JP2000345370A - Surface treatment of magnesium or magnesium alloy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of surface treatment excellent in corrosion resistance and adhesion with respect to a magnesium metal or its alloy being extremely light in weight and having strength but being seriously subject to corrosion. SOLUTION: An oxide ceramic film is formed on a magnesium metal or magnesium alloy by means of plasma chemical anodic oxidation, and a cationic electrodeposition coated film is formed on the oxide ceramic film. This procedure can provide surface treatment finish producing excellent mechanical strength and superior spread of electrodeposition coating. Further, the corrosion resistance of an oxide film can be improved and a uniform coated film can be produced by forming the cationic electrodeposition coated film after applying surface adjusting treatment to the oxide ceramic film.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an improved surface treatment method for magnesium or a magnesium alloy.

[0002]

2. Description of the Related Art Magnesium and magnesium alloys are very light and strong metals, but are very susceptible to corrosion. For this reason, it is hardly used for mechanical parts and automobile parts, etc., which emphasize functions, and the most widely used parts are mobile phone covers. In the case of cell phone covers, very colorful paint is applied, but not limited to this, the current surface treatment of magnesium and magnesium alloy products is to apply a spray coating after forming a chemical conversion film It is.

[0003]

The chemical conversion film is formed by immersion in a treatment bath such as chromate. This film is very thin (about 0.2 to 2 μm) and has a low corrosion resistance. There is a drawback that corrosion progresses promptly when the coating film is peeled off because of the badness. Therefore, it is difficult to use for applications requiring functionality.

[0004]

The present inventor has completed the present invention as a result of intensive studies in view of the above problems. The characteristic feature is that a high voltage (plasma) is applied in an electrolytic solution to form an oxide ceramic film on a magnesium metal or a magnesium alloy, and a cation electrodeposition film is formed thereon. .

[0005] Plasma chemical anodic oxidation is a process in which a high voltage pulse or a high voltage DC / AC superimposed current is passed through an electrolyte solution to a magnesium metal or magnesium alloy product supported on the anode side, thereby forming an oxide ceramic. It forms a film. As this electrolyte solution, an alkaline solution having a pH of 8 to 12, containing borate ions or sulfonate ions, phosphate ions, fluorine ions or chloride ions, and lithium, sodium or potassium ions, is preferably used. This oxide ceramic coating
In addition to a thick film having a thickness of 5 to 20 μm, an extremely strong protective film having good corrosion resistance can be obtained.

[0006] Then, by forming a composite film comprising a cationic electrodeposition coating process that can be performed continuously after this process, the corrosion resistance and adhesion of the conventional surface treatment are remarkably improved, and the performance is extremely improved. Good composite coating is obtained.

However, a film obtained by plasma chemical anodic oxidation has a large number of fine holes (10 nm in diameter and depth).
The diameter is about 10 to 10 μm, and the depth is 1
/ 2), when cation electrodeposition coating is carried out, irregular surface irregularities are formed on the painted surface, making it difficult to obtain a beautiful finish, and there is a possibility that the painted surface may peel off due to cracks in the irregularities. Therefore, the present inventors have developed a technique for further adjusting the surface of the oxide film. Surface treatment of the present invention, the fine holes of the film obtained by the plasma chemical anodization,
The surface of the film is smoothed by filling it with conductive salts or ions. The use of the conductive salt or ion is for attaching the electrodeposition paint. By doing so, the corrosion resistance of the oxide film is further improved, and the cationic electrodeposition coating can be performed very uniformly.

[0008] This surface conditioning, magnesium metal or magnesium alloy treated with a plasma-chemical anodized film,
Dissolve 1-5 g / L of metal ions such as Ag, Sn, Ni, etc. into a treatment liquid (liquid temperature 25 ± 5 ° C.) to which isopropyl alcohol, triethanolamine, acrylic acid resin or a mixture thereof is added as an additive. It is immersed, and a DC or AC current is applied for 30 to 60 seconds. Or,
Sodium silicate as main component 100-500ml /
L dissolution, immersed in a treatment liquid (60 ± 10 ° C.) to which sodium citrate and sodium metaborate are added as additives,
DC or AC is supplied for 5 to 30 minutes.

