U.S. patent application number 11/021722 was filed with the patent office on 2005-07-14 for method for anodizing aluminum materials.
Invention is credited to Beneke, Martin, Gerlach, Carmen, Kock, Erich.
Application Number | 20050150771 11/021722 |
Document ID | / |
Family ID | 34683935 |
Filed Date | 2005-07-14 |
United States Patent
Application |
20050150771 |
Kind Code |
A1 |
Kock, Erich ; et
al. |
July 14, 2005 |
Method for anodizing aluminum materials
Abstract
Structural components made of aluminum and aluminum alloys are
anodized in two sequential steps in two different electrolytes. The
first electrolyte is a mixed acid of two inorganic acids such as a
phosphoric-sulfuric acid mixture. The second electrolyte is a
further acid mixture of an organic acid and an inorganic acid to
form a tartaric sulfuric acid mixture. The two sequential anodizing
steps result in a surface texture that has three excellent surface
characteristics simultaneously, namely: a corrosion resistance, a
coating acceptance for lacquer coatings and an adhesive bonding
with other aluminum material components.
Inventors: |
Kock, Erich; (Bremen,
DE) ; Beneke, Martin; (Achim, DE) ; Gerlach,
Carmen; (Bremen, DE) |
Correspondence
Address: |
FASSE PATENT ATTORNEYS, P.A.
P.O. BOX 726
HAMPDEN
ME
04444-0726
US
|
Family ID: |
34683935 |
Appl. No.: |
11/021722 |
Filed: |
December 22, 2004 |
Current U.S.
Class: |
205/172 ;
205/175; 205/324 |
Current CPC
Class: |
C25D 11/04 20130101;
C25D 11/024 20130101; C25D 11/12 20130101 |
Class at
Publication: |
205/172 ;
205/175; 205/324 |
International
Class: |
C25D 011/04; C25D
011/12 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 23, 2003 |
DE |
103 61 888.0 |
Claims
What is claimed is:
1. A method for anodizing a surface of structural components made
of aluminum or aluminum alloys, said method comprising the
following steps: a) first preparing an inorganic acid mixture of at
least two inorganic acids as a first electrolyte, b) first exposing
said surface to said first electrolyte to provide an anodized first
surface coating, c) second preparing a further acid mixture of an
organic acid and an inorganic acid as a second electrolyte, and d)
second exposing said surface to said second electrolyte to provide
a second surface coating.
2. The method of claim 1, wherein said first preparing step is
performed by mixing a phosphoric acid and a sulfuric acid to form
said inorganic acid mixture as said first electrolyte.
3. The method of claim 2, wherein said inorganic acid mixture
contains between 50 to 250 g/l (gram per liter) phosphoric acid
(H.sub.3PO.sub.4) and 50 to 150 g/l of sulfuric acid
(H.sub.2SO.sub.4).
4. The method of claim 3, wherein said inorganic acid mixture
contains a mixture of 125 g/l of phosphoric acid (H.sub.3PO.sub.4)
and 75 g/l of sulfuric acid (H.sub.2SO.sub.4).
5. The method of claim 1, wherein said second preparing step
comprises mixing a tartaric acid and a sulfuric acid to form said
further acid mixture as an organic and inorganic acid mixture
forming said second electrolyte.
6. The method of claim 5, wherein said further acid mixture forming
said second electrolyte contains 20 to 150 g/l (L(+)tartaric acid,
and 20 to 150 g/l of sulfuric acid (H.sub.2SO.sub.4.)
7. The method of claim 6, wherein said second electrolyte contains
80 g/l of sulfuric acid (H.sub.2SO.sub.4).
8. The method of claim 1, further comprising performing said first
and second exposing steps each at an anodizing DC voltage profile,
and controlling said DC voltage profile such that an initial rising
DC voltage forming a ramp voltage as a function of time, is
followed by a constant DC voltage forming a plateau as a function
of time.
9. The method of claim 8, wherein said ramp voltage reaches said
plateau voltage at a DC voltage within the range of 3 to 25 V
DC.
10. The method of claim 8, wherein said ramp voltage has a rise
time within the range of 0.5 to 10 minutes.
