JP2008285716A - Method for forming corrosion resistant film on zinc metal surface - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a zinc metal surface treatment method excellent in safety and operability by which a coating having excellent rust prevention performance to white color rust can be provided. <P>SOLUTION: A method for forming a corrosion resistant film on a zinc surface is characterized in that (1) the zinc surface is subjected to oxidation treatment, (2) the zinc surface subjected to the oxidation treatment is brought into contact with a coating treatment liquid containing a silicic compound having a pH of 9 to 14, or the steps of (1) and (2) are simultaneously performed. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for forming a corrosion-resistant film on a zinc surface.

  Zinc-based plating has been adopted as an inexpensive, reliable and effective method for preventing rust and corrosion of steel products and parts. Furthermore, in order to improve the anti-corrosion and anti-corrosion ability of the zinc plating, a passivation treatment with a hexavalent chromium (Cr (VI)) compound, so-called chromate treatment, has been applied to the plating surface to protect it with excellent corrosion resistance. It is adopted as a method for forming a film.

  The chromate treatment can give excellent corrosion resistance in a simple and economical treatment step of dipping and drying in a predetermined solution containing a Cr (VI) compound following electrogalvanization. However, it is known that Cr (VI) easily passes through the cell membrane and has great toxicity, and in recent years, its use for surface treatment has begun to be regulated.

  For this reason, first, a surface treatment liquid mainly composed of a Cr (III) compound not containing Cr (VI) has been proposed. For example, Patent Document 1 describes a chemical conversion treatment liquid characterized by containing Cr (III) ions, silicates, fluorides and acids. However, the chemical conversion treatment liquid has many drawbacks such as poor liquid stability and complicated treatment methods. Further, since Cr (III) has a problem that it is oxidized to produce Cr (VI), a chemical conversion treatment liquid having a completely chromium-free composition is desirable.

  For this reason, several surface treatment techniques that do not use chromium at all have been proposed in the following documents.

  However, these treatment methods are not always satisfactory. Patent Document 2 contains at least one molybdic acid or molybdate in terms of molybdenum of 10 to 200 g / l, aluminum phosphate of 0.01 g / l or more, and an organic acid or inorganic acid is added to adjust the pH to 1 The rust prevention processing method of zinc covering steel materials which treats zinc covering steel materials with the acidic solution prepared in -6 is described. However, this method has a problem that the corrosion resistance is insufficient.

  In Patent Document 3, the metal substrate contains (A) an oxidizing substance, (B) a silicate and / or silicon dioxide, and (C) a titanium ion, and has a pH in the range of 0.5 to 6.0. A method of immersing in a treatment solution is described. However, the treatment liquid used in this method has a problem that titanium is precipitated as the oxidizing substance is consumed. Moreover, since the silicic acid compound in the treatment solution causes precipitation gelation, the stability of the bath is poor and is not suitable for actual use.

  In Patent Document 4, the object to be treated is immersed in a nitric acid solution to remove the oxide film on the surface of the zinc plating, washed with water, and then treated with a divalent manganese salt (such as manganese sulfate), an aluminum salt, and zinc. It describes a method of immersing in a chemical conversion treatment solution for zinc-based plated products, which contains at least one metal salt of a salt and an alkali silicate salt and is adjusted to pH 0.5-3. However, the chemical conversion treatment solution used in this method has a problem that the stability of the bath is poor because the alkali silicate salt and the metal polyvalent cation coexist and precipitate as a metal silicate.

In Patent Document 5, a surface of a metal member having a zinc surface is subjected to a chemical conversion treatment with zinc phosphate to form a chemical conversion treatment film, and then the surface of the chemical conversion treatment film is placed in an alcohol solvent or a mixed solvent of water and alcohol. A method of applying a non-chromium surface treatment solution containing a silica component or a component that changes to silica is described. Alcohol is easy to volatilize, the component concentration of the solution is likely to change, and is flammable, so explosion-proof equipment is required. Although the toxicity is not great, workers are always exposed to alcohol, and the working environment must be deteriorated. In any case, a capital investment is required to work well, and it is difficult to apply to the current plating process. Furthermore, it is practically difficult to replace the chromate treatment due to the high cost of the alcohol itself.
Japanese Patent Laid-Open No. 61-587 JP-A-57-5875 Japanese Patent Laid-Open No. 9-53192 JP 2002-47578 A JP 2006-225761 A

  The problem to be solved by the present invention is to provide a method for treating a zinc metal surface, which is excellent in safety and operability and can provide a coating excellent in rust prevention performance against white rust.

