U.S. patent application number 14/678868 was filed with the patent office on 2016-10-06 for process to mitigate grain texture differential growth rates in mirror-finish anodized aluminum.
The applicant listed for this patent is Apple Inc.. Invention is credited to William A. Counts, James A. Curran, Sean R. Novak.
Application Number | 20160289858 14/678868 |
Document ID | / |
Family ID | 57015610 |
Filed Date | 2016-10-06 |
United States Patent
Application |
20160289858 |
Kind Code |
A1 |
Curran; James A. ; et
al. |
October 6, 2016 |
PROCESS TO MITIGATE GRAIN TEXTURE DIFFERENTIAL GROWTH RATES IN
MIRROR-FINISH ANODIZED ALUMINUM
Abstract
Anodizing processes for providing durable and defect-free anodic
oxide films, well suited for anodizing highly reflective surfaces,
are described. In some embodiments, the anodizing electrolyte has a
sulfuric acid concentration substantially less than conventional
type II anodizing. In some embodiments, the electrolyte includes a
mixture of sulfuric acid and one or more organic acids. In further
embodiments, sulfuric acid is a relatively minor additive to an
organic acid, primarily serving to minimize discoloration. The
processes enables porous, optically clear, and colorless anodic
films to be grown in a manner similar to conventional Type II
sulfuric acid anodizing, but at lower current densities and/or
higher temperatures, without compromising film surface hardness.
The thickness uniformity of the resulting anodic oxide films can be
within 5% between grains of {111}, {110} and {100} surface
orientations. Furthermore, the anodic oxide films have minimal
incorporated sulfates, thereby avoiding certain cosmetic and
structural defects.
Inventors: |
Curran; James A.; (Morgan
Hill, CA) ; Counts; William A.; (Sunnyvale, CA)
; Novak; Sean R.; (San Jose, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Apple Inc. |
Cupertino |
CA |
US |
|
|
Family ID: |
57015610 |
Appl. No.: |
14/678868 |
Filed: |
April 3, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25D 11/10 20130101;
C25D 11/16 20130101; C25D 11/18 20130101; H05K 5/02 20130101; C25D
11/08 20130101 |
International
Class: |
C25D 11/10 20060101
C25D011/10; C25D 11/08 20060101 C25D011/08; C25D 11/16 20060101
C25D011/16 |
Claims
1. A method of forming an anodic film, the method comprising:
anodizing an aluminum alloy substrate in an electrolyte using a
current density of no greater than 1 A/dm.sup.2 and/or an
electrolyte temperature of no less than 30 degrees C. such that the
resultant anodic film has a hardness value of no less than 320
HV.sub.0.05 and a thickness variation of less than 5% between the
anodic film on grains having {111}, {110} and {100}
crystallographic orientations.
2. The method of claim 1, wherein the electrolyte has a sulfuric
acid concentration no greater than 7% by weight.
3. The method of claim 1, wherein the electrolyte comprises
sulfuric acid and an organic acid.
4. The method of claim 3, wherein after anodizing, the anodic film
has an average concentration of sulfur of no greater than 4% by
weight.
5. The method of claim 3, wherein the organic acid comprises at
least one of oxalic acid, glycolic acid, tartaric acid, malic acid,
citric acid, and malonic acid.
6. The method of claim 3, wherein the electrolyte has a higher
concentration of organic acid compared to sulfuric acid.
7. The method of claim 6, wherein the organic acid is oxalic acid,
wherein the electrolyte has an oxalic acid concentration of at
least 20 g/L and a sulfuric acid concentration between 5 g/L and 20
g/L, and wherein the resultant anodic film is grown to a thickness
of at least 10 micrometers and is colorless with an a* of less than
1 and a b* of less than 1 as measured in accordance with CIE 1976
L*a*b* color space.
8. The method of claim 1, wherein the aluminum alloy substrate
comprises zinc, copper and/or magnesium.
9. The method of claim 1, wherein the current density is no greater
than 0.75 A/dm.sup.2.
10. The method of claim 1, wherein the resultant anodic film is
grown to a thickness of at least 10 micrometers and is colorless
with an a* of less than 1 and a b* of less than 1 as measured in
accordance with CIE 1976 L*a*b* color space.
11. A method of forming an aluminum oxide coating, the method
comprising: anodizing an aluminum or aluminum alloy substrate in an
electrolyte with a sulfuric acid concentration ranging between 5
g/L and 70 g/L, wherein the electrolyte optionally includes one or
more organic acids at an organic acid concentration ranging between
10 g/L and 100 g/L.
12. The method of claim 11, wherein the electrolyte includes an
organic acid, and wherein the electrolyte has a higher sulfuric
acid concentration than organic acid concentration.
13. The method of claim 11, wherein sulfuric acid concentration
ranges between 5 g/L to 20 g/L and the organic acid concentration
is at least 20 g/L, resulting in a colorless aluminum oxide coating
with an a* of less than 1 and a b* of less than 1 as measured in
accordance with CIE 1976 L*a*b* color space, and wherein the
aluminum oxide coating is grown to a thickness of 10 micrometers or
greater.
14. The method of claim 11, wherein anodizing the aluminum or
aluminum alloy substrate comprises anodizing using a current
density no greater than 1 A/dm.sup.2.
