PATENT DOCUMENT

Publication Number: US-10351966-B2
Application Number: US-201615054004-A
Country: US
Kind Code: B2

Title: Process for cleaning anodic oxide pore structures

Abstract:
Processes for cleaning anodic film pore structures are described. The processes employ methods for gas generation within the pores to flush out contamination within the anodic film. The pore cleaning processes can eliminate cosmetic defects related to anodic pore contamination during the manufacturing process. For example, an anodic film that is adjacent to a polymer piece can experience contamination originating from a gap between the anodic film and polymer piece, which can inhibit colorant uptake of the anodic film in areas proximate the polymer piece. In some cases, an alternating current anodizing process or a separate operation of cathodic polarization is implemented to generate hydrogen gas that bubbles out of the pores, forcing the contaminates out of the anodic film.

Claims:
What is claimed is: 
     
       1. A method of cleaning an anodic oxide coating having pores, the anodic oxide coating overlaying an enclosure for a portable electronic device, wherein the enclosure includes a metal section that is separated from a non-metal section by a gap that contains contaminants, and wherein the metal section has a first coefficient of thermal expansion and the non-metal section has a second coefficient of thermal expansion that is different than the first coefficient of thermal expansion, the method comprising:
 forming the anodic oxide coating by exposing the metal section to an anodizing solution at a temperature that, due to the difference in the first and second coefficients of thermal expansion, causes the gap to enlarge such that the contaminants diffuse from the enlarged gap into at least some of the pores; and 
 subsequent to forming the anodic oxide coating, exposing the anodic oxide coating to an electrolytic cleaning solution concurrently with subjecting the anodic oxide coating to alternating polarization cycles, thereby generating a flow of bubbles from within the pores having the contaminants such that the flow of bubbles forcefully expel the contaminants from the pores. 
 
     
     
       2. The method of  claim 1 , wherein generating the flow of bubbles comprises generating gas at pore terminuses of the pores. 
     
     
       3. The method of  claim 2 , wherein the gas is hydrogen that is generated by cathodic polarization. 
     
     
       4. The method of  claim 3 , wherein the hydrogen is generated from hydrolysis of water from the electrolytic cleaning solution. 
     
     
       5. The method of  claim 3 , wherein the cathodic polarization is imposed as part of forcefully expelling the contaminants from the pores. 
     
     
       6. The method of  claim 1 , wherein the contaminants that are included in the gap are remnants of a machining process. 
     
     
       7. The method of  claim 2 , wherein the gas is oxygen generated by applying a sufficiently high anodic potential to the enclosure to generate the flow of bubbles from within the pores of the anodic oxide coating. 
     
     
       8. The method of  claim 1 , wherein the flow of bubbles forces the contaminants away from an exterior surface of the anodic oxide coating. 
     
     
       9. A method of cleaning an anodic oxide coating of an enclosure of an electronic device, wherein the enclosure includes a metal section that is separated from a non-metal section by a gap, and the gap includes contaminants that are remnants of a machining process, the method comprising:
 forming the anodic oxide coating over the metal section by exposing the metal section to an anodizing solution, wherein a temperature of the anodizing solution causes the gap between the metal section and the non-metal section to enlarge, thereby causing the contaminants to diffuse from the enlarged gap into pores of the anodic oxide coating; 
 exposing the anodic oxide coating to an electrolytic cleaning solution; and 
 during the exposing of the anodic oxide coating to the electrolytic cleaning solution:
 subjecting the anodic oxide coating to alternating anodic and cathodic polarization cycles, thereby generating a flow of bubbles from within the pores of the anodic oxide coating having the contaminants to forcefully expel the contaminants from the pores. 
 
 
     
     
       10. The method of  claim 9 , further comprising:
 incorporating a colorant within the pores of the anodic oxide coating, wherein the anodic oxide coating is cleaned prior to incorporating the colorant such that a region of the anodic oxide coating adjacent to the non-metal section uniformly uptakes the colorant. 
 
     
     
       11. The method of  claim 9 , wherein the metal section and the non-metal section are formed from different materials having different coefficients of thermal expansion. 
     
     
       12. The method of  claim 9 , wherein generating the flow of bubbles comprises generating hydrogen gas within the anodic oxide coating by cathodically polarizing the metal section. 
     
     
       13. A method of forming and cleaning an anodic oxide coating of an enclosure for a portable electronic device, wherein the enclosure includes a metal section that is separated from a non-metal section by a gap that includes contaminants, the method comprising:
 forming the anodic oxide coating that overlays the metal section by exposing the enclosure to an anodic polarization cycle, wherein the anodic polarization cycle is sufficient to cause the gap between the metal section and the non-metal section to expand, thereby causing diffusion of the contaminants from the expanded gap into pores of the anodic oxide coating; and 
 forcefully expelling the contaminants from the pores of the anodic oxide coating by exposing the enclosure to a cathodic polarization cycle, wherein during the cathodic polarization cycle, hydrogen gas is generated from within the pores that forcefully expels the contaminants from within the pores. 
 