  As a result of intensive studies for solving the above problems, the present inventor has obtained a pH 9 based on a silicate compound as a main component after oxidizing the zinc metal or the zinc metal surface subjected to electrogalvanization treatment or simultaneously with the oxidation treatment. It has been found that the treatment with the treatment solution of ˜14 enables a non-chromium rust prevention treatment having an extremely excellent rust prevention property industrially advantageously. The present invention is based on such knowledge.

  Accordingly, the present invention provides a method for forming a corrosion-resistant coating on the zinc surface shown in the following section.

Item 1. (1) Oxidizing the zinc surface,
(2) Zinc characterized by contacting the oxidized zinc surface with a coating treatment liquid containing a silicic acid compound having a pH of 9 to 14, or simultaneously performing the steps (1) and (2) A method of forming a corrosion-resistant film on the surface.

  Item 2. Item 2. The method according to Item 1, wherein the oxidation treatment is performed by electrically oxidizing the zinc surface in a conductive solution.

Item 3. The coating treatment liquid containing the silicic acid compound,
(I) water glass,
Item 3. The method according to Item 1 or 2, which is a sol obtained by hydrolyzing and condensing an alkoxysilane compound or (iii) colloidal silica.

  Item 4. Item 4. The method according to Item 3, wherein the water glass is an aqueous solution of at least one alkali silicate salt selected from the group consisting of lithium silicate, sodium silicate, potassium silicate, rubidium silicate, and cesium silicate.

Item 5. (1) Oxidizing the zinc surface by immersing the zinc surface in an aqueous solution containing an oxidizing agent or spraying an aqueous solution containing an oxidizing agent on the zinc surface;
(2) The method according to Item 1, wherein the oxidized zinc surface is contacted with a coating treatment liquid containing a silicic acid compound having a pH of 9 to 14.

  Item 6. Item 6. The method according to Item 5, wherein the oxidizing agent is hydrogen peroxide or permanganate.

  Item 7. Item 7. The method according to Item 5 or 6, wherein the concentration of the oxidizing agent in the aqueous solution is 0.001 to 35% by weight.

  Item 8. Item 8. The method according to any one of Items 1 to 7, wherein the coating treatment liquid contains a silicate compound in an amount of 0.5 to 30% by weight as silicon dioxide.

  Compatibility with a metal member having a zinc surface, such as a surface treatment agent, that is difficult to impart a practical level of rust prevention when a non-chromium surface treatment agent that forms a siliceous film on the zinc metal surface is applied by the method of the present invention. Even if it is the electrogalvanized metal member which is bad, surface treatment which improves the rust prevention performance with respect to the white rust can be given.

  Moreover, when the said process liquid is used, a coating layer can be dried on comparatively low temperature conditions.

  Moreover, according to this invention, the chemical conversion liquid which does not contain the chromium compound which has a bad influence on an environment and an ecosystem can be provided. As a result, it is possible to reduce chrome-based waste, enable a safe working environment for workers involved in the rust and corrosion prevention treatment of steel products, and provide health-safe products for consumers in the future. Can contribute.

  In addition, since the coating treatment liquid used in the method of the present invention does not contain alcohols or contains only a small amount of alcohol, there is no need for an investment such as an explosion-proof facility for preventing ignition, and a safe coating treatment. Can do.

  Furthermore, since the coating treatment liquid used in the method of the present invention does not gel and is stable over a long period of time, the management of the treatment liquid is extremely easy. Moreover, since the waste water treatment is easy, the handling is excellent. Therefore, it is a useful treatment method in which conventional chromate treatment equipment can be used and there is no problem in the environment.

  Below, the detail of this invention is demonstrated in more detail based on embodiment.

