Note: Descriptions are shown in the official language in which they were submitted.
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PROCEDURE FOR ANODISING ALUMINIUM OR ALUMINIUM ALLOYS
Field of the Invention
This invention refers to a process for anodising aluminium or aluminium alloy
parts, including pure or almost pure aluminium and all its combinations with
other
elements in any proportion.
Background
Traditionally, the acidic solutions used in anodising procedures are composed
of
sulphuric acid in high concentrations, or of chromic acid. The latter is the
main
component used in the aerospace industry. Sulphuric acid is not used in the
aerospace industry due to the low adherence in the treated parts, while
chromic acid
has a high toxicity in live beings and is hazardous for the environment.
The aqueous tartaric-sulphuric acid solution is an alternative method to
anodise
parts through an electrolytic process with low environmental impact. This
method is
described in patent number US 2002/0157961 Al. Another alternative method is
an
aqueous solution of sulphuric acid and boric acid described in US patent
4894127.
These methods do not provide the aluminium or aluminium alloy parts with the
same
properties pertaining to corrosion as the chromic acid treatment.
The procedure in this invention uses the aluminium or aluminium alloy parts,
described as anodes in an electrolytic cell with an aqueous acidic
electrolyte, in
order to create a superficial layer of aluminium oxide on said parts.
This superficial aluminium oxide improves the properties pertaining to
resistance
against corrosion and surface layer adherence of an aluminium or aluminium
alloy
part.
Detailed Description of the Invention
This invention refers to an anodising procedure for aluminium or aluminium
alloys in which the aluminium or aluminium alloy parts are submerged in an
aqueous
solution at a temperature between 0 C and 130 C, and where said solution
includes:
- sulphuric acid,
- tartaric acid, and
- at least one inorganic salt of an element selected between at least one
transition
metal, one lanthanide element, one actinide, and combinations of them, and
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apply a controlled potential difference.
The inorganic salt of the transition metal may be present in a concentration
between 5= 10-' and 1.5 M, preferably in a concentration between 1- 10-6 and 1
M.
In the invention's procedure, said inorganic salt of the transition metal may
be a salt of at least one metal selected between metals from the IIIB, IVB,
VB, VIB,
VIIB, VIIIB, IB, and IIB groups, a salt from a lanthanide or actinide element,
combinations of the previous ones, and preferably a molybdenum salt.
The previously mentioned inorganic salt or salts behave as corrosion
inhibitors by preventing the development of the different corrosion reactions
(depending on the type of inorganic salt), therefore improving the behaviour
of the
parts pertaining to corrosion.
The same aluminium or aluminium alloy parts to be anodised can be used as
an anode.
According to particular embodiments, the electrolyte is an aqueous acidic
electrolyte; preferably it is an aqueous solution of tartaric-sulphuric acid.
According to a preferred embodiment, the aqueous solution that acts as an
electrolyte has a concentration of sulphuric acid between 0.1 and 1.5 M,
preferably
between 0.2 M and 0.9 M, and a concentration of L(+)-tartaric acid between 0.1
and 1.5 M, preferably between 0.2 and 0.8M.
According to the most preferable embodiment, said solution has a
concentration of sulphuric acid between 0.2 M and 0.9 M, L(+)-tartaric acid
with a
concentration between 0.2 and 0.8M, and one or several inorganic salts
composed
of at least one or several transition metals in a concentration between 1- 10-
6 and 1
M.
During the anodising process, the temperature of the aqueous solution is
kept between 0 C and 140 C, preferably between 0 C and 130 C, even more
preferably between 5 C and 80 C, and most preferably between 30 and 40 C.
The electrolytic cell is subject to a potential difference 0.5V and 130V,
preferably between 1 V and 120 V, even more preferably between 2 V and 100 V,
and most preferably between 10 and 30 V according to the procedure.
The duration of the anodising procedure is between 1 and 130 minutes,
preferably between 5 and 120 minutes, and more preferably between 5 and 40
minutes.
This procedure has a duration cycle of about 40% less time as regards
traditional chromic acid anodising.
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The toxicity and hazardousness to the environment of the residues created
by this invention is greatly reduced when compared to those created by the
traditional anodising procedures.
Examples
Example I
Anodising a 2000 series aluminium alloy in a tartaric-sulphuric acid bath with
molybdenum salt
A 2000 series aluminium alloy part, 150x100x2 mm, is subjected to a
conventional cleaning and surface layer removal treatment: degreasing by
immersion for approximately 10 minutes, rinse in distilled water for
approximately 5
minutes, surface layer removal for approximately 10 minutes, and rinse in
distilled
water for 5 minutes.
