JPS6128751B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention mainly relates to a pretreatment method for improving the corrosion resistance, paintability, etc. of an alloyed galvanized steel sheet made of an iron-zinc alloy. More specifically, the present invention relates to a method for pre-treating galvannealed steel sheets for shipment as products by manufacturers of galvannealed steel sheets. In the present invention, an alloyed galvanized steel sheet is first subjected to an etching chromate treatment, and then an organic aqueous solution containing hexavalent chromium ions or hexavalent and trivalent chromium ions is applied to coat the alloyed galvanized steel sheet in two layers. It forms a film. The average composition of the plating layer of alloyed galvanized steel sheets is 5 to 20% iron and 80% zinc.
~95%, aluminum 0-1%, and lead,
Contains trace amounts of cadmium, tin and other unavoidable mixtures. Alloyed galvanized steel sheet is produced by heating the steel strip or steel sheet pulled up from the molten zinc bath using gas or electric means such as induction heating or resistance heating in a hot-dip galvanizing equipment. It can be obtained by carrying out an alloying reaction with the layer. When compared with normal hot-dip galvanized steel sheets whose plating layer mainly consists of zinc, the characteristics of alloyed galvanized steel sheets are: (1) The true surface area is large relative to the apparent unit surface area. (2) Since the chemical activity of the plating layer is lower than that of zinc alone, it has good corrosion resistance and is relatively less likely to react with organic acid groups in the paint resin to form metal soap. (3) Excellent weldability. That's a thing. Taking advantage of these excellent features, the fields of use of alloyed galvanized steel sheets are gradually expanding. Alloyed galvanized steel sheets are usually painted and put into practical use, but if they are shipped with chromate treatment, oils such as press oil and anti-corrosion oil are usually deposited on the steel sheet surface during the forming process. These adhered oils must be removed (degreased) in the pre-painting process. The degreasing agent most commonly used industrially has caustic soda, caustic potash, and their silicates, phosphates, etc. as the main ingredients, and a small amount of surfactant may be added to it, but the main point is When these alkaline degreasing liquids come into contact with an alloyed galvanized steel sheet that has been subjected to chromate treatment during the degreasing process, it dissolves a portion of the chromate film on the surface.
As a result, the corrosion resistance as a so-called bare board without painting, the corrosion resistance as a base for painting, and the painting performance deteriorate. To solve this problem, usually a chromate treatment is performed again after degreasing, or a phosphate treatment is performed. Alloyed galvanized steel sheets that are not subjected to chromate treatment are shipped as is or coated with anti-corrosion oil, and even if they are painted immediately after being degreased for practical use, they exhibit considerable paint adhesion and paint corrosion resistance. However, phosphate treatment is usually used to obtain higher coating performance. The present invention relates to simplifying (or omitting) the pre-painting process for consumers of such alloyed galvanized steel sheets, and to a pre-treatment method on the manufacturer's side to ensure even higher painting performance. It is something. On the consumer side, there is a trend to omit as much pre-painting treatment process as possible in order to save processes and prevent pollution, and there is a movement to promote the phosphate treatment process as an actual work. In addition, during the painting process, in areas that are physically difficult to paint due to the shape, such as the inside of the bag structure and the parts where the steel plates meet, which are not visible to the naked eye, it is necessary to give the steel plate surface considerable corrosion resistance before painting. is desirable. Alternatively, even if it is easy to paint, areas where it is desired to omit painting due to cost considerations or areas where the thickness of the coating film should be reduced should not be painted.In other words, if the corrosion resistance of the bare board is sufficiently ensured, it may be left unpainted or with a low coating thickness. In some cases, thickening the coating film may be sufficient. The present invention advantageously solves the above problems,
Its performance characteristics are that it can be painted immediately by the customer by degreasing and washing with water (hot water washing), and omitting the phosphate treatment process; It shows the same or better performance than galvanized steel sheets, and it shows particularly excellent performance for melamine alkyd paints.
