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Strong and biodegradable materials are key to the development of next-generation medical devices for interventional treatment. Biodegradable polymers such as polylactide (PLA) have controllable degradation profiles, but their mechanical strength is much weaker than some metallic materials such as iron; on the other hand, tuning the corrosion rate of iron to a proper time range for biomedical applications has always been a challenge. Very recently, we have achieved a complete corrosion of iron stent in vivo within the clinically required time frame by combining a PLA coating, which provides a new biomaterial type for the next-generation biodegradable coronary stents termed as a metal–polymer composite stent. The underlying mechanism of accelerating iron corrosion by a PLA coating remains an open fundamental topic. Herein, we investigated the corrosion mechanism of an iron sheet under a PLA coating in the biomimetic in vitro condition. The Pourbaix diagram (potential vs pH) was calculated to present the thermodynamic driving force of iron corrosion in the biomimetic aqueous medium. Electrochemical methods were applied to track the dynamic corrosion process and inspect various potential cues influencing iron corrosion. The present work reveals that acceleration of iron corrosion by the PLA coating arises mainly from decreasing the local pH owing to PLA hydrolysis and from alleviating the deposition of the passivation layer by the polymer coating.