JP2016037620A - Surface treated galvanized steel plate having excellent corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface treated galvanized steel plate of which surface treated coating film does not include a chromium compound, is a thin film, and realizes both corrosion resistance and black discoloration resistance.SOLUTION: On a surface of a galvanized steel plate where an Al oxide layer of a predetermined thickness is formed on a galvanized layer, a surface treatment liquid, in which a silane compound (A) having a hydrolyzable group which is obtained from a silane coupling agent (a1) having a glycidyl group, a tetraalkoxysilane (a2), and phosphonic acid (a3); a zirconium carbonate compound (B); a vanadic acid compound (C); and water are included at a specific rate, is applied, and then dried to form a surface treatment coating film.SELECTED DRAWING: None

Description

  The present invention is a surface-treated hot-dip galvanized steel sheet used for automobiles, home appliances, building materials, etc., and does not contain a chromium compound such as hexavalent chromium in the surface-treated film, and is particularly resistant to corrosion and discoloration in a wet environment. The present invention relates to a surface-treated hot-dip galvanized steel sheet having excellent properties.

  Conventionally, for the purpose of improving the corrosion resistance (white rust resistance, red rust resistance) on the surface of galvanized steel sheets used for steel sheets for household appliances, steel sheets for building materials, and steel sheets for automobiles, chromic acid, dichromic acid or Steel sheets subjected to chromate treatment with a surface treatment solution containing salts as main components have been widely used. However, due to recent global environmental problems, there is an increasing demand for adopting non-polluted surface-treated steel sheets that do not depend on chromate treatment, so-called chromium-free treated steel sheets.

  Many technologies related to chromium-free treated steel sheets have already been proposed. Technologies aiming at the passivating action of molybdic acid and tungstic acid belonging to the same group IVA as chromic acid, such as Ti, Zr, V, Mn, Ni, Co, etc. Using technology using metal salts of rare earth elements such as transition metals and La, Ce, technology based on chelating agents such as polyphenolic carboxylic acids such as tannic acid and compounds containing S and N, using silane coupling agents A technique for forming a polysiloxane film or a technique combining these techniques has been proposed.

Specific examples are as follows.
(1) Technology for forming a film from a treatment liquid containing a coating agent obtained by reacting an organic resin such as a polyvinylphenol derivative, an acid component, and an epoxy compound, a silane coupling agent, and a vanadium compound (Patent Document 1) ~ 4)
(2) Technology for forming a film containing an aqueous resin, a thiocarbonyl group, a vanadic acid compound, and phosphoric acid (Patent Document 5)
(3) Technology for forming a film from a treatment liquid containing a metal compound such as Ti and inorganic and organic acids such as fluoride and phosphate compounds (Patent Documents 6 to 12)
(4) Technology to form a composite film of rare earth elements such as Ce, La, Y, etc. and Ti, Zr elements, and to concentrate the oxide layer on the plating interface side and the hydroxide layer on the surface side in the film (patent Document 13) and technology for forming a composite film of Ce and Si oxide (Patent Document 14)

(5) Technology for forming an organic composite coating comprising a phosphoric acid and / or phosphoric acid compound film containing an oxide in the lower layer and a resin film on the upper layer (Patent Documents 15 and 16)
(6) Technology for forming a composite film comprising a specific inhibitor component and a silica / zirconium compound (Patent Document 17)
(7) Technology for forming a composite film comprising a water-soluble zirconium compound, a tetraalkoxysilane, a compound having an epoxy group, a chelating agent, vanadic acid, and a predetermined metal compound (Patent Document 18)
(8) Technology for forming a composite film composed of a specific silane compound, zirconium carbonate compound, vanadic acid compound, and nitric acid compound (Patent Document 19)

JP 2003-13252 A JP 2001-181860 A JP 2004-263252 A Japanese Patent Laid-Open No. 2003-155542 Japanese Patent No. 3549455 Japanese Patent No. 3302677 JP 2002-105658 A JP 2004-183015 A JP 2003-171778 A JP 2001-271175 A JP 2006-213958 A JP 2005-48199 A JP 2001-234358 A Japanese Patent No. 3596665 JP 2002-53980 A JP 2002-53979 A JP 2008-169470 A JP 2010-255105 A JP 2013-60647 A

