Note: Descriptions are shown in the official language in which they were submitted.
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"ANODIZATION METHOD FOR CORROSION PROTECTION OF ALUMINIUM
OR ALUMINIUM ALLOY ELEMENTS USED IN AN AIRCRAFT STRUCTURE"
CROSS-REFERENCE TO RELATED APPLICATIONS
This patent application claims priority from
Italian patent application no. 102018000007314 filed on
July 18, 2018, the entire disclosure of which is incorporated
herein by reference.
TECHNICAL FIELD
The present invention relates to an anodization method
for corrosion protection of aluminium or aluminium alloy
elements used in an aircraft structure.
BACKGROUND OF THE INVENTION
As is known, in order to protect aluminium or aluminium
alloy elements used in an aircraft structure, anodization
methods have been developed to provide a thin (a few microns)
protective layer of metal oxide, which protects the
underlying aluminium/aluminium alloy from corrosion. This
layer of metal oxide also facilitates the subsequent painting
of the aircraft structures and also increases the surface
electrical resistance of the aircraft structure.
Typically, the known anodization processes comprise a
plurality of steps, including:
a) subjecting the element to a degreasing step by means
of an alkaline bath for removing contaminating
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elements, such as for example oils, fats, lubricants,
protective layers, dusts and residues in general - then
subjecting it to a first washing in water;
b) subjecting the element to an acid pickling step. Then
extracting the element from the acid solution and
subjecting the element to a subsequent washing in
water. This step contributes to the removal of natural
oxide, thermal oxides, traces of materials deposited
as a result of mechanical processing, scratches,
discolourations, mild corrosion;
c) subjecting the washed element to a subsequent
electrochemical treatment step by dipping the element
in a chromic acid solution (using chromium with an
oxidation number of +6 - hexavalent chromium) and
applying an electric potential to this element;
d) subjecting the element to a subsequent second washing
in water;
e) dipping the element in a bath in which a solution of a
chromium compound with an oxidation number of +6
(hexavalent chromium) is present, in order to carry
out a post-anodization sealing step;
f) extracting the element from the bath of step e) and
subjecting it to a third final washing.
This method uses very dangerous compounds, such as
H2Cr04, commonly called chromic acid where the chromium has
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an oxidation number of +6; it is a highly oxidizing species.
The chemical reaction that occurs is the following:
Electrochemical reaction at the anode:
2A1 + 3H20 = A1203 + 6H+ + 6e
Electrochemical reaction at the cathode:
6H+ + 6e- = 3H2
Resulting anodization reaction:
2A1 + 3H20 => A1203 + 3H2
Aluminium chromate will also be formed according to the
following mechanism:
chromic anhydride -> chromic acid -> aluminium chromate
Cr03 -> H2Cr04 -> Al, (Cr04) 3
Furthermore, on the basis of experimental and
epidemiological evidence, chromium with an oxidation number
of +6 (hexavalent chromium) has been classified by IARC as
a human carcinogen (Class I).
With regard to the effects on health, several studies have
demonstrated that exposure to hexavalent chromium is one of
the possible causes of lung cancer, as it is mutagenic and
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carcinogenic. In fact, the respiratory system is the main
target of the toxic and carcinogenic action, and acute and
chronic occupational exposure occurs above all by absorption
through inhalation. The toxicity of the hexavalent form at
the intracellular level appears above all with the numerous
molecular and structural alterations caused by the unstable
[Cr(V) and Cr(IV)] and stable [Cr(III)] forms resulting from
the reduction process.
US2015020925 describes a method for the surface
treatment of an aluminum, magnesium or one of its alloys, to
protect the part from corrosion. The method consists in
consecutively immersing the part in the following two baths:
- first aqueous bath at low temperatures containing a
corrosion inhibiting metal salt and an oxidizing compound to
form a conversion coating on the surface of the part
containing oxides, hydroxides and fluorides based on
Zirconium / Chromium;
- a second aqueous bath, kept at a temperature below 80
C and containing an oxidizing compound and a rare earth salt
corrosion inhibitor. Through the presence of hydrogen
peroxide, the oxidation of chromium is obtained from the
trivalent to the hexavalent form and the formation of
chromates.
The method can be performed for the chemical conversion
of aluminum or its alloys and magnesium or its alloys, on
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parts that have not been previously treated or after
anodizing the piece to seal the anodic layer.