     
     
       14. The method of  claim 13 , wherein the metal section and the non-metal section are formed from different materials having different coefficients of thermal expansion. 
     
     
       15. The method of  claim 14 , wherein the anodic and cathodic polarization cycles are applied to the enclosure as an alternating current. 
     
     
       16. The method of  claim 14 , wherein the contaminants that are included in the gap are remnants of a machining process. 
     
     
       17. The method of  claim 15 , wherein the alternating current has a frequency of about 50 Hz. 
     
     
       18. The method of  claim 15 , subsequent to applying the alternating current to the enclosure, the method further comprises:
 removing the enclosure from an anodizing electrolyte; and 
 coloring the anodic oxide coating.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority under 35 U.S.C § 119(e) to U.S. Provisional Application No. 62/233,037, entitled “PROCESS FOR CLEANING ANODIC OXIDE PORE STRUCTURES,” filed on Sep. 25, 2015, which is incorporated by reference herein in its entirety. 
    
    
     FIELD 
     The described embodiments relate generally to anodic films and anodizing processes. More particularly, the present embodiments relate to cleaning anodic films such that they more readily and uniformly accept colorants. 
     BACKGROUND 
     Enclosures for electronic devices can have sections made of various different materials, such as metals, plastics, glass and ceramics. Each type of material has different functional attributes. Metals, for example, can have high tensile strength, can be electrically conductive and can provide a durable cosmetically appealing surface. Some metals, such as aluminum and aluminum alloys, can be anodized so as to form a hard anodic oxide coating that protects the underlying metal and can be readily colored to various shades. Non-metals, such as plastics, are generally radio frequency transparent, and therefore can be positioned over radio frequency transmitting antennas housed within the electronic device without interrupting radio frequency transmission, or can be used to electrically isolate various distinct metal parts of an aluminum enclosure to enable them to act as antennae. 
     One of the challenges with manufacturing of electronic device enclosures relates to the integration of metal sections with non-metal sections. Surface finishing operations, such as polishing and anodizing, are generally performed after the metal sections are secured to the non-metal sections. This means that contaminants from these manufacturing processes can get trapped within gaps between the metal and non-metal sections. These contaminants can leach out of the gaps and get trapped within the anodic oxide coating, eventually interfering with the anodic oxide coating coloring process, resulting in non-uniform coloring and cosmetically unappealing defects of the anodic oxide coating. 
     SUMMARY 
     This paper describes various embodiments that relate to anodic oxide coatings and processes for cleaning anodic oxide coatings. The processes involve the generation of gas within pores of the anodic oxide coatings so as to flush out contaminants that reside within the pores as a result, for example, of various preceding manufacturing operations. 
     According to one embodiment, a method of cleaning an anodic oxide coating on a substrate is described. The method includes immersing the substrate in a solution. The method also includes generating a flow of bubbles from within pores of the anodic oxide coating and out of the pores. The flow of bubbles force contaminants residing within the pores out of the pores. 
     According to another embodiment, a method of cleaning an anodic oxide coating on an enclosure for an electronic device is described. The enclosure includes a metal section having the anodic oxide coating and a non-metal section adjacent to the metal section. The method includes immersing the enclosure within a solution. The method also includes generating a flow of bubbles from within the anodic oxide coating. The flow of bubbles exit the anodic oxide through pores of the anodic oxide coating, thereby cleaning the anodic oxide coating. 
     According to a further embodiment, a method of anodizing a substrate is described. The method includes immersing the substrate in an anodizing electrolyte. The method also includes applying an alternating current to the substrate such that a potential at the substrate oscillates between anodic polarization cycles and cathodic polarization cycles. Surface portions of the substrate are converted to an anodic oxide during the anodic polarization cycles and hydrogen gas is generated within the anodic oxide during the cathodic polarization cycles. The hydrogen gas generates a flow of bubbles from within the pores that exit out of the pores. 
     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 pore cleaning processes described herein. 
         FIGS. 2A and 2B  show plan views of an enclosure having metal and non-metal sections. 
         FIGS. 2C and 2D  show plan views of a portion of an enclosure treated using a standard manufacturing process and an enclosure treated with an anodic pore cleaning process in accordance with some embodiments. 
         FIGS. 3A-3C  show cross-section views of a part undergoing a manufacturing operation illustrating how contaminants can get trapped within gaps between metal and non-metal sections. 
         FIGS. 4A-4C  show close-up cross section views of the part of  FIGS. 3A-3C  during an anodizing process and a dyeing process, illustrating how trapped contaminants can lead to visible defects. 
         FIGS. 5A and 5B  show cross section views of a part undergoing a pore cleaning process in accordance with some embodiments. 
         FIG. 6  shows a system suitable for performing an anodic pore cleaning process in accordance with some embodiments. 
         FIGS. 7A-7C  show plots of different AC waveforms for various pore cleaning process. 
         FIGS. 8A-8C  show plots of different DC processing sequences that can be used in various types of anodizing processes that include a pore cleaning process. 
         FIG. 9  shows a flowchart indicating a process for cleaning and anodic oxide coating. 
     
