Fig. 1. Typical microstructural images of Al-Zn-Mg aluminum alloy with two different treatments: (a) orientation map with NAT, (b) orientation map with AAT, (c) color code acted on characterize crystallographic orientations on standard stereographic projection (different color represents diverse orientation projections: red is [0 0 1]; blue is [1 1 1] and green is [1 0 1]) and distribution diagram of various grain diameter in Al-Zn-Mg aluminum alloy with NAT (d) and AAT (e), respectively.

Fig. 2. TEM images of Al-Zn-Mg alloy with (a, b) NAT and (c, d) AAT conditions.

Fig. 3. STEM-HAADF images of the IMPs of samples after NAT (a) and AAT (b) with corresponding EDS results of precipitates.

Fig. 4. Typical surface image of Al-Zn-Mg alloy after NAT immersed in (a) S1 solution, (b) S2 solution and (c) S3 solution at ambient temperature of 25 °C during experiments.

Fig. 5. SEM images of the microstructure of Al-Zn-Mg alloy after NAT immersed in (a, d, g) S1 solution, (b, e, h) S2 solution and (c, f, i) S3 solution at ambient temperature of 25 °C for (a, b, c) 12, (d, e, f) 72 and (g, h, i) 120 h with corresponding EDS analysis of framed region, (A) S1/120 h, (B) S2/120 h, (C) S3/120 h. The insets are higher magnification of micro-surface.

Fig. 6. SEM images of the microstructure of Al-Zn-Mg alloy after AAT immersed in (a, d, g) S1 solution, (b, e, h) S2 solution and (c, f, i) S3 solution at ambient temperature of 25 °C for (a, b, c) 12, (d, e, f) 72 and (g, h, i) 120 h immersion with corresponding EDS analysis of framed region, (A) S1/120 h, (B) S2/120 h, (C) S3/120 h. The insets are higher magnification of micro-surface.

Fig. 7. Bulk pH evolution of S1 solution (red line with circle), S2 solution (green line with diamond) and S3 solution (blue line with square) at ambient temperature of 25 °C.

Fig. 8. Typical polarization curves of Al-Zn-Mg alloy after (a) NAT and (b) AAT immersed in S1 solution (green line), S2 solution (red line) and S3 solution (blue line) at ambient temperature of 25 °C with a scan rate of 1 mV s-1.

Fig. 9. Representative Nyquist and Phase angle plots of the Al-Zn-Mg aluminum alloy with NAT under (a, b) S1 solution, (c, d) S2 solution and (e, f) S3 solution at ambient temperature of 25 °C for 4, 8, 12, 24, 48, 72, 96, 120 h, respectively. The colorful scattered symbols are experimental data and the corresponding fitting results are represented by black solid lines.

Fig. 10. Representative Nyquist and Phase angle plots of the Al-Zn-Mg aluminum alloy with AAT under (a, b) S1 solution, (c, d) S2 solution and (e, f) S3 solution at ambient temperature of 25 °C for 4, 8, 12, 24, 48, 72, 96, 120 h, respectively. The colorful scattered symbols are experimental data and the corresponding fitting results are represented by black solid lines.

Fig. 11. Equivalent circuit models used to fit EIS data under OCP condition at ambient temperature of 25 °C. Rs is electrolyte solution resistance, CPEdl is double-layer capacitance at high frequency, Rf is film resistance, CPEhole is associated with hole capacitance, Rhole is associated with hole resistance, CPEct is charge transfer capacitance at low frequency, Rct is charge transfer resistance, Ws is Warburg impedance, L is the inductance for fitting dispersive inductive tails and R is corresponding resistance at low frequency.

Fig. 12. Fitting values of Rct in EEC of Al-Zn-Mg alloy with NAT and AAT conditions, immersed in (a) S1 solution, (b) S2 solution and (c) S3 solution at ambient temperature of 25 °C, respectively.