U.S. patent application number 12/766733 was filed with the patent office on 2010-08-12 for corrosion resistant aluminum alloy substrates and methods of producing the same.
This patent application is currently assigned to Alcoa Inc.. Invention is credited to Albert Askin, Joseph D. Guthrie, Thomas L. Levendusky, Kevin M. Robare, Luis Fanor Vega, Clinton Zediak.
Application Number | 20100200415 12/766733 |
Document ID | / |
Family ID | 40343557 |
Filed Date | 2010-08-12 |
United States Patent
Application |
20100200415 |
Kind Code |
A1 |
Levendusky; Thomas L. ; et
al. |
August 12, 2010 |
CORROSION RESISTANT ALUMINUM ALLOY SUBSTRATES AND METHODS OF
PRODUCING THE SAME
Abstract
Aluminum alloy products comprising an aluminum alloy base and a
sulfate-phosphate oxide zone integral therewith are disclosed.
Methods of making the same are also disclosed.
Inventors: |
Levendusky; Thomas L.;
(Greensburg, PA) ; Vega; Luis Fanor; (Cheswick,
PA) ; Askin; Albert; (Lower Burrell, PA) ;
Guthrie; Joseph D.; (Murrysville, PA) ; Robare; Kevin
M.; (New Kensington, PA) ; Zediak; Clinton;
(Tarentum, PA) |
Correspondence
Address: |
INTELLECTUAL PROPERTY
ALCOA TECHNICAL CENTER, BUILDING C, 100 TECHNICAL DRIVE
ALCOA CENTER
PA
15069-0001
US
|
Assignee: |
Alcoa Inc.
Pittsburgh
PA
|
Family ID: |
40343557 |
Appl. No.: |
12/766733 |
Filed: |
April 23, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11846483 |
Aug 28, 2007 |
7732068 |
|
|
12766733 |
|
|
|
|
Current U.S.
Class: |
205/201 |
Current CPC
Class: |
C25D 11/16 20130101;
C23C 18/122 20130101; C25D 11/08 20130101; Y10T 428/12569 20150115;
Y10T 428/264 20150115; Y10T 428/265 20150115; C25D 11/24 20130101;
Y10T 428/249953 20150401 |
Class at
Publication: |
205/201 |
International
Class: |
C25D 11/18 20060101
C25D011/18; C25D 11/16 20060101 C25D011/16 |
Claims
1. A method comprising: producing a sulfate-phosphate oxide zone
having a thickness of at least about 5 microns in an aluminum alloy
base, wherein the producing step comprises electrochemically
oxidizing a surface of the aluminum alloy base via an electrolyte
comprising both phosphoric acid and sulfuric acid; and forming a
silicon-containing polymer zone integral with at least a portion of
the sulfate-phosphate oxide zone, wherein the aluminum alloy base,
the sulfate-phosphate oxide zone and the silicon-containing polymer
zone at least partially define a corrosion resistant aluminum alloy
substrate.
2. The method of claim 1, wherein the corrosion resistant aluminum
alloy substrate is capable of passing a copper-accelerated acetic
acid salt spray test, as defined by ASTM B368-97 (2003)e1.
3. The method of claim 1, wherein the electrochemically oxidizing
step comprises applying current to the aluminum alloy base at a
current density of at least about 12 amps per square foot.
4. The method of claim 1, wherein the electrochemically oxidizing
step comprises applying current to the aluminum alloy base at a
current density of at least about 18 amps per square foot.
5. The method of claim 1, wherein the electrolyte comprises at
least about 0.1 wt % phosphoric acid.
6. The method of claim 7, wherein the electrolyte comprises not
greater than about 5 wt % phosphoric acid.
7. The method of claim 1, wherein the electrochemically oxidizing
step comprising heating the electrolyte to a temperature of at
least about 75.degree. F.
8. The method of claim 1, wherein the electrochemically oxidizing
step comprising heating the electrolyte to a temperature of at
least about 90.degree. F.
9. The method of claim 1, further comprising: prior to the forming
a sulfate-phosphate oxide zone step, pretreating a surface of the
aluminum alloy base with a pretreating agent.
10. The method of claim 9, wherein the pretreating agent comprises
a chemical brightening composition that includes at least one of
nitric acid, phosphoric acid and sulfuric acid.
11. The method of claim 9, wherein the pretreating agent comprises
an alkaline cleaner.
12. The method of claim 9, further comprising: prior to the forming
a silicon-containing polymer zone step, applying at least one of a
dye and a nickel acetate solution to at least a portion of the
sulfate-phosphate oxide zone.
13. The method of claim 1, wherein the silicon-containing polymer
zone comprises at least one of polysiloxane and polysilazane.
14. The method of claim 13, wherein the forming a
silicon-containing polymer zone step comprises: depositing a
colloid on at least a portion of the sulfate-phosphate oxide zone;
and curing the colloid to form a gel comprising the
silicon-containing polymer coating on the surface of the aluminum
alloy base.
15. The method of claim 14, wherein the depositing step comprises:
applying a sufficient amount of the sol to both (a) fill pores of
the sulfate-phosphate oxide zone and (b) form a coating comprising
the silicon-containing polymer coating.
16. The method of claim 1, wherein the electrolyte consists of
essentially of sulfuric acid and phosphoric acid.
17. The method of claim 1, wherein the producing step comprises:
generating pores within the sulfate-phosphate oxide zone; and
wherein the forming step comprises: flowing at least some of a
silicon-containing polymer into the pores of the sulfate-phosphate
oxide zone.
16. The method of claim 17, wherein the pores have an average pore
size of from about 10 to about 15 nanometers.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of U.S. patent application
Ser. No. 11/846,483, filed Aug. 28, 2007, entitled "CORROSION
RESISTANT ALUMINUM SUBSTRATES AND METHODS OF PRODUCING THE SAME",
now U.S. Pat. No. ______, which is incorporated herein by reference
in its entirety.
