Journal of Science: Advanced Materials and Devices (2018) 412e418 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Combustion synthesis of Ni doped SnO2 nanoparticles for applications in Zn-composite coating on mild steel K Deepa, T.V Venkatesha* Department of Studies in Chemistry, School of Chemical Sciences, Kuvempu University, Shankaraghatta, 577451, Shimoga, Karnataka, India a r t i c l e i n f o a b s t r a c t Article history: Received June 2018 Received in revised form 14 November 2018 Accepted 18 November 2018 Available online 24 November 2018 Zinc (Zn)-composite coatings are still in demand as good corrosion barrier coatings to protect steel substrates from corrosion environment In this article, the Ni doped SnO2 nanoparticles were synthesized and used as a composite additive for Zn-coating The synthesis was carried out by the combustion method using citric acid as a fuel The Zn-Ni doped SnO2 composite coating was produced on mild steel by an electroplating technique The surface characterization and elemental analysis of the coated samples were examined by X-ray diffraction spectroscopy (XRD), scanning electron microscopic images (SEM) followed by energy dispersive spectroscopy (EDAX) The surface morphology of Zn-Ni doped SnO2 composite before and after corrosion showed a more compact surface structure with respect to the pure Zn-coat The corrosion resistance property of the Zn-Ni doped SnO2 composite coating was studied by Tafel polarization and electrochemical impedance spectroscopy © 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Ni doped SnO2 nanoparticles Zn-composite coating Corrosion behavior Tafel polarization EIS Introduction The steel materials have diverse applications throughout the world in various fields because of the ease of production, availability, low-price and better mechanical strength The main drawback of these materials is ‘corrosion’ in their applications which leads to economic problems The diversity in applications of steel made it so important to protect from corrosion process [1e3] The study on the protection of steel substrates from corrosion phenomena was an interesting research topic since many years Considering the corrosion problem of steel metals, investigations were focused on the development of protective layers on the surface of a steel substrate by an electrochemical process [4,5] The electroplating technique has been widely applied to the surface treatment of steel substrates to achieve better corrosion resistance properties of steel [6,7] The deposition of metallic layers on steel substrates involved the electrolysis of certain metals like Zn, Ni, Cu, Sn etc., provided a good corrosion protection under aggressive atmosphere [8e10] Indeed, the chrome coating provided an excellent corrosion passivation for the steel surface from the surrounding environment thereby corrosion resistance of the steel metal was sacrificial The chrome passivation, however, has been prohibited due to the toxicity * Corresponding author Fax: þ91 08282 256255 E-mail address: drtvvenkatesha@yahoo.co.uk (T.V Venkatesha) Peer review under responsibility of Vietnam National University, Hanoi towards the environment Thus, it is of essential to develop nontoxic and longer life spanned surface coating for steel surface protection [11,12] Among various coatings, zinc coating found much importance because of its broad range of applications in the automobile industry, construction platforms and also marine applications thanks to cost friendly and good mechanical property The presence of the salinity in the marine environment causes the deterioration of Zn-coated steel substrates which affects the service life of the Zn-coating [13] In recent years, efforts have been moved on to Zn-composite coatings due to their better corrosion resistance property compared to pure Zn-coating The extensive research on composite materials for Zn-composite coating was focused on the utilization of metal oxides [14], carbides [15], nitrides [16], polymers [17] These coatings improved the corrosion resistance properties of Zn-coating with respect to the pure Zn-coating in the presence of a corrosive atmosphere Amongst, the