Patent Publication Number: US-2022213608-A1

Title: Method for sealing pores

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of Chinese Invention Patent Application No. 202110007568.6, filed on Jan. 5, 2021. 
     FIELD 
     The disclosure relates to a method for sealing pores, and more particularly to a method for sealing pores formed in a metal layer or a metal oxide layer. 
     BACKGROUND 
     Magnesium alloys or aluminum alloys are usually used for manufacturing casings of portable electronic devices due to their good mechanical properties and low specific gravity. 
     Nevertheless, such alloys are highly reactive, and are prone to react with vapor in the air, resulting in corrosion on surfaces of the alloys. Therefore, when an alloy substrate, such as an aluminum alloy substrate, is used to form a casing, the alloy substrate is usually subjected to anodic oxidation to form a porous oxide layer on a surface thereof, thereby increasing the corrosion resistance of the alloy substrate. Subsequently, the porous oxide layer is subjected to a sealing process to seal pores in the porous oxide layer so as to improve surface evenness of the porous oxide layer and the aesthetic appearance of the casing. 
     The sealing process is conventionally conducted by immersing an alloy substrate formed with a porous oxide layer thereon into a solution containing colloids or metal ions for a period of time, such that the colloids or metal ions are deposited on the oxide layer to form a sealing layer that seals pores in the porous oxide layer. However, since the solution is usually acidic or basic, during the sealing process, the solution would infiltrate into the pores of the oxide layer, causing corrosion of the alloy substrate. Thus, the longer time the sealing process takes, the higher the probability of corrosion and the higher the production costs are. 
     SUMMARY 
     Therefore, an object of the disclosure is to provide a method for sealing pores that can alleviate at least one of the drawbacks of the prior art. 
     According to the disclosure, the method includes: providing an object which includes a substrate formed of a metal or alloy of the metal, and a passivation layer formed on the substrate, the passivation layer being formed of metal oxide and having a plurality of pores; 
     immersing the object as a cathode and an anode in a solution containing metal cations and anions; 
     providing an electric current between the anode and the object with an electric current density across the object being less than 0.5 A/dm 2 , such that the metal cations and anions in the solution undergo a redox reaction on the passivation layer; and 
     sealing the pores of the passivation layer with a metallic compound formed by the redox reaction of the metal cations and the anions in the solution. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Other features and advantages of the disclosure will become apparent in the following detailed description of the embodiments with reference to the accompanying drawing, of which: 
         FIG. 1  is a schematic view illustrating an embodiment of a method for sealing pores according to the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Before the disclosure is described in greater detail, it should be noted that where considered appropriate, reference numerals or terminal portions of reference numerals have been repeated among the FIGURE to indicate corresponding or analogous products, which may optionally have similar characteristics. 
     Referring to  FIG. 1 , an embodiment of a method for sealing pores according to the disclosure includes: providing an object  1  which includes a substrate  11  formed of metal or alloy of the metal, and a passivation layer  12  formed on the substrate  11 , the passivation layer  12  being formed of metal oxide and having a plurality of pores; immersing the object  1  as a cathode and an anode  2  in a solution  3  containing metal cations  31  and anions  32 ; providing a predetermined electric current between the anode  2  and the object  1 , and thus generating an electric field between the anode  2  and the object  1 , such that the metal cations  31  and the anions  32  in the solution  3  move onto the object  1  and undergo a redox reaction on the passivation layer  12 ; and sealing the pores of the passivation layer  12  with a metallic compound formed by the redox reaction of the metal cations  31  and the anions  32  in the solution  3 . 
     In this embodiment, the anode  2  is a carbon anode, a stainless steel anode, an aluminum anode or a lead anode. 
     The passivation layer  12  is formed by oxidizing the metal or metal alloy of the substrate  11 . In certain embodiments, the metal of the substrate  11  is aluminum, magnesium, or titanium, but are not limited thereto. 
     In some embodiments, the object  1  further includes a dye dispersed in the passivation layer  12 . 
