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.
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.
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.
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.
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.
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.
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.
These and other embodiments will be described in detail below.
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.
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.
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.
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.
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.
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.
These and other embodiments are discussed below with reference to
The methods described herein can be used to form durable and cosmetically appealing coatings for metallic surfaces of consumer devices.
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.
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.
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.
Methods described herein involve forming metal oxide coatings that provide improved corrosion protection for high performance alloys.
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.
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.
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.
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
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/dm2 and to about 2.0 A/dm2. 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.
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.
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.
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.
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.
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.
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.
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
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
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/dm2 and about 2.0 A/dm2 was used to form a porous metal oxide coating having a thickness between about 6 micrometers and about 30 micrometers.
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 Na2B4O5(OH)4.8H2O (sodium borate or borax), H3BO3 (boric acid), C4H6O6 (tartaric acid), (NH4)2.5B2O3.8H2O (ammonium pentaborate octahydrate), (NH4)2B4O7.4H2O (ammonium tetraborate tetrahydrate), and C6H10O4 (hexanedioic acid or adipic acid).
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.
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.
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.
As indicated by
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.
Results from described above with reference to
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.
This application claims priority under 35 U.S.C. §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. 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.
Number | Date | Country | |
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62249079 | Oct 2015 | US |