metal oxide nanoparticles received more attention due to their availability and low cost of preparation [18,19] Nowadays, doped metal oxides and mixed metal oxides exhibit remarkable physical and chemical properties Practically, Zn-1% Mn-doped TiO2 composite coating on the steel substrate has been studied by Kumar et al [20] They obtained a better corrosion resistance property in comparison to the Zn-composite coating The corrosion resistance property and tribological properties of ZnAl2O3-CrO3-SiO2 have been reported by Malatji et al [21] The observed results signified the enhanced anticorrosive property of https://doi.org/10.1016/j.jsamd.2018.11.005 2468-2179/© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) K Deepa, T.V Venkatesha / Journal of Science: Advanced Materials and Devices (2018) 412e418 Zn-mixed metal oxides composite coating A good improvement was reported for Zn-TiO2-WO3 composite coating [22] The present effort focused on increasing the service life of Zncoating with the reinforcement of Ni doped SnO2 nanoparticles as a composite additive in the Zn-matrix The tin metal and its oxides have many applications in various fields because of its better thermal stability and good mechanical property SnO2 is a n-type semiconductor metal oxide having a bandgap of 3.6 eV [23,24] The research on SnO2 metal oxide nanoparticles as a composite material in zinc coating has been investigated by Fayomi et al [25] They found that the anticorrosive and tribological properties of Zn-AlSnO2 composite coating were satisfactorily good compared with that of Zn-Al alloy coating To our best knowledge, no work has been found regarding incorporation of Ni doped SnO2 nanoparticles as a composite additive in Zn-coating for corrosion protection of steel 413 papers and acetone Finally, plates were rinsed with distilled water and used The zinc sheets were plunged in 5% HCl to activate the surface of the anode material each time [29] The bath solution prepared for Zn-Ni doped SnO2 composite coating has been stirred for 10 h to prevent the agglomeration of nanoparticles Scheme demonstrates the experimental setup carried out for generation of the Zn-Ni doped SnO2 composite coating The fabricated Zn and Zn-Ni doped SnO2 coatings were subjected to electrochemical corrosion studies of Tafel and electrochemical impedance spectroscopy (EIS) using potentiostat CHI660C electrochemical workstation The EIS studies were executed at the open circuit potential (OCP) with frequency ranging from 0.1 Hz to 10 kHz and amplitude of mV The morphology and composition of the deposits were scrutinized by XRD, scanning electron microscopic images and energy dispersive spectral investigation Results and discussion Experimental 3.1 XRD analysis 2.1 Materials and synthesis methods Nickel (II) chloride heptahydrate (NiCl2$7H2O) was supplied from Himedia Laboratories Pvt Ltd Mumbai Tin (II) chloride dihydrate (SnCl2$2H2O) was received from Merck Life Science Pvt Ltd Mumbai Citric acid anhydrous was arrived from Merck Specialties Pvt Ltd Mumbai and Millipore water In a typical synthesis of Ni doped SnO2 nanoparticles, salt precursors of SnCl2$2H2O, NiCl2$7H2O and citric acid as a fuel were taken as 1:1 ratio and completely dissolved in a dilute HNO3 solution to get a combustion mixture Afterward, the solution mixture was heated on a hotplate with constant stirring until a solution mixture converted into a gel form [26e28] The gel was transferred into a quartz crucible and kept into a preheated furnace maintained at 400 C Within a few seconds, precursor gel gets boiled and ignited Then the crucible was taken out and kept for cooling for few minutes at atmospheric temperature The product was finely grounded in an agate mortar and calcined for h at 500 C Scheme illustrated the experimental steps involved during the synthesis of Ni doped SnO2 nanoparticles The crystallite size of Ni doped SnO2 was determined by Powder x-ray diffraction analysis (PANalytical X'pert pro powder diffractometer, lKaCu ¼ 1.5418 Å) The surface morphology and percentage composition of the prepared products were studied by scanning electron microscopic photographs (FESEM-Carl ZEISS, Supra 