Influence of the natural aluminium oxide layer on ... - ALU-WEB.DE

APPLICATION-ORIENTED TECHNOLOGIES
Fig. 5: Impedance spectra <strong>of</strong> Al1050 at pH8 and in
solutions <strong>of</strong> <strong>the</strong> cleaner 1-3 (Table 2)
Fig. 6: SEM image (5 keV, 200x): Al2014 after immersion
in a solution <strong>of</strong> cleaner 2
corrosion product is most likely Al(OH) 3,
which only appears black because <strong>of</strong> <strong>the</strong> small
<strong>layer</strong> thickness.
The next step was to find suitable inhibitors
for <strong>the</strong> cleaning solution, which are able to protect
<strong>the</strong> <strong>aluminium</strong> materials, but <strong>the</strong>y should
not lower <strong>the</strong> pH-value to uphold <strong>the</strong> cleaning
effect. First experiments were done with sodium
di-hydrogen phosphate and benzotriazole.
The substances were added to a 2% solution
<strong>of</strong> cleaner 2 toge<strong>the</strong>r and each alone. In Fig. 7
<strong>the</strong> resistances R ox extracted from <strong>the</strong> impedance
spectra at low frequency regions (around
100 mHz) are presented. They correspond to
<strong>the</strong> quality <strong>of</strong> <strong>the</strong> <strong>oxide</strong> <strong>layer</strong> and <strong>the</strong> corrosion
resistance, consequently. Here, <strong>the</strong> inhibition
effect <strong>of</strong> both substances is obvious,
whereas <strong>the</strong> effect <strong>of</strong> each single substance is
limited. The following mechanism is assumed:
Phosphate is forming insoluble <strong>aluminium</strong>
phosphate, inhibiting <strong>the</strong> dissolution <strong>of</strong> <strong>the</strong>
<strong>aluminium</strong> bulk material. Benzotriazole is
well known as inhibitor for copper. It forms
conversion <strong>layer</strong>s on <strong>the</strong> copper rich phases,
blocking <strong>the</strong> reduction <strong>of</strong> oxygen. In comparison
<strong>the</strong> dissolution <strong>of</strong> <strong>the</strong> <strong>aluminium</strong> bulk material
dominates in <strong>the</strong> solution <strong>of</strong> cleaner 2.
Therefore, <strong>the</strong> single addition <strong>of</strong> benzotriazole
Cu Al O
Non affected area 5 80 5
Affected area 78 7 10
Table 3: Results <strong>of</strong> EDX measurements, amounts <strong>of</strong>
<strong>the</strong> elements Cu, Al and O in wt%
Fig. 7: R OX extracted from <strong>the</strong> impedance spectra
recorded in solutions <strong>of</strong> cleaner 2 with and without
benzotriazole and sodium di-hydrogen phosphate
to inhibit <strong>the</strong> inter-crystalline corrosion is insufficient
to stop <strong>the</strong> bulk corrosion, whereas
<strong>the</strong> single addition <strong>of</strong> phosphate can inhibit
<strong>the</strong> bulk corrosion, but not <strong>the</strong> inter-crystalline
corrosion.
Finally, Fig. 8 shows panels <strong>of</strong> Al 2014
completely cleaned (dip-cleaning assisted by
ultrasound for 10 min) with cleaner 2 with and
without <strong>the</strong> additionally added benzotriazole
and sodium di-hydrogen phosphate. The effect
is obvious [14].
4. Conclusions
Using <strong>the</strong> electrochemical methods <strong>the</strong> different
corrosion mechanisms <strong>of</strong> pure <strong>aluminium</strong>
and Al 2014 could be detected. However, <strong>the</strong>
behaviour <strong>of</strong> <strong>the</strong> free corrosion potentials is
difficult to interpret especially for <strong>aluminium</strong>
alloys. Here, <strong>the</strong> resulting potential is always
a mixture <strong>of</strong> <strong>the</strong> response <strong>of</strong> <strong>the</strong> several inter
metallic phases.