  Films formed by these technologies aim to suppress the occurrence of zinc white rust by the combined addition of organic or inorganic components. For example, the technologies (1) and (2) above are mainly used. Corrosion resistance is ensured by adding an organic resin. However, such an organic resin film has a problem of discoloration due to resin deterioration in an outdoor environment or a high-temperature and high-humidity environment. Moreover, high temperature baking is essential for film formation, and corrosion resistance cannot be ensured when it is produced by simple dryer drying that can ensure drying conditions of a plate reaching temperature of 30 ° C. or higher.

In the techniques (3) and (4), an inorganic single film that does not contain any organic component has been proposed. However, these metal oxide / metal hydroxide composite films cannot provide sufficient corrosion resistance to the galvanized steel sheet unless the film is thickened. That is, when manufactured by simple dryer drying, drying becomes insufficient, and corrosion resistance cannot be ensured.
The technique (5) has the same problems as the techniques (1) and (2) because a resin film is used as an upper layer in order to ensure corrosion resistance.

  In the technique of (6) above, a passivating action of a vanadate compound and a hardly soluble metal salt by a phosphate compound are used as an inhibitor component, and a composite film of a zirconium compound, fine particle silica, and a silane coupling agent is used as a skeleton film. By forming it, excellent corrosion resistance is expressed. However, with this technique, sufficient corrosion resistance cannot be imparted to the galvanized steel sheet unless the film is thickened. That is, when manufactured by simple dryer drying, drying becomes insufficient, and corrosion resistance cannot be ensured.

With the technologies (7) and (8), it is possible to provide a galvanized steel sheet that is thin and excellent in corrosion resistance. However, it is not possible to obtain discoloration resistance in a wet environment. It was found that the resistance to discoloration in a humid environment cannot be achieved.
As described above, it has been found that the chromium-free treated steel plates proposed so far cannot achieve both the corrosion resistance of the flat plate portion and the discoloration resistance in a wet environment.

  Therefore, the object of the present invention is to solve the problems of the prior art as described above, and to provide a high corrosion resistance and discoloration resistance in a wet environment with a thin film that does not contain a chromium compound such as hexavalent chromium in the surface treatment film. The object is to provide a surface-treated hot-dip galvanized steel sheet that can be made compatible and can be manufactured with simple equipment.

  As a result of intensive studies to solve the above problems, the inventors of the present invention have a specific silane compound, a zirconium carbonate compound, and a surface of a hot-dip galvanized steel sheet in which an Al oxide layer is formed on a galvanized surface layer. The present inventors have found that the above problems can be solved by forming a surface treatment film with a surface treatment liquid in which a vanadic acid compound and water are blended at a specific ratio.

The present invention has been made on the basis of such knowledge, and a glycidyl group is formed on the surface of a hot dip galvanized steel sheet in which an Al oxide layer having a thickness of 0.5 nm or more and less than 10.0 nm is formed on the surface of the galvanized layer. Coupling agent (a1), tetraalkoxysilane (a2) and phosphonic acid (a3) having hydrolyzable group, zirconia compound (B), and vanadic acid compound (C) and a surface treatment solution containing water and satisfying the following conditions (i) to (iv) are applied and dried, and the adhesion amount per side is 100 to 800 mg / m ^2. A surface-treated hot-dip galvanized steel sheet characterized by having a surface-treated film.
(I) The silane compound (A) is 30 to 70% by mass in the total solid content of the surface treatment liquid.
(Ii) The ratio (B / A) of the mass of the zirconium carbonate compound (B) in terms of ZrO _{2 and the} mass of the silane compound (A) is 0.3 to 2.0.
(Iii) The ratio (C / A) of the V-converted mass of the vanadic acid compound (C) to the mass of the silane compound (A) is from 0.010 to 0.15.