Then the part is completely submerged in an electrolytic cell, where the part
functions as an anode; the cathode is composed of AISI 321 stainless steel,
and it
has a geometric area equal to or larger than the anode's geometric area. The
electrolyte is an aqueous acidic solution made of 0.40 M sulphuric acid, 0.53
M L
(+)-tartaric acid, and 0.25M sodium molybdenum. The cell's temperature is at
37 C
1 C.
The potential difference increases from 0 to 14 V at a rate of 2.8 V-min-1,
and it stays at 14 V for 20 minutes, creating an oxide layer of approximately
2 m.
The part is rinsed in anodised water for approximately 5 minutes, and it is
sealed in anodised water at boiling point for approximately 40 minutes. Then
it is
dried with hot air.
Example 2
Anodising a 2000 series plaqued aluminium alloy part in a tartaric-sulphuric
acid bath with molybdenum salt.
A 2000 series plaqued aluminium part, 150x100x2 mm, is subjected to a
conventional cleaning and surface layer removal treatment, as described in
Example
1.
The part is completely submerged in an electrolytic cell, where the part
functions as an anode; the cathode is made of AISI 321 stainless steel, and it
has a
geometric area equal to or larger than the anode's geometric area. The
electrolyte
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and the anodising conditions are the same as those described in Example 1,
obtaining an oxide layer of approximately 2 m.
The anodised part is rinsed and sealed in the same manner as that
described in example 1.
Example 3 (comparative)
Anodising a 2000 series aluminium alloy in a tartaric-sulphuric acid bath.
A 2000 series aluminium alloy part, 150x100x2 mm, is subjected to a
conventional cleaning and surface layer removal treatment, as described in
Example
1.
Ttie part is completely submerged in an electrolytic cell, where the part
functions as an anode; the cathode is made of AISI 321 stainless steel, and it
has a
geometric area equal to or larger than the anode's geometric area. The
electrolyte
and the anodising conditions are the same as those described in Example 1,
obtaining an oxide layer of approximately 3 m.
The anodised part is rinsed and sealed in the same manner as that
described in example 1.
Example 4 (comparative)
Anodising 2000 series aluminium in a tartaric-sulphuric acid bath.
A 2000 series plaqued aluminium part, 150x100x2 mm, is subjected to a
conventional cleaning and surface layer removal treatment, as described in
Example
1.
Then the part is completely submerged in an electrolytic ceil, where the part
functions as an anode; the cathode is made of AISI 321 stainless steel, and it
has a
geometric area equal to or larger than the anode's geometric area. The
electrolyte is
an aqueous acidic solution made of 0.40 M sulphuric acid and 0.53 M L (+)-
tartaric
acid. The cell's temperature is maintained at 37 C 1 C.
The potential difference increases from 0 to 14 V at a rate of 2.8 V-min-1,
and it is maintained at 14 V for 20 minutes, creating an oxide layer of
approximately
3 m.
The anodised part is rinsed and sealed as in Example 1.
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Example 5 (comparative)
Anodising with chromic acid
A 2000 series plaqued aluminium part, 150x100x2 mm, is subjected to a
conventional cleaning and surface layer removal treatment, as described in
Example
1.
Then the part is completely submerged in an electrolytic cell, where the part
functions as an anode; the cathode is made of AISI 321 stainless steel, and it
has a
geometric area equal to or larger than the anode's geometric area. The
electrolyte is
an aqueous acidic solution with chromic acid. The cell's temperature is
maintained
between 35 C and 40 C.
The potential difference increases from 0 to 40 V at a rate of 5 V-min-1, and
it stays at 14 V for 45 minutes, creating an oxide layer of approximately 3
m.
The anodised part is rinsed and sealed as in Example 1.
Table 1. Comparison of properties in the parts treated according to the
previous
examples
Thickness (1)96 (1)336 (2)Adherence of (2)Adherence of
Part hours of hours of
( m) dried paint (Gt) damp paint /Gt)
exposure exposure
Ex.1 2 Pass Pass 0 0
Ex.2 2 Pass Pass 0 0
Ex.3 3 Pass Fail 0 0
Ex.4 3 Pass Fail 0 0
Ex.S 3 Pass Pass 0 0
(1) Assay in saline fog chamberaccording to standard ASTM B 117.
(2) Assay of paint adherence according to standard ISO 2409 (before and after
14
days of immersion in distilled water).
The parts treated according to the invention exceed 336 hours in saline fog
according to the requirement established in section 3.7.1.2 of the military
standard
MIL-A-8625-F for IC type anodic layers.
The comparison of the obtained results for the tests done on the example
parts according to the invention and the comparative example conclude that the
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oxides created by the invention have better properties pertaining to corrosion
than
the oxides created in aqueous acidic mediums without inorganic salts. These
properties are equal to or better than the ones obtained through anodising in
chromic acid.