    
    
     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 cleaning anodic oxide pore structures are described. Methods include generating bubbles within pores of an anodic oxide coating, during or after an anodizing process, that flush out contaminants residing within or around the pores. The pore cleaning processes are well suited for cleaning composite parts that include anodized metal portions that are adjacent to non-metal portions. Contaminants can get trapped within crevices between the metal and non-metal portions during various manufacturing processes, which can seep out of the crevices and work their way into the pores structures, subsequently inhibiting uniform dye uptake, and eventually leading to cosmetic defects. 
     The bubbles can be generated using any of a number of electrolytic techniques, ultrasonic techniques, low-pressure cycling techniques, or suitable combinations thereof. In some embodiments, the bubbles are created by gas evolution within the anodic coating itself near the underlying substrate at bases of the pores (also referred to as pore terminuses). In some cases, this involves an AC anodizing process that includes cathodic polarization cycles in periods between oxide growth. In one embodiment, a high potential (such as a 15-20 V AC anodizing potential) and/or current density (such as 1-4 A/dm 2 ) is applied. This high electrical potential and/or high current density can provide a forceful flow of bubbles and cleaning action. In other cases, AC polarization cycles are performed in operations other than the anodizing process, such as during a metal electrodepositing coloring process, if used. 
     As used herein, the terms anodic oxide, anodic oxide coating, 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 finishes for housing for computers, portable electronic devices, wearable electronic devices, and electronic device accessories, such as those manufactured by Apple Inc., based in Cupertino, Calif. 
     These and other embodiments are discussed below with reference to  FIGS. 1-9 . 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. 
     In some cases, the anodized metal sections of the enclosures for devices  102 ,  104 ,  106  and  108  are positioned adjacent to plastic sections or other non-metallic sections of the enclosures. This means that small gaps are formed at the interface between the metal and non-metallic sections. During manufacturing operations, such as surface finishing or machining operations, contaminants such as chemical residues can become trapped within these small gaps. These contaminants can work their way into the pores of the anodic oxide coating, thereby interrupting the uptake of dye within portions of the anodic oxide coatings adjacent to the plastic sections. The result is unsightly unevenly colored anodic oxide coatings. 
     To illustrate a specific example,  FIGS. 2A and 2B  show plan views of enclosure  200  of an electronic device, such as a portable phone.  FIG. 2A  shows an exterior view of enclosure  200 , which is observable to a user of the electronic device, and  FIG. 2B  shows an interior view of the enclosure  200 , which may not be readily observable to the user. In some embodiments, enclosure  200  corresponds to a rear portion of the electronic device, which couples with a front portion (not shown) to enclose internal components (e.g., processor circuits, memory circuits, flexible circuits, battery, etc.). Enclosure  200  includes metal sections  202 ,  204 , and  206  separated by non-metal sections  208  and  210 . Metal sections  202 ,  204 , and  206  can be coated with an anodic oxide coating, which can be colorized as described above. 
     In some cases, non-metal sections  208  and  210  are made of plastic, such as thermoplastic material that is molded onto surfaces of metal sections  202 ,  204 , and  206 . Non-metal sections  208  and  210  can allow radio frequency wave communication, to pass through enclosure  200  to and/or from an antenna housed within the electronic device. Thus, non-metal sections  208  and  210  can be referred to as radio-transparent sections of enclosure  200 . In some cases, metal sections  202 ,  204 , and/or  206  act as part of an antenna assembly of the electronic device, in which case non-metal sections  208  and  210  can serve to electrically isolate metal sections  202 ,  204 , and  206  from each other. In some embodiments, non-metal sections  208  and  210  are molded within recessed or protruding features  212  of metal sections  202 ,  204 , and  206  to improve engagement and adhesion of non-metal sections  208  and  210  to metal sections  202 ,  204 , and  206 . In some embodiments, non-metal sections  208  and  210  includes cosmetic portions  208   a  and  210   a , which are visible to a user of the electronic device, and structural portions  208   b  and  210   b , which may not be readily visible to the user and provide increased structural integrity and secure bonding with metal sections  202 ,  204 , and  206 . 
     During manufacturing operations, contaminants such as chemical residues can become trapped within small gaps between adjacent non-metal sections  208  and  210  and metal sections  202 ,  204 , and  206 , which can inhibiting uniform dye uptake and eventually lead to cosmetic defects within enclosure  200 . The manufacturing methods described herein can be implemented to reduce the occurrence of these defects. To illustrate,  FIGS. 2C and 2D  illustrate views of an exterior portion of enclosure  220  manufactured using a standard manufacturing process and an exterior portion of enclosure  250  manufactured using an anodic pore cleaning process described herein. 