BACKGROUND
[0002] Many metallic substrates, such as those including aluminum
alloys, may be anodized to increase corrosion resistance and wear
resistance of the substrate. Anodizing is an electrolytic
passivation process used to increase the thickness and density of
the natural oxide layer on the surface of metal parts. Anodic films
can also be used for a number of cosmetic effects, either via thick
porous coatings that can absorb dyes or via thin transparent
coatings that add interference effects to reflected light. Anodic
films are generally much stronger and more adherent than most
paints and platings, making them less likely to crack and peel.
Anodic films are most commonly applied to protect aluminum alloys,
although processes also exist for titanium, zinc, magnesium, and
niobium.
[0003] With respect to aluminum alloys, during anodizing an
aluminum oxide coating is grown from and into the surface of the
aluminum alloy in about equal amounts, so, for example, a 2 .mu.m
thick coating will increase part dimensions by 1 .mu.m per surface.
Anodized aluminum alloy surfaces can also be dyed. In most consumer
goods the dye is contained in the pores of the aluminum oxide
layer. Anodized aluminum surfaces have low to moderate wear
resistance, although this can be improved with thickness and
sealing. If wear and scratches are minor then the remaining oxide
will continue to provide corrosion protection even if the dyed
layer is removed.
[0004] While conventional anodizing processes may yield anodized
substrates having good abrasion resistance and ability to color the
surface with dyes, such substrates are not without their drawbacks.
For instances, many anodized substrates are unable to provide
durability and chemical stability in a corrosive environment, and
also are generally unable to provide hydration stability in humid
and outdoor environments. Protective compounds may be applied to
the anodized surfaces, but it is difficult to maintain adhesion and
chemical compatibility of these protective compounds with anodized
surfaces while maintaining suitable abrasion resistance and
coloring ability. In turn, the overall performance of the
corresponding finished products may be inadequate for certain
applications.
SUMMARY OF THE INVENTION
[0005] Broadly, the instant application relates to aluminum alloys
having sulfate-phosphate oxide zones included therein, wear and/or
corrosion resistant aluminum alloy products produced from the same,
and methods of producing the same. The sulfate-phosphate oxide
zones of the aluminum alloys may promote increased adhesion between
the aluminum alloy and polymers coated thereon. In turn, corrosion
resistant substrates may be produced. The corrosion resistant
substrates may be wear resistant, visually appealing (e.g., glossy)
and have a relatively smooth outer surface (e.g., have a low
coefficient of friction). In turn, the corrosion resistant aluminum
alloy substrates may have "slicker" surfaces, and thus reduced
material accumulation may be realized on the surface.
[0006] In one aspect, aluminum alloy products are provided. In one
embodiment, an aluminum alloy product includes an aluminum alloy
base and a sulfate-phosphate oxide zone integral with the base. In
one embodiment, the aluminum alloy product is a forged product. In
one embodiment, the aluminum alloy product is a wheel product.
[0007] The aluminum alloy base may be any suitable aluminum alloy,
but in some instance is a wrought aluminum alloy, such as any of
the DM, 3XXX, 5XXX, 6XXX and 7XXX series alloys, as defined by The
Aluminum Association, Inc. In one embodiment, the aluminum alloy is
a 6061 series alloy. In one embodiment, the aluminum alloy base 10
is a 2014 series alloy. In one embodiment, the aluminum alloy base
10 is a 7050 series alloy. In one embodiment, the aluminum alloy
base 10 is a 7085 series alloy.
[0008] The features of the sulfate-phosphate oxide zone may be
tailored. In one embodiment, the sulfate-phosphate oxide zone
comprises pores. The pores may facilitate, for example, flow of
polymer therein. In one embodiment, the pores have an average pore
size of at least about 10 nm. In one embodiment, the pores have an
average pore size of not greater than about 15 nm. In one
embodiment, the sulfate-phosphate oxide zone has a thickness of at
least about 0.0002 inch (about 5 microns). In one embodiment, the
sulfate-phosphate oxide zone has a thickness of not greater than
about 0.00065 inch (25 microns).
[0009] The aluminum alloy product may include a polymer zone. In
one embodiment, the polymer zone at least partially overlaps with
the sulfate-phosphate oxide zone. In one embodiment, the polymer
zone includes a silicon-based polymer. In one embodiment, the
silicon-based polymer is polysiloxane. In one embodiment, the
silicon-based polymer is polysilazane. The interface and/or
adhesion between the polymer zone and the sulfate-phosphate oxide
zone may be facilitated via the pores or the sulfate-phosphate
oxide zone.
[0010] In one embodiment, the polymer zone includes a coating
portion on a surface of the aluminum alloy base. In one embodiment,
the coating has a thickness of at least about 5 microns. In one
embodiment, the coating has a thickness of at least about 8
microns. In one embodiment, the coating has a thickness of at least
about 35 microns. In one embodiment, the coating is substantially
crack-free (e.g., as determined visually and/or via optical
microscopy). In one embodiment, the coating is adherent to a
surface of the aluminum alloy base. In one embodiment, all or
nearly all of the coating passes the Scotch 610 tape pull test, as
defined by ASTM D3359-02, Aug. 10, 2002. In one embodiment, all or
nearly all of the coating passes the Scotch 610 tape pull test
after army-navy humidity testing of 1000 hours, as defined by ASTM
D2247-02, Aug. 10, 2002. In one embodiment, the aluminum-alloy
base, the sulfate-phosphate oxide zone, and the polymer zone define
a corrosion resistant aluminum alloy substrate. In one embodiment,
the corrosion resistant substrate is capable of passing a
copper-accelerated acetic acid salt spray test (CASS), as defined
by ASTM B368-97 (2003)e1.
[0011] In another aspect, method of producing substrates having a
sulfate-phosphate oxide zone are provided. In one embodiment, a
method includes producing a sulfate-phosphate oxide zone in an
aluminum alloy base and forming a polymer zone integral with at
least a portion of the sulfate-phosphate oxide zone. In one
embodiment, the producing the sulfate-phosphate oxide zone step
comprises electrochemically oxidizing a surface of the aluminum
alloy base via an electrolyte comprising both phosphoric acid and
sulfuric acid. In one embodiment, the electrolyte comprises at
least about 0.1 wt % phosphoric acid. In one embodiment, the
electrolyte comprises not greater than about 5 wt % phosphoric
acid.
