Improving corrosion behavior and in vitro bioactivity of plasma electrolytic oxidized AZ91 magnesium alloy using calcium fluoride containing electrolyte Aidin Bordbar Khiabani, Arezoo Ghanbari, Benyamin Yarmand ⇑ , Ali Zamanian, Masoud Mozafari Nanotechnology and Advanced Materials Department, Materials and Energy Research Center (MERC), Karaj, Iran article info Article history: Received 27 June 2017 Received in revised form 20 September 2017 Accepted 15 October 2017 Available online 16 October 2017 Keywords: Plasma electrolytic oxidation AZ91 CaF 2 abstract Modified oxide layer was successfully prepared on AZ91 magnesium alloy by plasma electrolytic oxida- tion (PEO) using phosphate electrolyte containing calcium fluoride (CaF 2 ). Evaluating the corrosion behavior of the oxidized AZ91 magnesium alloy by potentiodynamic polarization in simulated body fluid (SBF) solution indicated that addition of CaF 2 to electrolyte leads to considerable decrease in corrosion rate caused by surface porosity, oxide layer thickness, and formation of MgF 2 phase. Electrochemical impedance spectroscopy (EIS) revealed that the resistance of the outer porous and the inner barrier parts of the oxide layer increased by 3 and 41 times, respectively. Presence of biological calcium ion along with fluorine and phosphorous ions in the oxide layer composition created a higher driving force for nucle- ation and growth of bioactive layer by decreasing the contact angle with SBF solution. Ó 2017 Elsevier B.V. All rights reserved. 1. Introduction Biodegradable magnesium alloys have been widely used in fab- rication of biomedical orthopedic implants owing to their similar characteristics to natural bone. In order to create a strong connec- tion to the bones and to prevent inflammation at the contact to the tissue, these implants should have a high corrosion resistance and bioactivity in physiological environment [1]. Plasma electrolytic oxidation (PEO) process is a novel method employed in surface modification of magnesium implants. In this process, an oxide layer is prepared as protector over the metal due to oxidation in electrolyte, the characteristics of which is affected by the electrolyte composition [2,3]. The results of various studies have demonstrated that addition of fluorine ion to the elec- trolyte using potassium fluoride leads to improved corrosion and tribological behavior of PEO treated magnesium alloys [4,5]. However, the toxicity of this precursor has limited its use specifi- cally in environmentally friendly PEO process. Among the precur- sors containing fluorine, calcium fluoride (CaF 2 ), due to its low toxicity, low price, and abundance, is considered an appropriate candidate to increase the corrosion resistance of PEO treated magnesium alloys [6]. Moreover, presence of biological calcium ion in this material plays a major role in improving the bioactivity behavior of oxidized magnesium alloys. This research is aimed at finding a proper candidate to enhance PEO treated AZ91 magnesium alloy’s corrosion and bioactivity behavior at the same time. To do so the effect of adding CaF 2 to the phosphate electrolyte on PEO treated AZ91 magnesium alloy’s characteristics is examined. 2. Material and methods 2.1. PEO treatments The PEO process was performed at constant current density of 100 mA/cm 2 , a duty cycle of 25%, stirring and cooling system. The polished AZ91 sheets were used as the working electrode and stainless steel as the counter electrode. Aqueous electrolyte was prepared using solution of 5 g/l K 3 PO 4 Á3H 2 O (Merck), 2 g/l KOH (BDH Chemical LTD), and 3 g/l CaF 2 (Merck) in distilled water, pH = 11.5. Sample treated in CaF 2 -free electrolyte coded as A, whereas sample oxidized in electrolyte containing CaF 2 coded as B. 2.2. Characterization and analysis The microstructural features and elemental composition of the specimens were analyzed using a scanning electron microscope (SEM, TESCAN Vega3) equipped with an energy dispersive X-ray https://doi.org/10.1016/j.matlet.2017.10.072 0167-577X/Ó 2017 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: byarmand@merc.ac.ir (B. Yarmand). Materials Letters 212 (2018) 98–102 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/mlblue