Corrosion and mechanical performance of novel electrochemical oxidation coatings on AZ31 magnesium alloys for biomedical applications

Magnesium-based implants offer significant benefits for biomedical applications due to their excellent biocompatibility and ability to biodegrade in physiological environments. However, their rapid corrosion can compromise mechanical integrity and hinder clinical translation. This study investigates the corrosion resistance and mechanical integrity of novel soft-sparking electrochemical oxidation (ECO) coatings on AZ31 magnesium alloys, highlighting their potential for biomedical applications. Unlike conventional plasma electrolytic oxidation (PEO), the soft-sparking ECO process operates under milder conditions and avoids dielectric breakdown, producing more uniform, adherent coatings even on complex geometries. Coatings measuring 5, 10, and 15 μm thick were made from five distinct electrolytes: phosphate (P), high phosphate (P(H)), phosphate-silicate (PS), phosphate-fluoride (PF), and phosphate-fluoride-silicate (PFS). These were evaluated regarding porosity, roughness, adherence, and corrosion performance in a 5 M NaCl solution. The most promising coating (PF) was selected for further electrochemical and mechanical analysis, including screw insertion, four-point bending, and scratch testing. Our findings reveal that the coatings reduce corrosion rates by up to 35 times compared to the uncoated alloy while maintaining excellent adhesion even under plastic deformation. Notably, this work presents the first systematic study integrating mechanical integrity assessments with corrosion analysis of soft-sparking ECO coatings on complex magnesium geometries, offering a novel surface modification approach for next-generation biodegradable Mg-based implants.