PATENT DOCUMENT

Publication Number: US-10174436-B2
Application Number: US-201615092563-A
Country: US
Kind Code: B2

Title: Process for enhanced corrosion protection of anodized aluminum

Abstract:
Processes for enhancing the corrosion resistance of anodized substrates are disclosed. In some embodiments, the process involves a second anodizing operation that targets an area of the substrate that is left inadequately protected by a first anodizing operation, and also targets defects that may have been arisen from intermediate processing operations such as laser-marking operations. The second anodizing operation can be conducted in a non-pore-forming electrolyte, and grows a thick protective barrier film over inadequately protected areas of the substrate, such as laser-marking treated areas.

Claims:
The invention claimed is: 
     
       1. An enclosure for a portable electronic device, the enclosure comprising:
 a metal substrate; 
 an anodic coating overlaying a surface of the metal substrate, wherein the anodic coating includes a defect region; and 
 a metal oxide plug that is disposed within the defect region of the anodic coating. 
 
     
     
       2. The enclosure of  claim 1 , wherein the anodic coating includes a porous portion and a barrier layer. 
     
     
       3. The enclosure of  claim 2 , wherein the metal oxide plug corresponds to a thickened portion of the barrier layer. 
     
     
       4. The enclosure of  claim 1 , wherein the defect region corresponds to a crack within the anodic coating. 
     
     
       5. The enclosure of  claim 1 , wherein the defect region is a crack propagating from a grain boundary within the metal substrate. 
     
     
       6. The enclosure of  claim 1 , wherein the defect region corresponds to a surface feature of the metal substrate. 
     
     
       7. An enclosure for a portable electronic device, the enclosure comprising:
 a metal substrate having a surface that includes a surface feature; and 
 a metal oxide coating covering the surface of the metal substrate, the metal oxide coating including (i) an interstice that is dependent upon the surface feature of the metal substrate, and (ii) a metal oxide plug disposed within the interstice. 
 
     
     
       8. The enclosure of  claim 7 , wherein the interstice is a crack having a width of about one micrometer or greater. 
     
     
       9. The enclosure of  claim 7 , wherein the metal oxide plug is non-porous. 
     
     
       10. The enclosure of  claim 7 , wherein the interstice defines a moisture ingress pathway to the metal substrate, and the metal oxide plug prevents moisture from reaching the metal substrate. 
     
     
       11. The enclosure of  claim 7 , wherein the surface of the metal substrate is a textured surface, and the surface feature corresponds to a peak of the textured surface. 
     
