Nucleation and growth of chemical conversion coating on Mg–4Zn alloy prepared by immersion in saturated NaHCO3 at ambient temperature and its corrosion behaviour in simulated body fluid (SBF) is investigated in this paper. Compact dense coating obtained up on 24 h of immersion at ambient temperature is found to provide complete coverage of the surface and a high initial corrosion resistance in SBF. With sustained exposure to SBF, the coating dissolves and slowly cracks to expose the substrate to the corrosion medium. The 24 h coated sample shows high levels of bio mineralisation, with flower shaped crystals of phosphates of Ca and Mg on the surface. The combined effects of slow first hydrogen evolution rate and precipitation of phosphates, and similarities to the carbonate coating developed in vivo makes the conversion coating discussed in this paper a suitable choice for Mg alloys for temporary bio implant applications. © 2019 Elsevier B.V.