  The surface-treated hot-dip galvanized steel sheet of the present invention has a high barrier property obtained by combining the Al oxide layer of the galvanized surface layer and the surface-treated film containing a specific component, and has a corrosion resistance and a wet environment comparable to a chromate film. The color discoloration resistance below can be made highly compatible, and can be manufactured with simple equipment.

The hot dip galvanized steel sheet used in the present invention is not particularly limited as long as it can be obtained by hot dip galvanization, but is usually a hot dip galvanized steel sheet (GI) or an alloyed hot dip galvanized steel sheet obtained by alloying it ( GA) is used.
An Al oxide layer having a thickness of 0.5 nm or more and less than 10.0 nm needs to be formed on the surface layer of the galvanized layer of the hot dip galvanized steel sheet. In hot dip galvanization, since a trace amount of Al is contained in the plating bath, a small amount of Al is contained in the galvanized layer, but Al contained in the galvanized layer has a strong affinity for oxygen. Therefore, an Al oxide layer is formed on the galvanized surface layer.

If the thickness of the Al oxide layer is less than 0.5 nm, sufficient corrosion resistance and discoloration resistance in a wet environment cannot be obtained. On the other hand, if it exceeds 10.0 nm, the adhesion to the film is lowered, so that the corrosion resistance is not. In addition, since the soluble components after the film formation increases as the adhesiveness decreases, the discoloration resistance in a wet environment decreases.
The thickness of the Al oxide layer is not only affected by the Al content of the galvanized layer, but can also be increased by leaving the hot-dip galvanized steel sheet in the air or heating it at several hundred degrees Celsius. By adjusting the above conditions (conditions for leaving in the atmosphere, heating conditions, etc.), the thickness can be adjusted to be thicker. On the other hand, the thickness of the Al oxide layer can be reduced by chemical etching by alkali degreasing or surface grinding. Therefore, for example, in the alkaline degreasing performed as a pretreatment for the surface treatment on the hot-dip galvanized steel sheet, the thickness of the Al oxide layer can be adjusted by adjusting the concentration of the alkaline degreasing solution and the treatment time.
The thickness of the Al oxide layer can be measured by cross-sectional TEM observation. In the present invention, the average value of the measurement values at five randomly selected points is used as the thickness of the Al oxide layer.

The surface-treated hot-dip galvanized steel sheet of the present invention is applied to the surface of the hot-dip galvanized steel sheet (the surface of the galvanized layer in which the Al oxide layer is formed on the surface layer) as shown below, It has a surface-treated film with an adhesion amount per side formed by drying of 100 to 800 mg / m ² .
The surface treatment liquid for forming a surface treatment film used in the present invention is a silane having a hydrolyzable group obtained from a silane coupling agent (a1) having a glycidyl group, a tetraalkoxysilane (a2) and a phosphonic acid (a3). A compound (A), a zirconium carbonate compound (B), a vanadate compound (C), and water are contained. This surface treatment liquid does not contain a chromium compound such as hexavalent chromium (excluding chromium compounds contained as inevitable impurities).

The silane compound (A) having a hydrolyzable group is obtained by reacting a phosphonic acid (a3) with a low condensate of a silane coupling agent (a1) having a glycidyl group and a tetraalkoxysilane (a2). Compound.
The silane compound (A) is a silane compound having a hydrolyzable group directly bonded to Si element, and the hydrolyzable group forms a silanol group by reacting with moisture. The silane compound (A) may be a group in which all of the groups bonded to the Si element are hydrolyzable groups, or a part of the groups bonded to the Si element may be hydrolyzable groups.

  The silane coupling agent (a1) having a glycidyl group contains a glycidyl group and a lower alkoxyl group having 1 to 5 carbon atoms, preferably 1 to 3 carbon atoms, as a hydrolyzable group in one molecule containing Si. If it exists, it will not specifically limit, For example, 3-glycidoxypropyl trimethoxysilane, 3-glycidoxypropyl triethoxysilane, 3-glycidoxypropylmethyl dimethoxysilane, 2- (3,4 epoxy cyclohexyl) ethyl Examples thereof include triethoxysilane, and one or more of these can be used.

The tetraalkoxysilane (a2) contains four lower alkoxyl groups as hydrolyzable groups, and has a general formula Si (OR) ₄ (wherein R is the same or different alkyl group having 1 to 5 carbon atoms). Group) is not particularly limited, and examples thereof include tetramethoxysilane, tetraethoxysilane, and tetrapropoxysilane, and one or more of these can be used.
Examples of the phosphonic acid (a3) include hydroxyethylene diphosphonic acid, nitrilotris (methylenephosphonic acid), phosphonobutanetricarboxylic acid, and ethylenediaminetetra (methylenephosphonic acid). One or more of these may be used. it can.

  The silane compound (A) having a hydrolyzable group includes a low condensate of the above-described silane coupling agent (a1) having a glycidyl group and tetraalkoxysilane (a2). This low condensate has a polysiloxane bond formed by the condensation reaction of the silane coupling agent (a1) and the tetraalkoxysilane (a2) as the main skeleton, and the terminal group bonded to the Si element is hydrolyzed. It may be a functional group, or a part of the group bonded to the Si element may be hydrolyzable.

As the silane compound (A) having a hydrolyzable group, a compound having a condensation degree of 2 to 30 can be used, and a compound having a condensation degree of 2 to 10 is particularly preferable. This is because if the degree of condensation is 30 or less, no white precipitation occurs and a stable silane compound (A) is obtained.
The silane compound (A) having a hydrolyzable group is prepared by combining a low condensate of a silane coupling agent (a1) and a tetraalkoxysilane (a2) with a phosphonic acid (a3) at a reaction temperature of 1 to 80 ° C. It can be obtained by reacting for about 20 minutes to 20 minutes and performing autoclaving.

In the silane compound (A) having a hydrolyzable group, the specific and condensed state of the hydrolyzable group can be measured using gel permission chromatography (GPC), NMR and IR.
The silane compound (A) having a hydrolyzable group is obtained by reacting a silane coupling agent (a1) having a glycidyl group, a tetraalkoxysilane (a2), and a phosphonic acid (a3). It is considered that (a1) and tetraalkoxysilane (a2) are coordinated by hydrolysis with water and phosphonic acid (a3). This hydrolysis reaction and coordination by phosphonic acid (a3) are obtained at the same time, and a silane compound that has extremely high stability at room temperature and can withstand long-term storage is produced.

  Phosphonic acid (a3) is also an effective component for securing corrosion resistance and storage stability. Although the reason is not necessarily clear, it is considered that the phosphonic acid (a3) coordinates to the silane coupling agent (a1) and the tetraalkoxysilane (a2), and the silane compound (A) is a polymer in the surface treatment liquid. It is thought that it has an action to suppress the formation of the material, and the quality at the time of preparation is maintained without deterioration even when the surface treatment liquid is stored for a long time after preparation due to such action. it is conceivable that. In addition, phosphonic acid (a3) is considered to coordinate with the vanadate compound (C) described later, and it is considered that vanadium dissolves in a corrosive environment to form a polysiloxane bond again.

The blending ratio of the silane coupling agent (a1) having a glycidyl group, the tetraalkoxysilane (a2), and the phosphonic acid (a3) is based on 100 parts by mass of the silane coupling agent (a1) from the viewpoint of corrosion resistance. The tetraalkoxysilane (a2) is preferably 25 to 75 parts by mass and the phosphonic acid (a3) is preferably 5 to 30 parts by mass.
Content of the silane compound (A) in a surface treatment liquid shall be 30-70 mass% in the total solid of a surface treatment liquid. If the content of the silane compound (A) is less than 30% by mass, the corrosion resistance cannot be ensured. On the other hand, if the content exceeds 70% by mass, the corrosion resistance decreases.
By mixing with the zirconium carbonate compound (B), the silane compound (A) has a barrier effect that once dried, it does not dissolve in water again.

  Examples of the zirconium carbonate compound (B) include salts of zirconium carbonate such as sodium, potassium, lithium, and ammonium (for example, zirconium ammonium carbonate, sodium zirconium carbonate, lithium zirconium carbonate), and one or more of these are used. be able to. Of these, ammonium zirconium carbonate is preferable from the viewpoint of corrosion resistance.

