Fine-grained, wrought Mg‒Zn‒Ca (ZX)-lean alloys attract great attention as temporary implant materials for the treatment of cardiovascular and musculoskeletal conditions [1, 2]. In order to ensure a safe, controllable and potentially tailorable biodegradation for such alloys, the influence of microstructural features on the biocorrosion processes needs to be understood. In particular, microstructural heterogeneities such as intermetallic phases require special attention, because they may control the degradation processes via micro-galvanic coupling. The low alloying-element content, as well as the sub-micrometer size and the low volume fraction of intermetallic phase (IMP) particles made either of Mg2Ca or a Ca‒Mg‒Zn compound in these lean alloys, however, make their study challenging and require methods of high spatial resolution (HR).
We present in this study a correlative approach, combining transmission electron microscopy (TEM)-based and electrochemical methods. Applied in concert we were able to reveal the role different types of nanometric IMPs relevant to the ZX system play in the active micro-galvanic couple. We observed for the first time directly and in statu nascendi that the dissolution of Zn-rich IMPs is governed by a dealloying mechanism. We describe in detail the impact of such dealloying mechanism, which controls a dynamically changing electrochemical activity of these IMPs.
With such insights obtained in vitro, the in vivo biocorrosion behavior could be evaluated and described in more detail. To this end, ZX-lean alloys designed to contain different types of IMPs were implanted transcortically in the femur of Sprague-Dawley rats and their degradation rate and morphology were studied over the implantation period of up to 1.5 years. Results show that the type of IMPs can significantly influence the degradation rate of ZX-lean alloys. Furthermore, histological analysis confirmed these materials to be well tolerated by the surrounding tissue, showing their suitability as implant materials with tailorable degradation rate.
Based on the results obtained we established a model for the active biocorrosion mechanisms in ZX-lean alloys, which decouples the impact of the overall Zn content from the impact of the type of IMP on the active biocorrosion mechanisms.
 J. Hofstetter et al. (2014), JOM, 66:566-572.
 J. Hofstetter et al. (2015), Acta Mater., 98:423-432.