JPS6242999B2 - - Google Patents

Description

[Detailed description of the invention]

Traditionally, galvanized steel sheets plated with zinc, aluminum, etc. have been used unpainted or painted for various purposes, but they are used for rust prevention or to improve the adhesion of the paint film to improve corrosion resistance. In order to increase the surface resistance of galvanized steel sheets, the surface of the plated steel sheet is treated with a chromate treatment solution containing chromic anhydride as the main component. Chromate treatment can also be carried out by an electrolytic treatment method in which electrolysis is carried out in a treatment solution using the base material to be treated as an electrode, or by a coating method in which the treatment solution is applied to the surface of the base material to be treated. Chromate treatment in which a chromate film is formed with etching through a chemical reaction between the surface of the base material and the treatment liquid, using the immersion method in which the substrate is immersed in the treatment solution or the spray method in which the treatment solution is sprayed onto the base material. (hereinafter referred to as reactive chromate treatment) is performed. In the case of reactive chromate treatment, the treatment solution is used repeatedly, and as shown in the following equation, metals (zinc, aluminum, etc.) on the surface of the base material to be treated dissolve into the chromate treatment solution and become metal ions, and at the same time, the treatment solution The hydrogen ions inside are consumed. Me＋nH ₂ Cr ₂ O ₇ →Me ⁺ⁿ +HCr ₂ O ₇ +H ₂ ↑ (However, n is 2 or 3) As a result, the free acid concentration in the liquid decreases depending on the amount of treated base material, and the chromate film is formed. The formation reaction gradually becomes less likely to occur, and eventually a normal chromate film is no longer formed, and the aged treatment solution must be disposed of as waste and replaced with a new treatment solution. This aging phenomenon of treatment liquids is generally a phenomenon that occurs when objects to be treated are repeatedly treated with a fixed amount of liquid, but especially in chromate treatment, the following various methods are used to increase the treatment effect. Depending on the processing method used, the aging of the processing solution becomes particularly significant. That is, in an attempt to obtain high corrosion resistance through chromate treatment, the chromate concentration in the chromate treatment solution is increased to increase the amount of chromium deposited, but in this case, the chromate film easily dissolves in water, Coloring, chromic acid stains, uneven chromium adhesion, and poor appearance may occur. Therefore, on the one hand, an etching agent is added to the chromate treatment solution in order to promote the chromate reaction. Mineral acids, organic acids, or their salts are used as etching agents.
Fluorine compounds with high solubility such as NH ₄ HF ₂ and Na ₂ SiF ₆ are also coming into use. Even when using a chromate treatment solution containing such etching agents, it is necessary to add a large amount of these etching agents to the chromate treatment solution in order to increase the amount of chromium deposited and obtain sufficient corrosion resistance. However, if a large amount of etching agent is added, the reaction between the chromate treatment solution and the surface of the plated steel sheet, which is the base material to be treated, will proceed rapidly, resulting in rapid aging of the treatment solution and shortening the lifespan of the treatment solution. In addition, there is a limit to the amount of etching agent that can be added, as the surface of the base material to be treated may be colored and chromic acid stains may occur. It is not easy to extend the life of the chromate film, and the corrosion resistance of the chromate film is still insufficient. It is also known to perform chromate treatment by adding a film forming agent such as silica sol or alumina sol to the chromate treatment solution, but the effect is not sufficient unless it is added together with an etching agent. In this way, aging of the treatment solution is a fate in reactive chromate treatment, and recently, pollution regulations have been tightened and treatment of hexavalent chromate waste fluid has become more difficult, so it is necessary to extend the life of the chromate treatment solution. However, the biggest challenge in reactive chromate treatment is to improve the treatment effect. Therefore, there has been a desire for a chromate treatment method that uses a slow-aging treatment solution that is easy to control concentration and contains an etching agent that forms a strong chromate film with excellent corrosion resistance even at low concentrations. As a result of intensive research, the present inventors have found that a