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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]