  In the present invention, the zinc surface is not limited to the surface of the steel material plated with zinc, as long as at least a part of the surface of the product, member, etc. targeted by the method of the present invention is made of zinc. Including the case.

  Further, the occurrence of white rust can be suppressed for a long time by applying the non-chromium rust preventive treatment method of the present invention to a die-cast member made of zinc metal or an alloy containing zinc as a main component.

  The method of the present invention is carried out by treating the zinc metal surface with a treatment solution having a pH of 9 to 14 containing a silicate compound as a main component after the oxidation treatment or simultaneously with the oxidation treatment.

1. Oxidation treatment In the method of the present invention, the oxidation treatment may be performed under any conditions as long as the surface zinc metal can be oxidized. Preferably, a method of electrically oxidizing in a conductive solution or an aqueous solution containing an oxidizing agent is used. This is a method for treating a zinc metal surface.

  In one embodiment of the invention, the zinc surface is first electrically oxidized in a conductive solution.

  The solution used for the electrical oxidation may be any solution as long as it can be electrolyzed. Preferably, sodium carbonate, potassium carbonate, sodium phosphate, potassium phosphate, sodium permanganate, potassium permanganate, lithium silicate, sodium silicate, potassium silicate, rubidium silicate, cesium silicate, titanium chloride, sodium molybdate, potassium molybdate , Sodium vanadate, potassium vanadate, sodium borate, potassium borate, sodium tungstate, potassium tungstate and the like.

  The voltage at the time of the electrical oxidation treatment may be any as long as the voltage between the two electrode terminals is within the range of 0.1 V to 10 V (preferably 1 V to 5 V).

  The time for applying the voltage is usually 0.1 second to 60 minutes, preferably 1 second to 10 minutes.

  During the oxidation treatment, the voltage may be kept constant or may be appropriately changed within the above range.

  The temperature during the electrical oxidation treatment may be any as long as it is within the range of 0 ° C to 100 ° C.

  In another embodiment of the present invention, the oxidation treatment can be performed by treating the zinc surface with an aqueous solution containing an oxidizing agent.

  Examples of the method for treating a zinc metal surface using an aqueous solution containing an oxidizing agent include immersion of the target product in an aqueous solution containing an oxidizing agent, spraying of an aqueous solution containing an oxidizing agent onto the zinc metal surface, and the like. .

  Examples of the oxidizing agent include hydrogen peroxide and permanganate.

  The hydrogen peroxide solution is preferably an aqueous solution containing 0.1 to 35% by weight of hydrogen peroxide. Particularly preferred is 1 to 10% by weight. If it is less than 0.1% by weight, the effect of oxidizing is small, and it takes too much time to oxidize. If it exceeds 35% by weight, the hydrogen peroxide solution becomes unstable. The oxidation treatment temperature may be any as long as it is within the range of 0 ° C to 100 ° C. If it is low temperature, processing time will become long, and when processing at high temperature, it can process in a short time. The treatment time is preferably 1 second to 30 minutes, particularly preferably 10 seconds to 15 minutes. After the oxidation treatment, it may be washed with water or the like, and may be dried with a dryer or hot air.

  The aqueous permanganate solution is preferably lithium permanganate, sodium permanganate, or potassium permanganate. The aqueous solution preferably contains 0.001 to 20% by weight, particularly preferably 0.01 to 5% by weight, of permanganate. If it is less than 0.001% by weight, the effect of oxidation is small, and it takes too long to oxidize. If it exceeds 20% by weight, the solubility of permanganate will be insufficient. The oxidation treatment temperature may be any as long as it is within the range of 0 ° C to 100 ° C. If it is low temperature, processing time will become long, and when processing at high temperature, it can process in a short time. The treatment time is preferably 1 second to 30 minutes, particularly preferably 10 seconds to 15 minutes. After the oxidation treatment, it may be washed with water or the like, and may be dried with a dryer or hot air.

  A corrosion-resistant film can be formed by subjecting the zinc surface oxidized by these steps to the coating treatment described below.

2. Coating treatment (Preparation of coating solution)
The coating treatment liquid used in the method of the present invention is a solution containing a silicic acid compound and having a pH of 9 to 14, preferably a pH of 10 to 12.5.