     In the step of providing the electric current between the anode  2  and the object  1 , an electric current density across the object  1  is controlled to be less than 0.5 A/dm 2 . In certain embodiments, the electric current density ranges between 0.02 A/dm 2  and 0.06 A/dm 2 . Since application of the electric current generates an electric field, electromigration near the object  1  is inhibited, so that electron loss occurring on a surface of the object  1  can be avoided, thereby reducing corrosion of the object  1 . Simultaneously, under the effect of the electric field, the metal cations  31  and the anions  32  in the solution  3  move toward and onto the object  1  and undergo redox reaction on the passivation layer  12 , so as to form the metallic compound that fills or covers the pores in the passivation layer  12  on the surface of object  1  (i.e., on a surface of the passivation layer  12 ). 
     To be more specific, when the electric current is too high, and the electric current density exceeds 0.06 A/dm 2 , the metal cations  31  will move too quickly, causing the metallic compound to be accumulated on a limited region of the surface of the passivation layer  12  (i.e., not being uniformly formed on the surface of the passivation layer of the object  1 ). Thus, the pores in the passivation layer  12  cannot be uniformly and fully filled or covered by the metallic compound. When the electric current is too low, and the electric current density is lower than 0.02 A/dm 2 , the metal cations  31  cannot move efficiently toward and onto the object  1 , which results in an inferior sealing effect. 
     In this embodiment, the metal cations  31  in the solution  3  is selected from at least one of nickel ions, chromium ions, or zirconium ions, and the anions  32  may be acetate ions, fluoride ions, or sulfate ions, but are not limited thereto. The pH value of the solution  3  may range between 3 and 7. In certain embodiments, the temperature of the solution  3  is controlled to range between 60° C. and 96° C. 
     Examples of the disclosure will be described hereinafter. It is to be understood that these examples are exemplary and explanatory and should not be construed as a limitation to the disclosure. 
     Example 1 
     A 5052 aluminum alloy substrate with a porous aluminum oxide layer formed thereon by virtue of anodic oxidation was used as a cathode. The 5052 aluminum alloy substrate and a carbon anode were immersed in a nickel acetate-based solution (TOP SEAL DX-500, Okuno Chemical Industries Co., Ltd.) under a temperature ranging between 90° C. and 95° C. A voltage of 1 V was applied to the carbon anode for 5 minutes, and an electric current density across the 5052 aluminum alloy substrate was 0.05 A/dm 2 . The nickel ions moved toward and onto the 5052 aluminum alloy substrate, and were reacted with the acetate ions in the solution to form a metallic compound of nickel acetate that was deposited on a surface of the porous aluminum oxide layer. The nickel acetate filled or covered pores in the porous aluminum oxide layer so that the pores were sealed, thereby obtaining a silver-colored aluminum alloy product. 
     Example 2 to Example 3 
     The procedures and conditions for preparing the silver-colored aluminum alloy products of Examples 2 and 3 are similar to those of Example 1, except that the voltage was applied for 10 minutes in Example 2 and 15 minutes in Example 3. 
     Example 4 to Example 6 
     A 5052 aluminum alloy substrate with a porous aluminum oxide layer formed thereon by virtue of anodic oxidation was used as a cathode. In each of Examples 4 to 6, a black dye is dispersed in the porous aluminum oxide layer. The 5052 aluminum alloy substrate and a carbon anode were immersed in a nickel acetate-based solution (TOP SEAL DX-500, Okuno Chemical Industries Co., Ltd.) under a temperature ranging between 90° C. and 95° C. A voltage of 1 V was applied to the carbon anode, and an electric current density across the 5052 aluminum alloy substrate was 0.05 A/dm 2 . The voltage was applied for 5 minutes in Example 4, 10 minutes in Example 5, and 15 minutes in Example 6. The nickel ions moved toward and onto the 5052 aluminum alloy substrate, and were reacted with the acetate ions in the solution to form nickel acetate that was deposited on a surface of the porous aluminum oxide layer. The nickel acetate filled or covered pores in the porous aluminum oxide layer so that the pores were sealed, thereby obtaining a black-colored aluminum alloy product. 
     Comparative Example 1 to Comparative Example 4 
     A 5052 aluminum alloy substrate with a porous aluminum oxide layer formed thereon by virtue of anodic oxidation was immersed in the nickel acetate-based solution as used in Example 1. In the nickel acetate-based solution, the nickel ions were reacted with the acetate ions to form nickel acetate that was deposited on a surface of the porous aluminum oxide layer so as to fill or cover pores in the porous aluminum oxide layer, thereby obtaining a silver-colored aluminum alloy product. Immersion of the 5052 aluminum alloy substrate in the nickel acetate-based solution was performed for 5, 10, 15, and 30 minutes in Comparative Examples 1, 2, 3 and 4, respectively. It is noted that, the aluminum alloy product obtained in Comparative Example 1 is sticky, which was considered as a disqualified product and cannot meet industrial requirements. Thus, the aluminum alloy product obtained in Comparative Example 1 was excluded from follow-up evaluation tests. 