[0012] In one embodiment, the electrochemically oxidizing step
comprises applying current to the aluminum alloy base at a current
density of at least about 12 amps per square foot. In one
embodiment, the electrochemically oxidizing step comprises applying
current to the aluminum alloy base at a current density of at least
about 18 amps per square foot. In one embodiment, the
electrochemically oxidizing step comprising heating the electrolyte
to a temperature of at least about 75.degree. F. In one embodiment,
the electrochemically oxidizing step comprising heating the
electrolyte to a temperature of at least about 90.degree. F.
[0013] In one embodiment, the polymer zone is a silicon-containing
polymer zone. In one embodiment, silicon-containing polymer zone
comprises at least one of polysiloxane and polysilazane. In one
embodiment, the forming the polymer zone step includes depositing a
colloid on at least a portion of the sulfate-phosphate oxide zone,
and curing the colloid to form a gel comprising the
silicon-containing polymer coating on the surface of the aluminum
alloy base.
[0014] In one embodiment, the colloid is a sol. In one embodiment,
the depositing step includes applying a sufficient amount of the
sol to both: (a) fill pores of the sulfate-phosphate oxide zone,
and (b) form a coating comprising the silicon-containing polymer
coating. In one embodiment, the method includes pretreating a
surface of the aluminum alloy base with a pretreating agent before
the producing the sulfate-phosphate oxide zone step.
[0015] In one embodiment, the pretreating agent comprises a
chemical brightening composition that includes at least one of
nitric acid, phosphoric acid and sulfuric acid. In one embodiment,
the pretreating agent comprises an alkaline cleaner. In one
embodiment, the method includes applying at least one of a dye and
a nickel acetate solution to at least a portion of the
sulfate-phosphate oxide zone before the forming a polymer zone
step.
[0016] As may be appreciated, various ones of the inventive aspects
noted hereinabove may be combined to yield various aluminum alloy
products having improved adhesive, corrosion and/or appearance
qualities, to name a few. Moreover, these and other aspects,
advantages, and novel features of the invention are set forth in
part in the description that follows and will become apparent to
those skilled in the art upon examination of the following
description and figures, or may be learned by practicing the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a schematic, cross-sectional view of one
embodiment of an aluminum alloy base including a sulfate-phosphate
oxide zone.
[0018] FIG. 2 is a schematic, cross-sectional view of one
embodiment of a corrosion resistant substrate.
[0019] FIG. 3 is a schematic view of various reaction mechanisms
that may occur in accordance with a sulfate-phosphate oxide zone
and a silicon-based polymer.
[0020] FIG. 4 is a flow chart illustrating methods of producing
aluminum alloys having a sulfate-phosphate oxide zone and corrosion
resistant substrates.
[0021] FIG. 5a is an SEM image (25000.times. magnification) of an
anodized 6061 series alloy that has been anodized with a
conventional Type II anodizing process.
[0022] FIG. 5b is an energy dispersive spectroscopy (EDS) image
obtained via x-ray analysis of the alloy of FIG. 5a.
[0023] FIG. 6a is an SEM image (25000.times. magnification) of a
6061 series alloy that has been surface treated with a mixed
electrolyte.
[0024] FIG. 6b is an energy dispersive spectroscopy (EDS) image
obtained via x-ray analysis of the alloy of FIG. 6a.
DETAILED DESCRIPTION
[0025] Reference is now made to the accompanying drawings, which at
least assist in illustrating various pertinent features of the
instant application. In one approach, the instant application
relates to aluminum alloys having a sulfate-phosphate oxide zone.
One embodiment of an aluminum alloy having a sulfate-phosphate
oxide zone is illustrated in FIG. 1. In the illustrated embodiment,
an aluminum alloy base 10 includes a sulfate-phosphate oxide zone
20. In general, and as described in further detail below, the
aluminum alloy base 10 may be modified with a mixed electrolyte
(e.g., sulfuric acid plus phosphoric acid) to produce the
sulfate-phosphate oxide zone 20. The sulfate-phosphate oxide zone
20 may promote, among other things, adhesion of the polymers to the
aluminum alloy base 10, as described in further detail below.
[0026] The aluminum alloy base 10 may be any material adapted to
have a sulfate-phosphate oxide zone formed therein via
electrochemical processes. As used herein, "aluminum alloy" means a
material including aluminum and another metal alloyed therewith,
and includes one or more of the Aluminum Association 2XXX, 3XXX,
5XXX, 6XXX and 7XXX series alloys. The aluminum alloy base 10 may
be from any of a forging, extrusion, casting or rolling
manufacturing process. In one embodiment, the aluminum alloy base
10 comprises a 6061 series alloy. In one embodiment, the aluminum
alloy base 10 comprises a 6061 series alloy with a T6 temper. In
one embodiment, the aluminum alloy base 10 comprises a 2014 series
alloy. In one embodiment, the aluminum alloy base 10 comprises a
7050 series alloy. In one embodiment, the aluminum alloy base 10
comprises a 7085 series alloy. In one embodiment, the aluminum
alloy base 10 is a wheel product (e.g., a rim). In one embodiment,
the aluminum alloy base 10 is a building product (e.g., aluminum
siding or composite panel).
[0027] In the illustrated embodiment, the aluminum alloy base 10
includes a sulfate-phosphate oxide zone 20. As used herein,
"sulfate-phosphate oxide zone" means a zone produced from
electrochemical oxidation of the aluminum alloy base 10, and which
zone may include elemental aluminum (Al), sulfur (S), phosphorous
(P) and/or oxygen (O) and compounds thereof. In one embodiment, and
as described in further detail below, the sulfate-phosphate oxide
zone 20 may be produced from an electrolyte comprising both
sulfuric acid and phosphoric acid.
[0028] The sulfate-phosphate oxide zone 20 generally comprises an
amorphous morphology that includes a plurality of sulfate-phosphate
pores (not illustrated). As used herein, "sulfate-phosphate oxide
pores" means pores of the sulfate-phosphate oxide zone 20 that
include elemental Al, O, S and/or P or compounds thereof and
proximal a surface thereof. As described in further detail below,
such sulfate-phosphate oxide pores may facilitate increased
adhesion between polymers and the sulfate-phosphate oxide zone 20
via chemical interaction between the polymer and one or more of the
Al, O, S, and P elements located on a surface thereof or proximal
thereto.
