FIELD

[0001] The described embodiments relate to colored anodized films and methods forming colored anodized films. More specifically, methods involve techniques for providing colored anodized films that are highly opaque.

BACKGROUND

[0002] 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 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 have a porous microstructure that can be infused with dyes to impart a desired color to the anodic films.

[0003] Conventional methods for coloring anodic films, however, have not been able to achieve an anodic film with an opaque and saturated colored appearance. In particular, the underling metal substrate can often be seen through a dyed anodic film such that the anodized substrate appears to have a slight grey or metallic appearance. Thus, it is not possible to achieve a truly opaque colored anodic film using conventional anodic film coloring techniques - especially when trying to achieve a pure and opaque white color. Rather, conventional techniques for producing white colored films result in films that appear to be off-white, muted grey, or transparent white. These near-white anodic films can appear drab and cosmetically unappealing compared to a desirable pure and opaque white.

SUMMARY

[0004] This paper describes various embodiments that relate to anodized films and methods for forming anodized films.

[0005] According to one embodiment, a method of coloring an anodic film is described. The anodic film includes a porous layer over a barrier layer. The method includes smoothing an interface surface between the barrier layer and a metal substrate. The method also includes depositing a pigment within pores of the porous layer of the anodic film. [0006] According to another embodiment, a metal article is described. The metal article includes a metal substrate. The metal article also includes an anodic film covering the metal substrate. The anodic film includes a porous layer having pores with pigment infused therein. The anodic film further includes a barrier layer positioned between the porous layer and the metal substrate. An interface surface between the barrier layer and the metal substrate is a sufficiently smooth to direct light incident a top surface of the anodic film toward the pigment within the pores. The bottoms of the pores of the porous layer can also be smooth.

[0007] According to a further embodiment, an enclosure for an electronic device is described. The enclosure includes a metal substrate and an anodic film covering the metal substrate. The anodic film includes a porous layer having pores with pigment positioned therein. The anodic film also includes a barrier layer positioned between the porous layer and the metal substrate. The barrier layer has a thickness greater than about 150 nanometers.

[0008] According to one embodiment, a method of forming a white appearing metal oxide film is described. The method includes forming a first layer of the metal oxide film by anodizing a substrate in a first electrolyte. The method also includes forming a second layer of the metal oxide film by anodizing the substrate in a second electrolyte different than the first electrolyte. The second layer is more porous than the first layer and has pore wall surfaces that diffusely reflect visible light incident an exterior surface of the metal oxide film so as to impart the white appearance to the metal oxide film.

[0009] According to another embodiment, an anodized substrate having a white appearance is described. The anodized substrate has an anodic coating including a first metal oxide layer having an exterior surface corresponding to an exterior surface of the anodized substrate. The anodic coating also includes a second metal oxide layer adjacent the first metal oxide layer. The second metal oxide layer is more porous than the first metal oxide layer and has pore wall surfaces that diffusely reflect visible light incident an exterior surface of the anodic coating so as to impart a white appearance to the anodic coating.

[0010] According to a further embodiment, an enclosure for an electronic device is described. The enclosure includes an aluminum alloy substrate. The enclosure also includes an anodic coating having a white appearance disposed on the aluminum alloy substrate. The anodic coating has a first metal oxide layer, a second metal oxide layer adjacent the first metal oxide layer, and a barrier layer. The second metal oxide layer pore wall structure that diffusely reflects incident visible light. The barrier layer is positioned between the second metal oxide layer and the aluminum alloy substrate, wherein a thickness of the barrier layer is between about 150 nanometers and about 800 nanometers.

[0011] 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.

[0012] 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.

[0013] 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.

[0014] These and other embodiments will be described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] 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.

[0016] FIG. 1 shows perspective views of devices having metallic surfaces that can be protected using anodic oxide coatings described herein.

[0017] FIG. 2 shows a cross section view of an anodized part illustrating how light can become trapped within a standard colored anodized film.

[0018] FIGS. 3A-3D show cross section views of an anodized part that is colorized in accordance with some embodiments.

[0019] FIG. 4 shows a flowchart indicating a process for forming and coloring an anodic film in accordance with some embodiments.

[0020] FIGS. 5 A and 5B show SEM images of cross sections of an anodized part before and after a barrier layer smoothing process in accordance with some embodiments.

[0021] FIG. 6 shows perspective views of devices having metallic surfaces that can be protected using anodic oxide coatings described herein.

[0022] FIG. 7 shows a cross section view of an anodized part illustrating how an anodized part using a conventional anodizing process can have a translucent appearance.

[0023] FIGS. 8A-8E show cross section views of an anodized part that having a multiple layered structure that provides a white appearance, in accordance with some embodiments.

[0024] FIG. 9 shows a flowchart indicating a process for forming a multiple layered anodic film having a white appearance, in accordance with some embodiments.

[0025] FIGS. 1 OA- IOC show SEM cross section images of different parts at various stages of forming a multiple layered anodic oxide coating, in accordance with some embodiments. [0026] FIGS. 11A-11D and 12A-12D show SEM cross section and top view images of a part indicating how a barrier layer smoothing process can affect a structure of and anodic film, in accordance with some embodiments.

[0027] FIGS. 13A-13D show how a circularly polarizing filter can be used to determine whiteness of a part, including parts having multiple layered anodic films in accordance with some embodiments.

[0028] FIGS. 14A-14B, 15A-15B and 16A-16B show SEM images of anodic film prior to and after barrier layer smoothing processes to illustrate the extent that a barrier layer smoothing process can smooth an interface surface of a barrier layer, in accordance with some embodiments.

[0029] FIG. 17 shows perspective views of devices having metallic surfaces that can be protected using anodic films described herein.

[0030] FIG. 18 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.

[0031] FIGS. 19A-19D show cross section views of an anodized part with enhanced corrosion and aesthetic characteristics, in accordance with some embodiments.

[0032] FIG. 20 shows a flowchart indicating a process for forming a metal oxide coating, in accordance with some embodiments.

[0033] FIGS. 21A and 21B show TEM cross section images of anodic films prior to a barrier layer thickening process, in accordance with some embodiments.

[0034] FIGS. 22 A and 22B 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.

[0035] FIGS. 23A and 23B show SEM cross section images of anodic films prior to and after a barrier layer thickening process, respectively, in accordance with some embodiments.

[0036] FIGS. 24-28 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

[0037] 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.

Anodized Films With Pigment Coloring

[0038] Described herein are processes for colorizing anodic films. In particular, methods for increasing the color saturation or intensity of a colorant deposited within the anodic film are described. In some embodiments, the colorant is a pigment having a relatively large particle diameter compared to the particle sizes of conventional dyes. In some cases, the pigment particles each have a particle diameter of about 20 nanometers or greater. In some embodiments, each of the pigment particles has a particle diameter of 50 nanometers or greater. The larger pigment particles can absorb and reflect more incident light and provide a more saturated color to the anodic film compared to dyes having smaller diameters. The constitution of the pigment will depend on a desired color for the anodic film. In some embodiments, titanium oxide (Ti0 ₂ ) pigment is used to provide a white appearance to the anodic film. In some embodiments, a carbon black colorant is used to provide a black appearance to the anodic film. In some cases, the pores of the anodic film are widened prior to infusion of pigment particles in order to accommodate the larger pigment particle sizes. The resultant colored anodic film can have about 1 weight % of colorant or greater - in some cases up to about 30 weight %.

[0039] According to some embodiments, the methods involve smoothing an interface surface of a barrier layer within the anodic film. Smoothing can also be described as flattening or creating a more a more even topology. The barrier layer generally corresponds to a non-porous layer of the anodic film that forms during an anodizing process. The interface surface corresponds to a surface of the barrier layer between the porous layer of an anodic film and the non-porous barrier layer. The interface surface generally has a roughened surface that has a series of scalloped- shaped hemispherical features attributed by the curved pore terminuses of the porous layer. This rough interface surface can trap incident light and prevent some light from reaching the colorant that is deposited within the pores of the anodic film. The methods described herein involve smoothing out the interface surface such that the interface surface reflects incident light onto the colorant. The smoothing process can also smooth out pore terminuses (bottoms of pores) of the porous layer. The smoothing can be accomplished by electrolyzing the anodic film in a solution that does not substantially dissolve the anodic film, but instead promotes smoothing and, in some cases, some growth of the barrier layer. The resultant anodic film can have a richer and more saturated color.

[0040] 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 oxide coating, anodic film, anodic layer, anodic coating, oxide film, oxide layer, oxide coating, etc. can be used interchangeably and can refer to suitable metal oxide materials, unless otherwise specified.

[0041] 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, California.

[0042] These and other embodiments are discussed below with reference to FIGS. 1 - 5B. 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.

[0043] 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 are often 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 anodic oxide coatings can be colorized to impart a desired color to the metal housing or metal sections, adding numerous cosmetic options for product lines.

[0044] Conventional anodic oxide coloring techniques involve infusing dyes, such as organic dyes, within the pores of the anodic oxide. Although conventional anodic oxide coloring techniques result in adding a colored finish to the metallic surfaces, the colored finish retains a metallic look. This is because the underlying metal substrate is still observable through the anodic oxide such that the anodic oxide finish has a silver or grey hue. It is difficult or impossible to create a pure colored anodic oxide finish that is not affected by the color of the underlying metal substrate using conventional dyeing techniques. Described herein are coloring techniques that can provide anodic oxide finishes to metal substrate, such as those on housing of devices 102, 104, 106 and 108, having more rich and saturated colors compared to conventional dyeing techniques.

[0045] FIG. 2 illustrates a cross section view of a surface portion of anodized part 200, showing how light can become trapped within a standard colored anodized film 204. Part 200 includes metal substrate 202 and anodic film 204. Anodic film 204 can be formed by an anodizing process, whereby surface portions of metal substrate 202 are converted to corresponding metal oxide material 201. Anodic film 204 includes porous layer 206 and barrier layer 208. Porous layer 206 includes pores 205 that are formed during the anodizing process. Barrier layer 208 corresponds to a non-porous layer positioned between substrate 202 and porous layer 206. Barrier layer 208, like porous layer 206, is made of metal oxide material 201 but does not include pores 205. Interface surface 203 of barrier layer 208 has a shape that is partially defined by pore terminuses 207 of pores 205. In particular, the curved shaped pore terminuses 207 can cause interface surface 203 to have a scalloped geometry or shape. In three dimensions, interface surface 203 can be characterized as having a series of curved, hemispherical, cup-like features.

[0046] The size of pores 205 will depend, in part, on the process conditions of the anodizing process. For type II anodizing processes, which involves anodizing in sulfuric acid solution, a typical diameter of pores 205 is on the scale of tens of nanometers - typically less than about 20 nanometers. Pores 205 can be filled with dye particles 209, typically organic dye particles to impart a particular color to anodic film 204 and provide a colored finish to part 200. Dye particles 209 have smaller diameters than the diameters of pores 205 so that dye particles 209 can fit within pores 205.

[0047] One of the challenges associated with coloring anodic film 204 is that it can be difficult to accomplish a visibly saturated, rich, highly opaque color. This is because metal oxide material 201 of anodic film 204 can be partially transparent to visible light. Thus, underlying metal substrate 202 can be visible through anodic film 204. This can result in part 200 appearing a particular color from infused dye particles 209 as well as a having a metallic appearance from underlying substrate 202, as viewed from surface 212. This can give part 200 a silver or grey hue, which can be more apparent for lighter shades of dye particles 209.

[0048] In addition, light incident anodic film 204 can become trapped within anodic film 204 due to the scalloped shaped interface surface 203 of barrier layer 208. To illustrate, light ray 210 that is incident on anodic film 204 can enter porous layer 206 and be locally scattered by the scalloped features of interface surface 203. This means that light ray 210 cannot not reach and reflect off dye particles 209, and therefore does not contribute to providing a desired color to anodic film 204. That is, light ray 201 becomes trapped within anodic film 204 by the scalloped topology of interface surface 203, thereby darkening the appearance of anodic film 204.

[0049] The methods described herein involved using different types of colorant and/or modifying features of anodic film 204 to increase the effectiveness of a colorant that is deposited within pores 205. In some embodiments, the methods involve using a pigment as a colorant instead of a conventional organic or inorganic dye. The pigments can have a larger particle diameter than that of dye particles 209 and provide better coverage of the anodic film. Alternatively or additionally, the methods involve smoothing interface surface 203 of barrier layer 208 to increase the amount of light that reaches dye particles 209 or other type of colorant.

[0050] FIGS. 3A-3D illustrate cross section views of a surface portion of part 300 undergoing an anodic film coloration process in accordance with some embodiments. FIG. 3A shows part 300 after an anodizing process, in which a portion of metal substrate 302 is converted to anodic film 304 that includes a corresponding metal oxide material 301. Metal substrate 302 can include an alloy, such as an aluminum alloy, to provide good strength and structural durability. If metal substrate 302 is aluminum or aluminum alloy, metal oxide material 301 includes aluminum oxide. [0051] In some embodiments, the anodizing process is performed in phosphoric acid and/or oxalic acid solution, which can result in anodic film 304 having a wider pores 305 compared to anodizing in sulfuric acid solution (e.g., type II anodizing). Wider pores can accommodate more colorant and larger sized colorant particles, the advantages of which will be described in detail below. Wider pores can also be accomplished by using higher anodizing voltages compared to standard type II anodizing processes. The voltage will vary depending on the type of anodizing solution and other process parameters. In particular embodiments, an applied voltage of greater than 50 volts is used. In one embodiment, a phosphoric acid solution is used and a voltage of about 150 volts is used. It should be noted that anodic film 304 that pores 305 that are too wide, or that have too many pores, could impact the structural integrity of anodic film 304. Thus, these considerations should be balanced when choosing the anodizing process parameters.

