Dopamine hydrochloride (98 wt.% purity), Tris(hydroxymethyl)aminomethane (99.8% purity), 8-hydroxyquinoline (8-HQ, 98 wt.% purity), and ethanol (AR) were purchased from Sigma-Aldrich Corporation. Hydrochloric acid (AR) and sodium chloride (AR) were obtained from Shanghai Richjoint chemical reagents co., Ltd.

Teflon sealed copper rod (99.999 wt.%) with an area of 0.0314 cm² was used as the observed electrode. First, to remove the oxide/hydroxide layer, the copper electrode was ground with 500- and 1,000-grit SiC papers. Then, 0.3 µm Al₂O₃ was used to polish the electrode surface. Finally, the polished copper surface was rinsed with deionized (DI) water (18.2 MΩ cm), pure ethanol, and deionized water successively to remove the loose oxides and alumina residue.

1) The copper electrode was immersed into 2 mg ml⁻¹ dopamine in 100 ml Tris–HCl (pH = 8.5) buffer solution for 12 h at room temperature. DA is therefore self-polymerized onto the copper surface, which is beneficial to the formation of a uniform and dense coating. The copper electrode with PDA coating, recorded as PDA@Cu in the following text, was first rinsed with deionized water and then air-dried. The self-polymerization process of DA is shown in Supplementary Figure S1A.

2) The electrode was immersed in 0.02 M 8-HQ anhydrous ethanol solution for 12 h, rinsed with DI water, air-dried, and recorded as 0.02 M 8-HQ@Cu.

3) After that, 0.2 g DA was dissolved in 100 ml Tris–HCl (pH = 8.5) buffer with magnetic stirring at room temperature. Following that, 20 ml 8-HQ (with different concentrations) anhydrous ethanol solution was slowly added to dopamine solution under constant stirring. The final electrode (PDA@0.02M 8-HQ@Cu) was obtained after the bare copper electrode was immersed in the mixture solution for 12 h at room temperature.

The composition of the coating was characterized using X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe, Japan). It provided 1,486.6 eV photons using an Al Ka X-ray source (40 W, 15 KV). The test voltage and the base pressure of the analysis chamber were set at 6 × 10⁻⁷ Pa and 2 × 10⁻⁸ Pa, respectively. The coatings of PDA@Cu and PDA@0.02M 8-HQ@Cu were scraped off from the copper surface to conduct structural characterization by FTIR (Thermo Fisher Nicolet iS5, United States ). The FTIR spectral range of 500–4,000 cm⁻¹ was recorded with 4 cm⁻¹ resolution in the transmission mode.

Raman spectra were collected using a confocal micro-Raman spectrometer (Super LabRam II system, Dilor, France). The laser and detector were 632.8 nm He-Ne laser and multichannel air-cooled 1,024 × 800 pixels charge-coupled device, respectively. The laser power, pinhole, and slit for Raman measurements were set at 5 mW, 1,000 μm, and 100 μm, respectively. Each Raman spectrum was acquired by 10 s three times and was calibrated using a silicon line (519.2 cm⁻¹).

The morphologies of electrodes were taken at 5 kV acceleration voltage by scanning electron microscopy (SEM, Hitachi S-4800).

To observe the electrochemical behaviors of the electrodes with coatings, by using a VersaSTAT4 electrochemical workstation (AMETEK Princeton Applied Research), OCP, EIS, and Tafel polarization curves were measured in 3.5 wt.% NaCl aqueous solution. A conventional three-electrode system was used, with copper, Pt wire, and saturated calomel electrode (SCE) as the working, counter, and reference electrodes. In detail, the electrodes were kept in the NaCl solution for 10,000 s before EIS tests to obtain a steady state of OCP. The EIS experiment was carried out in the range from 100 kHz to 0.01 Hz with an AC amplitude of 10 mV. The electrochemical polarization curve was acquired at an open-circuit voltage of OCP ± 0.25 V (vs. SCE) with a scan rate of 1 mV s⁻¹.

Bare copper, PDA@Cu, 8-HQ@Cu, and PDA@8-HQ@Cu specimens were scratched to expose the metallic substrates by using a ceramic knife (Analytik Jena). The scratched specimens were immediately immersed in 3.5 wt.% NaCl corrosive solution. The scratched portions of copper electrodes with and without treatment of corrosive media were observed using FE-SEM (Hitachi S-4800) and energy-dispersive X-ray (EDS) microanalysis (Hitachi S-4800).

