JP2008291310A - Magnesium material production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnesium material production method for producing a magnesium material having excellent corrosion resistance without affecting internal structure. <P>SOLUTION: The surface of a magnesium substrate (matrix) 100 is subjected to alkali treatment (an alkali treatment step), and the surface of the magnesium substrate subjected to the alkali treatment is irradiated with laser beam at a scanning rate in the range of 20 to 100 mms<SP>-1</SP>(a laser irradiation step). The laser average output is controlled to ≤11 W, and the frequency is controlled to 150 to 250 kHz. The whole of the surface in the magnesium substrate 10 is covered with a homogeneous MgO film 11, and high corrosion resistance is exhibited. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for producing a magnesium material having a corrosion-resistant coating on the surface of magnesium or a magnesium alloy.

  Since magnesium alloys are lightweight and have high strength, they are expected to be used particularly as members of transportation equipment. However, magnesium is low in corrosion resistance, and as a result, the expansion of applications is not progressing as expected. In order to improve the corrosion resistance, a surface treatment may be applied, and there are methods of forming a film by a chromate treatment or an anodic oxidation method. However, the development of alternative technologies is desired because of the social demands for chromium-free materials and the fact that the alloy system is limited to those containing Al elements.

By the way, in magnesium or an alloy thereof, it becomes clear that the MgO inner layer coating exists under the Mg (OH) ₂ passive coating covering the surface, and corrosion proceeds from a place where the formation of the MgO coating is insufficient. ing. Therefore, a technique for improving the corrosion resistance by forming a dense Mg (OH) ₂ film and firing it at a temperature of 673-773 K for 1 hour to form an MgO film has been proposed (Patent Document 1). As a method other than firing, an attempt has been made to modify the Mg (OH) ₂ coating to MgO by irradiating the surface of magnesium with laser light (Non-patent Document 1).
JP 2003-193259 A Tomoaki Yamazaki, Shogo Izumi, Masaaki Otsu, Noto Kawamura: Outline of the Japan Institute of Metals, Spring (138th) Meeting

  However, in the above-described firing method, the magnesium internal structure is changed by being exposed to high temperature for a long time (for example, coarsening of crystal grains, overaging due to precipitation of stable phase β phase in aging alloys) It has a problem that it leads to a decline in target characteristics. Also, in the method of irradiating with laser light, the irradiation conditions, particularly the laser scanning speed and the laser frequency, were inappropriate, so that a homogeneous MgO film could not be formed, and the surface film was extremely incomplete, so corrosion resistance Was not enough.

  The present invention has been made in view of such problems, and an object thereof is to provide a method for producing a magnesium material capable of producing a magnesium material having excellent corrosion resistance without affecting the internal structure. .

The method for producing a magnesium material of the present invention includes a step of subjecting a treated surface of a substrate (magnesium or an alloy thereof) containing magnesium and having a treated surface to an alkali treatment, and a treated surface of the substrate subjected to the alkali treatment to 20 mms ⁻¹ And a step of irradiating laser light at a scanning speed in the range of 100 mms ⁻¹ or less.

In the method for producing a magnesium material of the present invention, a uniform and dense Mg (OH) ₂ film is formed on the treated surface of the substrate containing magnesium by alkali treatment. Then, with respect to the Mg (OH) ₂ coating by laser light is irradiated at a predetermined scanning speed, Mg (OH) ₂ coating is reformed into homogeneous and dense MgO film. The laser beam output is preferably 11 W or less and the frequency is 150 kHz or more and 250 kHz or less.

  The alkali treatment is performed, for example, by immersing a substrate containing magnesium in an alkali solution. The alkaline solution preferably contains at least one of magnesium hydroxide, sodium chloride, magnesium chloride, and sodium hydroxide, and the immersion time is preferably 5 minutes to 15 minutes.