[0052] Prior to anodizing, a surface treatment can be applied to metal substrate 302. For example, a polishing operation can be used to create a highly reflective surface on metal substrate 302 such that, once anodized, surface 311 of metal substrate 302 retains the highly reflective surface property. In other embodiments, an etching (e.g., acidic or alkaline etching) is used to create a textured surface on metal substrate 302, which can also be retained by surface 311. Advantages of each of these pre-anodizing surface treatments will be described in detail below.

[0053] Anodic film 304 includes porous layer 306 and barrier layer 308. Porous layer includes pores 305 while barrier layer 308 is substantially free of pores 305. In some embodiments, a target thickness of the porous layer is between about 6 and 20 micrometers. Barrier layer 308 is positioned between porous layer 306 and metal substrate 302, with interface surface 303 of barrier layer 308 defining the junction region between porous layer 306 and barrier layer 308. Interface surface 303 is defined in part by the shape of pore terminuses 307. Thus, curved pore terminuses 307 can cause interface surface 303 to have a series of scalloped-shaped features, which in three dimensions corresponds to a series of cup-like features. As described above with reference to FIG. 2, this scalloped shaped interface surface 303 can trap incoming light.

[0054] FIG. 3B shows part 300 after an optional pore widening process, in which pores 305 are widened to accept more colorant. In some embodiments, the pore widening process includes exposing anodic film 304 to an electrolytic process in an acidic bath (e.g., phosphoric acid, sulfuric acid, sulfamic acid, oxalic acid) with a relatively weak voltage, which removes some of the metal oxide material 301 around pores 305. In a particular embodiment, a phosphoric acid solution having a concentration of between about 2% and 30% is used. In some embodiments, ultrasonic waves are applied while the voltage is applied. It should be noted, however, that other types of solutions and techniques can also cause widening of pores 305.

[0055] The resultant anodic film 304 is characterized as having a pore diameter D that is greater than the pore diameter d prior to the pore widening process. As with the anodizing process described above, a number of factors should be considered as to the extent of widening pores 305. That is, it may be desirable to widen pores 305 as much as possible to accommodate more colorant and thereby increase the relative amount of colorant within anodic film 304. However, widening pores 305 to a very large extent can negatively affect the structural integrity of anodic film 304.

[0056] It should be noted that the process conditions of the anodizing process (FIG. 3A) could depend on whether a pore widening process is performed. For example, in a particular embodiment, the anodizing process is performed in an oxalic acid solution (e.g., about 30 g/L to about 50 g/L concentration held at about 20 degrees C to about 40 degrees C) using a voltage of about 30V to about 80V. This oxalic acid anodizing process can result in a pore diameter d that is smaller than that using a similar process with a phosphoric acid solution. However, pore widening performed after such an oxalic acid anodizing process can result in the same or wider pore diameter D compared to that of a phosphoric acid anodizing process without a pore widening process. In addition, the oxalic acid anodizing and pore widening process can result in a more structurally sound anodic film 304 compared to that of a phosphoric acid anodizing process without a pore widening process.

[0057] FIG. 3C shows part 300 after a barrier layer smoothing process is performed, in which interface surface 303 of barrier layer 308 is smoothed. In addition, the shape of pore terminuses 307 can be smoothed and flattened compared to the curved shape prior to the smoothing process. In some embodiments, the smoothing process involves exposing part 300 to an electrolytic process where part 300 acts as an anode in a solution that promotes anodic film growth without substantially promoting anodic film dissolution, i.e., a non-pore-forming electrolyte. In some embodiments, the solution contains one or more of sodium borate (borax), boric acid and tartaric acid solution. Other suitable solutions are described below with reference to FIG. 4. In some embodiments, the solution contains between about 10 to 20 g/L of sodium borate and has a pH of about 9. In some embodiments, the solution contains between about 10 and 20 g/L of boric acid and has a pH of about 6. The temperature of the solution can vary. In some embodiments, the solution is held at about 25 degrees Celsius. In some embodiments, an alternating current (AC) of between about 100 and 400 volts is applied. In some embodiments, a direct current (DC) of between about 100 and 200 volts is applied. The voltage can vary depending on other process parameters - in some embodiments the voltage is between about 50 and 400 volts. The result is a flattening or partial flattening of the scalloped projections of interface surface 303.

[0058] Similar to an anodizing process, part 300 acts as the anode and a further portion of metal substrate 312 is converted to metal oxide material 301. Thus, the barrier layer smoothing process can be coupled with thickening of barrier layer 308. To some extent, the amount of smoothing of interface surface 303 can be proportion to the amount of thickening of barrier layer 308. The thickness t of barrier layer 308 can be measured using scanning electron microscopy (SEM) images of cross sections samples, which are described below with reference to FIGS. 5 A and 5B. In some embodiments, prior to the barrier layer smoothing process, barrier layer 308 has a thickness t of about 110 nanometers or less, and after the barrier layer smoothing process barrier layer 308 has a thickness t of greater than about 150 nanometers. In some embodiments, barrier layer 308 is grown to a thickness t of between about 150 and 500 nanometers. It should be noted that the smoothing process (FIG. 3C) can be performed before or after the pore widening process (FIG. 3B). In some cases, however, the smoothing process is performed after the pore widening process since doing so can result in a smoother barrier layer 308.

[0059] The thickening of barrier layer 308 can also be used to some advantage with regard to a final perceived color of anodic film 304 and part 300. For example, the thickness of barrier layer 308 can be tuned to create an interference effect with incoming light, adding a pre-determined perceived hue to anodic film 304. Such interference coloration effects are described in detail in application No. 14/312,502, which is incorporated by reference herein in its entirety. It should be noted that it may be beneficial to use generally higher voltages during electrolytic process when optimizing for smoothness of interface surface 303 compared to the voltages used for tuning a thickness of barrier layer 308 for a particular interference coloring effect described in Application No. 14/312,502.

[0060] FIG. 3D shows part 300 after colorant particles 312 are deposited within pores 305. Since the scalloped or cup-shaped geometry of interface surface 303 has been attenuated and flattened, incident light, which would previously have been trapped within anodic film 304, reflects off interface surface 303 and onto colorant particles 312. To illustrate, light ray 314 that is incident on anodic film 304 can enter porous layer 306, reflect off interface surface 303 onto colorant particle 312, and a portion of light ray 314 that is not absorbed by colorant particle 312 is reflected out of anodic film 304 where it is perceived as color. That is, smoothed interface surface 303 is sufficiently smooth to direct light ray 314 toward colorant particles 312. If colorant particles 312 primarily reflect visible wavelengths of light corresponding to a blue color, colorant particles 312 will appear a blue color. If colorant particles 312 absorb substantially all visible wavelengths of light, colorant particles 312 will appear black. Similarly, if colorant particles 312 reflect substantially all visible wavelengths of light, colorant particles 312 will appear white. In some cases, colorant particles 312 include a mixture of different particles that reflect different wavelengths of light, resulting a unique perceived color resulting from a blend of different colored colorant particles 312.

[0061] It should be noted that it could be beneficial for substrate surface 31 1 to have a particular surface geometry in order to enhance the light absorption and reflection qualities of colorant particles 312. For example, substrate surface 31 1 having a smooth and highly reflective geometry can efficiently reflect incoming light to colorant particles 312, thereby increasing an apparent color saturation, similar to as described above with respect to a smooth interface surface 303. This may be important for bright colors such as white and brighter shades of red, blue, yellow, etc. However, for darker colors such as black or dark brown, a textured substrate surface 31 1 that traps light may be more desirable. In these cases, substrate surface 31 1 can be textured, such as by a chemical etch process prior to anodizing.

[0062] Colorant particles 312 can be made of any suitable color-imparting material or combination of materials, including organic or inorganic dyes, metals or combinations of dyes and metals. In some embodiments, colorant particles 312 are pigment particles that are generally larger than organic dye particles. For example, titanium oxide (Ti0 ₂ ) pigment, which can be used to create a white appearing anodic film 304, can be available in particle sizes having a diameter of about 50 to 60 nanometers. This can be compared to many organic dyes that have a particle diameter of less than about 10 nanometers. Carbon black pigment, which can be used to create a black appearing anodic film 304, can be available in particle sizes having a diameter of about 70 to 80 nanometers. Other pigments, such as blue, red and yellow pigments, can have a particle diameter of about 50 to 100 nanometers.

[0063] Colorant particles 312 can be deposited within pores 305 using any suitable technique, and can depend on the type of colorant particles 312. Pigments are a typically suspended in a solution and infused within pores 305 by immersing or dipping part 300 within the pigment suspension. In some embodiments, the pigment particles are suspended in an aqueous solution. The concentration of pigment and the pH of the pigment suspension can vary depending upon the type of pigment used. In some embodiments, the concentration is between about 5% and 40% by weight. In one embodiment where titanium oxide particles are used, the pH of the pigment suspension is about 2. In one embodiment where carbon black particles are used, the pH of the pigment suspension is about 6. In one embodiment where blue, red and/or yellow particles are used, the pH of the pigment suspension is between about 2 and 11.

[0064] After the pigment particles are sufficiently infused within pores 305, part 300 is removed from the solution and either allowed to dry naturally under ambient conditions or an accelerated drying process is utilized, such as by directing heated air at part 300 or placing part 300 in an oven. Removing the moisture can cause the pigment particles to gather and clump together, resulting in agglomerated pigment particles having diameters larger than that of individual pigment particles. These agglomerated pigment particles can have diameters of about 50 nanometers or more, in some embodiments about 75 nanometers or more. Because of their larger sizes, the agglomerated pigment particles can result in an even greater perceived coloration of anodic film 304. In particular, anodic film 304 can have a more opaque appearance compared to the appearance of anodic film 304 prior to drying.

[0065] One of the advantages of using pigment particles over organic dye particles is that the larger pigment particles can result in a richer, more saturated appearance. The optional pore widening process described above with reference to FIG. 3B may be necessary in order to fit and adequate amount of the larger pigment particles within pores 305. In some embodiments, the amount of pigment, or any colorant particles 312, incorporated within anodic film 304 is measured by weight % relative to the anodic film 304. Since the weight % of colorant particles 312 can be proportional to an amount of perceived color saturation, this measurement can be used to predict how saturated and opaque anodic film 304 will appear. In a particular embodiment where titanium oxide pigment particles are used, the weight % of titanium oxide particles is greater than about 1.0 weight % - in some embodiments between about 1.5 weight % and about 5.0 weight %. In some embodiments, the weight % of the titanium oxide particles is between about 1.5 weight % and about 30 weight %.

[0066] Another advantage in using certain pigment particles (e.g., titanium oxide and carbon black) over conventional organic dyes is the organic dyes can be susceptible to fading when exposed to ultraviolet (UV) light. In contrast, pigments such as titanium oxide and carbon black are generally resistant to UV fading.

[0067] As described above, thickness t of barrier layer can be tuned to create light interference effects that can add a particular hue to anodic film 304. For example, it may be difficult to achieve a pure white color for anodic film 304 due to an inherent yellow hue of metal oxide material 301. Thus, thickness t of barrier layer 308 can be tuned to have a thickness sufficient to reflect blue wavelengths of light by interference coloring effects. The interference coloring does not generally provide a strong coloring effect, but rather a hue or tint to the overall appearance of anodic film 304. Thus, barrier layer 308 that provides a blue hue can counterbalance a yellow hue of metal oxide material 301, resulting in a more color neutral appearance. In this way, a pure white appearance for anodic film 308 can be achieved. In some embodiments, a final color of anodic film 304 is measured and characterized using a CIE 1976 L*a*b* color space model measurements, which is described in detail in Application No. 14/312,502.

[0068] FIG. 4 shows flowchart 400 indicating a process for forming and coloring an anodic film in accordance with some embodiments. At 402, a surface pretreatment is optionally performed on a metal substrate. The surface treatment can be a polishing process that creates a mirror polished substrate surface, corresponding to a very uniform surface profile. In other embodiments, the surface treatment is an etching process that creates a textured surface that can have a matte appearance. Suitable etching processes include an alkaline etch, where the substrate is exposed to an alkaline solution (e.g., NaOH) for a predetermined time period for creating a desired texture. Acidic etching solutions (e.g., NH ₄ HF ₂ ) can also be used. Polishing techniques can include chemical polishing, which involves exposing the metal substrate to sulfuric acid and/or phosphoric acid solutions. In some embodiments, the polishing includes one or more mechanical polishing processes. In some embodiments where a final white or other bright appearance to an anodic film is desired, the substrate is preferably polished rather than etched in order to create an underlying light reflective substrate surface. In other embodiments, where a dark color or shade is desired, the substrate can be etched in order to purposely create an underlying light trap that traps incoming light.

