U.S. patent application number 15/333072 was filed with the patent office on 2017-05-04 for anodic films for high performance aluminum alloys.
The applicant listed for this patent is Apple Inc.. Invention is credited to Jody R. AKANA, Takahiro OSHIMA, Masashige TATEBE.
Application Number | 20170121837 15/333072 |
Document ID | / |
Family ID | 58456159 |
Filed Date | 2017-05-04 |
United States Patent
Application |
20170121837 |
Kind Code |
A1 |
TATEBE; Masashige ; et
al. |
May 4, 2017 |
ANODIC FILMS FOR HIGH PERFORMANCE ALUMINUM ALLOYS
Abstract
Anodic films that provide improved corrosion resistance to high
performance aluminum alloys, and methods for forming the same, are
described. According to some embodiments, the anodic films have a
dense porous layer and a thickened barrier layer. The porous layer
can act as a cosmetic portion of the anodic film and have pores
that have a colorant infused therein. The thickened barrier layer
can distribute defects within the anodic film associated with
alloying elements of the high performance aluminum alloy in a
larger non-porous film compared to conventional anodic films,
thereby lessening the chance of corrosion inducing agents of
reaching the high performance aluminum alloy. The anodic films have
superior scratch and chemical resistance, as well as enhanced
cosmetic aspects, well suited for consumer products, such as
housings for electronic products.
Inventors: |
TATEBE; Masashige; (Kakogawa
City, JP) ; OSHIMA; Takahiro; (Tokyo, JP) ;
AKANA; Jody R.; (San Francisco, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Apple Inc. |
Cupertino |
CA |
US |
|
|
Family ID: |
58456159 |
Appl. No.: |
15/333072 |
Filed: |
October 24, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62249079 |
Oct 30, 2015 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25D 11/12 20130101;
H05K 5/0243 20130101; H05K 5/04 20130101; C25D 11/10 20130101; Y10T
428/131 20150115; C25D 11/08 20130101; C25D 11/16 20130101; C25D
11/24 20130101; C25D 11/06 20130101; C25F 3/20 20130101; G06F
1/1633 20130101; C25D 11/22 20130101 |
International
Class: |
C25D 11/12 20060101
C25D011/12; H05K 5/04 20060101 H05K005/04; C25D 11/24 20060101
C25D011/24; H05K 5/02 20060101 H05K005/02; C25D 11/08 20060101
C25D011/08; C25D 11/10 20060101 C25D011/10 |
Claims
1. A method of anodizing an aluminum alloy substrate, the method
comprising: forming a metal oxide film on the aluminum alloy
substrate by anodizing the aluminum alloy substrate in a first
electrolyte, the metal oxide film including a porous layer and a
barrier layer; and increasing a thickness layer of the barrier
layer by anodizing the aluminum alloy substrate in a second
electrolyte different than the first electrolyte, wherein a final
thickness of barrier layer ranges between about 50 nanometers to
about 500 nanometers, wherein the porous layer includes pores
having diameters ranging between about 10 nanometers to about 30
nanometers.
2. The method of claim 1, wherein a hardness of the metal oxide
film on the aluminum alloy substrate is about 200 HV or
greater.
3. The method of claim 1, wherein the first electrolyte includes
sulfuric acid, oxalic acid, or a mixture of sulfuric acid and
oxalic acid.
4. The method of claim 1, wherein the second electrolyte includes
at least one of Na.sub.2B.sub.4O.sub.5(OH).sub.4.8H.sub.2O (sodium
borate or borax), H.sub.3BO.sub.3 (boric acid),
C.sub.4H.sub.6O.sub.6 (tartaric acid),
(NH.sub.4).sub.2.5B.sub.2O.sub.3.8H.sub.2O (ammonium pentaborate
octahydrate), (NH.sub.4).sub.2B.sub.4O.sub.7.4H.sub.2O (ammonium
tetraborate tetrahydrate), or C.sub.6H.sub.10O.sub.4 (hexanedioic
acid or adipic acid).
5. The method of claim 1, wherein a thickness of the porous layer
is between about 6 micrometers and about 30 micrometers.
6. An anodized part, comprising: an aluminum alloy substrate; and
an anodic film disposed on the aluminum alloy substrate, the anodic
film including: an exterior oxide layer having an outer surface
corresponding to an outer surface of the anodized part, wherein the
exterior oxide layer includes pores having diameters ranging
between about 10 nanometers to about 30 nanometers, and a barrier
layer positioned between the exterior oxide layer and the aluminum
alloy substrate, wherein a thickness of the barrier layer ranges
between about 50 nanometers to 500 about nanometers.
7. The anodized part of claim 6, wherein the pores have diameters
ranging between about 10 nanometers to about 20 nanometers.
8. The anodized part of claim 6, wherein the pores are defined by
pore walls having thicknesses ranging between about 10 nanometers
to about 30 nanometers.
9. The anodized part of claim 6, wherein the aluminum alloy
substrate includes a 7000 series aluminum alloy or a 2000 series
aluminum alloy.
10. The anodized part of claim 6, wherein the aluminum alloy
substrate includes at least 4.0% by weight of zinc and at least
0.5% by weight of copper.
11. The anodized part of claim 6, wherein a thickness of the
exterior oxide layer ranges between about 6 micrometers to about 30
micrometers.
12. The anodized part of claim 6, wherein the anodic film has a
hardness value of about 200 HV or greater.
13. The anodized part of claim 6, wherein the anodic film has a
black dye incorporated therein, wherein an L* value of the anodic
film changes by no more than 9 after a salt-spray test per ASTM
B117 standards or after an ocean water test per ASTM D1141-98
standards.
14. An enclosure for an electronic device, the enclosure
comprising: an aluminum alloy substrate having at least 4.0% by
weight of zinc; and an anodic coating disposed on the aluminum
alloy substrate, the anodic coating including: an exterior oxide
layer having sealed pores having diameters ranging between about 10
nanometers to about 30 nanometers, and a barrier layer positioned
between the exterior oxide layer and the aluminum alloy substrate,
wherein a thickness of the barrier layer ranges between about 30
nanometers to about 500 nanometers.
15. The enclosure of claim 14, wherein a thickness of the exterior
oxide layer ranges between about 6 micrometers to about 30
micrometers.