According to the method for producing a magnesium material of the present invention, the surface of the substrate that has been subjected to alkali treatment is irradiated with laser light at a scanning speed of 20 mms ⁻¹ or more and 100 mms ⁻¹ or less. A magnesium material having an MgO coating can be produced.

  Embodiments of the present invention will be described below.

  The magnesium material according to an embodiment of the present invention has, for example, a dense and uniform highly corrosion-resistant MgO coating on the surface of a plate-like magnesium substrate. This magnesium material is used, for example, as a casing for transportation equipment and electronic equipment. The magnesium substrate is not limited to magnesium alone (Mg) but may be a magnesium alloy. Magnesium alone means a substance having a purity of 99.9% or more. Magnesium alloy refers to Mg as alloy elements: Al: 0.001 to 10 (atomic%), Zn: 0.001 to 3 (atomic%), Mn: 0.001 to 0.5 (atomic%), Si: 0.001 to 1 (atomic%), rare earth elements (Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb): 0.1 to 5 (atomic%) ), And may contain inevitable impurities.

  Note that the MgO film in the present embodiment is an oxide containing 90% or more of Mg as a cation, and optionally containing Al, Zn, Mn, and a rare earth element.

The magnesium material, subjected to alkali treatment to the surface of the magnesium substrate (substrate) (alkaline treatment step), a laser beam at a scanning speed of the surface to 20Mms ^-1 or more 100Mms ^-1 or less in the range of the magnesium substrate, on which the alkali treatment is made (Laser irradiation process).

(Alkali treatment process)
In the present embodiment, first, the magnesium substrate is immersed in an alkaline solution. That is, when the magnesium substrate is converted into a mass in 1 L of water as a chemical component, Mg (OH) ₂ : 0.001 to 0.012 g, NaCl: 0.001 to 300 g, MgCl ₂ : 0 to 400 g, Mg ( OH) ₂ : 0.001 to 0.012 g, NaOH: It is immersed for 10 seconds or more and 1 hour or less in an aqueous solution containing 0 to 500 g. In addition, it is preferable that immersion time shall be 5 to 15 minutes. This is because immersion in less than 5 minutes results in insufficient formation of a magnesium hydroxide film serving as a precursor of the corrosion-resistant MgO film, and the effect does not change even if treatment for more than 15 minutes is performed.

By this alkali treatment, a uniform and dense Mg (OH) ₂ film is formed on the surface of the magnesium substrate.

(Laser irradiation process)
After the magnesium substrate on which the Mg (OH) ₂ coating is formed is taken out of the alkaline solution, a Q-switch type or equivalent pulse type laser beam is applied to the surface of the magnesium substrate at a scanning speed of 40 mm / s or more and 120 mm / s or less. Irradiate in range. The average output at this time is, for example, 12 W or less and a frequency range of 100 kHz to 400 kHz. As a result, the Mg (OH) ₂ coating is oxidized, and a uniform crystalline MgO coating having a thickness of 0.5 to 3 μm is formed on the surface of the magnesium substrate.

  The irradiation conditions are preferably an average laser output of 11 W or less, a frequency of 150 kHz to 250 kHz, and a laser scanning speed of 20 mm / s to 100 mm / s. If the laser average power exceeds 11 W, melt marks due to the laser are formed on a part of the surface and the corrosion resistance is lowered, and when the frequency is less than 150 kHz, the peak power may be excessive and surface melting may occur. This is because when the frequency exceeds 250 kHz, the peak output is lowered and the modification of magnesium hydroxide to MgO becomes insufficient. Further, when the laser scanning speed is less than 20 mm / s, aggregation of formed MgO occurs, and when the laser scanning speed exceeds 100 mm / s, the modification from the magnesium hydroxide film to the MgO film becomes insufficient. It is.

