Abstract: The degradation behavior of biodegradable metallic materials (BMMs) critically determines their clinical applicability. As an emerging BMM candidate, the long-term degradation mechanisms and the discrepancies between in vitro and in vivo behaviors of molybdenum (Mo) remain insufficiently understood. This study systematically investigates the biodegradation mechanisms of pure Mo in simulated physiological solutions (PBS and HBSS) and living systems. Results demonstrate time-dependent corrosion characteristics of pure Mo. In vitro, HBSS exhibited significantly suppressed corrosion rates owing to Ca-P biomineralization forming protective composite oxide films, while PBS accelerated degradation by cracking induced by intrinsic stress in the film layer. The fundamental divergence between in vitro and in vivo degradation originates from the continuous oxidation of intermediate-valence Mo oxides (Mo⁴⁺/Mo⁵⁺) and dynamic reconstruction of Mo_xO_y/Ca-P composite layers in physiological environments. XPS analysis revealed gradient valence distributions in both systems, whereas in vivo specimens showed reduced surface Mo⁶⁺ enrichment with depth of descent, indicating precise modulation regulation of oxidation pathways by the biological microenvironment. Pure Mo consistently demonstrates uniform corrosion patterns in physiological conditions, with its self-repairing oxide films and Cl^- exclusion effects effectively preventing localized corrosion. This work pioneers the elucidation that Mo’s long-term degradation kinetics are primarily governed by film intrinsic stability, while local environments exert secondary modulation through interfacial modification. These findings provide fundamental insights into the biodegradation behavior of refractory metals and establish theoretical foundations for alloy design, structural optimization, and medical device development of Mo-based BMMs.

Key words: Biodegradable metallic materials, Molybdenum, Uniform corrosion, In vitro-in vivo degradation, Biomineralization