Bare Cu and modified coppers were scratched and corroded in 3.5 wt.% NaCl solution for 10 h. Then linear sweep voltammetry (LSV) was performed by a VersaSTAT4 electrochemical workstation to evaluate self-healing behavior in three-electrode electrochemical cells. The potential range was from 0 to 1.0 V, and the scan rate was set at 50 mV/s (Bernsmann et al; Park and Braun., 2010, 2011; Johansen et al., 2012).

The successful preparation of PDA@0.02M 8-HQ@Cu is confirmed by XPS and ATR-FTIR. Figures 1A,B show C 1s and N 1s XPS spectra of PDA@Cu and PDA@0.02M 8-HQ@Cu. From Figure 2, the contents of various C species are given in detail in Table 1. Some obvious differences could be found in C=C, C-C, C-N, C-O, and C = O groups after the introduction of 8-HQ, for PDA@0.02M 8-HQ@Cu in comparison with PDA@Cu. Direct evidence of the existence of 8-HQ in the composite coating could be offered via the comparison of the N 1s spectra. As shown in Figure 2C, N-C (399.7 eV) and N-H (400.9 eV) groups are attributed to PDA modified at the Cu surface, while in Figure 2D, the appearance of the C = N (398.7 eV) group in PDA@0.02M 8-HQ@Cu indicates the successful dopant of 8-HQ into PDA coating.

To further ascertain the structure of the coating, the FTIR experiment was conducted. As shown in Figure 1, the FTIR spectrum of 8-HQ presents a peak of C-H stretching at 3,057 cm⁻¹, two bands at 1,287 and 1,274 cm⁻¹ correspond to C–N stretching, and a peak at 1,580 cm⁻¹ is due to C = N stretching (Luo and Mather., 2013; Mahmoud et al., 2009), and the peaks at 1,222 and 1,206 cm⁻¹ are also from C-OH stretching of 8-HQ (Daradmare et al., 2016). All corresponding bands are observed in the FTIR spectrum of PDA@0.02M 8-HQ coating. Additionally, as compared with the FTIR spectrum of PDA, a new band at 1,613 cm⁻¹ occurring in the FTIR spectrum of PDA@0.02M 8-HQ coating is assigned to the superposition of phenylic C = C stretching (Liu et al., 2013), and intermolecular hydrogen bonds in PDA present a broad band in the 3,000- to 3,400-cm⁻¹ region (Jiang et al., 2013). The previous FTIR investigation depicts the successful formation of a composite coating of PDA and 8-HQ at the copper surface.

Similarly, the Raman experimental results could also show the coating formation. From Supplementary Figure S2, Raman spectra of PDA@0.02M 8-HQ@Cu present a peak at 950 cm⁻¹ from 8-HQ and two peaks at 1,382 and 1,603 cm⁻¹ from PDA.

Recorded in 3.5 wt.% NaCl solution, Figure 3 shows the curves plotted by open circuit potential (OCP vs. SEC) versus time for bare copper, PDA@Cu, and different PDA@8-HQ@Cu formed by changing the amount of 8-HQ. OCPs of bare Cu, PDA@Cu, and PDA@8-HQ@Cu with different concentrations of 8-HQ (i.e., 0.005 M, 0.01 M, 0.015 M, 0.02 M, and 0.025 M) are −0.209 ± 0.003, −0.196 ± 0.002, −0.202 ± 0.003, −0.198 ± 0.003, −0.191 ± 0.003, −0.172 ± 0.002, and −0.187 ± 0.001 mV, respectively. Obviously, the low OCP value of PDA@Cu is due to the defects in PDA coating, which could be observed in the SEM image (Supplementary Figure S3). From another aspect, some cracks in the PDA coating formed by self-polymerization would result in the occurrence of metal corrosion in chloride ions. By introducing an inhibitor (8-HQ) to synthesize a composite coating, the PDA coating with 8-HQ at the copper surface could dramatically prevent the attack of chloride ions, and the OCP value is relatively higher due to the defects filled by 8-HQ. In Figure 3, the OCP values first shift to the anodic direction and then to the cathodic direction with the increasing 8-HQ concentration. The RSD results in Figure 3 are acquired by three measurements.

Due to external factors, the inhibitor in the composite coating will be exposed, and the free copper (II) ions will electrostatically adsorb with 8-HQ and complex to repair the cracks caused by external factors. Then, 8-HQ will complex with copper (II) to heal the cracks caused by external force factors. The test results showed that the corrosion inhibition effect of the coating is bad at low concentrations. The optimal concentration of 8-HQ to stuff the defects in the coating is 0.02 by observation of OCP at a more positive value, presenting the best anticorrosion ability. When a lower concentration of 8-HQ was used, the coating defects could not be filled completely, and when the 8-HQ concentration was excessive, 8-HQ would precipitate in the crystal form during the reaction, which also affected the integrity of the coating.