     
       12. The enclosure of  claim 7 , wherein the surface feature corresponds to a grain boundary of the metal substrate.

Description:
FIELD 
     The described embodiments relate generally to anodic films and anodizing processes. More particularly, the present embodiments relate to processes for repairing defects, such as cracks, within anodic films so as to enhance the corrosion protection properties of the anodic films. 
     BACKGROUND 
     Anodizing is a method of providing a protective anodic oxide film on a metal substrate, often used in industry to provide a protective and sometimes cosmetically appealing coating to metal parts. When subjected to any of a number of manufacturing processes, such as laser marking or other thermal operations, however, anodic oxide films can crack due to internal stresses. Machining and handling can also form cracks or crack-like defects. Substrate geometry can also increase the likelihood of an overlying anodic oxide film to have cracks or crack-like defects. For example, anodic films formed on corners or convex curvatures of a substrate can develop cracks along these corners and curvatures. 
     Although stress-induced cracks are generally very small, if the cracks span the entire thickness of an anodic oxide film, they can present pathways for water or other corrosion-inducing agents to reach the underlying metal substrate through an otherwise protective metal oxide film. Over time and repeated exposure to water or other corrosion-inducing agents during the service life of a part, corrosion of the underlying metal substrate can quickly escalate and further compromise the protective properties of the anodic oxide film. What is needed, therefore, are manufacturing methods for repairing cracks within anodic oxide films, thereby enhancing the corrosion protection of the anodic oxide films. 
     SUMMARY 
     This paper describes various embodiments that relate to anodic oxide films and processes for enhancing the corrosion protection properties of anodic oxide films. The processes involve blocking pathways for water or other corrosion-inducing agents from reaching an underlying substrate via a crack or defect within an anodic oxide film. In particular embodiments, the methods involve thickening a barrier layer, or a portion of the barrier layer, of the anodic oxide film. 
     According to one embodiment, a method of anodizing a substrate is described. The method includes performing a first anodizing operation on the substrate. The first anodizing operation forms an anodic film on the substrate. The method also includes performing an intermediate operation on the anodic film. The intermediate operation forms a defect within the anodic film that provides a pathway through the anodic film to substrate. The method further includes performing a second anodizing operation on the substrate. The second anodizing operation forms a metal oxide plug within the anodic film that blocks the pathway. 
     According to another embodiment, an enclosure for an electronic device is described. The enclosure includes a metal substrate. The enclosure also includes an anodic coating covering a surface of the metal substrate. The anodic coating includes a defect region. The enclosure further includes a metal oxide plug within the defect region of the anodic film. 
     According to a further embodiment, an enclosure for an electronic device is described. The enclosure includes a metal substrate having a surface with a surface feature. The enclosure also includes an anodic coating covering the surface of the metal substrate. The anodic coating includes a crack located near the surface feature of the metal substrate. The anodic coating includes a metal oxide plug within the crack. 
     These and other embodiments will be described in detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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. 
         FIG. 1  shows perspective views of devices having anodized surfaces that can be treated using the methods described herein. 
         FIG. 2A  shows a cross-section view of an anodized part, which includes an anodic oxide film with stress-induced cracks. 
         FIG. 2B  shows a cross-section view of the anodized part of  FIG. 2A  after a barrier layer thickening process has been performed. 
         FIG. 2C  shows a cross-section view of the anodized part of  FIG. 2A  after a sealing process followed by a barrier layer thickening process. 
         FIGS. 3A and 3B  show cross-section views of an anodized part before and after a barrier layer thickening process has been performed. 
         FIGS. 4A and 4B  show cross-section views of an anodized part after a sealing process and a barrier layer thickening process, respectively. 
         FIG. 5  shows a graph indicating open circuit potential measurement data for aluminum alloy substrates with and without undergoing a barrier layer thickening process. 
         FIG. 6  shows a graph indicating open corrosion current density measurement data for aluminum alloy substrates with and without undergoing a barrier layer thickening process. 
         FIG. 7  shows images of a bare substrate, a substrate having only a barrier layer, and a substrate having a sealed conventional porous anodic film, after undergoing a neutral salt spray test to compare corrosion resistances. 
         FIGS. 8A-8C  show perspective views of sections of a metal alloy substrate, illustrating how second phase particles can affect the corrosion protection of a barrier-type film. 
         FIGS. 9A and 9B  show perspective views of sections of an anodized metal alloy substrate, illustrating how a thick barrier layer and overlying porous anodic film have improved corrosion protection. 
         FIG. 10  shows a flowchart indicating a process for improving the corrosion protection of an anodized metal substrate by barrier layer thickening. 
         FIG. 11  shows a flowchart indicating a process for improving the corrosion protection of an anodized metal substrate by localized barrier layer thickening. 
         FIG. 12  shows cross-section scanning transmission electron microscopy (STEM) images of anodized aluminum alloy samples with and without barrier layer thickening, as imaged with a high-angle annular dark field (HAADF) detector. 
         FIG. 13  shows cross-section STEM images of anodized aluminum alloy samples with and without barrier layer thickening, as imaged using bright field (BF) mode imaging. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
     Processes for improving the corrosion protection properties of anodic oxide films are described. In general, anodic oxide films are made of metal oxide material, which is generally harder than the underlying metal of the substrate. Therefore, anodizing is commonly used in industry to provide hard protective coatings for metal parts. Some manufacturing processes, however, such as laser-marking, thermal operations, machining and handling, can put stress on the anodic oxide films and may cause them to crack or acquire other defects. Although many of these cracks can be very small—on the scale of micrometers or tens of micrometers wide—they can allow water or other corrosion-inducing agents to reach the underlying metal substrate. Thus, these cracks can act as pathways for corrosion-inducing agents to reach the underlying metal substrate. This can eventually cause the metal substrate to corrode, especially if the metal substrate is relatively susceptible to corrosion, such as some alloys that have certain alloying elements. 
