The polarization curve of (CoCrFeNi)95Nb5 HEA coating in 3.5 wt.% NaCl solution is shown in Figure 1 For comparison,Figure 1 also exhibits the polarization curves of other HEA coatings prepared by other processes in 3.5wt% NaCl solution, and the corrosion dynamics parameters of all HEA coatings obtained by linear fit are given in Table 1. As can be seen, the (CoCrFeNi)95Nb5 HEA and Al2CrFeCoCuTiNi HEA coatings have higher corrosion potential than other coatings, and the (CoCrFeNi)95Nb5 HEA coating also possesses the lowest corrosion current density, indicating that the (CoCrFeNi)95Nb5 HEA coating has excelltnt comprehensive corrosion resistance in 3.5wt.% NaCl solution. Furthermore, although the pitting potential of (CoCrFeNi)95Nb5 HEA coating is similar to that or other CoCrFeNi-based coating,the lower passivation current density of (CoCrFeNi)95Nb5 HEA coating means the better pitting resistance.
Figure 1. Polarization curves of (CoCrFeNi)95Nb5 HEA coating and the reported HEA coatings in 3.5 wt.% NaCl solution.
Table 1. Corrosion dynamics parameters of different HEA coatings in 3.5 wt.% NaCl solution.
Figure 2 shows the SEM images of the surface of the (CoCrFeNi)95Nb5 HEA coating after electrochemical experiment in 3.5 wt.% NaCl solution. Obviously, a large number of tiny dish-shaped pitting pits with diameter of 1~5 um are observed on the surface of the coating (Figure 2a), and such pitting pits are only distributed in gray phase region. Figure 2b presents a large corrosion pit where many secondary pitting holes are formed on the inner wall of the pit. Furthermore, those pitting holes are mainly displayed in the interdendritic region; however, the dendritic phase is undamaged, indicating that selective corrosion occurs on the surface of the HEA coating. According to the foregoing EDS results, it can be inferred that in the 3.5 wt.% NaCl solution, the interdendritic phase rich in Nb and Cr is more easily corroded, while the dendritic phase rich in Co, Ni, and Fe is less susceptible to corrosion.
Figure 2. SEM images of corrosion morphology after electrochemical test: (a) dish-shaped pitting pits, (b) a large corrosion pit.
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