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During the last decade, magnesium and its alloys have been extensively studied to develop a new generation of biodegradable medical implants. The fast degradation rate of pure magnesium and related alloys in the physiological environment poses significant challenges to devices made of these materials for biomedical applications. In this study we have designed and fabricated biodegradable helical stents made of AZ31 magnesium alloy, and have explored their in vitro corrosion behavior in Dulbecco's Modified Eagle's Medium (DMEM). The corrosion rate was significantly reduced by surface modifications of the helical stent, achieved through applying a biocompatible Parylene C polymer coating, or via appropriate chemical etching of the devices in inorganic solutions. The corrosion rates of the coated AZ31 Mg helical stents were compared with uncoated samples used as a control. The results achieved indicated that all tested surface modifications successfully inhibited metal corrosion rates in vitro. Materials coated with Parylene C coating revealed a maximum corrosion rate reduction of 70% to 85% in DMEM solution.