**3.3. Corrosion property of sol-enhanced coatings**

placement, the better wear resistance of coating. The nano-hardness and scratch displacement of Au-Ni coating was 2.55 ± 0.13 GPa and 58.8 ± 3.7 nm, respectively. At a low sol concentration, nano-hardness increases and scratch displacement decreases gradually with increasing TiO2 content. 12.5 mL/L TiO2 sol enhanced composite coating to the highest nano-hardness of 3.20 ± 0.15 GPa (26% increase) and the lowest scratch displacement of 22.5 ± 4.3 nm (reduced to 38%). However, further increasing the concentration of TiO2 to 50 mL/L led to a decrease of nano-hardness to 2.66 ± 0.12 GPa, although it was still higher than that of the un-doped Au-

Ni coatings. Meanwhile, the scratch displacement increases to 32.3±2.1 nm.

114 Electrodeposition of Composite Materials

**Figure 8.** Schematic drawings of the enhancement mechanism of TiO2 sol in the composite coating

The enhancement mechanism of TiO2 sol on the composite coating can be elaborated in Fig. 8. The improved nano-hardness of sol-enhanced Au-Ni-TiO2 coating could be attributed to the combined effects of grain refinement and dispersion strengthening. The highly dispersed reinforced phase should play a more important role as the grain size change is rather small. When proper TiO2 sol was added into electrolyte, a good dispersion strengthening and grain Corrosion resistance is another important property for many coating applications. It was generally understood that materials with two phase microstructure may promote galvanic corrosion in corrosive environments therefore reducing their corrosion resistance. However, the nano-dispersion of a second phase can largely avoid galvanic corrosion and does not reduce the corrosion resistance of the composite coatings.

Fig. 9 shows the surface morphologies of Ni-B and sol-enhanced Ni-B-TiO2 composite coatings after salt spray test for 120 h. The traditional Ni-B coating presented a corroded surface and the rust area can be clearly seen in Fig. 9a. The sol-enhanced Ni-B-12.5 mL/L TiO2 coating displays an improved corrosion resistance as only two small corrosion pits can be seen by the white arrows in Fig. 9b. However, the sol-enhanced Ni-B-50 mL/L TiO2 coating surface exhibited a corroded surface, similar to the un-doped Ni-B coating. A large area of coating surface was covered by rusts as shown in Fig. 9c.

**Figure 9.** Surface morphologies of coatings after salt spray test in 5 wt. % NaCl solution for 120 h without removing the corrosion products: (a) Ni-B, (b) Ni-B-12.5 mL/L TiO2 coating, and (c) Ni-B-50 mL/L TiO2 coating.

The corrosion behaviors of coatings have a close relationship with the sol content due to its influence on the coatings microstructure. As it is well known that the corrosion resistance of a coating largely affected by its compactness, porosity is often the cause that a coating failure from corrosion. During the sol-enhanced electroplating process, the in-situ formed nanopar‐ ticles well distributed in the grain boundary areas can decrease the quantity of defects in the coating layer, making the coating more compact and less penetrable. Additionally, the nanoparticle itself is an inert compound, in the form of uniformly distributed nanoparticles in the coating, does not form micro galvanic cells. Instead, it may play a role of reducing the reactivity of matrix metal, therefore improving the corrosion resistance of the nano-composite coatings. However, when excessive sol was added into the electrolyte, the nanoparticles tend to agglomerate which increases the quantity of defects (voids) and lead to a porous structure in the coating, resulting in significant deterioration of corrosion resistance [19].