The potentiodynamic polarization curves (Figure 4) of the bare and modified copper electrodes were acquired in 3.5 wt.% NaCl aqueous solution. The resultant electrochemical parameters are listed in Table 2. As described in the literature (Amin and Khaled., 2010; Yu et al., 2010; Li et al., 2011; Gerengi et al., 2016), copper corrosion reactions in chloride could be remarked as follows:

The anodic dissolution reactions are given as follows: C u → C u + + e − , (1) C u + + C l − → C u C l , (2) C u C l + C l − → C u C l 2 (3) C u C l 2 − → + C u 2 + 2 C l − + e − . (4)

The cathodic oxygen reduction reaction is given as follows: O 2 + 2 H 2 O + 4 e − → 4 O H − . (5)

By Tafel line extrapolation, the corrosion current density (j _corr ), corrosion potential (E _corr ), anodic Tafel slopes (β _a ), and cathodic Tafel slopes (β _c ) could be obtained. The j _corr value of the coated coppers is smaller than that of bare copper. The lowest j _corr could be reached at PDA@0.02M 8-HQ@Cu, which shows a decrease in magnitude by two orders, compared to bare copper (from 0.776 to 0.0071 µA). The results also suggest that the inhibition efficiency of the composite coating with 0.02 M 8-HQ exhibits the greatest resistance to salt corrosion, and the inhibition efficiency could reach 99.09%. In addition, all coatings retard both the cathodic and anodic reactions to some extent, and the differences in β _c are less noticeable than β _a . This indicates that after modified copper surface with PDA or PDA@8-HQ, the reduction of dissolved oxygen and the diffusion of CuCl₂ ⁻ are diminished (Zucchi et al., 2004).

To evaluate the protective ability of the composite coating for copper, EIS is an effective method (King et al., 2014). Supplementary Figure S4 shows the Nyquist plots of the coated copper electrodes in 3.5 wt.% NaCl solution, and the inset plot shows that of the bare copper. A capacitive loop in the high-frequency range could indicate the solution resistance, and a straight line in the low-frequency range, Warburg impedance, is associated with the diffusion of CuCl₂ ⁻ (Babouri et al., 2015). After modification by PDA and PDA@8-HQ at the copper surfaces, in both EIS spectra, the Warburg impedances disappear and the radius of the capacitive loops increases. PDA@0.02M 8-HQ@Cu presents the highest impedance modulus.

In the Bode plots in Figure 5A, the impendence value at low frequency (|Z|_0.01Hz) increases from 3.48 to 5.16 with respect to bare copper. In the phase angle plots of Figure 5B, bare copper, PDA@Cu, and PDA@8-HQ@Cu have two time constants. PDA@ 0.02 M 8-HQ@Cu shows the maximum phase angle, manifesting the best inhibition efficiency. The result is in good consistency with that of potentiodynamic polarization.

For better understanding of the corrosion mechanism, ZsimpWin software was used to fit impedance spectra. The equivalent circuit model shows the minimum error and chi-square value (χ²) less than 1 × 10⁻³. The fitted electrical circuits are shown in Figure 6, and the corresponding electrochemical parameters are listed in Table 3. The most suitable fitting circuit model for bare copper is R(Q{R[Q(RW)]}), while the equivalent circuit model of R{Q[R(QR)]} is picked out for PDA@Cu. Additionally, R(QR)(QR) is a better mode for fitting the PDA@8-HQ coating. In the equivalent circuits, R _s stands for NaCl solution resistance, and R _ct and R _f represent charge transfer resistance and the resistance of the film, respectively. Constant phase elements (Q _c and Q _f ) are with respect to copper oxide, PDA coating, and PDA@8-HQ coating. W is Warburg impedance. The Q impedance is defined as Z _Q = Y ₀ ⁻¹ (jω) ^-n (Shi et al., 2017), in which Y ₀ is the values of Q, j is the imaginary number, ω (ω = 2πƒ) is the angular frequency, and n is the phase (–1≤n ≤ 1), which is related to the inhibitor adsorption, surface inhomogeneity, and porous layer formation. When n is −1, 0, 0.5, and 1, the Q represents inductance, resistance, Warburg impedance, and capacitance, respectively. As seen in Table 3, the n value of modified samples ranges from 0.5 to 1, meaning the relatively slow corrosion process (Li et al., 2014).

