JPS581093A - Method for forming protective film on surface of magnesium material - Google Patents

Abstract

PURPOSE:To reduce required treatment voltage and its time, and to save the quantity of power consumption, by using DC low voltage of a specific waveform as applied voltage, in an Mg material surface protective film forming method by spark discharge. CONSTITUTION:A solution containing alkali metal silicate or this silicate and alkali metal hydroxide is used as an electrolytic bath, in which an Mg material as the anode, and iron, Ni, etc. as the cathod are soaked respectively. Subsequently, DC voltage of a square waveform, a saw tooth waveform or a single phase half-waveform is applied until spark discharge is generated, and once spark discharge is generated, the voltage is further boosted. In this way, a desired protective film is formed on the surface of the anode Mg material.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for forming a protective film on the surface of a magnesium material, that is, magnesium metal or magnesium alloy.
In particular, the present invention relates to a method of forming an inorganic protective film with excellent corrosion resistance, chemical resistance, and durability on the surface of a magnesium material by applying electricity to the magnesium material as an anode during electrolysis and using spark discharge.

From a chemical standpoint, magnesium material is a metal that is extremely susceptible to corrosion. However, magnesium material has various advantageous properties including lightness. Therefore, in order to eliminate this chemical disadvantage and further expand its range of use as an industrial material, advanced anti-corrosion technology is required. Research on the law is being conducted.

Corrosion prevention treatment methods for such magnesium materials can be broadly classified 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 μm), and thicker films with repeated coatings are required. is necessary. Although the overcast film exhibits resistance to chemical attack, it deteriorates particularly under high temperature conditions and its adhesion to the magnesium material deteriorates.

There are many methods for forming chemical films, including the chromic acid method, selenite method, stannate method, phosphate method, and heptadide method, but the most common method is to use dichromate. Chromate treatment is used to form an anti-corrosion film by immersing the material in a solution containing the components and utilizing a chemical reaction. Although this method is excellent in terms of economy and workability, it is inferior in corrosion resistance and is currently used as a base treatment for painting in a high humidity atmosphere, in which case adhesion to the magnesium material is most important. It is.

There are two types of electrochemical conversion film forming methods: an anodic oxide film forming method and a spark discharge method to which the present invention relates. A typical method for forming an anodic oxide film is 4;jH,A in an alkaline bath.
There are methods such as DOW 12 method, acid bath method, DOW 17 method, DOW 9 method, and (1!r 22 treatment), but the anodic oxide film produced by the peeling method is a colored, matte and opaque film. The corrosion resistance is comparable to that of the above-mentioned chromate film.

In contrast to these methods, prior art methods for forming a thick inorganic protective film by spark discharge to which the present invention relates include U.S. Pat. No. 3,832,293 and U.S. Pat.
There are methods disclosed in No. 99 and No. 4184926.

The method described in the above-mentioned US Pat. The spark starting voltage is approximately 2
The voltage is 20 V, and then it is necessary to increase the pressure to 35° to too much V.Although it depends on the bath composition, it is necessary to increase the pressure to 1500 V in order to obtain a sufficient film.

The method described in the above-mentioned United States Patent No. 3,834,999 uses an alkali metal hydroxide and S i O3
” and at least one anion, e.g. BO, -1B
O, -', B2O,7-21, As04-', co, -
2, Cj r 04-2.0r20.1-21, Mob
,-”,Po4-3,Ti30.-2,wo,,-”,
W, 02. According to this method, the spark discharge starting voltage is:
The voltage is about 250 V, and depending on the material to be treated, it is necessary to increase the voltage to 400 to 600 V in order to form the desired film. 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.

The method described in the above-mentioned US Pat. No. 4,184,926 involves first treating a magnesium material with an aqueous solution of hydrofluoric acid to form a fluoromagnetic layer and a ram layer, and then treating the magnesium material with an alkali metal silicate and an alkali metal hydroxide. This method uses an electrolytic bath consisting of an aqueous solution, and according to this method, the voltage required until a visible spark appears on the anode surface, that is, 1
A voltage of 50 to 300 V must be applied and maintained.

The protective film formed by the spark discharge method is glass-like, thicker and superior in corrosion resistance, chemical resistance, durability, etc. when compared to films formed by other methods.

Therefore, an object of the present invention is to improve the spark discharge method described above according to the prior art to reduce the required processing voltage and time, thereby reducing power consumption, ancillary equipment, and equipment costs, and reducing processing costs. The goal is to make it cheaper.

According to the invention, this purpose is such that the electrolytic bath is a silicate-containing aqueous solution or an aqueous solution containing a silicate and an alkali metal hydroxide, and the applied voltage has a rectangular waveform, a sawtooth waveform or a single-phase half-wave waveform. This is achieved by a waveform DC low voltage.

In the method of the present invention, spark discharge starts at about 40 V when the applied voltage is a single-phase half-wave DC voltage. The desired protective film can be obtained by increasing the voltage to about 50 to 50V and maintaining this.

In the method of the present invention, when the applied voltage is a DC voltage with a rectangular waveform or a sawtooth waveform, the applied voltage can be further reduced significantly. That is, in this case about 15
Spark discharge starts when V is applied.

