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

APPLICATION-ORIENTED TECHNOLOGIES
Corrosion behaviour <strong>of</strong> <strong>aluminium</strong>
materials in aqueous cleaning solutions
Silvio Koehler, Georg Reinhard, Excor Korrosionsforschung GmbH
Especially in <strong>the</strong> car and aviation industry
<strong>aluminium</strong> and its alloys are <strong>the</strong> most important
materials. One advantage <strong>of</strong> <strong>aluminium</strong>
is <strong>the</strong> generally passive and corrosion resistance
in aqueous solutions within a defined
pH range except for pitting corrosion due to
some reactive species, such as chloride. While
pure <strong>aluminium</strong> is covered by a homogenous
<strong>oxide</strong> <strong>layer</strong>, <strong>aluminium</strong> alloys <strong>of</strong>ten consist <strong>of</strong>
several phases (aggregates and mixed phases
<strong>of</strong> alloying elements) leading to an inhomogeneous
<strong>oxide</strong> <strong>layer</strong>. The effect <strong>of</strong> alloying elements
on <strong>the</strong> breakdown <strong>of</strong> <strong>the</strong> passive film
was extensively studied using various grades
<strong>of</strong> <strong>aluminium</strong> and different metals [1,2].
At <strong>the</strong> cleaning <strong>of</strong> <strong>aluminium</strong> materials
most <strong>of</strong> <strong>the</strong> problems arise because <strong>of</strong> <strong>the</strong>
wrong pH-value <strong>of</strong> <strong>the</strong> respective cleaning
bath. The Pourbaix diagram <strong>of</strong> <strong>aluminium</strong> can
be used for a rough estimation, but it is not valid
by 100% for <strong>aluminium</strong> alloys. The second
problem is <strong>the</strong> present <strong>of</strong> corrosion promoting
anions, not only chloride. In combination with
an unsuitable pH-value <strong>the</strong> follows for <strong>the</strong> material
to clean can be dramatically, also in <strong>the</strong>
present <strong>of</strong> small quantities.
Before <strong>the</strong> corrosion ability <strong>of</strong> <strong>the</strong> <strong>aluminium</strong>
materials Al1050 and 2014 was characterized
in <strong>the</strong> aqueous solutions <strong>of</strong> commercial
available cleaning concentrates, <strong>the</strong> effect <strong>of</strong>
<strong>the</strong> pH and <strong>the</strong> chloride concentration were
investigated in model electrolytes with pH
values between 3 and 10 at room temperature.
In ano<strong>the</strong>r set <strong>of</strong> experiments sodium chloride
was added to solutions <strong>of</strong> pH8. In all solutions
<strong>the</strong> free corrosion potential as well as time
depending impedance spectra were recorded
using <strong>the</strong> <strong>aluminium</strong> materials as working
electrode in a top part measuring cell.
For pure <strong>aluminium</strong> (Al 1050) <strong>the</strong> results
measured in <strong>the</strong> model electrolytes with
<strong>the</strong> different pH values were in accordance
to <strong>the</strong> Pourbaix diagram. Fur<strong>the</strong>r, passive
<strong>aluminium</strong> is ra<strong>the</strong>r stable up to high chloride
contents. But in <strong>the</strong> industrial aqueous
cleaning solutions Al1050 corrosion attacks
were observed, although <strong>the</strong> pH value <strong>of</strong> <strong>the</strong>
cleaning solution was in <strong>the</strong> passive region
<strong>of</strong> <strong>aluminium</strong>. Surfactants and complexing
agents included in <strong>the</strong> cleaners seem to have
a corrosion promoting effect. In <strong>the</strong> case <strong>of</strong>
<strong>the</strong> <strong>aluminium</strong> alloy Al 2014 in all solutions
(model electrolytes as well as cleaning baths)
a corrosion attack was observed. It could be
shown by means <strong>of</strong> SEM that in <strong>the</strong> case <strong>of</strong>
Al 2014 <strong>the</strong> selective corrosion around <strong>the</strong>
cathodic copper-rich inter-metallic dominates
<strong>the</strong> corrosion performance.
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. Promising results were observed
at combinations <strong>of</strong> substances creating
a converting <strong>layer</strong> on <strong>the</strong> <strong>aluminium</strong> basis
material with substances inhibiting <strong>the</strong> copper
rich phases selectively.
1. Introduction
Aluminium materials are widely used in <strong>the</strong>
automotive, aerospace and railways industries
[3]. Therefore, protection concepts for semifinished
as well as final products made <strong>of</strong> <strong>aluminium</strong>
and its alloys are requested.
One way to protect <strong>aluminium</strong> surfaces
during <strong>the</strong>ir storage into moist air is <strong>the</strong> use
<strong>of</strong> volatile corrosion inhibitors. These are inhibiting
substances with a sufficient vapour
pressure, which can be incorporated in <strong>the</strong>
matrix <strong>of</strong> <strong>the</strong> packaging materials like paper
and films. The VCI can be released by sublimation
and/or be transporting by water vapour
into <strong>the</strong> package. In <strong>the</strong> case <strong>of</strong> densely closed
packages <strong>the</strong> VCI enrich and saturate <strong>the</strong> inner
atmosphere. Through a reversible adsorption
on <strong>the</strong> metal surface <strong>the</strong>y inhibit corrosion as
far as <strong>the</strong> packaging is closed [4,5]. Besides <strong>the</strong>
densely closed packages <strong>the</strong> right combination
<strong>of</strong> inhibitors has to be used depending on <strong>the</strong>
composition <strong>of</strong> <strong>the</strong> <strong>aluminium</strong> alloy [6].
The surfaces to protect have to be accessible
and clean. To install an effective cleaning
process a plenty <strong>of</strong> parameters and requirements
have to take into account especially
<strong>the</strong> compatibility <strong>of</strong> cleaner and material [7].
At <strong>the</strong> cleaning <strong>of</strong> <strong>aluminium</strong> materials most
<strong>of</strong> <strong>the</strong> problems arise because <strong>of</strong> <strong>the</strong> wrong
pH-value <strong>of</strong> <strong>the</strong> respective cleaning bath. The
second problem is <strong>the</strong> present <strong>of</strong> corrosion
promoting anions like chloride, because <strong>of</strong>
<strong>the</strong> long-term use <strong>of</strong> <strong>the</strong> respective cleaning
baths. In combination with an unsuitable pHvalue
<strong>the</strong> follows for <strong>the</strong> material to clean can
be dramatically, also in <strong>the</strong> present <strong>of</strong> small
quantities. It is known that most <strong>of</strong> <strong>the</strong> cleaning
processes take place at temperatures around
60°C. Here, <strong>the</strong> pH value is shifted to more
alkaline values compared to room temperature.
Fur<strong>the</strong>r, <strong>the</strong> solubility <strong>of</strong> solids (e. g. surface
<strong>oxide</strong> <strong>layer</strong>s, inorganic particles) as well
as <strong>the</strong> velocity <strong>of</strong> <strong>the</strong>ir dissolution is increased
and intended for good cleaning results with-
in short contact times [8]. It is not <strong>the</strong> purpose
<strong>of</strong> this paper to describe <strong>the</strong> mechanism during
<strong>the</strong> cleaning process itself. Especially <strong>the</strong>
electrochemical measurements should be presented
as tools to answer questions about <strong>the</strong>
compatibility <strong>of</strong> cleaning bath and material to
clean and to evaluate <strong>the</strong> current ‘corrosiveness’
<strong>of</strong> a cleaning bath similar to <strong>the</strong> chip/
filter paper test according to DIN 51360 part
2 finally with <strong>the</strong> aim to obtain conditions <strong>of</strong>
<strong>the</strong> metal surface that <strong>the</strong> application <strong>of</strong> VCI
is successful.
2. Experimental
Test panels (50 x 100 mm) <strong>of</strong> <strong>the</strong> <strong>aluminium</strong>
alloys mentioned in Table 1 were used for <strong>the</strong>
experiments. At first <strong>the</strong> panels were cleaned
with methanol and acetone assisted by an ultrasonic
bath. After this <strong>the</strong>y have been stored
in a desiccator for at least 12 h. So a reproducible
formation <strong>of</strong> a native <strong>oxide</strong> <strong>layer</strong> was assured.
For <strong>the</strong> electrochemical measurements
<strong>the</strong> panels were put in a cylindrical top cell
realizing a circular electrode area <strong>of</strong> 1.76 cm²
acting as working electrode. As counter and
reference electrode a platinum net and saturated
calomel electrode (SCE) were used. All
electrical measurements were performed using
<strong>the</strong> impedance measurement system IM6
(Zahner Instruments, Kronach, Germany) at
room temperature (22°C). In a first set <strong>of</strong> experiments
<strong>the</strong> influence <strong>of</strong> <strong>the</strong> pH value on <strong>the</strong>
corrosion were investigated. Therefore, aqueous
solutions (de-ionized water) including
0.01M KNO 3 as supporting electrolyte <strong>of</strong> <strong>the</strong>
Material Si Fe Cu Mn Mg Cr Zn Ti Zr
1050 0.09 0.25 0.001 0.01 0.01 0 0.01 0.01 0
2014 0.88 0.11 4.85 0.85 0.73 0.004 0.05 0.02 0.01
Table 1: Composition <strong>of</strong> <strong>the</strong> investigated <strong>aluminium</strong> alloys
50 ALUMINIUM · EAC CONGRESS 2011