  In a preferred embodiment of the present invention, the silicate compound includes, for example, at least one alkali silicate salt selected from the group consisting of lithium silicate, sodium silicate, potassium silicate, rubidium silicate, and cesium silicate, an alkoxysilane compound, and silicon dioxide. Etc.

In the present invention, the concentration of silicate compounds in the coating treatment solution, as SiO _2, is usually 0.5 to 30 wt%, preferably 1 to 5 wt%.

  Examples of the coating treatment liquid containing a silicic acid compound include water glass; a sol obtained by hydrolyzing and condensing an alkoxysilane compound; or colloidal silica.

  When water glass is used as the coating treatment liquid, the water glass is, for example, an aqueous solution of at least one alkali silicate salt selected from the group consisting of lithium silicate, sodium silicate, potassium silicate, rubidium silicate, and cesium silicate. Can be mentioned.

  When using a sol obtained by hydrolyzing and condensing an alkoxysilane compound as the coating treatment liquid, examples of the alkoxysilane compound include tetramethoxysilane, tetraethoxysilane, tetra-n-propoxysilane, and tetra-i-propoxysilane. , Tetra-n-butoxysilane, tetra-i-butoxysilane, tetra-t-butoxysilane, methyltrimethoxysilane, methyltriethoxysilane, methyltri-n-propoxysilane, methyltri-i-propoxysilane, methyltri-n- Examples thereof include butoxysilane and methyltri-t-butoxysilane, but are not limited thereto. These may be used alone or in combination of two or more. Of these, tetraethoxysilane is particularly preferred.

  Examples of the method for hydrolyzing the alkoxysilane compound include hydrolysis using a basic catalyst containing lithium, sodium, potassium, rubidium, cesium and the like.

  Examples of the basic catalyst include lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, cesium hydroxide, lithium carbonate, sodium carbonate, potassium carbonate, lithium hydrogen carbonate, sodium hydrogen carbonate, potassium hydrogen carbonate, Examples thereof include, but are not limited to, rubidium carbonate, rubidium hydrogen carbonate, cesium carbonate, cesium hydrogen carbonate and the like. These basic catalysts may be used alone or in combination of two or more. Preferred basic catalysts are lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, cesium hydroxide.

  The usage ratio of these basic catalyst and alkoxysilane compound is 0.5 to 5 mol, preferably 1 to 3 mol, with respect to 1 mol of the former.

  In the present invention, as the basic catalyst, in addition to the above basic catalyst, another basic catalyst may be used in combination.

  Examples of other basic catalysts in the present invention include magnesium hydroxide, calcium hydroxide, barium hydroxide, tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, tetrabutylammonium hydroxide, ammonia, pyrrolidine. , Broadly mean substances that show basicity in water, such as piperazine, ethylamine, diethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine.

  The usage ratio of the other basic catalyst and the alkoxysilane compound is 0.1 to 2 mol, preferably 0.2 to 1 mol, with respect to 1 mol of the former.

  In a preferred embodiment of the present invention, a sol that is a coating treatment liquid is obtained by adding the above basic catalyst and alkoxysilane to water at 60 ° C., dissolving under stirring, hydrolyzing the alkoxysilane, and adjusting the pH to 9 It can be prepared by adjusting to -14.

  The coating treatment liquid used in the present invention may contain a metal corrosion inhibitor, a wettability improver, a chelating agent, a dye, a pigment, a thickener, a dispersant and the like as optional components.

(Method of contacting the coating solution)
Examples of the method for bringing the coating treatment liquid into contact with the oxidized zinc surface include an immersion method, but are not limited thereto. Moreover, you may use application methods, such as a brush coating method and a spray method, as needed.

  The dipping method is performed by immersing the zinc surface portion of the article to be treated in a treatment liquid tank for a certain time and then pulling it up. In the immersion treatment, the entire surface of the article to be treated may be immersed in the treatment liquid, or a part of the article to be treated may be immersed on the zinc surface without touching the treatment liquid.

  By this step, a wet gel film is formed on the zinc surface. In this case, the thickness of the wet gel film obtained increases as the viscosity, surface tension and density of the treatment liquid increase.