Although there is no particular limit to the final voltage required to form the desired film, an inorganic (glass-like) film can be formed by increasing the voltage to 30 to 100 V and maintaining this voltage.

To further explain the method of the present invention, an aqueous solution containing a silicate or a silicate and an alkali metal hydroxide is used as an electrolytic bath, or at least one oxyacid salt is added to the aqueous solution. The magnesium material to be treated is immersed in the solution as an electrolytic bath, and the magnesium material to be treated is used as an anode and iron, stainless steel, or nickel is used as a cathode, and a DC voltage with a single-phase half-wave, rectangular, or sawtooth waveform is applied to generate a spark discharge. Apply the voltage until spark discharge occurs.1. If it is, further increase the voltage to the above voltage and approximately 1~! If the temperature is maintained for 0 minutes, the desired protective film will be formed on the surface of the anode magnesium material.

When carrying out the method of the present invention, there is no need for preliminary surface preparation such as treatment with an aqueous solution of fluoromagnesium to form a fluoromagnesium layer. That is, the magnesium material can be used as it is without any pre-treatment for stationery.

Of course, conventional degreasing, washing, pickling with ferric nitrate or the like, or the formation of a chemical film as described above can also be carried out as a pretreatment.

Such preliminary surface conditioning has little effect on the inherent corrosion resistance of the protective coating.

The silicate used in the method of the invention has the general formula M2
0 ・n S 102 (M represents an alkali metal, n
represents a positive number from 0.5 to 20], such as sodium silicate, sodium metasilicate, potassium silicate, lithium silicate, colloidal silica, etc. can be mentioned. These silicates can be used alone or as a mixture of two or more. The silicate concentration is 52/
It is possible to use one or more up to a saturation concentration, but the preferred range is 10 to 300 y/l. The higher the silicate concentration, the lower the spark discharge starting voltage, but at 200171 or more, almost no effect on the spark discharge starting voltage is observed.

When using an aqueous silicate solution or an aqueous solution containing a silicate and an oxyacid salt as the electrolytic bath, the silicate used preferably has a molar ratio (51o2/M, O) of 2.5 or less. In this case, it is preferable to use a relatively high silicate concentration (50171 or more). In electrolytic treatment using a silicate with a molar ratio of 2.5 or more, if no alkali metal hydroxide is contained, initial It is necessary to make the current density very high, and in the current density range of 0, 1 to 10 A/dm2 according to the method of the present invention described later, spark discharge does not substantially occur and colloidal substances with poor adhesion are deposited. Alternatively, only an anodized film is formed on the surface of the magnesium material.

Such a film does not have the corrosion resistance targeted by the present invention.

The electrolytic bath used in the method of the invention may contain at least one alkali metal hydroxide. In this case, the pH of the bath will be very high, the value being determined by the individual hydroxide concentration or the hydroxide combination. The inclusion of alkali metal hydroxide makes it possible to use silicate in various molar ratios, and depending on the relative amount to silicate, the concentration of silicate used can be reduced. , it also helps to reduce the spark discharge voltage. Preferred alkali metals used in the process of the invention are sodium, potassium and lithium, preferably containing at least 5y/m of alkali metal hydroxide. The hydroxide can be used up to a saturation concentration, but the preferred range for the protective coating according to the method of the present invention is 10 to 100, depending on the relative amount to the above-mentioned silicate concentration.
f/l.

By adding an oxyacid to an electrolytic bath consisting of a silicate or an aqueous solution containing a silicate and an alkali metal hydroxide, the spark discharge starting voltage is lowered, the silicate concentration is reduced, and the film is improved. It is possible to homogenize and smooth the film, and also to color the film. Among such oxyacid salts, mention may be made of tungstates, stannates, molybdates, phosphates, vanadates, borates, chromates and permanganates. These can be used alone or in combination of two or more, and the concentration is 0.2 y/or more. For example, stannate has the effect of smoothing the film and making it gray,
Also, depending on the concentration, vanadate gives the film a golden color, brown color, or
It has a grayish-black or black color with a certain effect.

The pH of the electrolytic bath is preferably 8.5 or higher, and 8.5
Below, undesirable phenomena such as gelation may occur. For this purpose, pH adjusters and stabilizers can be added to prevent gelation and precipitation.

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 electrolytic bath, and a DC voltage of the above waveform is gradually applied until a spark discharge occurs. The voltage may be increased to a predetermined voltage while maintaining the same, and the voltage may be maintained until a film of a predetermined thickness is formed.

For example, in galvanostatic electrolysis, the applied voltage is continuously varied to maintain a constant anode current density, producing intense spark discharge on the anode surface, and then the voltage is maintained until the film reaches the desired thickness. Continue to turn on the power while doing so. If constant current electrolysis cannot be carried out, if a voltage is applied to produce a spark discharge so that the anode current density is as follows, a decrease in current density will be observed as a film is formed by the discharge. Then, a voltage is applied so that the initial current density is achieved, and this operation is repeated to form the film to a desired thickness.

