U.S. patent application number 11/461853 was filed with the patent office on 2006-11-23 for color stabilization of anodized aluminum alloys.
This patent application is currently assigned to GENERAL MOTORS CORPORATION. Invention is credited to Sheila Farrokhalaee Kia, Hong-Hsiang Kuo, Yar-Ming Wang.
Application Number | 20060260947 11/461853 |
Document ID | / |
Family ID | 34217098 |
Filed Date | 2006-11-23 |
United States Patent
Application |
20060260947 |
Kind Code |
A1 |
Kia; Sheila Farrokhalaee ;
et al. |
November 23, 2006 |
Color Stabilization of Anodized Aluminum Alloys
Abstract
A process is disclosed for stabilizing certain colored anodized
coating on aluminum articles against degradation by ultraviolet
radiation. Anodized articles colored by an electrolytic or
interference process can thereafter be stabilized by heat treating
them at temperatures of the order of 350.degree. F. for a period of
an hour or so. The process is particularly useful for use on
electrolytically colored, anodized vehicular external body panels
made from suitably formable sheet metal aluminum alloys.
Inventors: |
Kia; Sheila Farrokhalaee;
(Bloomfield Hills, MI) ; Kuo; Hong-Hsiang; (Troy,
MI) ; Wang; Yar-Ming; (Troy, MI) |
Correspondence
Address: |
GENERAL MOTORS CORPORATION;LEGAL STAFF
MAIL CODE 482-C23-B21
P O BOX 300
DETROIT
MI
48265-3000
US
|
Assignee: |
GENERAL MOTORS CORPORATION
P.O. Box 300 Mail Code 482-C23-B21
Detroit
MI
48265-3000
|
Family ID: |
34217098 |
Appl. No.: |
11/461853 |
Filed: |
August 2, 2006 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10650204 |
Aug 28, 2003 |
|
|
|
11461853 |
Aug 2, 2006 |
|
|
|
Current U.S.
Class: |
205/173 |
Current CPC
Class: |
C25D 11/18 20130101;
C25D 5/50 20130101; C25D 11/22 20130101 |
Class at
Publication: |
205/173 |
International
Class: |
C25D 11/22 20060101
C25D011/22 |
Claims
1. A method of making an aluminum alloy article having a
decoratively colored, anodized surface in which the color is
stabilized against degradation by ultraviolet radiation, said
method comprising: anodizing said surface of said article to form
an anodized layer on said surface, said layer being characterized
by porous crystalline columns of aluminum oxide; electrolytically
depositing coloring particles in the pores of said anodized layer
for coloring said layer, the particles providing said layer with a
color for decorative purposes, the color being susceptible to
degradation by ultraviolet radiation; and before said colored layer
is exposed to color-degrading ultraviolet radiation, heating said
colored layer at a temperature above 300.degree. F. for a period
sufficient to stabilize said color layer against said
radiation.
2. The method as recited in claim 1 in which said aluminum article
is a sheet metal panel formed of an aluminum alloy of the AA5xxx
series of alloy compositions and said heating period is in excess
of 45 minutes.
3. The method as recited in claim 1 in which said aluminum article
is a sheet metal panel formed of an aluminum alloy of the AA6xxx
series of alloy compositions and said heating period is in excess
of 45 minutes.
4. The method as recited in claim 1 in which said aluminum article
is a sheet metal panel formed of an aluminum alloy of the AA6111
composition and said heating period is in excess of 45 minutes.
5. The method as recited in claim 1 in which said aluminum article
is a sheet metal panel formed of an aluminum alloy of AA5083 or
AA5657 composition and said heating period is in excess of 45
minutes.
6. A method of making an aluminum alloy article having a
decoratively colored, anodized surface in which the color is
stabilized against degradation by ultraviolet radiation, said
method comprising: anodizing said surface of said article in an
aqueous sulfuric acid bath to form a colorable anodized layer on
said surface, said layer being characterized by porous crystalline
columns of aluminum oxide; electrolytically depositing metal
particles in the pores of said anodized layer in an amount for
decorative coloring of said layer, the decorative color being
susceptible to color degradation by ultraviolet radiation; and
before said colored article is exposed to color-degrading
ultraviolet radiation, heating said colored layer at a temperature
above 300.degree. F. for a period sufficient to stabilize said
color layer against said radiation.