[0069] At 404, the substrate is anodized. In some embodiments, the anodizing is performed in a phosphoric acid or oxalic acid solution, which can generally form wider pores than sulfuric anodizing processes. In a particular embodiment, a phosphoric acid anodizing process using a voltage of between about 80 and 100 is used to form an anodic film having a target thickness of about 10 micrometers. In some embodiments, an oxalic acid anodizing process using a voltage of between about 20 and 120 is used. During the anodizing process, an anodic film having a porous layer and a barrier layer is formed.

[0070] At 406, the pores of the porous layer are optionally widened in order to accommodate more colorant in a subsequent colorant infusing process. The process can include an electrolytic process within an acidic bath with a relatively weak applied voltage. The resultant anodic film has a pore diameter that is greater than the pore diameter prior to the pore widening process. The pore widening process may be more beneficial for those coloring process that include larger pigment particles.

[0071] At 408, the interface surface of the barrier layer positioned between the anodic film and the substrate is smoothed in order to remove a scalloped shape of the interface surface. The interface smoothing process can involve exposing the substrate to an anodic process, whereby the substrate is immersed in an electrolytic solution that promotes metal oxide material grown without significant dissolution of metal oxide material. In some embodiments, the solution contains sodium borate, boric acid or tartaric acid solution. In some embodiments, the tartaric acid is added to a sodium borate solution or boric acid solution. In some embodiments, one or more of the following chemicals are be used in solution for the barrier layer smoothing electrolytic process: Na ₂ B ₄ 05(OH) ₄ ·8Η ₂ 0 (borax), H ₃ BO ₃ (boric acid), Η ₄ )2θ·5Β ₂ θ3·8Η ₂ 0 (Ammonium pentaborate octahydrate), ( Η ₄ ) ₂ Β ₄ θ7·4Η ₂ 0 (ammonium tetraborate tetrahydrate), C ₆ Hio0 ₄ (hexanedioic acid), C ₆ Hi ₆ N ₂ 0 ₄ (ammonium adipate), ( H ₄ ) ₂ C ₄ H 06 (ammonium tartrate), C ₆ H ₈ 07 (citric acid), C ₄ H ₄ O ₄ (maleic acid), C ₂ H ₄ 0 ₃ (glycolic acid), C ₆ H ₄ (COOH) ₂ (phthalic acid), Na ₂ C0 ₃ (sodium carbonate), [SiO _x (OH) - _2x ] _n (silicic acid), H ₃ P0 ₄ (phosphoric acid), H ₃ NS0 ₃ (sulfamic acid), H ₂ S0 ₄ (sulfuric acid), and (COOH) ₂ (oxalic acid).

[0072] In some embodiments, the anodic film will have residues of one or more of these chemicals after the barrier layer smoothing operation is complete, and thus can be one method of detecting whether such a barrier layer smoothing operation was performed. For example, presence of borax or boric acid residues may persist within the anodic film, which can be detected by chemical analysis of the anodic film.

[0073] The applied voltage of the barrier layer electrolyzing process can vary depending on a desired amount of smoothing and/or a desired final thickness of the barrier layer. In some embodiments, the applied voltage is greater than about 50 volts. In some embodiments, the applied voltage is between about 50 and 400 volts. In some embodiments, a final thickness of the barrier layer is chosen to create a predetermined color hue by light interference effects.

[0074] At 410, a colorant is deposited within the pores of the anodic film. The colorant imparts a color to the anodic film by absorbing certain wavelengths of visible light and reflecting other wavelengths of visible light. In some embodiments, the colorant includes an organic dye or metal. In some embodiments, the colorant includes pigment particles having a particle diameter greater than about 50 nanometers. In some embodiments, the colorant includes a combination of pigment, dye and/or metal colorant. The chemical composition of the colorant will depend, in part, on a desired final color of the anodic film. The smoothed interface surface of the barrier layer that underlies the porous layer of the anodic film acts to reflect light onto the colorant, thereby enhancing the coloring effect of the colorant. In particular, more the more reflective interface surface can cause more light to be absorbed and reflected by the colorant.

[0075] The type of colorant will depend on a desired final color of the anodic film. In some embodiments, a carbon black colorant is used to impart a black color to the anodic film with a target L* value of about 30 or less, where L* corresponds to an amount of lightness measured using CIE D65 color space standards. In some embodiments where a desired color of the anodic film is white (e.g., using Ti0 ₂ pigment), measurement using standard CIE D65 color space techniques may effective to some extent but may be limited in other aspects. For example, an J* value can be used to determine an amount of lightness of the anodic film (i.e., the amount of light reflected by the anodic film and underlying substrate). However, L* value alone may not be an accurate indication of an amount of white color saturation. That is, high L* values can also be attributed by a highly reflective underlying substrate surface, but the part will appear to have some silver or greyness from the underlying substrate and may not appear as a saturated white color. Another method of measuring whiteness of an anodized part is using ASTM E313 standard practice, which is used to calculate yellowness and whiteness indices. A further way to measure whiteness is by using human visual inspection, where colorized anodized parts are visually compared to one another for perceived whiteness and color saturation.

[0076] At 412, the pores of the anodic film are optionally sealed using a sealing process. The sealing process can lock in the colorant and provide a more durable anodic film. Any suitable sealing process can be used. In a particular embodiment, a sealing solution containing Okuno Chemical H298 (manufactured by Okuno Chemical Industries Co., Ltd., based in Japan).

[0077] FIGS. 5 A and 5B show SEM images of cross sections of anodic film samples prior to and after exposure to a barrier smoothing process in accordance with some embodiments. FIG. 5A shows anodic film 502, positioned on substrate 504, after an anodizing process but prior to a barrier layer smoothing process. Anodic film 502 includes porous layer 506 and barrier layer 508. As shown, interface surface 510 of barrier layer 508 and pore terminuses 512 have scalloped shapes, with each scallop feature corresponding to a hemispherical cup-like feature in three-dimensions. This scalloped geometry can cause incident light to become trapped within anodic film 502. Barrier layer 508 has a thickness of about 106 nanometers, as measured by the SEM image.

[0078] FIG. 5B shows anodic film 522, positioned on substrate 524, after a barrier layer smoothing process. Anodic film 522 includes porous layer 526 and barrier layer 528. As shown interface surface 530 of barrier layer 528 and pore terminuses 532 are smooth and relatively flat compared to the curved, scalloped geometry prior to the smoothing process (FIG. 5A). Barrier layer 528 has a thickness of about 484 nanometers, indicating that the smoothing process is associated with a thickening of barrier layer 508. White Anodic Films With Multiple Layers

[0079] Described herein are processes for providing a white color to anodic films. In particular embodiments, the anodic films have multiple layers, where a first layer, which can correspond to an outer or external layer of the anodic film, has a relatively high density of metal oxide material, thereby providing a hardness and chemical resistivity to the anodic film. A second layer, beneath the first layer, can include a pore wall structure that diffusely reflects incoming visible light, thereby providing a white appearance to the anodic film. The pore wall structure of the second layer can include pore wall surfaces that are at non-orthogonal orientations with respect to the outer surface of the anodic film, thereby providing a structure for diffusely reflecting incident light. In some cases, the anodic films include a smoothed barrier layer that defines a flat interface surface between the barrier layer and an underlying substrate. The flat interface surface can specularly reflect incoming light, thereby increasing a brightness and enhancing the white appearance of the multiple layered anodic film. The barrier layer smoothing process can also flatten pore terminuses of the second layer, thereby providing additional flat surfaces for specularly reflecting incoming light.

[0080] Methods for forming the multiple layered anodic films can include performing a first anodizing process using a first electrolyte and a second anodizing process using a second electrolyte different than the first electrolyte. In some embodiments, the first electrolyte includes oxalic acid, which can form a dense and chemically resistant first layer. In some embodiments, the first electrolyte includes sulfuric acid, which can form a substantially colorless and cosmetically appealing anodic film. In some embodiments, the second electrolyte includes phosphoric acid, which can form an irregular pore structure that includes light diffusing pore walls. The second anodizing process can result in a more porous second layer than the first anodizing process. In embodiments where a barrier layer smoothing process is used, the anodic film can be exposed to a third anodizing process that is performed in a non- dissolution (i.e., non-pore forming) electrolyte. In particular embodiments, the non- pore forming electrolyte includes borax or boric acid. The multiple layered anodic film can be sealed using a sealing process so as to further increase its chemical resistance and the corrosion resistance. The resultant white appearing anodic film can have a hardness of at least 150 HV (Vickers Pyramid Number as measured using Vickers hardness test) in order to withstand abrasion forces that may occur during normal use of a consumer product (e.g., an electronic device as described above). The resultant white appearing anodic film can also be characterized as having an J* value of at least 80 (in some cases at least 85), a b* value between about -3 and about +6, and an a* value of between about -3 and about +3. In some embodiments, a suitable white color can be achieved without infusing a dye or pigment within the multiple layered anodic film. In some embodiments, a suitable white color is achieved by infusing a dye or pigment within the multiple layered anodic film.

[0081] 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 magnesium. 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.

[0082] 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, California.

[0083] These and other embodiments are discussed below with reference to FIGS. 6 - 16B. 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.

[0084] The methods described herein can be used to form durable and cosmetically appealing coatings for metallic surfaces of consumer devices. FIG. 6 shows consumer products that can be manufactured using methods described herein. FIG. 6 includes portable phone 602, tablet computer 604, smart watch 606 and portable computer 608, which can each include housings that are made of metal or have metal sections. Aluminum alloys, such as 5000 series, 6000 series or 7000 series 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. In some cases the anodic oxide coatings are colorized to impart a desired color to the metal housing or metal sections.

[0085] Conventional anodic oxide coloring techniques involve infusing dyes, such as organic dyes or metal-based colorants, within the pores of the anodic oxide. It can be difficult, however, to impart an opaque white appearance to anodic oxide coatings. This is, in part, because white colorants can be composed of relatively large particles that can be difficult to infuse within the nano-scale diameter pores of anodic oxide coatings. Conventional colorizing techniques often result in off-white or silver colored anodic oxide coatings. This is because the underlying metal substrate can still be observable through the anodic oxide such that the anodic oxide finish retains a metallic look. Described herein are improved techniques for providing opaque white anodic oxide finishes to metal substrate, such as those on housing of devices 602, 604, 606 and 108.

[0086] In general, white is the color or appearance of an object if the material of the object diffusely reflects back most of visible light that strikes it. Non-dyed anodic oxide coatings may have a slight whitened or colored appearance, depending on the anodizing conditions and processing parameters for forming the anodic oxide coating. However, many types of non-dyed anodic oxide coating can be generally characterized as translucent in that the underlying metal substrate is typically clearly visible through the non-dyed anodic oxide coating.

[0087] To illustrate, FIG. 7 shows a cross section view of a surface portion of anodized part 700. Part 700 includes metal substrate 702 and metal oxide coating 704. Metal oxide coating 704 is composed of a metal oxide material 703 and includes pores 706 formed during the anodizing process. In this way, pores 706 are defined by pore walls 705, which are composed of metal oxide material 703. The size of pores 706 can vary depending on the anodizing process conditions. For example, some type II anodizing processes, as defined by MIL-A-8625 industry standards, which involve anodizing in a sulfuric acid bath, can result in pores 706 having diameters of about 20 nanometers (nm) to about 30 nanometers. Metal oxide coating 704 is uncolorized in that pores 706 do not include dye or metal colorants. Thus, much of the visible light incident metal oxide coating 704 can pass through metal oxide coating 704. For example, light ray 708 can enter outer surface 710 of metal oxide coating 704 and pass through metal oxide material 703 and pores 706, reflect off of underlying metal substrate 702, and reflect back out of metal oxide coating 704. In this way, underlying metal substrate 702 can be visible through metal oxide coating 704, thereby giving anodized part 700 a metallic look.

[0088] It should be noted that metal oxide coating 704 includes a porous layer 701 (defined by thickness 712), which includes pores 706 and a barrier layer 709, (defined by thickness 714), which corresponds to a generally non-porous portion of metal oxide coating 704 between metal substrate 702 and porous layer 704. Both porous layer 701 and the barrier layer 709 include metal oxide material 703 from converting surface portions of metal substrate 702 to a corresponding metal oxide material 703. Interface surface 716 of barrier layer 709, defined on one side by metal substrate 702 and on another side by barrier layer 709, has a shape that is partially defined by pore terminuses 718 of pores 706. In particular, the curved shaped pore terminuses 718 can cause interface surface 716 to have a scalloped geometry or shape. In three dimensions, interface surface 716 can be characterized as having a series of curved, hemispherical, cup-like features.

[0089] One of the challenges associated with imparting a white appearance to metal oxide coating 704 is that many white colorants, such as titanium oxide particles, can be too big to fit within pores 706. Thus, conventional methods can make it impossible to accomplish a visibly saturated, rich, highly opaque white color to metal oxide coating 704. Even when some whitening is accomplished, significant amounts of incoming light can still pass through metal oxide coating 704 so as to give part 700 a silver hue as viewed from surface 710. In addition, light incident metal oxide coating 704 can become trapped within metal oxide coating 704 due to the scalloped shaped interface surface 718, thereby darkening the appearance of metal oxide coating 704 and preventing a brightness necessary for a providing white appearance.

[0090] The methods described herein involve forming a multiple layered anodic coating that can provide a saturated, opaque and bright white appearance. FIGS. 8A- 8E illustrate cross section views of part 800 undergoing an anodizing process for forming a white appearing multiple layered coating, in accordance with some described embodiments.

