JPH01208494A - Formation of fluororesin coating film on surface of zinc, zinc alloy or galvanized material - Google Patents

Abstract

PURPOSE:To form a coating layer having high heat and corrosion resistances on the surface of a Zn member by forming an anodic oxide film on the surface of the member with an electrolytic soln. of a prescribed pH contg. phosphate and chromate, immersing the member in a fluororesin soln. to coat the oxide film with the fluororesin and carrying out heat treatment. CONSTITUTION:An anodic oxide film is formed on the surface of a Zn (alloy) member or a galvanized member by AC electrolysis with an electrolytic soln. contg. 0.1-3mol/l (expressed in terms of PO4) phosphate and 0.1-3mol/l (expressed in terms of CrO4) chromate and adjusted to 6-11pH with NaOH and/or KOH. The member is then immersed in a soln. of fluororesin such as polyvinyl fluoride in a proper solvent to coat the oxide film with the fluororesin and the resulting film is cured by heat treatment at <=170 deg.C and cooled. A fluororesin coating film having superior heat and corrosion resistances as well as superior adhesion can be formed on the surface of the member.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention provides a method for imparting high corrosion resistance, chemical resistance, and heat resistance to the surface of zinc, zinc alloy, or galvanized materials. This invention relates to a method for forming a novel fluororesin coating on a substrate.

(Conventional technology) Conventionally, zinc, zinc alloy or galvanized material (hereinafter referred to as

It is also called a zinc member. ) Forming a fluororesin film on the surface of the fluororesin requires a comparatively high baking temperature because the melting point of zinc is low and zinc and fluororesin react to form a compound that inhibits adhesion. Painting with high-quality fluororesin was considered inappropriate.

On the other hand, in Japanese Patent Publication No. 51-43045, a zinc alloy was treated with chromic acid, a special double structure non-adhesive fluororesin paint was applied on the chromate film, and the zinc alloy was further treated with chromic acid. 140-170℃ without damaging the physical properties
A method for forming a fluororesin film having practically sufficient hardness, corrosion resistance, and chemical resistance by baking at a relatively low temperature is disclosed.

In addition, Japanese Patent Publication No. 57-60434 describes a method for coating zinc-plated products with a protective layer of fluororesin by treating the zinc-plated products with a chromate solution at pH 2 or less and containing formic acid as a reducing agent. The chromated body is coated with polyvinyl fluoride dispersed in a high fluoride point solvent and then coated with at least 100%
A method is disclosed for coating galvanized metal objects with a protective layer of fluororesin, heat treated at temperatures of .degree.

On the other hand, Japanese Patent Publication No. 12250/1983 discloses that a corrosion-resistant and wear-resistant film can be formed on the surface of zinc and zinc alloy by forming an anodic oxide film without using a fluororesin film layer.

When anodizing zinc and zinc alloys, this is mainly composed of phosphate anions, chromate anions, and at least one selected from molybdate anions, tungstate anions, and vanadate anions. An electrolytic solution containing various anions and at least one cation selected from sodium and potassium is used, and its feature is that alternating current electrolysis is carried out at low voltage.

(Problems to be Solved by the Invention) In the above-mentioned conventional method of forming a fluororesin film on zinc, zinc alloy, or galvanized material, the former method (Japanese Patent Publication No. 51-43045) was developed by DuPont Co., Ltd. Modified fluororesin paints such as Teflon S-based paints must be used, which makes resin selection difficult and restrictive, and since these paints also contain other resin components, they have poor heat resistance and corrosion resistance. . Also, the latter method (Special Public Interest Publication No. 57-6
No. 0434), the condition is that after the chromate treatment, the chromate film is immersed in the fluororesin dispersion in a state where the film is not completely dried, that is, water remains in the hollow space of the chromate film layer. be.

This is because it is advantageous for the remaining water to be replaced with the polyvinyl fluoride resin dispersion in the high-fluorescence point solvent in the next step. Therefore, there are constraints on the work.

Continuous equipment that covers everything from electroplating to painting is required.

In addition, the above-mentioned Special Publication No. 51-43045 and Special Publication No. 57-
Both methods of No. 60434 involve coating a fluororesin on a chromate film provided on a zinc member.

Since the chromate film is formed in the form of a gel on the surface of the zinc component, it has poor heat resistance.Although it exhibits excellent corrosion resistance at room temperature, it does not fully maintain the heat resistance that is the original property of fluororesin. It cannot be expressed, and the conditions for using the product are also self-limited.

Further, the above-mentioned Japanese Patent Publication No. 43-12250 does not suggest the use of a fluororesin, and merely forming an anodic oxide film cannot bring out the above-mentioned properties of a fluororesin.

In view of the above circumstances, the inventors of the present invention have conducted extensive studies and found that
We have discovered that the anodic oxide film itself, which is formed under special conditions on the surface of zinc members, is heat resistant and strongly binds the fluororesin, making it extremely excellent as a base coat for fluororesin coating. This is what we have come to.