In addition, the corrosion resistance of the bare board (unpainted board) also shows good performance in the salt spray test specified in JIS Z-2371, so it may be sufficient for long-term use without painting depending on the practical application. The technical point of the present invention is to use an alloyed galvanized steel plate as the material, and in the first stage of chromate treatment, an etching chromate treatment suitable for the alloyed galvanized iron plate is applied, and excess chromate solution is removed. After that, a non-etching chromate treatment solution containing a water-soluble organic polymer or the like is immediately applied, followed by drying according to a conventional method. Next, the composition of the processing liquid related to the present invention will be described. The main components of the etching chromate treatment solution are (1) chromic acid or dichromate or their salts, (2) etching agents such as fluorides, and (3) inorganic acids such as phosphoric acid and sulfuric acid. and less than 1.5
The pH value is more preferably 1 or less.
If the pH value exceeds 1.5, the etching ability of the surface to be treated decreases, making it difficult to obtain a chromate film that firmly adheres to the substrate and exhibits good properties. The second stage treatment liquid is mainly composed of (1) chromic acid or dichromic acid or their salts, (2) water-soluble organic substances, and (3) inorganic acids for pH adjustment, and has a pH of 1 or more. Preferably it is 1.5 or more. The chromic acid, dichromic acid, or their salts in the second-stage treatment liquid become hexavalent or partially trivalent chromium ions in the treatment liquid, and have both hexavalent and trivalent ion ions from the beginning. Even if it is added in a form with
There is no harm in devising ways to make it more affordable. Water-soluble organic substances include polyacrylic acid, polyvinyl alcohol,
Hydroxyethyl ether of cellulose, copolymers of styrene and maleic anhydride are suitable;
As a reducing agent for valent chromium ions, pyrogallol,
Aldehydes are preferred. Note that it is not inconvenient even if the reduction residue co-precipitates in the film. In addition, phosphoric acid, boric acid, etc. can be used as inorganic acids for pH adjustment. If too much phosphoric acid is used, it can act as an etching agent for alloyed galvanized steel sheets and impair the stability of water-soluble organic polymers, etc. Therefore, it is necessary to keep the concentration to the minimum necessary for pH adjustment. It is necessary that the pH of the liquid is 1 or more. FIG. 4 shows a flow sheet of the treatment method according to the present invention. The strip 3 leaving the alloying furnace 1 of the galvanizing line is treated in a surface treatment section 2.
FIG. 5 is a detailed view of the surface treatment section 2. In the first stage chemical conversion treatment, the strip 3 is coated with an etching chromate treatment liquid by a spray 4, and the excess treatment liquid remaining on the strip 2 is removed by a rubber roll 5. After being removed, in the second stage chemical conversion treatment, it is applied with a spray 6 of a non-etching chromate treatment solution containing a water-soluble organic polymer, etc.
Subsequently, it is squeezed with a roll 7 and dried with hot air in a drying device 8. In this treatment method, the two types of chromate treatment solutions do not mix with each other in the treatment tank, and therefore troubles caused by agglomeration and precipitation of organic polymers can be avoided. If the second stage chromate treatment is performed immediately after the first stage treatment by simply squeezing the strip without drying it, the water-soluble organic polymer will agglomerate on the strip; Since it becomes part of the chromate film, there is no problem at all. The present invention is a continuous galvanizing process that is carried out continuously in the plating process at the outlet side of a continuous hot-dip galvanizing apparatus such as the Sendzimer type, non-oxidation furnace type, or Wheeling type, that is, the part after the hot-dip galvanizing part. However, it is the most advantageous in terms of performance such as corrosion resistance and paintability, and manufacturing cost. If continuous in-line processing is not possible due to equipment reasons, the first stage of etching chromate treatment is carried out in-line, and the alloyed galvanized steel sheet obtained is separated into strips or cut plates. The second stage treatment, that is, application of an organic aqueous solution containing hexavalent chromium or hexavalent and trivalent chromium may be performed using the above equipment. In this case, in order to form the second-stage treated film uniformly and with good adhesion on the first-stage treated film, it takes about 48 hours after the first-stage treatment is completed, depending on the atmospheric conditions. It is necessary to perform the second stage of processing within the next few days. The most desirable application condition of the present invention is inline 1