     Techniques described herein involve increasing the protective characteristics of an anodic oxide film by thickening a barrier layer, or a portion of the barrier layer, of the anodic oxide film. Conventionally, the barrier layer corresponds to a thin non-porous region of the anodic oxide film proximate to the metal substrate, and of thickness approximately equivalent to half an anodic cell pore wall thickness (typically 10-20 nanometers thick in Type II anodic oxides). Thickening this non-porous barrier layer, or a portion thereof, can block off the pathways created by the cracks within the anodic oxide film where corrosion-inducing agents can enter and reach the underlying metal substrate. 
     The barrier layer thickening can be accomplished by performing an additional anodizing process on the already-formed anodic oxide film. The additional anodizing process can be a non-pore-forming anodizing process so as to increase the thickness of the non-porous barrier layer. In some embodiments, the additional anodizing process is performed prior to an anodic film sealing process, resulting in a generally uniform thickening of the barrier layer. In some embodiments, the additional anodizing process is performed after an anodic film sealing process, resulting in localized thickening of the barrier layer—in particular, at locations corresponding to the crack or defects within the anodic oxide film. 
     The methods described herein may be particularly useful in applications that include metal substrates made of certain types of aluminum alloys that are relatively sensitive to corrosion. For example, some aluminum alloys that include relatively high amounts of zinc and magnesium (e.g., some 7000 series alloys) may be more susceptible to the above-described corrosion compared to aluminum alloys having lower amounts of zinc and magnesium (e.g., some 6000 series alloys). Thus, the methods described herein can provide a more robust corrosion protection layer on these corrosion sensitive alloys. It should be noted, however, that the methods described herein are not limited to use on any particular aluminum alloy, or on any particular metal. That is, the methods can be used on any suitable anodizable metal substrate. 
     As used herein, the terms anodic oxide, anodic oxide coating, anodic oxide film, anodic film, anodic layer, anodic coating, oxide film, oxide layer, oxide coating, metal oxide, etc. can be used interchangeably and can refer to suitable metal oxide materials, unless otherwise specified. 
     Methods described herein are well suited for providing cosmetically appealing surface finishes to consumer products. For example, the methods described herein can be used to form durable and cosmetically appealing anodized substrates used in enclosures, or portions of enclosures, for computers, portable electronic devices, wearable electronic devices, and electronic device accessories, such as those manufactured by Apple Inc., based in Cupertino, Calif. 
     These and other embodiments are discussed below with reference to  FIGS. 1-13 . 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. 
     The methods described herein can be used to form durable and cosmetically appealing anodic oxide coatings for metallic surfaces of consumer devices.  FIG. 1  shows consumer products than can be manufactured using methods described herein.  FIG. 1  includes portable phone  102 , tablet computer  104 , smart watch  106  and portable computer  108 , which each can 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 by infusing dyes within pores of the anodic oxide coatings, adding numerous cosmetic options for product lines. 
     During manufacturing, the anodized metal portions of devices  102 ,  104 ,  106  and  108  can be exposed to a number of mechanical, chemical and thermal processes. Such processes can include machining, surface finishing, chemical cleaning, laser marking, and other thermal operations. These manufacturing processes can sometimes cause stress-induced fractures, or cracks, to form within the anodic oxide coatings that are supposed to protect the underlying metal substrates. 
       FIG. 2A  shows a cross-section view of anodized part  200 , which includes anodic oxide film  202  with some of these stress-induced cracks. Anodic oxide film  202  is formed on metal substrate  204 , and can be formed by converting surface portions of metal substrate  204  to its corresponding metal oxide. For example, an aluminum or aluminum metal substrate  204  can be converted to a corresponding aluminum oxide film  202 . One of the most common methods of converting metal to its metal oxide is by anodizing, which is an electrolytic process whereby metal substrate  204  acts as an anode. The type of anodizing or oxidative operation can vary depending on desired structural and cosmetic qualities of anodic oxide film  202 . Most conventional anodizing processes are pore-forming anodizing processes, whereby metal oxide material is simultaneously being formed and dissolved by the electrolyte, forming elongated pores within anodic oxide film  202 . For example, type II anodizing (as defined by MIL-A-8625) generally involves anodizing within a sulfuric acid electrolyte and typically results in a porous, relatively transparent and cosmetically appealing anodic oxide film  202 . 
     In many applications, it is ideal for anodic oxide film  202  to be defect-free, such as represented by defect-free region  206 . As shown, defect-free region  206  is an area of anodic oxide film  202  that does not have defects that span thickness  207  of anodic oxide film  202 . An anodic film that is completely defect-free, however, is very rarely achieved, if ever—at least over a large area. Note that anodic oxide film  202 , including defect-free region  206 , does not show pores that are formed within anodic oxide film  202  during typical anodizing processes, such as type II anodizing. These pores, because of their small size, are not generally considered defects. Furthermore, these pores can be sealed using a sealing process, which will be described in detail below. 
     As described above, cracks within an anodic oxide film can result from any of a number of mechanisms. For example, cracks  208  can result from mechanically or thermally induced strain of part  200 , such as by machining, surface finishing or handling of part  200 . Cracks  210  can be induced by laser-marking, where the metal substrate  204  at interface  211  between metal substrate  204  and anodic oxide film  202  is melted by a laser wavelength to which anodic oxide film  202  is largely transparent in order to produce a dark or black mark  212 . Such laser-marking operations can cause intense localized heating and strain of anodic oxide film  202 , which can result in the formation of cracks  210 . Cracks  208 ,  210 ,  214  and  216  can also be referred to generally as defect regions within anodic oxide film  202 . 
     Concentrations of second-phase particles in metal substrate  204 —such as might occur on grain boundary  213 —may result in crack or defective region  214  to form within anodic oxide film  202  due to excessive dissolution or inhibited growth of anodic oxide film  202  near grain boundary  213 . That is, grain boundary  213  can correspond to a surface feature, albeit very small, on metal substrate  204  that can cause a corresponding defective region or crack  214  to form within anodic oxide film  202 . That is, crack  214  can propagate from grain boundary  213  within metal substrate  204 . In addition, the shape of metal substrate  204  can also affect the anodic oxide film  202 . For example, protruding feature  215 , which can correspond to corner or edge of metal substrate  204 , can also result in crack  216  to form within anodic oxide film  202  simply due to the convex geometry of protruding feature. That is, protruding feature  215  has a convex radius, which can cause a gap or crack  216  to develop during an anodizing process. In some cases, protruding feature  215  is a deliberately engineered macroscopic feature of metal substrate  204 . In other cases, protruding feature  215  is a microscopic texture, such as the peaks on a textured surface (e.g., blasted surface) of metal substrate  204 . 