40 VP) followed by energy dispersive spectroscopy Fig depicts the XRD patterns of as-prepared Ni doped SnO2 nanoparticles The characteristic peaks corresponding to Miller indices at (110), (101), (200), (111), (210), (211), (220), (002), (310), (112), (301), (202) and (321) confirmed that the prepared product is a tetragonal structured SnO2 [30e32] No impurity peaks were appeared indicating the formation of a single phase tetragonal shaped Ni doped SnO2 The DebyeeScherer equation was applied to calculate the crystallite size of the particles: D¼ Kl bCos q (1) where D is the diameter of the crystallite size, l is the wavelength of the radiation source, K is the shape factor (0.9), q is the Bragg's angle and b is the angular peak width at half maximum intensity (FWHM) The calculated average crystallite size was found to be 27.70 nm 3.2 SEM with EDAX studies The surface morphology and elemental analysis of the prepared nanoparticles were displayed in Fig As can be seen that the particles appeared like agglomerated spherical shaped flakes like morphology of Ni doped SnO2 nanoparticles The elemental composition showed that the presence of Ni, Sn and O with the percentage of constituents and there was no foreign elements were observed 2.2 Fabrication of Zn and Zn-Ni doped SnO2 composite coatings 3.3 Characterization of the coatings The electroplating bath composition and parameters were listed in Table Steel substrates with dimensions of   0.1 cm3 were used as cathode substrates and zinc sheets of the same dimension were used as anode materials Before electroplating experiment, the surface cleaning of steel plates was carried out using emery The XRD patterns of the Zn and Zn-Ni doped SnO2 coatings are represented in Fig The crystallite size was calculated using Debye Scherer equation and the obtained size for zinc coating was 33.88 nm and for Zn-composite coating it was 28.48 nm The characteristic Scheme Experimental sequence for synthesis of Ni doped SnO2 nanoparticles 414 K Deepa, T.V Venkatesha / Journal of Science: Advanced Materials and Devices (2018) 412e418 Table Bath composition and operating parameters Bath constituents 1(a) 1(b) ZnSO4-200 g/L Na2SO4-40 g/L NaCl-15 g/L H3BO3-12 g/L SLS-0.5 g/L 1(a)ỵ Ni doped 1(a)ỵ Ni doped 1(a)ỵ Ni doped 1(a)ỵ Ni doped Deposit code ZO SnO2 0.5 g/L SnO21 g/L SnO2 1.5 g/L SnO2 g/L ZI ZII ZIII ZIV Operating parameters Anode: zinc plate (99.99% pure) Cathode: mild steel plate Current density: A/dm2 Plating time: 10 Stirring speed: 300 rpm pH: Temperature: 303 K Scheme Electroplating setup for the Zn-Ni doped SnO2 composite coating surface leads to a compact structured surface and less number of surface pores SEM photographs of pure Zn coatings and Zn-Ni doped SnO2 composite coatings were represented in Fig The pure zinc deposit was accompanied with some gaps and micro holes on its surface as displayed in Fig 4(a) These micro-holes were greatly reduced and nearly absent in the Zn-Ni doped SnO2 composite deposit, which exhibited fine compact structured surface morphology as shown in Fig 4(b) [35,36] It can be seen that the reduced grain size leads to the formation of tiny Zn fibers like compact surface morphology in the case of Zn-Ni doped SnO2 composite coated steel surface compare to pure Zn deposit The energy dispersive spectrum demonstrated in Fig indicates the presence of Ni doped SnO2 nanoparticles in the Zn-composite matrix 3.4 Electrochemical corrosion studies Fig XRD patterns of Ni doped SnO2 nanoparticles peaks at (102) and (103) planes showed the highest intensity in case of pure Zn-coating and they are decreased for Zn-Ni doped SnO2 composite coating This finding indicated that the presence of Ni doped SnO2 nanoparticles inhibited the crystal growth thereby reduced the grain size [33,34] The reduction in grain size on the zinc 3.4.1 Tafel The Tafel plots were recorded for the study of the corrosion resistance property in terms of the polarization resistance behavior of the electrodeposited samples in 3.65% NaCl solution as corrosion media In order to measure the corrosion resistance property of the deposits, the electrolytic cell was used, in which the platinum wire is