Next, the step of cationic electrodeposition coating will be described. The epoxy cationic electrodeposition resin used in the present invention may be a polyamine resin such as an amine-added epoxy resin conventionally used in the field of cationic electrodeposition coatings, for example, polyepoxide and primary mono- and polyamines, secondary amines. Polyamines or adducts with primary and secondary mixed polyamines (see, eg, US Pat. No. 3,984,299); adducts of polyepoxides with secondary mono- and polyamines having ketiminated primary amino groups (eg, US Pat. US Pat. No. 4,017,438); a reaction product obtained by etherification of a polyepoxide with a ketiminated hydroxy compound having a primary amino group (see, for example, JP-A-59-43013). These polyamine resins can be cured using a polyisocyanate compound blocked with an alcohol to form an electrodeposition coating film.

Further, an amine-added epoxy resin which can be cured without using a blocked isocyanate compound can also be used. For example, a resin in which a β-hydroxyalkyl carbamate group is introduced into an added polyepoxide (for example, see JP-A-59-1984) JP-A-155470); a resin that can be cured by a transesterification reaction (for example, see JP-A-55-80436) can also be used.

The above-mentioned polyepoxides used in the production of resins include, for example, polyglycidyl ethers of polyphenols which can be produced by reacting polyphenols with epichlorohydrin in the presence of an alkali. Representative examples include bis (4-hydroxyphenyl) -2,2-propane, bis (4-hydroxyphenyl) -1,2, -ethane, bis (4-hydroxyphenyl) -methane,
Glycidyl ethers of polyphenols such as 4'-dihydroxyphenyl ether, 4,4 'dihydroxydiphenyl sulfone, phenol novolak, and cresol novolak, and polymers thereof.

Among the above-mentioned polyepoxides, those particularly preferred in view of price and corrosion resistance are those having a number average molecular weight of at least about 380, preferably in the range of about 800 to 2000.
And an epoxy equivalent of 190 to 2000, preferably 400
A polyglycidyl ether of a polyphenol in the range of 1000, particularly a polyepoxide represented by the following general formula (Formula 1).

(Equation 1)

The cationic electrodeposition paint used in the present invention is prepared, for example, by dissolving the epoxy cationic electrodepositable resin in a water-miscible organic solvent and adding water and an acid (for example, acetic acid,
An aqueous bath can be obtained by mixing with a water-soluble organic or inorganic acid such as formic acid, lactic acid, phosphoric acid, and sulfuric acid) and neutralizing the mixture. In addition to the above-mentioned resin component, a color pigment, an anticorrosion pigment, an extender pigment, an additive, and the like, which are generally used in the field of paint, can be added to the above-mentioned electrodeposition paint as needed.

Further, when an epoxy cationic electrodeposition resin of a type which is cured by a combined use with a curing agent is used as the resin, a polyisocyanate compound such as isophorone diisocyanate is used as a curing agent in the composition.
A predetermined amount of a blocked product such as 4,4′-diphenylmethane diisocyanate can be blended. As a method and an apparatus for performing electrodeposition coating on an object to be coated using the electrodeposition coating composition, known methods and apparatuses conventionally used per se in cathodic electrodeposition coating can be used. In this case, it is desirable to use a stainless steel or a carbon plate as the anode and the anode as an object to be coated. Electrodeposition conditions that can be used are not particularly limited, but generally, the temperature is 20 to 30 ° C., and the voltage is 100 to 400.
V, preferably 200 to 300 V, current density: 0.01 to
3 A / dm ² , energization time: 1 to 5 minutes, electrode area ratio (A /
C): 1/2 to 1/6, distance between poles: 10 to 100 cm,
It is desirable to perform electrodeposition while stirring.

The coating film deposited on the cathode substrate can be cured by baking at about 130 to 230 ° C. for about 10 to 30 minutes after washing.

