Environmental Engineering Reference
In-Depth Information
h e use of tartaric and sulphuric acids for anodization is a well-known
modii cation of the standard sulphuric acid anodization technique
[8] and
typically provides a passivation oxide layer with high corrosion resistance.
h e sealing quality and the corrosion resistance of AA2024 T3 anodized
in sulphuric acid with or without addition of tartaric acid were intensively
investigated by i eld emission scanning electron microscopy (FESEM) and
electrochemical impedance spectroscopy (EIS) techniques [6].
h e main results obtained were:
1. Anodizing of AA2024 T3 in the presence of tartaric acid led
to the formation of anodic i lms with lower porosity than
those obtained in diluted sulphuric acid (DSA).
2. h e characterization of hydrothermal sealing by EIS mea-
surements showed that the sealing quality was better for the
anodic i lms formed in TSA than in DSA in agreement with
the lower pore volume of the anodic i lms formed in TSA.
3. h e corrosion behavior evaluated by EIS measurements
revealed that the sealed i lms formed in TSA are signii cantly
more resistant to corrosion than the sealed i lms formed in
DSA. h is enhancement of corrosion resistance is mainly
associated with the higher density of the formed porous
layer and a higher resistance of the barrier layer obtained
at er hydrothermal sealing of specimens anodized in TSA.
Anodizing by even more diluted sulphuric acid has been introduced [6]
to obtain thin anodic i lms (1-5 μm) which should increase the fatigue resis-
tance for specii c structural materials, but the corrosion resistance obtained
by this treatment is lower than in the case of chromic acid anodization.
Here, the anodizing process was performed in a 20 L TSA
bath (0.53 M C
4
H
6
O
6
/ 0.41 M H
2
SO
4
) at 37
C following Iglesias-Rubianes
et al.,
who already discussed in detail the growth of anodic layers on
AA2024 T3 by this process even though in a dif erent scale [9]. Due to
the fact that AA2024 normally contains 4-4.5% of copper, the inl uence of
Cu
2+
ions in the anodic layer is substantial with respect to the morphology,
phase composition and dielectric properties. It is known from literature
that within barrier i lms, Al
3+
and O
2−
ions are mobile [10]. When copper
is present in the bulk aluminium it will be incorporated into the anodic
i lms as CuO islands. h e Cu
2+
ions are able to migrate outwards towards
the bulk about three times faster than the Al
3+
ions. h e presence of this
copper oxide—sometimes also referred to as “conductive oxide”—locally
increases the conductivity of the alumina layer [10]. Due to a relatively
°
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