KR101516379B1 - Surface treatment method for magnesium or magnesium alloy - Google Patents

Abstract

The present invention relates to a surface treatment method for magnesium or magnesium alloy, which treats magnesium or magnesium alloy with heat by humidifying and forms a brucite film on surfaces. The thermal treatment is performed in the range of 75-160°C. The humidification is 85-RH% at less than or equal to 100°C, and 50-100 RH% at more than or equal to 100°C. The surfaces of the brucite film are surface-treated by an alkali aqueous solution forming silicate.

Description

TECHNICAL FIELD [0001] The present invention relates to a surface treatment method of magnesium or magnesium alloy,

The present invention relates to a method for surface treatment of magnesium or magnesium alloy, and more particularly, to a method for surface-treating magnesium or magnesium alloy by humidifying heat treatment followed by silicate treatment to improve corrosion resistance.

Generally, magnesium or magnesium alloy is one of the new materials used in the manufacture of automobile parts and electronic devices such as computers and mobile phones because of its high mechanical strength, high dimensional stability and good mechanical properties.

In addition, magnesium or magnesium alloy can be lightly used, has a high impact resistance, is excellent in vibration absorbing property and electromagnetic wave shielding property, and can be widely used in household electric appliances.

However, such magnesium has a low corrosion resistance because it has a standard electrode potential of -2.363 V for a hydrogen electrode at 25 ° C. and has a disadvantage in that the corrosion rate increases when contacted with other electrically positive elements.

Therefore, many studies have been made to improve the corrosion resistance of magnesium alloys.

As a method for improving the corrosion resistance of a magnesium alloy, there is known a method of improving the corrosion resistance of a magnesium alloy by adding an alloying element or improving a corrosion resistance by performing a surface treatment process.

Examples of methods for improving the corrosion resistance of a magnesium alloy through a surface treatment process include anodizing and chromate treatment.

The anodizing treatment is a surface treatment method of improving the corrosion resistance, abrasion resistance and electrical resistance by forming a magnesium oxide layer on the surface of a material by using an electrochemical method. This method has the advantages of low manufacturing cost, simple process and easy management, but it is disadvantageous in that the process efficiency is not good and it takes time.

The chromate treatment is a method of chemical conversion using a chemical reaction between a treatment liquid and aluminum without using an electrical method, which enables a simple treatment at a relatively low cost, has a good corrosion resistance and uniform appearance.

However, since the chromate treatment uses a heavy metal such as chromium, there is a problem that the environment is contaminated.

Another method for surface treatment of magnesium alloys is to mechanically polish the surface of the magnesium alloy or to paint the surface of the alloy by pretreating the coating. However, the surface of the magnesium alloy treated with such a method is always There is a surface layer due to the oxidation of magnesium and the surface layer is not densified and the coating adhesion is low, so there is a limit to improve the corrosion resistance.

Therefore, in order to improve the corrosion resistance, which is an important disadvantage of the magnesium alloy, development of a new surface treatment method such as making the oxide film on the surface of the alloy to be dense is required.

In order to solve the above problems, the present invention provides a method for forming a dense corrosion-resistant film on the surfaces of various kinds of magnesium alloys such as magnesium plate, casting material, extruded material and three-dimensional molded product after moistening heat treatment of various magnesium alloys and post- In addition, the present invention provides a method for improving adhesion to paint materials.

In one or more embodiments of the present invention, magnesium or a magnesium alloy is heat treated in a humidified atmosphere to form a brucite coating on the surface, wherein the heat treatment is performed at a temperature range of 75 to 160 ° C, Is 85 to 100% RH at 100 DEG C or lower, and 50 to 100% RH at 100 DEG C or higher, and a surface treatment method of magnesium or magnesium alloy for surface treating the surface of the brucite coating with an alkaline aqueous solution forming a silicate is provided .

The alkaline aqueous solution forming the silicate may be an aqueous alkaline solution containing 1 to 5% by weight of potassium hydroxide or 1 to 10% by weight of sodium hydroxide and 1 to 5% by weight of tetraethylorthosilicate.

In addition, a humidifying heat treatment is performed for 120 hours or more at a temperature of 100 占 폚 or less, and a humidifying heat treatment is performed for 2 hours or less at a temperature of 100 占 폚 or more.

Before the heat treatment, a pickling treatment may be performed using any one of hydrofluoric acid, chromic acid, nitric acid, and mixed acid of hydrofluoric acid.

The magnesium alloy may be at least one selected from AZ31, AZ61, AZ31 added with Ca, or AM60.

According to the embodiment of the present invention, corrosion-resistant coating of magnesium and magnesium alloy of three-dimensional structure is possible.

In addition, by controlling the temperature and humidity, the thickness of the film can be adjusted in various ways, and corrosion can be prevented by generating a crack on the surface by X-cut.

In addition, the surface reliability of the magnesium coating can be improved by surface-treating the surface of the brucite coating with an aqueous solution of silicate (KT).

1 is a photograph showing a brucite crystal film of a magnesium alloy according to an embodiment of the present invention.
2 is a graph showing an X-ray diffraction pattern of a magnesium alloy according to an embodiment of the present invention.
FIG. 3 is a photograph showing a result of 48-hour salt spray test on a bismuth crystal coating film of a magnesium alloy according to an embodiment of the present invention.
FIG. 4 is a photograph showing a result of post-silicate treatment of a bismuth crystal film-coated specimen of a magnesium alloy according to an embodiment of the present invention.

Advantages and features of the present invention and methods of achieving them will become apparent with reference to the embodiments described in detail below. However, it is to be understood that the present invention is not limited to the disclosed embodiments, but may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. It is intended that the disclosure of the present invention be limited only by the terms of the appended claims.

In the case of the aluminum alloy according to the embodiment of the present invention, a naturally occurring oxide film is dense and different from magnesium in that the corrosion resistance is maintained, and the magnesium alloy is exposed to a high temperature and high humidity environment, thereby forming a dense oxide film .

Examples according to the present invention are magnesium sheet, magnesium alloy, AZ31, AZ61, magnesium sheet materials including Ca (AZ31 flame retardant added with Ca), AM60 and the like, extruded materials and three-dimensional molded articles (kitchen container, Mg PCB, Various magnesium alloys are used. It is possible to perform humidification heat treatment even in a state where a hot lubricant or the like used for warm molding remains on the surface. Depending on the pickling conditions used in the surface pretreatment of the magnesium alloy, the growth of the coating can be accelerated or hindered. The magnesium alloy thus produced can also maintain corrosion resistance.

The embodiment of the present invention improves adhesion and corrosion resistance with the coating layer by treating the surface of magnesium hydroxide in the surface of the magnesium alloy with silicate to simultaneously solve the coating adhesion and corrosion resistance of the pressurized humidifying thermal barrier film produced for producing the corrosion- And the like.

Hereinafter, a surface treatment method of a magnesium alloy according to an embodiment of the present invention will be described.

Magnesium or a magnesium alloy is heat-treated in a humidified atmosphere to form a brucite coating of Mg (OH) _{2 on} the surface. This makes it possible to form a blackened film having a high corrosion resistance on the surface of the magnesium alloy.

The temperature is not lower than 75 ° C and not higher than 160 ° C. The humidifying atmosphere is 85 RH% or higher and 100 RH% or lower at a low temperature of 100 ° C or lower, and 50 RH or higher and 100 RH or lower at a high temperature of 100 ° C or higher. At 120 ° C or lower, the coating should be humidified at a temperature of 120 ° C or higher, and at a temperature of 100 ° C or higher, the black coat formed even after a short time of 2 hours or less is stable for at least 48 hours. The blackening film produced at a low temperature is thin and corrosion resistance is not exhibited, and the blackening film produced at a high temperature of 160 캜 or higher becomes bulky and crumbles.

In the surface treatment method of magnesium alloy according to the embodiment of the present invention, the surface of the brucite coating is coated with 1 to 5% by weight of potassium hydroxide or 1 to 10% by weight of sodium hydroxide and 1 to 5% by weight of tetraethylor Lt; RTI ID = 0.0 > alkaline < / RTI >

Hereinafter, embodiments according to the present invention will be described in detail.

In the case of performing the humidification heat treatment according to one embodiment of the present invention, there is a difference in the thickness of the growing film even when the blackening treatment is performed under the same conditions according to the magnesium alloy.

Pure Mg and various magnesium alloys such as AZ31, AZ61, AZ91D, and AM60 have a significant difference in the thickness of the coating at the same temperature and humidity, but they all agree in that corrosion resistance appears due to progress of blackening.

As described above, the corrosion-resistant films appearing in various alloys are all Brucite crystal films of Mg (OH) ₂ as shown in Figs. 1 and 2. Conventional magnesium oxide films are amorphous and not dense. On the other hand, the coatings shown in the humidification heat treatment test according to the embodiment of the present invention are brittle crystalline films of Mg (OH) _{2 in} the X-ray diffraction pattern (XRD pattern) Which is thicker than the natural oxide film of 30 nm, which is suitable for maintaining the corrosion resistance.

As shown in FIG. 3, when the brucite crystal film is formed on the surface by performing the humidification heat treatment on the magnesium alloy, corrosion is not caused by the salt spray test for 48 hours, and the corrosion resistance is excellent.

When the humidity was fixed at 85 RH% in the humidifying heat treatment and the temperature was changed to 120 ° C., 130 ° C., 140 ° C., 145 ° C. and 150 ° C., respectively, the cross section of the blackening film formed was observed at 140 ° C. and 145 ° C. The thickness of the film is rapidly changed.

As described above, there is a difference in corrosion pattern of the corrosion-resistant film in the temperature range of 20 占 폚, 130 占 폚, 140 占 폚, 145 占 폚 and 150 占 폚. That is, when the film thickness is thin at 140 ° C, line defect occurs in the corrosion product, and when the thickness of the blackening film becomes thicker than 145 ° C, the pitting defect proceeds. However, these defects do not increase the size of defects over time.

In addition, when the humidification heat treatment is performed while changing the humidity at a temperature of 150 캜 to 55 RH%, 65 RH%, 75 RH%, 85 RH%, 95 RH% and 100 RH%, respectively, Can be reduced. The fact that the thickness of the coating is reduced even though the applied humidity is increased can indirectly indicate that a dense coating is formed.

In addition, in the embodiment of the present invention, it has been observed that the blackening can be easily prevented by performing the humidifying heat treatment after the acid etching of the magnesium alloy. That is, even if only the acid treatment according to various acid types is added before the humidification heat treatment step, the blackening growth can be prevented.

Possible pickling processes utilize mixed acids of hydrofluoric acid, chromic acid, nitric acid and hydrofluoric acid. Foshan easily reacts with the surface of magnesium alloy to generate MgF ₂ coating, so blackening can be easily prevented by preventing the change to brucite. This is important when one side benefits heat conduction and the other side benefits heat dissipation.

Meanwhile, in the method of surface treatment of magnesium alloy according to the embodiment of the present invention, it is preferable that the surface of the brucite coating is post-treated with an alkaline aqueous solution forming a silicate. By forming a silicate on the surface on which brucite is formed by the alkaline aqueous solution forming the silicate, the adhesion of the coating film is excellent and the coating reliability of the magnesium alloy can be enhanced.

For example, as in the case of the magnesium alloy surface treatment method according to the embodiment of the present invention, the surface of the brucite coating is surface treated with an aqueous alkaline solution forming a silicate, and a magnesium salt solution in which the surface of the brucite coating is treated with an alkaline aqueous solution In the case of performing the paint surface peeling test using the tape as the surface of the magnesium specimen not subjected to the surface treatment with the alkaline aqueous solution forming the silicate, as shown in Fig. 4, The layer was peeled off.

Here, the paint surface peeling test using the tape is a general method used for testing the degree of peeling of the paint surface, and thus a detailed description thereof will be omitted.

On the other hand, in the surface treatment method of a magnesium alloy, a method of forming a brucite coating on the surface by heat treatment of magnesium after surface treatment with an alkaline aqueous solution for forming a silicate on the surface of magnesium may be used. The corrosion resistant property can be reduced because the thickness of the corrosion resistant coating layer is small.

As described above, the surface reliability of the magnesium coating can be improved by surface-treating the surface of the brucite coating with an alkaline aqueous solution forming a silicate.

While the present invention has been described in connection with certain exemplary embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined by the appended claims.

It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive. The scope of the present invention is defined by the appended claims rather than the detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be interpreted as being included in the scope of the present invention .

Claims (5)

Magnesium or magnesium alloy is subjected to a pickling treatment using a mixed acid of hydrofluoric acid or hydrofluoric acid and the pickled magnesium or magnesium alloy is heat treated in a humidifying atmosphere to form a brucite coating on the surface,
The above-mentioned heat treatment is performed in a temperature range of 75 to 160 ° C, and the humidifying atmosphere is 85 to 100 RH% at 100 ° C or lower, and 50 to 100% RH at 100 ° C or higher,
Wherein the surface of the brucite coating is surface-treated with an alkaline aqueous solution forming a silicate. The method according to claim 1,
The alkaline aqueous solution forming the silicate may contain,
A surface treatment method of a magnesium or magnesium alloy which is an aqueous alkaline solution comprising 1 to 5% by weight of potassium hydroxide or 1 to 10% by weight of sodium hydroxide and 1 to 5% by weight of tetraethyl orthosilicate. 3. The method of claim 2,
A method for surface treatment of a magnesium or magnesium alloy which is subjected to a humidification heat treatment at a temperature of 100 ° C or lower for 120 hours and a humidification heat treatment at a temperature of 100 ° C or higher for 2 hours or less. delete 4. The method according to any one of claims 1 to 3,
Wherein the magnesium alloy is at least one selected from AZ31 flame retardant having AZ31, AZ61, Ca added thereto, or AM60, or a magnesium or magnesium alloy having at least one selected from AM60.

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