[0029] The sulfate-phosphate oxide zone 20 may include an amorphous
and porous morphology, which may facilitate increased adhesion
between polymer and the aluminum alloy via an increased surface
area. Conventionally anodized surfaces generally include columnar
morphology (e.g., for a Type II, sulfuric acid only anodized
surface), or a nodal morphology (e.g., for a phosphoric acid only
anodized surface). Conversely, the porous, amorphous morphology of
the sulfate-phosphate oxide zone 20 generally comprises a high
surface area relative to such conventionally anodized surfaces.
This higher surface area may contribute to increased adhesion
between polymer coatings and the aluminum alloy base 10.
[0030] Increased adhesion of polymers to the aluminum alloy base 10
may be realized by tailoring the pore size of the sulfate-phosphate
oxide pores. For example, the pore size of the sulfate-phosphate
oxide pores may be tailored so as to facilitate flow of certain
polymers therein by creating sulfate-phosphate oxide pores having
an average pore size that is coincidental to the radius of gyration
of the polymer to be used to coat the aluminum alloy base 10. In
one embodiment, the average pore size of the sulfate-phosphate
oxide pores may be in the range of from about 10 nm to about 15
nanometers, and the polymer may be a silicon-containing polymer,
such as polysilazane and polysiloxane polymers. Since this average
pore size range is coincidental to the radius of gyration of such
polymers, these polymers (or their precursors) may readily flow
into the sulfate-phosphate oxide pores. In turn, the polymers may
readily bond with the sulfate-phosphate oxides associated therewith
(e.g., during curing of the polymer, described in further detail
below).
[0031] As used herein, "average pore size" means the average
diameters of the sulfate-phosphate oxide pores of the
sulfate-phosphate oxide zone as measured using microscopic
techniques. As used herein, "radius of gyration" means the mean
size of the polymer molecules of a sample over time, and may be
calculated using an average location of monomers over time or
ensemble:
R g 2 = def 1 N ( k = 1 N ( r k - r mean ) 2 ) ##EQU00001##
where the angular brackets . . . denote the ensemble average.
[0032] To promote chemical interaction between surfaces of the
sulfate-phosphate oxide zone and the polymer, the ratio of sulfur
atoms to phosphorous atoms may be tailored. In one embodiment, the
polymer is a silicon-based polymer and the ratio of sulfur atoms to
phosphorous in the sulfate-phosphate oxide zone 20 is at least
about 5:1 (S:P), such as at least about 10:1 (S:P), or even at
least about 20:1 (S:P). In this embodiment, the ratio sulfur atoms
to phosphorus atoms in the sulfate-phosphate oxide zone 20 may not
exceed about 100:1 (S:P), or even not greater than about 75:1
(S:P).
[0033] The thickness of the sulfate-phosphate oxide zone 20 may be
tailored so as to produce a zone having sufficient surface area for
bonding with a polymer. In this regard, the sulfate-phosphate oxide
zone 20 of the corrosion resistant substrate 1 generally has a
thickness of at least about 5 microns (0.00020 inch), such as a
thickness of at least about 6 microns (0.00024 inch). The
sulfate-phosphate oxide zone generally has a thickness of not
greater than about 25 microns (about 0.001 inch), such as not
greater than about 17 microns (about 0.00065 inch).
[0034] As noted above, aluminum alloys include sulfate-phosphate
oxides may be utilized to produce wear/corrosion resistant aluminum
alloy products. One embodiment of a wear/corrosion resistant
substrate is illustrated in FIG. 2. In the illustrated embodiment,
the substrate 1 includes an aluminum alloy base 10, a
sulfate-phosphate oxide zone 20, and a silicon-containing polymer
zone 30. A first portion of the silicon-containing polymer zone
overlaps with at least a portion of the sulfate-phosphate oxide
zone 20, and thus defines a mixed zone 40. In other words, the
sulfate-phosphate oxide zone 20 and the silicon-containing polymer
zone 30 at least partially overlap, and this overlap defines a
mixed zone 40. Thus, mixed zone 40 includes both sulfate-phosphate
oxides and silicon-containing polymer. A polymer-free zone 60 may
make up the remaining portion of the sulfate-phosphate oxide zone
20. A coating 50 may make up the remaining portion of the
silicon-containing polymer zone 30. The coating 50 is located on an
outer surface of the aluminum alloy base 10, and, since the coating
50 is integral with the sulfate-phosphate oxide zone 20 via the
mixed zone 40, the coating 50 may be considered integral with the
aluminum alloy base 10 via the mixed zone 40. In turn, increased
adhesion between the coating 50 and the aluminum alloy base 10 may
be realized relative to conventional anodized products.
[0035] As noted above, the sulfate-phosphate oxide zone 20
generally is porous. Thus, various amounts of silicon-containing
polymer may be contained within the pores of the sulfate-phosphate
oxide zone 20. In turn, adhesion between the sulfate-phosphate
oxide zone 20 and the coating 50 may be facilitated. In particular,
chemical bonding between the silicon-containing polymer and the
sulfate-phosphate oxide zone 20 is believed to provide adhesive
qualities heretofore unknown with respect to electrochemically
treated aluminum substrates due to, for example, the molecular
structure of the formed Al--O--P--O--Si compounds. It is believed
that the Al--O--P--O--Si molecular structure is more stable than
the molecular arrangements achieved with conventional anodizing
processes (e.g., Al--O--Si, Al--O--P, Al--O--S, independently, and
Al--O--S--O--Si). For example, the substrate 1 may be able to pass
the ASTM D3359-02 (Aug. 10, 2002) tape adhesion test, in both dry
and wet conditions. Examples of chemical reactions that may occur
between polymers and the sulfate-phosphate oxides are illustrated
in FIG. 3. Starting from their original colloid compositions, the
chemical reactions that occur upon contact with water and
subsequent curing may lead to a sequence of hydration and
condensation reactions with the evolution of water, resulting in
one or more new chemical structures within the sulfate-phosphate
oxide zone involving sulfate-phosphate oxides and a silicon-based
polymer. For example, the end products 310, 320 illustrated in FIG.