[0091] FIG. 8A shows part 800 after metal substrate 802 is anodized using a first anodizing process. Metal substrate 802 can be any suitable anodizable material, such as suitable aluminum and aluminum alloys. In some embodiments, metal substrate 802 is a 5000 series, 6000 series or 7000 series aluminum alloy. The first anodizing process converts a portion of metal substrate 802 to first metal oxide layer 804. First metal oxide layer 804 is composed of metal oxide material 803, the composition of which depends on the composition of metal substrate 802. For example, an aluminum alloy metal substrate 802 can be converted to a corresponding aluminum oxide material 803. First metal oxide layer 804 includes porous portion 801 (defined by thickness 812) and the barrier layer 809 (defined by thickness 814). The porous portion 801 includes pores 806, which are formed during the anodizing process, and are formed within metal oxide material 803. The barrier layer 809 is generally free of pores 806 and is situated between metal substrate 802 and porous portion 804. The thickness of first metal oxide layer 804, corresponding to thicknesses 812 of the porous portion plus thickness 814 of barrier layer 809, can vary depending on the application. In some embodiments, barrier layer has a thickness 814 of about 100 nanometers or less. In some embodiments, the thickness of first metal oxide layer 804 is between about 3 micrometers and about 15 micrometers.

[0092] In some embodiments, first metal oxide layer 804 has a pore structure that provides high mechanical strength and chemical resistance to first metal oxide layer 804. This can be accomplished by adjusting process conditions of the first anodizing process. For example, anodizing in a bath including oxalic acid can result in pores 806 that are generally wider than those formed in an electrolytic bath including sulfuric acid. For example, in some embodiments using oxalic acid-based anodizing results in pores 806 having diameters 820 between about 30 nanometers and about 100 nanometers, compared to sulfuric acid-based anodizing that can result in pores 806 having diameters 820 between about 10 nanometers and about 40 nanometers.

[0093] Although pores 806 are generally wider using oxalic acid anodizing, the density of pores 806 is less compared to the density of pores 806 using sulfuric acid anodizing. That is, the density of metal oxide material 803 and the width of pore walls 805 can be generally greater when oxalic acid anodizing compared to sulfuric acid anodizing. This greater relative density of metal oxide material 803 (using oxalic acid-based anodizing) can result in metal oxide layer 804 being harder and more chemically resistant than a sulfuric acid-based oxide film, which can be useful in applications where first oxide layer 804 corresponds to an exterior surface of a consumer product (e.g., devices of FIG. 1). In some embodiments, good results were found when the electrolyte has a relatively low concentration of oxalic acid, such as about 10 g/1 of oxalic acid or less - which is lower than conventional oxalic acid anodizing processes.

[0094] It should be noted that oxalic acid-based anodizing can, in some cases, cause first metal oxide layer 804 to have a yellow hue, sometimes associated with using an organic acid-based anodizing bath. Since this may serve against providing a white appearing anodic coating, it may be preferable to use a sulfuric acid-based anodizing process in some cases. However, in some embodiments, the oxalic acid- based anodizing can result in a sufficiently white an colorless anodic film. In some cases, such a yellow hue can be offset using barrier layer thickening techniques, which will be described below with reference to FIG. 13C.

[0095] FIG. 8B shows part 800 after a second anodizing process is performed, causing more of metal substrate 802 to be converted to second metal oxide layer 822. Second metal oxide layer 822 grows beneath first metal oxide layer 804 and reforms barrier layer 809 (defined by thickness 827) adjacent metal substrate 802. Thus, the thickness of the multiple layered anodic oxide coating can be defined by thickness 812 of first metal oxide layer 804, thickness 833 of second metal oxide layer 822, and thickness 827 of barrier layer 809.

[0096] As shown, pores 823 within second metal oxide layer 822 are generally wider than pores 806 of first metal oxide layer 804. In some embodiments, the diameter 824 of pores 823 are about 100 nanometer or more, in some embodiments between about 100 nm and about 300 nm. In addition, second metal oxide layer 822 has a pore walls 825 that are irregular, in that pore walls 825 have pore wall surfaces 826 are oriented non-orthogonally with respect to the outer surface 810. This anodic pore structure can be accomplished, for example, by performing the second anodizing process in a bath including phosphoric acid.

[0097] The irregular pore structure of second metal oxide layer 822 can impart a white appearance to the anodic coating by diffusely reflecting incoming visible light. This is illustrated by first light ray 828 entering outer surface 810 of first metal oxide layer 804, reflecting off of pore wall surfaces 826 of second metal oxide layer 822, and exiting outer surface 810 at a first angle. Second light ray 829 enters outer surface 810 of first metal oxide layer 804, reflects off of pore wall surfaces 826, and exits outer surface 810 at a second angle different than the first angle. Third light ray 830 enters outer surface 810 of first metal oxide layer 804, reflects off of pore wall surfaces 826, and exits outer surface 810 at a third angle different than the first angle and the second angle. In this way, pore wall surfaces 826 within second metal oxide layer 822 can diffusely reflect visible light and impart a white appearance to the multiple layered anodic oxide coating of part 800. In some embodiments, a good whitening results were found when the second anodizing process involves using an electrolytic bath having a relatively low concentration of phosphoric acid, such as about 17 g/1 of phosphoric acid or less - which is much lower than conventional phosphoric acid anodizing processes.

[0098] It should be noted that first metal oxide layer 804, which can generally have more mechanical strength and be more dense (i.e., have more volume percent of metal oxide material) than second metal oxide layer 822 can provide structural integrity to the anodic film, while underlying second metal oxide layer 822, while generally more porous than first metal oxide layer 804, can provide the porous structure suitable for providing a white appearance to the anodic film.

[0099] At FIG. 13C, barrier layer 809 is optionally smoothed and thickened in order to enhance the whitening of the multiple layered anodic oxide coating. The smoothing of barrier layer 809 can smooth out interface surface 816 of barrier layer 809, which previously had a scalloped geometry. This can cause incoming light that does not diffusely reflect off of pore wall surfaces 826 to specularly reflect of flat interface surface 816. For instance, light ray 817 enters outer surface 810 of first metal oxide layer 804, passes through first metal oxide layer 804 and second metal oxide layer 822, reflects off of interface surface 816, and exits outer surface 810 at a first angle. Light ray 819 enters outer surface 810 of first metal oxide layer 804, passes through first metal oxide layer 804 and second metal oxide layer 822, reflects off of interface surface 816, and exits outer surface 810 at the same first angle as light ray 817. Additionally or alternatively, the barrier layer smoothing process can flatten or smooth pore terminuses 818 of pores 823, such that flattened pore terminuses 818 can also specularly reflect incoming light. In this way, the smooth (i.e., flat) interface surface 816 and/or pore terminuses 818 can cause light that does not diffusely reflect off of pore wall surfaces 826 to specularly reflect off interface surface 816 and/or pore terminuses 818, resulting in brightening and enhancing the white appearance of the multiple layered anodic coating. That is, the specular reflectivity of flattened interface surface 816 increases the lightness of the whiteness caused by diffuse reflection off of pore walls 826 (see e.g., light ray 829) to produce a bright white appearance. In some embodiments, the barrier layer smoothing process is necessary in order to accomplish a particular level of lightness, which can be measured using, for example, L* values as defined by CIE 1976 L*a*b* color space model standards.

[0100] The barrier layer smoothing process can be accomplished by anodizing part 800 using a third anodizing process that promotes anodic film growth without substantially promoting anodic film dissolution, i.e., a non-pore-forming electrolyte. In some embodiments, the non-pore forming electrolyte contains one or more of Na ₂ B ₄ 05(OH) ₄ ·8Η ₂ 0 (sodium borate or borax), H _8B 0 ₃ (boric acid), C ₄ H ₆ 0 ₆ (tartaric acid), ( Η ) ₂ θ·5Β ₂ θ ₃ ·8Η ₂ 0 (ammonium pentaborate octahydrate), ( Η ₄ ) ₂ Β θ7·4Η ₂ 0 (ammonium tetraborate tetrahydrate), C ₆ Hio0 ₄ (hexanedioic acid or adipic acid), C ₆ Hi ₆ N ₂ 0 ₄ (ammonium adipate), ( H ₄ ) ₂ C ₄ H 06 (ammonium tartrate), C ₆ H ₈ 07 (citric acid), C ₄ H 0 ₄ (maleic acid), C ₂ H 03 (glycolic acid), C ₆ H ₄ (COOH) ₂ (phthalic acid), Na ₂ C0 ₃ (sodium carbonate), [Six(OH) ₄ - _2x ] _n (silicic acid), and H ₃ NS0 ₃ (sulfamic acid). In some embodiments, the non-pore-forming electrolyte includes borax, boric acid or adipic acid. Below are listed some example process parameters used for a barrier layer smoothing process, in accordance with some embodiments.

Example 1

Example 2

[0101] Barrier layer 809 can be smoothed to differing amounts, depending on a desired final smoothing outcome and process limitations. In some embodiments, the barrier layer smoothing process is performed until interface surface 816 achieves a profile variance of no more than about 30 nanometers, where the profile variance is defined as a distance d between an adjacent peak and valley of the interface surface 816 over a predefined distance across part 800. In some embodiments, the profile variance is no more than about 6% of the thickness t of barrier layer 809. Profile variance can be measured, for example, using a scanning electron microscope (SEM) cross section image of the part 800. SEM cross section images of some samples are described below with reference to FIGS. 9A-9B, 10A-10B and 11 A-l IB.

[0102] In addition to smoothing barrier layer 809, the barrier layer thickening process can also thicken barrier layer 809 to thickness t. That is, thickness t is greater than thickness 814 (in FIG. 8A) prior to the barrier layer smoothing process. This aspect can be used to compensate for any discoloration of the anodic oxide coating. For example, as described above, anodizing in organic acids such as oxalic acid can cause first metal oxide layer 804 to have a yellow hue. To offset this yellowing, barrier layer 809 can be used to reflect light via thin film interference. For example, 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, according to CIE 1976 L*a*b* color space model measurements. Thus, thickness t of barrier layer can be tuned to create light interference effects that add a blue hue (negative b* value) to offset a yellow hue (positive b* value) of first barrier layer 804. Likewise, thickness t of barrier layer can be tuned to create light interference effects that add a magenta hue (positive a* value) to offset a green hue (negative a* value) of first barrier layer 804. In this way, a more color-neutral anodic coating conducive to a white appearance can be achieved.

[0103] Thus, the final thickness t of barrier layer 809 can be chosen so as to sufficiently smooth barrier layer 809 as well as to reflect a desired range of wavelengths of light by thin film interference. In some embodiments, thickness t of barrier layer 809 is at least about 200 nanometers. In some embodiments, thickness t is about 300 nanometers or more. In some embodiments, thickness t is about 400 nanometers or more. In some embodiments, thickness t about is between about 150 nanometers and 800 nanometers.

[0104] FIG. 8D shows part 800 after an optional pigment infusing process is performed, which involves depositing particles 821 within the multiple layered anodic oxide coating. Particles 821 should have a white appearance or otherwise be highly optically reflective. In some embodiments, particles 821 are composed of one or more of a titanium oxide (e.g., Ti0 ₂ ), an aluminum oxide (e.g., A1 ₂ 0 ₃ ) and a zinc oxide (e.g., ZnO). Particles 821 can be infused using any suitable method. In some cases, part 800 is immersed in a solution that has particles 821 suspended therein. In some embodiments, the solution is an aqueous solution with a controlled pH conducive to promoting diffusion of particles 821 within pores 806. Particles 821 thereby become infused within pores 806 and get trapped such that, when part 800 is removed from the solution, at least some of particles 821 remain within pores 806. In some embodiments, particles 821 become infused within both second metal oxide layer 822 and first metal oxide layer 804.

[0105] Particles 821 can diffusely reflect incoming visible light (e.g., light ray 832), thereby further enhancing the whiteness of the multiple layered anodic oxide coating. Thus, incoming light can diffusely reflect off of pore walls 826 of second metal oxide layer 822 (e.g., light ray 829), diffusely reflect off of particles 821 (e.g., light ray 832), and specularly reflect off of flattened interface surface 816, resulting in a bright and white appearance. Note that the particle infusing process shown in FIG. 8D is optional. That is, in some embodiments, a white enough multiple layered anodic oxide coating is achieved without infusing particles 821. In some embodiments, however, the addition of particles 821 may be beneficial to achieving adequate levels of whiteness.

[0106] FIG. 8E shows part 800 after an optional pore sealing process is performed in order to enhance the chemical resistance and corrosion resistance of the anodic oxide coating. The sealing process can hydrate the metal oxide material 803 of at least top portions of pore walls 805 of first metal oxide layer 804. In particular, the sealing process can convert metal oxide material 803 to its hydrated form 834, thereby causing swelling of pore walls 805 and sealing of pores 806. The chemical nature of hydrated metal oxide material 834 will depend on the composition of metal oxide material 803. For example, aluminum oxide (A1 ₂ 0 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 806 of first metal oxide layer 804 is hydrated, while in some embodiments substantially the entire length of pores 806 of first metal oxide layer 804 is hydrated. In some cases, a portion of pores 823 of second metal oxide layer 804 are also hydrated. Any suitable pore sealing process can be used, including exposing part 800 to hot aqueous solution or steam. In some cases additives are added to the aqueous solution, such as nickel acetate or other commercial additives, such as Okuno Chemical H298 (manufactured by Okuno Chemical Industries Co., Ltd., based in Japan).