The density of the coating treatment liquid of the present invention used in the dipping method, brush coating method or spray method is usually from 1.001 to 1.7 g / cm ³ (15 ° C.), preferably from 1.05 to 1.15 g / cm. ³ (15 ° C.). Therefore, a wet gel film having a desired thickness can be obtained by appropriately controlling the viscosity of the treatment liquid and the pulling speed in the coating treatment liquid having the above density.

  It is important in terms of processing efficiency to obtain a desired film thickness by a single dipping or coating (brush coating, spraying method, etc.), but a desired film thickness can be obtained by a single dipping or coating. Can not be obtained, a layer having a desired thickness can be obtained by repeating dipping or coating a minimum number of times.

(Drying process)
When the wet gel film formed on the zinc surface in the above process is dried at room temperature, a ceramic coating layer having excellent corrosion resistance is formed. When heat treatment is performed, in addition to removal of moisture remaining in the dry gel film, residual organic substances are removed, so heat treatment is performed as necessary.

(Thickness control of ceramic coating layer (corrosion resistant coating))
To control the film thickness of the ceramic coating layer (corrosion-resistant film) formed on the surface of the workpiece, immerse the workpiece in the coating solution bath, pull it up, and repeat the coating process up to the drying process a desired number of times. Well, the film thickness increases in proportion to the number of coatings.

In order to increase the film thickness in a single coating process, the viscosity of the processing solution and the pulling rate should be increased as described above. However, when the critical film thickness is exceeded, it occurs with the volatilization of the solvent from the film. Since the film is cracked by the tensile stress acting parallel to the surface to be coated, it is necessary to set the film thickness to be coated at one time optimally. For this purpose, an optimum condition may be found by using the viscosity of the processing liquid and the pulling speed as parameters. Similarly, when the treatment liquid is applied by a spray method, the film thickness can be controlled by the number of times of application. However, industrially, it is desirable that the number of times of dipping or coating is small in both the dipping method and the spray method. The thickness of the ceramic coating layer obtained by the method of the present invention is usually 10 to 2000 nm, preferably 50 to 1000 nm.

  Magnesium silicate, calcium silicate, and zinc silicate precipitate when magnesium and calcium, which are water hardness components, or zinc that may be precipitated from the surface of zinc plating are mixed in a coating solution containing a silicate compound. there is a possibility. These precipitates can be removed by filtration or the like, but when the filtration cannot be performed, a metal inhibitor having a chelating effect can be added to the coating treatment liquid. The chelating agent can be used without particular limitation as long as it does not impair the white rust prevention effect and the aesthetic appearance of the metal surface. Examples thereof include ethylenediaminetetraacetic acid and its salt, nitrilotriacetic acid and its salt, citric acid, and oxalic acid.

  In order to improve the wettability of the galvanized surface, a wettability improver such as a surfactant can be added to the coating treatment liquid without particular limitation as long as it does not impair the white rust prevention effect and the aesthetic appearance of the metal surface. Examples of such additives include alkyl ether ethylene oxide, ethylene oxide alkylamine, long chain fatty acid or salt thereof, alkylbenzene sulfonic acid or salt thereof, alkyl sulfate ester or salt thereof, long chain alkyl ammonium salt, and long chain alkyl. A pyridinium salt etc. can be mentioned.

  In the above description, the method of the present invention has been described using the embodiment in which the zinc surface is oxidized and then treated with a coating solution containing a silicic acid compound having a pH of 9 to 14 to form a corrosion-resistant film. This method includes an embodiment in which the oxidation treatment and the coating treatment are performed simultaneously.

  For example, a stable film can be formed by applying a voltage between both electrodes while the zinc surface is immersed in the coating solution.

  In the embodiment, the same conditions as in the coating process can be applied to the composition of the coating liquid. Moreover, the conditions similar to the said electrical oxidation process can be used for the voltage between electrodes, processing time, temperature, etc.

EXAMPLES The present invention will be described in detail below using examples, but the present invention is not limited to the following examples.
1. Evaluation of corrosion resistance by film formation and salt spray test
Preparation of Aqueous Solution Containing Oxidizing Agent An aqueous solution containing oxidizing agents having two compositions shown in Table 1 below was prepared.