     At  FIG. 2C , enclosure  220  includes metal sections  222  and  224  coupled together by non-metal section  226 . Non-metal section  226  can correspond to a radio-transparent section of enclosure  220 . As with enclosure  200 , non-metal section  226  can correspond to a radio-transparent section of enclosure  220  that allows radio-frequency transmission to pass to and/or from an antenna housed within enclosure  220 , or can serve to electrically isolate the two adjoining metal sections  222  and  224  to allow one of them to serve as an antenna. Metal sections  222 ,  224 ,  226  are each anodized and colorized such that a colored anodic surface coating is formed thereon. During the anodizing process or other operations, contaminants can get trapped within gaps between metal sections  222  and  224  and non-metal section  226 . These contaminants disrupt the uptake of colorant during a subsequent coloring process, which causes visual defects  228  to form within the anodic coating of near metal sections  222  and  224  the interfaces between metal sections  222  and  224  and non-metal section  226 . 
     At  FIG. 2D , enclosure  250  includes metal sections  252  and  254  coupled together by non-metal section  256 . Enclosure  250  has been treated using a pore cleaning process described herein to clean the anodic coating of metal sections  252  and  254 . As with enclosures  200  and  220 , non-metal section  256  can correspond to a radio-transparent section of enclosure  250  that allows radio-frequency transmission to pass to and/or from an antenna housed within enclosure  250 , or can serve to electrically isolate the two adjoining metal sections  252  and  254  to allow one of them to serve as an antenna. Metal sections  252  and  254  are anodized and colorized to form a colored anodic surface coating. The pore cleaning process cleans the pores from contaminants such that interfaces between non-metal section  256  and adjacent metal sections  252  and  254  can properly uptake colorant, creating a defect-free and uniformly colored anodic coating on metal sections  252  and  254 . 
       FIGS. 3A-3C  show cross-section views of part  300  undergoing a manufacturing operation illustrating how contaminants can get trapped within gaps between metal and non-metal sections of part  300 .  FIG. 3A  shows part  300  prior to the manufacturing operation. Part  300  includes first metal section  302  and second metal section  304  separated by non-metal section  306 . In some embodiments, part  300  corresponds to an enclosure for an electronic device. In some embodiments, first metal section  302  and second metal section  304  are each made of aluminum alloy, and non-metal section  306  is made of plastic. Non-metal section  306  can be radio-transparent so as to let radio frequency waves pass therethrough. Gaps  308  exist at the interfaces between first metal section  302  and non-metal section  306 , and between second metal section  304  and non-metal section  306 . 
       FIG. 3B  shows part  300  undergoing a manufacturing operation in which part  300  is immersed in solution  310 . The manufacturing process can be, for example, a pre-anodizing operation that conditions first metal section  302  and/or second metal section  304  prior to anodizing. For example, solution  310  can be a chemical etching solution, such as a hot phosphoric acid solution, that polishes exposed surfaces of first metal section  302  and/or second metal section  304 . In a particular chemical polishing embodiment, solution  310  is at temperatures of around 80 degrees Celsius. Since first  302  and second  304  metal sections are made of different materials than non-metal section  306 , the hot solution  310  can cause differential thermal expansion of first  302  and second  304  metal sections compared to non-metal section  306 . This can distort the shape of non-metal section  306 , thereby widening gaps  308 . As such, solution  310  can easily enter gaps  308 . 
     Note that the manufacturing operation is not limited to chemical polishing operation, or even limited to pre-anodizing operations. For example, the manufacturing operation can be the anodizing process itself where part  300  is immersed within a solution  310  corresponding to an electrolytic bath. Solution  310  can be an etching solution, such as a hot caustic solution, for cleaning oils and contamination off of part  300  after a machining operation. Furthermore, solution  310  may not necessarily be a hot solution. For example, some anodizing processes dictate an electrolytic bath temperature around 25 degrees Celsius. It is noted, however, that higher temperature solutions  310  may cause more distortion and widening of gaps  308  such that solution  310  can more easily enter gaps  308 . 
       FIG. 3C  shows part  300  after removal from solution  310 . As shown, some solution  310  remains within gaps  308  and has become entrapped as contaminants  311 . The constitution of contaminants  311  will depend on the type of solution  310 . For example, a phosphoric acid solution or sulfuric acid solution can cause contaminants  311  to include phosphates or sulfates, respectively. If solution  310  is a hot solution and part is returned to room temperature, gaps  308  can return to their pre-widened state, thereby further entrapping contaminants  311  within gaps  308 . In some embodiments, part  300  is rinsed in cold water to remove residues of solution  310  from external surfaces  312  of part  300 . The cold temperature can cause opposite distortion of non-metal section  306 , narrowing or closing gaps  308  and exacerbating the entrapment of contaminants  311  within gaps  308 . Once removed from the cold rinse(s), gaps  308  can reopen during subsequent higher temperature processes, such as an anodizing process and/or an anodic film coloring process, when contaminants  311  will be released and enter the porous structure of an anodic coating. This is shown in  FIGS. 4A-4C . 