16. The enclosure of claim 14, wherein the anodic coating as
measured from an exterior surface of the anodic coating has a
hardness value of about 200 HV or greater.
17. The enclosure of claim 14, wherein the pores have diameters
ranging from about 10 nanometers and about 20 nanometers.
18. The enclosure of claim 14, wherein the pores are defined by
pore walls having thicknesses ranging from about 10 nanometers to
about 30 nanometers.
19. The enclosure of claim 14, wherein the aluminum alloy substrate
has at least 5.4% by weight of zinc.
20. The enclosure of claim 14, wherein the aluminum alloy substrate
includes at least 0.5% by weight of copper.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority under 35 U.S.C.
.sctn.119(e) to U.S. Provisional Patent Application No. 62/249,079,
filed Oct. 30, 2015, and entitled "ANODIZED FILMS WITH PIGMENT
COLORING," which is incorporated herein by reference in its
entirety and for all purposes.
[0002] Any publications, patents, and patent applications referred
to in the instant specification are herein incorporated by
reference in their entireties. To the extent that the publications,
patents, or patent applications incorporated by reference
contradict the disclosure contained in the instant specification,
the instant specification is intended to supersede and/or take
precedence over any such contradictory material.
FIELD
[0003] The described embodiments relate to anodized films with
enhanced corrosion protection properties that are useful for
protecting high performance aluminum alloys. Described are methods
for forming anodic films that include structural features that
reduce deleterious of defects within the anodic films, thereby
increasing corrosion protection of an underlying alloy
substrate.
BACKGROUND
[0004] Anodizing is an electrochemical process that thickens a
naturally occurring protective oxide on a metal surface. An
anodizing process involves converting part of a metal surface to an
anodic film. Thus, an anodic film becomes an integral part of the
metal surface. Due to its relative hardness, an anodic film can
provide corrosion resistance and wear protection for an underlying
metal. In addition, an anodic film can enhance a cosmetic
appearance of a metal surface. For example, anodic films can have a
porous microstructure that can be infused with dyes to impart a
desired color to the anodic films.
[0005] When conventional anodizing methods are applied to some high
performance aluminum alloys, however, certain types of defects can
form within the anodic film. These defects can act as entry points
for water or other corrosion-inducing agents to enter the anodic
film and cause corrosion of the underlying metal substrate. What is
needed therefore are improved methods for forming corrosion
preventing and cosmetically appealing anodic films on high
performance alloys.
SUMMARY
[0006] This paper describes various embodiments that relate to
anodic films on high performance alloys, such as high strength
aluminum alloys, and methods for forming the same. The anodic films
can provide increased corrosion protection for the high performance
alloys.
[0007] According to one embodiment, a method of anodizing an
aluminum alloy substrate is described. The method includes forming
a metal oxide film on the aluminum alloy substrate by anodizing the
aluminum alloy substrate in a first electrolyte. The metal oxide
film includes a porous layer and a barrier layer. The method also
includes increasing a thickness layer of the barrier layer by
anodizing the aluminum alloy substrate in a second electrolyte
different than the first electrolyte. A final thickness of barrier
layer ranges between about 30 nanometers to 500 about
nanometers--in some cases, ranging between about 50 nanometers to
about 500 nanometers. The porous layer includes pores having
diameters ranging between about 10 nanometers to about 30
nanometers--in some cases, ranging between about 10 nanometers to
about 20 nanometers. In some embodiments, the pores are defined by
pore walls have thicknesses ranging between about 10 nanometers to
about 30 nanometers.
[0008] According to another embodiment, an anodized part is
described. The anodized part includes an aluminum alloy substrate
and an anodic film disposed on the aluminum alloy substrate. The
anodic film includes an exterior oxide layer having an outer
surface corresponding to an outer surface of the anodized part. The
exterior oxide layer includes pores having diameters ranging from
about 10 nanometers to about 30 nanometers. The anodic film also
includes a barrier layer positioned between the exterior oxide
layer and the aluminum alloy substrate. A thickness of the barrier
layer ranges between about 30 nanometers and 500 about
nanometers--in some cases, ranging between about 50 nanometers to
about 500 nanometers.
[0009] According to a further embodiment, an enclosure for an
electronic device is described. The enclosure includes an aluminum
alloy substrate having at least 4.0% by weight of zinc--in some
cases, at least 5.4% by weight of zinc. The enclosure also includes
an anodic coating disposed on the aluminum alloy substrate. The
anodic coating includes an exterior oxide layer having sealed pores
defined by pore walls. The sealed pores have diameters ranging
between about 10 nanometers to about 30 nanometers--in some cases,
ranging between about 10 nanometers and about 20 nanometers. The
anodic coating also includes a barrier layer positioned between the
exterior oxide layer and the substrate. A thickness of the barrier
layer ranges between about 30 nanometers to 500 about
nanometers--in some cases, ranging between about 50 nanometers to
about 500 nanometers.
[0010] These and other embodiments will be described in detail
below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The disclosure will be readily understood by the following
detailed description in conjunction with the accompanying drawings,
wherein like reference numerals designate like structural
elements.
[0012] FIG. 1 shows perspective views of devices having metallic
surfaces that can be protected using anodic films described
herein.
[0013] FIG. 2 shows a cross section view of an anodized part
illustrating how using a conventional anodizing process on high
performance alloys can cause defects within an anodic film.
[0014] FIGS. 3A-3D show cross section views of an anodized part
with enhanced corrosion and aesthetic characteristics, in
accordance with some embodiments.
[0015] FIG. 4 shows a flowchart indicating a process for forming a
metal oxide coating, in accordance with some embodiments.
[0016] FIGS. 5A and 5B show TEM cross section images of anodic
films prior to a barrier layer thickening process, in accordance
with some embodiments.
[0017] FIGS. 6A and 6B show TEM cross section images of the anodic
films of FIGS. 5A and 5B after a barrier layer thickening process,
in accordance with some embodiments.
[0018] FIGS. 7A and 7B show SEM cross section images of anodic
films prior to and after a barrier layer thickening process,
respectively, in accordance with some embodiments.
[0019] FIGS. 8-12 show aluminum alloy samples with and without a
thickened barrier layer before and after a salt-spray test and an
ocean water test, indicating the effectiveness of a thickened
barrier layer for protecting an underlying substrate from
corrosion.