11. The method of claim 8, wherein said plateau voltage is
maintained for a duration within the range of 5 to 90 minutes.
12. The method of claim 8, further comprising performing said first
exposing step at an anodizing DC voltage profile by controlling
said anodizing DC voltage to increase from 0 to 15 V DC within 5
minutes and then holding said DC voltage at 15 V DC for 15
minutes.
13. The method of claim 8, further comprising performing said
second exposing step at an anodizing DC voltage, and controlling
said anodizing DC voltage to increase from 0 to 13 V DC within 3.5
minutes and then holding said DC voltage at 13 V DC for 25
minutes.
14. The method of claim 8, further comprising performing said first
and second exposing step at a temperature within the range of 20 to
70.degree. C. for anodizing.
15. The method of claim 8, further comprising performing said first
exposing step at room temperature for anodizing.
16. The method of claim 8, further comprising performing said
second exposing step at a temperature of 35.degree. C. for
anodizing.
17. The method of claim 8, further comprising continuing said
exposing steps until an anodized layer thickness is achieved within
the range of 1 to 10 .mu.m for each layer formed by said first and
second exposing steps.
18. The method of claim 8, wherein said inorganic acid mixture for
said first exposing step contains between 50 to 250 g/l of
phosphoric acid (H.sub.3PO.sub.4).
19. The method of claim 8, wherein said further acid mixture of
said inorganic acid and of said organic acid for said second
exposing step contains between 20 to 150 g/l of L(+)tartaric
acid.
20. A structural component made of any one of aluminum and aluminum
alloys comprising an anodized surface texture produced according to
the steps of claim 1.
21. A method for anodizing a surface of structural components made
of aluminum or aluminum alloys, said method comprising the
following steps: a) first preparing an acid mixture of an organic
acid and an inorganic acid as a first electrolyte, b) first
exposing said surface to said first electrolyte to provide a first
surface coating, c) second preparing an inorganic acid mixture of
at least two inorganic acids as a second electrolyte, and d) second
exposing said surface to said second electrolyte to provide an
anodized second surface coating.
Description
PRIORITY CLAIM
[0001] This application is based on and claims the priority under
35 U.S.C. .sctn.119 of German Patent Application 103 61 888.0,
filed on Dec. 23, 2003, the entire disclosure of which is
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The invention relates to a method for anodizing aluminum
materials, more specifically structural components made of aluminum
and/or aluminum alloys.
BACKGROUND INFORMATION
[0003] It is known to anodize structural components made of
aluminum materials including aluminum and aluminum alloys in two
different sequential steps, whereby the respective different
electrolytes comprise at least two components. The surface of the
structural component is oxidized by applying a DC voltage to an
electrolytic bath. At least one of the two electrolytes is a
mixture of at least two inorganic acids forming an inorganic acid
mixture.
[0004] The coating of structural components of aluminum materials
with an anodized layer is intended to change the surface
characteristics of these structural components. Anodized layers are
porous layers which, depending on the respective production
parameters of the coating method result in a different surface
morphology or surface textures. For example, the purpose of an
anodized layer is primarily to provide a corrosion protection of
the basic material of which the structural component is made. A
further purpose of the surface texture of such components is to
provide a good adhesion strength if such components are to be
adhesively bonded to one another, for example in the form of
several plies. Yet another desirable characteristic of the surface
texture is a good adhesion of lacquer coatings. So far, all three
characteristics have not been achieved simultaneously without
rather undesirable side effects.
[0005] The more important conventional anodizing methods for
aluminum materials will now be described. The chromic acid
anodizing (CAA) as described in German Industrial Standard Sheet
DIN EN 3002 provides an anodized layer which has good corrosion
resistance while also exhibiting useful adhesion characteristics as
well as lacquer coating retaining characteristics. However, there
is a serious drawback because the electrolyte in the anodizing bath
contains chromate which is a carcinogenic. Therefore, even though
the CAA method is currently widely used in aircraft construction,
it will hardly be possible to keep using this method in the
future.
