JPS6012438B2 - Method of forming a colored protective film on the surface of magnesium material - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for forming a colored protective film on the surface of magnesium and magnesium alloys (hereinafter referred to as magnesium materials), in particular, by applying electricity to a magnesium material as an anode in an electrolytic bath, and by spark discharge. The present invention relates to a method for forming an inorganic colored protective film with excellent corrosion resistance, chemical resistance, and durability on the surface of a magnesium material.

From a chemical standpoint, magnesium material is a metal that is extremely susceptible to corrosion.

However, magnesium materials have a variety of advantageous properties, including light weight. Therefore, in order to eliminate this chemical defect and further expand the scope of use as an industrial material, advanced anticorrosion technology is required, and various anticorrosion methods have been studied. The anticorrosion treatment methods for magnesium materials can be roughly divided into painting methods, chemical film forming methods, and electrolytic conversion film forming methods. Corrosion prevention by painting is achieved by coating with an organic film such as paint or enamel, but since the formation of pinholes is unavoidable, corrosion resistance decreases with thin films (less than 20 Am), and thicker films with repeated coatings are required. is necessary.

Such coatings also exhibit resistance to chemical attack;
Particularly under high temperature conditions, it deteriorates and its adhesion to the magnesium material deteriorates. Chemical film formation methods include chromic acid method,
There are many methods such as the selenite method, the stannic acid method, the phosphate method, and the fluoride method, but the general method is to warm dichromate in a solution containing dichromate as the main component and cause a chemical reaction. Chromate treatment is used to form an anti-corrosion film.

Although this method is excellent in terms of economy and workability, it is inferior in corrosion resistance, and in high-temperature atmospheres, decolorization occurs and corrosion progresses significantly. Therefore, ``If corrosion resistance is desired, the current situation is to apply a coating on top of the chemical coating and use it as a base treatment for coating, and in this case, adhesion to the magnesium material is most important. Electrochemical conversion coating formation There are two methods: an anodic oxide film forming method and a spark discharge film forming method to which the present invention relates.

A typical method for forming an anodic oxide film is known as alkali. A. E. method, DOW12 method, DOW17 for acidity
method, DOW$ method, Cr22 treatment, etc., but the anodic oxide film obtained by such method is a colored, dull and opaque film, and its corrosion resistance is on the same level as the above-mentioned chromate film. In contrast to these methods, prior art methods relating to the formation of inorganic colored protective films by spark discharge to which the present invention relates include U.S. Pat.
There are methods disclosed in No. 83499y and No. 4184926.

The method described in Uekoshi U.S. Pat. This electrolytic method consists of applying a voltage of at least 220 V between the anode and cathode to produce a spark discharge, and then increasing the pressure to 350 to 1500 V until a film is deposited on the surface. The formation of vanadium, arsenic, boron, chromium, titanium, tin,
By adding large amounts of alkali metal salts of antimony, tungsten, and molybdenum.

However, examples of colored films disclosed in this document include:
There is only one example of a black film on an aluminum material with the addition of sodium vanadate, and the color tone of the film caused by other additives and the color tone on a magnesium material is unknown. The method described in US Pat. No. 3,834,999 comprises an alkali metal hydroxide and Si03 and at least one anion, such as
03,C^]4,Cr207,Moo4,P04,Ti
This method uses a strong alkaline electrolyte containing 307, W04, and W7024.According to this method, the colored film on the magnesium material is There is no mention of. Furthermore, the various methods described in the above-mentioned US patents require relatively long periods of time (30 to 60 minutes) to form the desired corrosion-resistant coating. In the method described in the above-mentioned US Pat. No. 4,184,926, in order to form a corrosion-resistant film on a magnesium material, the magnesium material is first treated with an aqueous hydrofluoric acid solution to form a fluoromagnesium layer, and then an alkali metal silicate layer is formed on the magnesium material. and an alkali metal hydroxide aqueous solution, and a voltage is applied between the magnesium material anode and the cathode until visible sparks are generated, forming a silicate film on the fluoromanesium layer. By maintaining this voltage until.

Formation of a colored film using this method requires a secondary treatment, which involves forming a milky white film in advance by the method described above and then treating it in the same manner using a military starvation bath containing potassium silicate and potassium vanadate. . Although such a method can provide color shading ranging from white to black, it requires secondary treatment to form a colored film and has problems such as the use of toxic hydrofluoric acid for preliminary surface conditioning. The protective film formed by the spark discharge method is glass-like,
It is thicker than films formed by other methods and has excellent corrosion resistance, chemical resistance, and durability.