DETAILED DESCRIPTION
[0020] Reference will now be made in detail to representative
embodiments illustrated in the accompanying drawings. It should be
understood that the following descriptions are not intended to
limit the embodiments to one preferred embodiment. To the contrary,
it is intended to cover alternatives, modifications, and
equivalents as can be included within the spirit and scope of the
described embodiments as defined by the appended claims.
[0021] Described herein are processes for providing anodic films
that provide superior corrosion protection and cosmetic qualities
to high performance aluminum alloys. In particular embodiments, the
anodic films have a dense exterior porous layer, which can
correspond to an outer layer of the anodic film. The pore walls of
the porous layer can be thicker than conventional anodic films,
thereby providing a high hardness and high chemical resistivity.
The porous layer can include pores that can hold colorants, such as
dyes or pigments, thereby providing cosmetic qualities to the
anodic film. The anodic films can also include a thickened
non-porous barrier layer positioned beneath the porous layer. The
dense porous layer and the thickened barrier layer can ameliorate
corrosion susceptibility due to the presence of defects within the
anodic film associated with certain alloying elements of high
performance aluminum alloys. In these ways, the anodic films can
provide a cosmetically appealing and high corrosion resistance
coating to the underlying high performance aluminum alloy.
[0022] Methods for forming the anodic films can include using a
first anodizing electrolyte to form a porous layer, and a second
anodizing electrolyte to thicken an existing a non-porous barrier
layer. In some embodiments, the first electrolyte includes oxalic
acid or sulfuric acid, under conditions that can form a relatively
dense and chemically resistant porous layer. The second electrolyte
can include a non-dissolution chemical, such as borax or boric
acid. The anodic film can be sealed using a sealing process to
further increase its chemical resistance and corrosion resistance.
The resultant anodic film can have a hardness of at least 200 HV
and a corrosion resistance of about 312 hours using salt spray
testing. In some embodiments, the anodic film is colorized using a
dye or pigment. In some embodiments, a final color of the anodic
film is determined by adjusting one or more of the first
electrolyte, the thickness of the barrier layer, the smoothness of
the barrier layer, or type of colorant infused within pores of the
anodic film.
[0023] The present paper makes reference to anodizing of aluminum
and aluminum alloy substrates. It should be understood, however,
that the methods described herein may be applicable to any of a
number of other suitable anodizable metal substrates, such as
suitable alloys of titanium, zinc, magnesium, niobium, zirconium,
hafnium, and tantalum, or suitable combinations thereof. As used
herein, the terms anodized film, anodized coating, anodic oxide,
anodic coating, anodic film, anodic layer, anodic coating, anodic
oxide film, anodic oxide layer, anodic oxide coating, metal oxide
film, metal oxide layer, metal oxide coating, oxide film, oxide
layer, oxide coating etc. can be used interchangeably and can refer
to suitable metal oxides, unless otherwise specified.
[0024] Methods described herein are well suited for providing
cosmetically appealing surface finishes to consumer products. For
example, the methods described herein can be used to form durable
and cosmetically appealing finishes for housing for computers,
portable electronic devices, wearable electronic devices, and
electronic device accessories, such as those manufactured by Apple
Inc., based in Cupertino, Calif.
[0025] These and other embodiments are discussed below with
reference to FIGS. 1-12. However, those skilled in the art will
readily appreciate that the detailed description given herein with
respect to these Figures is for explanatory purposes only and
should not be construed as limiting.
[0026] The methods described herein can be used to form durable and
cosmetically appealing coatings for metallic surfaces of consumer
devices. FIG. 1 shows consumer products that can be manufactured
using methods described herein. FIG. 1 includes portable phone 102,
tablet computer 104, smart watch 106 and portable computer 108,
which can each include housings that are made of metal or have
metal sections. Aluminum alloys can be a choice metal material due
to their light weight and ability to anodize and form a protective
anodic oxide coating that protects the metal surfaces from
scratches. The choice of aluminum alloy type will depend on desired
physical and cosmetic characteristics. For example, some 6000
series aluminum alloys can provide excellent corrosion resistance
and be anodized to form cosmetically appealing anodic oxide
coatings. For example, some 6000 series aluminum alloys (e.g., some
6063 aluminum alloy substrates) can be anodized using type II
anodizing (as defined by U.S. military specification MIL-A-8625),
which involves anodizing in sulfuric acid based solution, to form
relatively translucent and cosmetically appealing anodic oxide
coating.
[0027] Some 2000 and 7000 series aluminum alloys are considered
high performance aluminum alloys since they can have high
mechanical strength. For this reason, it may be desirable to form
housings for electronic devices using these high performance
aluminum alloys. However, these high performance aluminum alloys
can be more susceptible to corrosion due to the relatively high
concentrations of certain alloying elements. Anodizing can help
protect exposed surfaces of these high performance aluminum alloys.
However, anodizing high performance aluminum alloys can result in
anodic oxide coatings that include defects, thought to be related
to some of these alloying elements. For example, some 2000 series
aluminum alloys can have relatively large concentrations of copper,
and some 7000 series aluminum alloys can have relatively large
concentrations of zinc. These defects within the anodic oxide
coatings can act as entry points for water or other corrosion
inducing agents to penetrate the anodic oxide coatings and reach
the underlying aluminum alloys substrates.
[0028] Described herein are improved techniques for providing
improved anodic oxide coatings for high performance aluminum alloys
that prevent or reduce the occurrence of such corrosion-related
defects. Note that the methods can also be used for providing
anodic oxide coatings for aluminum alloys that are not considered
high performance, or other suitable anodizable substrates. For
example, although some 2000 series and some 7000 series aluminum
alloys may benefit from the anodic oxide coating described herein,
6000 series aluminum alloys (e.g., 6063 aluminum alloys) may also
benefit from having the anodic oxide coating described herein over
conventional anodic oxide coatings.
[0029] FIG. 2 shows a cross section view of a surface portion of
anodized part 200. Part 200 includes metal substrate 202 and metal
oxide coating 204. Metal substrate can be composed of a high
strength aluminum alloy, which includes alloying elements that
enhance the mechanical strength of metal substrate 202. Metal oxide
coating 204 can be formed using an anodizing process whereby a
surface portion of metal substrate 202 is converted to a
corresponding metal oxide material 203 (e.g., aluminum oxide).