       FIGS. 4A-4C  show close-up cross section views of part  300  during an anodizing process and a dyeing process, illustrating how trapped contaminants  311  can lead to visible defects.  FIG. 4A  shows part  300  undergoing an anodizing process whereby portions of first metal section  302  and second metal section  304  are converted to first anodic coating  314  and second anodic coating  316 , respectively. First anodic coating  314  and second anodic coating  316  have porous structures in that pores  318  form therein during the anodizing process. Surface portions of first metal section  302  and second metal section  304  are consumed such that the contaminants  311  leach out of gaps  308  and contaminate pores  318  adjacent to non-metal section  306 . In particular, contaminants  311  can become displaced on external surfaces of first  314  and second  316  anodic coatings and/or within pores  318  adjacent to non-metal section  306 . 
     In some embodiments, the anodizing process is a Type II anodizing process as defined by U.S. military specification MIL-A-8625, whereby a resultant anodic coating possesses a very fine pore structure such that pores  318  readily absorb dyes, and is therefore well suited for coloring. Unfortunately, this fine pore structure also makes it easier for contaminants  311  to enter pores  318 . 
       FIG. 4B  shows part  300  as the anodizing process progresses and more of first metal section  302  and second metal section  304  are converted to first anodic coating  314  and second anodic coating  316 , respectively. In some embodiments, the growth of  314  and second  316  anodic coatings results in an external surface of non-metal section  306  recessed with respect to external surfaces of first  314  and second  316  anodic coatings. In some cases, non-metal section  306  returns to its pre-distorted state such that gaps  308  are narrowed, which can cause more of contaminants  311  to be forces out of gaps  308  and onto and/or into adjacent pores  318 . 
     At  FIG. 4C , part  300  is exposed to a dyeing process whereby colorant  320  is deposited onto first  314  and second  316  anodic coatings and/or infused within pores  318 . Colorant  320  can be an organic dye, inorganic dye or a metallic colorant. Contaminants  311  inhibit the uptake of colorant  320  within pores  318  and/or on first  314  and second  316  anodic coatings, resulting in areas of first  314  and second  316  anodic coatings near non-metal section  306  having a visibly different color than surrounding dyed portions. This uneven coloration can be an unacceptable cosmetic defect, wherein either light or dark “staining” of the dyed surface is visible. Some these defects can be attenuated by more thorough cleaning process—in some cases using more than fifteen consecutive rinsing processes between the anodizing and dyeing processes. However, these rinses can be time consuming and may still not eliminate these defects. 
     The present paper provides a way of eliminating these defects by cleaning the anodic oxide pore structure of contaminants  311 , or avoiding the uptake of contaminants  311  during the anodizing process. 
       FIGS. 5A and 5B  show cross section views of portions of part  500  undergoing a pore cleaning process in accordance with some embodiments. Part  500  includes first metal section  502  and second metal section  504  separated by non-metal section  506 . In a particular embodiment, first  502  and second  504  metal sections are each made of aluminum alloy, and non-metal section  506  is made of plastic. Non-metal section  506  can be radio-transparent so as to let radio frequency waves pass, and one or both of first  502  and second  504  metal sections may act as part of an antenna assembly of an electronic device. Gaps  508  exist at the interfaces between first metal section  502  and non-metal section  506 , and between second metal section  504  and non-metal section  506 . Gaps  508  can have contaminants  511  trapped therein. Contaminants  511  can be trapped during, for example, a pre-anodizing process as described above. 
       FIG. 5A  shows part  500  undergoing a modified anodizing process that includes anodizing as well as anodic pore cleaning. Part  500  is immersed in an anodizing electrolytic solution (not shown) and an electric potential is applied such that portions of first  502  and second  504  metal sections are converted to first anodic coating  514  and second anodic coating  516 , respectively. During the anodizing, part  500  acts as an anode electrode. The applied current releases hydrogen (H 2 ) at the cathode (not shown) and oxygen (O 2 ) at the surface of anode part  500 , creating a build up of metal oxide corresponding to first  514  and second  516  anodic coatings. 
     In addition to the anodizing process, the current polarization is periodically switched such that part  500  periodically acts as a cathode, during which time growth of first  514  and second  516  anodic coatings is halted. Thus, during the modified anodizing process part  500  switches from acting as an anode, referred to as an anodic polarization cycle, and as a cathode, referred to as a cathodic polarization cycle. During the cathodic polarization cycle, hydrogen is released from part  500 —in particular, from within pores  518  near underlying metal sections  502  and  504 , also referred to as pore terminuses. The hydrogen gas is a result of the breakdown of water from the anodizing solution. This evolved gas is release from pores  518  in the form of bubbles  517 , which force out any contaminants  511  trapped within pores  518  and/or the exterior surfaces of first  514  and second  516  anodic coatings. In this way, the porous structure of first  514  and second  516  anodic coatings is purged and cleared of contaminants  511 . 
     The switch between cathodic and anodic polarizations can be achieved using a number of techniques. One technique involves applying an alternating current (AC) to part  500  during the anodizing process, instead of a conventional applied direct current (DC) for anodizing. Another technique involves using DC current that is switched between negative and positive polarities at one or more periods during the immersion of part  500  in the anodizing process tank. Details and variations of some of these techniques are described below with reference to  FIGS. 6, 7A-7C and 8A-8C . 
