Improving corrosion and microbial corrosion resistance of CoCrFeNi high entropy alloy in marine environment by trace amount of Ce addition

• As the Ce doping content increases, the corrosion resistance and microbiologically influenced corrosion resistance of CoCrFeNi HEA initially increase and then decrease. • The addition of Ce induced the conversion of Al 2 O 3 inclusions to CeAlO 3 , and the inclusion diameter was reduced by half. • CoCrFeNiCe 0.003 exhibits the best pitting potential of 929.7 mV in 3.5 wt% NaCl solution and protection potential of 880 mV under inoculated conditions. • Our findings propose a microbiologically influenced corrosion prevention strategy, which involves modifying inclusions to inhibit the initiation of pitting. Microorganisms in marine environments are able to accelerate the local dissolution of passive films formed on high entropy alloy surfaces, significantly reducing the corrosion resistance. The addition of trace amounts of Ce has been reported to modify inclusions, thereby enhancing the pitting resistance of passive films. Herein, the effect of Ce addition in CoCrFeNiCe x (x = 0, 0.001, 0.003 and 0.005) HEAs on microbiologically influenced corrosion (MIC) behaviors were investigated in this work. Through Scanning electron microscopy, energy dispersive spectroscopy, electrochemical tests, and X-ray photoelectron spectroscopy (XPS) analysis, we revealed that Ce addition synergistically optimizes inclusion morphology and passive film stability. With the addition of Ce reaching 0.003 in molar ratio, polygonal Al 2 O 3 inclusions in CoCrFeNi HEA were converted into spherical CeAlO 3 and the size of inclusions decreased approximately 50 % (1.4 μm), enhancing pitting resistance ( E pit = 930 mV). In inoculated medium, CoCrFeNiCe 0.003 exhibited shallower pits (0.36 μm) and higher protection potential ( E pp = 880 mV), compared to those of CoCrFeNi (0.53 μm and −231 mV). Excessive Ce addition (0.005 in molar ratio) led to the formation of new inclusion, increasing the pitting sensitivity of the alloy. These findings outline a new strategy by inclusions modification to increase the resistance to MIC.