Metal oxide coating 204 includes pores 206, formed during the
anodizing process, defined by pore walls 205. The size of pores 206
can vary depending on the anodizing process conditions. For
example, some type II anodizing processes can result in pores 206
having diameters of about 20 nanometers to about 30 nanometers.
Metal oxide coating 204 includes porous layer 201 (defined by
thickness 212) and a barrier layer 209 (defined by thickness 214),
which corresponds to a generally non-porous portion of metal oxide
coating 204 between metal substrate 202 and porous portion 201.
Thickness 214 of barrier layer 209 is typically on the order of 10
nanometers to about 30 nanometers.
[0030] As shown, defects 207 can form within metal oxide coating
204. Defects 207 can correspond to inconsistencies within the
structure of metal oxide material 203--in some cases defects 207
are in the form of cracks. Defects 207 can be associated with the
type and amount of alloying elements within metal substrate 202.
For example, defects 207 can be associated with relatively high
concentrations of zinc or copper, which can be found in some 7000
series alloys and some 2000 series alloys, respectively. Defects
207 can be small, sometimes in the order of nanometers or tens of
nanometers (e.g., as small as around 10 nm). However, some defects
207 are large enough to span thickness 214 of barrier layer 209.
For example, defects 207 can connect with each other, thereby
spanning thickness 214. In some cases, defects 207, such as cracks,
can become bigger during manufacture process or service lifetime of
anodized part 300. For example, thermal cycling can cause small
crack defects to become larger. In this way, defects 207 can act as
entry points for water or other corrosion inducing agents to reach
metal substrate 202. For example, corrosion inducing agents can
enter exterior surface 210 of metal oxide coating 204 via pores
206, pass through barrier layer 209 via defects 207 and reach metal
substrate 202. In some cases, defects 207 can allow corrosion
inducing agents to reach metal substrate 202 even if pores 206 are
sealed using a hydrothermal sealing process. If metal substrate 202
is relatively susceptible to corrosion, such as some 7000 and 2000
series aluminum alloys, metal substrate 202 can corrode, thereby
degrading the adhesion of metal oxide coating 204 and the integrity
of anodized part 200.
[0031] Methods described herein involve forming metal oxide
coatings that provide improved corrosion protection for high
performance alloys. FIGS. 3A-3C illustrate cross section views of
part 300 undergoing an anodizing process, in accordance with some
described embodiments.
[0032] FIG. 3A shows part 300 after metal substrate 302 is anodized
using a first anodizing process. Metal substrate 302 can be any
suitable anodizable material, such as suitable aluminum, aluminum
alloys, magnesium, magnesium alloys, and suitable combinations
thereof. In some embodiments, metal substrate 302 is composed of a
high performance (e.g., high mechanical strength) aluminum alloy,
such as a 2000 series or a 7000 series aluminum alloy. In some
embodiments, metal substrate 302 is composed of a 6000 series
aluminum alloy, such as 6063 aluminum alloy. In some embodiments,
metal substrate 302 is composed of a 4000 series aluminum alloy,
such as a 4045 aluminum alloy. In some embodiments, the aluminum
alloy includes at least 4.0% by weight of zinc (e.g., minimum zinc
in some 7000 series aluminum alloys). In some applications, the
aluminum alloy includes at least 5.4% by weight of zinc in order to
achieve at least a desired substrate hardness. In some embodiments,
the aluminum alloy includes at least 0.5% by weight of copper
(e.g., minimum copper in some 2000 series aluminum alloys).
[0033] The first anodizing process converts a portion of metal
substrate 302 to metal oxide coating 304, which include porous
layer 301 and barrier layer 309. Porous layer 301 includes pores
306, which are formed during the anodizing process, and barrier
layer 309 is generally free of pores 306 and is situated between
metal substrate 302 and porous layer 301. Porous layer 301 and
barrier layer 309 are both composed of metal oxide material 303,
the specific composition of which depends on the composition of
metal substrate 302. For example, an aluminum alloy metal substrate
302 can be converted to a corresponding aluminum oxide material
303.
[0034] In some cases where metal substrate 302 is composed of a
high performance substrate, defects 307 can form within metal oxide
coating 304. Defects 307 can be associated with certain alloying
elements within metal substrate 302, such as copper in some 2000
series aluminum alloys and zinc in some 7000 series aluminum
alloys. In some cases, defects 307 can include the alloying
element(s) (e.g., zinc or copper). In some cases, defects 307 are
in the form of cracks or voids within metal oxide coating 304.
Defects 307 can be randomly distributed within metal oxide coating
304 and can in some cases connect with each other. As described
above, defects 307 can act as pathways for corrosion inducing
agents to reach metal substrate 302.
[0035] In some embodiments, the first anodizing process can produce
a porous layer 301 that has a higher density of metal oxide
material 303 compared to conventional anodizing processes. For
example, pore walls 305 between pores 306 can be thicker than pore
walls of a standard type II anodizing process. In particular,
thickness 320 of pore walls 305 can range between about 10
nanometers and about 30 nanometers. Diameters 322 of pores 306 can
range between about 10 nanometers and about 30 nanometers. In some
embodiments, diameters 322 of pores 306 range between 10 nanometers
to about 20 nanometers. In addition, thickness 312 of porous layer
301 can be relatively thick compared to conventional anodizing
processes. For example, thickness 312 of porous layer 301 can
ranges between about 6 micrometers and about 30 micrometers--in
some embodiments ranges between about 10 micrometers and about 15
micrometers.
[0036] It should be noted that oxalic acid based anodizing can, in
some cases, cause metal oxide coating 304 to have a yellow hue,
sometimes associated with using an organic acid-based anodizing
bath. This yellow color may be desirable or undesirable, depending
on the application. For example, for exterior surfaces of consumer
products, it may be desirable to have a yellow hue. In some cases,
the yellow hue may be insignificant if the anodic oxide coating 304
is to be colorized by a dye or pigment. In other cases, it may be
preferable to have a neutral color and undesirable to have a yellow
hue. If a neutral color is desirable, the yellow hue can be offset
using barrier layer thickening techniques, which will be described
below with reference to FIG. 3B.