     In an alternative embodiment, applying higher anodic potentials during an anodizing process could generate bubbles  517  of oxygen evolved from the terminuses of pores  518 . However, this embodiment would require high electric potentials that would be associated with a correspondingly high growth rate of first  514  and second  516  anodic coatings. In some situations, this high growth rate and/or high current density may be damaging to first  514  and second  516  anodic coatings as they are growing. 
     Further embodiments of the gas generation within pores  518  can include ultrasonic agitation of a solution, or the application of low-pressure cycles to a solution. In the case of ultrasonic agitation, the ultrasonic pressure can create bubbles  517  by cavitation. In the case of low-pressure cycles, part  500  can be immersed in a solution that is then placed under relatively low pressure or vacuum, which causes gases incorporated within first  514  and second  516  anodic coatings to outgas, thereby creating bubbles  517 . In some embodiments, the ultrasonic agitation and/or low-pressure cycles is/are combined with the electrolytic polarization cycling or high potential anodizing processes described above. 
       FIG. 5B  shows part  500  after the modified anodizing process is complete and colorant  520  is infused within pores  518  of first  514  and second  516  anodic coatings. Colorant  520  can be an organic dye, inorganic dye and/or an electrodeposited metal colorant. First  514  and second  516  anodic coatings are uniformly infused with colorant  520  in the absence of contaminants  511 , resulting in a consistently colored and cosmetically appealing part  500 . In some cases, contaminants  511  can still exist within gaps  508 . Therefore, one may be able to detect whether the pore cleaning processes described herein have been implementing by chemically identifying contaminants  511  within gaps  508  and establishing that pores  518  are substantially free of contaminants  511 . 
     In a particular embodiment, the pore cleaning operation is accomplished during the coloring operation subsequent to the anodizing. For example, an electrodeposition process can be used to electrolytically deposit a metal colorant  520  within pores  518 . For instance, copper and/or tin may be deposited from copper sulfate or tin sulfate solutions, respectively, under alternating current conditions. During cathodic polarization cycles, the metal is deposited within pores  518 —while during anodic polarization cycles, oxygen gas is generated at the terminuses of pores  518  that flushed out contaminants. 
     It should be noted that an electrodeposition process can be, in itself, less sensitive to the contaminant-related defects described above for non-metal dyes since electrodeposited metal colorants do not generally depend on absorption of the dyes into the pore structure, but instead achieve coloration from the optical response of metal deposited in the pore terminuses. Nevertheless, part  500  can be both electro-colored with metal colorant as well as dyed using a non-metal colorant to adjust a final color—especially where light shades of electro-coloring are applied. In such cases, the pore cleaning processes described herein can greatly improve the uniformity of dyed anodic coatings. 
     In addition to the aforementioned benefit to the cosmetics of a colored anodic coating of a part, and especially those parts that incorporate non-metal into their design, the pore cleaning processes can be of benefit in improving adhesive bonding to porous anodic oxide structures. For instance, a molded material can be injection molded to the anodic oxide coating to create a composite part. If the pores of the anodic oxide coating are free from contaminants, the molded material can be more effectively injected into the clean pores, creating a strong interlocking bond with the anodic oxide coating. The pore cleaning process may also overcome localized defects in the sealing of the anodic oxide surface, since typical pore contaminants include phosphates and sulfates, both of which can inhibit the typical hydrothermal sealing processes which are most widely used for the protection of anodic oxide coatings. 
     Note that pore cleaning processes described above may modify the structure or shape of anodic pore structures. In particular, electrolytic polarization cycling and/or high potential anodizing processes described above may change the structure or shape of the pore base and/or the pore walls as compared to standard Type II anodizing processes, which can be detected by analyzing the anodic oxide coatings using high resolution electron microscopy. 
       FIG. 6  shows system  600  suitable for performing an anodic pore cleaning process in accordance with some embodiments. System  600  includes tank  602  suitable for holding solution  604 . Part  606  can be supported by fixture  608  and immersed within solution  604 . Part  606  can be electrically coupled to power source  610  by wires  612   a  and  612   b . Power source  610  can include a rectifier that switches the type of current supplied to part  606  between AC and DC. System  600  can be used in a modified anodizing process that includes a pore cleaning process, a modified electrodepositing process that includes a pore cleaning process, or process solely dedicated to pore cleaning. That is, the pore cleaning can be performed during an anodizing process, during an electrodepositing process (e.g., for electro-coloring an anodic oxide film), or in a stand-alone process separate from other processes. 
     In conventional anodizing processes, a DC current is applied to part  606  causing release of oxygen (O 2 ) at the surface of part  606  and creating a build up of metal oxide corresponding to an anodic oxide film. In addition, hydrogen (H 2 ) is released at the cathode, which can correspond to tank  602  in the set up of system  600 . In an anodizing process modified to include a pore cleaning process, part  606  acts as both an anode and a cathode. This can be accomplished by using AC current, in which the current polarization is periodically reversed, or by using a reversing DC current. Some suitable AC and DC waveforms or processing sequences are described in detail below with reference to  FIGS. 7A-7C and 8A-8C . 
     During positive current polarization, also referred to as an anodic polarization cycle, part  606  acts as an anode thereby promoting anodic oxide film growth. During negative current polarization, also referred to as a cathodic polarization cycle, part  606  acts as a cathode such that oxide film growth is halted and hydrogen gas is released from part  606  to clean the pores. In this way, the modified anodizing process can generate hydrogen gas throughout the anodizing process thereby continuously cleaning the pores. In other respects, the anodizing process can be similar to conventional anodizing processes. For example, in one embodiment, solution  604  is a sulfuric acid solution maintained at a temperature ranging between about 10 to about 30 degrees Celsius. The voltage and current density of the cathodic cycle can be adjusted to generate a sufficient amount of gas and provide a strong enough flow of bubbles to force the contaminants out of the pores. Lower amplitude current densities may not create an amount of gas necessary to sufficient clean the pores, whereas too high of a current density may damage the pore structure. In a particular embodiment where an AC current is used, the root mean square (RMS) voltage ranges between about 8 and 25 volts, and the current density ranges between about 0.5 and about 3.0 A/dm 2 . The duration of modified anodizing process can vary depending on a desired thickness of the anodic film. In one embodiment with a target anodic film thickness of 10 micrometers, the AC current anodizing time period is about 30 minutes or more. 
     In other embodiments, the pore cleaning process is performed in an operation separate from the anodizing process. For instance, the pore cleaning process can be integrated into a metal deposition process used to color the anodic coating. In practice, this would occur after the anodizing process where part  606  is placed into solution  604  containing a metal salt solution (e.g., 5% or 10% tin sulfate solution). In conventional electrodepositing, a part acts as a cathode that attracts positively charged metal ions from solution, which then get deposited within the pores of the anodic oxide film. 
     In a modified metal deposition process that involves pore cleaning, an AC current can be used such that oxygen gas is evolved at part  606 , specifically at the terminuses of the pores, between periods of electrodepositing of metal. As with the modified anodizing process described above, a modified electrodepositing process can involve some periods of time mainly dedicated to pore cleaning and other periods of time mainly dedicated to electrodepositing metal. For example, the pore cleaning time periods can involve use of AC while electrodepositing time periods can involve use of DC. 
     In other embodiments, the pore cleaning process is performed in a separate solution than an anodizing solution or an electrodepositing solution (if used). For example, part  600  can be removed from the anodizing bath and immersed within solution  604  that is dedicated to a pore cleaning process. This can give flexibility as to the type of solution  604  used. For example, solution  604  can be ionic but pH neutral, or mildly acidic, reducing the risk of damaging the anodic oxide film during the cathodic cleaning cycles. In some embodiments, solution  604  is water. However, it may be preferable for solution  604  to be at least slightly ionic such that sufficient current can pass through solution  604 . Solution  604  may be selected so as to have a high dissolving power for a given contaminant—for example, a solution of dilute nitric acid may be selected to promote the dissolution of phosphates. 
     It should be noted that in conventional aluminum anodizing processes, fixture  608  and wires  612   a  and  612   b  are made of a metal that is corrosion resistant to the anodizing process, such as titanium. However, if metals, such as titanium, are exposed within the solution during the cathodic polarization cycle, the hydrogen gas will be mainly generated at these metal surfaces rather than at part  606 . This is because exposed titanium presents an easier path for current under a cathodic polarization cycle and can minimize or eliminate the rate of hydrogen evolution at part  606  to a degree where it is insufficient for pore cleaning. Thus, fixture  608  should allow a sufficient cathodic potential to be achieved for a substantial flow of gas to be generated within the pores. Therefore, in pore cleaning processes, exposed metal surfaces of fixture  608  and wires  612   a  and  612   b  should be coated with a non-conductive material, such as a polymer sheathing. Alternatively, fixture  608  and/or wires  612   a  and  612   b  can be made of the same material as part  606  (e.g., aluminum), which is also anodized during the modified anodizing process. However, an anodic film would build up on fixture  608  and/or wires  612   a  and  612   b , and therefore would need to be periodically replaced or cleaned of the anodic film buildup between anodizing processes. 
