Fig. 1. (a) Schematic diagram illustrating voltage vs. time curve under constant current mode on AZ31B alloy [69]. (b) Evolution of current density and voltage with time under constant voltage mode on AM50 alloy [2].

Fig. 2. OES spectra during different MAO process stages: (a) anodizing, (b) and (c) plasma discharge [71]. Evolution of discharge sparks at different MAO stages: (d) transition, (e)-(i) plasma discharge [78].

Fig. 3. (a) Surface and (b) cross-sectional morphology of MAO coatings on Mg-Li alloys [80].

Fig. 4. Schematically illustrating MAO coating formation process [97].

Fig. 5. Schematically illustrating discharge model [95].

Fig. 6. The coating formation mechanism at later MAO stage [100].

Fig. 7. Photographs of (a) anodic, (b) cathodic, and (c) and (d) both anodic and cathodic micro-discharges of AZ81 alloy at current density of 4 A dm-2 in electrolyte with pH of 12.1, 5.35, 8.2, and 9.15, respectively [101]. (e)-(k) Schematically illustrating mechanism of cathodic breakdown [102].

Fig. 8. (a) Voltage vs. time curve during MAO process on AZ31B alloy. (b)-(f) SEM images of MAO coatings formed with different KOH concentrations [105].

Fig. 9. Optical profilometry images of 3D surface morphology of MAO coatings formed in phosphate-based electrolyte (a) without additive, with (b) Na2SiO3, (c) NaAlO2, and (d) K2ZrF6 [111].

Fig. 10. XRD pattern of MAO coatings formed in electrolyte without Na3PO4 (Al), and with 5 g L-1 Na3PO4 (AlP5) and 10 g L-1 Na3PO4 (AlP10) [115].

Fig. 11. Surface and cross-sectional morphologies of MAO coatings formed in electrolyte (a, d) without Na3PO4, and with (b, e) 5 g L-1 Na3PO4 and (c, f) 10 g L-1 Na3PO4 [115].

Fig. 12. SEM images of surface morphology of MAO coatings formed at current density of (a) 5, (b) 10, (c) 15, and (d) 20 A dm-2 [127].

Fig. 13. (a) Fluorescence images of cells after 60 min of incubation. (b) ALP activity of murine BASCs after 7 days of incubation in extracts. ZK60: bare ZK60 alloy; Si-ZK60: MAO-coated ZK60 alloy; Ti: Ti-6Al-4V alloy. *p<0.01 compared with the control; #p<0.01 compared with bare ZK60 [137].

Fig. 14. SEM images of (a) MAO coating and (b, c) MAO/Ta2O5 composite coating. Optical images of (d) AZ31 substrate, (e) MAO coating, and (f) MAO/Ta2O5 composite coating immersed in Hank's solution for 294 h [138].

Fig. 15. (a) CT image and (b) 3D reconstruction of high purity Mg screws after implantation for 2, 4, and 8 weeks [129].

Fig. 16. Optical images of MAO coating developed in (a) base electrolyte, (b) 3% Cu3PO4, (c) 5% Cu3PO4, and (d) silicate-based electrolyte with different concentrations of KMnO4 [145,146].

Fig. 17. (a) Variation of temperature with time of MAO coating and Mg substrate [150]. Variation of (b) emittance and (c) reflectance of MAO coatings developed with different ZnSO4 concentration with wavelength [158].

Fig. 18. Thermal control performance of MAO coatings developed (a) with 3, 5, 7, 10 min in 10 g L-1 NH4VO3, (b) in 5, 8, 10 g L-1 NH4VO3 with 10 min [154].

Fig. 19. Dependence of absorbance/emissivity (αs/ε) and balanced temperature on coating thickness and roughness [161].

Fig. 20. 3D topography of wear track on (a) AZ31B, and MAO coating formed with current of (b) 2 A, (c) 3 A, and (d) 4 A under load of 5 N. Wear depth of MAO coating under loads of (e) 5 N and (f) 10 N [169].

Fig. 21. SEM images of wear track of MAO coating formed with current of (a, b) 2 A, (c, d) 3 A, and (e, f) 4 A [169].

Fig. 22. Element distribution within thickness of coatings developed with (a) 1, (b) 2, and (c) 4 g L-1 TiN nanoparticles [13].

Fig. 23. 3D surface profiles of coatings developed with (a) 0, (b) 1, (c) 2, (d) 3, (e) 4 g L-1 TiN nanoparticles [13].

Fig. 24. SEM images of MAO coatings prepared with (a) 0 g L-1, (b) 1 g L-1, (c) 2 g L-1, and (d) 3 g L-1 CeO2 particles [185].

Fig. 25. SEM images of (a-c) MAO coating and (d)-(f) MgFe-LDH/MAO composite coating [189].

Fig. 26. Optical images of MAO coating immersed in 3.5 wt.% NaCl solution for (a) 0 day, (b) 7 days, (c) 24 days, and (d) 28 days and MgFe-LDH/MAO coating immersed in 3.5 wt.% NaCl solution for (e) 0 day, (f) 7 days, (g) 24 days, and (h) 28 days [189].

Fig. 27. (a) Bode plots of |Z| vs. frequency, (b-d) Nyquist plots, (e) Bode plots of phase angle vs. frequency and (f) polarization curves. (g and h) Hydrogen evolution rates as a function of immersion time in 3.5 wt.% NaCl solution for 228 h [61].