[0037] In some embodiments, the dense metal oxide coating 304 can
be accomplished using a sulfuric acid based anodizing electrolyte.
In particular embodiments, the electrolyte has a sulfuric acid
concentration ranges between about 180 g/l and about 210 g/l. The
temperature of the sulfuric acid based electrolyte ranges between
about 10 degrees C. and about 22 degrees C. In anodizing voltage
ranges between about 6 volts to about 20 volt, and the current
density ranges between about 0.5 A/dm.sup.2 and to about 2.0
A/dm.sup.2. The anodizing process time can vary depending on a
target thickness of metal oxide coating 304. In a particular
embodiment, the anodizing time period ranges from about 10 minutes
to about 100 minutes.
[0038] The higher density and thicker pore walls 305 of metal oxide
coating 304 enhances the structural integrity of metal oxide
coating 304 compared to conventional metal oxide coatings, despite
the presence of defects 307. That is, defects 307 can be
distributed within a more structurally dense metal oxide coating
304, thereby reducing the chance of defects 307 acting as entry
points for corrosion inducing agents to reach metal substrate
302.
[0039] FIG. 3B shows part 300 after a second anodizing process is
performed in order to further enhance the corrosion protection
ability of metal oxide coating 304. The second anodizing process
can promote anodic film growth without substantially promoting
anodic film dissolution, thereby increasing the thickness 314 of
barrier layer 309 to thickness t. This can be accomplished using a
non-pore-forming electrolyte, such as one or more of
Na.sub.2B.sub.4O.sub.5(OH).sub.4.8H.sub.2O (sodium borate or
borax), H.sub.3BO.sub.3 (boric acid), C.sub.4H.sub.6O.sub.6
(tartaric acid), (NH.sub.4).sub.2.5B.sub.2O.sub.3.8H.sub.2O
(ammonium pentaborate octahydrate),
(NH.sub.4).sub.2B.sub.4O.sub.7.4H.sub.2O (ammonium tetraborate
tetrahydrate), and C.sub.6H.sub.10O.sub.4 (hexanedioic acid or
adipic acid). Suitable barrier layer thickening processes are
described in detail in U.S. provisional application No. 62/249,079,
filed Oct. 30, 2015, which is incorporated herein in its
entirety.
[0040] Thickened barrier layer 309 can enhance the corrosion
protection characteristics of metal oxide coating 304 by providing
a thicker physical non-porous barrier between pores 306 and metal
substrate 302. The anodizing process parameters can be chosen in
order to provide a barrier layer 309 thickness t ranging from about
30 nanometers to about 500 nanometers. In some embodiments,
thickness t of barrier layer 309 is chosen based on providing a
color to metal oxide coating 304 by thin film interference
coloring. It should be noted that the barrier layer thickening
process can be performed without substantial change in the pore
structure of metal oxide coating 304. That is, diameter 322 of
pores can remain substantially the same before and after the
barrier layer thickening process.
[0041] In some embodiments, metal oxide coating 304 is colorized by
infusing a colorant, such as a dye, pigment or metal, within pores
306 and to impart a particular color to part 300. In some
embodiments where metal oxide coating 304 has a colored hue from
use of an oxalic acid or other organic acid electrolyte (e.g., from
the first anodizing process), the colored hue combines with and
enhances the color of the colorant. For example, a yellow hue
caused by anodizing in an organic acid can combine with a red
colorant to impart a darker or more orange aspect to metal oxide
coating 304. Likewise, a yellow hue caused by anodizing in an
organic acid can combine with a blue colorant to impart a green
aspect to metal oxide coating 304. In this way, any suitable
combination of color hues caused by anodizing in an organic acid
and colorant can be used to impart a final color to metal oxide
coating 304.
[0042] In addition to thickening barrier layer 309, anodizing in a
non-pore-forming electrolyte can also smooth out the boundaries of
barrier layer 309. For example, interface surface 316, which is
defined by barrier layer 309 on one side and metal substrate 302 on
another side, can have a smoother profile compared to the scalloped
geometry prior to the barrier layer thickening process. The
smoother and flatter interface surface 316 and/or pore terminuses
318 can increase the amount of visible light incident metal oxide
coating 304 that is specularly reflected, thereby increasing the
brightness of anodized part 300. Additionally or alternatively, the
barrier layer smoothing process can flatten or smooth pore
terminuses 318 of pores 306, such that flattened pore terminuses
318 can also specularly reflect incoming light. In this way, the
smooth (i.e., flat) interface surface 316 and/or pore terminuses
318 can cause light to specularly reflect off interface surface 316
and/or pore terminuses 318, resulting in brightening the appearance
of metal oxide coating 304.
[0043] In some embodiments, the barrier layer smoothing process is
necessary in order to accomplish a particular level of lightness or
a particular color, which can be measured using, for example, L*,
a* and b* values as defined by CIE 1976 L*a*b* color space model
standards. In general, L* indicates a level of lightness, with
higher L* values associated with higher levels of lightness.
Objects that reflect a yellow color will have a positive b* value
and objects that reflect a blue color will have a negative b*
value. Objects that reflect a magenta or red color will have a
positive a* value and objects that reflect a green color will have
a negative a* value. Some of these aspects are described in U.S.
provisional application No. 62/249,079, filed Oct. 30, 2015, which
is incorporated herein in its entirety.
[0044] The flatness or smoothness of interface surface 316 can be
quantified as a profile variance defined by distance d between an
adjacent peak and valley of the interface surface 316. Profile
variance distance d can be measured, for example, from a
transmission electron microscope (TEM) cross section image of the
part 300. In some embodiments, interface surface 316 achieves a
profile variance of no more than 5-6 nanometers.
[0045] FIG. 3C shows part 300 after colorant particles 311 are
optionally deposited within pores 306 to give metal oxide coating
304 a desired color. Colorant particles 311 can be composed of any
suitable colorant material, including suitable dye, pigment or
metal material. In some embodiments, colorant particles 311 include
black dye, such as Okuno Black 402 (manufactured by Okuno Chemical
Industries Co., Ltd., based in Japan), in order to give metal oxide
coating 304 a saturated black appearance. In some cases, a smoothed
interface surface 316 can brighten the color provided by colorant
particles 311. In some cases, thin film interference effects of
barrier layer 309 create a color that combines with a color
provided by colorant particles 311 to result in a final color.
