For decades, biomaterials used as implants saved the life or improved its quality for millions around the globe. Biodegradable metals constitute a new class of biomaterials especially designed for temporary implants. Fe-based biodegradable metals are an important sub-class, but their development is limited by their tendency to degrade faster in vitro than in vivo. Blood is known to contain a high CO₂ concentration. This is susceptible to form various carbonates with different stability, thus affecting and potentially decrease the material degradation rate. Therefore, this work investigates the influence of CO₂ on the static degradation of Fe-21Mn-1C alloy in modified Hanks' solution. Results clearly evidenced that the presence of CO₂ led to different degradation patterns and rates. Indeed, in 5% CO₂ atmosphere, the degradation layer exhibited uniform, compact and dense manganese carbonate crystals, whereas, under ambient atmosphere, amorphous iron phosphorous compounds were dominant.