*2.1. Materials*

Aluminum 6061 alloy (AA 6061) specimens with the dimensions of 20 × 20 × 3.5 mm<sup>3</sup> were used as the anodized substrates. Specifically, the alloy composition by wt % included 0.8–1.2% Mg, 0.4–0.8% Si, 0.15–0.4% Cu, and 0.04–0.35% Cr, and Al was the balance. The contents of Fe, Mn, Zn and Ti in the alloy were not higher than 0.7, 0.15, 0.25 and 0.15 wt %, respectively. Prior to the experiment, all specimens were mechanically ground using 240, 400 and 800 grit silicon carbide paper and then washed with distilled water.

### *2.2. Experiment Process*

Two 300 × 300 mm<sup>2</sup> AISI 321 stainless steel sheets (BENLAIMETAL, Shanghai, China) were used as the cathode. Then, the MAO process was carried out in a stirred electrolyte consisting of 15 g/<sup>L</sup> Na2SiO3 (MACKLIN, Beijing, China), 5 g/<sup>L</sup> KOH (MACKLIN, Beijing, China), and 5 g/<sup>L</sup> (NaPO3)6 (MACKLIN, Beijing, China). Except for the adjusted cellulose concentrations (0, 0.25, 0.50, 0.75, and 1 g/L), the MAO processes of specimens were carried out at 20 ◦C for 30 min with a DC pulse supply at the frequency of 50 kHz, the constant current density of 1 <sup>A</sup>/cm2, and the duty cycle of 15%. To examine the electrolyte stability, the MAO processes were conducted within the same electrolyte after various storage periods for 0, 1, 7, 14 and 30 days.
