JPH05140759A - Underlayer for coating and its formation - Google Patents

Abstract

PURPOSE:To form an underlayer for coating by which a corrosion resistant coating material can be allowed to well exhibit its function. CONSTITUTION:The surface of a substrate with a zinc-contg. layer on the surface is pretreated by air oxidation or boiling in an alkali soln. to form a layer of active zinc oxide or active zinc hydroxide and hydrothermal synthesis is carried out in an alkali silicate soln. Synthetic hemimorphite film having a mineral compsn. basically represented by a chemical formula Zn4(OH)2Si2O7.H2O and similar in crystal structure to natural hemimorphite can be synthesized in a relatively short time. The resulting synthetic hemimorphite coating film as an underlayer has high bonding strength to the substrate, heat and corrosion resistances and excellent adhesion after coating.

Description

Detailed Description of the Invention

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a base forming technique for applying a corrosion resistant coating on a metal or other substrate.

2. Description of the Related Art Conventionally, as an anticorrosive paint for galvanized steel sheets, JP-A-59-212256 and JP-B-53-
As disclosed in Japanese Patent No. 37255 and the like, a zinc rich paint mainly composed of zinc powder and an organic or inorganic silicate is used. However, although the zinc rich paint itself has excellent weather resistance, it has a difficulty in adhesion strength with the base material because the basic composition is a mixture of zinc particles and a silicate, and the uniformity during application is poor. It has the drawback of being bad. In order to make up for the drawbacks during coating, it has hitherto been formed with phosphoric acid or chromic acid in order to form an underlayer that has better compatibility with paints than metal base materials, and that itself has corrosion resistance. It is being processed. Furthermore, in recent years, in order to increase the durability of a coated steel sheet, as an underlayer having excellent adhesion and corrosion resistance, a metal surface technology (Japanese Patent Laid-Open No. 54-13429 and 1987 (for example, vo
l. 38. No. 12. p. 12) ”discloses forming a silica gel coating layer together with an alkali silicate solution on a base material as a coating base layer, and performing eutectoid plating of zinc and silica gel.

The object of the present invention is that it is superior in corrosion resistance to an atmosphere such as tap water or rainwater, and in its adhesiveness to a paint, as compared with these conventional coating substrates. The purpose of the present invention is to provide an underlayer capable of sufficiently exhibiting the function of a corrosion-resistant paint.

The present invention has achieved its object by using a synthetic hemimorphite coating having a mineral composition close to a natural composition as a coating base. The synthetic hemimorphite film is basically composed of Zn ₄ (OH) ₂ Si ₂
It has a chemical formula of O ₇ · H ₂ O and has a crystal structure close to that of natural hemimorphite. In order to form this synthetic hemimorphite, a zinc layer having a thickness of 10 μm or more is formed on a base material, and a base material having a zinc or zinc alloy layer on at least the surface thereof is heated in air at 100 to 400 ° C. Pre-oxidation treatment for 1 to 3 hours at heating temperature, or pH 7 to 13
In a sodium hydroxide solution of 80 to 95 ° C. for 1 to 60 minutes to perform a hydroxylation treatment, and the surface is treated with an active zinc oxide (ZnO) layer or an active zinc hydroxide (Zn (OH) 2
After forming the layer ₂ ) or a mixed layer thereof, a method of immersing or coating in a silicic acid or silicate aqueous solution and heating in the presence of water or steam can be adopted. The active zinc oxide layer formed by the pre-oxidation or hydroxylation treatment is for facilitating the formation of the hemimorphite layer and strengthening the bond with the zinc layer, and the thickness is 0.1 to 20 μm. Appropriate, but can be thicker. Further, formation of a film of synthetic hemimorphite is carried out by immersing or coating a pre-oxidized or hydroxylated base material in an aqueous solution of silicic acid or silicate having a concentration of 50 to 15,000 ppm and applying 110 to 210
Hydrothermal synthesis is performed at a temperature of 1 hour or more at a saturated steam pressure or higher. In addition, the synthetic hemimorphite coating is an atypical complex consisting of zinc, silicic acid, and water on the surface of which zinc in the zinc-containing layer on the substrate surface is eluted by an electrochemical method such as electrolysis. It can also be formed by forming the compound and then thermally accelerating it in warm water. further,
As another method for forming a synthetic hemimorphite film, a solution obtained by dissolving zinc and silicon alkoxide in an organic solvent is stirred while aerating an inert gas containing water vapor,
After coprecipitating zinc hydroxide and silicic acid in the solution, remove excess water and evaporate the solvent to dilute the gel-like white precipitate (precursor of hemimorphite) with the alkali silicate solution. Then, it is also possible to adopt a method in which it is applied to the surface of the metal substrate and then aged in warm water. Also, the film of synthetic hemimorphite as the underlayer does not necessarily have to be a dense film, considering the compatibility with the coating material to be applied thereon, may be porous, and may be completely crystalline. Even if it is not in a solidified state, the thickness is 0.5μ
If it is m or more, the function as a base for coating can be sufficiently exhibited. However, if it is too thick, cracking will occur due to processing stress or thermal stress, so it should be 50 μm or less. further,
As a raw material for silicic acid or silicate, when M is Li, Na, K, Cs which is a monovalent alkali metal, or when ammonium or amine is used, silicic acid having a ratio of M ₂ O / SiO ₂ of 2 or less. , Silicates can be used. In producing this synthetic hemimorphite underlayer,
The base material is not limited to a galvanized steel sheet, and any base material can be applied as long as it has a layer having zinc on its surface.

