Pulsed Laser Deposition and Phosphate Chemical Conversion Coatings for Tailoring the Corrosion Behavior of AZ31 Resorbable Implants Manufactured via Superplastic Forming

Abstract Magnesium (Mg) alloys are promising candidates for biodegradable orthopedic implants, thanks to their mechanical properties and biocompatibility. Challenges related to the production of Mg-based prostheses are ( i ) the choice and the design of flexible manufacturing processes to produce complex shape components ( e.g., reproducing human bones) and ( ii ) the possibility of tailoring magnesium degradation rate in body fluids. Among flexible sheet metal forming processes, superplastic forming (SPF) allows to produce customized and complex parts characterized by a satisfying shape accuracy and surface quality. However, to be defined as a viable process for prostheses manufacturing, investigations about the effect of the processing route on the Mg alloy corrosion behavior as well as viable coatings for tailoring the implant degradation rate are needed. In the present work, electrochemical tests in simulated body environment were carried out on AZ31 samples before and after being subjected to the SPF process. Then, few micrometers thick hydroxyapatite coating obtained by Pulsed Laser Deposition (PLD) or by a phosphate chemical conversion (PCC) were investigated as possible solutions for controlling the degradation rate of the formed components. Chemical-physical characterizations (SEM analysis and Raman spectroscopy) and electrochemical tests (open circuit potential measurement, electrochemical impedances spectroscopy and polarization curves) were performed to assess the possible beneficial effect of the applied coating on the corrosion behavior of the superplastically formed AZ31 Mg components. Phosphate conversion coatings were effective in enhancing the corrosion resistance of AZ31 samples. This is evidenced by a two-order-of-magnitude increase in the overall impedance at low frequencies, from approximately 10 2 Ω ·cm 2 for the untreated surface to 10 4 Ω ·cm 2 with the phosphate conversion coating. This significant increase directly indicates a substantial slowdown in the kinetics of magnesium dissolution and water reduction.