U.S. patent application number 10/419152 was filed with the patent office on 2003-10-23 for method of anodizing a part made of aluminum alloy.
This patent application is currently assigned to MESSIER-BUGATTI. Invention is credited to Viola, Alain.
Application Number | 20030196907 10/419152 |
Document ID | / |
Family ID | 28686230 |
Filed Date | 2003-10-23 |
United States Patent
Application |
20030196907 |
Kind Code |
A1 |
Viola, Alain |
October 23, 2003 |
Method of anodizing a part made of aluminum alloy
Abstract
The invention provides a method of anodizing a part made of
aluminum alloy. According to the invention, the method comprises
the following steps in succession: providing an aqueous anodizing
bath essentially comprising sulfuric acid at a concentration lying
in the range 55 g/l to 85 g/l, and excluding the presence of any
phosphoric acid or any boric acid; maintaining said bath at a
constant temperature essentially lying in the range 15.degree. C.
to 27.degree. C.; dipping said part into said bath; applying a
voltage to said part dipped in said bath, the voltage lying
essentially in the range 5 V to 30 V, with low current density on
said part; and maintaining said part in said bath until a coating
of desired thickness has been obtained, which thickness lies
substantially in the range 1 .mu.m to 3 .mu.m.
Inventors: |
Viola, Alain; (Strasbourg,
FR) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W.
WASHINGTON
DC
20037
US
|
Assignee: |
MESSIER-BUGATTI
|
Family ID: |
28686230 |
Appl. No.: |
10/419152 |
Filed: |
April 21, 2003 |
Current U.S.
Class: |
205/324 ;
205/328 |
Current CPC
Class: |
C25D 11/08 20130101;
C25D 11/12 20130101; C25D 11/10 20130101 |
Class at
Publication: |
205/324 ;
205/328 |
International
Class: |
C25D 011/04 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 22, 2002 |
FR |
0204984 |
Claims
What is claimed is:
1/ A method of anodizing an aluminum alloy part, the method
comprising the following steps in succession: providing an aqueous
anodizing bath essentially comprising sulfuric acid at a
concentration lying in the range 55 g/l to 85 g/l, and excluding
the presence of any phosphoric acid or any boric acid; maintaining
said bath at a constant temperature essentially lying in the range
15.degree. C. to 27.degree. C.; dipping said part into said bath;
applying a voltage to said part dipped in said bath, the voltage
lying essentially in the range 5 V to 30 V, with low current
density on said part; and maintaining said part in said bath until
a coating of desired thickness has been obtained, which thickness
lies substantially in the range 1 .mu.m to 3 .mu.m.
2/ A method according to claim 1, in which the concentration of
sulfuric acid in the bath lies essentially in the range 57 g/l to
67 g/l, and is preferably close to 62 g/l.
3/ A method according to claim 1, in which the bath is maintained
at a constant temperature close to 22.degree. C.
4/ A method according to claim 1, in which the voltage applied to
the part dipped in the bath is fixed on a constant value throughout
the duration of the anodizing treatment, said constant value lying
in the range 5 V to 30 V.
5/ A method according to claim 4, in which the constant value of
the voltage lies in the range 7 V to 20 V.
6/ A method according to claim 1, in which the voltage applied to
the part dipped in the bath is initially fixed to a first constant
value, and then after a predetermined duration, to a second
constant value that is higher than the first, said first and second
constant values both lying in the range 5 V to 30 V.
7/ A method according to claim 6, in which the first constant
voltage value lies in the range 5 V to 11 V, and the second
constant voltage value lies in the range 15 V to 30 V.
8/ A method according to claim 1, in which the current density on
the part dipped in the bath remains well below 100 A/m.sup.2.
9/ A method according to claim 8, in which the current density is
essentially less than 80 A/m.sup.2, preferably lying in the range
30 A/m.sup.2 to 70 A/m.sup.2.
10/ A method according to claim 1, in which the aqueous anodizing
bath also contains an acid alcohol having one to three acid
functions, at a concentration lying in the range 12 g/l to 22
g/l.
11/ A method according to claim 10, in which the acid alcohol used
in the that is tartaric acid or citric acid, and the concentration
of said acid alcohol lies essentially in the range 12 g/l to 17
g/l.