Preparation of Coating Treatment Liquid A coating treatment liquid of the present invention containing a composition represented by the general formula M ₂ O · nSiO ₂ was prepared.

Here, the concentration (in terms of SiO ₂ ), the type of alkali metal M, and the value of n used in each example and comparative example are as shown in Table 2 below.

  Here, in Table 2, the coating treatment solutions used in Examples 1 to 6 and Comparative Examples 1 and 2 were prepared by dissolving water glass in water.

  About Examples 7-8 and Comparative Examples 3-4, hydroxide (MOH; M is lithium, sodium, potassium, or cesium) and tetraethoxysilane (tetraethoxysilane) as a basic catalyst in water It was prepared by adding the number of moles of hydroxide of alkali metal M so that the number of moles of hydroxide of alkali metal M would be n in Table 2, heating to 60 ° C. and stirring for 30 minutes. After the tetraethoxysilane was separated from the aqueous layer into the upper part, it was confirmed that it became a uniform and transparent state to prepare a treatment liquid.

In addition, the density of the coating processing liquid used in Examples 1 to 10 and Comparative Examples 1 to 4 was in the range of 1.02 to 1.06 g / cm ³ (15 ° C.).

As a test piece for forming a corrosion-resistant film, a steel plate to which 10 μm of 8 × 50 × 2 mm electrogalvanizing (no surface treatment after plating) was applied was used.

  Using the test pieces, Examples 1 to 8 and Comparative Examples 5 and 6 were subjected to oxidation treatment under the conditions shown in Table 2 below.

  Next, each test piece was brought into contact with the coating treatment liquid described in Table 2 by a dipping method, and then naturally dried for 10 minutes to form a ceramic coating layer.

  In any of the coating treatment liquids of Examples 1 to 10 and Comparative Examples 1 to 4, gelation of the silicate compound did not occur, and the coating treatment could be easily performed. These coating treatment solutions were stable with no gelation for at least half a year.

Rust prevention test The rust prevention (weather resistance) of each test piece obtained was evaluated by the occurrence of white rust 24 hours after the salt spray test (according to the test method of JIS Z 2371).
Evaluation criteria for white rust generation after 24 hours of salt spray test ◎: No white rust ○: White rust is 5% or less △: White rust is 50% or less ×: White rust is 50% or more Test results are shown in Table 2. Street.

As a test piece for forming a corrosion-resistant film by electrical treatment, a steel plate on which 10 μm of electrogalvanizing (not subjected to surface treatment after plating) of 8 × 50 × 2 mm was used.

  The electrical treatment solution was substantially the same as the coating treatment solution of Example 1, and was prepared from water glass. Under the conditions shown in Table 3, two test pieces were used as an anode and a cathode, immersed in an electrical oxidation treatment solution, and a voltage was applied between both electrodes for 10 seconds to perform the treatment. The anode-side test piece was washed with water and naturally dried for 10 minutes to conduct a rust prevention test.

2. Electrochemical evaluation of coating formation and corrosion resistance
Formation of Corrosion Resistant Coating Example 11
As a test piece (hereinafter also referred to as an electrode), a pure zinc rod (99.9%, manufactured by Niraco, Φ = 3.00 mm) whose side surface was hardened with an epoxy resin was used. The cross section of the test piece was wet polished with # 1500 emery paper, then buffed with alumina powder having a particle size of 0.015 μm, and polished to a mirror surface. The polished test piece was washed twice with methanol and distilled water, and immediately subjected to the following surface treatment.

  Distilled water was added to 30% hydrogen peroxide water (special grade made by Wako) to prepare 2 mol / L. The ground electrode was pretreated by immersing it in this solution for 10 minutes, and then rinsed with distilled water.

The pretreated electrode was immersed for 10 seconds in a thermostatic bath containing a sodium silicate aqueous solution (SiO ₂ = 20000 ppm, pH = 11.3) heated to 60 ° C. as a treatment liquid. After 10 seconds, the electrode was pulled up at a speed of 50 cm / sec and allowed to air dry for 10 minutes.

