transferred during the experiment (expressed in Coulombs), z is
the number of electrons exchanged during the oxidation reaction
(2.92225 for the AA 7075, [21]) and F the Faraday constant
(96,485 C/mol). The alloy density and the surface exposed area
were 2.81 g/cm3 and 1.0 cm2 respectively;
iv) volumetric expansion ratio (Vox / Vmet), as presented in the following
Formula (2) and inspired by some literature works [22,
23], based on a simple measure of the alloy converted depth
(‘Al’ line in Fig. 2) and the oxide thickness (‘Ox’ line in Fig. 2):
Volumetric expansion ratio ¼ Vox
Vmet
¼ Ox thickness surface area
Al converted depth surface area ð2Þ
The latter quantity, being compared with the theoretical limit of
pure alumina (Pilling–Bedworth ratio of 1.275) is useful to evaluate
the oxide compactness and porosity;
v) nano-hardness profiles acquired along the oxides cross section
using a CSM Nanoindentation Tester equipped with a Berkovich
tip and setting a maximum load of 50 mN, a loading/unloading
speed of 40 mN/min and 15 s as dwell time. The reported values,
evaluated directly by the software using the Oliver–Pharr method
[24], derive from three nano-hardness profiles each consisting
of five indentations. An average of all the indentations was also
reported.
3. Results and discussion
3.1. Direct current and multistep DC anodizing
The former part of this work has dealt with the study of the influence
of the anodizing bath composition, in particular evaluating H2SO4
(anodizing electrolyte) and Al3+ (bath contaminant and hard Lewis's
acid) role and establishing their optimal concentration range during
DC hard anodizing. These results, not presented exhaustively in this
transferred during the experiment (expressed in Coulombs), z is
the number of electrons exchanged during the oxidation reaction
(2.92225 for the AA 7075, [21]) and F the Faraday constant
(96,485 C/mol). The alloy density and the surface exposed area
were 2.81 g/cm3 and 1.0 cm2 respectively;
iv) volumetric expansion ratio (Vox / Vmet), as presented in the following
Formula (2) and inspired by some literature works [22,
23], based on a simple measure of the alloy converted depth
(‘Al’ line in Fig. 2) and the oxide thickness (‘Ox’ line in Fig. 2):
Volumetric expansion ratio ¼ Vox
Vmet
¼ Ox thickness surface area
Al converted depth surface area ð2Þ
The latter quantity, being compared with the theoretical limit of
pure alumina (Pilling–Bedworth ratio of 1.275) is useful to evaluate
the oxide compactness and porosity;
v) nano-hardness profiles acquired along the oxides cross section
using a CSM Nanoindentation Tester equipped with a Berkovich
tip and setting a maximum load of 50 mN, a loading/unloading
speed of 40 mN/min and 15 s as dwell time. The reported values,
evaluated directly by the software using the Oliver–Pharr method
[24], derive from three nano-hardness profiles each consisting
of five indentations. An average of all the indentations was also
reported.
3. Results and discussion
3.1. Direct current and multistep DC anodizing
The former part of this work has dealt with the study of the influence
of the anodizing bath composition, in particular evaluating H2SO4
(anodizing electrolyte) and Al3+ (bath contaminant and hard Lewis's
acid) role and establishing their optimal concentration range during
DC hard anodizing. These results, not presented exhaustively in this
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