12/ A method according to claim 1, in which the part to be treated
is subjected to preliminary degreasing/pickling treatment prior to
being dipped in the bath.
13/ A method according to claim 1, in which the treated part is
subjected to subsequent treatment of sealing the coating.
Description
[0001] The present invention relates to treating aluminum alloy
parts, in particular parts for use in making aviation components,
and more specifically the invention relates to a method of
anodizing a part made of aluminum alloy.
BACKGROUND OF THE INVENTION
[0002] In order to avoid using chemicals containing hexavalent
chromium, proposals have already been made for methods of anodizing
aluminum and its alloys by using an aqueous anodizing bath
containing sulfuric acid and boric acid. A good illustration of
such a sulfuric-boric anodization method is given in document U.S.
Pat. No. 4,894,127. In that known method, an aqueous electrolytic
bath is used that essentially comprises sulfuric acid at a
concentration lying in the range 30.5 grams per liter (g/l) to 52
g/l, and boric acid at a concentration lying in the range 5.2 g/l
to 10.7 g/l.
[0003] Such an anodization method is relatively effective in
applying a coating of aluminum oxide on an aluminum alloy with a
solution of sulfuric acid and boric acid. The resulting anodized
coating is at least comparable, and in terms of resistance to
corrosion, equivalent to the anodized and sealed coatings made in
baths containing an aqueous solution of sulfuric acid and chromic
acid. The superiority of the method of document U.S. Pat. No.
4,894,127 over other prior methods of sulfuric-boric anodization
lies in the thinness of the coatings obtained, in particular
coatings having a thickness of 1 micrometer (.mu.m) to 3 .mu.m,
which is particularly advantageous in the field of aviation.
Nevertheless, the compositions specified for implementing such a
method are very broad, and that can lead to characteristics being
obtained that present a wide range of variation in the resulting
layers. Furthermore, it is difficult to control the thickness of
the oxide obtained at the end of treatment. It should be observed
that in the context of that known method, a voltage is applied to
the part which is dipped in the electrolytic bath, which voltage
increases linearly from 5 volts (V) to 20 V, with current density
over said part remaining close to 100 amps per square meter
(A/m.sup.2).
[0004] Mention can also be made of document EP-A-0 048 909
describing another anodizing method using an anodizing bath which
is essentially constituted by sulfuric acid and phosphoric acid, in
particular with the following respective concentrations [50 g/l; 50
g/l], [63 .mu.l; 37 g/l], and [75 g/l; 25 g/l]. The presence of
sulfuric acid may nevertheless turn out to be undesirable in
certain situations, if it is desired to obtain coatings of small
thickness, in particular coating less than 3 .mu.m thick.
[0005] Reference may also be made to document U.S. Pat. No.
4,861,440 describing the use of an anodizing bath containing
sulfuric acid and at least one carboxylic acid, with the sulfuric
acid at high concentration (112 g/l to 150 g/l).
[0006] Other anodizing techniques also exist using an aqueous
anodizing bath essentially comprising sulfuric acid to the
exclusion of any other acid, with the sulfuric acid at high
concentration, in general about 200 g/l.
[0007] Thus, document U.S. Pat. No. 4,554,216 describes an
anodizing method using a bath comprising sulfuric acid at a
concentration of 166 g/l to 230 g/l. The aqueous bath is at low
temperature (0.degree. C. to 5.degree. C.), and the part dipped in
said bath is subjected to high current density (200 A/m.sup.2 to
300 A/m.sup.2).
[0008] It is common practice in the field of aviation to use
anodizing methods with electrolyte baths comprising sulfuric acid
at a concentration lying in the range 180 g/l to 250 g/l. Existing
techniques are always restricted to this high range of
concentrations for sulfuric acid in anodizing baths because of
selection is based on the curve plotting variations in electrical
conductivity as a function of sulfuric acid concentration. That
curve plotting variations in conductivity is substantially in the
form of a downwardly open parabola, presenting a maximum in the
zone corresponding to concentrations of sulfuric acid lying in the
range 180 g/l to 220 g/l. Specialists have therefore invariably
relied on seeking maximum electrical conductivity for the
electrolytic bath. It is known that such high electrical
conductivity is favorable to rapid growth in the thickness of the
oxide. That explains why high concentrations (not less than 200
g/l) have always been selected for the sulfuric acid present in the
electrolytic bath.