The synthetic hemimorphite layer referred to in the present invention has sufficient bondability due to the crystal structure connection with the base material through the zinc oxide or zinc hydroxide layer, and is itself plate-like or columnar. Crystals have a three-dimensionally oriented shape, exert a sufficient anchoring action on the coating film formed on the surface thereof, and have very good compatibility with the coating material.

EXAMPLE The surface of a sample piece of 50 mm × 30 mm × 1 mm was degreased with acetone and then oxidized in air at 150 ° C. for 2 hours to form a zinc oxide film or in a sodium hydroxide aqueous solution having a pH of 9.7. And boiled for 30 minutes to form a zinc hydroxide film. Then, the galvanized steel sheet is treated with a SiO ₂ concentration of 300 ppm or 1000
After immersing in 125 ml of a ppm aqueous solution of sodium metasilicate, hydrothermal treatment was carried out in an autoclave at 180 ° C. for 6 hours or 20 hours. FIG. 1 shows the surface morphology of the hemimorphite synthetic film observed by a scanning electron microscope. It can be seen that the substrate surface is covered with plate-like hemimorphite crystals. FIG. 2 shows the identification results of the synthetic film product by X-ray diffraction, together with the natural hemimorphite. These steel materials of the present invention were tested for primary corrosion resistance, secondary corrosion resistance, primary paint adhesion and secondary paint adhesion. The results are shown in Table 1. As the primary corrosion resistance, a salt water spray test according to JIS-Z-2371 was performed on each steel material of the present invention, and the time until the occurrence of white rust was measured. As the secondary corrosion resistance, after each steel material of the present invention is coated with a paint, a crosscut is put on the coated surface, and a salt spray test of JIS-Z-2371 is performed for 100 hours to determine the peeling width of the paint generated in the crosscut portion. It was measured. As for the primary coating adhesion, a steel sheet material of the present invention was subjected to a goose stitch test according to JIS-K-5400 to determine the damage state of the coating film surface. After each steel material of the present invention was coated with a coating material, 100 goggles were cut at 1 mm intervals on the coating film surface, and cellophane tape was adhered to the gobangs and then peeled off. The secondary coating adhesion was 3 in boiling water after coating each steel material of the present invention with the coating material.
After being immersed for a period of time, a goose-eye test was conducted in the same manner as the primary paint adhesion. The adhesiveness was determined based on the following evaluation criteria. Evaluation point 10: No abnormality at all 〃 8: 1 to 5 peeling of the coating film occurred. 〃 6: 6 to 15 peeling of the coating film has occurred. 〃 4: 16 to 35 coating film peeling occurred. 〃 2: 36 to 65 pieces of coating film peeled off. 〃 0: 66 or more peeled coating films have occurred.

[Table 1] From these results, the hemimorphite film formed according to the present invention was compared with the case where the treatment was not performed (Example 5) and other coating substrate treatments (Examples 6 and 7). It can be seen that also in ~ 4), the corrosion resistance and the paint adhesion are excellent.

According to the present invention, the following effects can be obtained. (1) A synthetic hemimorphite coating formed on zinc or a zinc-containing surface such as zinc plating can be expected to have a sufficient anchoring effect due to the three-dimensional structure of the surface, so that a base having excellent coating adhesion can be obtained. (2) The synthetic hemimorphite film formed on the base material has a sufficient effect of blocking the base material from wet air, salt water, tap water, or rainwater, and has excellent corrosion resistance as a base of the coating film. Have. (3) Furthermore, ZnO which has been pretreated with zinc or a zinc-containing surface such as zinc plating, or synthetic hemimorphite continuously formed on Zn (OH) ₂ has a texture as well as Zn.
It has a structure continuous with O or Zn (OH) ₂ and has higher bondability with the substrate.

[Brief description of drawings]

FIG. 1 shows a crystal structure of a coating synthesized on a galvanized steel sheet under a scanning electron microscope (SEM).

FIG. 2 shows the results of identifying a film product by X-ray diffraction.

Claims (4)

[Claims] 1. Basically, Zn ₄ (OH) ₂ Si ₂ O ₇
An undercoat layer for coating, which is composed of a synthetic hemimorphite film having a chemical formula of H ₂ O. 2. An active zinc oxide (ZnO) layer or an active zinc hydroxide (Z) is formed on the surface of a substrate having a zinc-containing layer on its surface.
After forming the n (OH) ₂ ) layer or a mixed layer thereof,
A method for forming a base layer for coating which forms a synthetic hemimorphite film by heating and aging it in an aqueous solution of silicic acid or silicate. 3. A base material having a zinc-containing layer on its surface is electrolyzed in an alkali silicate solution to produce a composite compound consisting of zinc and water on the surface of the base material, and then the composite compound is aged in warm water. Then, a method for forming an undercoat layer for coating, which forms a synthetic hemimorphite film on the surface of a substrate. 4. A solution obtained by dissolving an alkoxide of zinc and silicon in an organic solvent is stirred while aerating an inert gas containing water vapor, and zinc hydroxide and silicic acid are coprecipitated in the solution. A gel-like white precipitate obtained by removing excess water and volatilizing the solvent is diluted with an alkali silicate solution and applied to the surface of a metal substrate, and the metal material is aged in warm water to synthesize a synthetic hemi-mol. A method for forming an undercoat layer for painting that forms a fight film.