Pure <strong>aluminium</strong> behaved passively in a
pH range between 5 and 8. In <strong>the</strong> recorded
impedance spectra only <strong>the</strong> resistance <strong>of</strong> <strong>the</strong>
electrolyte and <strong>the</strong> capacity <strong>of</strong> <strong>the</strong> barrier <strong>layer</strong>
were visible. At pH values above 10 a uniform
dissolution <strong>of</strong> <strong>the</strong> <strong>oxide</strong> <strong>layer</strong> was observed
characterized by a decreasing <strong>oxide</strong> <strong>layer</strong> resistance
and an increasing surface capacity.
The investigated <strong>aluminium</strong> alloy Al 2014
is an inhomogeneous alloy and has a relatively
high copper content. The surface <strong>oxide</strong> <strong>layer</strong>
<strong>of</strong> <strong>the</strong> alloy is also inhomogeneous and <strong>the</strong>
bulk material consists <strong>of</strong> several phases. For
this alloy a uniform and/or selective corro-
sion was found depending on <strong>the</strong> pH value
already at room temperature. Below pH 3
<strong>the</strong> alloy showed a uniform corrosion. In <strong>the</strong>
pH range between 3 and 8 an increase <strong>of</strong> <strong>the</strong>
resistance in <strong>the</strong> lower frequency range combined
with an unchanged capacity at medium
frequencies were detected. This response can
be interpreted as selective corrosion around
<strong>the</strong> cathodic copper rich phases. This (maybe)
leads to a formation <strong>of</strong> corrosion products with
a black visual appearance. They precipitated
on <strong>the</strong> whole immersed surface. Additionally
measurements by SEM/EDX proved <strong>the</strong> selective
corrosion and dissolution <strong>of</strong> <strong>aluminium</strong>
around <strong>the</strong> copper rich phases. At pH values
above 10 a uniform dissolution was observed
for Al 2014.
Transferred to <strong>the</strong> industrial cleaning <strong>the</strong>se
results reveal <strong>the</strong> importance <strong>of</strong> <strong>the</strong> ‘right
choice’ <strong>of</strong> method and cleaner concentrate (pH
value!) as well as an experienced bath monitoring
(at least pH value and chloride content).
Fur<strong>the</strong>rmore, <strong>the</strong> experiments showed that
always <strong>the</strong> compatibility <strong>of</strong> <strong>the</strong> cleaner and
<strong>the</strong> material has to be tested before <strong>the</strong> application.
Therefore, <strong>the</strong> recording <strong>of</strong> electrochemical
impedance spectra at <strong>the</strong> free corrosion
potential is predestinated. A comparison
<strong>of</strong> three commercial available cleaners (all
should be suitable for <strong>aluminium</strong> according
to <strong>the</strong>ir product data sheet) showed, that in
cleaners with <strong>the</strong> pH value above 8.5 pure
<strong>aluminium</strong> corroded heavily. But also in cleaners
with a suitable pH-value a moderate at-
tack was observed, caused by a complexion <strong>of</strong>
<strong>aluminium</strong> by cleaner ingredients.
The addition <strong>of</strong> both, sodium di-hydrogen
phosphate and benzotriazole, can prevent
Al2014 in near neutral cleaning solutions. But
<strong>the</strong> single addition <strong>of</strong> benzotriazole to inhibit
<strong>the</strong> inter-crystalline corrosion is insufficient to
stop <strong>the</strong> bulk corrosion, whereas <strong>the</strong> single addition
<strong>of</strong> phosphate can inhibit <strong>the</strong> bulk corrosion,
but not <strong>the</strong> inter-crystalline corrosion.
References
[1] H. Zhan, J. M. C. Mol, F. Hannour, L. Zhuang,
H. Terryn and J. H. W. de Wit; “The influence <strong>of</strong>
copper content on intergranular corrosion <strong>of</strong> model
Fig. 8: Al 2014 test panels after dip cleaning in a 2 wt% solution <strong>of</strong> cleaner 2; left: without benzotriazole
and sodium di-hydrogen phosphate, right: with benzotriazole and sodium di-hydrogen phosphate
52 ALUMINIUM · EAC CONGRESS 2011