Coatings on implants: Study on similarities and differences between the PCL coatings for Mg based lab coupons and final components

Highlights: • Corrosion resistance of pure Mg, uncoated and PCL coated Mg-4Zn-0.2Ca alloy screws are investigated. • Corrosion of coated Mg alloys proceeds via penetration of medium through pores in the polymer and degradation of substrate. • PCL layer peels off faster in CP Mg than in Mg-4Zn-0.2Ca due to the faster corrosion rate of CP Mg. • PCL coating adheres stronger to threaded portion of fabricated screws than smooth regions upon immersion in SBF. • Corrosion resistance of a coated fully threaded screw is better than that of a partially threaded screw. 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.