[0006] The phosphoric acid anodizing (PAA) method is described in
British Patent Publication GB 1,555,940. This type of anodization
is specifically directed to the improvement of the adhesion
characteristics of the components anodized by this method. U.S.
Pat. No. 4,085,012 describes a similar method. The phosphoric acid
anodizing (PAA) provides an anodized layer having a surface
morphology or texture that is suitable for adhesively bonding
components to each other and also function as a base coating for a
subsequent application of a lacquer coating. However, a substantial
drawback of an anodized layer produced by an electrolyte containing
phosphoric acid, is seen in the rather limited corrosion protection
characteristic.
[0007] Anodizing with an electrolyte containing phosphoric and
sulfuric acids is described in a pamphlet TN-EVC 904/96. This type
of anodization provides excellent adhesion characteristics for
adhesive bonding as well as a base coating for lacquering. It is an
advantage of this type of anodizing that chromates are not used.
However, phosphoric sulfuric acid anodization produces coatings
that do not provide a sufficient corrosion protection.
[0008] A so-called boric sulfuric acid anodizing (BSAA) is
described in U.S. Pat. No. 4,894,127. Coatings produced by that
method have a good corrosion resistance and provide a good base
coating for the adhesion of lacquers. However, these coatings are
not suitable for adhesive bonding with other components.
[0009] Yet another known method employs a DC current sulfuric acid
anodizing which provides surface coatings having very high
corrosion resistance. However, these coatings are neither suitable
for adhesive bonding nor as a base coating for lacquering.
[0010] Yet another method is known as a so-called mixed acid
anodization using as an electrolyte a mixture of tartaric acid and
sulfuric acid (TSA) as described in European Patent Publication EP
1,233,084 A2. Anodized surfaces produced by that method have a good
corrosion resistance and are suitable for a base coating for
lacquering. However, the coatings are not suitable for adhesive
bonding.
[0011] In view of the foregoing the only conventional anodizing
method that achieves all three characteristics simultaneously is
the chromic acid anodizing. However, in view of its use of
carcinogenic chromates chromic acid anodizing will not be suitable
for continued use. Other conventional anodizing methods using a
single anodizing step satisfy at best two of the three
requirements. In this connection reference is made to U.S. Pat. No.
5,486,283 which uses a so-called duplex method which employs a
phosphoric-boric-sulfuric acid anodizing (PBSA). This PBSA
anodizing is performed in a two step operation. The electrolytes
used in both steps are inorganic electrolytes. The first anodizing
step of the PBS method uses an inorganic acid while the second
anodizing step uses an inorganic mixed acid. The coatings produced
by the PBSA method have a good corrosion resistance and serve
simultaneously as adhesion enhancing agents for lacquers and
adhesive bonds so that this method satisfies all three
requirements. However, there is still room for improvement.
OBJECTS OF THE INVENTION
[0012] In view of the foregoing it is the foregoing objects singly
or in combination:
[0013] to provide a two step anodizing method that is particularly
suitable for improving aluminum or aluminum alloy surfaces and
which provides a surface texture with a good corrosion resistance
while simultaneously serving as a suitable base coating for
subsequent lacquering and as an adhesive bonding base coating;
[0014] to provide a two step anodizing method that does not cause
any technical problems in the sequence of the anodizing steps while
simultaneously avoiding health problems;
[0015] to make it possible that two directly sequential anodizing
steps can be performed in an optimally short time;
[0016] to provide an anodizing method that is particularly suitable
for all aluminum materials that are used in aircraft construction,
particularly high strength aluminum alloys and weldable aluminum
alloys; and
[0017] to provide an anodizing method, the parameters of which are
readily adaptable to the requirements for achieving different
surface characteristics;
[0018] The invention further aims to avoid or overcome the
disadvantages of the prior art, and to achieve additional
advantages, as apparent from the present specification. The
attainment of these objects is, however, not a required limitation
of the claimed invention.
SUMMARY OF THE INVENTION
[0019] The above objects have been achieved according to the
invention by an anodizing method for surfaces of aluminum and
aluminum alloys which method comprises the following steps:
[0020] a) first preparing a mixed inorganic acid as a first
electrolyte,
[0021] b) first exposing said surface to said first electrolyte to
provide an anodized first surface coating;
[0022] c) second preparing a mixed acid of an organic acid and an
inorganic acid as a second electrolyte, and
[0023] d) second exposing said surface to said second electrolyte
to provide a second surface coating.
