Dual incorporation of SiO 2 and ZrO 2 nanoparticles into the oxide layer on 6061 Al alloy via plasma electrolytic oxidation: Coating structure and corrosion properties S. Fatimah, M.P. Kamil, J.H. Kwon, M. Kaseem, Y.G. Ko * School of Materials Science and Engineering, Yeungnam University, Gyeongsan, 38541, Republic of Korea article info Article history: Received 31 July 2016 Received in revised form 15 September 2016 Accepted 8 November 2016 Available online xxx Keywords: Coating materials Plasma electrolytic oxidation SiO 2 ZrO 2 Corrosion resistance Potentiodynamic polarization test abstract The present work investigated the inﬂuence of SiO 2 and ZrO 2 incorporated nanoparticles on the coating structure and the corrosion behavior of 6061 Al alloy coated by plasma electrolytic oxidation (PEO). To achieve this purpose, a set of PEO treatments were conducted in alternating current condition at a constant current density of 130 mAcm 2 in phosphate electrolytes containing each and both nano- particles, respectively. Microstructure observations revealed that when SiO 2 or/and ZrO 2 nanoparticles were added to the electrolyte, both size and fraction of micropores tended to be decreased, which would be attributed to the incorporation of nanoparticles. SiO 2 nanoparticles were preferentially embedded in the vicinity of micropores, meanwhile ZrO 2 nanoparticles preferentially ﬁlled the cracks. m-ZrO 2 was mostly converted to o-ZrO 2 while SiO 2 were partly converted to o-SiO 2 and another part to mullite. Based on potentiodynamic polarization tests in 3.5 wt% NaCl solution, the coatings containing both SiO 2 and ZrO 2 nanoparticles exhibited excellent corrosion protection properties due to the combination of their roles as the micropores blocker and cracks ﬁller so that the microstructural defects were minimized. © 2016 Elsevier B.V. All rights reserved. 1. Introduction Aluminium and its alloys have been used widely in various ﬁelds of engineering, such as aircraft, electronic, automobile, and archi- tectural application owing to the combination of light weight and high speciﬁc strength. However, low surface hardness and corro- sion resistance have limited their applications [1]. In recent years, plasma electrolytic oxidation (PEO) has attracted incredible interest as an innovative surface treatment to form protective coatings on valve metals such as Al, Mg, and Ti which exhibit higher mechanical properties as well as higher corrosion resistance compared to the preceding anodization process [2,3]. Recent studies demonstrated that the incorporation of highly stable metal oxides, such as TiO 2 , ZrO 2 , SiO 2 , CeO 2 , and MoO 2 as secondary particles was an effective method to enhance the corrosion protection even under extreme environments where the micropores generated by plasma spark during PEO process might be blocked to some extent [4e9]. In addition, despite of the ability of nanoparticles to block the micropores, Matykina et al. [10] re- ported that a signiﬁcant improvement of corrosion protection was obtained by the incorporation of ZrO 2 on the coatings of pure aluminum which was expected to produce zirconia-toughened alumina (ZTA) whose superior properties, such as corrosion resis- tance, fracture toughness, and wear resistance was desirable. Furthermore, Khristal et al. [11] reported that SiO 2 nanoparticles could decrease surface roughness of coatings by 30% which was favored for improving corrosion-protection properties. Recently, most of the studies [10,12] utilized the incorporation of one type of particles. Moreover, Gnedenkov et al. [12] reported that the addi- tion of SiO 2 or ZrO 2 to silicate-ﬂuoride electrolyte would lead to an increment of the coating thickness formed in MA8 Mg alloy via PEO process. However, little information was available regarding the microstructure and corrosion behavior of the coatings of Al alloys subjected to PEO when both SiO 2 and ZrO 2 nanoparticles were added into the electrolyte. Therefore, the main aim of the present study is to investigate the inﬂuence of double incorporation of SiO 2 and ZrO 2 nanoparticles into the coatings via PEO process in the phosphate-based electro- lyte in relation to their structural characteristic and corrosion properties. * Corresponding author. E-mail address: younggun@ynu.ac.kr (Y.G. Ko). Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom http://dx.doi.org/10.1016/j.jallcom.2016.11.098 0925-8388/© 2016 Elsevier B.V. All rights reserved. Journal of Alloys and Compounds xxx (2016) 1e7 Please cite this article in press as: S. Fatimah, et al., Dual incorporation of SiO 2 and ZrO 2 nanoparticles into the oxide layer on 6061 Al alloy via plasma electrolytic oxidation: Coating structure and corrosionproperties, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/ j.jallcom.2016.11.098