15. The method of claim 11, wherein anodizing the aluminum or
aluminum alloy substrate comprises anodizing using a temperature
ranging between 25.degree. C. and 40.degree. C.
16. The method of claim 11, wherein after anodizing, the aluminum
oxide coating has a hardness value of no less than 320
HV.sub.0.05.
17. The method of claim 11, wherein after anodizing, the aluminum
oxide coating has an average concentration of sulfur of no greater
than 4% by weight.
18. The method of claim 11, further comprising forming a highly
reflective surface on the aluminum or aluminum alloy substrate,
wherein after the anodizing the highly reflective surface is
visible through the aluminum oxide coating and is uniform in
thickness to within 5% on aluminum alloy substrate grains having
{111}, {110} and {100} crystallographic orientations such that an
interface between the aluminum or aluminum alloy substrate and the
aluminum oxide coating is substantially free of indentations.
19. A metal housing for an electronic device, the metal housing
comprising: an aluminum alloy substrate comprising copper, zinc
and/or magnesium; and an anodic oxide comprising no greater than 4%
by weight of sulfur, wherein the anodic oxide has a hardness value
of no less than 320 HV.sub.0.05.
20. The metal housing of claim 19, wherein the anodic oxide has a
thickness variation of less than 5% on aluminum alloy substrate
grains having {111}, {110} and {100} crystallographic orientations.
Description
FIELD
[0001] The described embodiments relate generally to anodized films
and methods for forming the same. More particularly, the present
embodiments relate to methods for producing defect-free anodized
films on highly polished metal substrates.
BACKGROUND
[0002] The surfaces of many products in the commercial and consumer
industries may be treated by any number of processes to alter the
surface and create a desired effect, either functional, cosmetic,
or both. One example of such a surface treatment is anodizing of a
metal substrate. Anodizing converts a portion of the metal
substrate into a metal oxide, thereby creating a metal oxide layer,
which is generally harder than the underlying metal substrate and
therefore acts as a protective layer. A well-known anodizing
method, often referred to as Type II anodizing, has been found to
provide metal oxide layers with good corrosion and wear resistance
for many consumer products.
[0003] The surface of the metal substrate can be treated prior to
an anodizing treatment to give the substrate a desired texture. In
some cases, the substrate is lapped or polished smooth, providing a
mirror shine finish to substrate. It has been found, however, that
on certain aluminum alloys (and notably on 7000-series aluminum),
conventional Type II anodizing of a highly polished substrate can
cause grain-to-grain thickness variations which give the anodized
surface an "orange-peel" like texture, and in some more severe
cases, can cause tiny but visible indentations or pits to form at
the metal/oxide interface, corresponding to the grain structure of
the underlying metal substrate. These tiny pits are scattered along
the entire surface the substrate. Although these pits are very
small, they can detract from the pristine look of the mirror
polished substrate.
SUMMARY
[0004] This paper describes various embodiments that relate to
anodizing processes and anodic oxide coatings using the same.
Although they may be applied to any aluminum alloy, they are of
particular relevance to certain alloys (such as the 7000-series
aluminum used by Apple Inc., based in Cupertino, Calif.) where
alloying elements such as zinc, copper, manganese and magnesium
result in certain defects in the anodic oxide film. The methods can
be used to provide durable and defect-free anodized films of great
thickness uniformity, specifically less than 5% variation in
thickness between grains of any surface orientation, giving
improved anodic oxide cosmetics, especially on highly polished
substrate surfaces.
[0005] According to one embodiment, a method of forming an anodic
film is described. The method includes anodizing a substrate in an
electrolyte comprising no greater than 7% sulfuric acid by weight,
using a current density of no greater than 1 A/dm.sup.2, such that
the resultant anodic oxide film is uniform in thickness to within
5%, irrespective of the surface orientation of grains, and has a
hardness of no less than 320 HV.sub.0.05.
[0006] According to another embodiment, a method of forming an
aluminum oxide coating is described. The method includes anodizing
an aluminum or aluminum alloy substrate in an electrolyte with a
sulfuric acid concentration ranging between 5 g/L and 70 g/L. The
electrolyte optionally includes one or more organic acids at an
organic acid concentration ranging between 10 g/L and 100 g/L.
[0007] According to a third embodiment, a method of forming an
anodic film is described. The method involves anodizing a substrate
in an electrolyte, which is predominantly comprised of organic acid
(ranging from 20 g/L to 100 g/L), with a relatively minor addition
of sulfuric acid (5 g/L to 20 g/L). This electrolyte yields a
colorless anodic oxide film of great thickness uniformity (less
than 5% variation from grain to grain), and hardness of no less
than 320 HV.sub.0.05, even when operated at high temperatures (25 C
or higher) and/or low current densities (1 A/dm.sup.2 or less).
[0008] According to a further embodiment, a metal housing for an
electronic device is described. The metal housing includes an
anodic film having no greater than 4% by weight of sulfur. This low
sulfur content avoids interfacial adhesion problems associated with
the accumulation of elements such as zinc at the interface between
the metal and the oxide during anodizing. The anodic film has a
hardness value of no less than 320 HV.sub.0.05 as measured by
Vickers hardness test.
