The potentiodynamic polarization curves of pure Ni and Ni–TiO2 nanocomposite coatings in 0.5 M NaCl, 1 M NaOH and 1 M HNO3 solutions are presented in Figs. 1–3. Corrosion characteristics such as corrosion potential (Ecorr), corrosion current (icorr) and anodic/cathodic Tafel slopes (βa and βc) were obtained from the intersection of cathodic and anodic Tafel curve tangents using the Tafel extrapolation method. Also, the corrosion rates (rcorr) and polarization resistance (Rp) a r e calculated from these data. Table 1 represents the results obtained from polarization tests. It is obvious that with increasing of TiO2 nanoparticle content in coating the corrosion current decreases and the corrosion potential shifts to a more positive potential. The data clearly reveals the improvement of corrosion protection by TiO2 nanoparticles. Polarization resistance (Rp) increases TiO2 nanoparticle content in coating. It can be concluded that codeposited TiO2 nanoparticles in Ni matrix of coating increases the corrosion resistance in salty, acidic and alkaline solutions. Obviously, the TiO2 nanoparticles played a major role for improving the corrosion protection in two mechanisms. Firstly, these TiO2 nanoparticles act as inert physical barriers to the initiation and development of defect corrosion, modifying the microstructure of the nickel layer and hence improving the corrosion resistance of the coating. Secondly, dispersion of TiO2 nanoparticles in the nickel layer results in formation of many corrosion micro cells in which the TiO2 nanoparticles act as cathode and nickel metal acts as anode because of the standard potential of TiO2 more positive than nickel. Such corrosion micro cells facilitated the anode polarization. Therefore, in the presence of TiO2, localized corrosion is inhibited, and mainly homogeneous corrosion occurs.
Fig. 1. Tafel polarization curves for pure Ni coating and Ni–TiO2 nanocomposite coatings in 1 M NaCl solution.
Fig. 2. Tafel polarization curves for pure Ni coating and Ni–TiO2 nanocomposite coatings in 1 M NaOH solution.
Fig. 3. Tafel polarization curves for pure Ni coating and Ni–TiO2 nanocomposite coatings in 1 M HNO3 solution.
Table 1
Corrosion characteristic obtained from potentiodynamic polarization measurement for pure Ni coating and Ni–TiO2 nanocomposite coatings.
Fig.3 shows the polarization curves of pure Ni coating and Ni–TiO2 nanocomposite coatings in 1 M HNO3 solution. Passive layer formation is observed in anodic region of polarization curves. This behavior is due to the formation of NiO passive film in acidic solution. Passive layer formation current density of Ni–TiO2 nanocomposite coatings (about 0.01 A/cm2) is less than that of pure Ni coating (about 0.02 A/cm2). On the other hand, passive layer formation potential of Ni–TiO2 nanocomposite coatings (about 290 mv) is more negative compare to that of pure Ni coating (about 480 mv). It can be concluded that the presence of TiO2 nanoparticles in Ni matrix coating aids in the formation of a passive layer. The passive current of Ni–TiO2 nanocomposite coatings (about 10−3 A/cm2) is more than that of pure Ni coating (about 10− 4 A/cm2). It means that passive layer corrosion rate of Ni–TiO2 nanocomposite coatings is more than pure Ni coating. The presence of TiO2 nanoparticles in the coating surface disturbs the continuity of NiO passive layer and accelerates the destruction of passive layer which causes the increasing of corrosion rate.
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