Additionally or alternatively, discoloration of metal oxide coating
304 by anodizing in an organic acid based electrolyte (e.g., oxalic
acid) can combine with the a color provided by colorant particles
311 to result in a final color. That is, a final color of metal
oxide coating 304 can be a result of one or more of the
above-described brightening and color imparting techniques.
[0046] FIG. 3D shows part 300 after a pore sealing process is
performed in order to effectively close pores 306, thereby
enhancing the corrosion resistance, as well as chemical resistance,
of metal oxide coating 304. In addition, the sealing process can
make the outer surface of metal oxide coating 304 compatible for
touching from a user, such as a user of an electronic device.
Furthermore, if pores 306 include colorant particles 311, the
sealing process can retain colorant particles 311 within metal
oxide coating 304. The sealing process can hydrate the metal oxide
material 303 of at least top portions of pore walls 305 of metal
oxide coating 304. In particular, the sealing process can convert
metal oxide material 303 to its hydrated form 334, thereby causing
swelling of pore walls 305 and sealing of pores 306. The chemical
nature of hydrated metal oxide material 334 will depend on the
composition of metal oxide material 303. For example, aluminum
oxide (Al.sub.2O.sub.3) can be hydrated during the sealing process
to form boehmite or other hydrated forms of aluminum oxide. The
amount of hydration and sealing can vary depending on the sealing
process conditions. In some embodiments, only a top portion of
pores 306 of metal oxide coating 304, while in some embodiments
substantially the entire length of pores 306 of metal oxide coating
304 is sealed. Any suitable pore sealing process can be used,
including exposing part 300 to hot aqueous solution or steam. In
some cases, additives are added to the aqueous solution, such as
nickel acetate or commercial additives, such as Okuno Chemical H298
(manufactured by Okuno Chemical Industries Co., Ltd., based in
Japan).
[0047] After sealing, the metal oxide coating 304 can provide
superior hardness and scratch resistance to part 300, as well as
provide a desired cosmetic appearance to part 300. The relatively
greater density of metal oxide material 303 makes metal oxide
coating 304 harder and more chemically resistant than conventional
anodic oxide coating, which can be useful in applications where
metal oxide coating 304 corresponds to an exterior surface of a
consumer product (e.g., devices of FIG. 1). In some embodiments,
the metal oxide coating 304 on part 300 is characterized as having
a hardness value of at least 200 HV.
[0048] Corrosion resistance of part 300 can be measured using
standardized salt spray testing, such as per ASTM B117, ISO9227,
JIS Z 2371 and ASTM G85 standards. In particular embodiments, part
300 has a salt spray test corrosion resistance measurement of about
336 hours using ASTM B117 standard salt spay techniques. Corrosion
resistance can also be measured using ocean water testing, such as
per ASTM D1141-98 standards. Examples showing improved corrosion
resistance of samples having anodic films with thickened barrier
layers tested under salt spray and ocean water procedures are
described below with reference to FIGS. 8-12. This is a dramatic
improvement in comparison to a part having a standard type II metal
oxide coating (i.e., without barrier layer thickening). In some
embodiments, the final thickness of metal oxide coating 304
(including thickness 312 and thickness t) is between about 6
micrometers and about 30 micrometers.
[0049] FIG. 4 shows flowchart 400, which indicates a process for
forming a metal oxide coating in accordance with some embodiments.
At 402, a substrate undergoes an optional surface pretreatment. In
some embodiments, the surface pretreatment involves polishing a
surface of the substrate to a mirror polish reflection. In some
embodiments, the substrate surface is polished until the surface
achieve a gloss value of 1500 gloss units or greater, as measured
at 20 degree reflection. In a particular embodiment, the gloss
value is about 1650 gloss units as measured at 20 degree
reflection. The level of flatness/smoothness of the substrate
surface prior to anodizing can be important in some embodiments in
order to help achieve a sufficiently smooth barrier layer after a
barrier layer thickening process is performed (see FIG. 3C). In
other embodiments, the substrate undergoes a texturing process,
such as an abrasive blasting and/or a chemical etching process, in
order to form a blasted or matte texture to the substrate surface.
Other surface pretreatment processes can include degreasing and
de-smutting (e.g., exposure to a nitric acid solution for 1-3
minutes). Care should be taken on mirror polished surfaces,
however, to assure the degreasing and de-smutting do not
significantly damage the mirror polished surface of the substrate.
The substrate can be composed of any suitable anodizable material,
such as a suitable aluminum alloy.
[0050] At 404, a metal oxide coating is formed using a first
anodizing process. In some cases, the first anodizing process
involves using a first electrolyte that includes oxalic acid or
sulfuric acid. In some embodiments, the first electrolyte includes
a mixture of oxalic acid and sulfuric acid. In some embodiments, an
electrolyte having sulfuric acid in a concentration between about
180 g/L and about 210 g/L held at a temperature between about 10
degrees C. and about 22 degrees C. using a current density between
about 0.5 A/dm.sup.2 and about 2.0 A/dm.sup.2 was used to form a
porous metal oxide coating having a thickness between about 6
micrometers and about 30 micrometers.
[0051] At 406, the barrier layer of the metal oxide coating is
thickened using a second anodizing process, which can also be
referred to as a barrier layer thickening process. The barrier
layer thickening process can be performed in a non-pore forming
electrolyte. In some embodiments, the non-pore forming electrolyte
contains a non-pore forming agent, such as one or more of
Na.sub.2B.sub.4O.sub.5(OH).sub.4.8H.sub.2O (sodium borate or
borax), H.sub.3BO.sub.3 (boric acid), C.sub.4H.sub.6O.sub.6
(tartaric acid), (NH.sub.4).sub.2.5B.sub.2O.sub.3.8H.sub.2O
(ammonium pentaborate octahydrate),
(NH.sub.4).sub.2B.sub.4O.sub.7.4H.sub.2O (ammonium tetraborate
tetrahydrate), and C.sub.6H.sub.10O.sub.4 (hexanedioic acid or
adipic acid).
[0052] In some embodiments, the barrier layer thickening process
involves anodizing in an electrolyte including a non-pore forming
agent in a concentration of between about 10 g/L and 30 g/L held at
an anodizing temperature of between about 8 degrees C. and 40
degrees C. for a time period of between about 1 minute to 2 minutes
using a voltage between about 100 V and about 400 V. The voltage of
the anodizing process can vary depending, in part, on a desired
interference coloring provided by the barrier layer. In some
embodiments, a voltage of between about 200 volts and about 500
volts, with low current density, is used. In a particular
embodiment, a DC voltage is applied and increased at a rate of
about 1 volt/second until a voltage of between about 200 volts and
about 500 volts is achieved, which is maintained for about 5
minutes.
[0053] At 408, the metal oxide coating is optionally colored using
any suitable coloring process. In some embodiments, dye, pigment,
metal or a suitable combination thereof is deposited within pores
of the metal oxide coating in order to achieve a desired color. At
410, the metal oxide coating is sealed to seal at least top
portions of the pores within the metal oxide coating. This can
increase the mechanical strength and corrosion resistance of the
metal oxide coating.
[0054] FIGS. 5A and 5B show TEM cross section images of a part
after a type II anodizing process and prior to a barrier layer
thickening process. The part includes substrate 502, anodic oxide
film 504 and barrier layer 506. Substrate 502 is composed of a 6063
aluminum alloy. Anodic oxide film 504 is formed from a sulfuric
acid based (type II) anodizing process and has pore diameters
ranging between about 10 nanometers to about 20 nanometers. The
thickness of barrier layer 506 ranges between about 10 nanometer
and about 20 nanometers.
[0055] FIGS. 6A and 6B show TEM cross section images of the part in
FIGS. 5A and 5B after a barrier layer thickening process. In this
example, a electrolyte having borax was used. The thickness of
barrier layer 506 was increased to between about 60 nanometers and
about 70 nanometers, and was found to provide good corrosion
protection for substrate 502.
[0056] FIGS. 7A and 7B show SEM cross section images of anodic
films prior to and after a barrier layer thickening process is
performed. FIG. 7A shows part 700 after an anodizing process
converts a portion of substrate 702 to metal oxide layer 704.
Substrate 702 is composed of a 6063 aluminum alloy and was anodized
using a sulfuric acid based (type II) anodizing process. The
resulting metal oxide film layer 704 has pores 706 and a barrier
layer 709 having a thickness between about 10 nm and about 20 nm.
FIG. 7B shows part 700 after a barrier layer thickening process,
and where barrier layer 709 is thickened to thickness 710 of
between about 300 nm and about 500 nm.
[0057] Corrosion resistance evaluation of the anodic film having
the thickened barrier layer can be determined using any suitable
testing process. For example, the anodized substrate can be
subjected to a salt-spray test or ocean water test and then
inspected by eye and/or by color measurements to determine whether
there is a color change. FIGS. 8-12 show aluminum alloy samples
with and without a thickened barrier layer before and after a
salt-spray test and an ocean water test, indicating the
effectiveness of a thickened barrier layer for protecting an
underlying substrate from corrosion. Prior to the salt-spray or
ocean water testing, each sample in FIGS. 8-12 was dyed using a
black dye (i.e., Okuno Black 402) to create a saturated black color
in order to easily determine any color changes due to corrosion.
After the salt-spray or ocean water testing, the color of each
sample was visually evaluated and measured using standard CIE 1976
L*a*b* color space model measurements.
[0058] FIG. 8 shows perspective views of custom 6063 aluminum alloy
samples 802, 804, 806 and 808 before and after a salt-spray testing
procedure in accordance with ASTM B117 standard procedures for 336
hours. Sample 802 includes an type II anodic film without a
thickened barrier layer that was not subjected to a salt-spray
testing procedure. Sample 804 includes an type II anodic film
without a thickened barrier layer after being subjected to the
salt-spray testing procedure. Sample 806 includes an type II anodic
film with a thickened barrier layer that was not subjected to a
salt-spray testing procedure. Sample 808 includes an type II anodic
film with a thickened barrier layer after being subjected to the
salt-spray testing procedure. Table 1 below summarizes L*a*b*
values before and after the salt-spray testing procedure.
TABLE-US-00001 TABLE 1 6063 Aluminum Alloy (Custom) Salt-Spray
Testing Non-thickened barrier layer Thickened barrier layer L* a*
b* L* a* b* Before 29.10 -0.52 -2.65 30.95 -.90 -3.48 After 19.22
-0.64 -5.24 30.02 -0.83 -3.13 Difference 9.88 0.12 2.59 0.93 -0.07
-0.35
[0059] As indicated by FIG. 8, the sample 804 without the thickened
barrier layer is visibly much darker after the salt-spray testing
compared to the sample 808 with the thickened barrier layer after
the same salt-spray testing. Table 1 indicates a much larger
difference in L* and b* values for the sample 804 without the
thickened barrier layer compared to L* and b* values for the sample
808 with the thickened barrier layer.
[0060] FIG. 9 shows perspective views of market grade 6063 aluminum
alloy samples 902, 904, 906 and 908 before and after a salt-spray
testing procedure in accordance with ASTM B117 standard procedures
for 336 hours. Sample 902 includes an type II anodic film without a
thickened barrier layer that was not subjected to a salt-spray
testing procedure. Sample 904 includes an type II anodic film
without a thickened barrier layer after being subjected to the
salt-spray testing procedure. Sample 906 includes an type II anodic
film with a thickened barrier layer that was not subjected to a
salt-spray testing procedure. Sample 908 includes an type II anodic
film with a thickened barrier layer after being subjected to the
salt-spray testing procedure. Table 2 below summarizes L*a*b*
values of a market grade 6063 substrate samples before and after
the salt-spray testing procedure.
TABLE-US-00002 TABLE 2 6063 Aluminum Alloy (Market Grade)
Salt-Spray Testing Non-thickened barrier layer Thickened barrier
layer L* a* b* L* a* b* Before 28.31 -0.75 -2.87 31.84 -1.24 -3.23
After 16.67 -1.69 -5.39 24.13 -1.98 -5.70 Difference 11.64 0.94
2.52 7.71 0.74 2.47
[0061] FIG. 9 indicates that sample 904 without the thickened
barrier layer was visibly darker after the salt-spray testing
compared to the sample 908 with the thickened barrier layer after
the same salt-spray testing. Table 2 indicates a much larger
difference in L* values for sample 904 without the thickened
barrier layer compared to L* values for the sample 908 with the
thickened barrier layer.
