Abstract:
A method for surface treatment is disclosed. The method is achieved by forming a MgO film on a metal surface through anode processing of Mg or Mg alloy in an alkaline solution. The alkaline solution includes a hydroxide, a thickening agent, and a film adjusting agent. As the method is performed, the target object is immersed in the alkaline solution, and the target object is connected to an anode with an average electric current density of 1˜5 A/dm, at a temperature of 0˜30° C., and within a time period of 10˜120 minutes to form a film of 5˜25 μm. The forming rate of the film of the method of the present invention is fast, and the formed film is of little stress.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a surface treatment method, particularly a surface treatment method applicable on Mg or Mg Alloy.  
       BACKGROUND OF THE INVENTION  
       [0002]     Mg alloys are high in strength and light in weight and are widely used on airplanes, vehicles, and electronic products. Since magnesium is capable of forming alloys of high strength with many types of metal, Mg alloys have a wide variety of applications. However, Mg alloys are generally not suitable for mass production due to their disadvantages, such as poor corrosion resistance, and poor wear resistance, etc. Furthermore, due to an ever increasing expansion on the applications of Mg alloys, the demand on the acid-corrosion-resistance of the alloys is also increasing day by day.  
         [0003]     In the past, a Mg alloy is usually protected against acid corrosion by coating a protective paint or forming a protective film on the surface of the alloy. In recent years, due to the improvement in technology, the use of forming a protective film on the surface of a Mg alloy has become the main stream technique.  
         [0004]     Traditionally, a micro-arc oxidation treatment is used to form a protective film on a Mg alloy. Such a technique is characterized in that a high voltage exceeding 600˜1000V is used, the treatment temperature is over 40° C., and a film forming electrolysis is carried out in a fluoride-containing weak alkaline electrolysis solution. However, a surface formed through such a technique is rough due to the formation of a large amount of penetration sparks, and requires an additional coating treatment. Furthermore, due to the use of fluoride as a main chemical agent, wastewater from such a treatment is difficult to be treated and has a greater pollution impact on the environment.  
         [0005]     Moreover, acidic mixture solvents, e.g. borate, sulfate, phosphorate ions, fluorate ions, and chlorine ions, etc., are used for acidic anodic treatment to form a protective film. However, since a Mg alloy dissolves rapidly in an acidic state, the surface of the resulting film is liable to become rough. As a result, the dimensional precision of the workpiece is affected, a large residual internal stress is developed, and the process variables have a narrow window.  
         [0006]     Therefore, at present, the industry is urgently in need of a Mg or Mg alloy surface treatment method to overcome the above-mentioned conventional drawbacks without the need of using high temperature and high pressure and capable of forming an oxidation film rapidly.  
       SUMMARY OF THE INVENTION  
       [0007]     A main objective of the present invention is to provide a surface treatment method capable of forming a uniform anodic film on the Mg-containing material, e.g. Mg or Mg alloy.  
         [0008]     A surface treatment method according to the present invention comprises the following steps: providing a metal of Mg or Mg Alloy, a tank, and a surface treatment composition in the tank, wherein said surface treatment composition includes a hydroxide, a film thickening agent, a film adjustment agent, wherein said tank is equipped with an electrode; sequentially immersing said Mg or Mg Alloy or the surface thereof in said surface treatment composition; flowing an electric current through said Mg or Mg Alloy as an anode and the electrode immersed in said surface treatment composition via said surface treatment composition; and terminating the electric current and removing the treated Mg or Mg Alloy from said tank.  
         [0009]     Furthermore, in a preferred surface treatment method for a metal of Mg or Mg Alloy according to the present invention, the temperature during the surface treatment reaction is not limited, and is preferably 0˜40° C. In one embodiment, the pH value of the surface treatment composition of the present invention is not limited, preferably larger than 9, more preferably larger than 10, and most preferably larger than 11.  