7. The method recited in claim 6 comprising sealing said colored
layer before said heating step.
8. The method as recited in claim 6 in which said aluminum article
is a sheet metal panel formed of an aluminum alloy of the AA5xxx
series or AA6xxx series of alloy compositions and said heating
period is in excess of 45 minutes.
9. A method of making an exterior vehicular aluminum alloy sheet
metal body panel having a colored anodized surface in which the
color is stabilized against degradation by ultraviolet radiation,
said method comprising: forming said body panel from an aluminum
alloy sheet material chosen for the forming of said panel;
anodizing said surface of said formed panel in an aqueous sulfuric
acid bath to form a colorable anodized layer on said surface, said
layer being characterized by porous crystalline columns of aluminum
oxide and having a thickness of 15 micrometers or greater;
electrolytically depositing metal particles in the pores of said
anodized layer in an amount for coloring said layer, the coloring
of said layer being susceptible to color degradation by ultraviolet
radiation; and before said colored layer is exposed to
color-degrading ultraviolet radiation, heating said colored layer
at a temperature above 300.degree. F. for a period sufficient to
stabilize said color layer against said radiation.
Description
[0001] This is a Continuation of application Ser. No. 10/650,204,
filed on Aug. 28, 2003.
TECHNICAL FIELD
[0002] This invention pertains to the coloring of anodized aluminum
or aluminum alloy articles. More particularly, this invention
pertains to the stabilization of colored anodized aluminum articles
against degradation of the coloring materials by ultraviolet
radiation.
BACKGROUND OF THE INVENTION
[0003] There are known practices for the anodization of surfaces of
aluminum and aluminum alloy articles. Depending upon the intended
use of the article it may be anodized for corrosion resistance,
wear resistance and/or appearance. The surface of the article is
cleaned and pretreated in preparation for anodization. There are
many anodization practices, but in general the article is immersed
as an anode in an aqueous electrolyte comprising an acid such as
sulfuric acid. Anodization processes for producing colored or
colorable anodized layers are often conducted with an electrolyte
bath with a temperature of about 25.degree. C. The passage of an
electrical current, usually direct current, through the bath
produces an adherent coating on the aluminum surface of closely
spaced, crystalline columns of aluminum oxide. The resulting
columnar coating is a result of competing chemical reactions
between the electrolyte and the aluminum surface. The
electrochemical effect is to oxidize aluminum atoms at the
workpiece surface to Al.sub.2O.sub.3 which build-up on the surface
as thin polygonal columns of the oxide with a vertical axial pore.
Coincident with the oxidation of the aluminum surface atoms, the
acid electrolyte tends to soften and dissolve the aluminum oxide
columns. An exemplary thickness of the anodized oxide columns is
about fifteen micrometers.
[0004] In some instances the composition of the aluminum alloy is
such that the anodized layer is colored as formed. Some aluminum
alloys, such as certain magnesium containing alloys, yield anodized
layers that are grey or even black when anodized in a sulfuric acid
electrolyte. Other aluminum alloys yield different colors. Sheets
of these inherently colored, anodized aluminum alloys are useful
for decorative purposes in architectural and building applications.
Still other aluminum alloys yield clear anodized coatings that can
be colored by dying or by electrolytic deposition of metal
particles.
[0005] Recently, automotive vehicle body panels have been formed of
sheet metal aluminum alloys and they have been painted to match
other body surfaces. However, there is an interest in anodizing and
coloring such aluminum alloy surfaces. But automotive outer
surfaces are continually exposed to the color degrading effects of
the ultraviolet portion of solar radiation. Most dyes used in
coloring anodized surface quickly fade during prolonged exposure to
sunlight and are unsuitable for vehicular external applications.
The coloring effect of metal particles deposited in the pores of
the alumina columns is more durable than typical dyes but still
fades to an unacceptable degree.
[0006] Accordingly, it is an object of this invention to provide a
method of stabilizing the color of anodized aluminum coating
against degradation by ultraviolet radiation.
