Atom probe tomography shows defect-driven degradation in plasma electrolytic oxidation-coated Mg-Ca alloys

Magnesium alloys are promising biodegradable implant materials, but often suffer from rapid corrosion and uncontrolled hydrogen evolution in physiological environments, calling for robust surface modification strategies to regulate degradation and gas release. Plasma electrolytic oxidation (PEO) coatings are produced on an ultra-high-purity Mg-0.45 wt. % Ca alloy using two phosphate-based electrolytes and characterized by advanced electron microscopy and atom probe tomography (APT) to resolve their multi-layered structure, phase composition, and nanoscale defects. Both coatings form a tri-layered structure: a dense crystalline MgO inner barrier, a porous intermediate MgO/amorphous phosphorus-rich zone, and an outer amorphous phosphorus-rich layer modulated by electrolyte ions. APT reveals nanoscale pores, Na segregation, and H-enriched surface products after corrosion. Early-stage in vitro testing in Hank's solution shows uniform dissolution of the dense outer layer of the coating formed in the ammonia-based electrolyte, accompanied by cracking and delamination in the intermediate layer. This links the observed defects to the degradation pathways and enhanced susceptibility to localized attack within the coating. These insights reveal how electrolyte chemistry controls phase balance, porosity, and corrosion resistance, guiding the design of PEO coatings for enhanced implant longevity. STATEMENT OF SIGNIFICANCE: Understanding and controlling the degradation of Mg-based biomaterials remains a central challenge for their clinical adoption. This study establishes a nanoscale mechanistic framework linking the chemistry of plasma electrolytic oxidation (PEO) coatings to the corrosion behavior of a Mg-Ca alloy in Hank's balanced salt solution (HBSS). By combining atom probe tomography (APT) with advanced microscopy analyses, we reveal that Na accumulation within defects and the amorphous coating regions are key drivers of localized corrosion and coating instability. These insights redefine the current understanding of PEO coating performance and establish new principles for desigining degradation-resistant, clinically viable, biodegradable implants.