Optimization of Al content in CoCrFeNiAlx HEAs for enhanced hot corrosion resistance and microstructure

Purpose This study aims to investigate the impact of Al content (x = 0.1, 0.3, 0.5) on microstructure evolution and hot corrosion resistance of CoCrFeNiAlx high-entropy alloys (HEAs) in molten Na2SO4 + 25 Wt.% NaCl salt at 900°C, and to identify composition-optimized alloys for high-temperature marine applications. Design/methodology/approach CoCrFeNiAlx alloys with varying Al contents (x = 0.1, 0.3, 0.5) were exposed to hot corrosion in Na2SO4 + 25 Wt.% NaCl at 900 °C for 100 h. Microstructural characterization and corrosion kinetics (weight gain measurement) were coupled with corrosion product analysis via X-ray diffraction and scanning electron microscopy-EDS. Findings An Alx = 0.5 composition triggered a dual-phase transition from FCC to FCC + BCC with dendritic BCC grains, resulting in an ultra-low weight gain of 9.9 mg/cm2 after 100 h at 900 °C in a Na2SO4-NaCl solution, compared to 31.2 mg/cm2 for Al0.1 and 24.6 mg/cm2 for Al0.3. Corrosion scales showed dual-layer structure: outer (Al2O3 + Cr2O3 + spinel) and continuous inner a-Al2O3 barrier. This dense Al2O3 layer blocked Cl-induced cyclic sulfidation/chlorination by halting molten salt penetration. Originality/value This study offers significant insights into the combined effects of Al content on the hot corrosion behavior of HEAs, which is helpful for further study of CoCrFeNiAl HEAs system.