Corrosion mechanism of sulfate, chloride, and tetrafluoroborate ions interacted with Ni-19 wt% Cr coating: A combined experimental study and molecular dynamics simulation

Abstract This study aimed to investigate the corrosion behavior of a relatively resistant nickel‑chromium coating in the presence of SO42−, Cl−, and BF4− ions. The electrochemical measurements including linear polarization, small amplitude cyclic voltammetry (SACV), and electrochemical noise (EN) analysis, were used, along with molecular dynamics simulation (MD) to evaluate the interactions between the coating and the corrosive ions. Results of the EN analysis showed that the charges of events exchanged in the HBF4 and HCl solutions were in the range of 1 μC to 1 mC; i.e., the coating was locally damaged in these media. However, the rates of pit progress in the coating exposed to BF4− and Cl− were not similar because the polarization resistances (Rp) of the coating in these two solutions were 1745.6 Ω·cm2 and 2766.2 Ω·cm2, respectively. The most negative adsorption energy, alongside the highest diffusion force of tetrafluoroborate ions, compared to chloride and sulfate, led to severe pitting corrosion of the coating. In the H2SO4 medium, the passivation mechanism prevailed over the pit growth, so that the Rp increased to 5747.3 Ω·cm2. Among the passive layers constructed on the coating surface, Cr2O3 (001) formed the low-energy chemical bonds with all three destructive species of SO42−, Cl−, and BF4−. Also, NiO (111) provided a compressed structure of Ni and Cr oxides, which could impede the penetration of corrosive ions in the alloyed coating.