chromate treatment solution in which a fluorine compound consisting of only fluorine ions or fluorine complex ions and barium ions is added as an etching agent to an aqueous chromic acid anhydride solution ages slowly and is resistant to water. The present invention was completed by discovering that it is possible to form a chromate film that is soluble and has excellent corrosion resistance, processability, coating adhesion, etc. An object of the present invention is to provide a method for chromate treatment of a galvanized steel sheet, which slows aging of the chromate treatment solution in the reactive chromate treatment of a galvanized steel sheet and can form a chromate film with excellent physical properties. That is, in the present invention, chromic anhydride (CrO ₃ ) is
The surface of a plated steel plate is coated with a chromate treatment solution containing 0.04 g/g/ or more of a fluorine compound consisting of only barium ions as cations and fluorine ions or fluorine complex ions as anions, expressed as fluorine element. The present invention relates to a surface treatment method for galvanized steel sheets, which is characterized by subjecting them to surface treatment. The present invention will be explained in more detail below using the drawings. The figure is a graph showing the effect of barium silicate, an example of a fluorine compound used in the present invention (hereinafter referred to as a barium-containing fluorine compound), in making a chromate film less soluble in comparison with that of other fluorine compounds. . As the chromic anhydride (CrO ₃ ) used in the present invention, the one conventionally used for surface treatment of steel sheets is sufficient. In order to properly form a chromate film containing chromic anhydride as the main component on the surface of the plated steel sheet, the concentration of chromic anhydride in the chromate treatment solution should be
The optimal amount is 10-50g/. If the concentration of chromic acid anhydride is less than 10g/, a chromate film with excellent corrosion resistance cannot be obtained, and if it exceeds 50g/, the chromate film will take on a yellowish tinge, and as the treatment solution rises, chromate stains will easily occur and the appearance will be impaired. This is not only undesirable, but also increases consumption of chromate, making it uneconomical. The barium-containing fluorine compound is used as an etching agent. This barium-containing fluorine compound is a fluorine compound having a fluorine ion or a fluorine complex ion as an anion and only barium as a cation, and is the most distinctive feature of the present invention. BaF ₂ , BaSiF ₆ , BaZrF ₆ , BaTiF ₆ and the like are used as such barium-containing fluorine compounds. The amount of barium-containing fluorine compound contained in the treatment solution 1 is 0.04 g/fluorine element.
That's all. This barium-containing fluorine compound brings about the following important effects in the present invention. The first
The effect of this is to make the formed chromate film less soluble and provide a high degree of corrosion resistance. The figure shows 30g of chromic anhydride (CrO ₃ ) and 1g of various fluorine compounds 1
A galvanized steel sheet was chromate treated using the chromate treatment liquid contained in
m ² ) was immersed in distilled water at 40°C for 2 minutes to determine the amount of chromium eluted. In the drawing, A is BaSiF ₆
(Contains 0.41g of F), ( _B ) contains Na ₂ SiF ₆ (Contains 0.61g of F), C (Contains 0.64g of _F ). This is a case in which it does not contain any elementary compounds. The figure shows that fluorine compounds in which the cation is only barium ion relative to the anion of fluorine ion or fluorine complex ion are compared to other highly soluble fluorine compounds whose cation is sodium or ammonium ion. It can be seen that this is highly effective in making the chromate film difficult to dissolve. The effect of the barium-containing fluorine compound on making the chromate film less soluble is fully manifested at a concentration of 0.04 g or more of fluorine element per chromate treatment solution, as shown in Examples below. As long as this concentration is 0.04 g or more, it does not need to be a saturated concentration as described as the third effect as far as the effect of making the chromate film difficult to dissolve is concerned. The second effect of the barium-containing fluorine compound is that its etching effect is moderately small, so the appearance is not damaged by excessive etching, and it slows down the aging of the chromate treatment solution, extending its lifespan. It is to extend it. The third effect of the barium-containing fluorine compound is that the solubility of this barium-containing fluorine compound in water is moderately low. In the case of processing a coated steel plate, the present invention has an appropriate etching power to form a proper chromate film without excessive etching, thereby making it unnecessary to control the concentration of the etching agent. The reason for this is that barium-containing fluorine compounds have a saturation concentration (number of grams of solute in 1 saturated solution) relative to the chromic acid aqueous solution.