[0107] After sealing, the multiple layered anodic coating of part 800 can have superior hardness and scratch resistance and appear an opaque white color. The sealing of pores 806 may also help retain particles 821 within the multiple layered anodic coating (in embodiments that include particles 821). In some embodiments, the multiple layered anodic coating of part 800 is characterized as having a hardness value of at least 150 HV. In some embodiments, the multiple layered anodic coating of part 800 is characterized as having an L* value of 80 or higher, a b* value between about -3 and about +6 and an a* value of between about -3 and about +3. Note that in some embodiments, the barrier layer smoothing process can be necessary to achieve a certain level of lightness, related to the whiteness, of the multiple layered metal oxide film. For example, one multiple layered metal oxide coating sample was characterized as having an L* value of 74.16, a b* value of 1.75, and an a* value of 0.05, and visually appeared grey prior to performing the barrier layer smoothing process. After the barrier layer smoothing process, the multiple layered metal oxide coating sample was characterized as having an L* value of 84.30, a b* value of 1.85, and an a* value of -0.38, and visually appeared white. Thus, the barrier layer smoothing process can be used to increase the lightness (L*) and/or reduce discoloration (b* or a*) of the multiple layered anodic film.

[0108] In some cases, the whiteness of the anodic coating can be characterized using whiteness index (WI) ratings. One equation used for the measuring WI is the CIE standard illumination D65 formulae for whiteness W ₁₀ :

Wio = Yio800(x _n;10 - xio) + 1700(y _n;10 - yio) where Y is the Y tristimulus value (relative luminance), (x,y) is the chromaticity coordinate in the CIE 1931 color space, (x _n ,yn) is the chromaticity coordinate of the perfect diffuser (reference white), and the subscript ten (10) indicates the CIE 1964 standard observer.

[0109] In general, the higher the W ₁₀ value, the greater the whiteness. In some embodiments, the multiple layered anodic coating of part 800 has a W ₁₀ value of at least 75. It should be noted that in some embodiments these whiteness index values can be achieved without the use colorants (e.g., dyes, pigments or metal colorant) within the anodic oxide coating. In other embodiments, the anodic coating should include a colorant, such as pigment particles described above with reference to FIGS. 8D and 8E, in order to achieve these whiteness index values.

[0110] Thickness 812 of first metal oxide layer 804, thickness 833 of second metal oxide layer 822, and thickness t of barrier layer 809 can vary depending on desired mechanical or color properties of the multiple layered anodic coating. In particular embodiments, thickness 812 of first metal oxide layer 804 is between about 3 micrometers and about 15 micrometers, thickness 833 of second metal oxide layer 822 is between about 2 micrometers and about 15 micrometers, and thickness t of barrier layer 809 is at least about 200 nanometers (in some embodiments up to about 800 nanometers). In some embodiments, a final thickness of the multiple layered anodic coating of part 800 (including thickness 812, thickness 833 and thickness t) is between about 5 micrometers and about 30 micrometers.

[0111] It should be noted that thickness t of barrier layer 809 is dependent, in part, on the voltage used during the barrier layer smoothing process, with higher voltages associated with a thicker barrier layer 809. If the voltage used in the barrier layer smoothing process is too high, this could cause first metal oxide layer 804 and/or second metal oxide layer 822 to breakdown. Thus, the voltage should be kept sufficiently low to prevent such breakdown. This means that a maximum thickness t of barrier layer 809 is limited. In some embodiments, thickness t is grown to a maximum of about 800 nanometers. As described above, however, thickness t should be large enough to be associated with sufficient flattening of interface surface 816. This means that in some embodiments, thickness t should range between about 150 nanometers and about 800 nanometers. In some embodiments, thickness t of the barrier layer is at least about 6% of a total thickness of the anodic coating (t + 833 + 812).

[0112] FIG. 9 shows flowchart 900, which indicates a process for forming a multiple layered anodic coating having a white appearance, in accordance with some embodiments. At 902, 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 smoothing process is performed (see FIG. 13C). 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, 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.

[0113] At 904, a first layer of a metal oxide film 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 has an oxalic acid concentration of between about 5 g/1 and about 60 g/1. In some embodiments, the oxalic acid concentration is about 10 g/1 or less - which is lower than conventional oxalic acid anodizing. In some embodiments, the temperature of the electrolyte during anodizing is between about 20 degrees C and about 40 degrees C, using an anodizing voltage of between about 40 volts and about 100 volts, using an anodizing current density of between about 1 A/dm ² and about 4 A/dm ² . The anodizing time period will vary depending on a desired thickness of the first metal oxide layer. In some embodiments, the first anodizing time period is between about 1 minute and 5 minutes.

[0114] At 906, a second layer of the metal oxide film is formed using a second anodizing process. The second layer can be structurally different than the first layer in that the second layer can have more pore wall surfaces that diffusely reflect visible light incident an exterior surface of the metal oxide film compared to the first layer. For example, the first layer of the metal oxide film can have pore walls that are substantially orthogonal to the exterior surface of the anodic coating, whereas pore wall surfaces of the second layer of the metal oxide film can be oriented non- orthogonally with respect to the exterior surface such that light can reflect off the pore wall surfaces (see FIGS. 8A-8D).

[0115] In some embodiments, the second electrolyte includes phosphoric acid in a concentration of between about 15 g/1 and about 250 g/1. In some embodiments, the phosphoric acid concentration is about 17 g/1 or less - which is lower than conventional phosphoric acid anodizing. In some embodiments, the temperature of the second electrolyte during anodizing is between about 5 degrees C and about 70 degrees C, using an anodizing voltage of between about 70 volts and about 150 volts, using an anodizing current density of between about 0.5 A/dm ² and about 5 A/dm ² . In some embodiments, the electrolyte temperature is maintained at about 60 degrees C or higher during the anodizing, which is higher than conventional voltages used in phosphoric acid anodizing. The anodizing time period will vary depending on a desired thickness of the second metal oxide layer. In some embodiments, the second anodizing time period is between about 25 minute and 50 minutes.

[0116] At 908, the barrier layer of the multiple layered metal oxide film is smoothed using a third anodizing process, which can be referred to as a barrier layer smoothing process. The third anodizing process can be performed in a non-pore forming electrolyte such that the additional metal oxide material is non-porous, effectively thickening the substantially non-porous barrier layer. In some embodiments, the non-pore forming electrolyte contains one or more of Na ₂ B ₄ 0 ₅ (OH)4 ^e 8H ₂ 0 (sodium borate or borax), Η _8Β θ3 (boric acid), C ₄ H ₆ O ₆ (tartaric acid), (ΝΗ ₄ ) ₂ θ·5Β ₂ θ3·8Η ₂ 0 (Ammonium pentaborate octahydrate), ( Η ₄ ) ₂ Β ₄ θ7·4Η ₂ 0 (ammonium tetraborate tetrahydrate), C ₆ Hio0 ₄ (hexanedioic acid), C ₆ Hi ₆ N ₂ 0 ₄ (ammonium adipate), ( H ₄ ) ₂ C ₄ H ₄ 0 ₆ (ammonium tartrate), C ₆ H ₈ 0 ₇ (citric acid), C4H4O4 (maleic acid), C ₂ H ₄ O ₃ (glycolic acid), C ₆ H ₄ (COOH) ₂ (phthalic acid), Na2CC"3 (sodium carbonate), [Six(OH)4-2 _X ] _n (silicic acid), and H3NSO3 (sulfamic acid).

[0117] In particular embodiments, the third anodizing process involves anodizing in an electrolyte having borax in a concentration of between about 10 g/1 and 20 g/1 (at a pH between about 9 and 9.2) held at an anodizing temperature of between about 20 degrees C and 30 degrees C. In another embodiment, an electrolyte having boric acid in a concentration of between about 10 g/1 and 20 g/1 (at a pH of about 6) held at an anodizing temperature of between about 20 degrees C and 30 degrees C was used. 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 550 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 300 volts and about 500 volts is achieved, which is maintained for about 5 minutes.

[0118] At 910, a white pigment is optionally infused within the metal oxide film. Any suitable white coloring agent can be used. In some embodiments, the white pigment includes particles composed of a titanium oxide (e.g., T1O ₂ ), an aluminum oxide (e.g., AI ₂ O ₃ ), a zinc oxide (e.g., ZnO), or any suitable combination thereof. In some embodiments, the white pigment is infused by exposing the metal oxide film to an aqueous solution having white pigment particles suspended therein such that the pigment particles deposit into and get trapped within the anodic pores of at least the second layer.

[0119] At 912, the multiple layered metal oxide film is optionally sealed to seal at least top portions of the pores of the first layer. This can increase the mechanical strength and corrosion resistance of the multiple layered metal oxide film. In some embodiments, a target hardness of the multiple layered metal oxide film is at least about 150 HV, suitable for use in housing for electronic devices. In addition, the sealing process can retain the white pigment particles (if used) within the anodic pores of the metal oxide film.

[0120] FIGS. lOA-lOC show SEM cross section images of different parts at various stages of forming a multiple layered anodic oxide coating, in accordance with some embodiments. FIG. 10A shows part 500 after a first anodizing process converts a portion of substrate 502 to first metal oxide layer 504. Substrate 502 is composed of an aluminum alloy and the anodizing process involved using a sulfuric acid-based bath. The resulting first metal oxide layer 504 has pores 506 and barrier layer 509. Pores 506 have diameters of about 40 nm to about 50 nm. First oxide layer 504 has a thickness 508 of about 14.5 micrometers and barrier layer 509 has a thickness 510 between about 50 nm and about 70 nm.

[0121] FIG. 10B shows part 530 after two anodizing processes have been performed. Like part 500, a first anodizing process using a sulfuric acid-based electrolyte is used to form first metal oxide layer 504, which as a thickness 508 of about 4.7 micrometers. In addition, a second anodizing process is performed, whereby another portion of substrate 502 is converted to second metal oxide layer 512. The second anodizing process involved using a phosphoric acid-based bath and has a thickness 511 of about 6.7 micrometers. Barrier layer 509 is grown to a thickness 516 of about 150 nm.

[0122] As shown, pores 514 within second metal oxide layer 512 are generally wider than pores 506 of first metal oxide layer 504. In particular, the diameters of pores 514 are about 100 nanometers or more, compared to pores 506 having diameters between about 40 nm and about 50 nm. Likewise, the pore walls between pores 514 of second metal oxide layer 512 are generally wider (thicker) than the pore walls between pores 506 of first metal oxide layer 504. In addition, second metal oxide layer 512 has irregular pore walls with surfaces that are oriented non-orthogonally with respect to the outer surface 518.

[0123] FIG. IOC shows part 540 after three anodizing processes have been performed. Like parts 500 and 530, a first anodizing process using an sulfuric acid- based electrolyte is used to form first metal oxide layer 504 (in this case having a thickness 508 of about 5.1 micrometers) and a second anodizing process using an phosphoric acid-based electrolyte is used to form second metal oxide layer 512 (in this case having a thickness 508 of about 4.5 micrometers). In addition, a barrier layer smoothing and thickening anodizing process is performed, where barrier layer 509 is smoothed and thickened to thickness 520 of about 550 nm. The barrier layer process involved exposing part 540 to a non-pore-forming anodizing process using a non- dissolution electrolyte (e.g., borax, boric acid, etc.) The resulting multilayered anodic coating (first oxide layer 504 + second oxide layer 512 + smoothed barrier layer 509) has a white appearance as viewed from outer surface 518.

[0124] FIGS. 11A-11D and 12A-12D show SEM cross section and top view images of a part indicating how a barrier layer smoothing process can affect a structure of and anodic film, in accordance with some embodiments. The part includes an anodic film 602, formed using a phosphoric acid-based anodizing process, and barrier layer 604. FIGS. 11 A-l ID show images of the part before a barrier layer smoothing and thickening process, and FIGS. 12A-12D show images of the part after a barrier layer smoothing and thickening process is performed. The barrier layer was smoothed using borax-based barrier layer smoothing process.

[0125] FIGS. 11A and 11B show cross section images of the part at different magnifications, with FIG. 11B at a higher magnification. As shown in FIG. 11B, barrier layer 604 prior to the barrier layer smoothing process has an uneven and inconsistent boundary. FIG. 12B shows that barrier layer 604, after the barrier layer smoothing process is performed, has a much more even boundary that is more conducive to producing a white appearance. The barrier layer smoothing process also involves thickening barrier layer 604 (i.e., from about 200 nm thick to about 800 nm thick). Whiteness measurements of the part prior to the barrier layer smoothing process is characterized as having a whiteness value W ₁₀ of 64.7, and after the barrier layer smoothing process having whiteness value W ₁₀ of 70.48. This data indicates that the barrier layer smoothing process can significantly increase the whiteness of an anodic film.

[0126] FIGS. l lC and 11D show top views of the anodic film at different magnifications, with FIG. 11C at a higher magnification. FIGS. 12C and 12D show top views of the anodic film after the barrier layer smoothing process. As shown, the barrier layer smoothing process did not significantly change the pore structure of anodic film 602. In particular, the pore diameters were between about 200 nm and about 260 nm before and after the barrier layer smoothing process. This data indicates that the integrity of anodic film 602 is not significantly affected by the barrier layer smoothing and thickening process.