3 may be produced.
[0036] As used herein, "silicon-containing polymer" means a polymer
comprising silicon and that is suited for integrating with at least
a portion of the sulfate-phosphate oxide zone 20 (e.g., via
chemical bonding and/or physical interactions). In this regard, the
silicon-containing polymer should have a radius of gyration that is
coincidental with the average pore size of the sulfate-phosphate
oxide zone 20. Furthermore, since the silicon-containing polymer
zone 30 may act as a barrier between outside environments and the
aluminum alloy base 10, the silicon-containing polymer should
generally be fluid impermeable. For appearance purposes, the
silicon-containing polymer may be translucent, or even transparent,
so as to facilitate preservation of the original specularity and
aesthetic appearance of the finished product. Particularly, useful
silicon-containing polymers having many of the above qualities
include polysiloxanes (Si--O--Si) and polysilazanes (Si--N--Si).
Polysiloxane polymers are available from, for example, SDC Coatings
of Anaheim, Calif., U.S.A. Polysilazane polymers are available
from, for example, Clariant Corporation of Charlotte, N.C.,
U.S.A.
[0037] The selection of siloxane polymers versus silazane polymers
may be dictated by the desired performance characteristics of the
final product. Due to the dispersive nature of the siloxane
precursor, which involves condensation during reaction with the
sulfate-phosphate oxide zone 20, the resulting coefficient of
thermal expansion of the polysiloxane compound may induce residual
stresses at the surface of the coating 50, which may translate into
surface fissures and/or cracks in the finished product, as
described in further detail below. To avoid fissures and cracks
with coatings 50 comprising polysiloxane, the thickness of the
coating 50 may be restricted to not greater than 10 microns, or
even not greater than 8 microns. Thus, for enhanced corrosion
resistance, the barrier properties of the coating 50 may need to be
increased via, for example, increased thickness. Substrates
including coatings 50 produced from polysilazanes may have higher
thicknesses than coatings produced with polysiloxanes and having
similar fluid impermeable characteristics. It is believed that the
flexibility and chemical composition of polysilazanes allow the
production of end product 320, illustrated in FIG. 3, which, in
turn, allows longer molecular chain lengths, and thus increased
coating thicknesses with little or no cracking (e.g., fissure-free,
crack-free surfaces). In one embodiment, the coating 50 is
sufficiently thick to define a corrosion resistant substrate. The
corrosion resistant substrate may be corrosion resistant while
retaining a smooth surface and a glossy appearance (e.g., due to
transparency of the coating 50 in combination with the appearance
of the mixed zone 40). As used herein, "corrosion resistant
substrate" means a substrate having an aluminum alloy base, a
sulfate-phosphate oxide zone 20, and a silicon-containing polymer
zone 30, and which is able to pass a 240 hour exposure to
copper-accelerated acetic acid salt spray test, as defined by ASTM
B368-97 (2003)e1 (hereinafter the "CASS test"). In one embodiment,
the corrosion resistant substrate is capable of substantially
maintaining a glossy and translucent appearance while passing the
CASS test. In this regard, the silicon-containing polymer may
comprise a polysilazane and the coating 50 may have a thickness of
at least about 8 microns. In one embodiment, the coating 50 has a
thickness of at least about 35 microns. In one embodiment, the
coating 50 has a thickness of at least about 40 microns. In one
embodiment, the coating 50 has a thickness of at least about 45
microns. In one embodiment, the coating 50 has a thickness of at
least about 50 microns. In some embodiments, the coatings 50 may
realize little or no cracking. In this regard, it is noted that
polysilazane has a coefficient of thermal expansion that is closer
to the coefficient of thermal expansion of the aluminum alloy base
10 than polysiloxane coatings. For example, coatings comprising
polysilazane may have a coefficient of thermal expansion of at
least about 8.times.10.sup.-5/.degree. C. and aluminum-based
substrates may comprise a coefficient of thermal expansion of about
22.8.times.10.sup.-6/.degree. C. Hence, the ratio of the
coefficient of thermal expansion of the polysilazane coating to the
coefficient of thermal expansion of the substrate may be not
greater than about 10:1, such as not greater than about 7:1, or not
greater than 5:1, or not greater than about 4:1, or not greater
than about 3.5:1. Thus, in some instances, the coating 50 may
comprise a coefficient of thermal expansion that is coincidental to
a coefficient of thermal expansion of the aluminum alloy base 10
and/or the sulfate-phosphate oxide zone 20 thereof. Hence, coatings
50 comprising polysilazane may act as an impermeable or
near-impermeable barrier between the aluminum alloy base 10 and
other materials while maintaining a glossy appearance and a smooth
outer surface. Nonetheless, the polysiloxane coatings generally
should not be too thick, or the coating may crack. In one
embodiment, the coating 50 comprises polysilazane and has a
thickness of not greater than about 90 microns, such as a thickness
of not greater than about 80 microns.
[0038] As noted above, the coating 50 may have sufficient thickness
to facilitate production of a corrosion resistant substrate and the
corrosion resistant substrate may be capable of passing the CASS
test. In other embodiments, the corrosion resistance of the coating
50 may be a lesser consideration in the final product design. Thus,
the thickness of the coating 50 may be tailored based on the
requisite design parameters. In one embodiment, the coating 50
comprises polysiloxane and has a thickness of not greater than
about 10 microns, such as a thickness of not greater than about 8
microns.
[0039] Polymers other than silicon-based polymers may be used to
produce a polymer-containing zone. Such polymers should posses a
radius of gyration that is coincidental to the average pore size of
the sulfate-phosphate oxide zone 20. Materials other than polymers
may also be used to facilitate production of wear resistant and/or
corrosion resistant substrates. For example, the sulfate-phosphate
oxide zone 20 may optionally include dye and/or a nickel acetate
preseal. With respect to dyes, ferric ammonium oxalate, metal-free
anthraquinone, metalized azo complexes or combinations thereof may
be utilized to provide the desired visual effect.