         [0010]     Preferably, said Mg and Mg alloy used in the present invention is a Mg-riched alloy or casting Mg alloy. In a treatment method according to the present invention, the average current density of the electric current used is not limited, preferably is 1˜10 A/dm 2 , and more preferably is 1˜5 A/dm 2 . Furthermore, the treatment time for the Mg or Mg Alloy in the surface treatment composition of the present invention is not limited, preferably is 5˜240 minutes, and more preferably is 10˜120 minutes.  
         [0011]     The film thickening agent used in the present invention can be any conventional film thickening agent, preferably aluminate, silicate, vanadate, molybdate, tungstate, or a combination thereof, with an arbitrarily applicable concentration in the surface treatment composition. In a preferred embodiment of the present invention, the concentration of the film thickening agent is 10˜150 g/L. The main objective of using the film thickening agent is for growing a film. The film forming rate increases along with an increase in the concentration of the film thickening agent. Under suitable conditions, the objective of the film thickening can be achieved without the need of using high temperature and ultra-high voltage.  
         [0012]     A film adjustment agent suitable for use in the present invention is not particularly limited, and is preferably potasium dihydrogen phosphate, sodium dihydrogen phosphate, tripotassium phosphate, trisodium phosphate, oxalic acid, succinic acid, fatty acid, malic acid, or a combination thereof, with an arbitrarily applicable concentration in the surface treatment composition. In a preferred embodiment of the present invention, the concentration of the film adjustment agent is 10˜300 g/L. The film adjustment agent is capable of accelerating the forming rate of the film, promoting the formation of a uniform and fine film, as well as reducing the stress in the film.  
         [0013]     Furthermore, the hydroxide used in the present invention can be any conventional hydroxide, preferably sodium hydroxide, potassium hydroxide, or a mixture thereof, with an arbitrarily applicable concentration in the surface treatment composition. In a preferred embodiment of the present invention, the concentration of the hydroxide is 10˜100 g/L.  
         [0014]     In a preferred embodiment of the present invention, said Mg or Mg Alloy is connected to the positive electrode of a rectifier while undergoing the surface treatment. However, in embodiments according to the present invention, the rectifier used is not limited, preferably is a d.c. rectifier, and a pulse rectifier, and more preferably is a pulse rectifier. The d.c. rectifier, for example, can be a common constant current, constant voltage, or constant current density type rectifier; a constant current, constant voltage, or constant current density recycle type d.c. rectifier; or a constant current, constant voltage, or constant current density PR-type d.c. rectifier. The pulse type rectifiers can be pulse type rectifiers of different wave forms. The voltage used in the method of the present invention is only 100˜300V, and the operation is carried out at room temperature at a low energy consumption.  
         [0015]     The method of the present invention uses an anode oxidation film formed on the surface of Magnesium to achieve corrosion resistance. After oxygen atoms are developed near the anode in the electrolyte solution, the atoms in the substrate migrate to the surface of the anode and form an oxidation film (film forming) on the substrate. The formed film is slightly dissolved by the electrolyte solution (chemical dissolution). An oxidation film starts to develop when the film formation rate is greater than the film dissolution rate, thereby forming an oxidation film of the substrate. This is called an anode treatment.  
         [0016]     In the present invention the Mg or Mg alloy is capable of forming a film mainly constituted of magnesium oxide ceramic ingredient in an alkaline solution by anodic oxidation treatment, and the magnesium oxide ceramic ingredient is slightly soluble in an alkaline solution by chemical dissolution. Since the surface treatment composition according to the present invention contains no fluoride, such a surface treatment composition will not cause severe environmental pollution. Furthermore, the present invention uses a film thickening agent and a film adjustment agent to achieve an increase in the film forming rate, a reduction in film dissolution rate, a uniform and fine film, a stable dimensional precision of the workpiece, and a reduction on the internal stress of the film. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0017]      FIG. 1  shows a cross-sectional schematic view of a film formed on the surface of a metal of Mg or Mg alloy according to the method of the present invention;  
         [0018]      FIG. 2  shows a Bode diagram of an electrochemical a.c. impedance test performed on a film formed on the surface of a metal of Mg or Mg alloy according to a preferred example of the present invention;  
         [0019]      FIG. 3  shows a TEM photo of an anode film formed on the surface of a metal of Mg or Mg alloy according to a preferred example of the present invention;  
         [0020]      FIG. 4  shows a TEM photo of a locally enlarged A layer in  FIG. 3 ; and  
         [0021]      FIG. 5  shows a TEM photo of a locally enlarged C layer in  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     The present invention uses a film thickening agent and a film adjustment agent at different concentrations in an anodic treatment on a material of Mg alloy AZ31 (wherein said Mg alloy includes more than 90% of Mg, 3% of Al, and 1% of Zn). Even though the present invention uses the Mg alloy AZ31 as illustrated in the following examples, the composition of a Mg or Mg Alloy applicable in the present invention is not limited to AZ31, but is limited by the scope defined by the claims of the present invention. In one embodiment, the present invention uses silicate as the film thickening agent. In another embodiment, the present invention uses vanadate as the film thickening agent. In both cases, a film is capable of being formed on the surface of the Mg alloy. Furthermore, a film thickening agent according to the present invention is not limited to silicate and vanadate.  
         [0023]     In a preferred embodiment, a method according to the present invention comprises: firstly, providing a hydroxide, a film thickening agent, and a film adjustment agent; mixing the above-mentioned chemical agents to form a surface treatment composition; loading the prepared surface treatment composition into an electrolysis tank; next, mounting a Mg or Mg Alloy on a workpiece, and then mounting the workpiece on an anode location in the electrolysis tank containing said surface treatment composition; using a rectifier to apply an electric current on said anode in order to perform a film forming reaction on the surface of the Mg or Mg Alloy; after a specified reaction time, removing the workpiece together with the Mg or Mg Alloy from the electrolysis tank; and washing the surface of the Mg or Mg Alloy with water to complete a surface treatment operation for the Mg or Mg Alloy.  
         [0024]     In an embodiment according to the present invention, the rectifier can be a d.c. rectifier or a pulse rectifier. In an embodiment, a d.c. rectifier is set to a current density of 1˜5 A/dm 2 ; in another embodiment, a pulse rectifier is set to a current density of 1˜5 A/dm 2 , a frequency of 10˜2000 Hz, and a duty cycle of 0.1˜1.  
         [0025]      FIG. 1  shows a cross-section of a schematic diagram of a film formed on the surface of the Mg or Mg Alloy according to a preferred embodiment of the present invention. The film formed on the surface of the Mg or Mg Alloy according to the present invention is examined by an electrochemical AC current impedance spectrum and a TEM. The examination results indicate that the film has a three-layered structure, including two barrier layers and a porous layer. As shown in  FIG. 1 , the topmost layer of the film is a porous layer containing MgO and Mg 2 SiO 4 , the intermediate layer is a barrier layer formed of a dense MgO structure, and the bottom layer is a barrier layer formed of a nano crystalline MgO, wherein the porous layer is advantageous for the anchoring of a coating for the corrosion resistance of the substrate, and the barrier layers are capable of adjusting the film strength, and increasing the toughness and corrosion resistance of the film. Therefore, a film formed according to the method of the present invention, due to the multi-layered structure thereof, has the functions of buffering the internal stress of the film, accelerating the film forming rate, and increasing the denseness and corrosion resistance of the film.  
         [0026]      FIG. 2  shows a Bode diagram of an electrochemical a.c. impedance examination on a film formed on a Mg Alloy according to a preferred example of the present invention.  FIG. 2  indicates that peaks appear at locations of 10 −1  Hz and 10 2  Hz frequency. This indicates that the film formed by the anodic treatment according to the present invention has separately two layer structures at said locations. The frequency range of 10 −3  Hz to 10 4  Hz is the range for the barrier layers, and obviously there are only two peaks exist within this range, according to  FIG. 2 . This indicates that indeed the film has two barrier layers each of a different structure.  