SUMMARY OF THE INVENTION
[0007] This invention provides a method of treating an
electrolytically colored, anodized aluminum alloy article against
degradation of the color by sun light. Following the deposition of
the coloring particles in the pores of the anodized coating the
article is heated to a temperature above about 300.degree. F. for a
time sufficient to stabilize the color finishes.
[0008] The color characteristics of a newly colored anodized
workpiece can be quantitatively determined by a colorimeter or the
like. It is found that exposure of such a workpiece to intense
ultraviolet radiation results in a substantial loss of color
intensity or change in color, particularly in the initial period of
exposure to the UV radiation. It is believed that this initial
color change may be attributed to aging of hydrated material
associated with the oxide layer and/or the electrolytically
deposited metal particles. The porous anodized layer of crystalline
aluminum oxide columns are formed from an acidic electrolyte such
as aqueous sulfuric acid. The electrolytic deposition of the
coloring particles is also accomplished using an aqueous acid bath
containing a salt of the coloring metal. Either process can form
hydrates associated with the aluminum oxide or the particles.
Alteration or drying of the oxide by radiation could affect the
color of the layer. It is now found that a suitable heat treatment
of the newly formed article to age or dry the hydrate can
significantly reduce later unwanted color loss or change. Whatever
the mechanism, it is found that heating the colored workpiece to,
for example, 350.degree. F. for 60 minutes avoids or reduces the
abrupt initial change in color upon exposure to sunlight.
[0009] The process has been demonstrated using aluminum sheet metal
alloys such as AA5083, 5657 and 6111, which have been anodized in
an aqueous sulfuric acid electrolyte bath and then colored in a tin
sulfate/sulfuric acid electrolyte to produce workpiece specimens
with a decorative bronze color. Both plain electrolytic coloring
and pore widening interference coloring of nominally 15 micrometer
thick anodized layers were obtained. After sealing of the surfaces,
the colored specimens were heated at 350.degree. F. for 60 minutes.
Exposure to the intense UV radiation of a Xenon lamp yielded only a
gradual fading of the color. There was no abrupt immediate
degradation such as what is experienced with un-heat treated
samples anodized and colored by the same processes.
[0010] Thus, the process of this invention provides a way of
producing color stable, decorative formed and anodized articles
such as exterior body panels for automotive vehicles.
[0011] Other objects and advantages of the invention will be
apparent from detailed descriptions of specific embodiments which
follow.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a graph summarizing the color change (dE) from
anodized AA6111 sheet surface samples that have been
electrolytically (interference) colored using a stannous
sulfate/sulfuric acid electrolyte, heated at 350.degree. F. for 60
minutes and exposed to the UV radiation from a Xenon lamp
(wavelength 280 to 340 nm) for increasing doses up to about 1200
KJ/m.sup.2. Comparison samples of non-heat treated colored anodized
specimens were also exposed to the radiation. The data is presented
as a plot of dE versus Xenon exposure (KJ/m.sup.2) for the heated
sample (square data points) and unheated sample (diamond data
points).
[0013] FIG. 2 is a graph, like FIG. 1, of color change data for
heated and unheated electrolytic colored anodized AA5083 sheet
specimens.
[0014] FIG. 3 is a graph, like FIG. 1, of color change data for
heated and unheated electrolytic colored anodized AA5657 sheet
specimens.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0015] The practice of the invention will be illustrated by
anodizing and coloring certain aluminum alloys for processing in
accordance with this invention. Alloys were selected from those
used in the formation of external body panels or trim for
automotive vehicles. However, the practice of the invention is not
limited to these alloys but it is generally applicable to colored
anodized aluminum alloys.
[0016] Two of the exemplary alloys are AA5083 and AA5657. The third
example is AA6111. The AA5XXX alloys are a series of aluminum base
alloys that contain magnesium. The 5083 alloy is used in cold
rolled sheet metal form for high elongation stretch forming of
automotive body panels such as deck lids and door panels. This is
an aluminum alloy that contains about 4% magnesium along with some
manganese. Aluminum alloy 5657 is an alloy with less magnesium that
has been used for bright trim pieces on automotive vehicles, but
can also be used for making body panels. Aluminum alloy 6111 is an
aluminum-based alloy containing magnesium and silicon and it is
used in sheet metal form for stamping automobile body panels.