     Each type of cracks  208 ,  210 ,  214  and  216  can extend through thickness  207  of anodic oxide film  202 , and thus can present a pathway for corrosion-inducing agents to reach metal substrate  204 . It should be noted that a metal substrate  204  made of aluminum alloy is often inherently corrosion resistant, and even a defective anodic oxide film  202  can provide a sufficient barrier against most corrosive environments. However, for certain types of aluminum alloys having certain alloying elements, everyday substances that part  200  can be exposed to (e.g., water, sweat) can be sufficient to cause corrosion at these crack  208 ,  210 ,  214  and  216 —at least under long-term exposure. This has been simulated using tests that accelerate exposure to such substances in order to evaluate chemical sensitivity of part  200 . 
     Many anodizing processes include hydrothermal sealing processes, whereby anodic oxide film  202  is immersed in a hot aqueous solution or steam. The hydrothermal sealing process hydrates exposed portions of the metal oxide material of anodic oxide film  202 , causing the metal oxide material to swell and close up pores with anodic oxide film  202  that were formed during the anodizing process. This greatly improves the corrosion resistance of anodic oxide film  202  since the anodic pores can also provide pathways for corrosion-inducing agents to reach metal substrate  204 . However, the pores formed during the anodizing process are extremely small, typically on the scale of less than 20 nanometers in diameter. Cracks  208 ,  210 ,  214  and  216 , however, are typically too wide for conventional hydrothermal sealing processes to sufficiently close and provide significant protection of metal substrate  204  against corrosion-inducing agents. Thus, even after a conventional sealing process, cracks  208 ,  210 ,  214  and  216  can still provide pathways to metal substrate  204 . 
     The techniques provided herein address these problems by performing a barrier layer thickening process that thickens the barrier layer over metal substrate  204  at cracks  208 ,  210 ,  214  and  216 . The barrier layer thickening is accomplished by performing an additional anodizing operation on the already anodized part  200 . The additional anodizing process can be performed before a sealing operation or after a sealing operation, both of which can enhance the corrosion protection of anodic oxide film  202 .  FIGS. 2B and 2C  illustrate part  200  after barrier layer thickening processes been performed prior to sealing ( FIG. 2B ) and after sealing ( FIG. 2C ). 
       FIG. 2B  shows part  200  after a barrier layer thickening process has been performed and prior to a sealing operation. The barrier layer thickening can involve exposing part  200  to an additional anodizing process that promotes the growth of anodic oxide film  202 , but does so in such a way that promotes non-porous growth. In particular, the thin barrier layer formed during the previous anodizing process used to form anodic oxide film  202  is thickened, thereby forming thickened barrier layer  218 . As shown, thickened barrier layer  218  substantially uniformly covers metal substrate  204 —effectively closing off access for corrosion-inducing agents to reach metal substrate  204  via cracks  208 ,  210 ,  214  and  216 . For laser marking induced cracks  210 , mark  212  remains dark and visible to a user of part  200 . After thickened barrier layer  218  is formed, part  200  may undergo an optional sealing process to close of the nano-scale pores within anodic oxide film  202 . 
       FIG. 2C  shows part  200  after undergoing an alternative process than shown in  FIG. 2B . In particular, part  200  has undergone a sealing process (e.g., a hydrothermal sealing process as described above), followed by a barrier layer thickening process. The sealing process closes off the nano-scale pores formed during the anodizing process to form anodic oxide film  202 . The sealed anodic oxide film  202  provides limited access to metal substrate  204  during the additional anodizing process, such that barrier layer  220  is preferentially thickened at portions of cracks  208 ,  210 ,  214  and  216  near metal substrate  204 . This forms thickened barrier layer sections  222 , which are localized at  208 ,  210 ,  214  and  216  near metal substrate  204 . Thickened barrier layer sections  222  can block off access for corrosion-inducing agents to reach metal substrate  204  via cracks  208 ,  210 ,  214  and  216 . 
     Reference will now be made to  FIGS. 3A, 3B, 4A and 4B , which show detailed aspects of barrier layer thickening processes, in accordance with some embodiments.  FIG. 3A  shows part  300  after an anodizing process has been performed to convert a portion of metal substrate  304  to anodic oxide film  302 . This anodizing process forms a porous anodic oxide film  302  in that pores  305  are formed within porous portion  306  of anodic oxide film  302 . Any suitable anodizing process can be used. In some embodiments, a type II anodizing process involving the use a sulfuric acid based electrolytic bath is used. The size and shape of pores  305  can vary depending on anodizing processing parameters, such as type of electrolytic bath, current density and voltage. In some embodiments, pores  305  have an average diameter of less than about 50 nanometers in diameter—in some embodiments, between about 10 nanometers and about 20 nanometers in diameter. Barrier layer  308  corresponds to a thin non-porous portion of anodic oxide film  302  at an interface between metal substrate  304  and anodic oxide film  302 . In this way, thickness  307  of barrier layer  308  is defined by metal substrate  304  on one side and pore terminuses  309  on another side. After a typical pore-forming anodizing process in sulfuric acid-based bath, barrier layer  308  has a thickness of a few nanometers or a couple of tens of nanometers. 
     As described above, anodic oxide film  302  can include defects such as crack  310 , which can span the entire thickness  311  of anodic oxide film  302  and exposed a portion of metal substrate  304 . Crack  310  can be formed by any mechanism, including mechanically or thermally induced strain, laser marking, defects related to grain boundaries of metal substrate  304 , and/or geometry of metal substrate  304 , as described above. Crack  310  can act as a pathway for moisture or other corrosion-inducing agents to pass through anodic oxide film  302  and reach underlying metal substrate  304 . 
       FIG. 3B  shows the part  300  after an additional anodizing process is performed, which increases thickness  307  of the barrier layer  308  portion of anodic oxide film  302 , including at the interface of metal substrate  304  and anodic oxide film  302  at the site of crack  310 . In some embodiments, barrier layer  308  is substantially uniformly thickened. In other embodiments, barrier layer  308  is thickened more at the site of crack  310 . In any case, this thickening of barrier layer  308  can create metal oxide plug  312 , within crack  310  and over previously exposed portions of metal substrate  304 . As with barrier layer  308 , metal oxide plug  312  is composed of a metal oxide material as a result of the oxidative conversion of metal substrate  304 . 
