Patent Publication Number: US-10760176-B2

Title: Process for reducing nickel leach rates for nickel acetate sealed anodic oxide coatings

Description:
FIELD 
     This disclosure relates generally to anodizing systems and methods. In particular, methods and systems for providing sealed anodic films that are resistant to leaching of nickel are described. 
     BACKGROUND 
     Sealing is an essential aspect of any cosmetic anodizing process for aluminum alloys—necessary to ensure the corrosion resistance of the surface, and to protect the anodic oxide against uptake of dirt and loss of any incorporated coloring agents. Most sealing processes involve exposing the anodic coating to hot aqueous solutions that cause hydration of the pore structure. Although pure boiling water or steam may be used, additives are often added for efficiency and for improved process control and consistency, allowing lower temperatures to be used. 
     One way to increase the time efficiently of the pore sealing process is to use solutions such as nickel acetate and chromate solutions. For example, nickel acetate sealing solutions can provide exceptionally good sealing and can also be very time efficient, sometimes providing a good seal in a matter of seconds. However, use of these sealing solutions can have some disadvantages. For example, nickel originating from the nickel acetate sealing solution can leach out from the sealed anodic films, which may not be desirable in certain types of products. 
     SUMMARY 
     This paper describes various embodiments that relate to anodizing processes and anodic oxide films using the same. The methods described are used to form an anodic oxide film on a metal alloy substrate such that the anodic oxide film is resistant to leaching of any soluble compounds during service, making it better suited to use in wearable devices or devices which are to be in frequent contact with skin. 
     According to one embodiment, a method of reducing a leach rate of a leachable material from a sealed anodic film is described. The method includes immersing the sealed anodic film in a solution suitable for dissolving an amount of the leachable material so as to provide a diffusion path for removal of an amount of the leachable material such that the sealed anodic film achieves a target leach rate or less. The target leach rate is associated with a predetermined amount of the leachable material leached from the sealed anodic oxide film over a predetermined period of time. 
     According to another embodiment, a method of treating a sealed anodic film is described. The method includes heating the sealed anodic film in an aqueous solution having a temperature of at least 80 degrees Celsius for at least 20 minutes such that the sealed anodic film has a nickel leach rate of no greater than 0.06 micrograms/square centimeter/week. 
     According to a further embodiment, a method of treating a sealed anodic film is described. The method includes performing an anodic film modification process on the sealed anodic film. The anodic film modification process forms localized damage in the sealed anodic film. The method also includes exposing the sealed anodic film to a heated aqueous solution having a temperature sufficiently high to repair at least some of the localized damage. 
     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. 
         FIGS. 1A-1B  show stylized cross section views of a surface portion of a part showing aspects of a nickel acetate anodic film sealing process. 
         FIG. 2  shows a stylized cross section view the part in  FIGS. 1A-1B  after undergoing a post-sealing thermal process to remove at least some of the leachable nickel. 
         FIG. 3  shows a graph indicating leach rate results of anodic film samples that have undergone different post-sealing thermal treatments. 
         FIG. 4  shows a schematic view of a system suitable for exposing a part to a post-sealing thermal process. 
         FIG. 5  shows a flowchart indicating a post-sealing thermal process for removing leachable nickel. 
         FIG. 6  shows a flowchart indicating a post-sealing thermal process for completing a sealing process and removing leachable nickel. 
         FIG. 7  shows a flowchart indicating a post-sealing thermal process for repairing structural damage within the sealed anodic film. 
         FIG. 8  shows three flowcharts comparing different types of anodic film treatment processes. 
     
    
    
     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, they are intended to cover alternatives, modifications, and equivalents as can be included within the spirit and scope of the described embodiments. 
     Described herein are processes for providing a sealed anodic film that is resistant to leaching of certain unwanted compounds when exposed to moisture conditions. Examples of such unwanted compounds can include nickel and nickel-containing compounds. Nickel can become incorporated in a sealed anodic film during a sealing process that uses a nickel-containing solution, such as a nickel acetate solution. Some of this nickel can slowly leach from the sealed anodic film when the sealed anodic film is exposed to even relatively low amounts of moisture. This portion of nickel within the sealed anodic film can be referred to as leachable nickel. 
     Methods described herein involve post-sealing thermal processes that remove at least some of the leachable nickel from sealed anodic films as a means of reducing subsequent in-service nickel leach rates. In some embodiments, the post-sealing thermal process involves exposing a sealed anodic film to a heated solution having a temperature sufficiently high to cause dissolution and diffusion of leachable nickel of leachable nickel away from the sealed anodic film and into the heated solution. By dissolving the soluble forms of nickel under these conditions, the resulting sealed anodic film may be rendered far less prone to leaching nickel and nickel compounds during its service life. In some embodiments, the sealed anodic film is immersed in a bath of the heated solution. In other embodiments, the heated solution is only partially immersed or introduced to the sealed anodic film in vapor form. The heated solution can be an aqueous solution, such as water, or non-aqueous solution that provides sufficient dissolution and diffusion of leachable nickel out of the anodic film. 
     The heated solution can be heated to a temperature higher than the conventionally recommended exposure limit for sealed anodic films. For example, the process can be performed at solution temperatures of 50 degrees Celsius or more, in some cases 80 degrees Celsius or more. In some embodiments, the sealed anodic films are exposed to solutions at temperatures up to the solution boiling point (e.g., about 100 degrees Celsius for water). These temperatures are generally recommended to be avoided for seal anodic films in air, or even hot air at high relative humidity conditions since it is widely recognized that such temperatures can cause cracking or crazing of the sealed anodic film. However, it was found that by exposing the sealed anodic films to heated solutions under certain conditions—namely hot hydrating conditions—the sealed anodic films experience no significant cracking or crazing damage. 
     It is further observed that the post-sealing thermal process at higher temperatures may be used to repair some of the minor structural damage that may have been introduced in an intermediate operation, such as laser marking or anodic film surface finishing. The post-sealing thermal process can reduce the corrosion susceptibility of areas where laser marking or surface finishing has been performed. 
     The methods described herein are not limited the reducing leaching of nickel and nickel-containing compounds. That is, the methods can also be used to remove other types of unwanted constituents within a sealed anodic film. For example, the methods can be used to remove compounds relating to the anodizing process (such as sulfate or other anions incorporated during anodizing), to coloring processes (such as dyes, or pigments), or to other sealing solutions (such as other metal acetates, or chromates). 
     The present paper makes specific reference to aluminum oxide films formed from aluminum and aluminum alloy substrates. It should be understood, however, that the methods described herein can be applicable to the treatment of any of a number of other suitable metal oxide films, such as those formed from anodizable metals and metal alloys (e.g., containing titanium, zinc, magnesium, niobium, zirconium, hafnium and tantalum). As used herein, the terms anodic film, anodic layer, and anodic coating, oxide film, oxide layer, oxide coating can be used interchangeably and can refer to any suitable metal oxide material, unless otherwise specified. 
     Methods described herein are well suited for providing durable, chemically clean, and cosmetically appealing surface finishes to consumer products, particularly where frequent, direct skin contact is expected. For example, the methods described herein can be used to form durable and cosmetically appealing finishes for housing or 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.  1 A- 8 . 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. 
       FIGS. 1A-1B  show stylized schematic cross-section views of a surface portion of part  100  showing aspects of nickel acetate anodic film sealing processes. Substrate  104  can be made of any suitable anodizable material, such as aluminum and aluminum alloy.  FIG. 1A  shows part  100  after an anodizing process, wherein a portion of substrate  104  is converted to a corresponding metal oxide or anodic film  102 . The anodizing process forms anodic pores  106  within anodic film  102 , which have openings at exposed surface  108  of anodic film  102 . Anodic pores  106  generally have an average diameter in the nanometers (e.g., 10-150 nm). In some case, anodic pores  106  are utilized to hold colorant  107  (e.g., dye or pigment) that can give part  100  a desired color. Again, this is shown schematically as a reservoir of colorant, uniformly filling the pore, though it is noted that actual colorants may vary significantly in their distributions—from organic dyes, adhered to the walls in just the outermost microns, though inorganic pigments, distributes throughout pores, to metals deposited near the bases of the pores. Anodic pores  106 , however, can also be avenues for corrosion of underlying substrate  104 . In addition, any colorant  107  that resides within anodic pores  106  can leach out of anodic film  102  via the openings of anodic pores  106  at exposed surface  108 . Furthermore, dirt and grime can collect within anodic pores  106 , which can dirty the look of anodic film  102 . 
     One common technique to address these issues is implementation of an anodic pore sealing process.  FIG. 1B  shows part  100  after such a sealing process. Sealing generally involves hydrating the metal oxide material of anodic film  102  into a corresponding hydrated metal oxide material  110 , thereby sealing anodic pores  106 . In effect, the pore walls between anodic pores  106  swell such that the openings of anodic pores  106  at exposed surface  108  close off. Barrier  112  at the outermost region of anodic film  102  proximate to exposed surface  108  can also be formed. In this way, colorant  107  within anodic pores  106  is sealed within closed off anodic pores  106 . Since anodic pores  106  are sealed, dirt, grime and corrosion promoting materials are also prevented from entering anodic pores  106 . 
     Examples of hydrothermal sealing processes include exposing anodic film  102  to a boiling aqueous solution (e.g., 98±2 degrees Celsius) or steam, sufficient to form hydrated metal oxide material  110 . For example, if anodic film  102  includes aluminum oxide (Al 2 O 3 ), hydrated metal oxide material  110  can include boehmite AlO(OH) and/or gibbsite (Al(OH) 3 ). This hydrothermal sealing mechanism is most efficient at relatively high solution temperatures, such as within a few degrees of boiling water. The predominant reaction—hydration to boehmite AlO(OH)—typically occurs at temperatures over 80 degrees Celsius. At lower temperatures, the dominant hydration product can be gibbsite (Al(OH) 3 ) and the process is far less efficient. 
     In some cases, the sealing solution includes additives, such as nickel acetate or chromate, to increase the efficiency of the sealing process. These additives may change the reaction chemistry. In the case of chromates, for instance, aluminum oxidichromate or aluminum oxichromate may be formed in preference over boehmite. Nickel acetate may catalyst or accelerate the hydrothermal sealing mechanism, but it is also believed that nickel hydroxide (Ni(OH) 2 ) may be co-precipitated with the boehmite formation. A typical nickel acetate based sealing chemistry comprises about 1.4-1.8 g/L of nickel, and is operated at pH of 5.5-6.0 and a temperature of 85-90 Celsius—significantly lower than the temperatures required for efficient hydrothermal sealing in pure water (or steam). The high efficiency of a hot nickel acetate seal also results in a more wear-resistant oxide film (as assessed by Taber abrasion) than a hot water sealed film. 
     Nickel acetate based sealing chemistry provides exceptionally good sealing and is also very time efficient in terms of exposure times. For example, nickel acetate sealing can provide an effective barrier  112  in a matter of seconds. Typically, about 1-2 minutes of sealing are recommended per micrometer of coating thickness, such that a 15 minute nickel acetate sealing operation typically provides sufficient sealing of an aluminum oxide anodic film  102  to resist most everyday corrosive environments. It is notable that in a nickel acetate based sealing operation, the openings of the pores are typically plugged within a minute of immersion. This minimizes the leaching of colorants such as organic dyes, and is thus desirable in maintaining precise color control. 
     Nickel acetate sealing always incorporates nickel  116  into the anodic film  102 , particularly in hydrated metal oxide material  110  near exposed surface  108 . In some aluminum oxide anodic films  102 , nickel  116  is incorporated to about 1-3 weight percent (as evaluated in 20 kV surface Energy Dispersive Spectroscopy). Nickel  116  is, for the most part, fixed into the microstructure of hydrated metal oxide material  110  (e.g., boehmite) and is likely in the form of a mixture of hydroxide and acetates. However, the nickel  116  may be in other forms, such as in ionic form or in other compound form. Some of this incorporated nickel  116  can be susceptible to slow leaching under certain conditions. For example, some of nickel  116  can leach from anodic film  102  when exposed to certain conditions—notably moisture or humidity, and especially at low pH—conditions which might be encountered in contact with a user&#39;s skin. This can cause some problems in cases where anodic film  102  is in contact with skin since nickel  116  at some levels can cause irritation in certain, sensitized individuals. An allergic response to nickel is a common cause of contact dermatitis. Standards exist for the acceptable levels of leachable nickel for objects in skin contact, based on test methods such as EN 1811, where the object is placed in an artificial sweat solution for a week and the concentration of nickel leached into the solution is quantified. Although a typical nickel acetate sealed anodic oxide would meet most standards, it is still desirable to further reduce nickel leach rates, to further reduce the likelihood of any allergic responses among users. 
     It is a goal in embodiments described herein to reduce the amount of leachable nickel within anodic film  102  to a predetermined acceptable level. Leachable nickel can refer to that portion of incorporated nickel  116  that most readily leaches from anodic film  102  under certain conditions. The remaining portion of nickel  116  that remains within anodic film  102  when these certain conditions are applied can be referred to as non-leachable nickel. It is not fully understood why some portions of nickel  116  are more leachable than others. For example, the microstructure of anodic film  102  may influence the leachability of nickel  116  in certain regions of anodic film  102 . Additionally or alternatively, certain types of chemical interactions such as bonding of nickel  116  in certain regions of anodic film  102  can influence the leachability of nickel  116 . Without intending to be bound by theory, it is believed that most of the leachable nickel resides mainly near exposed surface  108 . 
     The methods described herein involve removing at least a portion of the leachable nickel within anodic film  102 , which can be achieved by exposing anodic film  102  to a post-sealing thermal process.  FIG. 2  shows part  100  after undergoing a post-sealing thermal process where at least some of the leachable nickel has been removed. As shown, the amount of nickel  116  within anodic film  102  has been significantly reduced. As described above, the mechanism through which leachable nickel leaches from anodic film  102  and the location in anodic film  102  where the leachable portion of nickel  116  resides is not fully understood. As described above, the leachable nickel is believed to mainly reside near exposed surface  108 . Therefore,  FIG. 2  shows most of the reduction of nickel  116  at or near exposed surface  108 . However, some of the leachable nickel may also reside further deep within anodic film  102 . 
     The post-sealing thermal process can involve exposing anodic film  102  to a heated solution such that at least some of the leachable nickel is dissolved and diffused out of anodic film  102  and into the heated solution. In this way, the leaching of the leachable nickel that would normally occur during normal use of part  100  is previously performed in an accelerated manner, resulting in part  100  be pre-leached of most, if not all, of leachable nickel. In some embodiments, the solution is an aqueous solution, while in other embodiments a non-aqueous solution is used. The solution, however, should be suitable for dissolving the leachable nickel and for providing a pathway for diffusion out of anodic film  102 . Since the sealing process has already been performed and anodic pores  106  have already been sealed, this post-sealing thermal dissolution process does not generally require the same high level of solution purity required in a sealing process, nor does it require the same high temperatures or degree of temperature control. It may thus be overflowed and replenished more frequently at lower cost than a conventional hot water seal, or alternatively, it may be replenished less frequently if cost or environmental constraints require this. For example, tap water may be used in some cases. It is nevertheless preferably to use higher purity water to minimize corrosion of certain aluminum alloys, and to use higher temperature for efficiency of the process. In some embodiments, the solution is a deionized water solution. Additives to promote the dissolution of specific leachable materials may also be included in the post-sealing thermal solution, preferably selected so as not to induce any significant damage to the bulk aluminum oxide of anodic film  102 . Examples include dilute acid (e.g., 2% nitric acid), hydrogen peroxide, or ammonia solutions to help dissolve soluble nickel compounds. 
     The temperature and time period of the post-sealing thermal solution can vary depending on a desired amount of leachable nickel removal and time constraints for performing the post-sealing thermal operation. In general, the higher the post-sealing solution temperature, the more leachable nickel removed and the quicker the removal. In addition, the longer the post-sealing thermal process, the more leachable nickel that is removed. However, production and manufacturing requirements can place time constraints on the post-sealing operation whilst the cost and practical difficulties of maintaining the process increase significantly as the temperature approaches its boiling point. Therefore, a balance must be determined based on the pressing constraints for a given production process. In particular embodiments, the temperature ranges between about 80 and 90 degrees Celsius. However, lower or higher temperatures can be used. It is of particular note that temperatures as low as 50 to 70 degrees Celsius have been shown to provide removal of some of the leachable nickel, and that there is no abrupt change in the process efficiency at 80 Celsius, indicating that the mechanism is independent of that of hydrothermal sealing processes. Use of these lower temperatures, however, will generally take longer and therefore may not be preferable in certain situations where the speed of the post-sealing thermal process is important. 
     In some embodiments, the temperature of the post-sealing solution is high enough to further hydrate and seal anodic film  102 , thereby enhancing the previously performed nickel acetate sealing process ( FIG. 1B ). For example, temperatures of 80 degrees Celsius and higher may be sufficiently high to further hydrate anodic film  102 . In some embodiments, the solution temperature is within 5 degrees Celsius of the boiling point of the solution. For example, a water solution can be heated to 100±5 degrees Celsius. These higher temperatures can be used to repair damage, such as small cracks, that can be formed within sealed anodic film  102  during one or more optional post-sealing operations. Details of this repairing function are described further below. 
     As noted above, heating to or beyond a threshold temperature for hydrothermal sealing is not a requirement, however, for removal of leachable nickel. For example, hydration of alumina to boehmite proceeds at temperatures of about 80 degrees Celsius or more. Because the thermal process for effective dissolution of nickel can occur above and below this temperature threshold with similar efficiency, it may be surmised that this process operates independently from the mechanism of hydrothermal sealing. Thus, temperatures of less than 80 degree C. can result in efficient nickel dissolution. For example, temperatures of about 70 degrees Celsius and lower may not be high enough to provide further sealing, but still may be sufficiently high to efficiently remove a desired amount of leachable nickel. 
     A particular embodiment, however, relies on operating within the temperature range of efficient hydrothermal sealing. As such, when the nickel leaching process is itself contributing to the final seal, it is possible to significantly reduce the duration of the initial nickel seal. For instance, a mere 30 second nickel seal may be used—well below the 1-2 minutes per micrometer anodic film thickness conventionally recommended for such a seal. A very brief nickel acetate seal such as this serves primarily to block the pore openings, and limit leaching of colorants during subsequent sealing. This reduced nickel acetate exposure time in itself reduces the amount of nickel incorporated into the anodic oxide, lowering the level of leachable nickel, and further lowering the final level of leachable nickel after the subsequent post-sealing thermal process. By compensating for the reduce nickel acetate sealing duration with hydrothermal sealing during the nickel leaching process, the same final seal integrity (as measured by admittance testing or acid dissolution testing) may be achieved. 
     The post-sealing thermal process does not generally negatively affect retention of colorant  107  within anodic pores  106  since anodic pores  106  have already been sealed. In embodiments where the temperature of the post-sealing thermal process is high enough to promote further hydrothermal sealing, the further sealing may even correct for any incomplete sealing of anodic pores  106  during the sealing process ( FIG. 1B ), thereby facilitating retention of colorant  107  in service. Moreover, some colorants (e.g., metallorganic dyes including heavy metals, heavy-metal base pigments, or metals deposited in pores  106 ) may themselves present undesirable leach levels in service, and in a further embodiment, the leaching of leachable colorants may itself be the objective of the post-seal thermal treatment. It should be noted, however, that the embodiments described herein are not limited to colored anodic films. That is, anodic films without colorants can also benefit from the nickel removal processes described herein. 
     The amount of leachable nickel that is removed from anodic film  102  may not be easily measured using bulk material analyses that measure a total amount of nickel  116  content within anodic film  102 . For example, inspection using a scanning electron microscope (SEM) may not be able to detect a reduction of apparent nickel  116  content within anodic coating  102  after the post-sealing thermal process is complete. This may be because the leachable nickel may only be a small percentage of the total amount of nickel  116  within anodic film. Therefore, other methods, such as measuring a nickel leach rate under predetermined conditions can be used to determine the amount of leachable nickel remaining within anodic film  102  after the post-sealing thermal process. The previously mentioned EN 1811 is a notable example of a test method widely applied to evaluate nickel leach rates from objects. 
     It should be noted that the post-sealing thermal process could additionally or alternatively be used to remove other leachable materials other than nickel from anodic film  102 . These other leachable materials could have been incorporated into anodic film  102  during a sealing process, during an anodizing process and/or during an anodic film coloring process. For example, metal acetates and/or chromates could have been incorporated within anodic film  102  during a sealing process. Sulfates and/or other anions could have been incorporated within anodic film  102  during an anodizing process. Furthermore, metal-organic dye compounds and/or metal-based pigments (e.g., heavy metal-based pigments). 
       FIG. 3  shows a graph indicating leach rate results of sealed anodic film samples A-J that have undergone different post-sealing thermal treatments. The graph of  FIG. 3  shows normalized amounts of nickel released from samples A-J under the same testing conditions. The nickel release rate is obtained by immersing the sealed anodic samples, of a known surface area, within an aqueous solution (most typically, an artificial sweat solution of a certain composition and pH, representative of a relevant population, is used) at a certain temperature (e.g., room temperature), for a certain period of time (e.g., one week) and measuring the amount (e.g., a nickel ion concentration within a given volume of the solution—as measured by a technique such as atomic absorption spectroscopy or inductively coupled plasma-mass spectroscopy) of nickel that is released in the aqueous solution. The amount of nickel within the water solution can be measured using, for example, liquid chromatography mass spectrometry. The relative amount of nickel released can be calculated as amount of nickel released per area (e.g., cm 2 ) of the anodic film. In some embodiments, the amounts of nickel release rates are measured in the order of micrograms or nanograms. 
     All samples A-J have undergone the same, conventional, nickel acetate based sealing process (i.e.,20 minutes for a 10 micrometer thickness of anodic oxide). Samples A and F have not undergone any post-sealing thermal process, and samples B-E and G-J have undergone post-sealing thermal processes in water. Samples F-J have anodic pores infused with dye and samples A-E have no in pore-fused dye. Samples B and G have undergone a 90 degree C. post-sealing thermal process for 30 minutes. Samples C and H have undergone a 90 degree C. post-sealing thermal process for 60 minutes. Samples D and I have undergone a 90 degree C. post-sealing thermal process for 120 minutes. Samples E and J have undergone an 80 degree C. post-sealing thermal process for 60 minutes. 
     As shown, samples B-E and G-J, which have undergone post-sealing thermal processes, released significantly lower amounts of nickel compared to samples A and F, which have not undergone post-sealing thermal processes. In some cases, the nickel release rate was reduced by 1 or 2 orders of magnitude. The graph of  FIG. 3  indicates that higher post-sealing solution temperatures and longer post-sealing times result in more removal of leachable nickel. The temperature and exposure time for the post-sealing thermal process can be chosen based on a desired outcome, in particular, an anodic film having a predetermined target nickel leach rate or below. In some embodiments, the target nickel leach rate is about 0.06 micrograms nickel/cm2/week, or less. In some embodiments, the target nickel each rate is about 0.03 micrograms nickel/cm2/week, or less. In some embodiments, the target nickel each rate is about 0.02 micrograms nickel/cm2/week, or less. In some embodiments, the target nickel each rate is about 0.01 micrograms nickel/cm2/week, or less. In some embodiments, a post-sealing thermal process using a 90 degree C. solution temperature for 30 minutes (samples B and G) is sufficient to accomplish a target nickel leach rate. In some embodiments, a post-sealing thermal process using an 80 degree C. solution temperature at least 20 minutes is used to accomplish a target nickel leach rate. In some embodiments, a post-sealing thermal process using a 95 degree C. solution, or higher, for about 100 minutes is used to accomplish a target nickel leach rate as well as provide further hydrothermal sealing. 
     Although  FIG. 3  shows nickel leach rates for anodic samples that have undergone hot water post-sealing thermal process for 30 minutes and higher using temperatures of 80 degrees or higher, lesser time periods and/or lower temperatures can be used. As described above, the hot water solutions can be as low as 50-70 degrees Celsius. In addition, effective post-sealing nickel removal can occur in time periods of 20 minutes or less, depending on the temperatures. In some embodiments, effective leachable nickel removal occurred using a temperature of at least 80 degrees Celsius for 20 minutes or more. 
     It should be noted that immersing a sealed anodic film to temperatures around or above the sealing temperature (e.g., around 80-100 degrees C.), as described herein, goes against conventional practice and recommendations. Although a warm water rinse after sealing is sometimes recommended to reduce smut residues or facilitate a drying process, the water temperature and amount/length of exposure is limited. For instance, Henkel&#39;s Bonderite (see Henkel Technical Process Bulletin, Bonderite M-ED 9000 Anodizing Seal, Issued Jun. 10, 2013) seal&#39;s technical process bulletin recommends a warm deionized water rinse be used after sealing to facilitate drying, specifying a temperature of 110-140 degrees F. (43-60 degrees C.). One reason that such an operation might not have been considered is that in general, well sealed anodized films have been observed to crack or craze when exposed to temperatures of 80 Celsius or more in vacuum, in air, or even in humid conditions (steam)—with the precise limit depending to some degree on the temperature of the initial sealing operation, and on the conditions of the subsequent heating (such as in the relative humidity of the air). The cracking is due to differential thermal expansion between the substrate and the anodic film. For example, aluminum substrates can have a coefficient of thermal expansion that are about five times greater than that of its corresponding anodic film. In the embodiments described herein, however, it is noted that exposure of previously sealed anodic films to hot aqueous solutions—even at boiling point—can result in substantially no cracking or physical/mechanical damage to the anodic film. 
     It should be noted that the thermal dissolution methods described herein are not limited to removing nickel. That is, the methods described herein can be exploited for the dissolution of any undesirable soluble components of a sealed anodic film. Examples include compounds incorporated from other seal chemistries (e.g., chromates, or other heavy metals or organic compounds), colorants, and also compounds incorporated from anodizing processes. It may also be exploited as a secondary reparatory hydrothermal sealing operation to repair localized damage, which a sealed anodic film might have experienced by such operations as laser marking. Similarly, anodic films that have been sealed and are then subjected to a surface finishing operation (e.g., lapping, buffing and/or polishing) may have had the integrity of their original seal compromised, and benefit from subsequent exposure to the post-sealing thermal processes described herein. The post-sealing thermal process may also help remove hot-water-soluble polishing or buffing compounds, which could otherwise cause discoloration and present a corrosion risk in the anodic coating. 
     It is further noted that the sealing and chemical resistance of an anodic film is not substantially degraded by the treatments described herein. The dissolution occurs on a physical or chemical scale that has no detrimental effect on anodic film microstructure. Surface plugging (as evaluated by dye uptake tests or the ability to immediately wipe off permanent marker with a wet paper towel) is maintained at the high level achieved by a preceding nickel acetate seal. Admittance tests show no increase in admittance and may even show an improvement if the hot water process is conducted at temperatures of over 80 Celsius (such that further hydrothermal may take place). It may thus be surmised that the soluble components of the anodic film, which are removed by the post-sealing thermal process, either plays no positive role in the original seal quality, or that their sealing function is readily replaced by hydration of any damaged sites in the anodic film. 
       FIG. 4  shows a schematic view of system  400  suitable for exposing part  100  to a post-sealing thermal process, in accordance with some embodiments. System  400  includes tank  404  suitable for containing solution  406  and part  100 . Heater  410  can be configured to heat solution  406  to a predetermined temperature as controlled by controller  408 . Tank  404  can include a temperature sensor, such as a thermocouple, that can monitor the temperature of solution  406  a post-sealing thermal process. In some embodiments, a stirring mechanism is used to stir solution  406 . 
     During the post-sealing thermal process, part  100  is immersed within solution  406 , which is heated to a temperature sufficiently high to induce dissolution and diffusion of at least some of the leachable nickel away from sealed anodic film  102  of part  100 . The leachable nickel can be in the form of nickel atoms/ions and/or nickel-containing compounds, such as nickel hydroxides or nickel acetates. Solution  406  can be any solution suitable for inducing dissolution and providing a diffusion path for leachable nickel within anodic film  102 . In some embodiments, solution  406  is an aqueous solution. In a particular embodiment, solution  406  is water, such as deionized water. In some embodiments, where the local water quality permits, and the substrate is sufficiently corrosion resistant, the water may even be tap water, since the purity constraints of a typical sealing process do not apply. 