[0009] According to another embodiment, a method of forming an
anodic film is described. The method includes anodizing an aluminum
alloy substrate in an electrolyte using a current density of no
greater than 1 A/dm.sup.2 and/or an electrolyte temperature of no
less than 30 degrees C. such that the resultant anodic film has a
hardness value of no less than 320 HV.sub.0.05.
[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 consumer products than can be manufactured
using anodizing methods described herein.
[0013] FIGS. 2A and 2B show cross section views of a part
undergoing a conventional Type II anodizing process.
[0014] FIGS. 3A and 3B show graphs indicating hardness of anodic
oxide coatings as a function of anodizing time and anodizing
current density.
[0015] FIGS. 4A and 4B show cross section views of a part
undergoing an anodizing process in accordance with some described
embodiments.
[0016] FIG. 5 shows a flowchart indicating an anodizing process in
accordance with some described embodiments.
DETAILED DESCRIPTION
[0017] 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.
[0018] The following disclosure relates to anodizing processes that
result in cosmetically appealing and durable anodic oxide films.
The anodizing processes described herein can be used as
alternatives to conventional Type II anodizing processes, which
have been found to cause certain visible defects associated with
the grain orientations of the underlying metal substrate. The
anodizing processes described herein can be used to form protective
coatings without introducing these visible defects even when
performed on highly visible surface, such highly polished and
reflective metal surfaces.
[0019] In some embodiments, the anodizing processes include using
an electrolyte with dilute concentrations of sulfuric acid compared
to Type II anodizing processes. In particular embodiments, the
sulfuric acid concentration is 70 g/L or less, and in some cases
ranges between 5 g/L to 20 g/L. This is compared to conventional
Type II anodizing electrolytes that typically have sulfuric acid
concentrations ranging between 10-20% by weight. In some
embodiments, the electrolyte includes a mixture of sulfuric acid
with one or more organic acids. In a particular embodiment, the
total concentration of organic acid within the electrolyte ranges
between 10-100 g/L. In a further embodiment, the electrolyte
mixture is predominantly comprised of organic acid (20 g/L to 100
g/L) with sulfuric acid as a relatively minor additive (5 g/L to 20
g/L).
[0020] Because the electrolyte has a lower concentration of
sulfuric acid, it dissolves the anodic oxide film during the
anodizing process at a lower rate than a conventional sulfuric acid
electrolyte, which enables porous, optically clear, and colorless
films to be grown in a manner similar to conventional Type II
sulfuric acid anodizing, but at lower current densities (1
A/dm.sup.2 or lower) and/or higher temperatures (25-40 degrees C.),
without compromising the metal oxide film surface hardness relative
to conventional type II sulfuric acid (specifically, the about 320
HV.sub.0.05 hardness measured on films grown to 10 micrometers
thickness at 20 degrees C. and 1.5 A/dm.sup.2 in 200 g/L sulfuric
acid). The lower sulfuric acid concentration electrolytes can also
result in minimal incorporation of acid anions into the
particularly when anodizing is also performed at relatively high
temperatures (e.g., 30 or 35 degrees C.) and/or relatively low
current densities (e.g., no greater than 1 A/dm2). Thus, the anodic
oxide films can have sulfur concentrations of less than 4% by
weight. This can be of particular benefit in avoiding a propensity
for low interfacial adhesion of anodic oxides to 7000-series
aluminum alloys (where zinc enrichment occurs at the oxide
interface, combining with sulfur to weaken the interface).
[0021] The anodizing methods described herein can be applied to
substrates made of any suitable anodizable material. Although
particular reference is made to 7000-series aluminum alloys, and to
alloys comprising zinc, copper, manganese and magnesium, the method
could be applied to other aluminum alloys where similar mechanisms
of differential growth rates on different grain orientations occur,
or where interfacial enrichment of alloying elements weakens an
anodic oxide adhesion. As described herein, the terms "anodic
film," "anodic oxide," "anodic layer," "anodic oxide," "anodic
oxide film," "anodic oxide layer," "anodic oxide coating" "metal
oxide," "metal oxide film," "metal oxide layer," and "metal oxide
coating" can be used interchangeably.
[0022] 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 and electronic device accessories, such
as those manufactured by Apple Inc., based in Cupertino, Calif.
[0023] These and other embodiments are discussed below with
reference to FIGS. 1-5. 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.
[0024] Methods described herein can be used to form durable and
cosmetically appealing coatings for metallic surfaces of consumer
devices. FIG. 1 shows consumer products than can be manufactured
using methods described herein. FIG. 1 includes portable phone 102,
tablet computer 104 and portable computer 106, which can each
include metal surfaces. Devices 102, 104 and 106 can be subject to
impact forces such as scratching, dropping, abrading, chipping and
gouging forces during normal use. Certain alloys (such as
7000-series aluminum) are selected for making the enclosures of
such devices, often driven by requirements such as high strength
and hardness. Metal surfaces of devices 102, 104 and 106 are
typically anodized in order to add a protective anodic oxide
coating to these metal surfaces. However, it has been found that
use of conventional Type II anodizing process can cause visual
defects on these anodized surfaces, detracting from the aesthetic
appeal of devices 102, 104 and 106. These visual defects can be
particularly severe with higher strength or hardness alloys. If the
metal surfaces are highly polished and reflective, these visual
defects can be even more apparent. In addition, depending on the
type of metal alloys used, conventional Type II anodizing can
result in an anodic oxide coating that is prone to chipping and
scratching from impact forces. The anodizing methods described
herein provide anodic oxide coatings having improved cosmetic
qualities and resistance to chipping and delamination compared to
anodic oxide coatings formed using conventional Type II anodizing
processes.