[0016]

EXAMPLES (Example 1) Next, the present invention will be described in more detail with reference to examples. In the examples, parts and% mean parts by weight and% by weight. I Plasma Chemical Anodizing of Magnesium Alloy The surface of a magnesium alloy (AZ91D plate material) was treated with an alkaline cleaning bath composed of the following components. Sodium hydroxide 50 g / L Trisodium phosphate 10 g / L Wetting agent (synthetic soap) 1 g / L Subsequently, etching was performed in a bath containing the following components. Phosphoric acid (85%) 380 ml / L Sulfuric acid (98%) 16 ml / L Water 604 ml / L Etching was performed at 20 ° C. for about 30 seconds. continue,
The surface was activated in hydrofluoric acid. Subsequently, anodization was performed using an electrolyte composed of the following components. Hydrofluoric acid (40%) 30 g / L Phosphoric acid (95%) 60 g / L Adjusted to pH 8.9 with ammonia Anodization was performed at DC using an alternating current of 50 Hz superimposed on DC. The voltage was increased to 240V. Oxidation lasted about 15 minutes. The thickness of the obtained plasma-chemical anodic oxide film was about 20 μm.

II Cation Electrodeposition Coating Next, electrodeposition coating was performed on the surface of the surface-treated magnesium alloy. (Production example of cationic electrodeposition paint) 1. Preparation of cationic epoxy resin (1) 930 parts of bisphenol-type epoxy resin (“Araldite # 6071” manufactured by Ciba-Geigy) (2) Bisphenol-type epoxy resin (manufactured by Ciba-Geigy) Araldite GY2600 ") 380 parts (3) Polycaprolactone diol (" Placcel # 205 "manufactured by Daicel) 550 parts (4) Dimethylbenzylamine acetate 2.6 parts (5) p-nonylphenol 79 parts (6) Monoethanol Amine methyl isobutyl ketone ketimine compound 71 parts (7) Diethanolamine 105 parts (8) Butyl cellosolve 180 parts (9) Cellosolve 525 parts Components (1) to (6) are combined, mixed and reacted at 150 ° C. for 2 hours. , Components (7) to (9) are added and reacted at 80 to 90 ° C. for 3 hours to obtain a resin solution having a solid content of 75%.

2 Formulation of electrodeposition paint (1) Production of emulsion (1) 82.6 parts as solid content of the above cationic epoxy resin (2) Ethylene glycol mono-2-ethylhexyl ether of 4,4'-diphenylmethane diisocyanate Block 5.0 parts (3) Methyl ethyl ketone ketoxime diblock of isophorone diisocyanate 12.4 parts (4) Polypropylene glycol 4000 0.5 part (5) Lead acetate 1.0 part (6) 10% acetic acid 9.3 parts ( 7) 185.75 parts of deionized water The components (1) to (4) are mixed uniformly, the components (5) to (6) are added and further mixed, and then the component (7) is added and mixed uniformly. Stir and mix to obtain an emulsion having a nonvolatile content of 32% (120 ° C., 1 hour).

(2) Production of pigment paste The above cationic epoxy resin (as solid content) 5.73 parts Formic acid (10%) 2.5 parts Titanium white 14.5 parts Carbon 0.54 parts Extender pigment (clay) A pigment paste consisting of 7.0 parts of lead silicate, 2.3 parts of dibutyltin oxide and 2.0 parts of a nonvolatile component of 50% (120 ° C., 1 hour) is obtained.

(Production of cationic electrodeposition coating composition) The above-mentioned emulsion (317.2 parts), the above-mentioned pigment paste (59.56 parts),
And 279.64 parts of deionized water are mixed to obtain eight kinds of cationic electrodeposition coating compositions (solid content: 20%). A magnesium plate (AZ91) subjected to a plasma chemical anodizing treatment was placed in an electrodeposition bath having a bath temperature of 28 ° C. comprising these electrodeposition coating compositions.
D) and a magnesium plate (AZ91D) whose surface has been subjected to a surface conditioning treatment was further immersed in the oxide ceramic film, and a current was passed between the anode and the counter electrode at 250 V for 2 minutes. Next, the film was washed with tap water for 3 minutes, drained for 5 minutes, and baked in a hot air drying oven at 180 ° C. for 30 minutes to obtain a coating film. Table 1 shows the results.

