10 s), which represents a combination between high current density
during time-on (high risk of flaws) and frequent pauses at low currents
(useful for the recovery, the heat dissipation and the homogenization of
the temperature on the component).
The sample obtained interrupting the treatment after 76 min (before
reaching a potential of 40 V) meets all the requirements expected:
in just over an hour it was possible to get a hard and compact oxide
(PC_6, Table 4) with a defects-free interfacial morphology, shown in
Fig. 6c, and properties comparable to those of the samples obtained
working at lower current values.
4. Conclusions
The most important results obtained by studying the hard anodizing
process of 7075-T6 alloy can be summarized as follow:
i) the brittleness and the poor interfacial adhesion of the AA 7075-
T6 anodic films are related to the presence of some intermetallic
phases, which are substrates unfit for this treatment: with direct
current procedures (simple DC or MSDC) only a properly selection
of the bath conditions and electrical parameters allow the
growth of interesting oxides;
ii) pulsed current cycles (PC), considering slow square pulse mode,
allow to obtain hard anodic oxide coating without affecting the
interfacial adhesion and then minimizing, thanks to the recovery
periods, the flaws growth on the intermetallic phases;
iii) high potential values, reached during the final stage of PC treatments,
cause an increase of the defectiveness due to an inhomogeneous
concentration of the electric field on the different alloy
constituents. Defectless thick coating (~80 μm) was obtained by
applying a current density of 24 mA/cm2 during the time-on, with
a duty cycle ranging between 50 and 75% (120–150 minute treatment);
slightly thinner oxides (50 μm) can be obtained in 1 h,
interrupting the PC treatment when the voltage approach the
value of 40 V.