Effect of Cr:Al Ratio on Corrosion Mechanism of Ni-Cr-Mo-Al Alloys in 3.5 wt.% NaCl Solution: Microstructure and Electrochemical and Passive Characteristics

In this study, the microstructure and electrochemical and passive characteristics of NiCr_25.2-xMoAl_x (x = 0, 1.25, 2.5, and 5 mol.%) alloys were investigated. The results show that Ni-Cr-Mo-Al alloys with varying Cr:Al ratios both had a single FCC structure without any second structure precipitates, and decreases in dislocation density and grain size were observed as the Al content in NiCrMoAl alloys increased. It was found from the electrochemical results that the NiCr_23.95MoAl_1.25 alloys had the maximum radius for a semicircle and the lowest <i>I</i>_corr, indicating an enhanced anti-corrosion performance (R_ct: 8.08 ± 0.368 × 10⁵ Ω cm², <i>I</i>_corr: 1.05 ± 0.003 × 10^-7 A/cm²). In this study, it was found that the anti-corrosion performance of the alloys had an approximate connection to the composition and density of passive films. Denser and more stable microstructures in NiCr_23.95MoAl_1.25 alloys were further proven by potentiostatic polarization tests and Mott-Schottky experiments, showing a lower stable current density and acceptor density (N_A: 9.79 ± 0.4 × 10^-20 cm^-3). In addition, the results of XPS show that the Al_1.25 specimen had the highest Cr₂O₃ in the passive film's content among the NiCrMoAl alloys. Cr₂O₃ was the main component, suggesting an enhanced protective influence of passive film. The present study can offer guidance for the application of nickel-based alloys with high anti-corrosion resistance in marine environments.