PSI - Issue 51

Nils Wegner et al. / Procedia Structural Integrity 51 (2023) 122–128 N. Wegner et al. / Structural Integrity Procedia 00 (2022) 000–000

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1. Introduction For the treatment of bone defects, biodegradable magnesium (Mg) implants are becoming the focus of research in surgical disciplines. Besides its biodegradability, Mg has mechanical properties comparable to those of human cortical bone, reducing the risk of bone resorption due to stress shielding. The inherent degradability allows the temporary support (functional phase) of the weakened bone so that the implant degrades after a defined time, making a second surgery for removal unnecessary (Chen et al., 2014). In the presence of aqueous body fluids, Mg degrades with the formation of magnesium hydroxide and hydrogen gas. According to Song and Atrens (2003), the reaction can be divided into three steps: First, Mg oxidizes to a single positively charged Mg + ion (1) before further oxidizing to a twice positively charged Mg 2+ ion with the formation of hydrogen gas and hydroxide ions (2). These reactions as well as the product formation are summarized in Equation 3. In addition to the reaction rate (i.e., corrosion rate), the corrosion morphology is of particular importance for the implant’s stability and, thus, for the duration of the functional phase. To define this phase and ensure patient safety, the corrosion rate and morphology must be determined in a reproducible manner (Singh Raman et al., 2015). Corresponding in vitro testing methods are time-consuming and cost intensive due to long implantation periods (Hou et al., 2019). Most of the common Mg alloys exhibit a time-dependent corrosion behavior, making the ability to perform a time-dependent evaluation indispensable. However, the time dependent corrosion behavior is caused by the formation of partially protective corrosion layers, detaching over time, and by micro-galvanic corrosion, leading to pitting corrosion as predominant corrosion mechanism (Ascencio et al., 2014)). In contrast to electrochemical methods (EIS, PDP), the immersion test allows a time-dependent evaluation (Nidadavolu et al., 2016). According to Equation 3, determination of the mass loss is feasible by measuring the produced hydrogen volume (Nidadavolu et al., 2016). The mentioned time and cost could be reduced by the application of short-time testing methods using, e.g., an acceleration of the corrosion process. One possibility is to apply an anodic current (through polarization), accelerating the formation of the positively charged Mg x+ ions (Equation 1 & 2) and the product formation (Equation 3). Different studies have shown that the hydrogen evolution rate (HER) is constant for a constant current density (Shi et al., 2014; Huang et al., 2021). Moreover, there is a linear relationship between the HER and mass loss (Shi et al., 2014), which is mainly related to an increase in hydrogen evolution on nobler impurities or phases (Thomas et al., 2015). The present study utilizes the relationship between the current density, HER, and mass loss to develop and validate a method for accelerating the corrosion process, enabling a time-efficient evaluation of the long-time immersion behavior and stability. 2Mg → 2Mg + + 2e − (1) 2Mg + + 2H 2 O → 2Mg 2+ + 2OH − + H 2 and 2H 2 O + 2e − → 2OH − + H 2 (2) 2Mg + 4H 2 O → 2Mg 2+ + 4OH − + 2H 2 with Mg 2+ + 2OH − → Mg(OH) 2 (3) 2. Materials and Methods The biomedical Mg-Y-RE-Zr alloy WE43MEO (Meotec, Aachen, Germany) with an elemental composition of 1.4-4.2% Y, 2.5-3.5% Nd, <1% (Al, Fe, Cu, Ni, Mn, Zn, Zr) and balance Mg (in wt.%) was used. Cylindrical specimens (gauge length l 0 = 9 mm, initial diameter d 0 = 4 mm) were machined from the chill-casted and extruded raw material. As a second material variant, specimens complemented with a plasma electrolytic oxidation surface (PEO, Kermasorb®, Meotec, Aachen, Germany) were used. For a defined and area-related corrosion, the specimens were coated with an anti-corrosion lacquer outside the gauge length. The raw material has a fine microstructure with a high number of intermetallic precipitates (Fig. 1 a–e). These are uniformly distributed in different sizes (100 nm to several micrometers), shapes (round, rod-shaped, and branched), and orientations (no preferred direction discernible in cross section). Whereas in the longitudinal section, the precipitates are elongated parallel to the extrusion direction resulting from the process temperatures and deformations (Zeng et al., 2019). These are classifiable as yttrium-rich (MgY, Mg 14 Y 5 , Mg 24 Y 5 ), neodymium-rich (Mg 41 Nd 5 , Mg 12 Nd), and zirconium-rich precipitates (Esmaily et al., 2020). The PEO layer (Fig. 1 f+g) has a thickness of about 25 ± 4 µm, exhibits the typical high porosity with different shapes, sizes, and positions as well as brittleness in the form of cracks (Darband et al., 2017). For mechanical stability, larger, branched pores with a connection to the substrate material are considered critical.