Influence of Nanoparticles on Corrosion Resistance of Electrodeposited Zn-Ni Coating on Steel Electrodes Sheong Wei NG Cologne University of Applied Sciences, Cologne/Germany, 2015

Introduction Zinc-Nickel (Zn-Ni) coatings have been receiving a lot of attention due to their higher corrosion resistance and better mechanical characteristics. The corro-sion resistance of the Zn-Ni coatings can be further improved by introducing nanoparticles (NPs) into the composite. Titanium dioxide (TiO2), silicon dioxide (SiO2) and aluminum oxide (Al2O3) are suitable for use as electrodeposited Zn-Ni-NP coatings.

Comparison of Zn-Ni and Zn-Ni-TiO2 coatings The Zn-Ni-TiO2 coating polarization curve (PC) shows a marked split toward more negative and positive potential values, which reduces the hydrogen reduction process and consequently slows down the corrosion rate of the coating. The reason behind the higher corrosion resistance is the physical barriers created by TiO2 NPs, which isolate the coating from corrosive environments [3].

Electrodeposition Electrodeposition (illustrated in Fig. 1) offers several advantages compared to other coating methods [2]: • Precise control • Low energy requirement • Uniform deposition • Low production cost for large area samples • Good reproducibility and versatility • Capability to coat complex component

geometries • Higher production rate Fig. 1: Illustration of metallic composite electrodeposition [1]

Fig. 2: Scanning electron microscope (SEM) images and PCs for Zn-Ni and Zn-Ni-TiO2 coatings (adapted from [3]).

Comparison of Zn-Ni and Zn-Ni-SiO2 coatings The shift of Zn-Ni-SiO2 coating anodic PC toward more positive values indicates a change in the content ratio of the alloy and higher corrosion resistance compared to Zn-Ni coatings [4]. The smaller surface area of smooth piled grain structure of Zn-Ni-SiO2 coatings, compared to branched acicular structure of Zn-Ni coatings, is the reason for their higher corrosion resistance.

Fig. 3: SEM images and anodic PCs for Zn-Ni and Zn-Ni-SiO2 coatings (adapted from [4]).

Effect of TiO2 NPs concentration on Zn-Ni coating corrosion resistance To study the effect of TiO2 NPs concentration (x) upon the Zn-Ni coating corrosion resistance, the following samples were prepared and investigated: • S0 : x=0 g/l • S3 : x=3 g/l • S5 : x=5 g/l • S10 : x=10 g/l The reducing trend of corrosion current (icor) (see Table 1), which is directly proportional to the corrosion rate, is due to the decreased surface contact of the Zn-Ni coating with the corrosive environment as x increases. However, the lowest icor is shown by S5 rather

than S10. As x increases to 10 g/l, TiO2 NPs disturb the crystallization process [5] and cause the surface area of coatings to crack (see Fig. 4). The cracked surface favors corrosion.

Fig. 4: SEM images and PCs for Zn-Ni and Zn-Ni-TiO2 coatings with different x (adapted from [5]).

Table 1: Corrosion potential (Ecor) and icor for the coatings

Conclusion • TiO2 and SiO2 NPs increase the corrosion resistance of Zn-Ni coatings

due to the physical barrier formed by the NPs, which isolate the coatings from the corrosive environment.

• Concentration of TiO2 NPs affects the morphology of Zn-Ni coatings

and improves their corrosion resistance. However, judging from their icor, the improvement is not significant.

• Cracking on the surface of the highly concentrated Zn-Ni-NPs coatings hinders their corrosion resistance.

References [1] F.C. Walsh, C. Ponce de Leon: A review of the electrodeposition of metal matrix composite coatings by inclusion of particles in a metal layer: an established and diversifying technology, Transactions of the IMF 92 (2014) 93-98 [2] R.K. Saha, T.I. Khan: Effect of applied current on the electrodeposited Ni-Al2O3 composite coating, Surface and Coating Technology 205 (2010) 890-895 [3] B.M. Praveen, T.V. Venkatesha: Electrodeposition and corrosion resistance properties of Zn-Ni/TiO2 nano composite coatings, International Journal of Electrochemistry 261 (2011) 407-411 [4] O. Hammani, L. Dhoubi, P. Bercot, E.M. Rezrari, E. Triki: Study of Zn-Ni alloy coatings modified by nano-SiO2 particles incorporation, International Journal of Corrosion 2012 (2012) [5] D. Blejan, D. Bogdan, M. Pop, A.V. Pop, L.M. Muresan: Structure, morphology and corrosion resistance of Zn-Ni-TiO2 composite coatings, Optoelectronics and Advanced Materials Rapid Communication 5 (2011) 25-29