This means that the processing in the first and second stages is performed continuously. It is preferable to carry out the first chromate treatment and then carry out the second step when the chromate treatment solution is not in a fluid state on the alloyed galvanized steel sheet or is in a semi-dry state. That is, it is important to carry out the second stage treatment in a state where the first stage chromate film retains sufficient crystal water. As mentioned above, it is possible to dry the chromate film after the first stage treatment and then perform the second stage treatment, but if the interval is longer than 48 hours, use salt water as shown in Table 1. The second non-etching chromate treatment should be performed within 48 hours after the first etchable chromate treatment as a reduction in spray corrosion resistance may be observed. Alloyed galvanized steel sheets have a porous surface with a large surface area due to the alloy formation process, and are suitable for storing a large amount of chromate treatment solution. However, since the surface is mainly composed of an iron-zinc alloy, it has less reactivity than zinc, so applying a chromate treatment solution with weak etching properties, such as chromic acid alone with a pH of 1.5 or higher, will cause the plating substrate to deteriorate. It is difficult to form a healthy chromate film that firmly adheres to the surface. To solve this difficulty, for example,
Like the chromate treatment solution described in No. 25625,
For example, it is necessary to use a strong etching agent such as phosphoric acid, nitric acid, hydrochloric acid, or fluoride. When chromate treatment solutions containing these strong etching agents coexist with water-soluble organic polymers, they can significantly inhibit the stability of the water-soluble organic polymers, cause undesirable reactions such as oxidative polymerization, and degrade the water-soluble organic polymers. In severe cases, the solution may become cloudy or the resin may precipitate. The present invention solves this problem by applying an organic aqueous solution containing chromate and chromate after chromate treatment, and this method greatly expands the selection range of water-soluble organic substances.
As will be described later, the range of effects expected from water-soluble resins has been expanded. Table 1 shows the treatments of the present invention, including etching chromate treatment alone, non-etching chromate treatment containing a water-soluble organic polymer alone, and etching chromate treatment followed by non-etching chromate treatment containing a water-soluble organic polymer, etc. The results of tests on corrosion resistance and paint adhesion of alloyed galvanized iron sheets are shown below. As is clear from Table 1, the combined effect of the two-stage treatment can be clearly seen.

[Table] By applying an organic aqueous solution containing hexavalent or hexavalent and trivalent chromium after chromate treatment, the negative effects of uneven coating of chromate treatment (variations in the amount of chromate deposited) can be corrected, and the corrosion resistance can be improved. Since a flexible resin film is formed, deterioration in corrosion resistance due to processing such as bending, drawing, and overhanging can be reduced. Furthermore, this resin film suppresses the deterioration of the underlying chromate film due to thermal effects (mainly due to the dehydration reaction of the chromate film). , is very small. In addition, this resin film has sufficient resistance to degreasing with an alkaline solution to protect the underlying chromate film, as shown in Figure 2, and as shown in Figure 3, corrosion resistance deteriorates due to alkaline degreasing. It is smaller than the chromate treatment method. In addition, paint adhesion can be significantly improved by selecting the properties of the resin film that match the compatibility with the paint applied on top of it, which has been a problem with conventional alloyed galvanized steel sheets. It was also possible to dramatically improve the adhesion with melamine alkyd type paints. It should be noted that simply applying an aqueous solution of hexavalent chromium or an organic polymer containing hexavalent and trivalent chromium directly to an alloyed galvanized steel sheet without performing the first (first stage) chromate treatment will not result in the above-mentioned problems. As alloyed galvanized steel sheets have poor reactivity compared to zinc, it is difficult to form a solid chromate film that firmly adheres to the plating base, and therefore corrosion resistance and other properties are insufficient. The present invention involves a two-stage treatment in which a chromate treatment is performed in the first step, for example, according to Japanese Patent Publication No. 44-26525, and then a water-soluble organic polymer containing hexavalent or hexavalent and trivalent chromium is applied in the second step. This has succeeded in bringing out the superior performance of step-by-step processing.
That is, in the present invention, by performing a two-stage treatment, a strong and robust chromate film is reacted in advance, and then a film containing an organic material containing hexavalent or hexavalent and trivalent chromium is overlapped to improve corrosion resistance. , which has dramatically improved performance such as paint adhesion. The amount of chromate film in the first stage is suitably 5 to 200 mg/m ² as the amount of chromium in the film. Film amount is 5
If the amount is less than mg/ ^m2 , the effect of the first stage chromate treatment will be lost, and even if it exceeds 200mg/ ^m2 , the improvement in corrosion resistance of the coating can be almost ignored compared to the coating amount of 200mg/ ^m2 . However, no effect on performance can be expected. Rather, it is technically difficult to uniformly coat the surface of an alloyed galvanized steel sheet with a coating amount of 200mg/m2 ^{or more} in one step, and it is easy to cause variations in corrosion resistance and chromate stains due to variations in the chromate coating thickness, making it impractical. It turned out that it wasn't. The amount of chromium in the second stage consisting of organic substances containing hexavalent or hexavalent and trivalent chromium is appropriate for the amount of chromium in the film, ^and the reason for this is that the chromate treatment in the first stage Same as in case. The advantages of the present invention are as described in detail, but in addition, by performing the chromate treatment in two stages, it is easy to produce a treated film with a uniform film thickness, good film surface appearance, and a large amount of chromium deposited. It also has the characteristics that can be obtained. Next, the composition of the processing bath related to the present invention will be described. First, regarding the etching chromate treatment bath composition, during the film formation stage, dissolution or oxidation of zinc and iron occurs at the anode portion of the surface of the treated material.
At the cathode, hexavalent chromium ions are reduced, and a chromate film mainly composed of trivalent and hexavalent chromium is formed on the surface. Alloyed galvanized steel sheet Since the chemical activity is lower than that of galvanized steel sheet, phosphoric acid, sulfuric acid, Types of nitric acid, hydrochloric acid, hydrofluoric acid, etc., and their salts (complex salts), singly or in combination, are used as hexavalent chromium,
For example, it is added to a solution mainly consisting of chromic anhydride. The chromate treatment bath composition applied to the first chromate treatment step of the present invention is also similar to these,
For example, the treatment bath and treatment conditions described in Japanese Patent Publication No. 44-26525 can be applied as is. That is, for example, CrO ₃ 15g/, K ₂ TiF ₆ 4g/, H ₃ PO ₄ 10c.c./
Also suitable are chromate solutions consisting of: Next, for an organic aqueous solution containing hexavalent chromium or hexavalent and trivalent chromium, the chromium concentration (in terms of Cr amount) is preferably in the range of 0.5 to 30 g/. If the amount is less than 0.5 g, there will be no effect of adding chromium, and if it is more than 30 g, the plate to be treated will be significantly colored and the stability of the solution will be impaired. As a chromium source, chromic anhydride, chromic acid sulfate, nitrate, ammonium salt, sodium or potassium chromate, dichromate, etc. are suitable, and when stability in the presence of organic matter is particularly important. Ammonium salts, dichromates, etc. are most suitable. Suitable water-soluble organic substances include polyacrylic acid, polyvinyl alcohol, cellulose hydroxyethyl ether, copolymers of styrene and maleic anhydride, and aldehydes, pyrogallol, etc. are suitable as reducing agents for hexavalent chromium ions, and their addition ratios A suitable amount is 0.5 to 30 g/. 30g/
If it exceeds 100%, it is difficult to uniformly apply the solution to the surface to be treated, and no specific improvement in performance can be observed. Furthermore, since the blocking phenomenon of the applied film becomes significant, the drying equipment becomes enormous and is not practical. As an organic aqueous solution composition containing hexavalent chromium and trivalent chromium, for example, Alodine NR-2 (trade name, manufactured by Nippon Paint Co., Ltd.) is commercially available and can be used as is in the present invention. By the way, Alodine NR
-2 is an aqueous solution of a water-soluble organic substance with a pH of 1.7 to 2.0, in which a portion of chromic acid is reduced to trivalent chromic acid with pyrogallol, and cellulose hydroxyethyl ether is added thereto. The treatment bath components used in the present invention have been described above, but the point of the present invention is not in specifying individual chemical compositions, but in determining the most suitable chemical composition for alloyed galvanized steel sheets without any constraints in the first stage. A strong chromate treatment is performed, and then in the second stage, the most appropriate organic aqueous solution is selected without any constraints imposed by the first stage treatment bath composition, and treated with a treatment bath to which a small amount of chromium compound is added. As a result, the individual effects of the first and second stage processing as well as their synergistic effects have been realized. Next, in a specific method of the present invention, the first-stage chromate treatment solution is applied onto an alloyed galvanized steel plate (steel strip) by dipping or spraying, and the excess solution is squeezed out using a roll or air knife. Alternatively, it may be applied using a roll heater. The steel plate and/or the treatment liquid are preferably heated to 40°C to 80°C.
After the film is dried after application, or after the film formation reaction has progressed sufficiently even if it is wet, a second stage organic aqueous solution containing hexavalent or hexavalent and trivalent chromium is applied by dipping or spraying, and then applied with a roll or air knife. The excess liquid may be squeezed out or applied using a roll coater. The appropriate bath temperature is room temperature to 60°C. 60℃
Exceeding this may cause problems with long-term continuous use in terms of bath stability. Although there is no particular limitation on the temperature on the low temperature side, a higher bath temperature is advantageous in order to facilitate the subsequent drying step. Please dry after application. Although the surface treatment method for alloyed galvanized steel sheets has been described in detail above, the present invention can of course also be applied to galvanized steel sheets in which the plating layer is mainly made of zinc.
In that case, since the surface of the galvanized steel sheet is more reactive to the chromate treatment solution than the galvanized steel sheet, a known chromate treatment solution suitable for the galvanized steel sheet that does not contain strong etching agents, such as anhydrous Chromic acid single bath, chromic acid anhydride ~
A fluoride bath may be applied to the first stage chromate treatment, and then, as described above, an organic aqueous solution containing hexavalent or hexavalent and trivalent chromium may be applied. Examples according to the present invention are shown below. Example 1 An alloyed galvanized steel sheet whose main components are Fe9.5~10%, Zn90~90.5%, and Al 0.25% was coated with chromic anhydride 15g/, K ₂ TiF ₆ 4g/, H ₃ PO ₄ 10
The sample was immersed in an aqueous solution of cc/ (bath temperature 50°C) for about 0.5 seconds, squeezed with a rubber roll to remove excess chromate treatment solution, and dried with hot air at 60°C to 65°C. (The amount of chromium in the chromate film at this stage was 73.6 mg/ ^m2 ) Next, an organic aqueous solution containing hexavalent and trivalent chromium was prepared using Allozin NR-2 (trade name,
(manufactured by Nippon Paint Co., Ltd.) was dropped onto the treated surface, excess liquid was removed with a rubber roll, and the product was dried with hot air at 60 to 65°C and used for testing. Example 2 Chromate treatment of galvanized steel sheet
After immersing for about 1 second in an aqueous solution (bath temperature 50°C) consisting of CrO ₃ : 16 g/, Na ₂ SiF ₆ : 1 g/, excess chromate treatment solution was squeezed out with a rubber roll, and then 60 to 65
Dry with hot air at ℃. Next, Allozin NR-2 shown in Example 1 was treated under the same conditions and subjected to a test. The amount of chromium deposited during the first stage treatment was 65.5mg/
It was ^m2 . Example 3 After the chromate treatment in Example 1, excess chromate treatment solution was removed using a rubber roll, and then Allozin NR-2 was applied without drying with a dryer. The amount of chromium deposited during the first stage treatment was 23.6 mg/m ²
It was hot. Example 4 The first stage chromate treatment was carried out under the same conditions as in Example 1, including drying. The amount of chromium deposited at this stage was 70.0 mg/m ² . Next, as an organic aqueous solution, polyacrylic acid 1
g/, containing 2 g/ of chromic anhydride partially reduced with formalin, and phosphoric acid with sufficient stirring.
