*2.1. Materials*

The AA6082 extruded bar was purchased from Metal Center (Trento, Italy), which mainly consists of (0.60%–1.20%) magnesium, (0.50) iron, (0.70%–1.30%) silicon, (0.10%) copper, (0.40%–1.00%) manganese and of a balance percentage of aluminum. The Mg(NO3)2·6H2O, (purity 98%), NH4NO3 (purity 95%)and NaOH (purity ≥98%) were purchased from Sigma-Aldrich Corporation (Saint Louis, MO, USA).

#### *2.2. Synthesis of Mg–Al Layered Double Hydroxide Film*

The AA608 specimens were initially ground with silicon carbon paper, starting from 500 grit paper to 1000, 2400, and 4000-grit respectively. The samples were cleaned with deionized water and further ultrasonically in pure ethanol for 15 min. Lastly, the specimens were immersed in a 0.1 M aqueous NaOH solution for one minute to etch the oxide layer on the surface of the alloy.

A 0.05 M (Mg(NO3)2·6H2O) solution was prepared in 1000 mL deionized water and the pH of the solution was adjusted to 10.0 by the dropwise addition of ammonium hydroxide solution. The solution was poured into a four bottleneck heating flask (Sigma-Aldrich Glassware) and pretreated aluminum samples (3.14 cm2) were immersed vertically in the solution. The experiments were performed at 40, 60, 80, and 100 ◦C reaction temperature, for 12, 18, and 24 h, under nitrogen bubbling. The samples were washed with deionized water after the experiments completion and were dried at room temperature. Figure 1 represents the general schematic diagram of the layered double hydroxide.

**Figure 1.** Schematic illustration of layered double hydroxides.