     Unlike the anodizing process for forming porous portion  306 , the subsequent anodizing process for thickening barrier layer  308  can involve a non-pore-forming anodizing process that promotes growth and thickening of barrier layer  308 . A non-pore-forming anodizing process generally involves growth of metal oxide material without substantial simultaneous dissolution of the metal oxide material. Thus, metal oxide plug  312  can be non-porous, thereby providing a better barrier between metal substrate  304  and the environment. This is in contrast to a dissolution anodizing process, such as the anodizing process used to form the first portion of anodic oxide film  302  at  FIG. 3A . 
     Non-pore-forming anodizing processes can involve the use of an electrolytic bath that promotes metal oxide growth without substantial dissolution and without substantial pore formation. Examples of non-pore-forming solutions which are suitable for the additional anodizing operation include electrolytes having any of a number of weak organic acids, such as one or more of formic acid, malonic acid, maleic acid and tartaric acid—as well as neutral and basic solutions such as one or more of ammonium adipate, sodium borate, sodium hydrogen phosphate, sodium hydroxide and sodium sulfate. In some embodiments, the non-pore-forming electrolyte includes one or both of tartaric acid and sodium tetra-borate. In a non pore-forming electrolyte, barrier layer  308  can achieve a thickness  307  of tens or hundreds of nanometers, depending on the voltage used during the anodizing operation. In a particular embodiment, an electrolyte with tartaric acid in a concentration of about 100 g/l was used. In another particular embodiment, substantially equivalent results (as the tartaric acid, 100 g/l) were obtained using sodium tetraborate in a concentration of about 15 g/l. The applied voltages can vary depending on a desired thickness of barrier layer  308 . In some embodiments, thickness  307  of barrier layer  308  in nanometers is typically about 1.3 times the applied voltage. In some embodiments, suitable voltages range between about 50 V and 300 V. In one embodiment, a voltage of about 200 V is used. 
     Although barrier layer  308  may not provide significant abrasion resistance on its own (i.e., without the presence of porous portion  306 ), it benefits from the mechanical protection of adjacent porous portion  306 . That is, the greater thickness  307  of barrier layer  308  (compared to a conventional barrier layer of just a few or tens nanometers) can provide improved corrosion resistance, while porous portion  306  can provide structural integrity and abrasion resistance. 
     As described above, the additional anodizing operation can be performed before and/or after a sealing process. When it is performed before sealing, it can uniformly thicken barrier layer  308  at terminuses  309  of substantially all pores  305 , as show in  FIGS. 2B and 3B ). In contrast, when the additional anodizing operation is performed after sealing, barrier layer growth can be localized in areas that lack adequate protection, i.e., at the base of cracks or other defects. 
       FIGS. 4A and 4B  show part  400  after a sealing process and a subsequent additional anodizing process has been performed.  FIG. 4A  shows part  400 , which includes anodic oxide film  402  formed over metal substrate  404 . Any suitable pore-forming anodizing process can be used, as described above. Anodic oxide film  402  includes porous portion  406  and barrier layer  408 . In some embodiments, pores  405  are infused with a colorant, such as a dye or metal colorant (not shown), which imparts a corresponding color to anodic oxide film  402 . After the sealing process is performed, the openings of pores  405  outer surface  414  of anodic oxide film  402  are sealed, thereby increasing the corrosion protection ability of anodic oxide film  402  and sealing in colorant (if any) within pores  405 . The sealing process typically involves a hydrothermal sealing operation, whereby anodic oxide film  402  is exposed to a heated aqueous solution or steam. This can hydrate the metal oxide material, converting exposed surfaces of anodic oxide film  402  into a corresponding metal oxide hydrate  416  form. In some embodiments, the aqueous solution or steam includes nickel acetate or other additive to enhance the sealing operation. 
     As shown, crack  410  can span the entire thickness  411  of anodic oxide film  402 . Although the sealing operation can sufficiently swell the metal oxide material of anodic oxide film  402  to seal pores  405 , it may not be sufficient to seal crack  410  if crack  410  is about one micrometer wide or wider. Thus, at  FIG. 4B , an additional anodizing operation is implemented to grow, repair or thicken portions of barrier layer  408  at the base of crack  410 . As with the additional anodizing process described above with reference to  FIGS. 3A and 3B , the barrier layer thickening process can be a substantially non-pore-forming process such dissolution of metal oxide material is minimized. 
     Since anodic oxide film  402  is sealed, the electrolyte used in the anodizing process may not readily access metal substrate  404  via pores  405 . The electrolyte, however, can more readily access metal substrate  404  via crack  410 . Thus, the anodizing preferably converts the portion of metal substrate  404  at the base of crack  410 , thereby forming a localized metal oxide plug  412 . Thickness  415  of metal oxide plug  412  can vary depending on process parameters of the non-pore-forming anodizing process, as described above with reference to  FIG. 3B . In some embodiments, thickness  415  of metal oxide plug  412  is tens or hundreds of nanometers. In some embodiments, thickness  407  of the portion of barrier layer  408  outside of the region around the base of crack  410  remains substantially the same as prior to the additional anodizing process is performed—in other embodiments, this thickness  407  increases. Generally, however, thickness  415  of metal oxide plug  412  is generally larger than thickness  407  of the remaining portions of barrier layer  408  where anodic oxide film  402  does not have a crack or other defect. 
     One of the benefits of methods described herein is most evident on an alloy, which can be inherently somewhat corrosion-prone. For example, some 7000 series aluminum alloys that include relatively high concentrations of magnesium and zinc alloying elements have high strength compared to typical 6000 series aluminum alloys. However, these alloying elements can make certain 7000 series aluminum alloys slightly more prone to corrosion. That is, a non-anodized surface of a 7000 series may be more prone to corrosion under certain environmental exposures (e.g., prolonged sweat, etc.) than an equivalent anodized surface of a 6000 series aluminum alloy. Metal alloys with anodized surfaces are generally well protected against corrosion, but can exhibit localized corrosion sensitivity after, for example, an infrared laser has been used to generate dark marks by locally melting the metal at the metal/oxide interface, as described above. In particular, a 72-hour ASTM B117 salt mist exposure yielded corrosion on about 10% of laser marked regions of an anodized 7000-series aluminum alloy part. However, when the part is protected by the embodiments described herein, this corrosion is eliminated under the same testing conditions. 
     Further quantitative evidence of the corrosion resistance improvement is offered by corrosion potential measurements and corrosion current density measurements, as indicated by Table 1 below. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Open circuit 
                 Corrosion 
               
