite coatings which are predominantly used in industrial applications, especially as a replacement for hard chromium coatings [3-5]. It should be mentioned that the role of reinforcing particles on the coating’s properties can be altered by the reinforcement type,
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size, and content. For instance, the corrosion resistance of Ni-matrix coatings can be improved significantly by incorporation of Al2O3, TiN, or CeO2 [3, 6, 7]; in addition, particles such as SiC,
resistance [4, 5, 8].
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TiO2, or B4C play a crucial role in the enhancement of the coating’s micro-hardness and wear
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Composite coatings can be made by several methods like physical vapor deposition [9], chemical vapor deposition [10], ion implantation [11], and electrodeposition [12], among which the last one has many advantages, such as low production cost, industrial capability, easy implementation, size and shape flexibility, and high production rates [13-15]. During the electrodeposition of Ni-matrix composites, insoluble particles are trapped in the growing Ni layer. Indeed, cations in the electrolyte are adsorbed on the dispersed particles, moving them to the cathode surface. Then, the positive particles are adsorbed on the cathode surface and trapped 2
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by the growing Ni layer, leading to the formation of a composite coating [16]. Based on previous studies [16-18], by decreasing the reinforcement particles size, the mechanical properties of the composite coatings, especially hardness, can be improved significantly, provided that the particle
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agglomeration is hindered by the utilization of suitable surfactants during deposition [2, 19]. This results from the enhancement of nucleation sites and the texture modification of the growing Ni crystals, being responsible for an increase in the particles fraction in the composite coating and a
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considerable decrease in the matrix grain size.
There are several studies on the individual utilization of Al2O3 and SiC particles as the
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reinforcement in Ni-matrix composite coatings due to their chemical stability, low cost and high hardness [1-4, 12-14, and 17-20]. In these reports, the effect of electrodeposition parameters (e.g. current density, bath conditions, and reinforcement content and size) on the physical and chemical properties of the composite coatings has been widely studied. However, a limited
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number of reports are available on the combination effect of Al2O3 and SiC particles on the coating’s properties. For instance, Masoudi and his co-workers [21] studied the structure, microhardness, wear resistance, and high-temperature oxidation behavior of Ni–Al2O3–SiC composite
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coatings electrodeposited onto pure copper. Also, the effect of the particle content on the structure, microstructure and chemical composition of the coatings has been recently reported by
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Alizadeh et al. [22]. In the present work, Ni-Al2O3-SiC composite coatings were deposited on steel substrates by a co-electrodeposition process using a Watt’s bath containing the various amounts of Al2O3–SiC nanoparticles. Afterwards, the effect of the nanoparticles concentration in the bath on the microstructure and corrosion behavior of the produced composite coatings was investigated.
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2. Experimental Procedures 2.1. Fabrication of nanocomposite coatings Ni–Al2O3–SiC nanocomposite coatings were prepared by a constant-current electrodeposition
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method from a typical Watt’s bath. The bath composition and plating conditions are listed in Table 1. Steel plates were used as cathodes and a pure Ni plate was selected as the anode. Prior to the co-deposition process, the steel plates were mechanically polished to 1000-grit finish,
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floated in an HCl solution for 1 min and degreased in acetone for 5 min. The surface area of the anode was approximately chosen three times greater than that of the cathode. Analytical reagents
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and de-ionized water were used to prepare the plating solution. Each experiment was carried out in a fresh solution and conducted at a current density of 1 A/dm2, wherein the distance between the electrodes was fixed around 3 cm. Prior to electrodeposition, the solution was stirred for 24 hours at 1200 rpm to obtain de-agglomerated nanoparticles. The average size of the as-received
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Al2O3 and SiC particles (purity >99.9%) was 40 nm and 45 nm, respectively. The concentration of the particles in the bath was varied from 0 to 40 g/L, wherein the Al2O3:SiC ratio was 1:1. To reduce agglomeration of the nanoparticles during electrodeposition, 0.1 g/L sodium dodecyl
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sulfate was added to the electrolyte and the solution was continuously stirred during the process.