[0127] It can be difficult to determine a level of whiteness of a part based on L*a*b* color space values alone since bright metallic surfaces can have similar L*a*b* measurements as white surfaces. FIGS. 13A-13D show how a circularly polarizing filter can be used to determine whiteness of a part, including parts having multiple layered anodic films, in accordance with some embodiments.

[0128] FIG. 13A shows a top view of part 1300, which is composed of an aluminum alloy substrate and which is anodized using a type II anodizing process. The type II anodizing process results in providing an anodic coating that is relatively transparent such that the silver metal appearance of the aluminum alloy substrate is highly visible through the anodic coating. First filter 1302 and second filter 1304, which are both circularly polarized filters of the same type, are positioned on top of part 1300. First filter 1302 has a first orientation (e.g., left circularly polarized) with respect to part 1300, and second filter 1304 has a second orientation (e.g., right circularly polarized) with respect to part 1300. First filter 1302 is oriented such that the bright silver appearance of part 1300is minimized barely visible through first filter 802 (i.e., has a dark appearance). Second filter 804 is oriented such that the bright silver appearance of part 1300is maximized and clearly visible through second filter 1304. L* measurements, which correspond to an amount of lightness, are taken of part 1300 through first filter 1302 and second filter 1304. The difference between the L* values are then quantified as AL* (L/R), where L corresponds to left circularly polarized and R corresponds to right circularly polarized. This AL* (L/R) can be used to distinguish between a white surface and a light reflection off of a metallic surface (e.g., a silver colored metal surface of an aluminum alloy). [0129] To illustrate, Table 1 below summarizes some color space value measurements for part 1300 and Table 2 summarizes the same color space value measurements for a white piece of paper.

Table 1 - Type II anodized aluminum alloy

[0130] Wio corresponds to a CIE standard illumination based on tristimulus value Y and chromaticity coordinate (x,y), as described above. In accordance with CIE D65 color space standards, L* corresponds to an amount of lightness, a* represents an amount of green or red/magenta, and b* represents an amounts of blue or yellow. Negative a* values indicate a green color while positive a* values indicate a red or magenta color. Negative b* values indicate a blue color and positive b* values indicate a yellow color. AL* corresponds to an amount of change of J* of the first filter compared to the second filter. W ₁₀ , J*, a* and b* measurements are taken directly at the surfaces of the anodized part 1300 and white piece of paper. The AL* value is based on measurements are taken through first filter 1302 and second filter 1304.

[0131] Tables 1 and 2 indicate that the type II anodized aluminum substrate has similar Wio, J*, a* and b* values as the white piece of paper. In fact, the Wio value, which is an indicator of whiteness, for the visibly silver anodized part 1300 is greater than the Wio value of the white piece of paper. This is because the anodized part 1300 has a high specular reflectance (i.e., high shine), which is associated with high lightness measurements. Thus, although Wio, J*, a* and b* values can be an indication of how colorless and bright a part is, these values may not fully indicate a level of whiteness of a part. In contrast, the AL* for the anodized part 1300 is much higher than that of the white piece of paper. In particular, the AL* value for the white piece of paper is relatively low (i.e., 1.1), whereas the AL* value of the bright silver anodized part 1300 is much higher (i.e., 34.3). That is, a small AL* value is associated with a white color.

[0132] FIG. 13B shows a top view of part 1310, which is composed of an aluminum alloy substrate (same type of aluminum alloy as the substrate of part 1300) having been treated with multiple anodizing processes to form a white multilayered anodic film. In particular, part 1310 includes a first layer formed by anodizing the substrate in an oxalic acid bath, a second layer formed by anodizing the substrate in a phosphoric acid bath, and a barrier layer that was smoothed and thickened using a barrier layer smoothing process. Circularly polarized filters 1312 and 1314 are positioned on top of part 1310 at opposing orientations, as described above. Table 3 below summarized whiteness measurements of part 810.

Table 3 - Multilayered anodic film on aluminum alloy

[0133] Table 3 indicates that part 1310 has a higher W ₁₀ value than the silver appearing type II anodized part 1300. In addition, part 1310 is characterized has having a much lower AL* value than the AL* value of part 1310. That is, part 1310 measures less change in an amount of lightness L*, as measured through opposite- oriented polarized filters, compared to part 1300. This indicates that less of the lightness J* of part 1310 is due to the specularly reflective underlying aluminum alloy substrate than the lightness L* of part 1300. In fact, to a human eye, part 1300 has a silver appearance while part 1310 has a distinctively white appearance.

[0134] FIG. 13C shows a top view of part 1320, which is composed of an aluminum alloy substrate (same type of aluminum alloy as the substrate of parts 1300 and 1310) having been treated with a different multiple anodizing processes than part 1310. In particular, part 1320 includes a first layer formed by anodizing the substrate in an oxalic acid bath, a second layer formed by anodizing the substrate in a phosphoric acid bath, a barrier layer that was smoothed and thickened using a barrier layer smoothing process, and white pigment (i.e., Ti0 ₂ ). Circularly polarized filters 1322 and 1324 are positioned on top of part 1320 at opposing orientations, as described above. Table 4 below summarized whiteness measurements of part 1320.

Table 4 - Multilayered anodic film with pigment on aluminum alloy

[0135] Table 4 indicates that part 1320, like part 1310, has a higher W ₁₀ value and much lower AL * value than the silver appearing type II anodized part 1300. Part 1320 also appears to a human eye to have a distinctively white appearance. In this embodiment, the addition of Ti0 ₂ pigment to the multilayered anodic film is shown to increase the W ₁₀ value and decrease the AL* value compared to part 1310 having a multilayered anodic film without pigment.

[0136] FIG. 13D shows a top view of part 1330, which is composed of an aluminum alloy substrate (same type of aluminum alloy as the substrate of parts 1300, 1310 and 1320) having been treated with a similar multiple anodizing processes as part 1320. In particular, part 1330 includes a first layer formed by anodizing the substrate in a sulfuric acid bath, a second layer formed by anodizing the substrate in a phosphoric acid bath, a barrier layer that was smoothed and thickened using a barrier layer smoothing process, and Ti0 ₂ pigment. Circularly polarized filters 1332 and 1334 are positioned on top of part 1330 at opposing orientations, as described above. Table 5 below summarized whiteness measurements of part 830.

Table 5 - Multilayered anodic film with pigment on aluminum alloy

[0137] Table 5 indicates that part 1330, like parts 1310 and 1320, has much lower AL * value than the silver appearing type II anodized part 1300. Part 1330 also has an even lower AL* value than that of part 1320, which also has a multilayered anodic film with Ti0 ₂ pigment. It is noted that part 1330 has a lower W ₁₀ value than parts 1310 and 1320 that also have multilayered anodic films, even though part 1330 appears to have a distinctively white appearance. This indicates that, in some embodiments, AL * values may be as important than W ₁₀ values in determining a whiteness of an anodic film. In any cases, the parts having the multilayered anodic films (parts 1310, 1320 and 1330) each have a higher W ₁₀ value than that of a single layered anodic film (part 1300).

[0138] The data of Tables 1-5 and FIGS. 13A-13D indicate that, in some embodiments, multilayered anodic films formed using the methods described herein can be characterized as having L * values of at least 80, a b * value between about -3 and about +6, and an a* value of between about -3 and about +3. In some embodiments, the multilayered anodic films are characterized as having W ₁₀ values of at least about 70 and a AL * value of no greater than about 10. It should be noted that L *, b *, a*, Wio, AL * values can vary while still appearing white (e.g., not silver) to a human eye, and that process parameters and can used to adjust different structural properties of a multilayered anodic film in order to achieve a particular white appearance and hardness value. For example, the thicknesses of the first and second anodic film layers can be adjusted, as can the smoothness and thickness of the barrier layer and the amount and type of pigment used (if a pigment is used).

[0139] FIGS. 14A-14B, 15A-15B and 16A-16B show SEM images of anodic film prior to and after barrier layer smoothing processes to illustrate the extent that a barrier layer smoothing process can smooth an interface surface of a barrier layer, in accordance with some embodiments.

[0140] FIGS. 14A-14B show SEM cross section images of part 1400 prior to a barrier layer smoothing process, with FIG. 14B showing a higher magnification. Part 1400 includes substrate 1402, which is composed of an aluminum alloy, and anodic film 1404, which was formed using a sulfuric acid-based anodizing process (using a voltage of around 20 V) and which as pores 1406. Barrier layer 1406 defines interface surface 1408 between barrier layer 1406 and anodic film 1404. As shown, interface surface 1410 has a scalloped structure in accordance with the terminuses (bottoms) of pores 1408. Lines 1412a and 912b demarcate the depth of the pore terminuses, which can be defined as thickness measurement of the curved bottom portions of pores 1408. The depth of pore 1408 terminuses is found to be around 12 nm.

[0141] It should be noted that the chemistry of electrolytic bath and the voltage used during the anodizing process for forming anodic film 1404 also has a relationship with the depth of pores 1408. For example, sulfuric acid-based anodizing generally results in pores that are smaller in diameter than pores formed form a phosphoric acid-based anodizing process. Also, higher voltages generally result in a less smooth interface surface. For example, a phosphoric acid-based anodizing process generally results in pores that are larger in diameter and that results in a barrier layer having less smooth interface than that of a sulfuric acid-based anodizing process (see FIGS. 15A and 15B).

[0142] FIGS. 15A and 15B show SEM cross section images of part 1400 after a barrier layer smoothing process. The barrier layer smoothing process involved applying a voltage of about 70 V to part 90 while immersed in a borax-base electrolyte. As shown, interface surface 1410 is significantly smoothed and no longer has a scalloped structure. That is, the terminuses of pores 1408 are flattened. This smoothing can also be characterized by a difference in the depth of the terminuses of pores 1408, as demarcated by lines 1502a and 1502b. In particular, the depth of the terminuses of pores 1408 is decreased to about 7 nm. According to some embodiments, the depth of the pore terminus is less than about 10 nm.

[0143] As described above, the chemistry of electrolytic bath and the voltage used during the anodizing process for forming an anodic film (prior to the barrier layer smoothing process) can affect the smoothness of an interface surface between the barrier layer and the porous portion of the anodic film. To illustrate, FIGS. 16A and 16B show top view and cross section SEM images, respectively, of part 1600. Part 1600 includes substrate 1602, anodic film 1604 and barrier layer 1606. Anodic film 1604 was formed using a phosphoric acid-based anodizing process using a voltage of about 100 V. The resulting anodic film 1604 has pores 1608 with diameters around 50 nm and about 100 nm having a pore terminus depth 1610 of about 40 nm and about 76 nm.

Anodic Films For High Performance Aluminum Alloys

[0144] 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.

[0145] 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.

[0146] 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. [0147] 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, California.

[0148] These and other embodiments are discussed below with reference to FIGS. 17 - 28. 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.

[0149] The methods described herein can be used to form durable and cosmetically appealing coatings for metallic surfaces of consumer devices. FIG. 17 shows consumer products that can be manufactured using methods described herein. FIG. 17 includes portable phone 1702, tablet computer 1704, smart watch 1706 and portable computer 1708, 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 MTL-A-8625), which involves anodizing in sulfuric acid based solution, to form relatively translucent and cosmetically appealing anodic oxide coating.

[0150] 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.

[0151] 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.

[0152] FIG. 18 shows a cross section view of a surface portion of anodized part 1800. Part 1800 includes metal substrate 1802 and metal oxide coating 1804. Metal substrate can be composed of a high strength aluminum alloy, which includes alloying elements that enhance the mechanical strength of metal substrate 1802. Metal oxide coating 1804 can be formed using an anodizing process whereby a surface portion of metal substrate 1802 is converted to a corresponding metal oxide material 1803 (e.g., aluminum oxide). Metal oxide coating 1804 includes pores 1806, formed during the anodizing process, defined by pore walls 1805. The size of pores 1806 can vary depending on the anodizing process conditions. For example, some type II anodizing processes can result in pores 1806 having diameters of about 20 nanometers to about 30 nanometers. Metal oxide coating 1804 includes porous layer 1801 (defined by thickness 1812) and a barrier layer 1809 (defined by thickness 1814), which corresponds to a generally non-porous portion of metal oxide coating 1804 between metal substrate 1802 and porous portion 1801. Thickness 1814 of barrier layer 1809 is typically on the order of 10 nanometers to about 30 nanometers.

[0153] As shown, defects 1807 can form within metal oxide coating 1804. Defects 1807 can correspond to inconsistencies within the structure of metal oxide material 1803 - in some cases defects 1807 are in the form of cracks. Defects 1807 can be associated with the type and amount of alloying elements within metal substrate 1802. For example, defects 1807 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 1807 can be small, sometimes in the order of nanometers or tens of nanometers (e.g., as small as around 10 nm). However, some defects 1807 are large enough to span thickness 1814 of barrier layer 1809. For example, defects 1807 can connect with each other, thereby spanning thickness 1814. In some cases, defects 1807, such as cracks, can become bigger during manufacture process or service lifetime of anodized part 1800. For example, thermal cycling can cause small crack defects to become larger. In this way, defects 1807 can act as entry points for water or other corrosion inducing agents to reach metal substrate 1802. For example, corrosion inducing agents can enter exterior surface 1810 of metal oxide coating 1804 via pores 1806, pass through barrier layer 1809 via defects 1807 and reach metal substrate 1802. In some cases, defects 1807 can allow corrosion inducing agents to reach metal substrate 1802 even if pores 1806 are sealed using a hydrothermal sealing process. If metal substrate 1802 is relatively susceptible to corrosion, such as some 7000 and 2000 series aluminum alloys, metal substrate 1802 can corrode, thereby degrading the adhesion of metal oxide coating 1804 and the integrity of anodized part 1800.