     As described above, the temperature of solution  406  can vary depending on a desired amount of removal of leachable nickel and process time constrains. In some embodiments, the composition and thickness of anodic film  102  may also factor in determining temperature and exposure time. The temperature and exposure time can be chosen to attain a predetermined nickel leach rate, which can be determined by nickel leach rate methods, such as described above with reference to  FIG. 3 . It should be noted that due to sample-to-sample variation, many samples should be evaluated to assess a given process configuration, and a substantial margin of error should be allowed for. In some embodiments, the predetermined nickel leach rate is no greater than 0.06 micrograms/square centimeter/week. In some embodiments, the temperature of solution  406  is chosen to be high enough cause further hydration of anodic film  102 , thereby repairing possible damage within anodic film  102  caused by other manufacturing processes such as laser marking or surface finishing. In particular embodiments, the temperature of solution  406  is held at a temperature of about 80 degrees Celsius or higher for a time period of at least 20 minutes. In further particular embodiments where solution  406  promotes efficient hydrothermal sealing, it may be used to compensate for a shorter initial sealing time. A nickel acetate process of less than one minute may be used, serving only to plug the openings of pores  106  and fix colorant  107 . This avoids colorant  107  leaching during a subsequent process, which serves a dual purpose of sealing anodic film  102  and removing leachable nickel. In this embodiment, the initial level of leachable nickel is lower, and correspondingly lower levels of leachable nickel are ultimately obtained, whilst the overall sealing process still benefits from the very high efficiency of a nickel acetate seal for fixing a specific color. 
     Anodic film  102  will generally not crack or craze despite exposure to these high temperatures because part  100  and anodic film  102  are immersed in solution  406  rather than in vacuum, air or steam environment. It is possible that anodic film  102  is more flexible and compliant while immersed within solution  406 , thereby making anodic film  102  less prone to cracking during the thermal process. If solution  406  is an aqueous solution, it is possible that in such a hydrating environment that solution  406  is helping to reseal any cracking that is occurring within anodic film  102  due to thermal stress. Regardless of the reason, anodic film  102  does not generally experience substantial cracking or crazing, despite conventional knowledge. 
       FIG. 5  shows flowchart  500  indicating a post-sealing thermal process for removing at least some of a leachable material, such as leachable nickel, within a sealed anodic film, in accordance with some embodiments. At  502 , an anodic film is sealed using an anodic film sealing process. In some embodiments, the sealing process includes using a nickel containing sealing solution. In particular embodiments, the sealing solution includes a nickel salt, such as nickel acetate, which can improve the sealing of anodic pores within the anodic film and decrease the time period for anodic pore sealing, and especially reduce the time taken to provide an adequate block at the pore openings, such that colorants are retained during subsequent sealing. In some embodiments, the anodic film is an aluminum oxide anodic film as part of an aluminum alloy part. In some embodiments, the anodic film has colorant infused within its anodic pores prior to sealing. In other embodiments, the anodic film is not colored. After sealing, the anodic film can be optionally rinsed using, for example, a warm water rinse, to remove residues (e.g., smut) or facilitate a drying process. 
     At  504 , an anodic film modification process is optionally performed. The anodic film modification process can include one or more processes to create a desired cosmetic effect or provide a functional purpose. For example, a laser marking process can be used to form markings on or within the anodic film. Alternatively or additionally, a polishing, lapping and/or buffing process can be used to polish an exposed surface of the anodic film to impart a shiny appearance to the anodic film. In some cases, the anodic film modification process can damage the anodic film to some degree. For example, lapping, buffing and polishing operations affect an exposed top surface of an anodic film, and therefor may negatively affect the quality of the sealed pores. Laser marking can introduce localized defects, such as microcracks (cracks in the scale of micrometers in length), within the structure of the anodic film. 
     At  506 , at least a portion of a leachable material within the anodic film is removed using a post-seal thermal process. In some embodiments, the leachable material is nickel that has been infused within the anodic film during, for example, the sealing process  502 . In some embodiments, the leachable material is a different material incorporated into the anodic film during the sealing process  502 , such as metal acetates or chromates. In some embodiments, the leachable material is one or more of a sulfate, an oxalate and other anions incorporated during a previously performed anodizing process. For example, a sulfate can originate from a sulfuric acid electrolyte and an oxalate can originate from an oxalic acid electrolyte in an anodizing process. In some embodiments, the leachable material is a metal pigment and/or a metal oxide dye compound infused within anodic pores during an anodic film coloring process. In some embodiments, the leachable material includes more than one of the above types of leachable materials. 
     In some embodiments, the leachable material removal process involves immersing the anodic film in a hot aqueous solution. The temperature of the hot aqueous solution and the time period for performing the post-seal thermal process can be chosen such that the anodic film attains a target leachable material leach rate or less. In some embodiments, the anodic film is immersed in an aqueous solution having temperature of at least 80 degrees Celsius for at least 20 minutes. In some embodiments where the leachable material includes nickel, the target nickel leach rate is about 0.06 micrograms per square centimeter per week or less. In some embodiments, the temperature of the post-seal thermal process is high enough to repair damage within the anodic structure of the anodic film. The damage can be in the form of localized cracks created during the anodic film modification at  504 . 
     In some embodiments, the post-seal thermal process is also used to seal a partially sealed anodic film.  FIG. 6  shows flowchart  600  indicating such a process. At  602 , an anodic film is partially sealed using an anodic pore sealing process, such as a nickel acetate sealing process. In contrast to a sealing process where the anodic film is completely sealed, a partial sealing process involves only partially sealing the pores of the anodic film. This can involve exposing the anodic film to the sealing solution for a shorter amount of time than typical sealing processes—as little as one minute or less (well below a typical time of 1-2 minutes per micrometer of anodic oxide film thickness). In some embodiments, the primary purpose of the partial sealing process is to seal or plug the anodic pores well enough to minimize leaching out of colorant during subsequent processing. In particular embodiments where a nickel acetate sealing process is used, the partial sealing is accomplished in one minute or less. 
     At  604 , an anodic film modification process is optionally performed, such as one or more of the laser marking, polishing, lapping and/or buffing process described above. The initial sealing process  602  can serve primarily to block the pore openings of the anodic from and prevent the leaching of colorant during the anodic film modification process  604 . 
     At  606 , at least a portion of the leachable material is removed from the anodic film and the sealing process is completed. That is, the post-sealing process can simultaneously remove some of the leachable material from the anodic film and complete the hydrothermal sealing process  602 . In some embodiments where the leachable material includes nickel from an nickel acetate sealing process, this post-seal process involve immersing the anodic film in an aqueous solution at temperatures of 95 degree Celsius or more for about 2 minutes per micrometer of anodic film thickness. 
     In some embodiments, repair of localized damage is of primary concern rather than a secondary concern.  FIG. 7  shows flowchart  700  indicating a post-sealing thermal process for repairing structural damage within a sealed anodic film, in accordance with some embodiments. At  702 , the anodic film is sealed using a sealing process. The sealing process can be a water based sealing process, or one that includes a catalyst such as nickel acetate or chromate. 
     At  704 , an anodic film modification process is performed on the sealed anodic film. As described above, the anodic film modification process can include a laser marking and/or surface finishing process, which can cause localized defects to form within the anodic film. At  706 , at least some of the damage formed within the sealed anodic film is repaired using a post-sealing thermal process. As described above, the temperature of the solution used for repairing structural defects may at or near the temperatures used for hydrothermal sealing, which can be higher than would be required for removing nickel or other constituents from the sealed anodic film. The flowchart of  FIG. 7 . illustrates that in some cases the primary purpose of the post-sealing thermal process is to repair localized damage within the sealed anodic film rather than removal of nickel. 
       FIG. 8  shows flowcharts  800 ,  802  and  804  comparing different types of anodic film treatment processes. Flowchart  800  indicates a conventional anodic film treatment process and flowcharts  802  and  804  indicate two different anodic film treatment processes that involve post-seal thermal processes in accordance with some embodiments. As shown, conventional process flowchart  800  involves anodizing a substrate to form an anodic film, optionally coloring the anodic film, optionally performing an anodic film modification process, sealing the anodic film, rinsing the anodic film, and then drying the anodic film. If a nickel acetate sealing process is used, the sealing solution typically has a temperature of 85 to 95 degrees Celsius. If a hot water sealing solution is used, the sealing solution typically has a temperature of above 95 degrees C. The anodic film is typically immersed in the sealing solution for about 2 minutes per micrometer of anodic film thickness. The rinsing can be used to remove smut residues. In some cases, the rinsing involves exposing the anodic film to deionized water having a temperature of about 50 to 60 degrees C. for only about 3 minutes to facilitate subsequent drying. 
     Unlike conventional process flowchart  800 , flowcharts  802  and  804  each include performing a post-seal thermal process after the sealing process. The post-seal thermal process can include heating the anodic film to temperatures of about 80 degrees Celsius, 90 degree Celsius, or higher, which is counter to conventional anodic film treatment and practice. The post-seal thermal process can include immersing the anodic film in an aqueous solution at these temperatures until most of a leachable material, such as nickel, is removed from the anodic film, which in some cases can take 15 minutes, 20 minutes, or more. The post-sealing thermal processes of  802  and  804  can also repair some or all of any damage within the anodic film induced by the anodic film modification process, which can include cracks or other local physical damage from laser marking or polishing operations. 
     The process of flowchart  802  includes completely sealing the anodic film prior to the post-sealing thermal process is performed. The process of flowchart  804  includes only partially sealing the anodic film prior to the post-sealing thermal process, then completing the sealing process simultaneously with removing a portion of the leachable material. In this way, the post-sealing thermal process in flowchart  804  further seals the anodic film and also reduces the level of leachable material that can be leached from the anodic film. Since the post-sealing thermal process completes the sealing, the time for the partial sealing process can be shortened. For example, a partial a nickel acetate sealing process can be accomplished in one minute or less, compared to a 1-2 minute per micrometer of anodic oxide thickness used for more traditional sealing under the same conditions. Flowcharts  802  and  804  each include an optional rinsing process to remove residues and a drying process. 
     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 targeted 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.