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2.2. Characterization of nanocomposite coatings To investigate the variation of the surface morphology, as well as the distribution and quantity of the nanoparticles in the nanocomposite coatings, a scanning electron microscope (SEM, TESCAN- VEGA3 SB) coupled with energy dispersive X-ray spectroscopy (EDS) was utilized. Furthermore, the microstructural analysis of the 20 g/l composite coating was conducted by a
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high resolution transmission electron microscope (TEM, JEOL- JEM 2010) in the bright-field
2.3. Aqueous corrosion behavior of nanocomposite coatings
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mode.
The corrosion behavior of the nanocomposite coatings was evaluated in a conventional threeelectrode cell with an Ag/AgCl as a reference electrode and a platinum plate as an auxiliary
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electrode. The samples were embedded in an epoxy resin, leaving an area of 1 cm2 exposed as
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the working electrodes. The measurements were conducted in a 3.5 wt. % NaCl solution at 25 ◦C using a potentiostat/ galvanostat device (Vertex, Ivium Technologies). Linear polarization curves were established from −100 mV to +100 mV vs. open circuit potential (OCP) at a scan rate of 0.05 mV/s; subsequently, Tafel and potentiodynamic polarization curves were set up at the sweep rate of 0.01 mV/s and 0.05 mV/s, respectively.
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The corrosion current density (icorr) of the nanocomposite coatings was calculated from the intersection of the cathodic and anodic Tafel curves using the Tafel extrapolation method (Fig. 1). To confirm the results, icorr was also estimated from the polarization curves based on the
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Stern–Geary equation (Eq. 1) [23]:
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(1)
where ba and bc are the anodic and cathodic Tafel slopes and Rp is the linear polarization resistance. The linear polarization resistance was calculated on the basis of the following equation: (2)
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where E is the potential in volt and i is the current density in A/cm2. The corrosion rate (RM) of the deposits was calculated by the Faraday’s law [24]:
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(3)
where M is the atomic weight of the metal, ρ is the density, n is the number of electrons
3. Results and discussion 3.1. Microstructural studies
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M/n is also sometimes referred to as the equivalent weight.
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exchanged in the dissolution reaction, and F is the Faraday constant (96.485 C/mol). The ratio
To determine the deposition effectiveness and to estimate the fraction of the nanoparticles deposited during electrodeposition, the SEM images of the cross sections were analyzed by a
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suitable image analyzer and the EDS analyses were conducted on the coating’s cross sections at different locations. Fig. 2 (a) displays the cross-sectional SEM image of the 20 g/L nanocomposite coating, wherein the microstructure is compact and the distribution of the
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reinforcing particles (dark points) in the Ni matrix is almost uniform. As it is clear, a desirable interface with no detectable porosity or delamination between the coating and substrate was
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obtained. The EDS point analyses of the coating cross section at three different locations were shown in Figs. 2 (b), 2 (c), and 2 (d). As can be seen in the EDS patterns, as well as the Ni peaks, peaks corresponding to Al (kα), Si(kα), C(kα), and O(kα) are visible even at locations with no detectable reinforcement at the present SEM resolution. That is, the Al2O3 and SiC particles were properly incorporated to the Ni matrix during deposition. Fig. 3 indicates the approximate quantity of the Al2O3 and SiC reinforcements in the deposited coatings at the different
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concentrations of the nanoparticles in the electrodeposition bath. Concerning the figure, by increasing the particle content in the electrolyte bath, their incorporation rate is enhanced and the quantity of the co-deposited Al2O3 and SiC nanoparticles initially increases and reaches the
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maximum value at 20 g/L (10 g/L Al2O3 + 10 g/L SiC). At this moment, the reinforcement
particles are saturated in the composite coating; that is, the optimum nanoparticle concentration in the electrodeposition bath is 20 g/L. At the higher concentrations of the nanoparticles, a
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reduction in the deposited particles is evident, which can be attributed to the increase of the nanoparticle agglomeration [25] and the enhancement of elastic collisions between the particles
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[14, 26]. It should be noticed that by increasing the particle contents in the electrolyte, a longer period is required for the nanoparticles to be embedded into the Ni matrix since the capturing capacity of the growing metal matrix remains virtually constant [27]. A more focus on Fig. 1 indicates that the quantity of the deposited Al2O3 nanoparticles is more than that of SiC, which
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can be due to the higher activity of Al2O3 in respect to SiC to attach the Ni matrix [19]. Fig. 4 demonstrates a bright-field transmission electron microscopy image of the 20 g/L nanocomposite coating. As it is evident, Ni crystals are in the nanometric scale, and dark
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nanoparticles are visible in the Ni matrix. That is, the Ni–Al2O3–SiC nanocomposite coating with the nanometric matrix and reinforcements is successfully deposited on the steel substrate by the
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electrodeposition technique. Moreover, the selected area diffraction (SAD) pattern of the figure indicates visible Ni-crystal diffraction rings and arrow-pointed feeble diffraction rings corresponding to the reinforcements, confirming the deposition of a nanocomposite Ni- matrix coating. It can also be seen that the nanoparticles are strongly embedded in the matrix with no sign of cracks and holes.