served as an auxiliary electrode, calomel electrode as a reference electrode and coated specimens as the working electrodes Initially, the coated samples were dipped in the corrosive electrolyte solution to attain the OCP The potential varies with respect to the time K Deepa, T.V Venkatesha / Journal of Science: Advanced Materials and Devices (2018) 412e418 415 Fig SEM image with EDAX analysis of Ni doped SnO2 nanoparticles and attained a steady state potential (referred to as OCP) [37,38] The electrode potential of the working electrode was polarized in the range of ỵ200 mV anodically and À200mv cathodically with respect to their OCP The Tafel plots are depicted in Fig The electrochemical parameters such as Ecorr (corrosion potential), Icorr (corrosion current), Rp (polarization resistance) were recorded and Fig XRD patterns of Zn and Zn-Ni doped SnO2 deposits Fig Energy dispersive spectrum of Zn and Zn-Ni doped SnO2 composite deposits Fig SEM images of (a) Zn-deposit (b) Zn-1.5 g/L Ni doped SnO2 composite deposit 416 K Deepa, T.V Venkatesha / Journal of Science: Advanced Materials and Devices (2018) 412e418 results were tabulated in Table It can be observed that the Ecorr value of Zn-coating was À1.138 V and it was slowly reduced with the addition of Ni doped SnO2 nanoparticles in Zn-coat (concentration varying from 0.5 to g/L) Among Zn-Ni doped SnO2 composite coatings, the Zn-1.5 g/L Ni doped SnO2 composite coating posses less Ecorr via À1.121 V, indicating the less response of a Zncomposite coated steel metal surface towards corrosion atmosphere Similarly, the corrosion current (Icorr) was higher for pure Zn deposit and it gradually reduced for Zn-composite deposits The optimum composite coating has been achieved at 1.5 g/L nanoparticles concentration The corrosion rate (CR) of the respective coatings was calculated by the following equation CRmpyị ẳ 0:13 Icorr ðEq:wtÞ d (2) The Zn-coating at Zn-1.5 g/L of Ni doped SnO2 nanoparticles concentration yields a good corrosion resistance property compared to the all other concentrations The increased amount of nanoparticles caused the reduced polarization resistance behavior This is due to the fact that the agglomeration of nanoparticles at higher concentration leads to the poor adhesion on the surface and slows down the deposition process [39,40] Hence the further addition of nanoparticles in Zn-coating was stopped after the Zn-2 g/L Ni doped SnO2 deposition are tabulated in Table The circuit was composed of the coating resistance (Rcoat), the coating capacitance (Qcoat), the double layer capacitance (Qdl) and the charge transfer resistance (Rct) The capacitance was replaced by a constant phase element to achieve good results of the fitted circuit with experimental EIS plots The constant phase element (CPE) implies the departure from the ideal capacitance behavior of the working electrode thanks to the surface inhomogeneity and micro-roughness [41,42] The impedance obtained by CPE was given by Àn ZCPE ¼ Y À1 ðiuÞ (3) where Y0 is CPE constant, i2 ¼ À1, an imaginary number, u is the angular frequency and n represents the component of CPE which provides the details regarding the degree of the inhomogeneity of the metal surface, micro-roughness and porosity [43] The Qdl and Qcoat values of a pure Zn-coating was higher compared to that of the Zn-Ni doped SnO2 composite coating The presence of Ni doped SnO2 nanoparticles in the Zn-composite provided a more stability for coated surface and formed a strong corrosion barrier under corrosive atmosphere The lower Rct value 3.4.2 Electrochemical impedance spectroscopy The corrosion resistance property of the prepared coated samples was examined by EIS test carried out in 3.65% NaCl solution at OCP value of the respective coated sample EIS measurements were recorded as Nyquist and bode plots at the frequency range of 0.1 Hze10 kHz with amplitude of mV as shown in Figs and Noted that the measured Nyquist plots shown in Fig was matched with the suitable equivalent circuit model using Z-simp win 3.21 software given in Fig The obtained experimental data Fig Nyquist plots of Zn and Zn-Ni doped SnO2 composite coatings Fig Tafel plots of