3.1 Fourier transform infrared spectroscopy
Figure 1 presents the FT-IR spectra of the CeO2 nanoparticles, PPy and CeO2-PPy nanocomposite. The peaks at the range from 3420 cm-1 to 3450 cm-1 represented physically adsorbed water molecules during the synthesized process18. Compared with CeO2 nanoparticles, we can find some new peaks associated with PPy appeared in the spectrum of the CeO2-PPy nanocomposite. In the case of the CeO2-PPy nanocomposite, the characteristic absorption bands at 1550 cm-1 were assigned to the fundamental vibration of the pyrrole ring19. The typical characteristic peaks appearing at 1190 cm-1 were mainly ascribed to the C-N bond stretching vibration and out-of-plane deformation C-C bond vibration20. The above results indicate that the PPy was successfully composited with CeO2. However, after the composition of CeO2 nanoparticle with PPy, the characteristic peak position and intensity variations occurred. With the addition of CeO2, the characteristic peak of PPy being shifted to lower wave numbers, which likely due to the physicochemical interaction between the CeO2 and PPy molecules during the process of redox reaction.
3.2 X-ray diffraction
To verify that CeO2 was successfully grafted with PPy, the crystalline structures of CeO2, PPy, and CeO2-PPy were characterized by XRD, as shown in Fig. 2. The distinct diffraction peaks of CeO2 were at 2θ = 28.6°, 33.1°, 47.5°,56.3°, and 59.1°, which correspond to the (111), (200), (220), (311), and (222) planes (ICDD card no. 43-1002). No other diffraction peaks were found in the pattern, which indicates the high purity of CeO2. PPy showed a broad diffraction peaks at 2θ = 24.5°and 43.3°, indicating an amorphous structure due to the presence of the five-membered ring backbone. However, the diffraction peak intensity and position of CeO2-PPy shifted to some extent than pure CeO2 and PPy, which may be contributed to the physicochemical interactions between CeO2 and PPy.
3.3 X-ray photoelectron spectroscopy
To confirm the oxidation of pyrrole monomers by cerium oxide, the cerium oxide-polypyrrole composite particles were investigated by XPS measurement. The XPS survey spectra and high resolution deconvoluted Ce 3d and N 1s spectra are presented in Fig. 3. The Ce 3d3/2 peak was present at the binding energy of 890 eV. The binding energy at 886.6 and 882.2 eV correspond to the Ce (III) 3d94f2O2p5 state and the Ce (III) 3d94f1O2p6 state according to previous reports21. The above results show that the cerium ions were reduced to the trivalent ion. Additionally, the slightly weak peak at 884.3 eV was attributed to the coordination of Ce3+ with nitrogen in the five-membered ring backbone of polypyrrole. Moreover, N 1s spectrum displays three different chemical states, These three types are as following binding energies: one at 399.8 eV was ascribed to the C-N+, the other one at 400.4 eV belonged to the C-N, the last one about at 401.6 eV corresponded to the C = N22. In conclusion, these results fully confirm the successful synthesis of cerium oxide polypyrrole composite material, cerium oxide itself as an oxidant is reduced to trivalent, polypyrrole loaded on cerium oxide surface.
3.4 Scanning electron microscopy and energy dispersive X-ray spectroscopy
SEM was performed to observe the morphology of CeO2 and CeO2-PPy, and the images are shown in Fig. 4. In Fig. 4 (a), we can clearly observe that the CeO2 exhibited a smooth rod-like structure with a diameter of about 200 nm. In Fig. 4 (b), CeO2-PPy shows a rougher surface than CeO2, which dues to the PPy particles symmetrically cover the whole surface of CeO2 nanorods. In addition, the PPy particles of CeO2-PPy appeared as a bunch of bulges, which effectively lengthened the pathways of corrosive ions penetrating the coating to the surface of the steel plate. Additionally, the EDS in Fig. 5 shows the elements of C, N, O, and Ce atoms existed on the surface of CeO2-PPy composite particles. EDS showed the relative contents of various elements, among which cerium was the least, which was consistent with the results of XPS characterization. It is further proved that cerium oxide was reduced and pyrrole was oxidized to polypyrrole during the synthesis of cerium oxide polypyrrole composite particles.
3.5. Electrochemical impedance spectroscopy
Electrochemical impedance spectroscopy (EIS) was used to study the corrosion resistance of the pure epoxy coating, epoxy-CeO2 coating and epoxy-CeO2-PPy coating, as shown in Fig. 6, and 7. The experimental data are represented by Nyquist and Bode diagrams, and suitable equivalent circuits were selected to match them. The dots represent the raw data, and the lines stand for data fitted by ZView software. During the preparation of the sample, the addition of CeO2 and was CeO2-PPy both are 6wt%.