[0017] In preparation for anodizing small sheet metal sections of
these alloys were cleaned in trisodium phosphate for five minutes
at 60.degree. C. and rinsed in water. The cleaned specimens were
etched in sodium hydroxide solution for five minutes at 60.degree.
C. and rinsed and then de-smutted (deoxidized) in nitric acid
solution for one minute with a water rinse.
[0018] The several sheet metal aluminum alloy samples were anodized
in aqueous sulfuric acid electrolyte under conditions to produce
clear anodized coatings that were about 15 micrometers in
thickness. The electrolyte bath for anodizing contained about 165
grams of sulfuric acid per liter of bath. The respective aluminum
alloy workpieces were arranged as anodes in the bath with stainless
steel cathode bars. The anodization was conducted at a temperature
of about 25.degree. C. and 16 volts direct current for several
minutes until the desired clear coat anodized thickness was
obtained. A current density of about 12-15 amperes per square foot
was employed for the respective samples except for the AA5083
samples. In the case of the AA5083 samples the high magnesium
content tended to produce a dark un-colorable anodized layer and a
current density of only about 5 amperes per square foot was
employed. Following anodization, the samples were rinsed. Thus,
each of the AA 5083, 5657 and 6111 sheet samples were provided with
a layer of a dense porous coating of columnar crystals of aluminum
oxide. These anodized coatings, about fifteen micrometers thick,
were to be colored by the electrolytic deposition of coloring metal
particles in the pores.
[0019] Some of the anodized aluminum alloy samples were colored by
a, more or less, standard electrolytic coloring process using a tin
sulfate solution. The bath was made up to contain 15 grams of tin
(as stannous sulfate) per liter of bath, 15 grams of sulfuric acid
per liter of bath and a stabilizing agent. The bath had a pH of
about 1. The electrolytic coloring was preformed at an ambient
temperature of about 25.degree. C. with the workpieces arranged as
cathodes for the direct current portion of the coloring
process.
[0020] The workpieces were immersed in the bath for about 60
seconds to allow the aqueous electrolyte to penetrate the pores of
the clear anodic coating. The workpieces were then pre-treated in
the bath at 8 volts direct current for 60 seconds. The coloring was
then affected by applying an alternating current power source to
the workpieces with the voltage cycling at 60 hertz, between +4
volts and -9 volts. This alternating current coloring was continued
for 15 seconds. During the cathodic portion of the AC cycling small
particles of tin were formed from the electrolyte in the pores of
the coating and deposited there to provide each of the respective
sample layers with a bronze color.
[0021] The colored anodized parts were then rinsed and immersed in
a nickel fluoride sealing bath at ambient temperature for 15
minutes. Finally the parts were removed from the ambient sealing
bath and sealed again with hot water for 15 minutes at 70.degree.
C.
[0022] Other samples of the respective anodized aluminum alloy
sheet specimens were colored by a variation of above described
electrolytic coloring process to produce an interference-type
coloring. This coloring process was similar to the above-described
process in that tin sulfate in a sulfuric acid electrolyte was
employed for the coloring process but the samples were pretreated
to somewhat enlarge the pores in the clear coating of columnar
aluminum oxide crystals before the deposit of the coloring tin
particles. After anodizing the samples to be interference colored
were immersed in an aqueous sulfuric acid bath for a few minutes at
a direct current voltage less than the anodizing voltage. The
conditions were selected to enlarge the pores of the existing
porous anodized layer rather than growing the existing layer. As is
known, the larger pores provide a larger cavity for the internal
reflection of light from deposited coloring particles. The
pretreated anodized samples were then electrolytically colored in
an acidic tin sulfate bath as described above.
[0023] The interference colored aluminum alloy specimens were also
sealed in a room temperature nickel fluoride bath followed by a hot
water seal for 15 minutes at 70.degree. C.
[0024] These electrolytic and interference colored samples each had
attractive colored surfaces that would be pleasing on an external
body surface of an automotive vehicle. However, experience has
shown that the color will fade quickly upon exposure to sunlight.
