The application of the pore-filling method to the study of the structure of porous anodic films on aluminium

Accurate evaluations of both porosity and tortuosity of anodic films grown on rolled AA 1050 and on rolled or machined AA 2024 T3

“…From cross‐sectional FEG‐SEM views, the thickness of the anodic film (Figure ) and the thickness of the barrier layer (Figure ) were first determined. Figure shows in particular that the film thickness changes little (slope 1) at the beginning of the voltage ramp (from 0 to 320 s, ie, from 0 to 16 V), whereas it increases significantly (slope 2), especially during the steady state, ie, after 320 seconds, for a voltage value at 16 V. In contrast, the thickness of the barrier layer regularly increases (slope 3) both during the transient regime (the voltage ramp) and during the steady‐state voltage regime (Figure ), in agreement with previous works …”

“…The reanodization slope (m 1 ) thus becomes steeper for films having increasing durations of the first anodization (Figure ). Otherwise, m 1 values seem proportional to the square root of the anodization voltage (Figure ), as proposed previously …”

“…A third technique known as "reanodization" (or also the "Dekker-Van Geel" technique) [26][27][28][29][30][31][32] was used; this is based on a second galvanostatic anodization (also called the "forming" step) of a first porous anodic film, in a mixed boric acid-tetraborate electrolyte (H 3 B0 3 0.5 mol L −1 ; Na 2 B 4 O 7 0.05 mol L −1 ). 28,30,32 At a constant forming current density (here J = 0.5 mA cm −2 ), 28,32 the voltage increases linearly with the gradual filling of the porosity, which results when the filling is integral with a slope break (from m 1 to m 2 ), also called the "knee voltage," at time t p (Figure 1).…”

“…Figure 4 shows in particular that the film thickness changes little (slope 1) at the beginning of the voltage ramp (from 0 to 320 s, ie, from 0 to 16 V), whereas it increases significantly (slope 2), especially during the steady state, ie, after 320 seconds, for a voltage value at 16 V. In contrast, the thickness of the barrier layer regularly increases (slope 3) both during the transient regime (the voltage ramp) and during the steady-state voltage regime ( Figure 5), in agreement with previous works. 29 In addition, FEG-SEM surface views provide the average pore diameter ( Figure 6) and the average porosity of the anodic film The first forming slope (m 1 ) corresponds to the filling rate of the film pores. The reanodization slope (m 1 ) thus becomes steeper for films having increasing durations of the first anodization ( Figure 11).…”

“…Otherwise, m 1 values seem proportional to the square root of the anodization voltage (Figure 12), as proposed previously. 29 Slope m 2 depicts the growth speed of the oxide above the filled anodic film, once the pores are completely occluded. The value of Previous results correspond to the first anodization durations of less than 180 seconds, with the slope change (the knee voltage) being graphically determined from the forming curves ( Figure 9).…”