Unique Role of Microstructure of a Bioimplant Zinc Alloy and Protein Content of Simulated Body Fluid in Corrosion and Fracture

Zinc (Zn) alloys are among the few metallic materials that possess characteristics suitable for their application as temporary, biodegradable implants. This study investigated the combined role of mechanical stress and corrosive simulated body fluid (SBF) that can cause catastrophic and sudden fracture, which is called stress corrosion cracking (SCC). This is a relatively rarely investigated domain of Zn alloys in the context of their application as implants, even though metallic implants are known to suffer SCC. While most in vitro studies of SCC of bioimplants have employed SBFs that simulate only the inorganic constituents, this study investigated the role of adding the major organic constituent of body fluid (i.e., protein) to SBF in corrosion, and SCC in particular. While protein addition reduced corrosion resistance (by >100%) during prolonged immersion, it unexpectedly improved resistance to SCC (by >50%). Our findings also indicate that, unlike other alloys, the Zn alloy showed minimal intergranular stress corrosion cracking (IGSCC) despite the presence of extensive grain boundary precipitates. Detailed electron microscopy provided the key to explaining this unexpected behavior.