Corrosion behaviour of polymer coated Mg alloy screw is compared with a laboratory coupon with emphasis on the adhesion of coating to surface features such as threads. The corrosion behaviour of poly (ε-caprolactone) (PCL) coated commercial pure Mg (CP Mg) and Mg-4Zn-0.2Ca alloy in simulated body fluid (SBF) is investigated. CP Mg with a high corrosion rate (8.426 mmyr− 1) formed large amount of corrosion products on the surface which pushed out the PCL layer. In contrast the Mg-4Zn-0.2Ca alloy with a corrosion rate of 2.481 mmyr− 1 held the PCL coating intact for up to 72 h. Fully and partially threaded screws of the Mg-4Zn-0.2Ca alloy were fabricated and coated with PCL. It was found that pH variation of the medium depends on PCL adhesion to screw, which was better in the threaded regions than in smoother areas. As a result, the fully threaded screws showed slower pH rise than partial threaded ones. The crests and troughs of the threads act as anchoring points for the polymer. It was found in the current study that the increased adhesion of PCL coating to threaded Mg alloy substrate dominates over the increased corrosion rates of threaded surface. © 2017 Elsevier Ltd