[0062] FIG. 10 shows perspective views of a 7000 series aluminum
alloy samples 1002, 1004, 1006 and 1008 before and after a
salt-spray testing procedure in accordance with ASTM B117 standard
procedures for 336 hours. Sample 1002 includes an type II anodic
film without a thickened barrier layer that was not subjected to a
salt-spray testing procedure. Sample 1004 includes an type II
anodic film without a thickened barrier layer after being subjected
to the salt-spray testing procedure. Sample 1006 includes an type
II anodic film with a thickened barrier layer that was not
subjected to a salt-spray testing procedure. Sample 1008 includes
an type II anodic film with a thickened barrier layer after being
subjected to the salt-spray testing procedure. Table 3 below
summarizes L*a*b* values of the 7000 series alloy substrate before
and after the salt-spray testing procedure.
TABLE-US-00003 TABLE 3 7000 Series Aluminum Alloy Salt-Spray
Testing Non-thickened barrier layer Thickened barrier layer L* a*
b* L* a* b* Before 29.06 -0.61 -2.88 32.01 -1.11 -3.71 After 22.65
-1.06 -4.13 31.33 -1.07 -3.33 Difference 6.41 0.45 1.25 0.68 -0.04
-0.38
[0063] FIG. 10 indicates that sample 1004 without the thickened
barrier layer was visibly darker after the salt-spray testing
compared to the sample 1008 with the thickened barrier layer after
the same salt-spray testing. Table 3 indicates a much larger
difference in L* values for sample 1004 without the thickened
barrier layer compared to L* values for the sample 1008 with the
thickened barrier layer.
[0064] FIG. 11 shows perspective views of market grade 6063
aluminum alloy samples 1102, 1104, 1106 and 1108 before and after
an ocean water testing procedure in accordance with ASTM D1141-98
standard testing procedures (without heave metal). The ocean water
testing procedures in accordance with ASTM D1141-98, Formula a,
Table x1.1, Section 6 was used. In particular, 42 grams of "sea
salt" was mixed with 1 liter of deionized water, having a pH of 8.1
to 8.3. The samples were dipped for 168 hours.
[0065] Sample 1102 includes an type II anodic film without a
thickened barrier layer that was not subjected to ocean water
testing procedure. Sample 1104 includes an type II anodic film
without a thickened barrier layer after being subjected to the
ocean water testing procedure. Sample 1106 includes an type II
anodic film with a thickened barrier layer that was not subjected
to a ocean water testing procedure. Sample 1108 includes an type II
anodic film with a thickened barrier layer after being subjected to
the ocean water testing procedure. Table 4 below summarizes L*a*b*
values of the market grade 6063 aluminum alloy substrate before and
after the ocean water testing procedure.
TABLE-US-00004 TABLE 4 6063 Aluminum Alloy (Market Grade) Ocean
Water Testing Non-thickened barrier layer Thickened barrier layer
L* a* b* L* a* b* Before 28.42 -0.83 -3.12 31.04 -1.14 -3.46 After
18.55 -0.96 -3.79 28.13 -1.78 -2.57 Difference 9.87 0.13 0.67 2.91
0.64 -0.89
[0066] FIG. 11 indicates that sample 1104 without the thickened
barrier layer was visibly darker after the ocean water testing
compared to the sample 1108 with the thickened barrier layer after
the same ocean water testing. Table 4 indicates a much larger
difference in L* values for sample 1104 without the thickened
barrier layer compared to L* values for the sample 1108 with the
thickened barrier layer.
[0067] FIG. 12 shows perspective views of market grade 4045
aluminum alloy samples 1202, 1204, 1206 and 1208 before and after
an ocean water testing procedure in accordance with the same ASTM
D1141-98 standard testing procedures described above with reference
to FIG. 11. Sample 1202 includes an type II anodic film without a
thickened barrier layer that was not subjected to ocean water
testing procedure. Sample 1202 includes an type II anodic film
without a thickened barrier layer after being subjected to the
ocean water testing procedure. Sample 1206 includes an type II
anodic film with a thickened barrier layer that was not subjected
to a ocean water testing procedure. Sample 1208 includes an type II
anodic film with a thickened barrier layer after being subjected to
the ocean water testing procedure. Table 5 below summarizes L*a*b*
values of the market grade 4045 aluminum alloy substrate before and
after the ocean water testing procedure.
TABLE-US-00005 TABLE 5 4045 Aluminum Alloy (Market Grade) Ocean
Water Testing Non-thickened barrier layer Thickened barrier layer
L* a* b* L* a* b* Before 30.87 0.33 -0.16 31.14 -0.01 -6.42 After
27.24 0.91 1.34 28.15 -0.21 -6.28 Difference 3.63 -0.58 -1.50 2.99
0.20 -0.14
[0068] FIG. 12 indicates that sample 1204 without the thickened
barrier layer was visibly darker after the ocean water testing
compared to the sample 1208 with the thickened barrier layer after
the same ocean water testing. Table 5 indicates a much larger
difference in L* values for sample 1204 without the thickened
barrier layer compared to L* values for the sample 1208 with the
thickened barrier layer.
[0069] Results from described above with reference to FIGS. 8-12
and Tables 1-5 indicate that the barrier layer thickening processes
described herein can improve the corrosion resistance and
discoloration of any of a number of types of aluminum alloy
substrates. In some embodiments, the L* value of the anodic film
changes by no more than 9 after a salt-spray test per ASTM B117
standards or after an ocean water test per ASTM D1141-98
standards.
[0070] The foregoing description, for purposes of explanation, used
specific nomenclature to provide a thorough understanding of the
described embodiments. However, it will be apparent to one skilled
in the art that the specific details are not required in order to
practice the described embodiments. Thus, the foregoing
descriptions of the specific embodiments described herein are
presented for purposes of illustration and description. They are
not intended to be exhaustive or to limit the embodiments to the
precise forms disclosed. It will be apparent to one of ordinary
skill in the art that many modifications and variations are
possible in view of the above teachings.
* * * * *