is about 0.6 (g/) (for example, in the case of barium silicate, it corresponds to 0.25 g/ in terms of fluorine element), and moreover, the chromate obtained by saturating a chromium aqueous solution with a barium-containing fluorine compound in this way Even when a plated steel plate is surface-treated with a treatment solution, the etching effect is moderate, and the chromate film formed has excellent corrosion resistance. These facts will be clarified later in Examples. Therefore, even if the barium-containing fluorine compound is dissolved in the chromate treatment solution in a saturated state, the amount of etching is moderately small and a film with excellent corrosion resistance can be obtained. The dissolved barium-containing fluorine compound is contained in excess in solid form and consumed as the chromate treatment progresses, and is constantly replenished by the successive dissolution of solids to maintain a saturated concentration, that is, a constant concentration. It is possible. Therefore, as long as a barium-containing fluorine compound is used as an etching agent and is added to the chromate treatment solution in excess of its saturation level, there is no need for the complicated work of controlling the concentration of the etching agent in the treatment solution, and sometimes saturation occurs. It is sufficient to replenish to maintain the above surplus. The chromate treatment solution used in the present invention is obtained by adding and adjusting a barium-containing fluorine compound to an aqueous solution of chromic anhydride so that each component has a predetermined concentration. The plated steel plate to be treated in the present invention is a plated steel plate obtained by hot-dipping or electroplating with zinc, aluminum, etc.
In addition to galvanized steel sheets that are simply galvanized, galvanized steel sheets that have been subjected to alloying treatment by further heating at a high temperature after galvanizing are also used as galvanized steel sheets (in the present invention, the above-mentioned treatment target Each steel plate is collectively called a plated steel plate). To apply the surface treatment to a plated steel plate using the chromate treatment liquid used in the present invention, follow the commonly practiced method. After degreasing the plated steel plate as necessary and washing it with water to clean the surface, apply an appropriate treatment. The surface is treated by dipping or spraying at a temperature of 50 to 70°C for 5 to 10 seconds, and if necessary, remove excess treatment liquid that has adhered to the steel plate with a rubber roll. You can remove it and dry it. The appropriate amount of chromate film to be deposited is 10 to 50 mg/m ² in terms of chromium. The present invention will be further explained below with reference to Examples and Comparative Examples. Examples 1 to 8, Comparative Examples 1 to 7 Various plated steel plates with a thickness of 0.4 mm were subjected to continuous chromate treatment by dipping or spraying using a treatment solution having the composition shown in Table 1 and treatment conditions. After rolling and drying, a chromate-treated steel plate was obtained. A test piece was taken from this treated steel plate, and tested for (1) coloration, (2) corrosion resistance, and (3) coating performance, as will be explained in detail later. The test method for each test item of the chromate-treated test piece is as follows. (1) Coloring condition The coloring condition of the surface of the treated steel sheet after chromate treatment was observed with the naked eye. (2) Corrosion resistance As shown below, a salt spray test was conducted on test pieces under three conditions in accordance with JIS Z 2371 (salt spray test method), and the state of white rust formation was observed with the naked eye. (a) Flat part: The flat test piece was subjected to the test as it was, and the salt water spray test was conducted for 180 hours. (b) Erichsen extrusion section: A 4 mm test piece was extruded using an Erichsen extrusion tester and subjected to a salt spray test for 130 hours. (c) Bent part: A test piece bent at 180° was subjected to a salt spray test for 50 hours. (3) Paint film performance Acrylic paint was applied to the test piece and heated to 350℃.
It was baked and dried for 35 seconds to form a coating film with a thickness of 20 μm. About this painted test piece JIS G 3312
According to the test method of (Colored Galvanized Iron Sheets), (a) a cross-over test, (b) a Dupont impact test, and (c) a 500-hour salt spray test were conducted, and the condition after the test was observed with the naked eye.

【table】

[Table] The observation results for each of the above items (1), (2), and (3) are evaluated in four stages as shown in Table 2, and the symbols indicating each stage (◎, 〇, △, and x).

[Table] As can be seen from Table 3, the chromate-treated steel sheets treated with the chromate treatment method according to the present invention have better coloring, corrosion resistance in various parts, and coating film performance than those treated with the conventional chromate treatment method. It is excellent in both respects. Furthermore, as can be seen from Comparative Example 3, when the amount of chromic anhydride is less than 10 g per treatment solution, the corrosion resistance effect is insufficient.

[Table] Examples 9 to 11, Comparative Examples 8 and 9 Galvanized steel sheets with a thickness of 0.4 mm and a width of 20 cm were continuously surface-treated by a spray method using a treatment solution with a composition shown in Table 4 and under treatment conditions. . And each example,
The amount of processing solution used in the comparative examples was 20 in each case, and the processing solution was not renewed. However, the amount of processing solution taken out to the processing plate and the etching agent were replenished to maintain the initial concentration, and the processing was continued continuously. When performing chromate treatment, the pH of the treatment solution is the condition that makes it difficult for the chromate reaction to occur and for the formation of a chromate coating to occur.
The degree of aging was compared based on the amount of processing of each plated steel plate until the temperature reached 3.0. The results are shown in Table 4.

【table】

[Table] In Table 4, in each of the examples, the aging of the treatment solution is slow, the life is extended, and the amount of steel plate processed is large. And Example 9
When barium-containing fluorine compounds are added in excess, as in ~11, it is only necessary to add them occasionally to maintain the excess state during processing, and controlling the concentration requires less effort than in other cases. Nakatsuta. The chromate treatment method according to the present invention uses a treatment solution containing chromic anhydride as the main component and a barium-containing fluorine compound as an etching agent, so that the chromate film is strong and has excellent corrosion resistance, paint film adhesion, etc. Furthermore, the aging of the treatment solution is slow and has a long life, and the process can be easily controlled, making it possible to supply surface-treated galvanized steel sheets with excellent performance easily and at low cost, which is of industrial benefit. It's large.

[Brief explanation of the drawing]

The figure is a graph showing the effect of barium silicate, an example of the barium-containing fluorine compound used in the present invention, in making the chromate film less soluble in comparison with that of other fluorine compounds.

Claims (1)

[Scope of Claims] 1. 10 to 50 g of chromic anhydride (CrO ₃ ) and a fluorine compound consisting of only barium ions as cations and fluorine ions or fluorine complex ions as anions, expressed as fluorine element. 0.04g/
A method for surface treatment of a plated steel sheet, characterized by subjecting the plated steel sheet to surface treatment with a chromate treatment solution containing each of the above. 2. The method for chromate treatment of a plated steel sheet according to claim 1, which uses a chromate treatment solution in which the fluorine compound is _BaF2 . 3. The method for chromate treatment of a plated steel sheet according to claim 1, which uses a chromate treatment solution in which the fluorine compound is BaSiF ₆ . 4. A chromate treatment method for a plated steel sheet according to claim 1, which uses a chromate treatment solution in which the fluorine compound is BaZrF ₆ . 5. A chromate treatment method for a plated steel sheet according to claim 1, which uses a chromate treatment solution in which the fluorine compound is _BaTiF6 . 6 Fluorine compounds expressed as fluorine element 0.04
The method for chromate treatment of a plated steel sheet according to any one of claims 1 to 5, which uses a chromate treatment solution containing a chromate treatment solution having a concentration of 1 g/g/g/ or more and a saturation concentration or more.