[0009] Known techniques of sulfuric anodic oxidation nevertheless
suffer from three drawbacks comprising difficulty in monitoring the
thickness of the coating, while obtaining porosity that is always
high because of the high concentration of sulfuric acid together
with roughness that is uncontrolled. Controlling the thickness or
the weight of the coating is indeed difficult, since two phenomena
are taking place concurrently during the anodizing process, namely
an electrolytic phenomenon which corresponds to growing an
interface layer, and a chemical phenomenon of dissolving the
barrier layer formed both at the interface between the substrate
and the coating that has been formed and at the surface of the
porous layer in contact with the electrolyte. The porosity of the
resulting coating layer, which is known to depend on the chemical
composition of the electrolyte, and in particular on its sulfuric
acid concentration, is thus always high, thereby giving an effect
that is unfavorable on the whole on the characteristics of the
resulting layer.
[0010] The person skilled in the art knows that anodization
performed in an acid medium (pH<2.5) is essentially porous, and
that if it is desired to avoid high porosity on treated parts, it
is necessary to make use of anodizing techniques in a medium that
is more neutral, in order to obtain barrier anodization with a
non-porous layer.
[0011] The technological background of the invention is also
illustrated by the following documents: U.S. Pat. No. 3,563,867,
U.S. Pat. No. 6,149,795, U.S. Pat. No. 4,968,398, and
JP-A-2000/026997.
OBJECTS AND SUMMARY OF THE INVENTION
[0012] An object of the present invention is to propose an
anodizing method that provides better performance, that is
essentially related to sulfuric anodic oxidation techniques but
that allows better monitoring of the thickness or the weight of the
coating, while not obtaining high porosity on the treated
parts.
[0013] In accordance with the invention, this problem is solved by
a method of anodizing a part made of aluminum alloy, the method
comprising the following successive steps:
[0014] providing an aqueous anodizing bath essentially comprising
sulfuric acid at a concentration lying in the range 55 g/l to 85
g/l, and excluding the presence of any phosphoric acid or any boric
acid;
[0015] maintaining said bath at a constant temperature essentially
lying in the range 15.degree. C. to 27.degree. C.;
[0016] dipping said part into said bath;
[0017] applying a voltage to said part dipped in said bath, the
voltage lying essentially in the range 5 V to 30 V, with low
current density on said part; and
[0018] maintaining said part in said bath until a coating of
desired thickness has been obtained, which thickness lies
substantially in the range 1 .mu.m to 3 .mu.m.
DETAILED DESCRIPTION OF THE INVENTION
[0019] Particular features relating to the above-specified steps of
the anodizing method of the invention are described in greater
detail below, and provision can be made in particular for the
aqueous anodizing bath also to contain an acid alcohol having one
to three acid functions in order to limit dissolution of the
resulting coating layer, for the purpose of having perfectly
uniform porosity throughout the thickness of the layer while not
losing electrical conductivity in the bath which favors good growth
of said layer.
[0020] The anodizing method of the invention is described in
greater detail below, setting out the various ranges associated
with each of the parameters of the method, each time giving
preferred values as they have been determined in testing carried
out by the Applicant.
[0021] The first step of the anodizing method of the invention
consists-in providing an aqueous anodizing bath essentially
comprising sulfuric acid at a concentration lying in the range 55
g/l to 85 g/l, and excluding the presence of any phosphoric acid or
boric acid. Certain authors prefer the specified concentrations to
be given in weight percentages: specifically, the above-mentioned
limits given in g/l correspond to concentration values lying in the
range 5.36% to 8.2% by weight.
[0022] It is important to observe that the above-mentioned range of
concentrations is well below the concentrations used in the
above-mentioned known sulfuric anodization techniques, which
concentrations lie in the range 180 g/l to 220 g/l. It has
therefore been necessary to overcome a prejudice, to avoid seeking
maximum electrical conductivity for the electrolytic bath, and
consequently to depart from the range that has always been
recommended for high concentrations of sulfuric acid.
[0023] The sulfuric acid concentration of the bath is preferably
lies essentially in the range 57 g/l to 67 g/l, with a highly
preferred value being situated in the vicinity of 62 g/l (i.e.
slightly more than 6% by weight).
[0024] The second preparatory step of the anodizing method of the
invention consists in maintaining the above-mentioned aqueous
anodizing bath at a constant temperature which lies essentially in
the range 15.degree. C. to 27.degree. C. The bath is preferably
maintained at a constant temperature close to 22.degree. C.
[0025] The aluminum alloy part for treatment is thus dipped into
the aqueous anodizing bath as prepared in this way.
[0026] Fundamentally, a voltage essentially lying in the range 5 V
to 30 V is then applied to said part dipped in said bath, with the
current density on said part being low.
[0027] The voltage applied to the part dipped in the bath may be
fixed to a constant value throughout the duration of the anodizing
treatment, said constant value then lying in the range -5 V to 30
V. Under such circumstances, it is advantageous to select a
constant value for the voltage lying in the range 7 V to 20 V.
[0028] Nevertheless, it has been found to be more advantageous for
further improving good control over the growth of the oxide coating
to make provision for the voltage applied to the part dipped in the
bath to begin by being fixed to a first constant value, and then
after a predetermined duration to a second constant value that is
higher than the first, said first and second constant values both
lying in the range 5 V to 30 V.
[0029] In this respect, it has been found particularly advantageous
to select a first constant voltage value lying in the range 5 V to
11 V, with this low value serving to control moderate growth of the
oxide layer, and a second constant voltage value lying in the range
15 V to 30 V, in order to obtain the desired properties for the
barrier layer. The use of two successive voltage levels, of
duration that is essentially a function of the thickness desired
for the coating, makes it possible to build the barrier layer more
quickly while keeping control over the growth of the coating. The
Applicant has performed numerous tests and has observed, in
particular on the basis of curves plotting rate of growth, that
excellent results are obtained with a first level of 10 V for a
duration of about 25 minutes (min), followed by a second level of
20 V for a duration of 15 min.
[0030] As mentioned above, relatively low current density is used
on the part which is dipped in the electrolytic bath. The term
"low" means in this case that the current density is substantially
less than 100 A/m.sup.2.
[0031] In practice, it has been found advantageous to make
provision for the current density to be essentially less than 80
A/m.sup.2, and for it to lie preferably in the range 30 A/m.sup.2
to 70 A/m.sup.2. Optimum values for current density with the
above-mentioned voltage values lie around 34 A/M.sup.2 to 35
A/M.sup.2.
[0032] In an anodizing method as described above, the part for
treatment is maintained in the electrolytic bath until the desired
thickness of coating has been obtained, which thickness lies
substantially in the range 1 .mu.m to 3 .mu.m.
[0033] In this respect, it is advantageous to observe that
relatively low concentrations of sulfuric acid have already been
used in hard anodizing techniques, but the coating thicknesses
involved have nothing to do with the presently-desired values since
those coatings were very thick, being about 250 .mu.m. The other
parameters of hard anodizing also lie well outside the ranges
provided in the present method (high voltage up to 120 V, high
current densities of about 250 A/m.sup.2, and low temperatures
lying in the range -5.degree. C. to +5.degree. C.): the present
anodizing method therefore cannot be compared with prior hard
anodizing techniques.
[0034] In the context of more advanced developments of the
above-described anodizing method, it has also been found
advantageous to envisage adding an acid alcohol to the aqueous
anodizing bath, the acid alcohol having one to three acid
functions, and being at a concentration that remains low, lying in
the range 12 g/l to 22 g/l.
[0035] This addition of an acid alcohol at low concentration
appears to be advantageous in limiting dissolution of the layer and
in increasing the wettability of the electrolyte, and thus
obtaining good uniformity of porosity throughout the thickness of
the layer, and this is achieved without spoiling the electrical
conductivity of the electrolytic bath. Acid alcohols are
particularly advantageous under such circumstances since they are
completely miscible with the electrolyte at ambient temperature.
High stability is thus obtained in the rate of dissolution with
highly satisfactory maintenance of electrical conductivity.
[0036] The acid alcohol used in the bath is preferably tartaric
acid (an acid alcohol having two acid functions, of formula
C.sub.4H.sub.6O.sub.6) or citric acid (an acid alcohol having three
acid functions, of formula C.sub.6H.sub.8O.sub.7). The
concentration of tartaric acid or of citric acid then preferably
lies essentially in the range 12 g/l to 17 g/l, with the optimum
concentration observed during testing lying in the vicinity of 17
g/l.
