work, have allowed to understand how the most important effect of
these two acid species is the growth of an extremely defected oxide
when their concentrations are close or exceed a threshold value
(240 g/L for H2SO4 and 10 g/L of Al3+). All subsequent anodizing tests
were carried out using solutions containing 5–8 g/L of Al3+ and
190 g/L of H2SO4, which represent the optimal conditions in terms of
oxide defectiveness achieved. Natural continuation of the preliminary
work was an intense study of the influence of the bath temperature,
an even more critical process parameter for the hard anodizing treatment,
especially for the peculiar 7075-T6 alloy.
These tests, performed applying direct current (24 mA/cm2 for
95 min, as summarized in Table 1), have allowed to highlight how the
brittleness and the poor mechanical properties of the anodic oxide of
the AA 7075-T6 can be ascribed to the alloy/oxide interface defects
which grow when not appropriated bath conditions are set, and that
are emphasized when high voltage is reached (72 V for the laboratory
system employed).
At the higher potential values, sometimes reached during the final
anodizing stages (for example if an extremely thick oxide is desired),
all the problems related to the different response of the electric field
on the intermetallics become more relevant with consequent formation
of large interfacial flaws often associated to a critical stress state and the
development of gaseous O2. Setting lower temperatures does not permit
an optimal oxygen diffusion toward the oxide facilitating a deep
longitudinal cracking on the conical asperity defects (as shown in
Fig. 3a), while at higher temperatures poor and too porous coatings
are obtained (as reported in Table 3, at T N 0 °C), because at the higher
temperatures the dissolution effect of the acid bath becomes stronger.
With this analysis was therefore possible to identify −2 °C as the
most promising anodizing temperature for the AA 7075-T6 even if, by
reducing the acid dissolution and accelerating the film growth (respect
to 0 °C or 2 °C), this temperature (DC condition) induces a nonlinear
alloy/oxide interface (Fig. 3b).