As for blank copper, R _f and Q _f are attributed to the inevitable layer of corrosion products (Refait et al., 2020). Nevertheless, diffusion in such a relatively thin layer then becomes possible, resulting in the presence of W. As for the PDA-modified copper electrode, R _f and Q _f are caused by a composite film containing copper oxides and PDA. This thick PDA film therefore hinders the diffusion process causing W disappearance. For PDA@0.02M 8-HQ coating, since 8-HQ can fill the cracks of PDA, the dense protection layer shows the greatest inhibition efficiency up to 97.58%. Therefore, the first time constant is correlated with the film of PDA@0.02M 8-HQ coating, and the second time constant is then the double layer (Farahati et al., 2019).

Figure 7 shows FE-SEM images of copper with and without PDA or 8-HQ coatings, scratched by external forces, which were taken before and after corroded in 3.5 wt.% NaCl aqueous solution for 10 h. In Figures 7B,D, after immersion, there are many pitting corrosions that occurred at the scratched bare copper and the scratched 0.02M 8-HQ@Cu surface, and the visible corrosion products around the scratched spots indicate severe corrosion. Much seriously, in Figure 7F, the PDA coating has partly peeled off from the copper surface after long-term immersion in NaCl media. Obviously, the protection efficiencies of copper from corrosive solution by only PDA coating or 8-HQ are not satisfactory. Figure 8A shows SEM images and EDS results recorded before immersion of the scratched PDA@ 0.02M 8-HQ@Cu in salt solution. The exposure of copper in the scratched portion of PDA@ 0.02M 8-HQ@Cu could be visible.

In Figure 8B of scaled SEM images, after the scratched PDA@0.02M 8-HQ@Cu in 3.5 wt.% NaCl solution for 10 h, some nanorods produced in the vicinity of the scratched trace could be found, and the EDS result indicates that the nanorods are mainly composed of O, N, C, and Cu elements. Supplementary Figure S5 shows the section views of the PDA@0.02M 8-HQ@Cu. After immersion in 3.5 wt.% NaCl aqueous solution for 10 h, nearly no corrosion occurred in the scratched portion due to the self-healing ability of composite coating. It shows that when the coating is damaged by external forces, 8-HQ embedded in the PDA coating would combine with copper ions through electrostatic attraction, forming insoluble clathrate of bis(8-hydroxyquinoline) copper [Cu (HQ)₂]. The preeminent self-healing ability of such PDA@0.02M 8-HQ composite coating will also be validated by following an electrochemical experiment.

Potentiodynamic polarization was used to characterize self-healing property. PDA@Cu and PDA@0.02M 8-HQ@Cu were scratched with a knife, and then the scratched coatings were immersed in 3.5 wt.% NaCl solution for 0 h, 10 h, and 20 h. As shown in Figures 9A,B and Table 4, compared to the intact PDA@Cu and PDA@0.02M 8-HQ@Cu, j _corr values of both scratched coatings increased. However, j _corr value of the scratched PDA@Cu sample kept increasing with increasing immersion time, indicating further corrosion in NaCl aqueous solution. By contrast, for PDA@0.02M 8-HQ@Cu, j _corr value remarkably decreased (down to 0.003 μA cm⁻²) as soaking time increased, and after self-healing for 20 h, the inhibition efficiency of the scratched PDA@0.02M 8-HQ@Cu recovered to the original level of the unscratched one.

Figures 9C,D show the Bode plots of self-healing tests of PDA@Cu and PDA@0.02M 8-HQ@Cu. At the beginning stage, the impendence values at low frequency for scratched PDA@Cu and PDA@0.02M 8-HQ@Cu reduce an order of magnitude compared to undamaged coatings. Similarly, after self-healing for 20 h, the impendence value of scratched PDA@0.02M 8-HQ@Cu increased to the same level as the unscratched coating, hinting that the scratches have been repaired by 8-HQ molecules. By contrast, in the case of scratched PDA@Cu, the impendence value decreased with the time undergoing. From Table 5, EIS measurements with RSD about 2.69% repeated for five batches of scratched samples reveal excellent reproducibility. Figure 9E shows OCP (vs. SCE) curves for scratched PDA@Cu and scratched PDA@0.02M 8-HQ@Cu in 3.5 wt.% NaCl solution over the immersion time. Both OCP values (vs. SCE) of scratched PDA@Cu and scratched PDA@0.02M 8-HQ@Cu present a sharp fall in the first 3 h, indicating corrosion occurrence in the cracks by the penetration of the corrosive media. After 3 h, OCP (vs. SCE) of scratched PDA@Cu remained decreased, while OCP (vs. SCE) of scratched PDA@0.02M 8-HQ@Cu increased due to the self-healing behavior. After 12 h, the OCP value of scratched PDA@0.02M 8-HQ coating reached 181 mV (vs. SCE), which was close to the undamaged coating (ca. 172 mV). The self-healed coating exhibited a stable protection performance for the copper substrate after immersion in salt solution for 20 h.