Example 12
Pretreatment was performed using an aqueous solution of 1/40 N potassium permanganate (for COD measurement manufactured by Kanto Chemical) instead of 2 mol / L hydrogen peroxide solution, and the same operation as in Example 9 was performed. Formed.

Example 13
As a test piece, a pure zinc rod (99.9% made by Niraco, Φ = 3.00 mm) whose side surface was hardened with an epoxy resin was used. The cross section of the test piece was wet polished with # 1500 emery paper, then buffed with alumina powder having a particle size of 0.015 μm, and polished to a mirror surface. The polished test piece was washed twice with methanol and distilled water, and immediately subjected to the following surface treatment.

  Sodium carbonate (Kanto Chemical Special Grade) was weighed so as to have a concentration of 0.5 mol / L, and distilled water was added to prepare an electrolytic solution. The polished electrode is immersed in an electrolytic solution, and a potential is applied and scanned at a rate of 1 mV per second from a natural potential to +1.0 V (vs. SSE, the same applies hereinafter) using a potentiostat. gave.

The pretreated electrode was rinsed with distilled water, and the pretreated electrode was immersed in an aqueous sodium silicate solution (SiO ₂ = 20000 ppm) immediately heated to 60 ° C. for 10 seconds. After 10 seconds, the electrode was pulled up at a speed of 50 cm / sec and allowed to air dry for 10 minutes.
Comparative Example 8
The electrode was polished in the same manner as in Example 11 and pretreatment and surface treatment were not performed, and Comparative Example 8 was used for measuring corrosion resistance.
Comparative Example 9
The electrode was polished in the same manner as in Example 12. The ground electrode was immersed in an aqueous solution of sodium silicate (SiO ₂ = 20000 ppm) immediately heated to 60 ° C. for 10 seconds without pretreatment. After 10 seconds, the electrode was pulled up at a speed of 50 cm / sec and allowed to air dry for 10 minutes.

Measurement of corrosion resistance Corrosion resistance was determined by measuring electrochemical impedance using a potentiostat with a built-in function generator. Air was bubbled into a 5.0% NaCl aqueous solution for 30 minutes or more to saturate the air. One hour after immersing the electrodes of Examples and Comparative Examples in this 5.0% NaCl aqueous solution, an alternating voltage of 5 mV was applied in a wide frequency range at a natural immersion potential, and electrochemical impedance measurement was performed. The value of the difference in impedance at a frequency of 1 mHz and 10 kHz was used as the corrosion resistance.

  As is clear from the results in Table 4, the corrosion resistance value is increased in the case of only the surface treatment by performing the pretreatment before the surface treatment.

Claims (8)

(1) Oxidizing the zinc surface,
(2) Zinc characterized by contacting the oxidized zinc surface with a coating treatment liquid containing a silicic acid compound having a pH of 9 to 14, or simultaneously performing the steps (1) and (2) A method of forming a corrosion-resistant film on the surface. The method according to claim 1, wherein the oxidation treatment is performed by electrically oxidizing a zinc surface in a conductive solution. The coating treatment liquid containing the silicic acid compound,
(I) water glass,
The method according to claim 1 or 2, which is (ii) a sol obtained by hydrolyzing and condensing an alkoxysilane compound or (iii) colloidal silica. 4. The method according to claim 3, wherein the water glass is an aqueous solution of at least one alkali silicate salt selected from the group consisting of lithium silicate, sodium silicate, potassium silicate, rubidium silicate and cesium silicate. (1) Oxidizing the zinc surface by immersing the zinc surface in an aqueous solution containing an oxidizing agent or spraying an aqueous solution containing an oxidizing agent on the zinc surface;
(2) The method according to claim 1, wherein a coating treatment liquid containing a silicic acid compound having a pH of 9 to 14 is brought into contact with the oxidized zinc surface. 6. The method of claim 5, wherein the oxidant is hydrogen peroxide or permanganate. The method according to claim 5 or 6, wherein the concentration of the oxidizing agent in the aqueous solution is 0.001 to 35% by weight. The method according to any one of claims 1 to 7, wherein the coating treatment liquid contains 0.5 to 30 wt% of a silicic acid compound as silicon dioxide.