[0025] Aluminum and aluminum alloys can exhibit a highly reflective
surface when lapped or polished to a smooth finish. This
mirror-like finish may be protected against abrasive wear by
applying a substantially transparent anodic oxide, such as that
formed by Type II sulfuric acid anodizing (or simply Type II
anodizing) in accordance with the Aluminum Anodizing Council's
(AAC) Military Specification Mil-A-8625. Using Type II anodizing on
certain alloys of aluminum, however, can create defects related to
the different crystallographic orientations of the grains within
the aluminum substrate.
[0026] To illustrate, FIGS. 2A and 2B show cross section views of
part 200 undergoing a conventional Type II anodizing process. FIG.
2A shows part 200, which includes metal substrate 201, prior to
anodizing. Metal substrate 201 can be made of any suitable
anodizable material, typically an aluminum alloy. The following
description is particularly relevant to high strength aluminum
alloys (such as 7000-series aluminum), where alloying elements such
as zinc, copper, manganese and magnesium result in anisotropic
anodizing behavior. Surface 202 of metal substrate 201 can be
lapped or polished to a mirror shine. Metal substrate 201 has
grains 206a, 206b, 206c, 206d and 206e along surface 202 that are
defined and separated by grain boundaries 204. Grains 206a, 206b,
206c, 206d and 206e are inherent crystallographic structures having
different crystallographic orientations within metal substrate 201.
Grains 206b and 206d have {111} crystallographic orientations while
grains 206a, 206c and 206e have crystallographic orientations that
are different than {111}, such as {110} and {100} crystallographic
orientations. Grains 206b and 206d having {111} crystallographic
orientations are generally dispersed throughout metal substrate
201. The size and distribution of grains 206a, 206b, 206c, 206d and
206e can vary depending on the type of metal and the temper of
metal substrate 201.
[0027] FIG. 2B shows metal substrate 201 after a conventional Type
II anodizing process. Anodizing processes, in general, involve
converting a portion of metal substrate 201 to a corresponding
metal oxide, referred to as anodic oxide coating 208. Thus, metal
substrate 201 can be referred to as an underlying metal substrate
201. Interface 210 between anodic oxide coating 208 and metal
substrate 201 takes on the same geometry of surface 202 prior to
the anodizing process. Thus, interface 210 takes on the polished,
mirror shine, highly reflective quality of surface 202. In some
cases, anodic oxide coating 208 is transparent to some of the light
incident surface 212 of anodic oxide coating 208 such that the
highly reflective surface of interface 210 is visible through
anodic oxide coating 208.
[0028] However, due to their {111} orientation in aluminum alloys
having zinc, grains 206b and 206d undergo the conversion process
faster than grains 206a, 206c and 206e. Thus, anodic oxide coating
208 is thicker a locations corresponding to grains 206b and 206d.
In some cases, it was found that grains 206b and 206d having {111},
or near {111} orientation, are anodized about 20% faster than
grains 206a, 206c and 206e having different orientations. Note that
the grain orientations that experience accelerated growth are not
limited to grains having {111} orientation. For example, it has
been found that {111} oriented grains experienced accelerated
growth in zinc-rich aluminum alloys, and {110} oriented grains
experienced accelerated growth in copper-rich aluminum alloys.
[0029] The localized thicker anodic oxide coating manifests as
protrusions 214 of surface 212 of anodic oxide coating 208, and
indentations or pits 216 within the reflective surface of interface
210 corresponding to intrusions of anodic oxide coating 208. Pits
216 can have sizes matching the sizes of corresponding grains 206b
and 206d. Pits 216 may be widely and asymmetrically distributed
over the surface interface 210 with a bimodal or tri-modal
distribution, and a distribution that widens with increasing
thickness of anodic oxide coating 208. The variations in oxide
thickness from grain to grain are visually perceived as an "orange
peel" texture in the metal/oxide interface, detracting from the
smooth, mirror-like reflective quality of the surface of interface
210. Specifically, pits 216 can appear as scattered tiny bright
spots that interrupt the mirror-like appearance of part 200 as
viewed from surface 212. In this way, pits 216 can be referred to
as visible defects within part 200. These visible defects can
become very noticeable when the average thickness of anodic oxide
coating 208 exceeds about 6 micrometers. For many applications,
however, the thickness of anodic oxide coating 208 should be
greater than about 10 micrometers in order to provide good wear
protection. Thus, these visual defects would be very apparent in
these applications. It should be noted that if substrate 201 had a
rougher texture, such as a blasted finish, or if anodic oxide
coating 208 is dyed, pits 216 may be visually apparent as a general
non-uniformity or blotchiness as viewed from surface 212, which is
also undesirable.
