A preliminary study of calcium containing plasma electrolytic oxidation coatings on AM50 magnesium alloy

Abstract

Magnesium alloys owing to their light weight, excellent castability and good mechanical properties are employed for applications in automotive and electronic industries [1]. Bio-degradable nature of magnesium alloys makes them an attractive candidate for implant materials [2]. The corrosion resistance of magnesium alloys is of concern and a variety of surface modification technologies is contemplated for enhancing their service life and durability, especially in aggressive environments [3–5]. In recent times, plasma electrolytic oxidation (PEO) has been the most preferred process for the treatment of magnesium alloys [6]. The majority of the research papers published on the PEO coating of magnesium alloys are based on either ammonium hydroxide or potassium hydroxide electrolytes [7–10]. For the bio-medical applications, it is significant to prepare the coating containing Ca and P, which provides better biological compatibility and biological activity. Wen et al. [11] have recently reported a cathodic hydroxyapatite deposition procedure on magnesium alloys. As the PEO coatings offer a much superior corrosion resistance than the conversion coatings or cathodically deposited coatings, in the current work we have made an attempt to produce a PEO coating containing calcium and/or calcium compounds from a calcium hydroxide (Ca(OH)2)-based electrolyte on an AM50 magnesium alloy and compare the corrosion performance with that of a coating obtained from a conventional KOH-based electrolyte with the same level of phosphate ions under identical processing conditions. A successful development of a calcium containing coating in this preliminary investigation shall lead us to explore this coating further on the bio-degradable magnesium alloys for implant applications. Specimens of size 15 mm 9 15 mm 9 4 mm of a cast AM50 magnesium alloy were used as substrate for the PEO processing. They were ground successively with 500, 800, 1200 and 2500 grit emery sheets and cleaned with acetone before the PEO treatment. The PEO process was carried out using a pulsed DC electrical source with a pulse ratio of ton:toff = 2:20 ms in two alkaline electrolytes viz., (a) 2 g/L KOH ? 10 g/L Na3PO4 and (b) 2 g/L Ca(OH)2 ? 10 g/L Na3PO4. The coatings were obtained at a constant current density of 30 mA cm for 15 min. The temperature of the electrolytes was always kept at 10 ± 2 C by a water cooling system. The surface morphology of the PEO-coated specimens was examined in a Cambridge stereoscan scanning electron microscope (SEM), and X-ray diffraction (XRD) was performed using a Bruker X-ray diffractometer with Cu Ka radiation to determine the phase composition. The elemental composition of the PEO-coated specimens was assessed in a Zeiss Ultra 55 scanning electron microscope equipped with an energy dispersive X-ray spectrometer (EDS). Electrochemical impedance spectroscopy (EIS) tests were carried out using a Gill AC potentiostat/frequency response analyser with a three electrode cell setup. The measurements were performed at open circuit potential with an AC amplitude of 10 mV over a frequency range of 0.01 Hz to 30 kHz on the PEO-coated specimens exposed to 0.1 M NaCl solution for different durations viz., 0.5, 2, 5, 10, 25 and 50 h. As the calcium hydroxide coating survived the 50-h EIS test, the test was extended up to 150 h to understand its long-term stability in this corrosive environment. P. Bala Srinivasan (&) J. Liang C. Blawert M. Stormer W. Dietzel Institute of Materials Research, GKSS-Forschungszentrum Geesthacht GmbH, 21502 Geesthacht, Germany e-mail: bala.srinivasan@gkss.de