[Structure of the invention]

(Means for Solving the Problems) To summarize the present invention, in forming a fluororesin film on the surface of zinc, zinc alloy, or galvanized material, (1) first po, then 0. 1-3 mol/l phosphate.

Electrolysis was carried out by alternating current using an electrolytic solution containing 0.1 to 3 mol/l of chloro-11 acid salt as CrO4 and adjusted to pH 6 to 11 with either sodium hydroxide or potassium hydroxide, or both. to form an anodic oxide film on the surface of the zinc, zinc alloy or galvanized material, (ii) then apply a fluororesin by immersing it in a fluororesin dispersion, (iii) after that. The present invention relates to a method for forming a fluororesin coating on the surface of zinc, zinc alloy, or galvanized material, comprising: heat-treating at a temperature of at least 170°C.

Hereinafter, the configuration of the present invention will be explained in detail.

Examples of zinc members to which the method for forming a fluororesin film of the present invention is applied include die-cast products made of zinc or alloys whose main component is zinc, castings, and galvanized products in which a galvanized layer is applied to the surface of a steel material. is included. In the case of galvanized products, the zinc layer reacts with the treatment solution during the anodic oxidation treatment, so it is desirable that the plating film thickness be approximately 10 μs or more. The plating film may be formed by electroplating or hot-dip plating.

As a treatment solution for anodizing the surface of these zinc members, an electrolytic solution containing phosphate and chromate is used. In this case, P04 is 0.1 to 3 mol/l, preferably 0.6 to 1.0 mol/l, and CrO4 is 0.1 to 3 mol/l.
One containing 3 mol/l, preferably 0.4 to 0.8 mol/l is used.

In addition, the electrolytic solution used in the present invention has a pH of 6 to 1 using either sodium hydroxide or potassium hydroxide, or both.
Adjusted to 1. In this case, even if the pH is higher or lower, the amount of zinc dissolved from the zinc member will increase, and the formation of the film will be delayed or will not be formed at all.

It is preferable that the electrolytic solution contains 0.3 mol/l or less of fluoride as F (fluorine). Containing a small amount of fluorine is effective in promoting film formation and shortening processing time.

The bath temperature of the electrolyte is in the range of 30°C to 90°C, preferably 5°C.
It is preferable to maintain the temperature within the range of 0°C to 70°C.

AC electrolysis is performed using the above electrolytic solution and using the zinc member to be treated as one or both electrodes. As the electrolysis time progresses, a resistive film is formed on the zinc surface and the current value tends to decrease, but so-called constant current electrolysis is performed so that the voltage is increased to keep the current value constant. When the voltage to maintain a constant current increases rapidly, a spark discharge occurs on the electrode surface.

In the present invention, it is important to stop electrolysis before this spark discharge begins. Using the above electrolyte, the voltage is 20
It is appropriate to stop the electrolysis when the voltage reaches V or less, preferably 15 to L7V, in order to form the base film in the subsequent coating of the fluororesin dispersion. If electrolysis is carried out to the point of spark discharge, the adhesion of the coating will actually deteriorate.

After the anodizing process has been completed, the zinc member is pulled out of the electrolyte, thoroughly washed with water, and then dried.

The coating formed on the surface of the zinc member by anodizing in this way is ZnCr0. , ZnPO4, CrPO4, Z
nO.

It is estimated that it is made of a composite such as CrO3, and its surface is brown or black, the film thickness is about 2 to 5 microns, and it has excellent corrosion resistance and heat resistance. When the above-mentioned anodic oxide film is observed with an N1 microscope, fine racks are formed in the film, and these have an anchoring effect that improves the permeability of the fluororesin emulsion applied in the next process and the adhesion of the fluororesin. It is assumed that it gives

For coating with fluororesin, dissolve fluororesin such as polyvinyl fluoride or polyvinylidene fluoride in an appropriate solvent and apply 20 to 50% of the fluororesin.
This is carried out by immersion treatment using a dispersion solution having a concentration of . After removing the resin from the dispersion, remove excess resin by air blowing if necessary, and then heat at 170°C to 28°C.
Heat treatment is performed at a temperature of 0° C. for 2 to 5 minutes, followed by cooling to harden the coating film. In this way, a fluororesin coating film with excellent adhesion, heat resistance, and corrosion resistance is formed on the surface of the zinc member.

〔Example〕

Hereinafter, the present invention will be explained in more detail based on Examples, but the present invention is not limited to these Examples unless the gist of the present invention is exceeded.

Example 1 Sample: Zinc alloy die-cast product (ZDC2) Electrolyte: 50 g of chromic anhydride, sodium dihydrogen phosphate Ax dihydrate 24
Dissolve 0g in 1Ω water, add sodium hydroxide and
Let's call it H7.