         [0027]      FIG. 3  is a TEM photo diagram of the film formed on a Mg alloy by the anodic treatment according to a preferred example of the present invention. From the diagram, the film formed according to the method of the present invention includes three different structures, the A, B, C layers as shown in  FIG. 3 . A local enlargement diagram of the A layer is shown in  FIG. 4 , from which it can be seen that the A layer is a porous topmost layer structure. The bright spots shown in  FIG. 5 , a local enlargement diagram of the C layer, are nano MgO crystals. Thus, the C layer is a bottom layer structure with nano MgO crystals.  
         [0028]     The reaction conditions and parameters of embodiments according to the present invention are shown in the following. The A-series examples use a d.c. rectifier for supplying electric current, and the B-series examples use a pulse rectifier for supplying electric current. Furthermore, comparative Examples A, and B are control sets for the A-series examples, which use a d.c. rectifier; however and a surface treatment composition free of a film thickening agent or a film adjustment agent. The anode film formed on the Mg-containing alloy in Example A1 has the best corrosion resistance among the films formed in the A-series examples.  
       EXAMPLE A1  
       [0029]     Surface treatment composition: sodium hydroxide (70 g/L) was used as a hydroxide, sodium metasilicate (64 g/L) was used as a film thickening agent, and trisodium phosphate (19 g/L) and oxalic acid (80 g/L) were used as a film adjustment agent. This example used a d.c. rectifier and adopted the following conditions: temperature 20° C., electric current density 1.6 A/dm 2 , and reaction time 30 minutes.  
       EXAMPLE A2  
       [0030]     Surface treatment composition: sodium hydroxide (70 g/L) was used as a hydroxide, sodium vanadate (50 g/L) was used as a film thickening agent, and trisodium phosphate (19 g/L) and oxalic acid (80 g/L) were used as a film adjustment agent. This example used a d.c. rectifier and adopted the following conditions: temperature 20° C., electric current density 1.6 A/dm 2 , and reaction time 30 minutes.  
         [0031]     Since sodium metasilicate is cheap and readily available, and the resulting anodic film has a fair performance, sodium metasilicate was used as a film thickening agent in the following examples. Meanwhile, various film adjustment agents with different concentrations of agents were used in the examples for investigating the role of each chemical agent in the resulting anodic treated films.  
       EXAMPLE A3  
       [0032]     Surface treatment composition: sodium hydroxide (70 g/L) was used as a hydroxide, sodium metasilicate (21 g/L) was used as a film thickening agent, and trisodium phosphate (95 g/L) and succinic acid (80 g/L) were used as a film adjustment agent. This example used a d.c. rectifier and adopted the following conditions: temperature 20° C., electric current density 1.6 A/dm 2 , and reaction time 30 minutes.  
       EXAMPLE A4  
       [0033]     Surface treatment composition: sodium hydroxide (70 g/L) was used as a hydroxide, sodium metasilicate (64 g/L) was used as a film thickening agent, and trisodium phosphate (95 g/L) and fatty acid (80 g/L) were used as a film adjustment agent. This example used a d.c. rectifier and adopted the following conditions: temperature 20° C., electric current density 1.6 A/dm 2 , and reaction time 30 minutes.  
       EXAMPLE A5  
       [0034]     Surface treatment composition: sodium hydroxide (10 g/L) was used as a hydroxide, sodium metasilicate (64 g/L) was used as a film thickening agent, and trisodium phosphate (19 g/L) and malic acid (80 g/L) were used as a film adjustment agent. This example used a d.c. rectifier and adopted the following conditions: temperature 20° C., electric current density 1.6 A/dm 2 , and reaction time 30 minutes.  
       EXAMPLE A6  
       [0035]     Surface treatment composition: sodium hydroxide (70 g/L) was used as a hydroxide, sodium metasilicate (64 g/L) was used as a film thickening agent, and trisodium phosphate (57 g/L) and potassium citrate (80 g/L) were used as a film adjustment agent. This example used a d.c. rectifier and adopted the following conditions: temperature 20° C., electric current density 1.6 A/dm 2 , and reaction time 30 minutes.  