               
                   
                   
                 potential mV 
                 current density 
               
               
                 Sample 
                 Process 
                 versus SCE 
                 A cm −2   
               
               
                   
               
             
            
               
                 1 
                 Bare 7000-series alloy 
                 −926 ± 3  
                 8.7E−07 ± 6.4E−08 
               
               
                 2 
                 Ano A + Seal 1 
                 −216 ± 52 
                 2.2E−10 ± 7.1E−11 
               
               
                 3 
                 Ano A + Seal 2 
                 −286 ± 8  
                 3.9E−10 ± 6.4E−11 
               
               
                 4 
                 Ano A + Seal 3 
                  −270 ± 104 
                 1.1E−10 ± 9.7E−11 
               
               
                 5 
                 Ano A + Seal 4 
                 −241 ± 13 
                 3.4E−10 ± 9.2E−11 
               
               
                 6 
                 Ano B 
                 −254 ± 13 
                 1.3E−10 ± 8.1E−11 
               
               
                 7 
                 Ano A + Barrier layer 
                  213 ± 48 
                 7.6E−12 ± 7.5E−11 
               
               
                   
                 thickening 
               
               
                   
               
            
           
         
       
     
     Table 1 shows open circuit potentials and corrosion current densities for substrate samples 1-7 of the same type of 7000-series aluminum processed in different ways. Sample 1 corresponds to a bare substrate that has not undergone an anodizing process. Sample 2 corresponds to a substrate after a pore-forming anodizing process (Ano A) and a first type of sealing process are performed. In some embodiments, the pore-forming anodizing process (Ano A) is a sulfuric acid-based anodizing process, and the sealing process is a hydrothermal sealing process. Sample 3 corresponds to a substrate after a pore-forming anodizing process (Ano A) and a second type of sealing process are performed. Sample 4 corresponds to a substrate after a pore-forming anodizing process (Ano A) and a third type of sealing process are performed. Sample 5 corresponds to a substrate after a pore-forming anodizing process (Ano A) and a fourth type of sealing process are performed. Sample 6 corresponds to a substrate after a non-forming anodizing process (Ano B) is performed. Sample 7 corresponds to a substrate after a pore-forming anodizing process and a barrier layer thickening process are performed. The barrier layer thickening process of sample 7 involves further anodizing the substrate with a non-pore-forming anodizing process using 200 V. 
     Sample 1 (bare substrate) has high open circuit potential, as indicated by the large negative value −926 mV+/−3. Anodizing using a pore-forming anodizing process (Ano A) or non-pore-forming anodizing process (Ano B) increases the open circuit potential, as indicated by samples 2-6. However, the open circuit potential of sample 7 indicates that anodizing using a pore-forming anodizing process (Ano A) followed by a barrier layer thickening process drastically increases the open circuit potential (213+/−48 mV). 
     Corrosion current density measurements yield similar results. The corrosion current density of sample 1 (bare substrate) is 8.7×10 −7  A cm −2 . Corrosion current density measurements of samples 2-6 show that anodizing reduces the corrosion current density by about three orders of magnitude, to between 1×10 −10  A cm −2  and 3.9×10 −10  A cm −2 . However, corrosion current density of sample 7 indicates that anodizing using a pore-forming anodizing process (Ano A) followed by a barrier layer thickening process drastically reduces the corrosion current density (7.6×10 −12  A cm −2 ). 
     The data of Table 1 are graphically illustrated in the graphs of  FIGS. 5 and 6 .  FIG. 5  shows graph  500 , which plots open circuit potential measurements for samples 1-7. Graph  500  illustrates how anodizing using either a pore-forming anodizing process or a non-pore-forming anodizing process (samples 2-6) increases the open circuit potential measurement of a substrate compared to a bare substrate (sample 1). However, anodizing using a pore-forming anodizing process and a barrier layer thickening process (sample 7) significantly increases the open circuit potential measurement of a substrate. 
       FIG. 6  shows graph  600 , which plots corrosion current density measurements for samples 1-7. Graph  600  illustrates how anodizing using either a pore-forming anodizing process or a non-pore-forming anodizing process (samples 2-6) decreases the corrosion current density measurement of a substrate compared to a bare substrate (sample 1). However, anodizing using a pore-forming anodizing process and a barrier layer thickening process (sample 7) significantly decreases the corrosion current density measurement of a substrate. 
       FIG. 7  shows images of different aluminum alloy substrate samples after undergoing a neutral salt spray test (in accordance with ASTM B117) to test the corrosion resistance of the substrate samples (100 mm by 50 mm plaques). Samples A, B, C and D are bare aluminum alloy substrates that have not been anodized. Samples E, F, G and H are aluminum alloy substrates that have been anodized using a non-pore-forming anodizing process (without anodizing using a pore-forming anodizing process. Thus, samples E, F, G and H each have barrier-type films without overlying porous portions. The barrier-type films were formed using a 75 V non-pore forming anodizing process, corresponding to resultant non-porous film of about 100 nm in thickness. Samples I, J, K and L are aluminum alloy substrates that have been anodized using a conventional pore-forming anodizing process (e.g., type II anodizing process). The samples were exposed to salt spray for different amounts of time: 0 hours, 12 hours, 24 hours and 48 hours. 
     As expected, the bare substrate samples A, B, C and D are shown to have more discoloration with more salt spray exposure time, indicating significant corrosion of the aluminum alloy. Those samples I, J, K and L protected by a conventional porous anodic oxide film show little discoloration, indicating the conventional porous anodic oxide film can provide good corrosion protection under the salt spray conditions. It should be noted, however, that samples I, J, K and L protected by a conventional porous anodic oxide may experience corrosion if the porous anodic film develops cracks or defects as described above. In particular, for example, if a region of the surface has been laser-marked, it will typically exhibit local corrosion.  FIG. 7  shows, unexpectedly, samples E, F, G and H protected by barrier-type films show significant discoloration, indicating corrosion at a similar rate to the bare unprotected substrate samples A, B, C and D. These results indicate that barrier-type films without overlying porous portions may not, in themselves, deliver significant corrosion protection in the corrosion scenarios of interest for use in enclosures for consumer electronic devices. 