[0040] Methods of producing corrosion resistant substrates are also
provided, one embodiment of which is illustrated in FIG. 4. In the
illustrated embodiment, the method includes the steps of producing
a sulfate-phosphate oxide zone on a surface of the aluminum alloy
base (220) and forming a silicon-containing polymer zone on the
sulfate-phosphate oxide zone (240). The method may optionally
include the steps of pretreating an aluminum alloy base (210)
and/or applying a dye to the sulfate-phosphate oxide zone (230).
The aluminum alloy base, the sulfate-phosphate oxide zone and the
silicon-containing polymer zone may be any of the above-described
aluminum alloy bases, sulfate-phosphate oxide zones and
silicon-containing polymer zones, respectively.
[0041] In one embodiment, and if utilized, a pretreating step (210)
may comprise contacting the aluminum alloy base with a pretreating
agent (212). For example, the pretreating agent may comprise a
chemical brightening composition. As used herein, "chemical
brightening composition" means a solution that includes at least
one of nitric acid, phosphoric acid, sulfuric acid, and
combinations thereof. For example, the methodologies disclosed in
U.S. Pat. No. 6,440,290 to Vega et al. may be employed to pretreat
an aluminum alloy base with a chemical brightening composition. In
one approach, and with respect to 6XXX series alloys, a phosphoric
acid-based with a specific gravity of at least about 1.65, when
measured at 80.degree. F. (about 26.7.degree. C.) may be used, such
as a phosphoric acid with a specific gravities in the range of from
about 1.69 to about 1.73 at the aforesaid temperature. A nitric
acid additive may be used to minimize a dissolution of constituent
and dispersoid phases on certain Al--Mg--Si--Cu alloy products,
especially 6XXX series forgings. Such nitric acid concentrations
dictate the uniformity of localized chemical attacks between
Mg.sub.2Si and matrix phases on these 6XXX series Al alloys. As a
result, end product brightness may be positively affected in both
the process electrolyte as well as during transfer from process
electrolyte to a rinsing substep (not illustrated). In one
approach, the nitric acid concentrations of may be about 2.7 wt. %
or less, with more preferred additions of HNO.sub.3 to that bath
ranging between about 1.2 and 2.2 wt. %. For 6XXX series aluminum
alloys, improved brightening may occur in those alloys whose iron
concentrations are kept below about 0.35% in order to avoid
preferential dissolution of Al--Fe--Si constituent phases. For
example, the Fe content of these alloys may be kept below about
0.15 wt % iron. At the aforementioned specific gravities, dissolved
aluminum ion concentrations in these chemical brightening baths
should not exceed about 35 g/liter. The copper ion concentrations
therein should not exceed about 150 ppm.
[0042] In another approach, the pretreating agent may include an
alkaline cleaner. As used herein, "alkaline cleaner" means a
composition having a pH of greater than approximately 7. In one
embodiment, an alkaline cleaner has a pH of less than about 10. In
one embodiment, an alkaline cleaner has a pH in the range of from
about 7.5 to about 9.5. In one embodiment, the alkaline cleaner
includes at least one of potassium carbonate, sodium carbonate,
borax, and combinations thereof. In another embodiment, an alkaline
cleaner has a pH of at least about 10.
[0043] In one embodiment, the pretreating step (210) includes
removing contaminates from a surface of the aluminum alloy base.
Examples of contaminates include grease, polishing compounds, and
fingerprints. After the pretreating step (210), such as via
chemical brighteners or alkaline cleaners, described above, the
absence of contaminants on the surface of the aluminum alloy base
may be detected by determining the wetability of a surface of the
aluminum alloy base. When a surface of the aluminum alloy base wets
when subjected to water, it is likely substantially free of surface
contaminants (e.g., an aluminum alloy substrate that has a surface
energy of at least about 72 dynes/cm).
[0044] Turning now to the producing a sulfate-phosphate oxide zone
step (220), the sulfate-phosphate oxide zone may be produced via
any suitable technique. In one embodiment, the sulfate-phosphate
oxide zone is produced by electrochemically oxidizing a surface of
the aluminum alloy base. As used herein, "electrochemically
oxidizing" means contacting the aluminum alloy base with a
electrolyte containing both (a) sulfuric acid and (b) phosphoric
acid, and applying an electric current to the aluminum alloy base
while the aluminum alloy base is in contact with the
electrolyte.
[0045] The ratio of sulfuric acid to phosphoric acid within the
electrolyte (sometimes referred to herein as a "mixed electrolyte")
should be tailored/controlled so as to facilitate production of
suitable sulfate-phosphate oxide zones. In one embodiment, the
weight ratio of sulfuric acid (SA) to phosphoric acid (PA) in the
electrolyte is at least about 5:1 (SA:PA), such as a weight ratio
of at least about 10:1 (SA:PA), or even a weight ratio of at least
about 20:1 (SA:PA). In one embodiment, the weight ratio of sulfuric
acid to phosphoric acid in the electrolyte is not greater than
100:1 (SA:PA), such as a weight ratio of not greater than about
75:1 (SA:PA). In one embodiment, the mixed electrolyte comprises at
least about 0.1 wt % phosphoric acid. In one embodiment, the mixed
electrolyte comprises not greater than about 5 wt % phosphoric
acid. In one embodiment, the mixed electrolyte comprises not
greater than about 4 wt % phosphoric acid. In one embodiment, the
mixed electrolyte comprises not greater than about 1 wt %
phosphoric acid. In one embodiment, the phosphoric acid is
orthophosphoric acid.
[0046] The current applied to the mixed electrolyte should be
tailored/controlled so as to facilitate production of suitable
sulfate-phosphate oxide zones. In one embodiment, electrochemically
oxidizing step (222) includes applying electricity to the
electrolyte at a current density of at least about 8 amps per
square foot (asf). In one embodiment, the current density is at
least about 12 asf. In one embodiment, the current density is at
least about 18 asf. In one embodiment, the current density is not
greater than about 24 asf. Thus, the current density may be in the
range of from about 8 asf to about 24 asf, such as in the range of
from about 12 asf to about 18 asf.