Spray an aqueous solution adjusted to pH 2.0 (ratio of hexavalent chromium to total chromium in this solution: 69% by weight) to the treated surface, remove excess liquid with a rubber roll, and then
The samples were dried with hot air at ~65°C and used for testing. Example 5 Chromate treatment and drying were carried out under the same conditions as in Example 1. The amount of chromium deposited at this stage was 8.5 mg/m ² . Next, as a water-soluble organic polymer, an aqueous solution containing 20 g of polyvinyl alcohol and 10 g of achromic acid partially reduced with formalin and adjusted to pH 2.5 with phosphoric acid with sufficient stirring (accounting for the total chromium in this solution) The treated surface was spray-coated with 80% by weight of hexavalent chromium, and the excess liquid was removed using a rubber roll. Example 6 Chromate treatment and drying were performed under the same conditions as in Example 1. The amount of chromium deposited at this stage is 56.5mg/m ²
It was hot. Next, an aqueous solution containing 0.5 g of hydroxyethyl cellulose as a water-soluble organic polymer and 1 g of chromic anhydride partially reduced with formalin and adjusted to pH 2.0 with phosphoric acid with sufficient stirring (total chromium in this solution) was added. The treated surface was spray-coated with 50% by weight of hexavalent chromium, the excess liquid was removed with a rubber roll, and then dried with hot air at 60 to 65° C., and then tested. Example 7 An alloyed galvanized steel sheet whose main components are Fe9.5~10%, Zn90~95%, and Al 0.25% was treated with chromic anhydride 15g/, Na ₂ SiF ₆ 4g/, H ₃ PO ₄ 10c.c./
After being immersed in an aqueous solution (bath temperature: 60°C) for about 1 second, the sample was squeezed with a rubber roll to remove excess chromate treatment solution, and then dried with hot air at 60 to 65°C. The amount of chromium deposited at this stage was 103.2 mg/m ² . Next, as water-soluble organic polymers, polyacrylic acid 0.5g/, polyvinyl alcohol 1.5g/, hydroxyethyl cellulose 0.3g/,
(NH ₄ ) ₂ Cr ₂ O ₇ 20g/, Cr(NO ₃ ) ₃・9H ₂ O3g/
An aqueous solution containing 100% phosphoric acid and adjusted to pH 2.8 with phosphoric acid was sprayed onto the treated surface with sufficient stirring, excess liquid was removed with a rubber roll, and the solution was dried with hot air at 60 to 65°C and used for testing. . Comparative Example The alloyed galvanized steel sheet shown in Example 1 was subjected to only the first chromate treatment under the same conditions. Test method The test material was degreased in advance by spraying with Ridrin #75.1 at 60°C for 1 minute and then subjected to the following test. Bare corrosion resistance test: In a salt spray test based on JIS Z-2371, evaluation is performed based on the 5 area % white rust generation time. Paint adhesion test: Apply 25μ of acrylic paint or melamine paint and place the painted board in warm water at 40℃.
After soaking for 144 hours, perform the following tests. Goban Erichsen: Goban 2m/m Erichsen 6m/m Overhang Impact: 1Kg x 40cm 1/2″ Bending: After physical property test of 4T or more, cellophane tape is peeled off. 1 if the entire paint film is peeled off, and there is no damage to the paint film at all. of
10, and a 10-level classification evaluation was performed. Paint corrosion resistance test: After applying 25μ of each acrylic paint or melamine paint and making cross cuts that reach the substrate on the painted board, conduct a salt spray test in accordance with JIS Z-2371 for 240 hours, and then remove the cross cuts with cellophane tape. do. Evaluation was performed on a 10-point scale, with 1 being complete peeling of the paint film and 10 being no damage to the paint film at all. The test results are shown in Table 2. As described above, excellent results were obtained using the method of the present invention.

[Table] Note that even when the present invention is applied to galvanized steel sheets that are not subjected to alloying treatment, excellent results in corrosion resistance and paintability can be obtained, so that the present invention is included within the scope of application of the idea of the present invention.

[Brief explanation of the drawing]

Fig. 1 is an explanatory chart showing the relationship between heat treatment and corrosion resistance, Fig. 2 is an explanatory chart showing chromium elution due to degreasing, Fig. 3 is an explanatory chart showing changes in corrosion resistance due to degreasing, and Fig. 4 is an explanatory chart showing the relationship between heat treatment and corrosion resistance. FIG. 5, which is an explanatory diagram of the apparatus for carrying out the method of the invention, is a detailed view of the surface treatment section shown in FIG. 4.

Claims (1)

[Claims] 1 Alloyed galvanized steel sheet with Cr content of 5 to 200
mg/m ² of an etching chromate treatment solution with a pH of 1.5 or less, and then a solution in which a part of hexavalent chromium is reduced to trivalent chromium using pyrogallol, one or two of aldehydes, or a solution of hexavalent chromium A solution in which an aqueous solution of a water-soluble organic substance of one or more of polyacrylic acid, polyvinyl alcohol, cellulose hydroxyethyl ether, and a copolymer of styrene and maleic anhydride is added to a chromic acid solution is Cr.
A method for surface treatment of an alloyed galvanized steel sheet, characterized in that the coating is applied in an amount of 5 to 200 mg/ ^m2 .