     Comparative Example 5 to Comparative Example 7 
     An object containing a 5052 aluminum alloy substrate, a porous aluminum oxide layer formed on the 5052 aluminum alloy substrate by virtue of anodic oxidation, and a black dye dispersed in the porous aluminum oxide layer was provided. The object was immersed in the nickel acetate-based solution as used in Example 1. In the nickel acetate-based solution, the nickel ions were reacted with the acetate ions to form nickel acetate that was deposited on a surface of the porous aluminum oxide layer to fill or cover pores in the porous aluminum oxide layer, thereby obtaining a black-colored aluminum alloy product. Immersion of the object in the nickel acetate-based solution was performed for 10, 15, and minutes in Comparative Examples 5, 6 and 7, respectively. 
     Evaluation Tests 
     A UV degradation test, a salt spray test, a thermal shock test, a temperature and humidity test, and a weight loss test were conducted to evaluate the corrosion resistance of the aluminum alloy products and adhesion of the nickel acetate deposited on the porous aluminum oxide layers in Examples 1 to 6 and Comparative Examples 1 to 6. The test results are summarized in Table 1. 
     UV Degradation Test 
     The UV degradation test was performed using a spectrophotometer (CM-2600d, Konica Minolta) to measure the L*a*b* color values of each of the aluminum alloy products in Examples 1 to 6 and Comparative Examples 1 to 6. The color value L* represents the perceptual lightness of the color (the higher the value, the lighter the color is), the color value represents the green-red chromaticity coordinate (negative values indicate green and positive values indicate red), and the color value b* represents the blue-yellow chromaticity coordinate (negative values indicate blue and positive values indicate yellow). 
     The UV degradation test includes the steps of: measuring initial color values of the aluminum alloy product using the spectrophotometer; conducting a UV radiation exposure cycle for 12 times; and then measuring the final color values of the aluminum alloy product. The UV radiation exposure cycle involves exposing the aluminum alloy product to UV radiation for 4 hours at 60° C., and then placing the aluminum alloy product in an environment without UV radiation at 50° C. for 4 hours. The color difference of the aluminum alloy product before and after the UV radiation exposure, i.e., ΔE value, was calculated using the formula: ΔE=√{square root over (ΔL* 2 +Δa* 2 +Δb* 2 )}, in which ΔL indicates the difference in the color value L* of a region of the aluminum alloy product before and after the UV radiation exposure, Δa indicates the difference in the color value a* of the region before and after the UV radiation exposure, and Δb indicates the difference in the color value b* of the region before and after the UV radiation exposure. The greater the ΔE value, the more severe the discoloration of the aluminum alloy product. 
     Salt Spray Test 
     A cycle of a salt spray test involves placing the aluminum alloy product in a salt spray chamber at 35° C., providing continuous salt water (5% NaCl) mist to the aluminum alloy product in the salt spray chamber for 24 hours; and drying the aluminum alloy product for 24 hours. The cycle was repeated 2 times. After the salt spray test, the aluminum alloy product was cleaned and was observed for any defects (e.g., corrosion, rust, etc.) occurred thereon. 
     Thermal Shock (TS) Test (TS Test) 
     A cycle of the TS test involves placing the aluminum alloy product in a TS tester, and exposing the aluminum alloy product to a temperature difference between −20° C. (i.e., minimum temperature) and 60° C. (i.e., maximum temperature) with a temperature transition rate of 20° C. per minute. In each of the cycle, the aluminum alloy product was kept at each of the maximum and minimum temperatures for 10 minutes. After performing 48 cycles of the TS test, which lasted for 24 hours, the appearance of the aluminum alloy product was observed whether or not nickel acetate was peeled off from the surface of the aluminum alloy substrate. 
     Temperature and Humidity Test (TH Test) 
     The temperature and humidity (TH) test was carried cut by placing the aluminum alloy product in a test chamber under 60° C. and 95% humidity for 4 days. After the TH test, the appearance of the aluminum alloy product was observed whether or not nickel acetate was peeled off from the aluminum alloy substrate, or if any defect, such as blisters or cleavages, occurred. 
     Weight Loss Test 
     The corrosion rate of the aluminum alloy product was evaluated by the weight loss test. First, the initial weight W 1  of the aluminum alloy product was measured. Then, the aluminum alloy product was immersed in a chromium trioxide (CrO 3 ) solution at 38° C. for 15 minutes, followed by cleaning and drying the aluminum alloy product. Thereafter, the weight W 2  of the aluminum alloy product was measured. Weight loss ratio (W L ) was determined by the formula: 
     