[0024] Thus, the first anodizing step is performed with an
inorganic acid mixture while the second anodizing step is performed
with a mixture of an organic and an inorganic acid.
[0025] The present method is suitable for all aluminum alloys,
particularly also for high strength aluminum alloys particularly
including corrosion sensitive aluminum alloys. Another advantage of
the invention is seen in that the materials used for preparing the
electrolytes are neither carcinogenic nor toxic. Still another
advantage of the invention is seen in that the individual
parameters of the electrolytes are easily adapted to the required
functional and textural characteristics of the coatings to be
produced. These parameters can be individually adapted in each of
the individual anodizing steps.
[0026] The present method produces in a two step operation an oxide
film particularly on aluminum alloys as they are used in the
aircraft construction. The first anodizing step produces a first
layer having preferably a thickness within the range of 1 to 2
.mu.m. This first layer has a substantial porosity which is
exceptionally well suited for adhesively bonding aluminum
components to one another. The second anodizing step forms a low
porosity layer having preferably a thickness within the range of 2
to 4 .mu.m underneath the first layer. Presumably, the second layer
formed in the second anodizing bath can grow below the first layer
due to the substantial porosity of the first layer and probably due
to a certain affinity between the aluminum material and the second
electrolyte both. Further, it is possible to reverse the step
sequence. More specifically, to perform the second step first and
the first step last. In both possibilities the highly corrosion
resistant layer will be formed between the aluminum surface and the
adhesion enhancing outer anodized layer. The corrosion resistance
of the second anodized layer can be further improved by a
subsequent densifying step, for example by a rolling or pressing
operation. As mentioned above, the present two step operation is
equally suitable for all aluminum alloys used in the aircraft
manufacture, particularly high strength aluminum alloys of the
series 2XXX and 7XXX as well as for the weldable aluminum alloys of
the series 6XXX. The preferred thicknesses of the layers produced
according to the invention are within the range of 1.5 to 10 .mu.m
for the individual layers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] In order that the invention may be clearly understood, it
will now be described in connection with example embodiments
thereof, with reference to the accompanying drawings, wherein:
[0028] FIG. 1 shows a schematic, sectional view of an aluminum
component coated according to the invention; and
[0029] FIG. 2 shows an anodizing voltage diagram as a function of
time.
DETAILED DESCRIPTION OF A PREFERRED EXAMPLE EMBODIMENT AND OF THE
BEST MODE OF THE INVENTION
[0030] FIG. 1 shows an aluminum component 1 provided with a first
anodized layer 2 and a second anodized layer 3. The layer thickness
is exaggerated having regard to the fact that the thickness of the
layers is within the range of a few microns, preferably in the
range of 1 to 10 .mu.m as mentioned above.
[0031] It will be noted that in FIG. 1 the anodized layer 2, which
is produced first, appears as top layer having a very rugged
surface 2A with deep pores 2B and roots 2C reaching into the second
layer 3 shown to adhere to the surface 1A of the component 1. The
second layer 3 is anodized after the first layer 2 has been
anodized. Yet, the second layer 3 appears between the first layer 2
and the surface 1A. It is assumed that the second anodized layer 3
can grow through the deep pores 2B and then adhere to the surface
1A and to the first layer 2 due to affinities between the aluminum
materials on the one hand and the two different electrolytes on the
other hand. In this context it is possible to achieve the same
results by first anodizing the second layer 3 on the surface 1A and
then anodizing the first layer 2 on the top of the so-called second
layer. However, bonding between the layers 2, 3 and the surface 1A
appears to be better if layer 2 is anodized first to generate the
shown intermeshing.
[0032] FIG. 2 illustrates the control of the DC anodizing voltage
as a function of time. A ramp voltage RV rises within a maximum of
10 minutes to a maximum voltage which becomes a constant or plateau
voltage PV which does not exceed 25 V maximally and which continues
to a maximum of about 90 minutes. Further details how these
parameters may differ for different anodizing requirements will be
described below.
