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The advantageous degradation properties of zinc in a biological environment are related to the presence of a protective corrosion layer composed of both organic and inorganic components. However, the mechanisms governing its formation and how the organic species influence its properties have not been established. Here we study the protective layer formation during anodic polarization in whole blood by in situ electrochemical impedance spectroscopy (EIS) as well as infrared spectroscopy and scanning electron microscopy. Simulated body fluid (m-SBF) was used as a reference media to determine the influence of the organic species present in whole blood. Protective zinc phosphate layers form on the Zn surface in both electrolytes, but of different nature and mechanisms. In m-SBF the passivating thin film formation occur already at open circuit potential, reducing the corrosion current compared to exposure in whole blood by a factor of 10 3 . The high corrosion current in whole blood can be explained by a process including rapid protein adsorption preventing the initial formation of a protective phosphate layer. EIS analysis detected an inductive arc in whole blood at low overpotentials, before the onset of protective film formation, indicating the presence of adsorbed Zn ions. The coverage of Zn ions approach 100% of the active surface at 110 mV. At this critical surface coverage a reaction between the adsorbed Zn ions and PO 4 3− takes place which results in formation of a protective, porous, film of ∼1 μm thickness. The inorganic component of the protective film formed in whole blood was characterized as Zn 4 (PO 4 ) 2 (OH) 2 ·3H 2 O. [ABSTRACT FROM AUTHOR]

During implantation load-bearing devices experience stress that may influence its mechanical and corrosion profile and potentially lead to premature rupture. The susceptibility to stress corrosion cracking (SCC) of the Mg-Al alloy AZ61 and Zn was studied in simulated body fluid (m-SBF) and whole blood by slow strain rate (SSR) testing in combination with electrochemical impedance spectroscopy (EIS) and further ex situ analysis including scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy. AZ61 was found to be highly susceptible to SCC. EIS analysis show that although the majority of cracking occurred during the apparent plastic straining, cracking initiation occurs already in the elastic region at ∼50% of the ultimate tensile strength (UTS). Shifts in EIS phase angle and open circuit potential can be used to detect the onset of SCC. Zinc demonstrated a highly ductile behavior with limited susceptibility to SCC. No significant decrease in UTS was observed in m-SBF but a decrease in time to failure by ∼25% compared to reference samples indicates some effect on the mechanical properties during the ductile straining. The formation of micro cracks, ∼10 μm deep, was indicated by the EIS analysis and later confirmed by ex situ SEM. The results of SSR analysis of zinc in whole blood showed a reduced effect compared to m-SBF and no cracks were detected. It appears that formation of an organic surface layer protects the corroding surface from cracking. These results highlight the importance of considering the effect of biological species on the degradation of implants in the clinical situation. Statement of Significance Strain may deteriorate the corrosion properties of metallic implants drastically. We study the influence of load on the corrosion properties of a magnesium alloy and zinc by a combination of electrochemical impedance spectroscopy (EIS) and slow strain rate analysis. This combination of techniques has previously not been used for studying degradation in physiological relevant electrolytes. EIS provide valuable information on the initial formation of cracks, detecting crack nucleation before feasible in slow strain rate analysis. This sensitivity of EIS shows the potential for electrochemical methods to be used for in situ monitoring crack formation of implants in more applied studies. [ABSTRACT FROM AUTHOR]