[Table 1]

In Table 1, the appearance of the coating film was determined by examining the coating film for abnormalities such as cracks, peeling, and blisters. The adhesiveness was evaluated by visually observing the coated surface when the coating was scratched at an angle of 60 degrees with a cutter and the attached gum tape was rapidly peeled off. For corrosion resistance (salt spray resistance), a cross-cut wound was made with a knife to reach the substrate, and this was subjected to a salt spray test for 480 hours in accordance with JIS Z-2371. Evaluated by width. In each case, along with the numerical values, when this rust or blister width is less than 0 to 1 mm, ◎,
When it was less than 1 to 2 mm, it was indicated by ○, when it was less than 2 to 6 mm, it was indicated by Δ, and when it was 6 mm or more, it was indicated by X.

Example 2 Surface Adjustment Treatment of III Oxide Ceramic Coating The magnesium alloy that had been subjected to the plasma chemical anodic oxidation treatment in Example 1 was subjected to a surface adjustment treatment. This treatment is a treatment bath (25 ° C.) in which 1.5% of isopropyl alcohol and 0.3% of triethanolamine are added as additives to a solution in which 3 g / L of Ni ions (Ni (CH ₃ COO) ₂ ) are dissolved. And a DC or AC current density of 0.8 A / dm ² for 30 minutes.
This was carried out by energizing for ~ 60 seconds. Similarly, the results are shown in Table 1.

Comparative Example Similarly, a magnesium alloy (A
Z91D plate material) and spray coating finish after forming a conventional chemical conversion film. (Conversion film treatment) Conditions: Treatment in a treatment bath containing sodium dichromate as a main component. (Blow-coating) conditions: After applying a special epoxy resin-based primer, drying at 130 ° C. for 20 minutes. Next, a thermosetting acrylic resin-based paint is applied, and then dried at 150 ° C. for 20 minutes. The results are also shown in Table 1.

In the present invention, since the object to be coated with the cationic electrodeposition coating is a cathode, the metal of the object to be coated remains inactive without being ionized at the time of electrodeposition coating. For this reason, since the electrodeposition coating film is formed without dissolving the metal of the material, an electrodeposition coating film having excellent anticorrosion properties can be formed.

The specific magnesium oxide film used in the present invention has a magnesium oxide layer formed by sequentially laminating a barrier layer, a low-porosity magnesium oxide layer, and a porous magnesium oxide layer on a magnesium metal surface, and In these pores, in addition to the magnesium oxide layer, there is a compound of acidic ions (phosphoric acid, boric acid, sulfuric acid, fluorine, chlorine, etc.) used in anodic oxidation and magnesium, and residues of acidic ions that cannot be completely removed by washing with water. Included,
In particular, such acidic ions have a bad influence on the corrosion resistance of magnesium. In the present invention, such acidic ions are used in a cationic coating film which is subsequently subjected to cationic electrodeposition coating (a basic amino acid is contained in the coating film). It is presumed that it is neutralized by preventing the progress of corrosion of magnesium.

[0026]

As described above, according to the surface treatment method of the present invention, an oxide ceramic film is formed on a magnesium metal or a magnesium alloy by plasma chemical anodic oxidation, and cation electrodeposition is performed on the oxide ceramic film. It forms a coating film. Further, the method of the present invention is to form a cationic electrodeposition film after subjecting the formed oxide ceramic film to a surface conditioning treatment.

Therefore, the thickness of the film is about 10 times that of a conventional case where the film is applied on a chemical conversion film, so that the film has excellent corrosion resistance, and there is almost no fear of corrosion even when the film is peeled off. In addition, the coating film itself is difficult to peel because of good adhesion to the paint. When the coating film is subjected to a surface conditioning treatment for closing the fine holes, the riding of the electrodeposited coating film is further improved, and a very uniform coated surface having no unevenness on the surface can be obtained.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Fuji Watanabe 33-1 Kanzakicho, Amagasaki City, Hyogo Prefecture Kansai Paint Co., Ltd. F-term (reference) 4K044 AA06 BA12 BA21 BB03 BC02 BC04 CA17

Claims (2)

[Claims] An oxide ceramic coating is formed on a magnesium metal or magnesium alloy by plasma chemical anodization, and a cationic electrodeposition coating is formed on the oxide ceramic coating. Surface treatment method for magnesium alloy. 2. After subjecting the oxide ceramic coating to a surface conditioning treatment, a cationic electrodeposition coating is formed.
The method for treating a surface of magnesium or a magnesium alloy according to claim 1.