Surface coating of magnesium alloy resorbable implants manufactured via superplastic forming

Magnesium alloys are considered as promising candidates for biodegradable orthopaedic implants, due to their mechanical properties and biocompatibility. Challenges related to the production of Mg-based prostheses are (a) the choice and the design of flexible manufacturing processes to produce complex-shaped parts (e.g. reproducing geometries of the human bones) and (b) the control of magnesium degradation rate in body fluids via specific surface coating techniques. Among flexible sheet metal forming processes, superplastic forming (SPF) allows to produce customized and complex-shaped implants characterized by a satisfying shape accuracy and surface quality. SPF can affect the microstructure and, thus, the corrosion resistance of the alloy, whose value is crucial in influencing the biodegradation rate. In the present contribution, a pulsed laser deposition of hydroxyapatite (HA)1 coating and a phosphate chemical conversion coating2 (see Scheme 1) were applied to AZ31 Mg alloy components, formed by SPF process, to decrease their degradation rate. Chemical-physical characterizations, electrochemical corrosion tests and electrochemical impedance spectroscopy were performed to assess the most promising deposition route and the possible beneficial effect of the applied coating on the corrosion behaviour of the superplastic formed AZ31 Mg components.