       
         
           
             
               
                 W 
                 L 
               
               = 
               
                 
                   
                     W 
                     1 
                   
                   - 
                   
                     W 
                     2 
                   
                 
                 A 
               
             
             , 
           
         
       
     
     in which A represents a surface area of the aluminum alloy product before immersion in the chromium trioxide solution. Sealing is determined to be complete if W L  of a sealed aluminum alloy product is not greater than 20 mg/dm 2 . 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 UV Degradation 
                 Salt 
                   
                 Weight 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Pore Sealing Parameters 
                 Test 
                 Spray 
                 TS 
                 TH 
                 Loss 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Temperature 
                 Time 
                 Voltage 
                   
                 After exposure 
                 Test 
                 Test 
                 Test 
                 Ratio 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 (° C.) 
                 (min) 
                 (V) 
                 Dye 
                 (ΔE) 
                 Appearance 
                 (mg/dm 2 ) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Example 
                 1 
                 90-95 
                 5 
                 1 
                 — 
                 0.756 
                 ∘ 
                 ∘ 
                 ∘ 
                 3.49 
               
               
                   
                 2 
                 90-95 
                 10 
                 1 
                 — 
                 0.709 
                 ∘ 
                 ∘ 
                 ∘ 
                 2.17 
               
               
                   
                 3 
                 90-95 
                 15 
                 1 
                 — 
                 0.702 
                 ∘ 
                 ∘ 
                 ∘ 
                 1.68 
               
               
                   
                 4 
                 90-95 
                 5 
                 1 
                 Black 
                 1.562 
                 ∘ 
                 ∘ 
                 ∘ 
                 6.64 
               
               
                   
                 5 
                 90-95 
                 10 
                 1 
                 Black 
                 1.545 
                 ∘ 
                 ∘ 
                 ∘ 
                 4.74 
               
               
                   
                 6 
                 90-95 
                 15 
                 1 
                 Black 
                 1.385 
                 ∘ 
                 ∘ 
                 ∘ 
                 2.99 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Comparative 
                 1 
                 90-95 
                 5 
                 0 
                 — 
                 — 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Example 
                 2 
                 90-95 
                 10 
                 0 
                 — 
                 0.960 
                 ∘ 
                 ∘ 
                 ∘ 
                 3.81 
               
               
                   
                 3 
                 90-95 
                 15 
                 0 
                 — 
                 0.938 
                 ∘ 
                 ∘ 
                 ∘ 
                 3.24 
               
               
                   
                 4 
                 90-95 
                 30 
                 0 
                 — 
                 0.711 
                 ∘ 
                 ∘ 
                 ∘ 
                 2.11 
               
               
                   
                 5 
                 90-95 
                 10 
                 0 
                 Black 
                 5.757 
                 ∘ 
                 ∘ 
                 ∘ 
                 6.79 
               
               
                   
                 6 
                 90-95 
                 15 
                 0 
                 Black 
                 4.02 
                 ∘ 
                 ∘ 
                 ∘ 
                 5.12 
               
               
                   
                 7 
                 90-95 
                 30 
                 0 
                 Black 
                 3.76 
                 ∘ 
                 ∘ 
                 ∘ 
                 3.15 
               
               
                   
               
               
                 *The symbol “∘” means good appearance. 
               
