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

following pH values were prepared: pH 3, pH
5, pH 8 and pH 10. The pH value was adjusted
with HNO 3 and NaOH, respectively. After
<strong>the</strong>se experiments 1-3 wt% aqueous solutions
(de-ionized water) including 0.01M KNO 3 as
supporting electrolyte <strong>of</strong> <strong>the</strong> commercial available
cleaner concentrates mentioned in Table
2 were prepared. In all aerated solutions <strong>the</strong>
free corrosion potential has been measured for
30 min. After this electrochemical impedance
spectra (EIS) were recorded at <strong>the</strong> respective
open circuit potentials in a frequency range <strong>of</strong>
100 kHz to 50 MHz after 5, 10, 15 and 30min
<strong>of</strong> immersion time. Thereby, <strong>the</strong> impedance
spectra were taken at o<strong>the</strong>r surface areas than
<strong>the</strong>se used for <strong>the</strong> recording <strong>of</strong> <strong>the</strong> free corrosion
potentials. The EIS curves were fitted using
<strong>the</strong> Thales s<strong>of</strong>tware (Zahner Instruments,
Kronach, Germany).
The samples <strong>of</strong> Al 2014 were investigated
by SEM (Scanning Electron Microscopy)
No. kind <strong>of</strong> cleaner suitable for* pH value at 25°C*
1 demulsifying Fe, (Al) 9.1 ± 0.3 (1%)
2 demulsifying Fe, Al, Zn 8.0 ± 0.2 (2%)
3 emulsifying Fe, Al, Zn 8.6 ± 0.3 (1%)
* according to product data sheet
Table 2: List <strong>of</strong> investigated cleaners
coupled with EDX (Energy-dispersive X-ray
spectroscopy). Here, surface regions affected
and not affected by cleaning solution 2 were
investigated and compared concerning <strong>the</strong> ratio
<strong>of</strong> <strong>the</strong> elements Al, Cu and O. The SEM
images were recorded with a LEO 440 (STS,
North Billerica, USA) at activation energies
<strong>of</strong> 5 keV and 20 keV, respectively. The EDX
spectra were recorded by <strong>the</strong> coupled SDD
(Silicon Drift Detector) AXAS (Ketek GmbH,
München, Germany).
In ano<strong>the</strong>r set <strong>of</strong> experiments 0.01M benzotriazole
and/or 0.01M sodium di-hydrogen
phosphate was added to a 2 wt% solution <strong>of</strong>
cleaner 2. The free corrosion potential as well
as electrochemical impedance spectra were
recorded as described above.
3. Results and discussion
At first <strong>the</strong> influence <strong>of</strong> <strong>the</strong> pH value and <strong>the</strong>
chloride concentration were investigated on
<strong>the</strong> alloy 1050, which consists <strong>of</strong> pure <strong>aluminium</strong>
by almost 99.5%. As expected from
<strong>the</strong> Pourbaix-diagram <strong>of</strong> <strong>aluminium</strong> [9] <strong>the</strong>
free corrosion potential E corr measured in <strong>the</strong>
pH range between 3 and 5 remains in a region,
where <strong>the</strong> <strong>aluminium</strong> surface is in a passive
state, while at pH 10 <strong>the</strong> free corrosion potential
corresponds to an active dissolution (see
Fig. 1).
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In alkaline solutions <strong>the</strong> <strong>oxide</strong> <strong>layer</strong> dissolves
under <strong>the</strong> formation <strong>of</strong> [Al(OH) 4 ]- complexes.
The impedance spectra recorded at pH10 (Fig.
2) show an increase <strong>of</strong> <strong>the</strong> C ox combined with
a decreasing resistance in <strong>the</strong> region <strong>of</strong> lower
frequencies. This behaviour is typical for a
uniform dissolution <strong>of</strong> <strong>the</strong> <strong>oxide</strong> <strong>layer</strong> [10].