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Fig. 5 shows the surface morphology of the coatings as a function of the particle concentration in the deposition bath. Obviously, the surface morphology of the Ni-matrix changes considerably by adding the nanoparticles. By embedding the Al2O3-SiC particles into
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the Ni matrix, the grain morphology of polyhedral Ni crystallites would change into the
protruding crystallite morphology and the surface heterogeneity increases considerably, which are compatible with the literature [28]. Moreover, it is apparent that in the various nanoparticle
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quantities, the coating’s morphology is quite different. Regarding Fig. 5 (b), the surface
morphology of the 5 g/L composite coating contains a high density of micro-cracks, which can
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significantly deteriorate its properties. This can be due to the texture modification from the soft [1 0 0] mode to the preferred [2 1 1] mode [29, 30], as well as a considerable reduction in the Ni matrix ductility at a very low crystallite size (less than 25 nm) [31]. At the sufficiently high amounts of the reinforcements (e.g. 20 g/L, Fig. 5 (c)), epitaxial growth prevails over the
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polynomial growth of the Ni crystals, and because of an increase in nucleation sites [32], the growing rate of Ni crystals is reduced considerably. In other words, the amount of residual stresses is reduced remarkably and micro-cracks are therefore diminished. Fig. 5 (d) reveals that
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at the high particles concentrations (40 g/L), due to the adsorption of highly agglomerated particles into the surface, the composite coating contains a high fraction of porosity which is
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rarely filled by the deposited Ni ions during electrodeposition. The hydrogen generation and high level of particle collision can also assist the porosity generation at the high particle concentrations [33].
3.2. Corrosion resistance
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Fig. 6 illustrates the potentiodynamic polarization curves of the nickel and Ni–Al2O3–SiC nanocomposite coatings in the 3.5 wt. % NaCl solution. As can be seen, the potential of the pure Ni coating electrode is -0.25 V vs. the reference Ag/AgCl electrode. With the addition of
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nanoparticles to the Ni matrix, due to the creation of micro-cracks in the coating surface and consequently the exposure of the under-layer steel surface, the measured corrosion potential initially moves to the active directions; then, because of the coating microstructure improvement
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and the defects reduction, it returns to the noble directions and reaches -0.22 V vs. Ag/AgCl at the concentration of 20 g/L. At the sufficiently high concentrations of the nanoparticles in the
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electrodeposition bath (e.g. 40 g/L), the corrosion tendency of the coating is re-increased and the corrosion potentials shift to the negative direction since the quantity of micro-pores and -holes is increased particularly due to the particle agglomeration. A further focus on the potentiodynamic curves discloses that by increasing the reinforcement content in the electrolyte, the passive
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potential range is initially reduced, then enhanced, and afterward decreased again. On the other hand, the inverse behaviors occur for the current density variations in the passive region. At the moderate concentrations of the nanoparticles (e.g. 20 g/L), the polarization curve shows the
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broadest passive region and the smallest passive current density. This designates that the corrosion resistance of the Ni–Al2O3–SiC nanocomposite coating deposited at the 20 g/L
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concentration is the best amongst the other coatings. In fact, at a sufficient quantity of the nanoparticles, owing to the formation of several micro-galvanic cells, the passivation process of the metal matrix is accelerated and the development of corrosion pits is prevented [3, 18], improving the corrosion resistance of the coating. At the low and high concentrations of the nanoparticles in the bath, due to the existence of microstructural defects (e.g. micro-cracks and micro-holes) in the deposited coatings, the electrolyte containing corrosive ions (e.g. chloride)
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can be easily diffused to the substrate surface through the coating’s defects, thereby reducing the passivation stability and increasing the current density in the passivation region. The linear polarization curves of the deposited coatings, in which the polarization resistances
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are estimated from their slopes, are illustrated in Fig. 7. Apparently, by incorporation of the nanoparticles, the polarization resistance increases considerably from 44.5 kΩ (pure Ni (0 g/L)) to 99.4 kΩ (20 g/L) (i.e. more than twice). At the sufficiently high concentrations of the
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nanoparticles in the electrolyte bath (> 20 g/L), the polarization resistance indicates a decreasing tend, which can be attributed to the increase in the coating porosity, as previously explained.