[0030] A further, un-related problem with using conventional Type
II anodizing with certain alloys is weakened interfacial adhesion
as a result of interfacial accumulation of alloying elements such
as zinc, which can combine with sulfur from the sulfuric acid of
the Type II electrolyte. These sulfur-containing agents can weaken
the bonding strength at interface 210 between substrate metal
substrate 201 and anodic oxide coating 208. This is described in
related U.S. patent application Ser. No. 14/474,021 filed Aug. 29,
2014, which is incorporated by reference herein in its
entirety.
[0031] One approach to mitigating the problem of differential
growth anodic oxide growth rates on grains of different
orientations is to limit the applied current density to less than
1.0 A/dm.sup.2, or to limit the applied voltage to less than 10 V.
However, these conditions limit the hardness and durability of the
resulting anodic oxide coating due to the excessive dissolution of
the anodic oxide material by the sulfuric acid during the prolonged
exposure that is required to grow an anodic oxide coating of
sufficient thickness (e.g., 10 micrometers or more). That is, the
resultant anodic oxide coating will not be hard enough to provide
sufficient abrasion and wear resistance for many consumer products,
especially at edges and corners of the consumer product. To
illustrate, FIGS. 3A and 3B show graphs indicating hardness of an
anodized 7003 aluminum substrate as a function of anodizing time
and current density using a conventional Type II anodizing
process.
[0032] The graph of FIG. 3A shows hardness data for anodic oxide
coatings grown to 9 micrometers in thickness using Type II
anodizing. This data shows that the hardness of an anodic oxide
coating decreases with increased anodizing processing time. This is
due to the dissolution of the anodic oxide material during the
anodizing process. The graph of FIG. 3B shows hardness data for
anodic coatings grown to 10 micrometers thickness using Type II
anodizing, with anodizing process times indicated. This data shows
that, for a given film thickness, lower current density
necessitates increased anodizing process time and results in
reduced surface hardness. For a number of applications, a current
density around 1.5 A/dm.sup.2 or higher is necessary in order to
provide a sufficiently durable anodic oxide coating. In order to
reduce the occurrence of the above-described pit defects, the
current density would have to be reduced to about 0.5 A/dm.sup.2,
which would result in a soft anodic oxide film that is not hard
enough for many consumer product applications.
[0033] The methods described herein address the above-described
issues associated with using conventional Type II anodizing
processes. The methods involve reducing the dissolving power of the
sulfuric acid electrolyte during the anodizing process by reducing
the concentration of sulfuric acid within the anodizing
electrolyte. FIGS. 4A and 4B show cross section views of part 400
undergoing an anodizing process in accordance with some
embodiments. FIG. 4A shows part 400, which includes metal substrate
401, after an optional surface finishing process. The optional
surface finishing process can include lapping, polishing and/or
buffing of surface 402. In some cases, surface 402 is polished to a
mirror-like shine. That is, surface 402 can be highly reflective to
incident light. In other embodiments, surface 402 is treated to
have a rough texture, such as by a blasting operation and/or
etching operation. Metal substrate 401 can be made of any suitable
anodizable material, such aluminum or aluminum alloy. In some
cases, substrate is made of an aluminum alloy with zinc, magnesium
and/or copper alloying elements. Metal substrate 401 has grains
406a, 406b, 406c, 406d and 406e defined and separated by grain
boundaries 404. Grains 406b and 406d have {111} crystallographic
orientations that undergo accelerated anodizing using Type II
anodizing conditions, described above. Grains 406a, 406c and 406e
have different crystallographic orientations than grains 406b and
406d and do not undergo accelerated anodizing using Type II
anodizing conditions.
[0034] FIG. 4B shows part 400 after an anodizing process, where a
portion of metal substrate 401 is converted to a corresponding
metal oxide, referred to as anodic oxide coating 408. If metal
substrate 401 is aluminum or aluminum alloy, anodic oxide coating
will include aluminum oxide. Remainder portion of metal substrate
401 is positioned below anodic oxide coating 408, and thus can be
referred to as an underlying metal substrate 401. Interface 410
between anodic oxide coating 408 and metal substrate 401 takes on
the same geometry of surface 402 prior to the anodizing process.
Thus, interface 410 takes on the polished, mirror shine, highly
reflective quality of surface 402. In some embodiments, anodic
oxide coating 408 is transparent to at least some of the light
incident surface 412 of anodic oxide coating 408 such that the
highly reflective surface of interface 410 is visible through
anodic oxide coating 408.
[0035] Anodic oxide coating 408 is formed using an anodizing
process with an electrolyte having a lower concentration of
sulfuric acid compared to electrolytes used in Type II anodizing.
The lower concentration of sulfuric acid reduces the dissolving
power of the sulfuric acid within the electrolyte and thereby
produces a harder anodic oxide coating 408. In addition, since the
sulfuric acid concentration is less than Type II anodizing,
accelerated anodizing due to different grain orientation is reduced
or eliminated. Thus, the thickness of anodic oxide coating 408
grown at {111} oriented grains 406b and 406d will be substantially
the same as the thickness of anodic oxide coating 408 at grains
406a, 406c and 406e. In this way, the above-described pits from
using Type II anodizing is dramatically reduced or eliminated and
the thickness of anodic oxide coating 408 is more uniform than that
of an anodic oxide coating formed using Type II anodizing. That is,
substrate 401 is substantially free of indentations and the highly
reflective surface at interface 410 remains uninterrupted and
pristine in appearance.