1A and 1B, the laser irradiation conditions are as described above (laser average output: 11 W or less, frequency: 150 kHz to 250 kHz, laser scanning speed: 20 mm / s to 100 mm / s). 2 (A) and 2 (B) are schematic views showing the state of film formation when performed under other conditions. In the case of the above-mentioned conditions, as shown in FIG. 1A, the homogeneous MgO film 11 covers the entire surface on the magnesium substrate 10, and the magnesium substrate 10 is not altered. Further, partially aggregated MgO12 is also observed as shown in FIG. 1B, but the film thickness is not affected and a film is formed as in FIG. FIG. 1B also shows no alteration to the magnesium substrate 10. On the other hand, in FIG. 2A, the MgO film 11 does not cover the entire surface, and the thickness is not constant. There are also many agglomerated MgO 12, and some remain as residual Mg (OH) ₂ 13. Further, when reaching FIG. 2B, the structure of the magnesium substrate 10 has been altered by the heat affected zone 14.

As described above, in the present embodiment, the magnesium substrate is subjected to alkali treatment to form a uniform and dense Mg (OH) ₂ film, and then a relatively low output laser beam is irradiated at a low scanning speed. It is possible to modify only the surface without affecting the internal structure of the substrate and form a dense and uniform highly corrosion-resistant MgO film. Although the substrate is described as a magnesium substrate, the shape is not limited to the substrate shape and is arbitrary.

  Further, specific examples of the present invention will be described.

C_r：腐食速度（mm year^-1）
V_h：発生水素量（ml）
M：試料金属の平均質量（g mol^-1）
r：試料金属のの密度（g cm^-3）
A：試料金属の初期表面積（mm²）
t：浸漬時間（min.） (Production conditions)
In terms of mass in 1L of water as a chemical component, magnesium simple substance (purity 99.9%) is immersed in an aqueous solution containing a substance of Mg (OH) ₂ : 0.012 g for 5 to 15 minutes. After forming a Mg (OH) ₂ film, a laser beam having an average output of 11 W and a frequency of 200 kHz was scanned at a speed of 25 to 100 mm / s.
(Corrosion test)
After immersing the magnesium material manufactured under the above manufacturing conditions in a 1 mass% NaCl aqueous solution, the corrosion rate was calculated, and the surface state was observed with an optical microscope.
In addition, the calculation method of the corrosion rate was performed by the following calculation formula.

C _r : Corrosion rate (mm year ^-1 )
V _h : Amount of generated hydrogen (ml)
M: Average mass of sample metal (g mol ^-1 )
r: Density of sample metal (g cm ^-3 )
A: Initial surface area of sample metal (mm ² )
t: Immersion time (min.)

(Example 1-1)
As Example 1-1, the immersion time in saturated magnesium hydroxide was 10 minutes, the scanning speed of the laser beam was 50 mm / s, and a magnesium material was produced according to the manufacturing conditions described above.

(Comparative Examples 1, 2, 3-1)
As Comparative Example 1, a magnesium material (purity: 99.9%) is simply polished (no alkali solution immersion, no laser irradiation), and as Comparative Example 2, the magnesium material (purity: 99.9%) is saturated. What was immersed in magnesium hydroxide aqueous solution for 10 minutes (without laser irradiation) was used as Comparative Example 3-1, which was prepared in the same manner as in Example 1 except that the laser scanning speed was 200 mm / s. .

The surface of the magnesium material obtained in Example 1-1, Comparative Example 1, Comparative Example 2, and Comparative Example 3-1 was observed with an optical microscope. 3 (A), (B), (C), and (D) are photographs taken with an optical microscope of Example 1-1, Comparative Example 1, Comparative Example 2, and Comparative Example 3-1, respectively. Compared with comparative example 1, in comparative example 2, the metal surface is colored yellow. This is presumably because the metal surface was chemically changed to Mg (OH) ₂ by immersion in the magnesium aqueous solution. In contrast to Comparative Example 2, Example 1-1 was irradiated with laser light at a scanning speed of 50 mm / s, but the color of the portion that was colored yellow was removed by irradiating the laser light. I was sorry. This is considered to be because the surface Mg (OH) ₂ was chemically changed to MgO by laser irradiation at a scanning speed of 50 mm / s. On the other hand, in Comparative Example 3-1, in which laser light was irradiated at a scanning speed of 200 mm / s compared to Comparative Example 2, yellow coloration remained partially despite laser light irradiation. . This is thought to be because the chemical change of the metal surface was not performed sufficiently by increasing the scanning speed.

(Examples 2-1 and 3-1)
Example 2-1 was prepared in the same manner as Example 1-1 except that the laser scanning speed was 25 mm / s, and Example 3-1 was the same as Example 1 except that the laser scanning speed was 100 mm / s. What was produced like 1 was used.

  The thicknesses of the corrosion-resistant coatings of Examples 1-1, 2-1, 3-1 and Comparative Example 3-1 were measured with a confocal optical microscope. Specifically, the light was incident perpendicularly on the sample, the reflected light from the air / film interface and the film / internal metal interface was detected, and the depth profile was measured. FIG. 4 shows the relationship between the thickness of the MgO film and the laser scanning speed at that time. In the case of Example 1-1 (laser scanning speed 50 mm / s), the MgO film had a thickness of about 1.1 μm, and the thickest MgO film was formed. In the case of Example 2-1 (laser scanning speed of 25 mm / s), it was about 0.8 μm, and in the case of Example 3-1 (laser scanning speed of 100 mm / s), it was about 0.6 μm. On the other hand, in the case of Comparative Example 3-1 (laser scanning speed is 200 mm / s), the thickness of the coating was about 0.4 μm.

Next, the components of the coating formed in each of Examples 1-1, 2-1, and 3-1 and Comparative Example 3-1 were examined by XRD (X-ray diffraction: X-ray diffraction apparatus). FIG. 5 shows the X-ray diffraction pattern. In Example 1-1, Example 2-1 (not shown), and Example 3-1 (not shown), diffraction lines due to MgO were observed, but not in Comparative Example 3-1. . The diffraction lines observed in Example 1-1, Example 2-1 (not shown), and Example 3-1 (not shown) are broad, suggesting that the crystal grain size is extremely small. The In Comparative Example 3-1, a diffraction line caused by MgO was not observed, and a diffraction line of Mg (OH) ₂ was observed. Therefore, a large amount of Mg (OH) ₂ remained in the magnesium metal surface coating. I guess that.

(Examples 1-2, 1-3, Examples 2-2, 2-3, Examples 3-2, 3-3, Comparative Examples 3--2, 3-3)
Example 1-2 was the same as Example 1-1 except that the immersion time in saturated magnesium hydroxide was 5 minutes, and Example 1-3 was carried out except that the immersion time in saturated magnesium was 15 minutes. The same as Example 1-1, except that the immersion time in saturated magnesium hydroxide was 5 minutes as Example 2-2, and saturated magnesium hydroxide as Example 2-3. The same as Example 2-1 except that the immersion time in 15 minutes was the same as Example 2-1, except that the immersion time in saturated magnesium hydroxide was 5 minutes as Example 3-2. As Example 3-3, the same one as in Example 3-1 was used except that the immersion time in saturated magnesium hydroxide was 15 minutes. Moreover, except that the immersion time in saturated magnesium hydroxide was 5 minutes as Comparative Example 3-2, except that the immersion time of saturated magnesium was 15 minutes as Example 3-3 Used the same as in Comparative Example 3-1.

  The corrosion resistance of Examples 1-1 to 1-3, 2-1 to 2-3, 3-1 to 3-3 and Comparative Examples 3-1 to 3-3 was examined by the above corrosion test. FIG. 6 shows the relationship between the corrosion rate and laser scanning rate of Examples 1-1 to 1-3, 2-1 to 2-3, 3-1 to 3-3, and Comparative Examples 3-1 to 3-3. It is. It was found that Examples 1-1 to 1-3 with the laser scanning speed of 50 mm / s exhibited the highest corrosion resistance. Moreover, the difference was hardly seen in the corrosion rate in Examples 1-1 to 1-3. Examples 2-1 to 2-3 on the condition of a laser scanning speed of 25 mm / s showed high corrosion resistance, although the corrosion resistance of Examples 1-1 to 1-3 was not observed. Moreover, in Example 2-1 to 2-3, the remarkable difference by the immersion time to saturated magnesium hydroxide was not seen. Examples 3-1 to 3-3 under the conditions of a laser scanning speed of 100 mm / s resulted in different corrosion resistance depending on the difference in immersion time. In Examples 3-1 and 3-3 in which the immersion time was 10 minutes and 15 minutes, high corrosion resistance was almost the same as in Examples 1-1 to 1-3, but the immersion time was 5 minutes. Was slightly inferior. However, in Examples 1-1 to 1-3, Examples 2-1 to 2-3, and Examples 3-1 to 3-3, the corrosion rate was 2 mm / year or less, indicating high corrosion resistance. On the other hand, in Comparative Examples 3-1 to 3-3, even in Comparative Example 3-3 having the highest corrosion resistance, corrosion proceeds to about 6 mm / year, and in Comparative Example 3-1 having the lowest corrosion resistance, the corrosion resistance decreases to about 7 mm / year. It became clear to do.

  FIG. 7A shows the results of surface observation by an optical microscope after the corrosion test of Example 1-1, and FIG. 7B shows the results of surface observation by an optical microscope after the corrosion test of Comparative Example 3-1. While Example 1-1 has a clean surface even after the corrosion test, Comparative Example 3-1 shows that magnesium metal-containing filamentous corrosion has occurred after the corrosion test.

It is a schematic diagram of the MgO film formed by laser irradiation of the present invention. It is a schematic diagram (comparative example) of the MgO film formed by laser irradiation. It is an optical microscope photograph of the metal surface by Example 1-1, Comparative Example 1, Comparative Example 2, and Comparative Example 3-1. It is a figure showing the thickness of the surface film by Example 1-1, Example 2-1, Example 3-1, and Comparative Example 3-1. It is a figure showing the X-ray-diffraction result of the surface film by Example 1-1, Example 2-1, Example 3-1, and Comparative Example 3-1. The relationship between the corrosion rate and the laser scanning rate according to Examples 1-1 to 1-3, Examples 2-1 to 2-3, Examples 3-1 to 3-1, and Comparative Examples 3-1 to 3-3 is shown. FIG. It is the photograph by the optical microscope after the corrosion test of Example 1-1 and 3-1.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Magnesium substrate (substrate), 11 ... MgO film | membrane, 12 ... Aggregation MgO, 13 ... Residual Mg (OH) ₂ , 14 ... Heat affected zone

Claims (6)

Applying alkali treatment to the treated surface of the substrate comprising magnesium and having a treated surface;
Irradiating the treated surface of the substrate subjected to the alkali treatment with a laser beam at a scanning speed in the range of 20 mms ^-1 or more and 100 mms ^-1 or less. The magnesium material according to claim 1, wherein the alkali treatment is performed by immersing the substrate in an alkali solution. The said alkaline solution is a solution containing at least 1 sort (s) among magnesium hydroxide, sodium chloride, magnesium chloride, and sodium hydroxide. The manufacturing method of the magnesium material of Claim 2 characterized by the above-mentioned. The method for producing a magnesium material according to claim 2 or 3, wherein the immersion time in the alkaline solution is 5 minutes or more and 15 minutes or less. The method for producing a magnesium material according to any one of claims 1 to 4, wherein the output of the laser beam is 11 W or less. The method for producing a magnesium material according to any one of claims 1 to 5, wherein the frequency of the laser light is in a range of 150 kHz to 250 kHz.

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