Anodic oxidation treatment: Constant current electrolysis is carried out at a bath temperature of 70°C, with the sample as one electrode and a current density of 20A/dn+" maintained by AC electrolysis.The voltage gradually increases from 6V to 15V after 8 minutes.
When the electrolytic solution reaches 1, the electrolysis is stopped and the sample is lifted out of the electrolyte, washed with water, and then dried.

A light chocolate-colored anodic oxide film with a thickness of 2.0 μm was obtained on the surface of the zinc alloy die-cast product.

Fluororesin coating: After applying polyvinylidene fluoride by dipping it in a dispersion of 30% dissolved in Resonal's Retentent Solvent, heat treatment at a temperature of 250°C for 5 minutes, immediately cool it in water, and apply it. The film was cured.

A zinc alloy die-cast product was obtained, the surface of which was coated with polyvinylidene fluoride having a thickness of 13 μm.

Example 2 Sample: Steel pipe (outer diameter 10 mm, wall thickness 0.7 rm) galvanized product.

A steel pipe on which a galvanized layer with a thickness of 25 μm was formed on the surface by electroplating was used.

Electrolyte Sodium dichromate tetrahydrate 100g, 98%
Dissolve 97 g of phosphoric acid and 5 g of sodium fluoride in IQ water and add potassium hydroxide to make ρ old 0.

Anodic oxidation treatment: A bath temperature is 60° C., a sample is placed on both electrodes, and constant current electrolysis is performed by maintaining the current density at 40 A/dm” by alternating current electrolysis.

The voltage gradually increases from 6 V, and when it reaches 17 V after 5 minutes, electrolysis is stopped, and the battery is pulled out of the solution, washed with water, and then dried.

The surface of the zinc layer exhibited a dark brown color, and an anodic oxide film with a thickness of 2.5 μm was formed.

Fluororesin coating: After applying polyvinyl fluoride by dipping it in a dispersion of 30% dissolved in Resonal's Retentent Solvent, it was heated with hot air at a temperature of 200°C for 3 minutes, and then cooled in water. The coated flu was cured.

A galvanized steel pipe was obtained, the surface of which was coated with polyvinyl fluoride having a film thickness of 13 μm.

Example 3 Sample: Same electrolyte as Example 2: Same anodic oxidation treatment as Example 2: Electrolysis is performed under the same bath temperature and current conditions as Example 2. After 10 minutes, the voltage reached 70V and spark discharge appeared. At this time, the electrolysis was stopped, the sample was taken out from the solution, washed with water, and dried.A gray-green anodic oxide film was formed on the surface of the sample. The coating thickness was 5μ.

Fluororesin coating: Processed in the same manner as in Example 2, and polyvinyl fluoride was coated on the upper layer of the anodic oxide film.

Comparative example 1 Sample: Same as Example 2 Chromate treatment: 20 g of sodium dichromate.

Anhydrous chromium 1.5 g/l, sodium sulfate 5 g
The sample was immersed in a chromate solution containing +acetic acid 5- dissolved in IQ, washed with water and dried to form a yellow chromate film on the surface.

Fluororesin coating: Processed in the same manner as in Example 2, and polyvinyl fluoride was coated on the chromate film.

Examples 1-3. For each sample obtained in Comparative Example 1, an evaluation test was conducted to evaluate the heat resistance and adhesion of the fluororesin coating on the zinc member. The test method and test results are as follows.

<Test method> One cycle is to hold the sample in a high temperature bath at 100°C for 1 hour and then let it cool for 1 hour at room temperature.This is repeated for 5 cycles, and the sample is further tested in a 2 mm square. Perform the cellophane tape test.

Test results> The test results are also shown in Table 1.

(Table 1) [Effects of the Invention] According to the method of forming fluororesin waves H of the present invention, a fluororesin coating film with extremely excellent adhesion can be formed on the surface of various zinc members. , a product with a highly heat-resistant and corrosion-resistant coating layer can be obtained.

Patent applicant: Usui Kokusai Sangyo Co., Ltd. Agent: Yoshio Mizuno, patent attorney

Claims (1)

[Claims] When forming a fluororesin film on the surface of zinc, zinc alloy, or galvanized material, (i) 0.1 to 3 mol/l phosphate as PO_4 and 0.1 to 3 mol/l as CrO_4; Using an electrolytic solution containing 3 mol/l of chromate and having a pH of 6 to 11 with either sodium hydroxide or potassium hydroxide, or both, the zinc, zinc alloy or Forming an anodic oxide film on the surface of the galvanized material, (ii) Next, applying a fluororesin by immersing it in a fluororesin dispersion, (iii) Then heat-treating at a temperature of at least 170°C. A method of forming a fluororesin film on the surface of zinc, zinc alloy, or galvanized material consisting of: