Featured Letter A highly compact coating responsible for enhancing corrosion properties of Al-Mg-Si alloy Mosab Kaseem, Kyungsu Choi ⇑ , Young Gun Ko ⇑ School of Materials Science and Engineering, Yeungnam University, Gyeongsan 38541, Republic of Korea article info Article history: Received 2 January 2017 Received in revised form 16 March 2017 Accepted 20 March 2017 Available online 21 March 2017 Keywords: Al-Mg-Si alloy Plasma electrolytic oxidation K 2 SiF 6 Ceramic Corrosion abstract The formation of a highly compact coating responsible for improvement in corrosion properties of Al-Mg-Si alloy was investigated. In this study, a plasma electrolytic oxidation (PEO) was performed in a citrate-aluminate electrolyte with potassium hexafluorosilicate (K 2 SiF 6 ), and the results were compared to the counterpart without K 2 SiF 6 . The addition of K 2 SiF 6 to the present electrolyte would give rise to the highly compact coating due to the insoluble compounds of SiO 2 and AlF 3 which were incorporated uni- formly throughout the coating. This would lead effectively to superior corrosion-protection properties to the case without K 2 SiF 6 . Ó 2017 Elsevier B.V. All rights reserved. 1. Introduction Recently, the formations of conformal coatings on light-weight metals and their alloys have been suggested extensively in order to improve both the mechanical and electrochemical properties for industrial applications [1]. In this regard, a plasma electrolytic oxi- dation (PEO) has regarded as one of the promising surface modifi- cation methods to fabricate the thick, compact, and hard ceramic coatings on light-weight metals and their alloys [2,3]. The funda- mental principle of the present method relied upon the genera- tions of the short-lived micro-discharges on the surfaces of metals which were immersed in a suitable alkaline-based elec- trolyte by applying an extremely high voltage. During PEO coating, both the extremely high temperature and pressure through the discharge channels of the coating would cause a localized melting of the oxide layer and the formations of a highly adhesive coating would be promoted such as outer porous layer and inner barrier layer. Hence, the use of PEO process to yield the desirable morphol- ogy and structure of the coating would be of great importance by taking the effects of the processing parameters into account because high corrosion-protection properties would be required in various recent industries. For instance, the surface reforming of Al and its alloys could be applied in electronic and automobile parts which needed electrochemical stability with high corrosion resistance [4]. Thus, a great deal of research effort has been aiming at enhancing the corrosion properties through PEO utilizing elec- trolyte modification which was connected to the significant changes in the structure and phase of the coating. Kaseem et al. [2] reported that the addition of sodium benzoate into the NaAlO 2 - KOH electrolyte resulted in higher ratio of a-Al 2 O 3 /c-Al 2 O 3 which would improve the corrosion properties of 6061 Al alloy. On the other hand in this direction, the chemical compounds with F to trigger the formation of protective coatings would be useful [5–7]. Ryu and Hong [5] demonstrated that the polarization resis- tance of AZ91 Mg alloy treated in the KF-KOH-NaAlO 2 electrolyte was higher than those in the KOH-NaAlO 2 electrolyte due to the formation of F-rich inner layer with suppressing the micro-pores. Wang et al. [6] discovered using PEO coatings with respect to the concentration of KF added to the electrolyte that corrosion resis- tance of AZ31 Mg alloy would depend primarily on the amount of MgF 2 compounds in the coating. It was likely based on the struc- tural morphologies of the coatings that the effect of F-rich com- pounds seemed to be localized to the ‘only’ inner layer. To improve electrochemical properties further after PEO, the condi- tions for a highly compact coating should be optimized with a spe- cial focus on the outer layer because the volume fraction of the outer layer was higher than that of the inner layer. Thus, the effect of potassium hexafluorosilicate (K 2 SiF 6 ) in the electrolyte on the corrosion behavior of Al-Mg-Si alloy via PEO under low current density condition will be investigated systematically in this study. The main purpose of the present study is to look into the structure- properties relationship of the highly compact coating formed on Al-Mg-Si alloy sample via PEO in the alkaline-based electrolyte http://dx.doi.org/10.1016/j.matlet.2017.03.106 0167-577X/Ó 2017 Elsevier B.V. All rights reserved. ⇑ Corresponding authors. E-mail addresses: choi6915@ynu.ac.kr (K. Choi), younggun@ynu.ac.kr (Y.G. Ko). Materials Letters 196 (2017) 316–319 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/mlblue