*2.1.2. Co-deposition of corrosion -resistant particles*

Apart from using the conventional electroplating technique (CEP), sediment co-deposition technique (SCD) has gained acceptance for higher level of co-depositing particles in the metal matrix with better dispersion and uniformity. Feng et al. [22] investigated the corrosion resistance and high-temperature oxidation resistance of Ni- Al2O3 nano- composite coatings using SCD technique. The incorporation of the nano-alumina particles in the Ni matrix refined the Ni crystallite and changed the preferential orientation of composite coating. Ni- Al2O3 composite coating produced compared with the one fabricated using CEP technique proved that the former had improved corrosion and oxidation resistance.

 11 Fayomi et al. [23] carried out a research on mild steel protection in chloride medium to develop ceramic composite coating that will reduce its susceptibility to corrosion attack. Zn-TiC/TiB composite coating was produced via electro-deposition method. The coatings showed better adhesion strength, improved hardness and enhanced corrosion resistance compared to the TiC/TiB -coated mild steel.

Three different nano- oxides of Al2O3, Cr2O3 and SiO2 have been reinforced in Zn matrix using electro-deposition process to produce composite coating on mild steel [24]. The incorporation of Al2O3, Cr2O3 and SiO2 nanoparticles in the ternary composite showed grain refinement, modified orientation of Zn matrix and good synergetic effect on the corrosion resistance of Znbased coatings. Zn-Cr2O3 nano- composite had the highest micro-hardness while Zn-Al2O3 nano-composite was found to exhibit the highest corrosion resistance coupled with lowest wear loss.

In the research work carried out by Yong et al [25], ultrasonic technique was used as a direct replacement of conventional mechanical agitation to foster effective stirring, dispersion and suspension of the second phase particles in the electrolytic bath. The XRD analysis of the developed nickel-based composite coatings containing TiN nano-particles shows that appli‐ cation of ultrasonic agitation and use of nano-particles of TiN refined the Ni grains, resulting in good corrosion resistance.

A comparative study of the properties of EN, EN-SiC, EN-PTFE and EN-SiC-PTFE composite coatings was conducted on the coating properties using electroless plating [26]. The results showed that electroless nickel (EN) composite coatings incorporated with PTFE and/or SiC particles demonstrated significantly improved mechanical and tribological properties as well as low surface energy, which are desired for anti-sticking and wear-resistant applications. It is evident that the Ni matrix substantially enhanced corrosion resistance by improving the autocatalytic properties and homogeneity.

Electro-deposition mechanism of Ni-Al composite coating was studied from a modified Watts solution by means of zeta potential analysis, voltammetry and electrochemical impedance spectroscopy (EIS) [27]. The findings showed that addition of Al particles shifts the reduction potential of Ni to more negative values, which is attributed to a decrease in the active surface area. Also the loop size of EIS curves increases with addition of Al particles to Ni electrolyte because of an increase in charge transfer resistance. It was also demonstrated that the codeposition behavior of Ni−Al composite coatings obeys the Guglielmi's model.

Shi et al. [28] studied effects of current density, stirring rate, nanoparticle concentration and temperature of plating bath on the composition of Ni–Co/SiC nano- composite coatings with various contents of SiC nano-particulates by electro-deposition technique. The result showed that Ni–Co/SiC nano- composite coatings have higher micro-hardness and better wear resistance than the Ni–Co alloy coating, which is attributed to the grain-fining and dispersive strengthening effects of the co-deposited hard SiC nano-particulates. The better corrosion resistance of the Ni-Co/SiC nano -composite coatings may be ascribed to the favorable chemical stability of the SiC nano-particulates, which help to reduce the whole size in the nano compo‐ site coatings and prevent the corrosive pits from growing up.

To improve the resistance of the hydro-transport pipe steel to corrosion and erosion in oil sand slurry, a Ni–Co–Al2O3 composite coating was fabricated by [29] using electrolytic deposition on X-65 pipe steel substrate. Electrolytic deposition of Ni–Co/Al2O3 composite coating showed significant improvement in the resistance of hydro-transport pipe steel to corrosion and erosion in oil sand slurry. The micro-hardness and wear resistance of the composite coating are much higher than the steel substrate and increase with the increasing Al2O3 particle amount in the coating.