Zn and Zn-composite coatings Fig Equivalent circuit model matched with Nyquist plots Table Tafel parameters Samples Ecorr (V vs SCE) Icorr  10À5 (A/cm2) ZO ZI ZII ZIII ZIV À1.13 À1.12 À1.11 À1.09 À1.10 3.49 2.83 1.88 9.88 1.24      10À5 10À5 10À5 10À6 10À5 ebc (VÀ1 dec) ba (VÀ1 dec) LP (U cm2) Corrosion rate  10À5 (g/h) 7.55 4.99 5.97 7.22 4.69 16.17 14.43 17.55 21.66 20.18 524 790 980 1522 1399 3.55 2.87 1.91 1.00 1.26 K Deepa, T.V Venkatesha / Journal of Science: Advanced Materials and Devices (2018) 412e418 417 for Zn-Ni doped SnO2 composite coating compared to pure Zncoating indicated a reduced number of surface active pores which are the cause of corrosion reactions The incorporation of Ni doped SnO2 nanoparticles in Zn-coating accumulates the surface pores and thereby slows down the corrosion reactions at the interface of the metal surface and electrolyte in aggressive media [36,44] The corrosion resistance property was satisfactory at the Zn-1.5 g/L Ni doped SnO2 composite coating Further increase of the Ni doped SnO2 concentration in the zinc matrix results in a decreased impedance of the deposit Hence, the 1.5 g/L concentration of the Ni doped SnO2 composite additive has been considered as an optimum concentration for the good Zn-composite coating The polarization resistance (RP) was given by the sum of the resistances of Rcoat and Rct The Zn-Ni doped SnO2 composite coating has higher RP value in view of more corrosion resistance property compared to pure zinc coating Similar results have been observed in bode plot and bode phase angle plot as displayed in Fig 9a,b in which higher modulus impedance was observed for the Zn-1.5 g/L Ni doped SnO2 composite coating and it was lesser for pure Zn-coating Also, a maximum phase angle was attained for the Zn-Ni doped SnO2 composite coating due to the more homogeneous surface with good corrosion resistance property of Zn- Ni doped SnO2 composite coating Fig Bode magnitude plot (a) and Bode phase angle plot (b) of Zn and Zn-Ni doped SnO2 composite coatings 3.4.3 Corrosion morphology The SEM images depicted in Fig 10 shows that the corroded surface morphology captured after corrosion studies in 3.65% NaCl solution The surface of pure Zn coated specimen was highly deteriorated and some cracks were also observed in Fig 10(a) This indicates the poor corrosion resistance property under corrosive environment The Zn-1.5 g/L Ni doped SnO2 composite coated surface exhibited a little effect on the corrosion reactions as shown in Fig 10(b) Here, less deterioration and no cracks were appeared on the surface The presence of Ni doped SnO2 nanoparticles in Zn-matrix provided a strong corrosion barrier in corrosion media Table EIS parameters Samples ZO ZI ZII ZIII ZIV Qcoat (Sn UÀ1 cmÀ2) 3.46 8.75 9.31 1.25 9.91      À3 10 10À4 10À5 10À5 10À5 n 0.8 0.7 0.7 0.8 0.8 Qdl (Sn UÀ1 cmÀ2) 2.89 1.25 3.72 1.90 5.41      À4 10 10À4 10À4 10À5 10À5 n 0.8 0.8 0.8 0.8 0.8 Cdl (F/cm2) 1.65 6.75 3.07 8.58 2.94      À4 10 10À5 10À4 10À6 10À5 Fig 10 SEM images of corroded (a) Zn deposit (b) Zn-1.5 g/L Ni doped SnO2 composite RPẳ(Rcoat ỵ Rct) (U cm2) 364.2 669.6 1241 2151 1617 418 K Deepa, T.V Venkatesha / Journal of Science: Advanced Materials and Devices (2018) 412e418 Conclusion The Ni doped SnO2 nanoparticles were prepared by combustion method The Zn-Ni doped SnO2 composite coating was fabricated by an electroplating technique X-ray-diffraction study revealed the nano size of the Ni doped SnO2 particles Surface morphology of Ni doped SnO2 showed the spherical nanoflakes structure The EDAX analysis confirmed the percentage composition of the prepared nanoparticles The Zn-Ni doped SnO2 composite coating exhibited an improved surface texture The incorporation of the Ni doped SnO2 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Zn -Ni doped SnO2 composite coatings, the Zn- 1.5 g/L Ni doped SnO2 composite coating posses less Ecorr via À1.121 V, indicating the less response of a Zncomposite coated steel metal... 417 for Zn -Ni doped SnO2 composite coating compared to pure Zncoating indicated a reduced number of surface active pores which are the cause of corrosion reactions The incorporation of Ni doped