KR20120004811A - Surface treatment method for magnesium alloy - Google Patents

Abstract

PURPOSE: A method for processing surface of magnesium alloy is provided to improve the formation time of a chemical conversion coating layer. CONSTITUTION: A method for processing surface of magnesium alloy comprises next steps. The preprocessing is performed d in order to eliminate the foreign material of the surface of magnesium alloy or surface segregation. The chemical conversion coating is formed on the surface of MG. The foreign material and segregation of the magnesium alloy surface are eliminated. The chemical polishing liquid comprises sulfuric acid of 90~110 weight%, sodium nitrate of 27~33 weight%, and sodium citrate of 95~105 weight%.

Description

SURFACE TREATMENT METHOD FOR MAGNESIUM ALLOY

The present invention relates to a method for surface treatment of magnesium alloys, and more particularly, to a novel surface treatment method for providing high corrosion resistance, high powder coating-film adhesion, and low surface electrical resistance to magnesium alloys. The present invention is applicable to the surface treatment of general magnesium alloy plate materials and magnesium casting products, and is particularly suitable for the surface treatment of magnesium alloy products produced by die casting.

 Magnesium alloys have been widely used in the automotive and electronics fields because of their advantages such as low specific gravity, strong strength, renewable compatibility, and the like. However, magnesium alloys have high chemical reactivity and low corrosion resistance and therefore should be used after the formation of a corrosion resistant coating film. In the automotive industry, corrosion resistance and adhesion with electrodeposited coatings are required, and in the case of electronics, a low surface electrical resistance is additionally required, in which the electromagnetic wave protection effect is not impaired.

Since magnesium has the lowest standard electrode potential among practical metals, corrosion resistance is very weak, so magnesium alloy products require surface treatment. In addition, surface treatment is also required in order to improve surface functions such as appearance characteristics and wear resistance characteristics of magnesium alloy products.

The process of surface treatment of magnesium alloy parts may be performed according to the chemical conversion method, depending on the conditions required by the product such as surface conductivity, corrosion resistance, and abrasion resistance, the type of alloy (or alloy composition), and / or assembling with other metals. Processes such as anodizing, electroplating, electroless plating or painting are optionally used. Among them, the chemical conversion method and the anodization method are most used. The pretreatment process of the chemical conversion or anodization treatment includes cleaning, surface adjustment treatment, and the like, and is generally finished by coating after chemical conversion or anodization treatment. In this case, the chemical conversion treatment is mainly used in an environment in which corrosion conditions such as painting or storing parts are not severe, and the anodizing treatment is mainly used in a severe corrosion environment in which wear resistance is required.

In particular, in the case of the chemical conversion treatment of magnesium alloy, it is common to aim at the formation of a chemical conversion coating as a coating (or coating) base layer, and this chemical conversion coating has a good adhesion, strength (durability), There is a need to meet the properties required by magnesium alloy products for specific applications, such as corrosion resistance and / or electrical resistance. No chemical conversion technique has yet been found to form a chemical conversion coating that completely satisfies all of these major properties. Efforts to obtain a chemical conversion coating that satisfies coating adhesion, strength (durability), corrosion resistance or other properties have been made through the development of a chemical composition with a new composition and improvement of the pretreatment process before chemical conversion. Therefore, there is a need in the art to develop a chemical conversion treatment liquid with a new composition and / or to improve the pretreatment process in order to obtain a high quality chemical conversion coating film.

On the other hand, magnesium alloy products are mainly molded by die casting and thixomolding methods. These molding methods require pouring the molten magnesium alloy into the mold (or mold) at high speed under high pressure, and in order to facilitate the desorption of the mold and the magnesium product, an aqueous or emulsion on the surface of the mold prior to the molding process. It is necessary to apply a mold casting coating. At this time, smut occurs due to the mold coating on the surface of the cast magnesium alloy product after the molding is completed. The mold coating chemically reacts with the molten magnesium alloy at high temperature and is partially contained on the surface of the magnesium product. In addition, magnesium alloys usually add aluminum (Al) or zinc (Zn) to increase castability and mechanical strength, which are unevenly distributed on the surface of the magnesium alloy due to uneven cooling and uneven solidification. There is a tendency. Thus, techniques for removing surface segregation due to non-uniform coagulation and distribution of contaminant layers such as smut and additional components such as aluminum or zinc, which are caused by the mold coating, are importantly recognized.

As mentioned above, magnesium alloy has a characteristic of being easy to oxidize in the air because it is a metal having a low standard potential among practical metals, which is very poor in corrosion resistance, and contaminated by smut and the like due to the process characteristics of forming magnesium alloy. Layer and / or surface segregation occurs.

Accordingly, one problem to be solved by the present invention is a surface of a magnesium alloy which provides high corrosion resistance, high coating film adhesion and / or low electrical resistance to the surface of the magnesium alloy through improvement of the chemical conversion treatment and its pretreatment process. It is to provide a treatment method.

The surface treatment method of the magnesium alloy provided according to the present invention includes a chemical conversion treatment, which is low in production cost and excellent in workability, and is useful for forming a chemical conversion coating film suitable for coating. In particular, the chemical treatment method provided according to the present invention uses a chromate method, but using a deposition method among a number of methods, that is, deposition method, brush method, spray method, roller treatment method, this deposition method is excellent corrosion resistance, It is more effective in imparting paint adhesion and low electrical resistance.

According to one aspect of the invention, there is provided a surface treatment method of magnesium alloy for forming a chemically treated coating film having good corrosion resistance and paint adhesion on the surface of the magnesium alloy, the surface treatment method, foreign matter or surface of the surface of the magnesium alloy Pretreatment steps for removing segregation, and a chemical conversion step of depositing a magnesium alloy having undergone the pretreatment steps in a chemical conversion treatment solution containing chromium manganate to form a chemical conversion coating film.

According to an embodiment of the present invention, the pretreatment steps, the magnesium alloy by depositing a magnesium alloy in a chemical polishing liquid containing 90 to 110 parts by weight of sulfuric acid, 27 to 33 parts by weight of sodium nitrate and 95 to 105 parts by weight of sodium citrate A chemical polishing step of removing foreign substances and segregation of the surface; And a surface adjustment step of depositing the magnesium alloy subjected to the chemical polishing step in a surface adjustment liquid containing 187 to 213 parts by weight of sodium hydroxide and 27 to 33 parts by weight of sodium carbonate, wherein the chemical conversion treatment includes 8 to 12 parts by weight of chromium manganate. , 19 to 21 parts by weight of phosphoric acid, and 0.8 to 1.2 parts by weight of ammonium fluoride to immerse the magnesium alloy passed through the pretreatment steps for 3 to 5 minutes to form the chemical conversion coating on the surface of the magnesium alloy.

According to another embodiment of the invention, the pretreatment step, Degreasing step of depositing the magnesium alloy in a degreasing solution containing 126 ~ 154 parts by weight sodium hydroxide and 48 ~ 52 parts by weight sodium carbonate; And an acid etching step of depositing the magnesium alloy subjected to the degreasing step into a pickling solution including 44 to 56 parts by weight of iron nitrate, 141 to 159 parts by weight of chromic anhydride, and 2.6 to 3.4 parts by weight of nitric acid. The chemical conversion step may be performed by depositing 3 to 5 minutes of the magnesium alloy subjected to the pretreatment steps in a chemical conversion solution containing 8 to 12 parts by weight of chromium manganate, 17 to 23 parts by weight of hydrofluoric acid, and 0.8 to 12 parts by weight of ammonium fluoride. The chemical conversion coating is formed on the surface of the alloy.

According to yet another embodiment of the present invention, the pretreatment steps include: a degreasing step of depositing the magnesium alloy in a degreasing solution including 91 to 109 parts by weight of sodium hydroxide and 51 to 69 parts by weight of sodium carbonate; And an acid etching step of depositing the magnesium alloy subjected to the degreasing step into a pickling solution including 66 to 74 parts by weight of iron nitrate, 97 to 103 parts by weight of chromic anhydride, and 4.2 to 5.8 parts by weight of nitric acid. The chemical conversion step is carried out by depositing a magnesium alloy subjected to the pretreatment steps for 3 to 5 minutes in a chemical conversion solution containing 18 to 22 parts by weight of chromium manganate, 17 to 23 parts by weight of phosphoric acid, and 1.8 to 2.2 parts by weight of ammonium fluoride. The chemical conversion coating is formed on the surface of the alloy. In this case, a first washing step of dipping the magnesium alloy in distilled water of 50 ° C. or more immediately before the degreasing step is performed. After the chemical conversion step, a second washing step of dipping the magnesium alloy in distilled water of 80 ° C. or more within 30 seconds is performed. Is performed.