The content of the zirconium carbonate compound (B) is such that the ratio (B / A) of the mass of ZrO ₂ converted to ZrO ₂ (ZrO ₂ converted mass) of the zirconium carbonate compound (B) and the mass of the silane compound (A) is 0. 3 to 2.0, preferably 0.35 to 1.5. If the mass ratio (B / A) is less than 0.3, corrosion resistance cannot be ensured. On the other hand, if the mass ratio (B / A) exceeds 2.0, the corrosion resistance decreases. Moreover, when mass ratio (B / A) exceeds 2.0, the discoloration resistance in a humid environment will also fall.

The vanadic acid compound (C) is present in the film formed on the surface of the zinc-based plated steel sheet, uniformly dispersed in a form that is easily dissolved in water, and exhibits a so-called inhibitor effect during zinc corrosion. In addition, the vanadic acid compound (C) is considered to be coordinated to the phosphonic acid (a3), and the vanadic acid compound (C) is excellent in that it is ionized and passivated in a corrosive environment. Demonstrate corrosion resistance.
Examples of the vanadic acid compound (C) include ammonium metavanadate, sodium metavanadate, vanadyl acetylacetonate, and one or more of these can be used.

  The content of the vanadic acid compound (C) is such that the ratio (C / A) of the V-converted mass of the vanadic acid compound (C) to the mass of the silane compound (A) is 0.010 to 0.15, Preferably it is set to 0.030-0.10. If the mass ratio (C / A) is less than 0.010, corrosion resistance cannot be ensured. On the other hand, if the mass ratio (C / A) exceeds 0.15, discoloration resistance in a wet environment is lowered.

A lubricant can be added to the surface treatment liquid in order to improve the lubricating performance. Examples of the lubricant include polyethylene wax, oxidized polyethylene wax, oxidized polypropylene wax, carnauba wax, paraffin wax, montan wax, rice wax, Teflon (registered trademark) wax, carbon disulfide, and graphite. Can be used.
The content of the lubricant is preferably 1 to 10% by mass, and more preferably 3 to 7% by mass in the total solid content of the surface treatment liquid. When the content of the lubricant is 1% by mass or more, an effect of improving the lubrication performance is obtained, and when it is 10% by mass or less, the corrosion resistance of the galvanized steel sheet is not lowered.

In addition, the surface treatment liquid contains a thickener for improving workability, a conductive substance for improving conductivity, a color pigment for improving design properties, a solvent for improving film forming property, and the like. You may add suitably as needed.
The surface treatment liquid can be obtained by mixing the above-described components in water such as deionized water or distilled water. What is necessary is just to select the solid content concentration of a surface treatment liquid suitably. Moreover, you may add alcohol, a ketone, a cellosolve-type water-soluble solvent, an antifoamer, a fungicidal agent, a coloring agent, etc. to a surface treatment liquid as needed. However, it is important to add these components (that is, components (A) to (C), water, and an additive component other than the lubricant) to the extent that the performance obtained in the present invention is not impaired. The total solid content is preferably less than 5% by mass.

The adhesion amount per one side of the surface treatment film is 100 to 800 mg / m ² , preferably 200 to 500 mg / m ² . If the coating amount per side is less than 100 mg / m ² , there is a concern about insufficient corrosion resistance. On the other hand, if it exceeds 800 mg / m ² , production with simple equipment such as dryer drying becomes difficult.

The surface-treated hot-dip galvanized steel sheet of the present invention is coated on the surface of the hot-dip galvanized steel sheet in which the Al oxide layer is formed on the surface of the galvanized surface, and then dried, so that one side after drying It is manufactured so that the amount of adhesion per unit is 100 to 800 mg / m ² .
As a method for applying the surface treatment liquid to the surface of the hot-dip galvanized steel sheet, any method such as a roll coating method, a bar coating method, a dipping method, a spray coating method, or the like can be employed. What is necessary is just to select an optimal method suitably according to the shape etc. of the hot dip galvanized steel plate to be processed. For example, if the hot dip galvanized steel sheet to be processed is a sheet, then roll coating or bar coating, or spraying a surface treatment solution on the surface of the hot dip galvanized steel sheet, and then spraying with a roll squeezing or gas at a high pressure. It is appropriate to use a spray coating method that adjusts the amount. Moreover, when the hot dip galvanized steel sheet is a molded product, a method of adjusting the coating amount by immersing it in a surface treatment liquid and pulling it up and blowing off excess surface treatment liquid with compressed air as necessary is suitable.

  In addition, before applying the surface treatment liquid to the hot dip galvanized steel sheet, pretreatment for the purpose of removing oil and dirt on the hot dip galvanized steel sheet surface may be performed as necessary. The pretreatment method is not particularly limited, and examples thereof include hot water washing, solvent washing, and alkaline degreasing washing. Hot-dip galvanized steel sheets are often coated with rust-preventive oil for the purpose of rust-prevention, and even when rust-preventive oil is not applied, the surface has oil or dirt attached during work. . By performing the above pretreatment, the surface of the galvanized layer is cleaned and easily wetted uniformly. When the surface treatment liquid gets wet uniformly on the surface of the hot dip galvanized steel sheet, pretreatment is not particularly necessary.

  Although there is no restriction | limiting in particular in the heating temperature at the time of drying after apply | coating a surface treatment liquid (maximum ultimate board temperature), Usually, it is about 30-200 degreeC. If the heating temperature is 30 ° C. or higher, no moisture remains in the film, and if the heating temperature is 200 ° C. or lower, the generation of cracks in the film is suppressed. This is because there is no problem. The heating time is appropriately selected depending on the type of hot-dip galvanized steel sheet used. From the viewpoint of productivity and the like, about 0.1 to 60 seconds is preferable, and about 1 to 30 seconds is more preferable.

  The surface-treated hot-dip galvanized steel sheet of the present invention has the advantage that the surface-treated film does not contain a chromium compound, has excellent corrosion resistance and resistance to discoloration in a wet environment, and can be manufactured with simple equipment. . The reason for such excellent performance is not necessarily clear, but is considered to be due to the following effects.

  In the present invention, it is considered that a high barrier property can be obtained by the effect of combining the Al oxide layer present on the surface layer of the galvanized surface and the surface treatment film. In the conventional chromate film, a film is formed by the reaction of chromic acid with zinc, so that a higher corrosion resistance is obtained when the Al oxide layer is not present on the plating surface layer. On the other hand, in the case of having a chromate-free film, particularly a surface-treated film that does not form many reaction layers with zinc as in the present invention, if the Al oxide layer is present on the plating surface layer with a certain thickness or more, the Al oxide It is considered that the barrier effect possessed by the layer itself and the following effects by the surface treatment film formed by the specific surface treatment liquid are combined to provide excellent corrosion resistance. Moreover, if the thickness of the Al oxide layer is excessive, the reactivity with the chemical conversion solution decreases, so the soluble components after film formation increase, and the discoloration resistance in a wet environment is considered to decrease. .

  First, among the components of the surface treatment liquid, the skeleton of the coating formed on the surface of the galvanized layer is constituted by the silane compound (A) and the zirconium carbonate compound (B). The hydrolyzable group of the silane compound (A) is considered to fix the coating component by reacting with the surface of the galvanized layer and three-dimensionally crosslink with the zirconium carbonate compound (B). Furthermore, it is considered that the glycidyl group of the silane coupling agent (a1) also reacts with the surface of the galvanized layer, and the bond strength of the film becomes stronger. The surface treatment film formed in this way, once dried, does not dissolve in water again and has a barrier effect, so that excellent corrosion resistance is obtained in combination with the barrier effect of the Al oxide layer. Further, since the film is not formed by a crosslinking reaction that requires a temperature like a resin, the film can be appropriately formed even by a simple drying means such as dryer drying.

  Among the components of the surface treatment liquid, the vanadic acid compound (C) is present in the film in a form that is easily dispersed in a form that is easily soluble in water, and exhibits a so-called inhibitory effect during zinc corrosion. That is, it is considered that the vanadic acid compound (C) is partially ionized and passivated in a corrosive environment, thereby suppressing zinc corrosion itself. Moreover, since it coordinates to phosphonic acid (a3), after ionization, the hydrolyzable group of the silane compound (A) is three-dimensionally cross-linked, thereby repairing a film defect portion and suppressing corrosion of zinc. Conceivable.

That is, the surface treatment liquid forms a dense film with the silane compound (A) and the zirconium carbonate compound (B) to obtain high corrosion resistance, and contains a vanadate compound (C) as a corrosion inhibitor in the film. Thus, a dense film following the hot dip galvanized steel sheet can be formed.
The hot-dip galvanized steel sheet of the present invention can be applied to various uses, and is suitably used for materials used in various fields such as architecture, electricity, and automobiles.

(1) Test plate (hot dip galvanized steel plate)
The following commercially available galvanized steel sheets were used as test plates.
(I) Hot-dip galvanized steel sheet (GI): Plate thickness 0.8 mm, plating basis weight 60/60 (g / m ² )
(Ii) Alloyed hot-dip galvanized steel sheet (GA): plate thickness 0.8 mm, plating basis weight 40/40 (g / m ² )
In addition, the plating basis weight is each plating adhesion amount on both surfaces of the steel sheet. For example, the plating basis weight 60/60 (g / m ² ) means that each of both surfaces of the steel sheet has a plating layer of 60 g / m ^2. means.
In addition, the thickness of the Al oxide layer on the plating surface layer was adjusted by adjusting the pretreatment alkaline degreasing solution concentration and the treatment time as described below.

(2) Pretreatment of test plate (cleaning)
Alkali degreasing (treated with “Fine Cleaner E6406” manufactured by Nihon Parkerizing Co., Ltd. according to the instruction manual) is applied to the surface of the above test plate (hot dip galvanized steel plate) to remove oil and dirt on the surface, and Al oxidation The thickness of the material layer was adjusted. Specifically, when alkali degreasing a plated steel sheet having an Al oxide layer formed on the plating surface layer, the higher the alkali degreasing solution concentration and the longer the treatment time, the smaller the thickness of the Al oxide layer. Then, alkali degreasing was performed under the conditions shown in Table 3 to adjust the thickness of the Al oxide layer. Next, after rinsing with tap water and confirming that the surface of the test plate is 100% wet with water, pour pure water (deionized water), and then dry the moisture in an oven at 100 ° C., This was used as a test plate.

(3) Compound for surface treatment liquid The following were used as the compound for the surface treatment liquid.
(3-1) Production / Production Example 1 of Silane Compound (A) (Silane Compound A1)
3-Glycidoxypropyltrimethoxysilane, tetraethoxysilane, and deionized water were mixed, and ammonia water was added dropwise to precipitate a silane compound. After washing with deionized water, nitrilotris (methylene sulfonic acid) was added as phosphonic acid and stirred to obtain silane compound A1.

Production Example 2 (Silane Compound A2)
A mixture of 3-glycidoxypropyltrimethoxysilane and tetraethoxysilane was added dropwise to a mixture of deionized water and hydroxyethylene diphosphonic acid as phosphonic acid with stirring at 20 ° C. over 1 hour. . Thereafter, aging was carried out at 25 ° C. for 2 hours to obtain a silane compound A2.
Production Example 3 (Silane Compound A3)
Silane compound A2 of Production Example 2 was further aged at 80 ° C. for 1 hour to obtain silane compound A3.

(3-2) Zirconium carbonate compound (B)
B1: Ammonium zirconium carbonate B2: Sodium zirconium carbonate (3-3) Vanadate compound (C)
C1: ammonium metavanadate C2: vanadyl acetylacetonate (V: 19.2% by mass)

(4) Preparation of surface treatment liquid The above compounds were mixed in water at the ratios shown in Tables 1 and 2 to obtain a surface treatment liquid having a solid content of 15% by mass.