            
           
         
       
     
     Referring to Table 1, it can be seen from the results of the salt spray test, the TS test, and the TH test that, in Examples 1 and 4 (i.e., voltage was applied for 5 minutes), the aluminum alloy products in Examples 1 and 4 have good appearance (without defects), which indicates that the aluminum alloy products in Examples 1 and 4 exhibit good corrosion resistance and salt spray resistance similar to those in Comparative Examples 2 to 7. Moreover, the nickel acetate formed on the aluminum alloy substrates of the aluminum alloy products in Examples 1 to 6 exhibits good adhesion, so that the metallic compound of nickel acetate can provide protection for the aluminum alloy substrate in an environment that is hot and humid or that has a great temperature change. 
     Additionally, it can be seen from Comparative Example 1 that, when the aluminum alloy substrate is sealed for 5 minutes without voltage application, the product thus obtained can not meet industrial requirements, whereas the aluminum alloy product of Example 1 (being sealed for 5 minutes using the sealing method of this disclosure) is a qualified product. It is indicated that the sealing method of this disclosure can shorten sealing time for obtaining a qualified aluminum alloy product, thereby improving the efficiency of the method of this disclosure. 
     The results obtained from the UV degradation test reveal that the ΔE values of Examples 4 to 6 are smaller than those of Comparative Examples 5 to 7, indicating that the aluminum alloy products in Examples 4 to 6 have a smaller color value change after UV radiation exposure and thus, exhibit an improved color constancy. In addition, the aluminum alloy products containing the dye (i.e., Examples 4 to 6 and Comparative Examples 5 to 7) have a larger ΔE value than the aluminum alloy products without the dye (i.e., Examples 1 to 3 and Comparative Examples 2 to 4), which is due to the aluminum alloy products without the dye having a lighter color and better reflectivity than the aluminum alloy products with the dye. 
     The results obtained from the weight loss test show that the weight loss ratios of Examples 1 to 6 are smaller than those of Comparative Examples 2 to 7, indicating that the aluminum alloy products in Examples 1 to 6 have a smaller weight change after immersion in the chromium trioxide (CrO 3 ) solution as compared to the aluminum alloy products in Comparative Examples 2 to 7, thereby suggesting that the aluminum alloy products in Examples 1 to 6 are more resistant to corrosion than the aluminum alloy products in Comparative Examples 2 to 7. Moreover, the aluminum alloy products containing the dye (i.e., Examples 4 to 6 and Comparative Examples 5 to 7) have a greater weight loss ratio than the aluminum alloy products without the dye (i.e., Examples 1 to 3 and Comparative Examples 2 to 4), which is due to the fact that the pores of the aluminum alloy products in Examples 4 to 6 and Comparative Examples 5 to 7 are filled with the dye, resulting in a smaller space available for the metallic compound. Therefore, the metallic compound provides less protection for the aluminum alloy products. 
     In view of the aforesaid, by virtue of the method for sealing pores of the present disclosure, with voltage application, the efficiency for sealing the pores is improved, and thus the aluminum alloy products have reduced discoloration and weight loss ratio, especially in the aluminum alloy products containing the dye. 
     In the description above, for the purposes of explanation, numerous specific details have been set forth in order to provide a thorough understanding of the embodiments. It will be apparent, however, to one skilled in the art, that one or more other embodiments may be practiced without some of these specific details. It should also be appreciated that reference throughout this specification to “one embodiment,” “an embodiment,” an embodiment with an indication of an ordinal number and so forth means that a particular feature, structure, or characteristic may be included in the practice of the disclosure. It should be further appreciated that in the description, various features are sometimes grouped together in a single embodiment, FIGURE, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of various inventive aspects, and that one or more features or specific details from one embodiment may be practiced together with one or more features or specific details from another embodiment, where appropriate, in the practice of the disclosure. 
     While the disclosure has been described in connection with what are considered the exemplary embodiments, it is understood that this disclosure is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements.