In-Situ Degradation Studies of the Bioresorbable Mg-Based Alloy ZX00
Considering cytotoxicity and the role in biochemical reactions, Mg would be an ideal bioresorbable implant material for bone fracture fixation. However, pure Mg does not show sufficient mechanical strength for load-bearing implants and the corrosion rates in physiological (aqueous) environments are significantly faster than desirable.
These problems can be at least partially be overcome by Mg-based alloys such as ZX00 (Mg0.5Ca0.5Zn). ZX00 shows promising degradation rates (approx. 20 µm/year), yet its strength does not satisfyingly fulfil the requirements for adult/elderly patients. One strategy to further strengthen the material is grain refinement via severe plastic deformation (e.g. high pressure torsion (HPT)). This will however in turn influence the degradation behavior.
Here, we study the influence of grain refinement on the in-vitro degradation of ZX00 in simulated body fluid (SBF). For dynamically investigating the degradation processes, two different in-situ methods, namely dilatometry and electrochemical impedance spectroscopy (EIS), are employed in comparing as-received (reference) with severely plastically deformed ZX00. Generally grain refined samples exhibit a faster degradation. Both, deformed and undeformed samples, tend to expand upon immersion in SBF, as shown by dilatometry, indicating a hydrogenation of the bulk as well as an interfacial formation of MgO and Mg(OH)2 phases.
The interfacial layer evolution can be specified further by EIS, allowing to estimate their thicknesses due to the different conductive characteristics. Interestingly, the HPT-deformed samples exhibit a stronger coverage with MgO already upon immersion, indicating a higher corrosion tendency also in air. In good accordance with literature reports for a related alloy , the undeformed reference material shows the typical stages of Mg(OH)2 layer growth in SBF, being initial formation of a dense layer, a temporary disruption phase (thickness decrease) and finally further porous layer growth. In contrast to this, no disruption phase is observed for the grain refined samples. This may be interpreted in terms of the fine-grained structure disturbing the initial formation of a dense Mg(OH)2 layer, leading to an incomplete coverage with electrolyte channels leading to the metallic surface. This will allow a permanently proceeding degradation without a disruption step, which happens only for an initially dense layer.
 Y. Liu et al., Electrochim. Acta 264 (2018) 101