       FIGS. 7A-7C  show plots of different AC waveforms for various pore cleaning process. The pore cleaning processes can be performed during an anodizing or metal electrodepositing process, or in a stand-alone pore cleaning process.  FIG. 7A  shows a standard sinusoidal AC current in which the current/voltage continuously oscillates between anodic polarization cycles (positive) and cathodic polarization cycles (negative). Note that the frequency of the AC current can vary. In some embodiments, a 50 Hz AC current was used. If the process is an anodizing process, oxide growth occurs during anodic polarization cycles while pore cleaning occurs during cathodic polarization cycles. If the process is a metal electrodepositing process, metal deposition occurs during cathodic polarization cycles while pore cleaning occurs during anodic polarization cycles. In some embodiments, it is preferred that the entire anodizing process is conducted under AC conditions, such that gas is evolved throughout the process and a single processing operation is involved for both the anodic oxide growth and pore cleaning. 
     Alternatively, an AC pore cleaning process can be combined with a DC anodizing process.  FIG. 7B  shows a variation of a modified anodizing process where standard DC anodizing occurs during a first period of time such that an initial anodic oxide film is grown. Then, the current is switched to an AC current for a pore cleaning period of time. The duration of the AC pore cleaning can vary depending on a number of factors such as the voltage and/or current density. In some embodiments, the AC pore cleaning is relatively brief (e.g., seconds or minutes) compared to the DC anodizing process.  FIG. 7C  shows another variation in which the AC pore cleaning is performed before the DC anodizing. This embodiment may be used in instances where the benefit may be accrued mainly from cleaning of the metal surfaces and the gap between adjacent metal and non-metal sections of a part prior to anodizing. 
       FIGS. 8A-8C  show plots of different DC processing sequences that can be used in various types of anodizing processes that include a pore cleaning process.  FIG. 8A  shows a processing sequence where the pore cleaning occurs during an initial time period, then the polarization is switched for anodizing for a remainder time period.  FIG. 8B  shows the reverse, wherein pore cleaning occurs after the anodizing.  FIG. 8C  shows a processing sequence where the voltage/current is oscillated between anodizing and pore cleaning modes. Note that similar DC processing sequences can be used during an electro-coloring process or during a stand-alone pore cleaning process. 
       FIG. 9  shows flowchart  900  indicating a process for cleaning an anodic oxide coating in accordance with some embodiments. At  902  a substrate is immersed in a solution. The solution can be an anodizing solution, where the pore cleaning process is performed during an anodizing process. The solution can be an electrodepositing solution, wherein the pore cleaning process is performed during a metal deposition process. The solution can be a solution dedicated to a pore cleaning operation. 
     At  904 , a flow of bubbles is created from within pores of an anodic oxide on the substrate. If an anodizing process, the flow of bubbles can be created before, during or after anodic oxide film growth. If a metal electrodepositing process, the flow of bubbles can be created before, during or after depositing of metal within the pores. If the pore cleaning process is performed separately, the pore cleaning process can be performed before or after anodizing or metal electrodepositing processes. The bubbles can be created from hydrogen or oxygen gas that is generated from within the pores, depending on whether the pore cleaning process involves cathodic polarization or high potential anodizing. In some cases, ultrasonic agitation or low-pressure cycling generates the bubbles. The flow of bubbles forces contaminants out of the pores and into the solution. 
     At  906 , the anodic oxide coating is optionally colored using a colorant. The coloring process can include immersing the anodized substrate in a heated solution containing an organic and/or inorganic dye for a period of time sufficient for the organic and/or inorganic dye to seep within pores of the anodic oxide coating, and adsorb onto the surface of the pore walls. Additionally or alternatively, the anodized substrate can be subjected to a metal electrodepositing process where a metal colorant is electrochemically driven into the pores. Since the pores have been cleaned of contaminants, the colorant can be evenly accepted throughout the anodic oxide coating, resulting in a uniformly and cosmetically appealing colored anodic oxide coating. After coloring, the anodic oxide coating can be optionally sealed using a sealing process to close the pores and lock in the colorant. 
     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: 20160225
Publication Date: 20190716
Grant Date: 20190716
Priority Date: 20150925
Inventors: CURRAN, JAMES A.
BURKE, WILLIAM D.
Assignee: APPLE INC
CPC Classifications: [{"code": "C25D11/024", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/24", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04M1/0283", "inventive": false, "first": false, "tree": "[]"}, {"code": "C25D11/243", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/20", "inventive": true, "first": true, "tree": "[]"}, {"code": "C25F1/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04M1/0202", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/246", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04M1/0283", "inventive": false, "first": false, "tree": "[]"}, {"code": "C25D11/243", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/20", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04M1/0202", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/246", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/24", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25F1/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/024", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 58408585