Figure 15 provides a deeper understanding of passivation mechanisms by illustrating the cation fractions of the elemental electronic states in the passive film. Coox2+/3+, Crox3+, Niox2+, Alox3+, and Tiox4+ were detected on the surfaces of all three samples, contributing to a highly compact passive film. The cation fraction of CO showed no significant difference among the three samples (Fig. 15a). Co3O4, as the primary oxide in CoCrFeNi2 and CrFeNi2 alloys, has limited effects on the corrosion resistance of passive films [87]. As shown in Fig. 15b, Crox3+ was the dominant passive component in the ABH-973-48 sample, twice as much as in the as-cast and as-built samples, which explains the improvement in corrosion properties. Furthermore, Crhy3+ can be seen in the passive film of ABH-973-48 (Fig. 15b), helping to facilitate secondary passivation.
In Figs. 15c and d, it is observed that Niox2+, Nisat2+, and Alox3+ content increased significantly after corrosion measurement. However, electrochemical passive films of Al and Ni elements after corrosion measurement are non-compact films with a negative impact on material corrosion resistance [44]. Unlike Al and Cr, minimal NiO oxides form in the passive film due to superior resistance [92].
In Fig. 15e, both the as-cast and ABH-973-48 samples contain an equal amount of Tiox4+. This encourages its combination with O2- ions to form TiO2 passive films incorporated into HEAs’ passive films that improve pitting resistance. The fraction of Ti0 is higher in ABH-973-48 than in as-cast samples, hindering point defect transport and improving passive film protection. In contrast, no O2- ion peak was observed in the passive film of the as-built sample (Fig. 15f). Our findings demonstrate that Ti and TiO2 play an essential role in enhancing anti-corrosion properties of the passive film in ABH-973-48 samples by reducing the adverse effects of Al2O3 on alloy passive films. XPS analysis showed that Cr2O3, Al2O3, and TiO2 make up the passive film on ABH-973-48 samples, making it more stable than the passive films on as-cast and as-built samples.