       EXAMPLE A7  
       [0036]     Surface treatment composition: sodium hydroxide (70 g/L) was used as a hydroxide, sodium metasilicate (64 g/L) was used as a film thickening agent, and trisodium phosphate (19 g/L) and potassium citrate (48 g/L) were used as a film adjustment agent. This example used a d.c. rectifier and adopted the following conditions: temperature 20° C., electric current density 1.6 A/dm 2 , and reaction time 30 minutes.  
       EXAMPLE B1  
       [0037]     Surface treatment composition: sodium hydroxide (70 g/L) was used as a hydroxide, sodium metasilicate (64 g/L) was used as a film thickening agent, and trisodium phosphate (19 g/L) and potassium citrate (80 g/L) were used as a film adjustment agent. This example used a pulse rectifier and adopted the following conditions: temperature 15° C., electric current density 1.6 A/dm 2 , frequency 1000 Hz, duty cycle 0.3, and reaction time 15 minutes.  
       EXAMPLE B2  
       [0038]     Surface treatment composition: sodium hydroxide (70 g/L) was used as a hydroxide, sodium metasilicate (64 g/L) was used as a film thickening agent, and trisodium phosphate (19 g/L) and potassium citrate (80 g/L) were used as a film adjustment agent. This example used a pulse rectifier and adopted the following conditions: temperature 15° C., electric current density 1.6 A/dm 2 , frequency 1000 Hz, duty cycle 0.3, and reaction time 45 minutes.  
       EXAMPLE B3  
       [0039]     Surface treatment composition: sodium hydroxide (70 g/L) was used as a hydroxide, sodium metasilicate (64 g/L) was used as a film thickening agent, and trisodium phosphate (19 g/L) and potassium citrate (80 g/L) were used as a film adjustment agent. This example used a pulse rectifier and adopted the following conditions: temperature 45° C., electric current density 1.6 A/dm 2 , frequency 1000 Hz, duty cycle 0.3, and reaction time 15 minutes.  
       EXAMPLE B4  
       [0040]     Surface treatment composition: sodium hydroxide (70 g/L) was used as a hydroxide, sodium metasilicate (64 g/L) was used as a film thickening agent, and trisodium phosphate (19 g/L) and potassium citrate (80 g/L) were used as a film adjustment agent. This example used a pulse rectifier and adopted the following conditions: temperature 15° C., electric current density 2.2 A/dm 2 , frequency 1000 Hz, duty cycle 0.3, and reaction time 15 minutes.  
       EXAMPLE B5  
       [0041]     Surface treatment composition: sodium hydroxide (70 g/L) was used as a hydroxide, sodium metasilicate (64 g/L) was used as a film thickening agent, and trisodium phosphate (19 g/L) and potassium citrate (80 g/L) were used as a film adjustment agent. This example used a pulse rectifier and adopted the following conditions: temperature 15° C., electric current density 2.2 A/dm 2 , frequency 10 Hz, duty cycle 0.3, and reaction time 15 minutes.  
       EXAMPLE B6  
       [0042]     Surface treatment composition: sodium hydroxide (70 g/L) was used as a hydroxide, sodium metasilicate (64 g/L) was used as a film thickening agent, and trisodium phosphate (19 g/L) and potassium citrate (80 g/L) were used as a film adjustment agent. This example used a pulse rectifier and adopted the following conditions: temperature 15° C., electric current density 1.6 A/dm 2 , frequency 1000 Hz, duty cycle 0.6, and reaction time 15 minutes.  
       EXAMPLE B7  
       [0043]     Surface treatment composition: sodium hydroxide (70 g/L) was used as a hydroxide, sodium metasilicate (64 g/L) was used as a film thickening agent, and trisodium phosphate (19 g/L) and potassium citrate (80 g/L) were used as a film adjustment agent. This example used a pulse rectifier and adopted the following conditions: temperature 15° C., electric current density 1.0 A/dm 2 , frequency 1000 Hz, duty cycle 0.3, and reaction time 15 minutes.  