     These results may be an indication of inhomogeneities in the alloy substrate. These aspects are illustrated in  FIGS. 8A-8C , which show perspective views of sections of metal alloy substrate  800 .  FIG. 8A  shows bare metal alloy substrate  800  in an inhomogeneous state, meaning second phase particles  802  (or clusters thereof) are distributed therein. Second phase particles  802  (or clusters thereof) are precipitates that contribute to the strength of metal alloy substrate  800 . For example, in some aluminum alloys second phase particles  802  can correspond to copper-rich regions within metal alloy substrate  800 . 
       FIG. 8B  shows metal alloy substrate  800  after a portion is converted to thin barrier-type film  804 , which corresponds to a non-porous anodic oxide film that can be formed using a non-pore-forming anodizing process, such as described above. Second phase particles  802  (or clusters thereof) within substrate  800  can result in corresponding discontinuities  808  within thin barrier-type film  804 . Discontinuities  808  correspond to inhomogeneities within thin barrier-type film  804 . For example, copper-rich regions as second phase particles  802  within some aluminum alloy substrates can result in copper-rich streak-like discontinuities  808  through barrier-type film  804 . In  FIG. 8B , thickness  806  of thin barrier-type film  804  is very thin (i.e., tens of nanometers), which can be comparable to the dimensions of discontinuities  808  or other inhomogeneities in thin barrier-type film  804 . This means that discontinuities  808  may span thickness  806  of thin barrier-type film  804 . In this way, discontinuities  808  can act as initiation sites for forming pathways for corrosion-inducing agents to pass through thin barrier-type film  804  and reach underlying substrate  800 . 
       FIG. 8C  shows metal alloy substrate  800  after a larger portion is converted to thick barrier-type film  810 . As with thin barrier-type film  804 , thick barrier-type film  810  corresponds to a non-porous anodic oxide film, which can be formed using a non-pore-forming anodizing process. However, thick barrier-type film  810  is grown to a larger thickness  812  compared to thickness  806  of thin barrier-type film  804 . Although thick barrier-type film  810  can significantly reduce the incidence or spatial distribution of discontinuities  808  within thick barrier-type film  810  compared to thin barrier-type film  804  (e.g., by a full order of magnitude)—in some cases leaving a single discontinuity  808  in every square millimeter of a surface of thick barrier-type film  810 . That is, barrier-type film  810  can lessen the number of discontinuities  808  that can act as corrosion initiation sites  809 . Surprisingly, however, the reduced number of initiation sites  809  has not been found to limit corrosion of metal alloy substrate  800  and general corrosion of metal alloy substrate  800  will still occur. That is, thick barrier-type film  810  by itself is ineffective in general corrosion prevention. This is one of the reasons why such non-porous anodizing can be disregarded as a solution for general corrosion protection for metal alloy substrate  800 . 
     In some embodiments described herein, an anodic oxide film having a thick barrier layer portion and a porous portion is found to provide significant reduction in corrosion of an underlying metal alloy substrate. To illustrate,  FIGS. 9A and 9B  show perspective views of sections of metal alloy substrate  900 . Metal alloy substrate  900  includes second phase particles  902  (or clusters thereof), which can contribute to the strength of metal alloy substrate  900 . 
     At  FIG. 9A , a portion of metal alloy substrate  900  is converted to porous anodic oxide film  903  using a pore-forming anodizing process, such as a type II anodizing (sulfuric acid) process. Type II anodizing, for example, typically results in porous anodic oxide film  903  having thickness  910  hundreds of times thicker than barrier-type films  804  or  810  described above with reference to  FIGS. 8A-8C . Porous anodic oxide film  903  includes porous portion  906 , as well as barrier layer  904  as a natural outcome of the pore-forming anodizing process. Barrier layer  904 , however, has an extremely low thickness  912 , i.e., on the scale of a few nanometers or a couple of tens of nanometers. Thus, when crack  909  (which in some cases has a width of about 1 micrometer or more) is formed within porous anodic oxide film  903  during, for example, one or more of the above-described crack-forming processes, barrier layer  904  may not deliver adequate corrosion protection to metal alloy substrate  900 —especially if metal alloy substrate  900  is sensitive to corrosion. In particular, many discontinuities  908  can span thickness  912  of barrier layer  904 , and thus can act as initiation sites for forming pathways for corrosion-inducing agents to pass through thin barrier layer  904  via crack  909  and reach underlying substrate  900 . 
       FIG. 9B  shows metal alloy substrate  900  after a barrier layer thickening process, whereby barrier layer  904  is increased to greater thickness  914 . In some embodiments, this is achieved using a non-pore-forming anodizing process, as described above. Thickened barrier layer  904  can lessen the number of discontinuities  908  that can act as corrosion initiation sites  911 . Compared to thick barrier-type film  810  of  FIG. 8C , however, the presence of the very thick porous portion  906  of anodic oxide film  903  is found to provide additional corrosion protection such that metal alloy substrate  900  is adequately protected despite the presence of crack  909 . These results indicate that, in some embodiments, it is when the statistically significant reduction in corrosion initiation sites  911  is combined with a thick anodic oxide film  903 , which has its own independent population of defects, that a significant benefit in corrosion protection arises. In another words, crack  909  in anodic oxide film  903  means that only a small fraction of the surface of anodic oxide film  903  is relevant, and corrosion is much more dependent on the number of initiation sites  911  compared to thick barrier-type film  810  of  FIG. 8C . An order of magnitude reduction in the spatial density of initiation sites  911  for corrosion now correlates to an order of magnitude improvement in corrosion resistance. 
       FIG. 10  shows flowchart  1000 , indicating a process for improving the corrosion protection of an anodized metal substrate, in accordance with some embodiments. In some cases, the metal substrate is an aluminum alloy that is sensitive to corrosion, such as some aluminum alloys having relatively high levels of magnesium and zinc (e.g., some 7000 series aluminum alloys). In some cases, a surface of the metal substrate has been finished using, for example, one or more abrasive blasting, laser texturing, polishing or buffing operations. 
