The duplex CrON coatings were prepared on H13 tool steels by combination of plasma nitriding and arc ion plating. The effects of oxygen flow rate on microstructure and corrosion behavior of duplex CrON coating in the molten aluminum were investigated. Results show that the as-deposited coatings with oxygen flow rate of 0, 50, and 100 mL/min, namely CrON-0, CrON-50, and CrON-100 coatings, mainly consist of B1-CrN phase. The major components of CrON coating change from nitride into oxide phase with increasing the oxygen flow rate, and the Cr₂O₃ crystalline phase is obviously observed in the as-deposited coatings with oxygen flow rate of 200 mL/min, namely CrON-200 coating. Oxygen addition in nitride restrains the columnar growth of grains resulting in the dense microstructure. The surface defects and roughness increase with the increase of oxygen content. The failure behavior of duplex CrON coatings is mainly corrosion pitting in molten aluminium. The CrON-50 coating reveals the best corrosion resistance due to the dense columnar microstructure and high thermal stability. The dense Cr₂O₃ layer in CrON-200 coating is also favourable for corrosion protection against the molten aluminum.