According to embodiments of the present invention, by improved chemical polishing before chemical conversion, it is possible to more effectively remove layers and surface segregation contaminated with the mold coating, and also to uniformly polish the work hardened layer on the surface of the die-cast casting. can do. In addition, through improved surface adjustment after chemical polishing, the formation rate of the chemical conversion coating layer can be further increased. In addition, through the improved pretreatment process including the high temperature first washing step and the second washing step, it is possible to simultaneously remove the foreign matter from the surface and increase the surface adhesion, thereby improving the coating (coating) adhesion. It is possible to provide a coating to the magnesium alloy. In addition, according to embodiments of the present invention, by completely removing impurities from the surface, it is possible to improve the corrosion resistance of the magnesium alloy product, by controlling the state of the coating film to improve the coating and adhesion, the coating surface This can not be peeled off so that the metal surface is not corroded.

1 is a flow chart for explaining the surface treatment method of the magnesium alloy according to the first embodiment of the present invention.
FIG. 2 is a micrograph showing the surface of the chemical conversion coating formed on the magnesium alloy by the method shown in FIG.
Figure 3 is a flow chart for explaining the surface treatment method of a magnesium alloy according to a second embodiment of the present invention.
4 is a micrograph showing the surface of the chemical conversion coating formed on the magnesium alloy by the method shown in FIG.
5 is a flowchart illustrating a surface treatment method of a magnesium alloy according to a third embodiment of the present invention.
6 is a micrograph showing the surface of the chemical conversion coating formed on the magnesium alloy by the method shown in FIG.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[First Embodiment]

1 is a flow chart for explaining the overall surface treatment method of the magnesium alloy according to the first embodiment of the present invention.

1, the surface treatment method of the magnesium alloy according to the present embodiment, the chemical polishing step (S11) for chemically polishing the surface of the magnesium alloy, and the surface adjustment for adjusting the surface of the chemically polished magnesium alloy And a chemical conversion treatment step S13 of forming a chemical conversion coating film by chemically treating the magnesium alloy that has undergone the surface adjustment step S12. After the chemical conversion step (S13), the magnesium alloy is dried. A painting process is performed on the surface of the dried magnesium alloy. In addition, water washing is performed on the magnesium alloy between the chemical polishing step (S11) and the surface adjustment step (S12), and between the surface adjustment step (S12) and the chemical conversion treatment step (S14). Although not shown, degreasing and / or washing with respect to the magnesium alloy may be performed even before the chemical polishing step (S11).

The specimen of magnesium alloy used in the surface treatment method of the present embodiment may be the most commonly used AZ91D magnesium alloy, which contains magnesium (Mg) as a main component, Al 9wt%, Zn 0.7 wt as additives %, Mn 0.15 wt%, other 0.006 wt%. In addition, the magnesium alloy specimens used above were molded by die casting, and contained surface contaminations such as Al and Zn, and contamination layers such as smut by a mold coating agent before surface treatment. Since the release agent of Al ₂ O ₃ and SiO ₂ component is used for easy desorption of the specimen from the mold after molding by the die casting, foreign substances are generated on the surface of the magnesium alloy specimen, and due to the difference in thermal flow Segregation occurs on the specimen surface.

In the chemical polishing step (S11), magnesium alloy specimens are deposited for about 30 seconds in a chemical polishing solution composed of 100 g / L sulfuric acid, 30 g / L sodium nitrate, and 100 g / L sodium citrate to deposit foreign substances and segregation on the surface of the magnesium alloy specimen. Remove

Next, after the washing step, in the surface adjustment step (S12), the magnesium alloy specimen is deposited for about 3 minutes in a liquid (ie, surface adjustment liquid) containing 200g / L sodium hydroxide and 30g / L sodium carbonate. The surface adjustment step (S12) is to adjust the surface of the magnesium alloy so that the chemical conversion treatment film formed on the surface of the magnesium alloy in the chemical conversion step (S13) can be grown densely.

Next, after the washing step, in the chemical conversion step (S13), magnesium alloy specimen is deposited for about 3 minutes in a chemical conversion solution containing 10 g / L manganese acid, 20 g / L phosphoric acid, 1 g / L ammonium fluoride And then hot drying for approximately 30 minutes above 80 ° C. Magnesium is an active metal, which has unstable corrosion resistance in the air, but in Cl- environment, corrosion resistance is very low, which leads to many restrictions on use. Impurities such as Fe, Ni, Cu, and the like have a very bad effect on the corrosion resistance of magnesium alloys in a brine atmosphere. The corrosion resistance of the magnesium alloy is passivated by forming a hydroxide film of Mg (OH) 2 which is very stable in strong alkalinity, but corrosion occurs in all regions other than strong alkalinity. Manganese chromic acid contained in the chemical conversion treatment liquid of the chemical conversion treatment step (S13), which is more harmless to the environment, forms a passive film on the surface of the magnesium alloy to impart corrosion resistance. In addition, the chemical polishing step (S11) is carried out to remove the layer and the surface segregation contaminated with the mold coating agent and to evenly polish the work hardened layer on the surface of the die-cast magnesium alloy product. The surface adjustment step (S12), which is aquatic after the chemical polishing step (S11), contributes to increasing the formation rate of the chemical conversion coating layer in the next chemical conversion treatment step (S13).

Figure 2 shows the surface shape of the magnesium alloy specimen obtained through the surface treatment as described above, referring to Figure 2, it can be seen a high quality porous surface suitable for the underlying layer of the coating (or coating).

Second Embodiment

Hereinafter, a method according to a second embodiment of the present invention for surface treatment of a magnesium alloy produced by die casting or thixomolding is described.

3 is a flow chart for explaining the overall surface treatment method of the magnesium alloy according to the second embodiment of the present invention.

Referring to FIG. 3, the surface treatment method of the magnesium alloy according to the present embodiment includes a degreasing step (S20), an acid etching step (S21), and a chemical conversion treatment step (S23). The magnesium alloy specimen is washed between the degreasing step (S20) and the acid etching step (S21). In addition, the magnesium alloy specimen is washed between the acid etching step (S21) and the chemical conversion step (S23). After the chemical conversion step S23, the magnesium alloy is dried. A painting process is performed on the surface of the dried magnesium alloy.

In the degreasing step (S20), a magnesium alloy specimen is deposited for 2 minutes in a degreasing solution consisting of 140 g / L sodium hydroxide and 50 g / L sodium carbonate.

After the next washing step, in the acid etching step (S21), the magnesium alloy is deposited for about 30 seconds in the pickling solution containing 50g / L iron nitrate, 150g / L chromic anhydride, 3g / L nitric acid.

Next, after the washing step, in the chemical conversion step (S23), magnesium alloy specimens for about 3 minutes in a non-chromate chemical treatment solution containing 10 g / L manganese acid, 20 g / L hydrofluoric acid, 1 g / L ammonium fluoride By depositing, a chemical conversion coating film having a good surface as shown in the micrograph of FIG. 4 is formed on the surface of the magnesium alloy specimen. Drying takes place at about 50 ° C. for about 30 minutes.

Third Embodiment

Hereinafter, a method of surface treatment of a magnesium alloy according to a third embodiment of the present invention suitable for surface treatment of a magnesium alloy plate.

5 is a flow chart for explaining the surface treatment method of the magnesium alloy according to the third embodiment of the present invention as a whole.

Referring to Figure 5, the surface treatment method of the magnesium alloy according to the present embodiment, the first washing step (S30-1), degreasing step (S30-2), acid etching step (S31) and chemical conversion treatment step (S33) And a second washing step (S34). The first washing step (S30-1) facilitates the removal of foreign matter in the next step (s), the second washing step (S34) performs a reaction stop and sealing process and make the surface with increased adhesion to paint Ensure good adhesion during work.

In the first washing step (S30-1), the magnesium alloy specimen is immersed in distilled water of 50 ℃ or more for 60 seconds.

Next, in the degreasing coalesced system (S30-2), magnesium alloy specimens are deposited for 3 minutes in a degreasing solution containing 100 g / L sodium hydroxide and 60 g / L sodium carbonate.

Next, in the acid etching step (S31), magnesium alloy specimens are deposited in the pickling solution containing iron nitrate 70g / L, chromic anhydride 100g / L, nitric acid 5 g / L for about 20 seconds.

Next, in the chemical conversion treatment step (S33), magnesium alloy specimens are deposited for about 3 minutes in a chemical treatment solution containing 20 g / L manganese acid, 20 g / L phosphoric acid, and 2 g / L ammonium fluoride.

Next, in the second washing step (S34), the specimen is immersed for 20 seconds in the second washing liquid consisting of secondary distilled water at 80 ℃. A drying step is then carried out, with the drying taking place at 50 ° C. for 30 minutes. The temperature and time of the second washing step greatly affect the peeling of the chemically treated film. [Table 1] below shows the degree of peeling different from the temperature and the time of the second washing step. Secondary washing should be done within 30 seconds in the washing liquid of the temperature higher than 80 ℃.

division  30 ℃ 40 ℃ 50 ℃ 60 ℃ 70 ℃ 80 ℃ 90 ° C 10 sec With peeling With peeling With peeling With peeling No peeling No peeling No peeling 20 seconds With peeling With peeling With peeling No peeling No peeling No peeling No peeling 30 seconds With peeling With peeling With peeling With peeling With peeling No peeling No peeling

As a result of experimenting with the first, second and third embodiments of the present invention under different conditions, it was confirmed that the chemical conversion treatment time affected the corrosion resistance and the smut generation of the chemical coating film. More specifically, in the corrosion resistance test, when the chemical conversion treatment time is 3 minutes or more, the corrosion resistance is maintained for 48 hours or more, but when the chemical conversion treatment time is about 60 seconds, the retention time of corrosion resistance is only 12 hours. Therefore, the chemical conversion treatment time was found to be 3 to 5 minutes, most preferably 3 to 4 minutes.

As described above, the present invention may be variously substituted, modified, and changed by those skilled in the art without departing from the technical spirit of the present invention.

S11: chemical polishing step, S12: surface adjustment step
S13, S23, S33: chemical conversion step S20, S30-2: degreasing step
S21, S31: acid etching step

Claims (5)

In the surface treatment method for forming a chemically treated coating film having good corrosion resistance and paint adhesion on the surface of a magnesium alloy,
Pretreatment steps for removing foreign matter or surface segregation of the surface of the magnesium alloy;
And depositing a magnesium alloy that has undergone the pretreatment steps in a chemical conversion solution containing chromium manganate, thereby forming a chemical conversion coating on the surface of the magnesium alloy. The method of claim 1, wherein the pre-treatment step, by depositing a magnesium alloy in a chemical polishing liquid containing 90 to 110 parts by weight of sulfuric acid, 27 to 33 parts by weight of sodium nitrate and 95 to 105 parts by weight of sodium citrate, A chemical polishing step of removing segregation; And a surface adjustment step of depositing the magnesium alloy subjected to the chemical polishing step in a surface adjustment liquid containing 187 to 213 parts by weight of sodium hydroxide and 27 to 33 parts by weight of sodium carbonate, wherein the chemical conversion treatment includes 8 to 12 parts by weight of chromium manganate. 3 to 5 minutes by depositing a magnesium alloy subjected to the pretreatment steps in a chemical conversion solution containing 19 to 21 parts by weight of phosphoric acid and 0.8 to 1.2 parts by weight of ammonium fluoride to form the chemical conversion coating on the surface of the magnesium alloy. Surface treatment method of the magnesium alloy characterized in that. The method of claim 1, wherein the pretreatment step, Degreasing step of depositing the magnesium alloy in a degreasing solution containing 126 ~ 154 parts by weight of sodium hydroxide and 48 ~ 52 parts by weight of sodium carbonate; And an acid etching step of depositing the magnesium alloy subjected to the degreasing step into a pickling solution including 44 to 56 parts by weight of iron nitrate, 141 to 159 parts by weight of chromic anhydride, and 2.6 to 3.4 parts by weight of nitric acid. The chemical conversion step may be performed by depositing a magnesium alloy having undergone the pretreatment steps for 3 to 5 minutes in a chemical conversion solution containing 8 to 12 parts by weight of chromium manganate, 17 to 23 parts by weight of hydrofluoric acid, and 0.8 to 1.2 parts by weight of ammonium fluoride. The surface treatment method of the magnesium alloy, characterized in that to form the chemical conversion treatment film on the surface of the alloy. The method of claim 1, wherein the pretreatment step comprises: a degreasing step of depositing the magnesium alloy in a degreasing solution containing 91 to 109 parts by weight of sodium hydroxide and 51 to 69 parts by weight of sodium carbonate; And an acid etching step of depositing the magnesium alloy subjected to the degreasing step into a pickling solution including 66 to 74 parts by weight of iron nitrate, 97 to 103 parts by weight of chromic anhydride, and 4.2 to 5.8 parts by weight of nitric acid. The chemical conversion step is carried out by depositing a magnesium alloy subjected to the pretreatment steps for 3 to 5 minutes in a chemical conversion solution containing 18 to 22 parts by weight of chromium manganate, 17 to 23 parts by weight of phosphoric acid, and 1.8 to 2.2 parts by weight of ammonium fluoride. The surface treatment method of the magnesium alloy, characterized in that to form the chemical conversion treatment film on the surface of the alloy. 5. The method of claim 4, wherein a first washing step of dipping the magnesium alloy in distilled water of 50 ° C. or more immediately before the degreasing step is performed, and after the chemical conversion treatment, the magnesium alloy is immersed in distilled water of 80 ° C. or more within 30 seconds. Surface treatment method of magnesium alloy, characterized in that the second washing step is performed.

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