Therefore, there is a need to develop a method which does
not use toxic/carcinogenic materials and allows the
formation of an oxide layer that provides good protection to
the underlying aluminium/aluminium alloy.
SUMMARY OF THE INVENTION
The above object is achieved by the present invention
in so far as it relates to an anodization method for
corrosion protection of an aluminium or aluminium alloy
element used in an aircraft structure, comprising the
following steps:
a) subjecting the element to a degreasing step by means
of an alkaline bath (block 100) for removing contaminating
elements;
b) subjecting the element to a subsequent first washing
in water (block 110);
c) subjecting the element to an acid pickling step (120)
by dipping the element in an acid solution and then
extracting the element from the acid solution and subjecting
the element to a subsequent washing in water;
d) subjecting the washed element to a subsequent
electrochemical treatment step in a tank (140) by dipping
the element in a solution of tartaric acid (C4H606) and
sulphuric acid (H2SO4) and applying an electric potential to
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said element;
e) subjecting the element to a subsequent second washing
in water (150);
f) dipping (block 170) the element in a bath in which
a solution of chromium, with an oxidation number of +3, and
zirconium ions and fluorides is present, in order to carry
out a post-anodization sealing step;
g) extracting the element from the bath of step f) and
subjecting it to a third final washing in water and a
subsequent dipping in a tank of boiling water, which provides
a second sealing step, and then drying the element (block
180).
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be illustrated with reference to
the accompanying figures wherein:
Figure 1 represents a non-limiting embodiment showing
the main steps of the method according to the present
invention; and
Figure 2 specifies one step of the method of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
With reference to Figure 1, the anodization method for
corrosion protection of an aluminium or aluminium alloy
element used in an aircraft structure comprises the following
steps:
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a) subjecting the element to a degreasing step by means
of an alkaline bath (block 100) for removing contaminating
elements, such as for example oils, fats, lubricants,
protective layers, dusts and residues in general. Typically,
step a) is carried out by dipping the element in the alkaline
bath for a time interval of 10 - 20 minutes. Typically, the
alkaline bath has a temperature of approximately 55 5
degrees Celsius.
b) subjecting the element to a subsequent first washing
in water (block 110). Typically, step b) is carried out with
water for a time ranging from 2 to 5 minutes at a temperature
below 35 degrees Celsius.
c) subjecting the element to an acid pickling step (block
120) by dipping the element for approximately 5-10 minutes
in an acidic solution based on ferric sulphate and a mixture
of acids maintained at a temperature comprised between 20 C
and 40 C and then extracting the element from the acidic
solution and subjecting the element to a subsequent washing
in water (block 130 following block 120) for 4 - 10 minutes
at room temperature and assessing the film of water. Checking
by assessing the film of water on the surface of the part
during the rinsing ensures the effectiveness of the
pretreatment. Step c) contributes to the removal of natural
oxide, thermal oxides, traces of materials deposited as a
result of mechanical processing, scratches, discolourations,
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mild corrosion;
d) subjecting the washed element to a subsequent
electrochemical treatment step in a tank (block 140) by
dipping the element in a solution of tartaric acid (C4H606)
and sulphuric acid (H2SO4) and applying an electric
potential, wherein the aluminium/aluminium alloy element
behaves as the anode (positive pole) whereas the negative
electrode (or cathode) is represented by the tank. Step d)
is carried out with a solution having a temperature comprised
between 36 and 39 degrees Celsius. The typical concentration
of tartaric acid is 72-88 g/1 and that of sulphuric acid is
36-44 g/l.
The chemical reaction occurring in step d) is the
following:
Electrochemical reaction at the anode:
2A1 + 3E170 = A1203 + 6H+ + 6e
Electrochemical reaction at the cathode:
6H+ + 6e- - 3H2
Resulting anodization reaction:
271 + 3H20 => A1203 + 3H2
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Aluminium sulphate, which contributes to the protection of
the underlying metal/metal alloy, will also be formed
according to the following mechanism:
3H2SO4 + 2A1 E-4 Al2 (SO4)3 + 3H2
Step d) is typically performed using the following
parameters (see Figure 2):
- applying on the element the voltage within one minute
from the dipping of the element in the solution;
- subsequently and without interruption applying on the
element an increasing voltage with a ramp not exceeding 3
volts per minute;
- subsequently and without interruption applying on the
element a constant voltage (approximately 14 Volts) for
approximately 20 minutes, and thereafter;
- gradually reducing the applied voltage to a null value
in approximately one minute;
- removing the element from the solution within 3
minutes from the switching off of the voltage
e) subjecting the element to a subsequent second washing
in water (block 150). Typically, step e) comprises an
optional rinsing step by dipping in industrial water (block
150a) at room temperature and a subsequent rinsing step by
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dipping in purified water at room temperature (block 150b).
Subsequently, the washed element is subjected to visual
inspection by an operator (block 160) and, if this step is
fulfilled, the following step is carried out:
f) dipping (block 170) the element in a bath in which
chromium, with an oxidation number of +3, zirconium ions and
fluorides (resulting from salts and
fluorozirconates/silicates) are present, in order to carry
out a first post-anodization sealing step;
The chemical reaction that occurs is the following:
4A1203 + 24F- + 3Zr+4 + 4Cr+3 4 8A1F3 + 3Zr02 + 2Cr203
Typically, the dipping time in step f) is comprised between
2 and 20 minutes. It is suggested a time comprised between
2 and 3 minutes.
g)
extracting the element from the bath of step f)
and subjecting it to a third final washing and a subsequent
dipping in a tank of boiling water (temperature comprised
between 95 and 100 degrees Celsius, pH ranging from 4.5 to
7, for approximately 30 minutes), which provides a second
sealing step, according to the following reaction:
Al2O3 + H20 -> 2 A10 (OH) (an aluminium oxide hydroxide-
bohmite)
Which results in a volume increase, which is responsible for
the "filling" of the pores of the anodic oxide. The reduction
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in porosity resulting from alumina hydration drastically
reduces the adsorption capacity, making the surface
insensitive to fingerprints, colour and grease stains, and
giving the oxide a greater resistance to corrosion. To
achieve the above reaction and to obtain A10 (OH) (aluminum
hydroxide oxide), the temperature has a fundamental
influence. If in fact the temperature were lower than the
one highlighted above, a layer of aluminum hydroxide [Al
(OH) 3] would be created with different characteristics and
with a percentage of hydration of the oxide that would not
guarantee the same resistance to corrosion.
Drying the element (block 180). Typically, the drying
can be carried out in a dust-free environment, for example
an oven that reaches a temperature between 45 and 65 degrees
Celsius for at least 20 minutes.
Between step b) and step d) the following further steps
may also be carried out, optional to step c), which is
carried out anyway:
rapid alkaline chemical etching (block 200) with a
caustic soda based solution in order to prepare/activate the
aluminium/aluminium alloy surfaces for the electrochemical
treatment in step d). Typically, this step is carried out by
dipping the element in an alkaline solution having a
temperature of approximately 60 degrees for 30-60 seconds;
and
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Aluminium desmutting and rinsing (block 210).
Typically, this step of clearing the blackening due to the
preceding alkaline bath is performed by dipping the element
in a clearing solution (based on ferric sulphate and a
mixture of acids) at room temperature (approximately 25
degrees) for 5-10 minutes.
From the above description it appears that the method
of the present invention, in particular the sealing (step
f), does not use highly toxic, and in particular carcinogenic
compounds, such as chromium with an oxidation number of +6.
The sealed oxide layer has a sensibly constant thickness of
a few microns (typically from 2 to 7 microns) and good
adhesion characteristics. The elements are therefore
effectively protected from corrosion.
The Applicant performed a series of mechanical fatigue
tests on samples subjected to the method of the present
invention. The tests were carried out on cylindrical
specimens treated according to the method of the present
invention in accordance with standard EN6072 provided in the
aeronautical field. Results were plotted in a Wohler curve
at various load levels.
By comparing the data, the treatment according to the
present method was thus shown not to alter the fatigue
resistance of the sample, which complies with the CS 25.571
aeronautical directives. Corrosion tests, which were carried
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out in a salt spray chamber for ASTM B 117, also performed
well. In this case, a salt spray chamber was used, which was
capable of providing a controlled saline environment fed by
a 5% NaCl salt solution. Tests for permanence in the salt
spray for 336 hours were brilliantly passed.
Tests for adhesion of the paint to the elements treated
according to the method of the present invention were also
carried out. These tests were carried out in accordance with
ISO 2409 standard after dipping in demineralized water at 23
degrees Celsius for 14 days. The test consists in squaring
off, after the dipping in water, the paint layer with a six-
blade cutter, arranging and pressing a layer of tape against
the squared paint and then tearing off the tape quickly. The
tests gave a positive result, with a detachment of the
coating paint of less than 5%.
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