The current density can be arbitrarily selected in the range of 0.1 to 10 A/d'm2, and this current density has almost no relation to the spark discharge voltage, but in the case of low current density, it can be selected up to a predetermined voltage. It takes a long time to apply and when the current density is high, problems such as an increase in the temperature of the electrolytic bath occur, so the
/dm2 is preferable.

The thickness of the film formed depends on the concentration of the electrolytic bath, the temperature of the electrolytic bath,
The temperature of the electrolytic bath is determined depending on the processing voltage, processing time, etc., and the temperature of the electrolytic bath is determined depending on the desired film, but is generally set at 5 to 80°C.

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

Example 1 50 y/y of potassium silicate and 50 y/y of sodium wood oxide
In an aqueous solution consisting of y/l, the surface area is 50'c++! A magnesium alloy plate AZ 310 with a thickness of 3 mm is used as an anode and an iron plate is used as a cathode. A single-phase half-wave DC voltage is applied to the anode current density of 1. If applied continuously while maintaining the voltage at OA/dm2, a spark discharge will occur at approximately 40 V. The voltage was increased to 60 V and maintained for 10 minutes. This energization is accompanied by intense spark discharge. A milky glass-like film was formed on the anode plate, and the thickness of the film was about 30 μm.

Example 2 Sodium metasilicate nonahydrate (molar ratio -0.9 to 1, I
A magnesium alloy plate AZ'31 (1!) with a surface area of 50 d1 and a thickness of 3 mm was immersed in a 150 y/l aqueous solution with the plate as an anode and the iron plate as a cathode, and a sawtooth wave DC voltage was applied to the anode current density of 0.5 A/dm2. If applied continuously while holding the voltage, a spark discharge will occur at about 20V.
The voltage was increased to 1, and current was applied for 5 minutes while maintaining this voltage.

This energization is accompanied by intense spark discharge.

A milky glass-like film is formed on the anode plate,
The thickness of this film was approximately 20 pm.

Example 3 A surface area of 50 cr. , a magnesium alloy casting plate AZ 910 with a thickness of 2 mm was used as an anode and an iron plate was used as a cathode, and the anode current density was 0.5 A/dm.
2 and gradually apply a single-phase half-wave DC voltage, a spark discharge will occur at about 40 V. The voltage was increased to 55 V, and the treatment was carried out for 10 minutes while maintaining this voltage.

The film thus formed on the anode plate had a thickness of about 30 μm and a dark brown glassy appearance.

Example 4 A cast magnesium alloy plate AZ 910 with a surface area of 50-1 and a thickness of 2 mm was immersed as an anode and an iron plate as a cathode in an aqueous solution containing 200 f/l of sodium silicate and 60 l/l of sodium hydroxide. , anode current density 0.8 A
/dm2 and gradually apply a rectangular waveform DC voltage one by one, a spark discharge will occur at about 15V.

Increase the voltage to 30 V, and while maintaining this voltage,
The army was processed for a minute. This energization is accompanied by intense spark discharge. A smooth, glass-like, milky white film was formed on the anode plate, and the thickness of the film was about 10 μm.

Example 5 Potassium silicate 10? /l and potassium hydroxide 4°f
A magnesium alloy plate AZ 31 C with a surface area of 50 cn and a thickness of 3 stops was used as an anode and a stainless steel plate was used as a cathode, and the anode current density was set to 0.5.
If a sawtooth wave DC voltage is gradually applied while maintaining the voltage at A/dm2, a spark discharge will occur at about +5V. The voltage was increased to 30 V, and the treatment was carried out for 10 minutes while maintaining this voltage. A milky white glass-like film was formed on the anode plate, and the thickness of this film was about 15 μm.

Example 6 Lithium silicate 80? A magnesium alloy plate A231Cf@ as an electrode and an iron plate as a cathode were immersed in an aqueous solution containing 5°9/l of sodium hydroxide and 10 f/l of sodium hydroxide, and the anode current density was set to 1. OA
/dm" and gradually apply a sawtooth waveform DC voltage, a spark discharge will occur at about 15V.
The voltage was increased to 0 V, and current was applied for 5 minutes while maintaining this voltage. This energization is accompanied by intense spark discharge.

A gray glassy film with a thickness of about 20 μm was formed on the anode plate.

The coatings formed by the methods described in Examples 1 to 6 above are durable coatings that have much greater resistance to strong acids and strong alkalis than magnesium materials coated by conventional methods. Met. Furthermore, each film has good adhesion to paint and excellent wear resistance.

Claims (1)

[Scope of claims]
In the method of forming a protective film on the surface of a magnesium material by spark discharge, the electrolytic bath is an aqueous alkali metal silicate solution or an aqueous solution containing an alkali metal silicate and an alkali metal hydroxide, and the applied voltage is rectangular. wave waveform,
A method characterized in that the DC low voltage has a sawtooth waveform or a single-phase half-wave waveform. (2. In the method described in claim 1, the electrolytic bath contains tungstate, stannate, molybdate, phosphate, vanadate, borate, chromate, and permanganate. A silicate aqueous solution containing at least one oxyacid selected from acid salts, or an aqueous solution containing at least one of these oxyacids, a silicate, and an alkali metal hydroxide. how to.