Thus, an object of the present invention is to eliminate the drawbacks of the prior art and to obtain thick, inorganic colored protective coatings of various colors, excellent in corrosion resistance, chemical resistance, and durability, which could not be obtained by conventional spark discharge methods. That is. Furthermore, the purpose of the present invention is to form the above-mentioned colored protective film in a short time by only primary treatment without any preliminary surface preparation, compared to conventional colored film forming methods. In order to achieve the above object, the inventors investigated various coating methods and found that one or two water-soluble salts selected from phosphates, vanadates, aluminates, and borates. We have discovered that by using an aqueous solution containing the above as an electrolyte and applying electricity to the magnesium material as an anode to generate a spark discharge on the surface of the anode, it is possible to obtain an inorganic colored protective film of various colors on the surface of the magnesium material.

To explain the method of the present invention in more detail, the above aqueous solution is used as an electrolyte, a magnesium material is used as an anode and iron, stainless steel or nickel is used as a cathode and immersed in the electrolyte, and a DC voltage is applied to increase the voltage until a spark discharge occurs. .

The voltage is further increased to obtain the desired film thickness and color tone. In this way, a colored protective film of the desired color tone is formed on the surface of the anode magnesium material. When implementing the present invention,
There is no need for preliminary surface preparation such as treatment with an aqueous hydrofluoric acid solution to form a fluoromagnesium layer.

That is, the magnesium material can be used as it is without any pretreatment. Of course, conventional degreasing and cleaning, pickling with ferric nitrate or the like, or the above-mentioned chemical film forming method can also be carried out as a pretreatment. Such preliminary surface conditioning has almost no effect on the inherent corrosion resistance of the colored protective film. The water-soluble oxyacid salts used in the present invention include phosphates having the general formula M3P04.
Orthophosphates represented by z&0 (M is Na, K, Li or NH4; z is 0 or a positive number), such as trisodium phosphate (Na3P04), triammonium phosphate [(NH
4) 3P04/Pan 20], tripotassium phosphate (K3P0
4), etc., as a vanadate, the general formula My03 (M
are the same as above), such as sodium metavanadate (NaV03), potassium metavanadate (KV03), etc.; Tetraborates represented by M2&07・zQ○ (M in both formulas; z is the same as above), such as sodium metaborate (NaB02・2day 20), lithium metaborate (LiB02・2LO), Sodium tetraborate (Na2 & 07, 10 days 20) etc., as an aluminate, meta-aluminate represented by the general formula MA-SO02-zH20 (M; z is the same as above), etc. ), potassium metaaluminate (KA〆02・3/2日20), etc. can be mentioned as representative examples.

These salts can be used alone or in combination of two or more. Although its density varies depending on the desired color tone, it can be used up to saturation density for one night or more. Further, in order to obtain colored films with various tones that cannot be obtained with the above-mentioned aqueous salt solution, a compound having a metal as a constituent component can be added to the above-mentioned oxyacid salt. Such compounds include hexavalent chromium, trivalent chromium, iron, copper, nickel, cobalt, silver,
Manganese, zinc, lead, tin, magnesium, molybdenum,
The composition contains at least one metal selected from tungsten and antimony, and one or more of these compounds can be used in combination. Examples of these compounds include sulfates of the above metals,
Any form of organic acid salts such as nitrates, carbonates, phosphates, oxyacids, halogen salts, acetates, cyanides, bases, oxides, etc. can be used. However, when the above-mentioned compound is added to an oxysalt-containing electrolytic bath, it must be used in a water-soluble form. That is, among the above-mentioned compounds, those which are water-insoluble need to be added after making a collar body such as an EDTA rust body, an amine key body, a cyan complex, etc., and making it water-soluble. The concentration of the compound added varies depending on the type of metal constituting the compound and the desired color tone of the film, but it can be used from 0.019/or higher as the metal to the saturation concentration of the compound. A colored film having a color tone approximately as described below is formed depending on the added metal, but the color tone and shading are not limited thereto since they vary depending on the electrolytic conditions, electrolytic concentration, etc.

Note that as chromium, any of hexavalent chromium such as chromate and dichromate, and trivalent chromium such as chromium nitrate and chromium sulfate can be used. Cu: brown, gray-green,
Gray Ni: brown, beige Cr: yellow-green, green Fe: gray-green, dark green Co: yellow, gray-yellow, blue Zn: white, gray Mn: yellow, red-brown Sn: gray-white, gray Ag: yellow Pb: white , gray Mg: grayish white, gray Mo: yellow, brown W: brown, gray Sb: yellow, gray The color tone of the colored film is determined by the relative amounts of oxyacid salts and additive compounds, processing voltage, processing time, and processing current as described above. Determined by density etc.

The pH of the electrolyte varies depending on the type and concentration of the oxyacid salt used and the type of additive compound, but it is desirable to maintain the axis in the range of 7 to 14.

When forming a colored protective film, a DC voltage having various waveforms such as a rectangular waveform, a sawtooth waveform, a single-phase half-wave waveform, and a single-phase full-wave waveform can be applied.