(5) Surface treatment method The above surface treatment solution was applied to each test plate by bar coating or spraying, and then dried in a hot air oven as it was without washing with water to form a film. The drying conditions were adjusted according to the furnace atmosphere temperature and the time in the furnace.
The bar coat treatment and spray treatment were performed as follows.
Bar coating treatment: The surface treatment solution was dropped onto the test plate and treated with a # 3-5 bar coater. By adjusting the count of the bar coater used and the solid content concentration of the surface treatment liquid, the coating amount was adjusted to be a predetermined amount.
-Spray treatment: The surface treatment solution was sprayed on the test plate, and the amount of the film was adjusted with a roll coater. By adjusting the conditions of the roll coater and the solid content concentration of the surface treatment liquid, adjustment was made so as to obtain a predetermined coating amount.

(6) Method of evaluation test (6-1) Corrosion resistance A test piece having a size of 70 mm × 150 mm was cut out from the test plate on which the surface treatment film was formed, and the back and end portions of the test piece were sealed with vinyl tape, A salt spray test (SST) based on JIS-Z-2371-2000 was performed. The time until the white rust occurrence area ratio in the salt spray test was 5% was measured, and the corrosion resistance was evaluated as follows.
◎: Time until white rust generation area ratio reaches 5% or more 240 hours ○: Time until white rust generation area ratio reaches 5% 120 hours or more, less than 240 hours △: White rust generation area ratio Time until 5% is 72 hours or more and less than 120 hours ×: Time until white rust occurrence area ratio is 5% is less than 72 hours

(6-2) Discoloration resistance in a wet environment Two test pieces having a size of 70 mm × 150 mm were cut out from the test plate on which the surface treatment film was formed, and the target surfaces were overlapped and tightened with a torque strength of 20 kgf. The specimen was held in a constant temperature bath at 50 ° C. and 98% RH for 4 weeks, and the color tone of the test piece before and after holding was evaluated. The color tone of the test piece before and after holding was measured with a spectrocolorimeter, the color tone was expressed by the L value of the Lab color system, and the resistance to discoloration was evaluated by the difference ΔL between the L values before and after holding as follows.
A: ΔL ≧ −2
○: −2> ΔL ≧ −5
Δ: −5> ΔL ≧ −10
×: ΔL <−10

(7) Measurement of the thickness of the Al oxide layer The cross section of the test plate is observed with a TEM to measure the thickness of the Al oxide layer, and the average value of the measurement values at five randomly selected points is used as the Al oxide layer. The thickness of the oxide layer.

The results of the above evaluation tests are shown in Table 1 and Table 2 together with the composition of the surface treatment liquid, the type and configuration of the hot dip galvanized steel sheet, and the surface treatment conditions. The drying temperature is the temperature reached on the surface of the test plate.
In the comparative example that does not satisfy the conditions of the present invention, either corrosion resistance or discoloration resistance in a wet environment is insufficient. On the other hand, the examples of the present invention have excellent corrosion resistance and discoloration resistance in a wet environment without containing a chromium compound in the surface treatment film.

Claims (1)

A silane coupling agent (a1) having a glycidyl group, tetraalkoxysilane (on the surface of a hot dip galvanized steel sheet on which an Al oxide layer having a thickness of 0.5 nm or more and less than 10.0 nm is formed on the surface layer of the galvanized layer, a hydrolyzable silane compound (A) obtained from a2) and phosphonic acid (a3), a zirconium carbonate compound (B), a vanadic acid compound (C), and water; A surface treatment melt characterized by having a surface treatment film of 100 to 800 mg / m ² formed by applying a surface treatment liquid satisfying the conditions of (iv) and drying. Galvanized steel sheet.
(I) The silane compound (A) is 30 to 70% by mass in the total solid content of the surface treatment liquid.
(Ii) The ratio (B / A) of the mass of the zirconium carbonate compound (B) in terms of ZrO _{2 and the} mass of the silane compound (A) is 0.3 to 2.0.
(Iii) The ratio (C / A) of the V-converted mass of the vanadic acid compound (C) to the mass of the silane compound (A) is from 0.010 to 0.15.