[0047] The voltage applied to the mixed electrolyte should also be
tailored/controlled so as to facilitate production of suitable
sulfate-phosphate oxide zones. In one embodiment, the
electrochemically oxidizing step (222) includes applying
electricity to the electrolyte at a voltage of at least about 6
volts. In one embodiment, the voltage is at least about 9 volts. In
one embodiment, the voltage is at least about 12 volts. In one
embodiment, the voltage is not greater than about 18 volts. Thus,
the voltage may be in the range of from about 6 volts to about 18
volts, such as in the range of from about 9 volts to about 12
volts.
[0048] The temperature of the electrolyte during the
electrochemically oxidizing step (222) should also be
tailored/controlled so as to facilitate production of suitable
sulfate-phosphate oxide zones. In one embodiment, the
electrochemically oxidizing step (222) includes heating the
electrolyte to and/or maintaining the electrolyte at a temperature
of at least about 75.degree. F. (about 24.degree. C.), such as a
temperature of at least about 80.degree. F. (about 27.degree. C.).
In one embodiment, the temperature of the electrolyte is at least
about 85.degree. F. (about 29.degree. C.). In one embodiment, the
temperature of the electrolyte is at least about 90.degree. F.
(about 32.degree. C.). In one embodiment, the electrochemically
oxidizing step (222) includes heating the electrolyte and/or
maintaining the electrolyte at a temperature of not greater than
about 100.degree. F. (about 38.degree. C.). Thus, the temperature
of the electrolyte may be in the range of from about 75.degree. F.
(about 24.degree. C.) to about 100.degree. F. (38.degree. C.), such
as in the range of from about 80.degree. F. to about 95.degree. C.,
or a range of from about 85.degree. F. (about 29.degree. C.) to
about 90.degree. F. (about 32.degree. C.).
[0049] In a particular embodiment, the electrochemically oxidizing
step (222) includes utilizing a mixed electrolyte having: (i) a
weight ratio of sulfuric acid to phosphoric acid of about 99:1
(SA:PA), and (ii) a temperature about 90.degree. F. In this
embodiment, the current density during electrochemically oxidizing
step (222) is at least about 18 asf.
[0050] After the sulfate-phosphate oxide zone is produced (220),
the method may optionally include the step of presealing the
sulfate-phosphate oxide zone (not illustrated) prior to or after
the applying a dye step (230) and/or prior to the forming a
silicon-containing polymer zone (240). In one approach, at least
some, or in some instances all or nearly all, of the pores of the
sulfate-phosphate oxide zone may be sealed with a sealing agent,
such as, for instance, an aqueous salt solution at elevated
temperature (e.g., boiling salt water) or nickel acetate.
[0051] Moving to the applying a dye step (230), in one embodiment
the applying a dye step (230) comprises applying at least one of
ferric ammonium oxalate, metal-free anthraquinone, metalized azo
complexes or combinations thereof to at least a portion of a
sulfate-phosphate oxide zone. The dye may be applied via any
conventional techniques. In one embodiment, the dye is applied by a
spray coating or dip coating.
[0052] Turning now to the forming a silicon-containing polymer zone
step (240), in one embodiment the forming a forming a
silicon-containing polymer zone step (240) includes depositing a
colloid (e.g., a sol) on at least a portion of the
sulfate-phosphate oxide zone (242), and curing the colloid (244).
In a particular embodiment, the colloid is a sol and the curing
step (244) results in the formation of a gel comprising the
silicon-containing polymer zone. The depositing step (242) may
accomplished via any conventional process. Likewise, the curing
step (244) may be accomplished via any conventional process. In one
embodiment, the depositing step (242) is accomplished by one or
more of spray coating or dip coating, spin coating or roll coating.
In another embodiment, the depositing step (242) is accomplished by
vacuum deposition from liquid and/or gas phase precursors. The
silicon-containing polymer zone may be formed on a dyed
sulfate-phosphate oxide zone or an undyed sulfate-phosphate oxide
zone.
[0053] Colloids used to form the silicon-containing polymer zone
generally comprise particles suspended in a liquid. In one
embodiment, the particles are silicon-containing particles (e.g.,
precursors to the silicon-containing polymer). In one embodiment,
the particles have a particle size in the range of from about 1.0
nm to about 1.0 micron. In one embodiment, the liquid is
aqueous-based (e.g., distilled H.sub.2O). In another embodiment,
the liquid is organic based (e.g., alcohol). In a particular
embodiment, the liquid comprises at least one of methanol, ethanol,
or combinations thereof. In one embodiment, the colloid is a
sol.
[0054] The viscosity of the colloid may be tailored based on
deposition method. In one embodiment, the viscosity of the colloid
is about equal to that of water. In this regard, the particles of
the colloid may more freely flow into the pores of the
sulfate-phosphate oxide zone. During or concomitant to the
depositing step (242), the colloid may flow into the pores of the
sulfate-phosphate oxide zone, and may thus seal the pores by
condensation of the colloid to a gel state (e.g., via heat). Water
released during this chemical reaction may induce oxide hydration
and, therefore, sealing of the pores. In a particular embodiment,
the colloid may flow into a substantial amount of (e.g., all or
nearly all) the pores of the sulfate-phosphate oxide zone. In turn,
during the curing step (244), the silicon-containing polymer is
formed and seals a substantial amount of the unsealed pores of the
sulfate-phosphate oxide zone. In this embodiment, the curing step
(244) may include applying a temperature of from about 90.degree.
C. (about 194.degree. F.) to about 170.degree. C. (about
338.degree. F.). In one embodiment, the curing step may include
applying a temperature of from about 138.degree. C. (about
280.degree. F.) to about 160.degree. C. (about 320.degree. F.).
[0055] In one embodiment, the curing step (244) results in the
production of a polysiloxane coating (e.g., via gelation of the
colloid). In one embodiment, the curing step (244) results in the
production of a coating comprising polysilazane. In this regard,
the colloid may include silane precursors, such as trimethoxy
methyl silanes, or silazane precursors, such as methyldichlorine or
aminopropyltriethoxysilane reacted with ammonia via ammonolysis
synthesis. As noted above, the use of polysilazanes versus
polysiloxanes is primarily a function of the desired corrosion
resistance and film thickness of the final product.
