Processing – structure – degradation relationships in spark-processed AZ91 magnesium alloy for biodegradable orthopaedic implants

Magnesium (Mg) and its alloys are promising candidates for next-generation biodegradable implants owing to their biocompatibility, suitable mechanical properties, and ability to gradually degrade in vivo, eliminating the need for secondary removal surgery. However, their rapid corrosion in physiological environments remains a challenge. Wire Electrical Discharge Machining (WEDM) is increasingly considered for manufacturing complex Mg-based medical devices, yet its influence on surface integrity and degradation behaviour requires careful evaluation. In the present study, the in vitro corrosion performance of polished (Control), and polished/WEDM-processed AZ91 Mg alloy was systematically investigated. Specimens were immersed in simulated body fluid (SBF) for up to 168 h, following ASTM G31-72, and characterised using SEM/EDX, XRD, and electrochemical impedance spectroscopy (EIS). WEDM processing produced a re-solidified layer with micro-cracks, selective evaporation of Al and Zn, while polished samples retained α-Mg traces. A higher maximum area polarisation resistance for polished samples of 280 Ω cm² was obtained after 72 h immersion in SBF (pH 7.4) maintained at 37 ± 0.5 °C. EIS revealed higher polarisation resistance for polished samples (peak 280 Ω cm² at 72 h) vs. WEDMed (70 Ω cm² at 168 h). Hydrogen evolution rate (HER) and pH monitoring indicated more rapid alkalisation for WEDMed samples, corroborating accelerated corrosion. Post-immersion analysis showed a Ca/P ratio of 1.63 on polished samples, indicating stable hydroxyapatite deposition, compared to 1.29 for WEDMed surfaces, signifying less crystalline Ca – P phases. XRD corroborated these trends, revealing well-crystallised apatite on polished surfaces and predominantly soluble magnesium salts on WEDMed alloys. While WEDM offers manufacturing precision, its surface defects compromise corrosion resistance and bioactivity, highlighting the need for post-processing treatments to optimise implant performance.