Improving the corrosion resistance and bioactivity of magnesium by a carbonate conversion-polycaprolactone duplex coating approach

Decreasing the rate of degradation of Mg has been the focus of developing various types of coatings for Mg. Polycaprolactone (PCL) coating though capable of providing short-term improvement in corrosion resistance for Mg, the porous nature of the PCL coating and the inside out corrosion of Mg has led to cracking and delamination of the PCL coating. Poor adhesion of the PCL coating on Mg is another limitation. To overcome these limitations, the present study aims to explore magnesium carbonate coating as a pre-treatment. The study proposes a duplex coating approach, which involves deposition of magnesium carbonate by chemical conversion method as the first layer over which PCL coating is formed by dip coating method. The morphological features, nature of functional groups, phase content, adhesive strength, etc., of the magnesium carbonate and duplex coating were evaluated by scanning electron microscopy, FT-IR spectroscopy, X-ray diffraction measurement and tensile test, respectively. The corrosion behaviour of magnesium carbonate and duplex coating in Hanks’ solution was evaluated by potentiodynamic polarization, electrochemical impedance spectroscopy and immersion tests. The bioactivity of the magnesium carbonate and duplex coating was evaluated by immersing them in simulated body fluid. The extent of apatite growth served as a measure of their bioactivity. The study reveals that MgCO3 coating served as an excellent base for the deposition of PCL layer and the duplex coating offered a good corrosion resistance in Hanks’ solution and exhibit better bioactivity in simulated body fluid. The study concludes that magnesium carbonate-PCL duplex coating is a viable approach to reduce the rate of corrosion of magnesium and to improve its bioactivity. Keywords: carbonate conversion coating; polycaprolactone; duplex coating; corrosion resistance; bioactivity