Fig. 1. (a) The synthesis process of PDA capsules and the loading process of BTA; (b, c) SEM and TEM images of SiO2 nanoparticles (b1, c1), PDA@SiO2 particles (b2, c2), PMs (b3, c3) and PMBs (b4, c4); (c5, c6) are the enlargements of (c3, c4), respectively.

Fig. 2. (a) FTIR spectra of PMB, PM, SiO2@PDA and SiO2 paticles; (b) UV-vis spectra of PMB, BTA and PM; (c) Raman spectra of PM and PMB; (d, e) TG (d) and DTG (e) curves of PMB, PM and BTA; (f) Release profiles of BTA from PM in 3.5 wt% NaCl and in different pH solutions.

Fig. 3. SEM images of cross sections of (a) EP, (b) PM/EP and (c) PMB/EP. (a2), (b2) and (c2) are the white regions in (a1), (b1) and (c1), respectively.

Fig. 4. Surface morphologies and roughness of as-prepared coatings recorded by SEM and SPM images: (a) EP, (b) PM/EP and (c) PMB/EP. (a2), (b2) and (c2) refer to the white regions in (a1), (b1) and (c1), respectively. The scanning area in SPM images for each coating is 20 μm × 20 μm.

Fig. 5. Bode and Nyquist plots of steel exposed to Blank (a), BTA (b), PM (c) and PMB (d) extracts for immersing for different time in 3.5 wt% NaCl solution; Polarization curves (e) and corresponding line chart of Ecorr and icorr (f) after the immersion of 24 h.

Fig. 6. Bode and Nyquist plots of steel electrodes coated with (a) pure EP, (b) PM/EP and (c) PMB/EP with artificial scratches during 15 days of immersion in 3.5 wt% NaCl solution.

Fig. 7. The equivalent electric circuit used for EIS fitting (a) and Ro (b) and Rct (c) obtained from fitting parameters with immersing for different time in 3.5 wt% NaCl solution; Raman spectra and corresponding optical images (d) of the scratched sample with immersing for 18 days in 3.5 wt% NaCl solution and in air for 7 days.

Fig. 8. Bode plots of steel electrodes coated with pure EP (a), PM/EP (b) and PMB/EP (c) during 40 days of immersion in 3.5 wt% NaCl solution; (d) Impedance modulus measured at 0.01 Hz (|Z|0.01 Hz); (e) Time-dependent properties of the breakpoint frequency (fb); (f) Charge transfer resistance at steel/solution interface (Rct) obtained from the equivalent circuits in Fig. S3.

Fig. 9. LEIS maps of EP (a), PM/EP (b) and PMB/EP (c) coatings around the scratches with different immersion time in 3.5 wt% NaCl solution.

Fig. 10. Optical images of mild steel with pure EP coating, PM/EP coating and PMB/EP coating after 288 h of salt spray test.

Fig. 11. Optical images of scratched pure EP, PM/EP and PMB/EP coatings after immersing for 288 h in 3.5 wt% NaCl solution.

Fig. 12. SEM images (a-c) and Raman spectra (d) of steel surface after peeling the coatings off: (a) blank EP, (b) PM/EP, (c) PMB/EP (Insets in (d) are the corresponding optical images of steel surface after removing the coatings. Scale bar is 500 μm).

Fig. 13. Self-healing mechanism of PMB in epoxy coatings.