[0033] Prior to exposing an aluminum or aluminum alloy component to
the two anodizing steps according to the invention, the surfaces of
the component are conventionally cleaned in a grease removing bath
and then in a pickling bath, whereby basic and/or acidic pickling
steps may be performed. After such cleaning the aluminum components
are exposed in sequence to two anodizing electrolyte baths
according to the invention. A first electrolyte for the first bath
is an inorganic acid mixture of a phosphoric-sulfuric acid and
results in the application of a highly porous anodized layer 2
having a thickness of, for example 1 to 2 .mu.m. This first layer
is particularly suitable for adhesive bonding of components. This
first layer 2 is also suitable as a base coating for a subsequent
lacquer coating of the components. The second anodizing step
involves the use of a second electrolyte that is a mixture of an
organic and an inorganic acid. For example, a tartaric acid is
mixed with a sulfuric acid. Exposing the compound to this second
electrolyte results in a low porosity second anodizing layer 3
having a preferred thickness within the range of 2 to 4 .mu.m. This
layer is formed below the first layer 2 as explained above and can
be subsequently densified, for example by a rolling operation
whereby the desired corrosion resistance is further improved.
[0034] According to the invention it is possible to individually
improve any one of the above mentioned three desirable
characteristics, namely the corrosion resistance and the lacquer
acceptance characteristic and the ability to increase an adhesive
bonding strength. This improvement can be made by selecting the
anodizing parameters such as the ratio of the acid mixtures in the
electrolytes, the voltage and current density in the electrolyte
baths, the duration of bath exposure, and the individual
electrolyte concentration in each individual bath.
[0035] The anodizing steps are performed in each bath at
temperatures within the range of 20 to 70.degree. C. The anodizing
is performed at a DC voltage profile as shown in FIG. 2 by the ramp
voltage RV and the plateau voltage PV. The plateau voltage PV is
within the range of 3 to 25 V. The time for increasing the ramp
voltage is within the range of 30 seconds to 10 minutes while the
duration of maintaining the plateau voltage PV is within the range
of about 5 to 90 minutes. The concentration of the electrolytes in
the first anodizing step which is the so-called PSA step is within
the range of 50 to 250 g/l of phosphoric acid and within the range
of 50 to 150 g/l of sulfuric acid. In the second anodizing step
referred to as TSA step, the electrolyte concentrations in the
second bath are as follows: 20 to 150 g/l of (L)+tartaric acid and
20 to 150 g/l of sulfuric acid.
[0036] In a preferred example embodiment the following anodizing
steps were performed at the following parameters. This example
embodiment of the two baths was suitable for any aluminum and
aluminum alloys: First anodizing step (PSA-step) 125 g/l of
H.sub.3PO.sub.4, 75 g/l of H.sub.2SO.sub.4. The anodizing takes
place with a DC anodizing voltage profile in which the ramp voltage
RV rises from 0 V to 15 V within 5 minutes followed by a plateau of
15 V for 15 minutes. The first anodizing is performed at room
temperature.
[0037] Second anodizing step (TSA step) (80 g/l of (L)+tartaric
acid, 40 g/l of H.sub.2SO.sub.4. The anodizing takes place at a DC
anodizing voltage profile in which the ramp voltage RV rises from 0
to 13 V in 3.5 minutes. The 13 V plateau is maintained for 25
minutes and the bath temperature is 35.degree. C.
[0038] The above example embodiment is particularly suitable for
creating a surface morphology or texture which is substantially
ragged or jagged. Such a surface texture is particularly suitable
for adhesive bonding and lacquer adhesion. The second layer that is
formed below or underneath the first layer has a high corrosion
resistance. The first step that produces the first layer 2 is the
PSA step. The second step that produces the layer 3 under the first
layer is the TSA step.
[0039] Although the invention has been described with reference to
specific example embodiments, it will be appreciated that it is
intended to cover all modifications and equivalents within the
scope of the appended claims. It should also be understood that the
present disclosure includes all possible combinations of any
individual features recited in any of the appended claims.
* * * * *