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The corrosion current density (icorr) estimated from the intersection of the anodic and cathodic Tafel curves using the Tafel extrapolation method and the one calculated from the linear polarization curves based on the Stern–Geary equation are summarized in Table 2. The results estimated by both the methods are quite similar, confirming the precision of the results. The data
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clearly indicates that despite the existence of defects in some of the prepared nanocomposite coatings, the corrosion rate of the Ni matrix is reduced considerably by the Al2O3–SiC reinforcement. It indicates the effectiveness of the nanoparticle incorporation on the corrosion
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resistance of the prepared coatings. The improvement of corrosion resistance could be simply as a result of limiting surface area of the exposed nickel-base to the corrosive electrolyte. Also, the
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nanoparticles act as inert physical barriers to the initiation and development of corrosion, improving the corrosion resistance of the coating [3]. Moreover, by adding the nanoparticles to the Ni matrix, the structure of Ni crystallites is modified and the crystallite size is considerably refined, improving the corrosion resistance of the coating [3]. According to our results, it can also be specified that among the composite coatings, the coating deposited in the bath containing 20 g/L nanoparticles shows the highest corrosion resistance. This is due to the fact that a deposit
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with a desirable microstructure is achieved at a moderate concentration of the reinforcement in the electrolyte bath. To determine the advantages or disadvantages of the simultaneous over separate incorporation
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of the Al2O3 and SiC nanoparticles into the Ni matrix during electrodeposition in terms of resistance to corrosion, the Ni/Al2O3, and Ni/SiC nanocomposites coatings were also
electrodeposited under the same condition in the electrolyte bath with 20 g/L reinforcement (20
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g/L Al2O3 for Ni/Al2O3 and 20 g/L SiC for Ni/SiC). Subsequently, their polarization behaviors were evaluated and compared with the Ni/Al2O3+SiC coating deposited in the electrolyte bath
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with 20 g/L Al2O3+SiC (10 g/L Al2O3+ 10 g/L SiC). Three important corrosion parameters, i.e. corrosion potential, corrosion rate, and breakdown potential in the 3.5 wt. % NaCl solution were compared (Table 3). According to the results, it is evident that the corrosion resistance of the produced Ni/Al2O3+SiC nanocomposite coating is higher than that of the Ni/Al2O3 or Ni/SiC
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coating. The lower corrosion rate, higher corrosion and breakdown potential, and developed passivation range of the Ni/Al2O3+SiC coating compared to those of Ni/Al2O3 or Ni/SiC are distinctive. Indeed, it can be inferred that the resistance of the Ni coatings to electrochemical
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corrosion in the 3.5 wt. % NaCl solution is improved significantly by the simultaneous incorporation of the Al2O3 and SiC nanoparticles into the Ni matrix with respect to the separate
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addition.
4. Conclusions
In the present work, Ni–Al2O3–SiC nanocomposite coatings were electro-deposited on a steel substrate in the conventional Watt’s bath containing dispersed Al2O3 and SiC nanoparticles.