     At  1002 , a porous anodic film is formed on the metal substrate. In some embodiments, a type II anodizing process is used, which can provide a relatively transparent and cosmetically appealing anodic film. At  1004 , an intermediate processing operation, such as machining, and/or laser marking or texturing, is optionally performed. At  1006 , the barrier layer of the porous anodic film is thickened. In some embodiments, the barrier layer is thickened to a pre-determined thickness, such as a target thickness of tens or hundreds of nanometers. The barrier layer thickening can be achieved by exposing the already anodized metal substrate to a non-pore-forming anodizing process. The final thickness of the barrier layer can depend on processing conditions—for example, the voltage used during the non-pore-forming anodizing process. After the non-pore-forming process is complete, the resultant anodic film retains its porous portion and also includes a thickened barrier layer. 
     At  1008 , the porous anodic film is optionally colored. For example, a colorant (e.g., dye or metal) can be infused within pores of the porous portion of the anodic film. At  1010 , the porous anodic film is optionally sealed using a sealing process. In some cases, the sealing is performed in a hot an aqueous solution (e.g., nickel acetate solution). Sealing the pores of the anodic film can increase the corrosion protection quality of the anodic film since the pores can act as pathways for corrosion-inducing agents (e.g., water, sweat) to come near, and possibly reach, the underlying metal substrate. The sealing process can also retain colorant within the anodic film if the anodic film is colored. At  1012 , a processing operation is optionally performed. In some embodiments, the processing operation includes an anodic film polishing process or other post-sealing manufacturing process. 
       FIG. 11  shows flowchart  1100 , indicating a process for improving the corrosion protection of an anodized metal substrate involving a post-sealing operation, in accordance with some embodiments. As with flowchart  1000  of  FIG. 10 , in some embodiments, the metal substrate is an aluminum alloy that is sensitive to corrosion, such as some aluminum alloys having relatively high levels of magnesium and zinc (e.g., some 7000 series aluminum alloys). Prior to anodizing, in some cases, a surface of the metal substrate has been finished using, for example, one or more abrasive blasting, laser texturing, polishing or buffing operations. 
     At  1102 , a porous anodic film is formed on the metal substrate using a pore-forming anodizing process, such as a type II anodizing process. At  1104 , the porous anodic film is optionally colored, for example, by infusing a colorant within the pores. At  1106 , the porous anodic film is optionally sealed to increase the corrosion protection quality of the anodic film and retain colorant within the anodic film if the anodic film is colored. At  1108 , an intermediate processing operation, such as machining, and/or laser marking or texturing, is optionally performed. 
     At  1110 , the barrier layer of the porous anodic film is locally thickened at locations where any cracks within the anodic film exist, and which are too wide for the sealing process to seal. This can create a sort of metal oxide plug that selectively protects the underlying metal substrate at locations corresponding to cracks and other defect within the anodic film. In some embodiments, the localized barrier layer is thickened to a pre-determined thickness, such as a target thickness of tens or hundreds of nanometers. The localized barrier layer thickening can be achieved by exposing the already anodized and sealed substrate to a non-pore-forming anodizing process. The final thickness of the metal oxide plug can depend on processing conditions—for example, the voltage used during the non-pore-forming anodizing process. At  1112 , an optional processing operation, such as an anodic film polishing process or other post-sealing manufacturing process, can be performed. 
       FIGS. 12 and 13  show scanning transmission electron microscopy (STEM) cross-section images of anodized aluminum alloy samples with and without barrier layer thickening. In  FIG. 12 , the samples were imaged using a high-angle annular dark field (HAADF) detector. Images  1202 ,  1204  and  1206  show an anodized aluminum alloy substrate (substrate), at three different magnifications, that has not undergone a barrier layer thickening operation. Images  1208 ,  1210  and  1212  show an anodized aluminum alloy substrate, at three different magnifications, after undergoing a barrier layer thickening operation. As shown, the barrier layer of the sample having undergone the barrier layer thickening (images  1208 ,  1210  and  1212 ) is about 100 nanometers in thickness, whereas the barrier layer of the sample without the barrier layer thickening (images  1202 ,  1204  and  1206 ) is about 10 nanometers or less. 
     In  FIG. 13 , the samples were imaged using bright field (BF) mode imaging. Images  1302 ,  1304  and  1306  show an anodized aluminum alloy substrate (substrate), at three different magnifications, that has not undergone a barrier layer thickening operation. Images  1308 ,  1310  and  1312  show an anodized aluminum alloy substrate), at three different magnifications, after undergoing a barrier layer thickening operation. As with the images of  FIG. 12 , these images show the barrier layer of the sample having undergone the barrier layer thickening (images  1308 ,  1310  and  1312 ) is about 100 nanometers in thickness, whereas the barrier layer of the sample without the barrier layer thickening (images  1302 ,  1304  and  1306 ) is about 10 nanometers. 
     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.

Metadata:
Filing Date: 20160406
Publication Date: 20190108
Grant Date: 20190108
Priority Date: 20160406
Inventors: CURRAN, JAMES A.
COUNTS, WILLIAM A.
PATERSON, AARON D.
Assignee: APPLE INC
CPC Classifications: [{"code": "H05K5/04", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F1/1613", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/20", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/18", "inventive": true, "first": true, "tree": "[]"}, {"code": "C25D11/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/246", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/08", "inventive": false, "first": false, "tree": "[]"}, {"code": "C25D11/10", "inventive": false, "first": false, "tree": "[]"}, {"code": "C25D11/243", "inventive": false, "first": false, "tree": "[]"}, {"code": "C25D11/04", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F1/1656", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/246", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/243", "inventive": false, "first": false, "tree": "[]"}, {"code": "C25D11/10", "inventive": false, "first": false, "tree": "[]"}, {"code": "C25D11/08", "inventive": false, "first": false, "tree": "[]"}, {"code": "C25D11/04", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F1/1656", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/18", "inventive": true, "first": true, "tree": "[]"}, {"code": "C25D11/18", "inventive": true, "first": true, "tree": "[]"}, {"code": "C25D11/12", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 59998635