Magnesium (Mg) degrades in the human body without any harmful reaction products [1]. In order to adjust the corrosion behavior for biodegradable implants, the surface is modified by plasma electrolytic oxidation (PEO) which enhances the corrosion and wear resistance [2]. Bone screw threads made of ZX10 (Mg-1Zn-0.3Ca) were produced and modified with PEO to improve the corrosion fatigue behavior. The surface modification was applied to specimens of ZX10 alloy with smooth (reference) and threaded measurement areas. The corrosion fatigue tests were performed in a stress-controlled manner at different stress horizons, a stress ratio of -1 (fully reversed loading) and a frequency of 6 Hz. A 0.9 wt.-% NaCl solution was used as the electrolyte. The relaxation time to reach the resting potential was 4 h. The smooth samples failed for the stress amplitude 20 MPa after about 1.5∙10⁷ load cycles and approx. 50 h in the medium. Mg-alloys and Mg PEO modified alloys in corrosive media show a reduced fatigue behavior in general, due to corrosion pitting and hydrogen embrittlement. Cracks and pores act as crack initiation areas under cyclic load inside the PEO. Those initiation areas lead to a reduction of fatigue strength which could be shown for threaded and smooth samples. Further, the ZX10 undergoes a softening and hardening within the first 1.2∙10⁴ cycles. The PEO react different with micro cracks within the first 1∙10² cycles which releases the substrate material to the media. As reaction to this, the electrical potential drops drastically. In addition to the this, the samples with threads failed significantly earlier with 6∙10⁶ load cycles. This behavior was expected due to the threads acting as notches, as the superposition of mechanical and corrosive loads weakens the failure-prone thread areas under the PEO. The PEO-modified surface was further subjected to a sealing process to improve the corrosion fatigue properties.

The authors would like to thank the AiF Projekt GmbH (project no. KK5072218FF1) and the Federal Ministry of Economics and Climate Protection for funding the research project “Development of a bioresorbable magnesium bone screw with customized corrosion behavior through the use of a PEO surface modification” and Königsee Implantate GmbH for providing samples as part of an excellent scientific collaboration.

References
[1] V. Labmayr; O. Suljevic; N. G. Sommer; U. Y. Schwarze; R. L. Marek; I. Brcic; I. Foessl; A. Leithner; F. J. Seibert; V. Herber; P. L. Holweg; Clinical orthopaedics and related research, 2024, 482, 184–197.
[2] S., Sowka; N. Wegner; A. Tiwari; A. Buling; J. Zerrer; F. Walther; Werkstoffprüfung 2023 - Werkstoffe und Bauteile auf dem Prüfstand, Hrsg.: M. Wächter, J. B. Langer, 2023, 1861-8154, 130-135.