Fabrication of duplex coatings on biodegradable AZ31 magnesium alloy by integrating cerium conversion (CC) and plasma electrolytic oxidation (PEO) processes

Abstract

In the present study, two types of duplex coatings were fabricated on pre-treated biodegradable AZ31 magnesium alloy by a combination of cerium conversion (CC) and plasma electrolytic oxidation (PEO) coating processes, namely CC followed by PEO coating (CCPEO) and PEO followed by CC coating (PEOCC). PEO coating was fabricated with an electrolyte system consisting of 7 g/l sodium silicate (Na2SiO3·10H2O) and 3 g/l potassium hydroxide (KOH). All the PEO coatings were formed using a pulsed DC power source at a constant current density of 60 mA/cm2 and a treatment time of 15 min. An electrolyte containing 10 g/l cerium nitrate hexahydrate (Ce (NO3)3·6H2O) with a pH of 3 adjusted by using H2O2 was used for the conversion coating process with the treatment time of 4 h. The phase composition, surface roughness, surface morphology, elemental composition and structural characteristics of the coatings were assessed by X-ray diffraction (XRD), an optical profilometer, scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) techniques. The electrochemical corrosion behavior of substrate and coatings was analyzed under a Kokubo 7.4 pH simulated body fluid (SBF) solution by open circuit potential (OCP) measurement, electrochemical impedance spectroscopy (EIS) with an equivalent circuit modelling and potentiodynamic polarization (PDP) methods. The scratch resistance and wettability of the coatings were analyzed by performing a scratch test using a Rockwell C diamond indenter and contact angle goniometer, respectively. The apatite forming ability of all the coatings and the substrate was studied after immersing in SBF solution for 14 days. Among all the coatings under this investigation, the CCPEO coating showed higher corrosion resistance in the SBF environment (icorr = 1.18 × 10−3 μA/cm2 and higher scratch resistance (Lc = 32.5 N). The high corrosion and scratch resistances of the CCPEO coating is attributed to its higher coating thickness and less surface porosity. The CCPEO coated sample also showed good apatite forming ability under physiological environment, making it a good candidate material for biodegradable implant applications.