[0037] Selecting tartaric acid and citric acid also appears to be
particularly advantageous insofar as it is desirable firstly to
have pH stabilized on a low value, and secondly to have a strong
oxidizer whose electrochemical activity is high without being
aggressive, and thus not generating pitting. It has thus been found
that sulfuric-tartaric or sulfuric-citric anodic oxidation appears
to be much more desirable than sulfuric-boric anodic oxidation as
used in prior techniques, in particular in the technique described
in document U.S. Pat. No. 4,894,127. In the present case, excellent
stability is obtained in the dissolution of the alumina layer.
[0038] Adding tartaric acid or citric acid to the bath in the
context of the present invention makes it possible to obtain
dissolution of the alumina layer which is less than that obtained
with sulfuric acid on its own, and in addition current density in
the bath does not drop off as would be the case with other acids,
such as boric acid, because of the action of surface tension.
[0039] It is also important to provide for the part that is to be
treated to be subjected to preliminary degreasing/pickling
treatment or deoxidation prior to being dipped in the bath.
[0040] In traditional techniques, use has been made of a first step
of degreasing, followed by rinsing, then followed by a second step
of pickling in an alkaline medium, or more generally in an aqueous
solution of sulfuric acid and chromic acid, finally followed by
further rinsing. Nevertheless, it has been found to be more
advantageous to use a solution capable of performing satisfactory
degreasing and pickling directly in a single step, and use may
advantageously be made of a solution comprising phosphoric acid
having anionic wetting agents added thereto such as the product
sold under the reference "NOVACLEAN AL-85" by the German supplier
Henkel Surface Technologies. The use of such a product makes it
possible to obtain pickling that is uniform, i.e. without the
presence of any surface etching.
[0041] Finally, it is advantageous to make provision for the
treated part to be subjected to subsequent treatment for sealing
the coating.
[0042] Sealing of the coating must perform two functions, namely
promoting both adhesion and resistance to corrosion. Sealing is
conventionally performed by soaking in hot water at a temperature
of not less than 97.degree. C., or in a dilute solution of
potassium bichromate. It is preferable to use a solution of
deionized water at a temperature lying in the range 85.degree. C.
to 98.degree. C., with soaking being performed for a duration which
is a function of the thickness of the coating that has been
obtained. In a variant, it is also possible to use sodium molybdate
(MoNa.sub.2O.sub.4, 2H.sub.2O) or manganese sulfate (MnO.sub.2S,
H.sub.2O), or indeed immersion in a particularly effective sealant
based on nickel acetates or on lithium acetates, for example using
the respective products "ANOSEAL 1000" or ENVIROSEAL 2500" sold by
the above-specified supplier Henkel Surface Technologies.
[0043] Among the numerous tests performed by the Applicant, mention
can be made of a first example of sulfuric anodic oxidation with an
aqueous anodizing bath comprising sulfuric acid at a concentration
of 62 g/l. It was found that the coating layer grew regularly and
good control was obtained over small thicknesses without any change
to the initial roughness of the substrate.
[0044] Other tests with a sulfuric-citric anodizing bath may also
be mentioned, with the sulfuric acid concentration being 62 g/l and
the citric acid concentration being 17 g/l. For the other operating
conditions, it can be mentioned that the constant temperature at
which the bath was maintained was 22.degree. C., and the applied
voltage was 10 V for a first period of 25 min followed by 20 V for
a second period of 15 min. That produced a low porosity coating of
thickness lying in the range 2.8 .mu.m to 3.2 .mu.m.
[0045] As an indication, the tests were performed using the
aluminum alloys bearing the references 2024 T 351 and 7175 T
7351.
[0046] After sealing, it was observed that the parts treated in
this way still presented highly satisfactory appearance after being
exposed for 500 hours to a saline mist. In addition, fatigue
weakening characteristics remained very satisfactory compared with
known sulfuric-boric or chromic anodic oxidation techniques.
[0047] A high performance anodizing method is thus provided which
makes it possible simultaneously to monitor the thickness or the
weight of the coating and its roughness, and to obtain low porosity
at the surface of the treated parts.
[0048] The invention is not limited to the method described above,
but on the contrary encompasses any equivalent anodizing method
coming within the general definition given at the beginning of the
description.
* * * * *