Strong deviations from <strong>the</strong> Pourbaix-diagram
mentioned above can be observed at <strong>the</strong>
investigation <strong>of</strong> <strong>aluminium</strong> alloys. The inter-
Fig. 1: Free corrosion potential E corr <strong>of</strong> Al 1050 in
solution <strong>of</strong> different pH value
Fig.2: Impedance spectra <strong>of</strong> Al 1050 at several pH
values after 30 min immersion time
pretation <strong>of</strong> <strong>the</strong> free corrosion potentials <strong>of</strong>
<strong>the</strong> <strong>aluminium</strong> alloys is ra<strong>the</strong>r complicated,
because <strong>the</strong>y are mixed potentials with portions
<strong>of</strong> bulk and/or selective corrosion processes.
In <strong>the</strong> case <strong>of</strong> <strong>the</strong> copper-rich Al2014
(Fig. 3) <strong>the</strong> curve in <strong>the</strong> beginning can be
interpreted by a selective corrosion around
<strong>the</strong> cathodic copper-rich inter-metallic phases
followed by a stabilization / passivation at pHvalues
between 3 and 10. In more acid as well
as more alkaline solutions <strong>the</strong> free corrosion
potentials are typical for <strong>the</strong> dissolution <strong>of</strong>
<strong>the</strong> bulk <strong>aluminium</strong> [11,12]. More details are
available from <strong>the</strong> impedance data. As already
mentioned <strong>the</strong> alloy Al2014 is characterized
by a higher content <strong>of</strong> copper, which forms inter-metallic
phases. In <strong>the</strong> impedance spectra
(Fig. 4) one can observe lower resistances in
<strong>the</strong> low-frequency region compared to pure
<strong>aluminium</strong>. Taking a look on <strong>the</strong> transient behaviour
<strong>the</strong>se resistances keep constant on a
low level at pH values below 3 and above 10.
The capacities belonging to <strong>the</strong>m keep con-
stant, too. Such a behaviour can be explained
by a selective corrosion around <strong>the</strong> copper rich
inter-metallic phases.
Transferring <strong>the</strong> findings to industrial
cleaning baths: Here, solutions <strong>of</strong> 3 commercial
available cleaning concentrates were pre-
Fig. 3: Free corrosion potential E corr <strong>of</strong> Al 2014 in
solution <strong>of</strong> different pH value
Fig. 4: Impedance spectra <strong>of</strong> Al 2014 at several pH
values after 30min immersion time
pared. All cleaners should be suitable to clean
<strong>aluminium</strong> according to <strong>the</strong>ir product data
sheets (see Table 2), however, at higher temperatures
as room temperature. A strong uniform
corrosion was observed already at room
temperature in <strong>the</strong> baths prepared <strong>of</strong> cleaner
1 and 3. This is explainable by <strong>the</strong> pH-value
situated in <strong>the</strong> active region <strong>of</strong> <strong>aluminium</strong>. In
<strong>the</strong> cleaner 2 with pH8 only a moderate attack
was observed (Fig. 5). Maybe <strong>the</strong> bath ingredients
accelerate <strong>the</strong> <strong>oxide</strong> <strong>layer</strong> dissolution
at local defects by complexion <strong>of</strong> <strong>aluminium</strong>
[13]. For <strong>the</strong> <strong>aluminium</strong> alloy Al2014 <strong>the</strong>
results found in <strong>the</strong> cleaning solutions were
identical to this in <strong>the</strong> respective pH-model
solutions. A black-colouring <strong>of</strong> <strong>the</strong> immerged
alloy surfaces as shown in Fig. 6 was observed
every time.
Surface areas <strong>of</strong> Al 2014 affected (Fig. 6,
black coloured area in <strong>the</strong> small picture) and
not affected (Fig. 6, grey coloured area in <strong>the</strong>
small picture) by cleaner 2 were investigated
by SEM and EDX. According to <strong>the</strong> measured
element ratios (Table 3) <strong>the</strong> selective corrosion
and dissolution <strong>of</strong> <strong>aluminium</strong> around <strong>the</strong> copper
rich phases could be proved. The black
ALUMINIUM · EAC CONGRESS 2011 51