After immersion for 1 day, a nearly complete semi-arc can be clearly observed in the Nyquist diagram of the pure epoxy coating, indicating that the coating has been corroded23. Meanwhile, the epoxy-CeO2 coating also presented a nearly complete semi-arc, but the radius of the arc was larger than that of the pure epoxy coating. However, the epoxy-CeO2-PPy coating showed very strong anti-corrosion properties. Remarkably, the epoxy-CeO2-PPy coating exhibited an almost straight Bode impedance line and the value of |Z|0.01Hz was larger than 109 Ω·cm2, indicating that the epoxy-CeO2-PPy coating has large capacitance characteristic due to the barrier property of the epoxy resin and fillers24.
After immersion for 10 days, the impedance values (|Z|0.01Hz ) of the three coatings all showed a small decrease, but those of the epoxy-CeO2-PPy coating were still significantly higher than those of the other coatings. This may be contributed to the relatively rougher surface of CeO2-PPy fillers than CeO2 fillers, which can effectively prolong the penetration paths of corrosive ions and effectively fill the hole defects of the epoxy resin .Thus, further blocking the invasion of ions.
Upon increasing the immersion time, a complete semi-arc appeared in the Nyquist plots of all three different coatings, which denoted that corrosive ions have passed through the coating and reached the surface of the substrate. After immersion for 40 days, the impedance value (|Z|0.01Hz ) of the epoxy-CeO2-PPy coating was 2.73⋅108 Ω·cm2, higher than the pure epoxy coating (1.04⋅107 Ω·cm2) by one order of magnitude. Furthermore, the breakpoint frequency (the frequency of the − 45◦ phase angle) showed the anti-corrosion performance of the coating by reflecting the microscopic delamination. The breakpoint frequencies of the pure epoxy coating, CeO2-Epoxy coating and Epoxy-CeO2-PPy coating were 14.7, 6.2, and 0.56 Hz, respectively, indicating that the epoxy-CeO2-PPy coating showed less corrosion..
An appropriate equivalent circuit model was selected in Fig. 8 to accurately analyze the corrosion behavior of the coating and steel plate interface. The equivalent circuit model included multiple components, including solution resistance (Rs), coating resistance (Rc), charge transfer resistance (Rct), double-layer capacitance (Qdl), and constant-phase components (Qc). At the initial stage of corrosion, the corrosion resistance of the coating mainly depended on the physical barrier property of the epoxy resin, and the equivalent circuit model was the R(QR), as shown in Fig. 8(a). Upon extending the immersion time, corrosive ions permeated the coating to the surface of the steel plate, and the equivalent circuit model changed to R(CR(QR), as shown in Fig. 8(b).
3.6 Protective mechanism for coatings
The protection mechanisms of three different coatings were shown in Fig. 8. During the curing of the epoxy resin, many micropores and defects formed in the coating due to solvent volatilization, which provided corrosive ions the opportunity to penetrate the coating to the surface of the steel plate through these micropores and defects. The following oxidation and reduction reactions occurred at the interface of coating/metal25, 26:
Fe→Fe2+ + 2e−, Fe2+→ Fe3++e− (1)
H2O + (1/2)O2 (g) + 2e−→2OH− (2)
2Fe2+ (aq) + O2 + 2H2O→2FeOOH + 2H+ (3)
Therefore in terms of pure epoxy coating, corrosive ions could reach the metal surface in a short time, leading to severe corrosion of the metal matrix. For the epoxy-CeO2 coating, the coarse surface structure of the CeO2 nano-fillers can effectively prolong the paths of corrosive ions penetrating the coating to a certain extent, but with the increase of immersion time, the corrosive ions eventually reached the metal surface, leading to the failure of the corrosion resistance of the coating27. Although the pure CeO2 nanofiller is chemically stable, it can only act as a physical barrier and does not provide long-term corrosion protection. However, the epoxy-CeO2-PPy coating exhibited longer-term protection than the pure epoxy coating and epoxy-CeO2 coating due to the synergistic passivation effect of the conductive polymer PPy and the ability of cerium ions to trap hydroxide ions. As a conductive polymer, PPy can accept electrons produced by metal anode reaction from the oxidation state to the reduced state, which leading to the increasing iron ions (Fe2+ and Fe3+) eventually transform into a dense passivated film(Fe2O3 + Fe3O4) and thus the development of corrosion was prevented28. The XPS and EDS results of the CeO2-PPy show that the surface of the composite fillers contains a certain amount of cerium ion (Ce3+). Since the cerium ions (Ce3+) and conducting polymers are bonded by weak interactions, cerium ions are released when PPy is converted from the oxidation state to the reduced state. Then the released Cerium ions (Ce3+) and the hydroxide ions of the cathode are deposited on the cathode surface through a series of reactions16, which inhibiting the continuation of the plate cathode reaction.