[0154] Methods described herein involve forming metal oxide coatings that provide improved corrosion protection for high performance alloys. FIGS. 19A-19C illustrate cross section views of part 1900 undergoing an anodizing process, in accordance with some described embodiments.

[0155] FIG. 19A shows part 1900 after metal substrate 1902 is anodized using a first anodizing process. Metal substrate 1902 can be any suitable anodizable material, such as suitable aluminum, aluminum alloys, magnesium, magnesium alloys, and suitable combinations thereof. In some embodiments, metal substrate 1902 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 1902 is composed of a 6000 series aluminum alloy, such as 6063 aluminum alloy. In some embodiments, metal substrate 1902 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).

[0156] The first anodizing process converts a portion of metal substrate 1902 to metal oxide coating 1904, which include porous layer 1901 and barrier layer 1909. Porous layer 1901 includes pores 1906, which are formed during the anodizing process, and barrier layer 1909 is generally free of pores 1906 and is situated between metal substrate 1902 and porous layer 1901. Porous layer 1901 and barrier layer 1909 are both composed of metal oxide material 303, the specific composition of which depends on the composition of metal substrate 1902. For example, an aluminum alloy metal substrate 1902 can be converted to a corresponding aluminum oxide material 303.

[0157] In some cases where metal substrate 1902 is composed of a high performance substrate, defects 1907 can form within metal oxide coating 1904. Defects 1907 can be associated with certain alloying elements within metal substrate 1902, such as copper in some 2000 series aluminum alloys and zinc in some 7000 series aluminum alloys. In some cases, defects 1907 can include the alloying element(s) (e.g., zinc or copper). In some cases, defects 1907 are in the form of cracks or voids within metal oxide coating 1904. Defects 1907 can be randomly distributed within metal oxide coating 1904 and can in some cases connect with each other. As described above, defects 1907 can act as pathways for corrosion inducing agents to reach metal substrate 1902.

[0158] In some embodiments, the first anodizing process can produce a porous layer 1901 that has a higher density of metal oxide material 303 compared to conventional anodizing processes. For example, pore walls 1905 between pores 1906 can be thicker than pore walls of a standard type II anodizing process. In particular, thickness 320 of pore walls 1905 can range between about 10 nanometers and about 30 nanometers. Diameters 1922 of pores 1906 can range between about 10 nanometers and about 30 nanometers. In some embodiments, diameters 1922 of pores 1906 range between 10 nanometers to about 20 nanometers. In addition, thickness 1912 of porous layer 1901 can be relatively thick compared to conventional anodizing processes. For example, thickness 1912 of porous layer 1901 can ranges between about 6 micrometers and about 30 micrometers - in some embodiments ranges between about 10 micrometers and about 15 micrometers. [0159] It should be noted that oxalic acid based anodizing can, in some cases, cause metal oxide coating 1904 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 1904 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. 19B.

[0160] In some embodiments, the dense metal oxide coating 1904 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/1 and about 210 g/1. 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 ² and to about 2.0 A/dm ² . The anodizing process time can vary depending on a target thickness of metal oxide coating 1904. In a particular embodiment, the anodizing time period ranges from about 10 minutes to about 100 minutes.

[0161] The higher density and thicker pore walls 1905 of metal oxide coating 1904 enhances the structural integrity of metal oxide coating 1904 compared to conventional metal oxide coatings, despite the presence of defects 1907. That is, defects 1907 can be distributed within a more structurally dense metal oxide coating 1904, thereby reducing the chance of defects 1907 acting as entry points for corrosion inducing agents to reach metal substrate 1902.

[0162] FIG. 19B shows part 1900 after a second anodizing process is performed in order to further enhance the corrosion protection ability of metal oxide coating 1904. The second anodizing process can promote anodic film growth without substantially promoting anodic film dissolution, thereby increasing the thickness 1914 of barrier layer 1909 to thickness t. This can be accomplished using a non-pore-forming electrolyte, such as one or more of Na ₂ B ₄ 05(OH) ₄ ·8Η ₂ 0 (sodium borate or borax), H ₃ BO ₃ (boric acid), C ₄ H ₆ 0 ₆ (tartaric acid), (ΝΗ ₄ )2θ·5Β ₂ θ3·8Η ₂ 0 (ammonium pentaborate octahydrate), (ΝΗ ₄ )2Β4θ7·4Η ₂ 0 (ammonium tetraborate tetrahydrate), and C ₆ Hio0 ₄ (hexanedioic acid or adipic acid).

[0163] Thickened barrier layer 1909 can enhance the corrosion protection characteristics of metal oxide coating 1904 by providing a thicker physical non- porous barrier between pores 1906 and metal substrate 1902. The anodizing process parameters can be chosen in order to provide a barrier layer 1909 thickness t ranging from about 30 nanometers to about 500 nanometers. In some embodiments, thickness t of barrier layer 1909 is chosen based on providing a color to metal oxide coating 1904 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 1904. That is, diameter 1922 of pores can remain substantially the same before and after the barrier layer thickening process.

[0164] In some embodiments, metal oxide coating 1904 is colorized by infusing a colorant, such as a dye, pigment or metal, within pores 1906 and to impart a particular color to part 1900. In some embodiments where metal oxide coating 1904 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 1904. 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 1904. 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 1904.

[0165] In addition to thickening barrier layer 1909, anodizing in a non-pore- forming electrolyte can also smooth out the boundaries of barrier layer 1909. For example, interface surface 1916, which is defined by barrier layer 1909 on one side and metal substrate 1902 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 1916 and/or pore terminuses 1918 can increase the amount of visible light incident metal oxide coating 1904 that is specularly reflected, thereby increasing the brightness of anodized part 1900. Additionally or alternatively, the barrier layer smoothing process can flatten or smooth pore terminuses 1918 of pores 1906, such that flattened pore terminuses 1918 can also specularly reflect incoming light. In this way, the smooth (i.e., flat) interface surface 1916 and/or pore terminuses 1918 can cause light to specularly reflect off interface surface 1916 and/or pore terminuses 1918, resulting in brightening the appearance of metal oxide coating 1904.

[0166] 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.

[0167] The flatness or smoothness of interface surface 1916 can be quantified as a profile variance defined by distance d between an adjacent peak and valley of the interface surface 1916. Profile variance distance d can be measured, for example, from a transmission electron microscope (TEM) cross section image of the part 1900. In some embodiments, interface surface 1916 achieves a profile variance of no more than 5-6 nanometers.

[0168] FIG. 19C shows part 1900 after colorant particles 1911 are optionally deposited within pores 1906 to give metal oxide coating 1904 a desired color. Colorant particles 1911 can be composed of any suitable colorant material, including suitable dye, pigment or metal material. In some embodiments, colorant particles 1911 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 1904 a saturated black appearance. In some cases, a smoothed interface surface 1916 can brighten the color provided by colorant particles 1911. In some cases, thin film interference effects of barrier layer 1909 create a color that combines with a color provided by colorant particles 1911 to result in a final color. Additionally or alternatively, discoloration of metal oxide coating 1904 by anodizing in an organic acid based electrolyte (e.g., oxalic acid) can combine with the a color provided by colorant particles 1911 to result in a final color. That is, a final color of metal oxide coating 1904 can be a result of one or more of the above-described brightening and color imparting techniques.

[0169] FIG. 19D shows part 1900 after a pore sealing process is performed in order to effectively close pores 1906, thereby enhancing the corrosion resistance, as well as chemical resistance, of metal oxide coating 1904. In addition, the sealing process can make the outer surface of metal oxide coating 1904 compatible for touching from a user, such as a user of an electronic device. Furthermore, if pores 1906 include colorant particles 1911, the sealing process can retain colorant particles 1911 within metal oxide coating 1904. The sealing process can hydrate the metal oxide material 303 of at least top portions of pore walls 1905 of metal oxide coating 1904. In particular, the sealing process can convert metal oxide material 303 to its hydrated form 334, thereby causing swelling of pore walls 1905 and sealing of pores 1906. The chemical nature of hydrated metal oxide material 334 will depend on the composition of metal oxide material 303. For example, aluminum oxide (AI ₂ O 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 1906 of metal oxide coating 1904, while in some embodiments substantially the entire length of pores 1906 of metal oxide coating 1904 is sealed. Any suitable pore sealing process can be used, including exposing part 1900 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).

[0170] After sealing, the metal oxide coating 1904 can provide superior hardness and scratch resistance to part 1900, as well as provide a desired cosmetic appearance to part 1900. The relatively greater density of metal oxide material 303 makes metal oxide coating 1904 harder and more chemically resistant than conventional anodic oxide coating, which can be useful in applications where metal oxide coating 1904 corresponds to an exterior surface of a consumer product (e.g., devices of FIG. 17). In some embodiments, the metal oxide coating 1904 on part 1900 is characterized as having a hardness value of at least 200 HV.

[0171] Corrosion resistance of part 1900 can be measured using standardized salt spray testing, such as per ASTM B117, IS09227, JIS Z 2371 and ASTM G85 standards. In particular embodiments, part 1900 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 Dl 141-98 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 1904 (including thickness 1912 and thickness t) is between about 6 micrometers and about 30 micrometers.

[0172] FIG. 20 shows flowchart 2000, which indicates a process for forming a metal oxide coating in accordance with some embodiments. At 2002, 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. 19C). 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.

[0173] At 2004, 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 ² and about 2.0 A/dm ² was used to form a porous metal oxide coating having a thickness between about 6 micrometers and about 30 micrometers.

[0174] At 2006, 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 ₂ B ₄ 0 ₅ (OH) ₄ ·8Η ₂ 0 (sodium borate or borax), H ₃ BO ₃ (boric acid), C ₄ H ₆ 0 ₆ (tartaric acid), ( Η ₄ )2θ·5Β ₂ θ3·8Η ₂ 0 (ammonium pentaborate octahydrate), ( Η ) ₂ Β ₄ 0 ₇ ·4Η ₂ 0 (ammonium tetraborate tetrahydrate), and C ₆ Hio0 ₄ (hexanedioic acid or adipic acid).

[0175] 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.

[0176] At 2008, 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 2010, 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.

[0177] FIGS. 21 A and 2 IB 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 2102, anodic oxide film 2104 and barrier layer 2106. Substrate 2102 is composed of a 6063 aluminum alloy. Anodic oxide film 2104 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 2106 ranges between about 10 nanometer and about 20 nanometers.

[0178] FIGS. 22A and 22B 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 2106 was increased to between about 60 nanometers and about 70 nanometers, and was found to provide good corrosion protection for substrate 2102. [0179] FIGS. 23A and 23B show SEM cross section images of anodic films prior to and after a barrier layer thickening process is performed. FIG. 23 A shows part 700 after an anodizing process converts a portion of substrate 2302 to metal oxide layer 2304. Substrate 2302 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 2304 has pores 2306 and a barrier layer 2309 having a thickness between about 10 nm and about 20 nm. FIG. 23B shows part 700 after a barrier layer thickening process, and where barrier layer 2309 is thickened to thickness 2310 of between about 300 nm and about 500 nm.

[0180] 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. 24-28 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 1916 L*a*b* color space model measurements.

[0181] FIG. 24 shows perspective views of custom 6063 aluminum alloy samples 2402, 2404, 2406 and 2408 before and after a salt-spray testing procedure in accordance with ASTM B1 17 standard procedures for 336 hours. Sample 2408 includes a type II anodic film without a thickened barrier layer that was not subjected to a salt-spray testing procedure. Sample 2404 includes a type II anodic film without a thickened barrier layer after being subjected to the salt-spray testing procedure. Sample 2406 includes a type II anodic film with a thickened barrier layer that was not subjected to a salt-spray testing procedure. Sample 2408 includes a type II anodic film with a thickened barrier layer after being subjected to the salt-spray testing procedure. Table 6 below summarizes L*a*b* values before and after the salt-spray testing procedure. Table 6 - 6063 Aluminum Alloy (Custom) Salt-Spray Testing

[0182] As indicated by FIG. 24, the sample 2404 without the thickened barrier layer is visibly much darker after the salt-spray testing compared to the sample 2408 with the thickened barrier layer after the same salt-spray testing. Table 6 indicates a much larger difference in J* and b* values for the sample 2404 without the thickened barrier layer compared to J* and b* values for the sample 2408 with the thickened barrier layer.

[0183] FIG. 25 shows perspective views of market grade 6063 aluminum alloy samples 2502, 2504, 2506 and 2508 before and after a salt-spray testing procedure in accordance with ASTM B1 17 standard procedures for 336 hours. Sample 2502 includes a type II anodic film without a thickened barrier layer that was not subjected to a salt-spray testing procedure. Sample 2504 includes a type II anodic film without a thickened barrier layer after being subjected to the salt-spray testing procedure. Sample 2506 includes a type II anodic film with a thickened barrier layer that was not subjected to a salt-spray testing procedure. Sample 2508 includes a type II anodic film with a thickened barrier layer after being subjected to the salt-spray testing procedure. Table 7 below summarizes L*a*b* values of a market grade 6063 substrate samples before and after the salt-spray testing procedure.