EXAMPLES
Example 1
Testing of Polysiloxane Coating with Conventional Type II Anodized
Sheet
[0056] A 6061-T6 aluminum alloy sheet is anodized via a
conventional Type II anodizing process in a sulfuric acid only
electrolyte (10-20 w/w % sulfuric acid, MIL-A-8625F). The sheet is
anodized at 75.degree. F. at a current density of 12 asf. The sheet
is dyed and sealed via a conventional nickel acetate sealing
process (e.g., sealing in an aqueous nickel acetate solution at
190.degree. F.-210.degree. F.). The sheet is coated with a sol
comprising polysiloxane, and the sol is then cured to form a gel
coating comprising polysiloxane on the sheet. The sheet has a dull
appearance and the gel coating does not pass ASTM D3359-02, Aug.
10, 2002 (hereinafter, the "Scotch Tape 610 test"), as coating is
removed from the substrate surface via the tape.
Example 2
Testing of Polysiloxane Coating to Conventional Type II Anodized
Sheet with Pretreatment
[0057] A 6061-T6 aluminum alloy sheet is prepared similar to
Example 1, except that the sheet is pretreated with an alkaline
cleaner and is chemically brightened prior to anodizing. The
anodizing conditions remain the same. The sheet is coated with the
sol composition of Example 1, and the sol is then cured to form a
gel coating comprising polysiloxane on the sheet. The sheet has
dull/matte appearance after curing. The sheet is tested in
accordance with ASTM D2247-02, Aug. 10, 2002 (hereinafter the
"army-navy test") for 1000 hours. The coated sheet does not pass
the army-navy testing as the coating is not adherent to the surface
as tested via the Scotch 610 tape test.
[0058] SEM micrographs of the surface treated sample reveal the
original topography of the sample under as-anodized conditions, as
exhibited in FIG. 5a. Additional x-ray analysis of this sample via
Energy Dispersive Spectroscopy (EDS) verifies the absence of
silicon on the sample surface as shown in FIG. 5b. The results of
this example, and Example 1, indicate that adhesion of silicon
polymers to Type II anodized surfaces is problematic, and that the
pretreatment consisting of alkaline cleaner and chemical
brightening does not have any significant effect on adhesion
properties.
Example 3
Adhesion Testing of Polysiloxane Coating to Surface Treated Sheet
Processed in Mixed Electrolyte
[0059] An aluminum alloy 6061-T6 test sheet is provided. The sheet
is pretreated with an alkaline cleaner and is chemical brightened.
The sheet is surface treated in a mixed electrolyte comprising 96
wt % sulfuric acid and 4 wt % phosphoric acid at about 90.degree.
F. and a current density of about 18 asf. A sulfate-phosphate oxide
zone is created in the processed sheet as determined by energy
dispersive x-ray (EDS) analysis. The thickness of each of the
sulfate-phosphate oxide zones is at least about 0.00020 inch (about
5 microns) as measured using an Eddy current probe. The sheet is
dyed in an aqueous dye solution. The sheet is then sealed in an
aqueous nickel acetate bath at about 190.degree. F. The sheet is
subsequently coated with the same sol of Example 1, and a gel is
formed on the sheet. The sheet is subjected to the army-navy test
for 1000 hours. The sheet passes the army-navy test as the coating
is adherent to the sheet using the Scotch 610 tape pull test.
Furthermore, the sheet has a bright, glossy appearance.
[0060] SEM micrographs of the surface treated sample reveal the
original topography of the sample under as-processed conditions, as
exhibited in FIG. 5a. Additional x-ray analysis of this sample via
Energy Dispersive Spectroscopy (EDS) verifies the presence of
silicon on the sample surface as shown in FIG. 5b. These results
indicate that adhesion of silicon polymers to aluminum alloys
surface treated with a mixed electrolyte comprising sulfuric acid
and phosphoric acid may realize increased adhesion between the
aluminum alloy base and the silicon polymer coating relative to
conventionally processed aluminum alloy substrates.
Example 4
Corrosion Testing of Polysiloxane Coating to Surface Treated Sheet
Processed in Mixed Electrolyte
[0061] An aluminum alloy 6061-T6 test sheet is provided and
prepared as provided in Example 3, except that the sheet is not
sealed in nickel acetate solution. The sheet is subjected to the
army-navy test for 1000 hours. The sheet passes the army-navy test
as the coating passes the Scotch 610 tape test. The sheet is
further subjected to a copper-accelerated acetic acid salt spray
test (CASS) in accordance with ASTM B368-97 (2003)e1 (hereinafter
the "CASS test"). The sheet does not pass the CASS test. It is
postulated that the silicon polymer coating of the gel does not
provide sufficient barrier characteristics against the copper ions
of the CASS test migrating through the coating and chemically
reacting with the aluminum alloy base.
Example 5
Corrosion Testing of Polysiloxane Coating to Surface Treated Sheet
Processed in Mixed Electrolyte
[0062] An aluminum alloy 6061-T6 test sheet is provided and
prepared as provided in Example 4, except that the sol coating is
applied multiple times to provide a gel coating having an increased
thickness. The final thickness of the gel coating is about 8
microns. The sheet is subjected to the army-navy test for 1000
hours. The sheet passes the army-navy test as the coating passes
the Scotch 610 tape test. The sheet is further subjected to the
CASS test. The sheet passes the CASS test. Unfortunately, the
coating contains cracking, giving it an undesirable appearance.
Example 6
Corrosion Testing of Polysilazane Coating to Surface Treated Sheet
Processed in Mixed Electrolyte
[0063] An aluminum alloy 6061-T6 test sheet is provided and
prepared as provided in Example 4, except that the coating is a
polysilazane-based coating. The coating is applied multiple times
to provide a gel coating having an increased thickness. The final
thickness of the gel coating is about 8 microns, but the coating
comprises polysilazanes instead of the polysiloxanes of Example 5.
The sheet is subjected to the army-navy test for 1000 hours. The
sheet passes the army-navy test as the coating passes the Scotch
610 tape test. The sheet is further subjected the CASS test. The
sheet passes the CASS test. The coating is crack-free.
[0064] While various embodiments of the present application have
been described in detail, it is apparent that modifications and
adaptations of those embodiments will occur to those skilled in the
art. However, it is to be expressly understood that such
modifications and adaptations are within the spirit and scope of
the present invention.
* * * * *