EFFECT OF TiO2 NANOPARTICLES IN NI MATRIX ON MECHANICAL AND CORROSION RESISTANCE PROPERTIES

Nickel (Ni) is one of the most important hard coatings. Improvement in its tribological and mechanical properties would greatly enhance its use in industry. Nanocomposite coatings of metals with various reinforced nanoparticles have been developed in last few decades. Titania (TiO 2 ) exhibit excellent mechanical properties. It is believed that TiO 2 incorporation in Ni matrix will improve the properties of Ni coatings significantly. The main purpose of the current work is to investigate the mechanical and anti-corrosion properties of the electroplated Nickel nanocomposite with a different percentage of TiO 2 . The results showed that the content of TiO 2 and the microhardness in the Ni-TiO 2 coatings first increased and reached the maximum at the loading of 20 g/L, then decreased, due to agglomeration of dioxide of Titania for high concentration of TiO 2 . The [200] preferred orientation for Ni pure gradually evolved to [111] orientation with increasing TiO 2 nanoparticle loading. At 20 g/L of titania , we obtained the minimum crystallite size (27 nm). The anti-corrosion property of nanocomposite coating was carried out in 3.5 % NaCl electrolyte, and the result also showed that the nanocomposite coatings improve the corrosion resistance significantly. This present work reveals that incorporation of TiO 2 in nickel nanocomposite coating improves corrosion resistance and mechanical properties of both hardness and corrosion resistance performances, and the improvement becomes at it maximum for 20 g/L of TiO 2 .


Introduction
Since the late1990s, metal-ceramic composite coatings have been used on automobile parts, appliances, metal furniture, beverage containers, fasteners, and various other industrial products [1][2][3]. Composite coatings are formed by components with characteristic dimensionality, such as micro/manometer-size setting in different matrixes [4]. Electrodeposition is a technique for the preparation of excellent performance composite coatings. A characteristic feature of this process is that ceramic particles (ex. SiC, CNTs, TiN, and TiO2) suspended in a liquid medium migrate under the influence of an electric field (electrophoresis) and are deposited on an electrode. All charged ceramic particles used to form stable suspensions can be used in electrodeposition. However, electrodeposition is affected by parameters such as current density, particle concentration, and bath temperature. For example, Bebea and al. [5] found that Co-ZrO2 composite coatings were uniform and well bonded to the substrate, and the thickness of the coating increased with increasing current density. Xia and al. [6]  that entered and homogeneously dispersed in a composite coating increased the number of nuclei for nucleation of Ni grains and inhibition of grain growth. Parida and al. [7] reported a Ni-TiO2 composite coating on steel substrates directly prepared by electrodeposition from a bath containing a dispersion of TiO2 power in Watt's bath. TiO2 particles less than 100 nm in size were homogeneously co-deposited with Ni on steel substrate, and microhardness values increased after incorporation of TiO2 compared with a pure Ni deposition.

Experimental procedure
Pure nickel coating and Ni-TiO2 composite coatings were fabricated on mild steel substrate by electrodeposition. The electrolyte was the traditional Watts bath ( Table 1). The electrolytes were prepared by adding a range of loadings of 15, 20 and 25 g/L of constituent TiO2 nanoparticles (50 nm) in Watts's bath, respectively. During electrodeposition, pure nickel, and mild steel worked as an anode and a cathode. All the coatings were electrodeposited with the direct current and deposition time of 30 min. The electrolyte composition and preparation parameters are listed in Table 1. Before electrodeposition, the mild steel specimens were first polished, and then they were degreased in alkaline solution (NaOH 15 g/L + Na2CO3 50 g/L) for 10 min and rinsed in deionized water. After that, they were pickled in 10% HCl solution to remove oxide traces and followed by another rinse in deionized water. The electrolyte was magnetically stirred for 12 h to disperse the nanoparticles. After electrodeposition, the prepared specimens were cleaned with deionized water. The morphology and chemical composition were characterized by Field Emission Scanning Electron Microscopy (FSEM, JSM-7600 F). Structural investigations phase compositions of the coatings were analyzed by XRD method using a Bruker diffractometer (D8 advance model) with Cu K radiation (1.5406 Å). The microhardness was examined by Vickers microhardness tester type Wolpert Wilson Instruments model 402UD under loading 25g and holding 15s, and the average microhardness was calculated from 5 measurements. The mass of the electrodeposits was evaluated by weighing the samples before and after the electrodeposition. The anti-corrosion behavior was investigated by potentiodynamic polarization. Potentiodynamic polarization measurements were conducted by using a standard three-electrode cell with the coated samples as a working electrode, Platinum as auxiliary electrode and saturated calomel Ag/AgCl electrode as a reference electrode. All these electrodes were immersed in 3.5 wt % NaCl electrolyte. This cell was connected at voltalab 20 (PGP201) device working at a scanning rate of 0.5 mV/s and a potential range from -700 to -100 mV. Corrosion rate (mm/y), corrosion potential Ecorr (mV), current density Icorr (mA/cm2) and Tafel slopes (mV/s) were calculated by using Tafel extrapolation technique provided by Volta Master4 software. 3 Results and discussion 3.1 Surface morphology and composition of the coatings Fig. 1 shows surface morphologies of the coatings electrodeposited at different TiO2 nanoparticle loadings in the electrolyte. As shown in Fig. 1, Ni-TiO2 coating showed a typical regular spherical crystal structure. The results showed that the suspended TiO2 nanoparticle loadings in electrolyte strongly influenced the surface morphology evolution of the coatings. Fig, 2 shows that relationship between TiO2 contents and TiO2 concentration of nanoparticles was not linear. It was found that the TiO2 contents first increased then decreased. The maximum value of TiO2 content was obtained for 20g/L. This could be attributed to the agglomeration of TiO2 nanoparticles when the TiO2 content increased significantly [8]. Meanwhile, the agglomeration phenomenon raised the roughness and non-homogeneity of Ni-TiO2 coating [9].   The evolution of crystallite size of the coatings is shown in Fig. 4. It demonstrated that crystallite size was strongly dependent on the TiO2 contents in the coatings. As mentioned above, the incorporated TiO2 nanoparticles precluded the further growth of Ni crystallites and worked as new nucleus sites, then promoted the Ni crystallites refinement. More incorporated TiO2 nanoparticles would induce more Ni crystallites refinement. Thus, the Ni-TiO2 coating with 20 g/L of TiO2 possessed the smallest crystallite size. The evolution of microhardness of the coatings is shown in Fig. 5. As the TiO2 nanoparticle loadings increased from 0 to 20 g/L, the microhardness increased from 239 to 553.4 HV25. However, the microhardness decreased to 421 HV25 with further increasing the TiO2 nanoparticle loading to 25 g/L. It could be found that the Ni-TiO2 coating with the minimum crystallite size exhibited the maximum microhardness [12,13,14]. Grain size (nm)

Concentration of TiO 2 (g/l)
Grain size of the coating electrodeposition at different TiO2 nanoparticle loadings It could be found that the grain size decreased obviously due to the incorporation of TiO2 nanoparticles. And the decreased grain size contributed to the increase of microhardness, which was consistent with Hall-Petch behavior. Generally, dispersion strengthening and grain refinement strengthening are the main mechanisms for strengthening of metal and alloys [15]. Due to the incorporation of TiO2 nanoparticles in Ni-TiO2 coatings, the Ni matrix crystallite size decreased a lot. The grain refinement strengthening mechanism was quite effective for the improvement of microhardness of the Ni-TiO2 coatings. As strong obstacles for dislocation movement, the incorporated TiO2 nanoparticles in Ni matrix contributed to the dispersion strengthening. Thus, the Ni-TiO2 coating electrodeposited at 20 g/L possessed the maximum value of microhardness due to the smallest crystallite size and the most strong [111] orientation. Microhardness of the coating electrodeposited at different TiO2 nanoparticle loadings

Corrosion behavior of the coatings
Potentiodynamic polarization curves of the coatings are presented in Fig. 6, and the corrosion parameters (corrosion potential Ecorr and Corrosion current Icorr) are presented in be found that the corrosion current had decreased trend with increasing the TiO2 nanoparticle loading in electrolyte.
In the microstructure evolution, the crystallite size refinement played important roles in the improvement of the anti-corrosion behavior. From Fig. 7 and Table 2, it could be found that the decreased crystallite size enabled the Ni-TiO2 coatings with superior anti-corrosion behavior. This was because that the smaller crystallite size could facilitate the passivation of the nickel matrix [16,17]. In the FCC crystalline structure of Ni matrix, the dissolution of atoms of the (111) planes needed more energy than that of (200) planes [17]. Therefore, the formation of [111] orientation was beneficial to the anti-corrosion behavior of the Ni-TiO2 coatings. Furthermore, the uniformly distributed nanoparticles in the coating could act as physical barriers [2] to hinder the defect corrosion. Thus, the incorporation of TiO2 nanoparticles was beneficial to the anti-corrosion behavior of the Ni-TiO2 coatings.

Conclusions
Ni-TiO2 nanocomposite coatings were fabricated by electro deposition in watts bath. TiO2 nanoparticles reinforced Ni nanocomposite coatings revealed a much smoother and compact surface microstructure with excellent mechanical properties. Meanwhile, the corrosion resistance has been enhanced significantly. Effectively, the incorporated TiO2 nanoparticles precluded the further growth of Ni crystallites and worked as new nucleus sites, then promoted the Ni crystallites refinement. The crystallite size refinement played important roles in the improvement of the microstructure, microhardness and anti-corrosion behavior of Ni-TiO2 nanocomposite coatings. The Ni-TiO2 coating electrodeposited at 20 g/L of TiO2 possessed the maximum value of hardness and a good tendency for corrosion resistance.