Table 7 - 6063 Aluminum Alloy (Market Grade) Salt-Spray Testing

[0184] FIG. 25 indicates that sample 2504 without the thickened barrier layer was visibly darker after the salt-spray testing compared to the sample 2508 with the thickened barrier layer after the same salt-spray testing. Table 7 indicates a much larger difference in J* values for sample 2504 without the thickened barrier layer compared to J* values for the sample 2508 with the thickened barrier layer.

[0185] FIG. 26 shows perspective views of a 7000 series aluminum alloy samples 2602, 2604, 2606 and 2608 before and after a salt-spray testing procedure in accordance with ASTM B1 17 standard procedures for 336 hours. Sample 2602 includes a type II anodic film without a thickened barrier layer that was not subjected to a salt-spray testing procedure. Sample 2604 includes a type II anodic film without a thickened barrier layer after being subjected to the salt-spray testing procedure. Sample 2606 includes a type II anodic film with a thickened barrier layer that was not subjected to a salt-spray testing procedure. Sample 2608 includes a type II anodic film with a thickened barrier layer after being subjected to the salt-spray testing procedure. Table 8 below summarizes L*a*b* values of the 7000 series alloy substrate before and after the salt-spray testing procedure.

Table 8 - 7000 Series Aluminum Alloy Salt-Spray Testing

[0186] FIG. 26 indicates that sample 2604 without the thickened barrier layer was visibly darker after the salt-spray testing compared to the sample 2608 with the thickened barrier layer after the same salt-spray testing. Table 8 indicates a much larger difference in J* values for sample 2604 without the thickened barrier layer compared to L * values for the sample 2608 with the thickened barrier layer.

[0187] FIG. 27 shows perspective views of market grade 6063 aluminum alloy samples 2702, 2704, 2706 and 2708 before and after an ocean water testing procedure in accordance with ASTM Dl 141-98 standard testing procedures (without heave metal). The ocean water testing procedures in accordance with ASTM Dl 141-98, Formula a, Table xl . l, 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.

[0188] Sample 2702 includes a type II anodic film without a thickened barrier layer that was not subjected to ocean water testing procedure. Sample 2704 includes a type II anodic film without a thickened barrier layer after being subjected to the ocean water testing procedure. Sample 2706 includes a type II anodic film with a thickened barrier layer that was not subjected to a ocean water testing procedure. Sample 1108 includes a type II anodic film with a thickened barrier layer after being subjected to the ocean water testing procedure. Table 9 below summarizes L*a*b* values of the market grade 6063 aluminum alloy substrate before and after the ocean water testing procedure.

Table 9 - 6063 Aluminum Alloy (Market Grade) Ocean Water Testing

[0189] FIG. 27 indicates that sample 2704 without the thickened barrier layer was visibly darker after the ocean water testing compared to the sample 2708 with the thickened barrier layer after the same ocean water testing. Table 9 indicates a much larger difference in J* values for sample 2704 without the thickened barrier layer compared to J* values for the sample 2708 with the thickened barrier layer.

[0190] FIG. 28 shows perspective views of market grade 4045 aluminum alloy samples 2802, 2804, 2806 and 2808 before and after an ocean water testing procedure in accordance with the same ASTM Dl 141-98 standard testing procedures described above with reference to FIG. 27. Sample 2802 includes a type II anodic film without a thickened barrier layer that was not subjected to ocean water testing procedure. Sample 1202 includes a type II anodic film without a thickened barrier layer after being subjected to the ocean water testing procedure. Sample 2806 includes a type II anodic film with a thickened barrier layer that was not subjected to a ocean water testing procedure. Sample 1208 includes a type II anodic film with a thickened barrier layer after being subjected to the ocean water testing procedure. Table 10 below summarizes L*a*b* values of the market grade 4045 aluminum alloy substrate before and after the ocean water testing procedure. Table 10 - 4045 Aluminum Alloy (Market Grade) Ocean Water Testing

[0191] FIG. 28 indicates that sample 2804 without the thickened barrier layer was visibly darker after the ocean water testing compared to the sample 2808 with the thickened barrier layer after the same ocean water testing. Table 10 indicates a much larger difference in J* values for sample 2804 without the thickened barrier layer compared to J* values for the sample 2808 with the thickened barrier layer.

[0192] Results from described above with reference to FIGS. 24-28 and Tables 6- 10 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 J* value of the anodic film changes by no more than 9 after a salt-spray test per ASTM B 117 standards or after an ocean water test per ASTM Dl 141-98 standards.

Representative Embodiments

[0193] In some embodiments, a method of coloring an anodic film, the anodic film including a porous layer over a barrier layer, is described. The method includes: smoothing pore terminuses of pores of the anodic film and an interface surface between the barrier layer and the porous layer; and depositing a pigment within pores of the porous layer. In some embodiments, the method further includes forming the anodic film using an anodizing process, wherein prior to forming the anodic film, polishing the metal substrate such that the metal substrate has a uniform metal surface prior to anodizing. In some embodiments, the method further includes widening the pores so as to allow more pigment to be deposited within the pores. In some embodiments, the pigment has a diameter of at least 20 nanometers. In some embodiments, smoothing the interface surface includes: electrolyzing the anodic film in a non-pore-forming electrolyte. In some embodiments, the non-pore-forming electrolyte includes sodium borate or boric acid. In some embodiments, the electrolyzing involves using a voltage greater than about 50 volts. In some embodiments, smoothing the interface surface includes: increasing a thickness of the barrier layer, wherein the thickness of the barrier layer is increased to a predetermined thickness that adds a colored hue to the anodic film by thin film interference. In some embodiments, the pigment is composed of titanium oxide, thereby giving the anodic film a white appearance. In some embodiments, the pigment is composed of carbon black, thereby giving the anodic film a black appearance.

[0194] In some embodiments, a metal article is described. The metal article includes: a metal substrate; and an anodic film covering the metal substrate, the anodic film including: a porous layer having pores with pigment infused therein, and a barrier layer positioned between the porous layer and the metal substrate, wherein an interface surface between the barrier layer and the metal substrate is a sufficiently smooth to direct light incident a top surface of the anodic film toward the pigment within the pores. In some embodiments, a thickness of the barrier layer is greater than about 150 nanometers. In some embodiments, the pigment is composed of titanium oxide to impart a white appearance to the anodic film or carbon black to impart a black appearance to the anodic film.

[0195] In some embodiments, an enclosure for an electronic device is described. The enclosure includes: a metal substrate; and an anodic film covering the metal substrate, the anodic film including: a porous layer having pores with pigment positioned therein, and a barrier layer positioned between the porous layer and the metal substrate, wherein the barrier layer has a thickness greater than about 150 nanometers. In some embodiments, a thickness of the barrier layer is between about 150 nanometers and 500 nanometers. In some embodiments, the pigment is composed of titanium oxide, thereby giving the anodic film a white appearance. In some embodiments, a weight % of titanium oxide within the anodic film is greater than about 1.0 weight %. In some embodiments, a weight % of titanium oxide within the anodic film is between about 1.5 and about 30. In some embodiments, the pigment is composed of carbon, thereby giving the anodic film a black appearance. In some embodiments, a thickness of the porous layer is between about 6 micrometers and 20 micrometers.

[0196] In some embodiments, a method of forming a white appearing metal oxide film is described. The method includes: forming a first layer of the metal oxide film by anodizing a substrate in a first electrolyte; and forming a second layer of the metal oxide film by anodizing the substrate in a second electrolyte different than the first electrolyte, wherein the second layer is more porous than the first layer and has pore wall surfaces that diffusely reflect visible light incident an exterior surface of the metal oxide film so as to impart the white appearance to the metal oxide film. In some embodiments, the method further includes smoothing a barrier layer of the metal oxide film by anodizing the substrate in a non-pore-forming electrolyte, wherein the barrier layer has a profile variance of no more than about 6% of a thickness of the barrier layer. In some embodiments, the thickness of the barrier layer is between about 150 nm and about 800 nm. In some embodiments, the non-pore-forming electrolyte includes at least one of boric acid, borax, ammonium pentaborate octahydrate, ammonium tetraborate tetrahydrate, hexanedioic acid, ammonium adipate, ammonium tartrate, citric acid, maleic acid, glycolic acid, phthalic acid, sodium carbonate, silicic acid or sulfamic acid. In some embodiments, the first electrolyte includes oxalic acid or sulfuric acid, and the second electrolyte includes phosphoric acid. In some embodiments, the white appearing metal oxide film is characterized as having an J* value of at least 80, a b* value between about -3 and about +6, and an a* value of between about -3 and about +3. In some embodiments, the white appearing metal oxide film is characterized as having a W ₁₀ value of at least about 70 and a AL* value of no greater than about 10.

[0197] In some embodiments, an anodized substrate having a white appearance is described. The anodized substrate includes an anodic coating including: a first metal oxide layer having an exterior surface corresponding to an exterior surface of the anodized substrate; and a second metal oxide layer adjacent the first metal oxide layer, wherein the second metal oxide layer is more porous than the first metal oxide layer and has pore wall surfaces that diffusely reflect visible light incident an exterior surface of the anodic coating so as to impart the white appearance to the anodic coating. In some embodiments, the anodic coating has a W ₁₀ value of at least 75. In some embodiments, a thickness of a barrier layer of the anodic coating is between about 150 nanometers and about 800 nanometers. In some embodiments, the anodic coating has a hardness value of about 150 HV or greater. In some embodiments, the first metal oxide layer has a thickness of about 3 micrometers or greater, and the second metal oxide layer has a thickness of about 2 micrometers or greater. In some embodiments, a barrier layer of the anodic coating has a profile variance of no more than about 6% of a thickness of the barrier layer. In some embodiments, the anodized substrate has an J* value of 80 or higher. In some embodiments, the anodized substrate has a. b* value between about -3 and about +6. In some embodiments, the anodized substrate has an a* value of between about -3 and about +3.

[0198] In some embodiments, an enclosure for an electronic device is described. The enclosure includes: an aluminum alloy substrate; and an anodic coating having a white appearance disposed on the aluminum alloy substrate, the anodic coating including: a first metal oxide layer, a second metal oxide layer adjacent the first metal oxide layer, wherein the second metal oxide layer pore wall structure that diffusely reflects incident visible light, and a barrier layer positioned between the second metal oxide layer and the aluminum alloy substrate, wherein a thickness of the barrier layer is between about 150 nanometers and about 800 nanometers. In some embodiments, the thickness of the barrier layer is at least about 6% of a total thickness of the anodic coating. In some embodiments, the anodic coating has an J* value of 80 or higher, a b* value between about -3 and about +6 and an a* value of between about -3 and about +3. In some embodiments, the anodic coating has a W ₁₀ value of at least 75.

[0199] In some embodiments, 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 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. In some embodiments, a hardness of the metal oxide film on the aluminum alloy substrate is about 200 HV or greater. In some embodiments, the first electrolyte includes sulfuric acid, oxalic acid, or a mixture of sulfuric acid and oxalic acid. In some embodiments, the second electrolyte includes at least one of Na ₂ B ₄ 05(OH) ₄ ·8Η ₂ 0 (sodium borate or borax), H ₃ BO ₃ (boric acid), C ₄ H ₆ 0 ₆ (tartaric acid), ( Η ₄ )2θ·5Β ₂ θ3·8Η ₂ 0 (ammonium pentaborate octahydrate), ( Η ) ₂ Β ₄ θ7·4Η ₂ 0 (ammonium tetraborate tetrahydrate), or C ₆ Hi ₀ O (hexanedioic acid or adipic acid). In some embodiments, a thickness of the porous layer is between about 6 micrometers and about 30 micrometers.

[0200] In some embodiments, 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 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. In some embodiments, the pores have diameters ranging between about 10 nanometers to about 20 nanometers. In some embodiments, the pores are defined by pore walls having thicknesses ranging between about 10 nanometers to about 30 nanometers. In some embodiments, the aluminum alloy substrate includes a 7000 series aluminum alloy or a 2000 series aluminum alloy. In some embodiments, the aluminum alloy substrate includes at least 4.0 % by weight of zinc and at least 0.5 % by weight of copper. In some embodiments, a thickness of the exterior oxide layer ranges between about 6 micrometers to about 30 micrometers. In some embodiments, the anodic film has a hardness value of about 200 HV or greater. In some embodiments, the anodic film has a black dye incorporated therein, wherein an J* 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 Dl 141-98 standards.

[0201] In some embodiments, an enclosure for an electronic device is described. The enclosure includes: 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. In some embodiments, a thickness of the exterior oxide layer ranges between about 6 micrometers to about 30 micrometers. In some embodiments, the anodic coating as measured from an exterior surface of the anodic coating has a hardness value of about 200 HV or greater. In some embodiments, the pores have diameters ranging from about 10 nanometers and about 20 nanometers. In some embodiments, the pores are defined by pore walls having thicknesses ranging from about 10 nanometers to about 30 nanometers. In some embodiments, the aluminum alloy substrate has at least 5.4 % by weight of zinc. In some embodiments, the aluminum alloy substrate includes at least 0.5 % by weight of copper. [0202] 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.