Impact of TiO2/CeO2 nano-particles applied in PEO on hydrophobicity and corrosion resistance of Mg AZ21 alloy

Abstract A superhydrophobic TiO2/CeO2 surface was fabricated on Mg AZ21 alloy by using the plasma electrolyte oxidation (PEO) and the presence of nano-particles dramatically enhanced the hydrophobicity and corrosion resistance of the surface. Introduction Mg Alloys have attracted much attention due to their unique features such as a high ratio of strength to weight which led to using in a wide range of aerospace and automobile industries. Using PEO with simultaneous secondary surface treatment is one of the best routes to reach some brilliant surface features. In this study, the effect of TiO2 and CeO2 NPs on the hydrophobicity of Mg AZ21 surfaces fabricated by PEO was investigated [1]. Experimental An electrolyte with components of 16.8 g/l CaN2O6.4H2O plus 19.2 g/l (NH4) H2PO4 was prepared while another electrolyte was provided with the addition of 6 g/l TiO2 plus 3 g/l CeO2 in it. PEO was applied on 1×1 cm2 AZ21 (580 volts in 3 minutes) and eventually, the prepared surface was treated with trimethoxy(propyl)silane/ethanol solution. After that, the wettability and corrosion behavior of the hydrophobic surface was tested by using FIBRO SYSTEM AB (PG-X, Sweden model) and EG G Princeton Applied PARSTAT 2263 (USA) devices, respectively. Results PEO with using TiO2/CeO2 NPs and without NPs were shown in Fig. 1a and b, respectively. As can be seen, using NPs have a major impact on water contact angle (WCA) which led to increasing WCA from 96.7˚ to 150.8˚. In this case, there are some probable reasons in favour of supporting what happened exactly which resulted in higher WCA in the sample with NPs. First, the presence of NPs on the surface will bring about more surface roughness than the situation there are not any extra particles on the surface. Concerning Cassie-Baxter theory [1], if roughness increases, WCA will noticeably reach higher amounts. In other words, NPs can form various air-trapped areas on the surface and all of them act as a repellent force against any liquid. The second reason is attributed to the formation of a hierarchical structure on the surface due to the deposition of TiO2/CeO2 NPs [1]. As can be seen in Fig. 2a related to the sample with NPs, the hierarchical structure with anticipated roughness is obvious whereas there is no similar structure in the sample lacking NPs (Fig. 2b). Also, fabricating a superhydrophobic surface on AZ21 remarkably enhances its corrosion resistance. Fig. 3 compared the potentiodynamic polarization curves of neat AZ21, superhydrophobic PEO AZ21, and superhydrophobic PEO AZ21 with TiO2/CeO2 NPs. As can be seen in Fig. 3, neat AZ21 had the most negative corrosion potential (Ecorr) while superhydrophobic PEO AZ21 with TiO2/CeO2 NPs experienced the most positive Ecorr. In general, thanks to lower contact between water and surface in superhydrophobic surfaces, intake of water into the sample is noticeably lower than other surfaces and as a result superhydrophobic surfaces often show better corrosion resistance. To be precise, the more WCA surface shows, the more corrosion resistance is likely to be observed [1]. Conclusion -Superhydrophobic surfaces can dramatically enhance the corrosion resistance of samples. -Using nanoparticles is remarkably helpful and empirical to reach better results in the fabrication of superhydrophobic surfaces