       EXAMPLE B8  
       [0044]     Surface treatment composition: sodium hydroxide (70 g/L) was used as a hydroxide, sodium metasilicate (64 g/L) was used as a film thickening agent, and trisodium phosphate (19 g/L) and potassium citrate (80 g/L) were used as a film adjustment agent. This example used a pulse rectifier and adopted the following conditions: temperature 15° C., electric current density 1.6 A/dm 2 , frequency 1000 Hz, duty cycle 0.3, and reaction time 10 minutes.  
       EXAMPLE B9  
       [0045]     Surface treatment composition: sodium hydroxide (70 g/L) was used as a hydroxide, sodium metasilicate. (64 g/L) was used as a film thickening agent, and trisodium phosphate (19 g/L) and potassium citrate (80 g/L) were used as a film adjustment agent. This example used a pulse rectifier and adopted the following conditions: temperature 15° C., electric current density 1.6 A/dm 2 , frequency 100 Hz, duty cycle 0.3, and reaction time 15 minutes.  
         [0000]     Control A:  
         [0046]     Surface treatment composition: sodium hydroxide (70 g/L) was used as a hydroxide, and trisodium phosphate (50 g/L) was used as a film adjustment agent. This example used a d.c. rectifier and adopted the following conditions: temperature 20° C., electric current density 1.6 A/dm 2 , and reaction time 30 minutes.  
         [0000]     Control B:  
         [0047]     Surface treatment composition: sodium hydroxide (20 g/L) was used as a hydroxide, and sodium metasilicate (80 g/L), sodium carbonate (53 g/L), and boric acid (12.5 g/L) were used as a film thickening agent. This example used a d.c. rectifier and adopted the following conditions: temperature 20° C., electric current density 1.6 A/dm 2 , and reaction time 30 minutes.  
         [0048]     Table 1 lists the test results of corrosion resistance for the films prepared in the examples and controls, wherein the salt spray test for corrosion resistance used 5% NaCl aqueous solution. A film is rated “Pass” if no corrosion spot is formed at 35° C. after 100 hours in the test. Generally speaking, the thickness of the formed film is not related to the corrosion resistance of the film per se. The structure and the denseness of the formed film per se are important factors affecting the corrosion resistance of the film. Thus, the test results of the corrosion resistance (the salt spray test) for the formed films in the examples according to the present invention are shown together with the impedance values in Table 1. When the impedance value is high, i.e. a higher denseness, the formed film will also have a better corrosion resistance.  
         [0049]     The corrosion resistance of the film formed in Example A1 is the best in the A-series examples, but is generally lower than that of the film formed in the B-series examples. Therefore, a pulse rectifier seems to be a better choice for the method of the present invention. However, the films formed by using a d.c. rectifier in the method of the present invention still have good corrosion resistance, referring to the test results of the A-series examples in Table 1.  
                                                           TABLE 1                           Test results for corrosion resistance                Film thickness,   Impedance,   Result of   Roughness,       Example   μm   Ω   brine spray test   μm                    A1   10   900K   Pass           A2   9.5   880K   Pass       A3   10.3   400K       A4   5   350K       A5   1.5   180K       A6   9.7   500K       A7   10   800K   Pass       B1   7.2   3000K    Pass   0.59       B2   13.6   2400K    Pass       B3   5.3   700K       B4   12.5   2600K    Pass   0.9       B5   8.5   550K       0.7       B6   8.7   1400K    Pass       B7   4.8   1200K    Pass   0.51       B8   5.5   1300K    Pass   0.47       B9   12.8   1100K    Pass   1.37       A   No film       Not Pass           deposited       B   10-60   150K   Not Pass                  
 
         [0050]     The above-mentioned examples are for illustrative purpose only and not for limiting the scope of the present invention, which is defined in the claim appended.