[0036] The concentration of sulfuric acid can vary depending on a
desired hardness and reduction of pit defects. In some embodiments
where substrate 201 is made of an aluminum alloy, the sulfuric acid
concentration was reduced to less than about 70 g/L, or less than
about 7% by weight. In some embodiments the sulfuric acid
concentration ranged between about 50-60 g/L. In other embodiments,
a sulfuric acid concentration as low as about 5 g/L is found to be
sufficient. These are well below any recited literature for Type II
anodizing electrolytes. For example, conventional Type II anodizing
typically includes using an electrolyte having a sulfuric acid
concentration of ranging between about 180-210 g/L, or about 10-20%
by weight.
[0037] The rate of dissolution of the anodic oxide coating 408
during anodizing is significantly lower in the lower sulfuric acid
electrolytes than in conventional Type II electrolytes. This
reduced rate of anodic oxide dissolution results in lower surface
porosity and greater surface hardness of anodic oxide coating 408
compared to anodic oxides grown to equivalent thicknesses in Type
II electrolytes, even when the current density or growth rates for
the latter is four or five times higher. In this way, the dilute
sulfuric acid concentration electrolyte enables an anodizing
process with results similar to more conventional Type II
anodizing. That is, the resultant anodic oxide coating 408 is a
reasonably hard (i.e., .gtoreq.320 HV.sub.0.05), clear, porous
oxide film, which is also well suited to dyeing and sealing
processes. As known in the art, HV.sub.0.05 refers to a Vickers
hardness testing scale, specifically at a load of 50 g. This may be
measured on a polished surface, or directly on an anodized surface
when that same has been formed on a polished substrate. It is
recognized that at thicknesses of 10 micrometers or less,
contributions from the substrate hardness will have an influence on
the measured surface hardness, and the measured value may not
reflect the true, absolute hardness of corresponding bulk material.
However, throughout this paper, quoted hardnesses are measured in
the same way, allowing meaningful comparisons of relative hardness
values.
[0038] In some embodiments, the electrolyte includes other acids,
such as one or more organic acids. It has been found in some cases
that adding an organic acid to the electrolyte can increase the
hardness of the final anodic oxide coating 408. However, organic
acids can also affect the appearance of the anodic oxide coating
408, such as give anodic oxide coating 408 a yellow, gold, bronze
or brown hue depending on the type and amount of organic acid.
Therefore, the use or organic acid and the type of organic acid
will depend on various factors such as a desired final hardness and
color of anodic oxide coating 408. In some cases, suitable organic
acids include one or more of oxalic acid, citric acid, malic acid,
malonic acid, glycolic acid, acetic acid and tartaric acid.
Operating voltages for a 0.5-2 A/dm.sup.2 current density anodizing
in a mixed (dilute sulfuric acid and organic acid) electrolyte can
be similar to those of conventional Type II anodizing (e.g., 5-30
V, sometimes preferably 10-25 V), rather than the higher voltages
typically required for anodizing in more conventional organic acid
electrolytes in the absence of the sulfuric acid. In particular
embodiments, oxalic acid added at a concentration of between 10-100
g/L is found to provide good hardness without too much
discoloration. In some embodiments, an oxalic acid concentration of
between 10-30 g/L is preferable. In some embodiments, other organic
acids or mixtures of organic acids can be added to a dilute
sulfuric acid electrolyte at similar concentrations. In a
particular embodiment, sulfuric acid is added as a relatively minor
additive (e.g., 5 g/L to 20 g/L) to an organic acid (at 20 g/L to
100 g/L), so as to reduce discoloration to negligible degree (i.e.,
each of a* and b*<1, as measured in accordance with CIE 1976
L*a*b* color space techniques), enabling the use of an organic acid
(and the corresponding benefits of high hardness at low current
density or high anodizing temperature, and minimal sulfate anion
incorporation), without the usual problem of discoloration
associated with anodizing in an organic acid.
[0039] The lower anodic oxide dissolution rate using dilute
sulfuric acid or mixed electrolyte makes it possible to extend the
range of anodizing process parameters to include lower current
densities (e.g., 1 A/dm.sup.2 or lower), and/or higher electrolyte
temperatures (e.g., 25.degree. C. to 40.degree. C.) whilst
maintaining anodic oxide coating 408 surface harnesses equal to or
better than those achieved with Type II anodizing under more
conventional conditions, such as the 320 HV.sub.0.05 achieved with
10 micrometer oxide growth at 1.5 A/dm.sup.2 and 20.degree. C. This
expansion of the processing parameter window to lower current
densities, or to higher temperatures, without sacrificing surface
hardness relative to conventional Type II anodizing, enables tuning
of the anodizing process to give anodic oxide coating 408 a high
degree of clarity (transparency) and great thickness uniformity
across surfaces comprising grains of varying crystallographic
orientations (specifically less than 5% thickness variation between
the film formed on grains of {111}, {110} and {100} orientation).
This enables the mirror-like finish of substrate 401 to be
protected against abrasion or wear with minimal loss of reflection
specularity, gloss, or distinctness of image. The gloss, measured
at 20 degrees, on a given lapped surface, is in excess of 1300
gloss units, when anodizing is performed in the preferred
embodiments herein, whereas the more typical electrolyte used in a
type II process (e.g., 200 g/l sulfuric acid) yields a maximum of
about 1100 gloss units on an equivalent lapped surface at 1
A/dm.sup.2 and 20 degrees C. To match the 1300 gloss units, the
conventional sulfuric acid's temperature would have to be raised to
25 degrees C., and the current density lowered to 0.5 A/dm.sup.2,
with a resulting surface hardness reduced to about 250
HV.sub.0.05.
[0040] The expansion of anodizing process parameters to lower
current densities or to higher temperatures, without sacrificing
hardness, is also of benefit in minimizing anion incorporation into
anodic oxide coating 408. The reduced concentration of sulfuric
acid also helps in this regard. A purer anodic oxide coating 408
results, with less incorporation of organic acid anions (such as
oxalates in cases where the acid includes oxalic acid) than would
be encountered when anodizing in the pure organic acid, and also
with less incorporation of sulfate ions than that encountered when
anodizing in more conventional and more concentrated sulfuric acid
electrolytes. This is itself of benefit in terms of increasing the
hardness anodic oxide coating 408 since the incorporation of
sulfate anions can compromise the hardness of the resulting oxide
film. In addition, this provides benefits in terms of the clarity
and optical transparency of anodic oxide coating 408. In some
cases, it may also be of benefit chemically by minimizing
interactions of undesirable compounds (e.g., oxalates or sulfates)
with other chemicals during subsequent processing operations (e.g.,
dyeing and sealing of the oxide film), or during use of part 400.
For example, corrosion can be minimized, and the leaching of
compounds such as oxalates to skin contacts during use of part 400
may be minimized.
[0041] By minimizing organic acid anion incorporation, this
approach of using a mixed acid electrolyte enables clearer oxides
to be produced than would result from the organic acid components
alone. For example, anodizing in an electrolyte having a
concentration of about 30 g/L oxalic acid can result in yellow
discoloration of the anodic oxide coating 408, whereas the addition
of 5 g/L to 20 g/L of sulfuric acid results in a clear, colorless
anodic oxide coating 408 In particular embodiments, the
colorlessness of anodic oxide coating 408 is measured as having an
a* of <1 and a b* of <1, as measured in accordance with CIE
1976 L*a*b* color space. This is desirable in many cosmetic
anodizing operations, where a clear anodic oxide coating 408 is
preferred, either for use in its own right, or as a neutral base
color for subsequent coloration using dyes.
[0042] Similarly, by minimizing the incorporation of inorganic acid
anions, relative to more conventional sulfuric acid anodizing
processes, this approach of using a mixed acid electrolyte enables
anodic oxide coating 408 to be formed on alloys such as AA7003,
without delamination risk that would otherwise result from
interactions of sulfates with zinc enriched at interface 410.
Details regarding the relationship between sulfur/sulfates and
delamination are provided in the U.S. patent application Ser. No.
14/474,021 referenced above. As a result, the level of sulfur in
the resulting anodic oxide coating 408 can be less than 4% by
weight, in some cases less than 3% by weight. This is compared to
anodic oxide coatings formed using conventional Type II anodizing
that generally have sulfur concentrations of greater than 10% by
weight, more typically about 13% by weight.
[0043] For many cosmetic applications, anodic oxide coating 408
grown in the dilute sulfuric acid electrolyte can exhibit uniform
pore structure similar to anodic oxide coating using Type II
anodizing. Thus, anodic oxide coating 408 is suitable for
permeation by dyes or other colorants, making it possible to
achieve a wide spectrum of colors through post-anodizing
operations. Moreover, due to the reduced dissolving power of the
electrolyte for the growing anodic oxide material of anodic oxide
coating 408 during anodizing, the outermost surface of the anodic
oxide coating 408 may present an even more uniform pore structure
than that of a film grown in Type II anodizing electrolyte at a
given temperature. This ensures uniformity of color of anodic oxide
coating 408, even when a very light dye is applied.
[0044] A further possible benefit of the anodizing processes
described herein is that they may reduce in-process corrosion of
certain corrosion-sensitive alloys. In particular, the increased
pH, reduced sulfate concentration, and possible inhibitive action
of certain organic acids such as tartaric acid may all contribute
to this benefit, as may the reduced potential or local
over-potentials associated with anodizing at a lower applied
voltage or current density.
[0045] Table 1 below summarizes a comparison of a sample (1)
anodized using a conventional Type II sulfuric acid anodizing
process (as exemplified by a very typical 1.5 A/dm.sup.2 process at
20 degrees C. in 200 g/L sulfuric acid) to samples (2), (3) and (4)
anodized using improved anodizing processes according to some
embodiments described herein. Sample (2) was anodized using dilute
sulfuric acid electrolyte with no organic acid. Sample (3) was
anodized using a mixed electrolyte having a concentration of 60 g/L
of sulfuric acid and 30 g/L of oxalic acid. Sample (4) was anodized
using a mixed electrolyte having a concentration of 10 g/L of
sulfuric acid and 30 g/L of oxalic acid.
[0046] Table 1 shows that the anodizing processes used for samples
(2), (3) and (4) can be performed using lower current density
and/or increased temperature compared to sample (1) using
conventional Type II anodizing, and still result in an anodic oxide
coating having a surface hardness of 320 HV.sub.0.05 or greater. In
particular, a current density no greater than 1 A/dm.sup.2, in some
embodiments 0.75 A/dm.sup.2, and electrolyte temperatures of up to
35 degree C., in some embodiments up to 40 degrees C., can be used.
Reduced current density, and to some degree the increased
temperature, reduces the grain-to-grain thickness variation. This
is exemplified by the column in Table 1 indicating Thickness
Variation of a resultant anodic oxide coating formed on a 7005
aluminum alloy (AA7003), with the thickness variation measured
across the substrate surface having {111} grain and a {100} grain
orientations. As indicated, sample (2) have a thickness variation
of 4% and samples (3) and (4) each have a thickness variation of
2%, compared to 20% thickness variation of Type II sample (1). This
very small thickness variation improves the cosmetics of the
anodized surfaces, as described above.
[0047] Table 1 also shows that the sulfur content within the anodic
oxide coatings formed using dilute sulfuric acid electrolyte (2)
and mixed acid electrolytes (3) and (4) is generally much less than
that of anodic oxide coating formed using Type II anodizing (1),
which correlates to improved adhesion and reduced risk of
delamination for certain metal alloys, notably those such as 7000
series aluminum alloys where zinc accumulates at the interface. In
some embodiments, the anodizing process is tuned to result in an
anodic oxide coating having a sulfur concentration of less than 4%
by weight, in some cases 3% by weight or less.
TABLE-US-00001 TABLE 1 Current Surface Thickness S Conc. Density
Temp. Hardness variation Incorporation (g/L) (A/dm.sup.2) (.degree.
C.) (HV.sub.0.05) (%) (Wt %) 1 Type II H.sub.2SO.sub.4 200 1.5 20
320 20 10 2 Dilute H.sub.2SO.sub.4 60 .ltoreq.1 20 320 4 <4 3
Dilute H.sub.2SO.sub.4 + 60/30 .ltoreq.1 30 350 2 <4 oxalic acid
4 Dilute H.sub.2SO.sub.4 + 10/30 .ltoreq.1 35 320 2 .ltoreq.3
oxalic acid
[0048] FIG. 5 shows flowchart 500 indicating an anodizing process
in accordance with some described embodiments. At 502, an optional
pre-anodizing surface treatment process is performed on a
substrate. The surface treatment process can include one or more of
lapping, polishing, buffing, blasting, chemical etching and laser
etching processes. In some embodiments, the surface of the
substrate is lapped and or polished to a mirror shine such that the
surface of the substrate is highly reflective of incident light. In
some embodiments, the substrate is made of an aluminum alloy, such
as aluminum alloys containing zinc and/or copper alloying agents.
In some embodiments, the substrate is made of a 6000 series or 7000
series aluminum alloy.
[0049] At 504, the substrate is anodized in a dilute sulfuric acid
electrolyte. The concentration of sulfuric acid of the electrolyte
is sufficiently low to prevent formation of visually apparent
defects caused by accelerated anodic oxide growth at certain grain
orientations of the substrate using Type II anodizing electrolytes.
That is, the anodized substrate is free of the scattered tiny pits
observed on substrates anodized using Type II anodizing processes.
Thus, if the substrate has a mirror shine prior to anodizing, the
anodized substrate will retain the uninterrupted mirror shine. In
some embodiments, the sulfuric acid concentration of the
electrolyte is no greater than 7% sulfuric acid by weight. In some
embodiments, the dilute sulfuric acid electrolyte includes organic
acid to enhance the hardness of the resulting anodic oxide coating.
In some embodiments, the resultant anodic oxide coating has a
hardness of no less than 320 HV.sub.0.05, in some cases no less
than 400 HV.sub.0.05. In some embodiments, the anodic oxide coating
has an average concentration of sulfur of no greater than 4% by
weight. For certain alloys, particularly those where zinc becomes
enriched at the metal/oxide interface, reduction of the sulfur
concentration within the oxide to the level is a necessary to avoid
a weakened interface between the oxide and the metal, and to ensure
that the anodic coating is resistant to delamination and
chipping.
[0050] At 506, a post-anodizing process is optionally performed on
the anodic oxide coating. The post-anodizing process can include a
coloring process whereby the anodic oxide coating is dyed to a
predetermined color. In some embodiments, the pore structure (e.g.,
pore size and pore uniformity) can be similar to the pore structure
of an anodic oxide coating formed using a Type II anodizing
process. Thus, the coloring process can be similar to one used in a
Type II anodic oxide coating. Any suitable coloring process can be
used, including organic dye infusion and/or electrolytic coloring.
In some embodiments, the anodic oxide coating is sealed using a
suitable pore sealing process.
[0051] 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 meant 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.
* * * * *