In accordance with this invention, it is found that suitable
heating of the colored anodized workpieces is effective in
stabilizing the colored surfaces against color change when the
surface is exposed to intense ultraviolet radiation. It is not
known exactly how the heating stabilizes the color. But it is
believed that if sufficient heat is applied residual water in the
pores of the anodized coating is removed and the pigment containing
anodized layer is otherwise aged or stabilized so that there is
less degradation in color by subsequent exposure of ultraviolet
light. As will be shown in the subsequent description of
experiments below heating at 350.degree. F. for 60 minutes markedly
stabilizes the colored anodized samples. It is likely that a
heating regimen at a lower temperature would also be suitable.
[0025] As stated representative samples of electrolytically and
interference colored anodized aluminum alloys were heated at
350.degree. F. for sixty minutes to stabilize and age the colored
coating. These samples and corresponding untreated samples were
exposed to the UV radiation of a Xenon lamp (wavelength, 280 to 340
nanometers) for increasing periods of exposure up to about 1200
KJ/m.sup.2. The initial color of the specimens and the color
changes in the specimens due to the UV radiation were measured in a
CIELAB.TM. colorimeter.
[0026] The CIELAB.TM. calorimeter is an opponent color system in
which color from a source is translated into distinctions between
light and dark, red and green, and blue and yellow. In an analysis
of a colored specimen, the CIELAB instrument indicates these values
with three data axes: L*, a* and b*. The central vertical axis
represents lightness, signified as L*, whose values run from 0
(black) to 100 (white). Two orthogonal horizontal axes represent
color. They are based on the fact that a color can't be both red
and green, or both blue and yellow, because these colors oppose
each other. On each axis the values run from positive to negative.
On the a-a' axis, positive values indicate amounts of red while
negative values indicate amounts of green. On the b-b' axis, yellow
is positive and blue is negative. For both axes, zero is neutral
gray.
[0027] Thus the color analysis of a particular specimen has a color
characterized by specific values of L*, a*, and b*. For purposes of
considering changes in the color of a specimen, for example, before
and after exposure to a known energy of UV radiation, the following
computation involving the changes in the three characteristic
values may be made: dE= {square root over
(V(.DELTA.L*).sup.2+(.DELTA.a*).sup.2+(.DELTA.b*).sup.2)}.
[0028] After initial color values were obtained, samples were
removed after increasing periods of exposure to the high intensity
lamp and color values again measured. This test is an accelerated
test for determining color change attributable to exposure to
sunlight and like weathering. FIGS. 1-3 summarize change in color
(dE) for the different treated and untreated aluminum alloy sample
after periods of UV exposure.
[0029] FIG. 1 is a graph of dE versus UV exposure in KJ/m.sup.2 for
tin sulfate interference colored AA6111 specimens. In FIG. 1, the
filled diamond data points show the progressive increase in dE for
the non-heat treated AA6111 interference color anodized comparison
samples. The filled square data points are the dE values obtained
from the AA6111 samples heat treated after anodizing and
interference coloring in accordance with this invention. It is seen
that the heat treatment markedly lowers the dE values of the
samples produced by the invention.
[0030] FIG. 2 is a graph of dE values of heat-treated and non
heat-treated AA5083 electrolytically color anodized samples. The
filled diamond data points show the increase in dE for the AA5083
electrolytic color anodized comparison samples that were not
heat-treated in accordance with this invention. The filled square
data points are the dE values obtained from the AA5083 samples
heated after anodizing and electrolytic coloring in accordance with
this invention. It is seen that the heat treatment markedly lowers
the dE values of the samples produced by the invention.
[0031] FIG. 3 is a graph of dE values of heat-treated and non
heat-treated AA5657 anodized and electrolytically colored samples.
The filled diamond data points show the increase in dE for the
AA5657 anodized and colored comparison samples that were not
heat-treated in accordance with this invention. The filled square
data points are the dE values obtained from the AA5657 samples
heated after anodizing and coloring in accordance with this
invention. It is seen that the heat treatment markedly lowers the
dE values of the samples produced by the invention.
[0032] Thus, the aging or stabilizing heat treatment of anodized
and colored aluminum alloy samples markedly improves their
resistance to color change or degradation by weathering. The
practice of the invention has been illustrated by some specific
examples but the scope of the invention is not intended to be
limited by these illustrations.
* * * * *