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Subsequently, their microstructure and corrosion behavior were investigated. The results can be summarized as follows: 1) At the present electrodeposition condition, the successful deposition of the Ni–Al2O3–SiC
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nanocomposite coatings with a compact microstructure, proper distribution of the reinforcements and desirable interface between the coating and substrate was obtained especially at the medium concentration of the nanoparticles (20 g/L) in the electrolyte bath.
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2) The optimum concentration of the nanoparticles in the electrolyte bath to attain the maximum amount of incorporation of the reinforcements into the Ni matrix was 20 g/L. At this
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concentration, a nanocomposite coating with 4.75 vol. % Al2O3 + 2.13 vol. % SiC was deposited. 3) By nanoparticles addition to the electrodeposition bath, the surface morphology of the Nimatrix changed and its heterogeneity increased considerably. The protruding crystallite morphology with no evidence of micro -cracks and -pores were attained at 20 g/L reinforcement
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concentration in the electrolyte bath.
4) Despite the existence of cracks and holes in some of the prepared Ni/Al2O3+SiC nanocomposite coatings, their corrosion rate was considerably less than pure Ni coating.
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5) By incorporation of the nanoparticles, the coating polarization resistance was increased significantly (more than twice). However, at the sufficiently high concentrations of the
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nanoparticles in the electrolyte bath (> 20 g/L), a decreasing trend was appeared in the polarization resistance.
6) At the low concentrations of the reinforcement (e.g. 5 g/L), micro-cracks were developed in the coating and in comparison to the pure Ni coating, the corrosion potential was moved to the active regions, the passive potential range was reduced, and the passive current density was increased.
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7) In the composite coatings deposited at the high reinforcement concentrations in the bath (e.g. 40 g/L), the high levels of porosity and micro-holes were detected. These coatings indicated the lower corrosion resistance and inferior passivation behavior in comparison to the coatings
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deposited at the moderate concentrations of the nanoparticles in the electrolyte bath (e.g. 20 g/L). 8) By the simultaneous incorporation of Al2O3 and SiC into the Ni-matrix, the resistance of the Ni coatings to electrochemical corrosion in the 3.5 wt. % NaCl solution was improved
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significantly with respect to the separate incorporation of Al2O3 or SiC.
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Table 1
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Electrodeposition bath composition and plating conditions.
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Table 2 Corrosion current density (icorr) estimated from intersection of the anodic and cathodic Tafel curves using the Tafel extrapolation method (a) and linear polarization curves based on the
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Table 3 Corrosion resistance comparison of the produced Ni/Al2O3+SiC, Ni/Al2O3, and Ni/SiC coatings
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in the 3.5 wt. % NaCl solution
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Figure Captions Fig. 1. Estimation of corrosion current density from Tafel extrapolation method
(b), 2 (c), and 3 (d) of 20 g/L nanocomposite coating.
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Fig. 2. Cross-sectional SEM image (a) and the corresponding EDS point analyses at locations 1
Fig. 3. Approximant quantity of deposited nanoparticles as a function of their concentration in the electrodeposition bath.
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Fig. 4. Bright-field TEM image of the coating electro-deposited in the electrolyte bath containing 20 g/L nanoparticles.
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Fig.5. Surface morphology of the prepared coatings as a function of particles concentration in the bath: (a) pure Ni (0 g/L); (b) 5 g/L; (c) 20 g/L; (d) 40 g/L.
Fig. 6. Potentiodynamic polarization curves of the pure nickel and Ni-Al2O3-SiC nanocomposite coatings in the 3.5 wt. % NaCl solution.
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Fig. 7. Linear polarization curves of the pure nickel and Ni-Al2O3-SiC nanocomposite coatings
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in the 3.5 wt. % NaCl solution.
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Highlights Synergism effect of Al2O3+SiC on corrosion resistance enhancement in 3.5 % wt. NaCl Corrosion rate reduction by nanocomposite formation in comparison to pure Ni coating
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Maximum nanoparticles incorporation (4.75 % Vol. Al2O3 + 2.13 % Vol SiC) at 20 g/L Proper microstructure and good corrosion behavior at moderate concentrations (20 g/L)
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Development of micro-cracks and -holes at low and high concentrations, respectively