Method for treating surface of magnesium or magnesium alloy

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

The present invention relates to a surface treatment method, particularly a surface treatment method applicable on Mg or Mg Alloy.

BACKGROUND OF THE INVENTION

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.

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.

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.

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.

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

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.

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.

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.

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/dm², and more preferably is 1˜5 A/dm². 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.

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.

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.

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.

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.

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.

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

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;

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;

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;

FIG. 4 shows a TEM photo of a locally enlarged A layer in FIG. 3; and

FIG. 5 shows a TEM photo of a locally enlarged C layer in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

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.

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.

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/dm²; in another embodiment, a pulse rectifier is set to a current density of 1˜5 A/dm², a frequency of 10˜2000 Hz, and a duty cycle of 0.1˜1.

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 Mg₂SiO₄, 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.

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 10² 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 10⁴ 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.

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.

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

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/dm², and reaction time 30 minutes.

EXAMPLE A2

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/dm², and reaction time 30 minutes.

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

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/dm², and reaction time 30 minutes.

EXAMPLE A4

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/dm², and reaction time 30 minutes.

EXAMPLE A5

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/dm², and reaction time 30 minutes.

EXAMPLE A6

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/dm², and reaction time 30 minutes.

EXAMPLE A7

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/dm², and reaction time 30 minutes.

EXAMPLE B1

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/dm², frequency 1000 Hz, duty cycle 0.3, and reaction time 15 minutes.

EXAMPLE B2

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/dm², frequency 1000 Hz, duty cycle 0.3, and reaction time 45 minutes.

EXAMPLE B3

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/dm², frequency 1000 Hz, duty cycle 0.3, and reaction time 15 minutes.

EXAMPLE B4

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/dm², frequency 1000 Hz, duty cycle 0.3, and reaction time 15 minutes.

EXAMPLE B5

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/dm², frequency 10 Hz, duty cycle 0.3, and reaction time 15 minutes.

EXAMPLE B6

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/dm², frequency 1000 Hz, duty cycle 0.6, and reaction time 15 minutes.

EXAMPLE B7

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/dm², frequency 1000 Hz, duty cycle 0.3, and reaction time 15 minutes.

EXAMPLE B8

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/dm², frequency 1000 Hz, duty cycle 0.3, and reaction time 10 minutes.

EXAMPLE B9

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/dm², frequency 100 Hz, duty cycle 0.3, and reaction time 15 minutes.

Control A:

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/dm², and reaction time 30 minutes.

Control B:

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/dm², and reaction time 30 minutes.

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.

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

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.

Claims

1. A surface treatment method, which comprises:

(A) providing a metal of Mg or Mg Alloy, a surface treatment composition, and a tank, wherein said surface treatment composition comprises a hydroxide, a film thickening agent, and a film adjustment agent, wherein said tank is provided with an electrode and said surface treatment composition therein;

(B) immersing said Mg or Mg Alloy or a surface thereof into said surface treatment composition in said tank;

(C) flowing an electric current through said Mg or Mg Alloy and said electrode in said tank; and

(D) terminating the electric current, and removing the treated Mg or Mg Alloy from said tank.

2. The method as claimed in claim 1, wherein the Mg or Mg Alloy during said Step (C) for is at 0˜40° C.

3. The method as claimed in claim 1, wherein said surface treatment composition in Step (B) has a pH value greater than 9.

4. The method as claimed in claim 1, wherein said film thickening agent in Step (A) is selected from the group consisting of aluminate, silicate, vanadate, molybdates, tungstates, and a combination thereof.

5. The method as claimed in claim 1, wherein said film adjustment agent in Step (A) is selected from the group consisting of potasium dihydrogen phosphate, sodium dihydrogen phosphate, tripotassium phosphate, trisodium phosphate, oxalic acid, succinic acid, fatty acid, malic acid, and a combination thereof.

6. The method as claimed in claim 1, wherein said hydroxide in Step (A) is selected from the group consisting of sodium hydroxide, potassium hydroxide, and a combination thereof.

7. The method as claimed in claim 1, wherein said surface treatment composition in Step (A) has a concentration of said hydroxide of 10˜100 g/L.

8. The method as claimed in claim 1, wherein said Mg and Mg alloy in Step (A) is a Mg-riched alloy or casting Mg alloy.

9. The method as claimed in claim 1, wherein said surface treatment composition in Step (A) has a concentration of said film thickening agent of 10˜150 g/L.

10. The method as claimed in claim 1, wherein said surface treatment composition s in Step (A) has a concentration of aid film adjustment agent of 10˜300 g/L.

11. The method as claimed in claim 1, wherein said electric current in said Step (C) is flowed at an average electric current density of 1˜10 A/dm2.

12. The method as claimed in claim 1, wherein said Step (C) is carried out in said surface treatment composition for 5˜240 minutes.

13. The method as claimed in claim 1, wherein the Mg-containing material is connected to a positive electrode of a rectifier in Step (C).

14. The method as claimed in claim 13, wherein said rectifier is selected from the group consisting of a d.c. rectifier and a pulse rectifier.

15. The method as claimed in claim 14, wherein said d.c. rectifier is selected from the group consisting of a constant current type d.c. rectifier, a constant voltage type d.c. rectifier, and a constant current density recycle type d.c. rectifier.