At this time, a DC voltage with a pulse waveform such as a rectangular waveform or a sawtooth waveform is used because it is easy to form colored films with various shades of color, it is easy to obtain a homogeneous film, and the spark discharge voltage can be significantly reduced. It is advantageous to apply In the electrolytic treatment, as described above, the magnesium material to be treated is used as an anode and iron, stainless steel or nickel is used as a cathode, and is immersed in the above electrolysis method, and a DC voltage is gradually applied until a spark discharge occurs, and then the spark discharge is maintained. The voltage may be increased to a predetermined voltage while maintaining the voltage, and the voltage may be maintained until a colored protective film of desired thickness and color tone is formed.

For example, in the galvanostatic electrolysis method, the applied voltage is continuously changed to maintain a constant anode current density to produce intense spark discharge on the anode surface, and then the coating is heated until the desired thickness and color are achieved. Continue energizing while maintaining the voltage. If constant current electrolysis cannot be performed, first apply the voltage to a certain anode current density, and a rapid drop in current value will be observed as a film is formed. Further voltage is applied to achieve the density. By repeating this operation, intense spark discharge is generated on the anode surface, and the film has a desired thickness and color tone. Current density is 0.1~
The current density can be arbitrarily selected within the range of 10 A/d, and this current density has little to do with the spark discharge voltage, but in the case of low current density, it takes time to reach the specified voltage. In addition, in the case of high current density, problems such as the smoothness of the colored protective film and an increase in the temperature of the exposure bath occur, so the current density is 0.5 to 2 A/
It is preferable to do it after d. The thickness of the film formed is determined by the electrolytic concentration, electrolytic temperature, processing voltage, processing time, processing current density, etc. The electrolytic temperature is determined depending on the desired film, but it is usually 5-8000.

The invention will now be explained in more detail with reference to examples.

(Example 1) A magnesium alloy plate AZ- with a surface area of 50 mm and a thickness of 3 mm was placed in an aqueous solution consisting of 40 mm/s of sodium metaaluminate.
If 31C is used as an anode and an iron plate is used as a cathode, and a sawtooth waveform DC voltage is continuously applied while maintaining the anode current density IA'd, a spark discharge will occur at about 30V.

The voltage was increased to 40V and maintained for 10 minutes. This energization is accompanied by intense spark discharge. A smooth, milky inorganic film was formed on the anode plate, and the thickness of the film was about 10 mm. (Example 2) A magnesium alloy plate AZ-31C with a surface area of 50 mm and a thickness of 3 ribs was used as an anode and an iron plate was used as a cathode in an aqueous solution consisting of 50 mm/kg of potassium metavanadate, and a single-phase full-wave DC voltage was obtained. is the anode, current density is 0.
Approximately 150 if applied continuously while maintaining the 7-axis/d force.
A spark discharge occurs at V.

The voltage was increased to 200V and maintained for 10 minutes. This energization is accompanied by intense spark discharge. A brown, smooth, glass-like film is formed on the anode plate, and the thickness of this film is approximately 10 mm.
It was 〆m. (Example 3) A magnesium alloy casting AZ-91C with a surface area of 50cm and a thickness of 2 was used as an anode and an iron plate was used as a cathode. If a sawtooth wave DC voltage is gradually applied while maintaining the anode current density at IA/d, a spark discharge will occur at about 25V.

The voltage was increased to 35 V, and the treatment was carried out for 2 minutes while maintaining this voltage. This energization is accompanied by intense spark discharge. A yellow-green smooth glass-like film was formed on the anode plate, and the thickness of the film was about 25 mm. (Example 4
~16) The colored protective films formed by the methods described in Examples 1 to 12 above have much greater resistance to strong acids and strong alkalis than magnesium materials coated by conventional methods. It was a durable film.

Claims (1)

[Claims] 1. In a method of forming a colored protective film on the surface of a magnesium material by spark discharge by applying electricity to a magnesium material as an anode in an electrolytic bath, the electrolytic bath contains phosphates, vanadates,
1 of the oxyacid salts selected from aluminates and borates
A method characterized in that the aqueous solution contains a species or two or more water-soluble salts, and the applied voltage is a DC voltage with an arbitrary waveform. 2. In the method of forming a colored protective film on the surface of the magnesium material by spark discharge by applying electricity to a magnesium material as an anode in an electrolytic bath, the electrolytic bath contains phosphates, vanadates,
1 of the oxyacid salts selected from aluminates and borates
or two or more water-soluble salts, and at least one metal selected from copper, nickel, iron, cobalt, chromium, zinc, manganese, tin, silver, lead, magnesium, molybdenum, tungsten, and antimony. A method characterized in that the aqueous solution is composed of one or more kinds of compounds, and the applied voltage is a DC voltage with an arbitrary waveform.