JP2005169765A - Coated zn-al alloy plated steel sheet excellent in corrosion resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a coated Zn-Al alloy plated steel sheet excellent in corrosion resistance and coating film adhesion without relying on a chromate film, a Cr type rustproof pigment or the like. <P>SOLUTION: The coated Zn-Al alloy plated steel sheet is constituted by forming an undercoating film 4 compounded with a magnesium salt and/or a phosphate on a Zn-Al alloy plating layer 2 through a chemical forming film 3 containing a titanium compound and/or a zirconium compound. As the titanium compound or the zirconium compound, at least one of a compound selected from hexafluorotitanic acid, hexafluorozirconic acid and metal salts of them is used. The chemical forming film may be an organic-inorganic composite film containing an organic resin such as a phenol resin, an acrylic resin, an acrylolefin resin, a polyurethane resin and the like. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a coated Zn—Al-based alloy-plated steel sheet that exhibits excellent corrosion resistance even without containing a chromium compound with a large environmental load.

  Conventionally, hot-dip galvanized steel sheets with good corrosion resistance have been used as coating raw materials for exterior materials, interior materials, and exterior materials. However, with the progress of air pollution, acidification of the atmosphere and rainwater due to sulfur oxides, nitrogen oxides, etc. has been remarkable in recent years. As a result, corrosion of the hot dip galvanized layer is promoted, and there is a concern about durability as an interior building material, an exterior building material, and the like. For example, the corrosion resistance of the flat portion is such that corrosive ions such as Cl ions penetrate the coating film to promote corrosion of the hot dip galvanized layer, and the coating film is pushed up by the volume-expanded zinc-based corrosion product. As observed.

As a material exhibiting corrosion resistance superior to that of a hot dip galvanized steel sheet, a ratio of using a hot dip Zn-Al alloy-plated steel sheet as a coating original sheet is increasing. In the hot-dip Zn—Al-based alloy-plated steel sheet, when the Al content of the hot-dip coating layer is increased, the corrosion resistance of the flat part and the part with the coating film wrinkles is improved. However, even if the Al content is increased, the corrosion resistance of the processed part and the cut end face is not always satisfied. For example, when a coated hot-melt Zn-55% Al-plated steel sheet is bent, cracks occur in the Zn-Al alloy plating layer with poor ductility, and the underlying steel exposed through the cracks is likely to be the starting point of corrosion.
Corrosion starting from a defective plating layer is suppressed by forming a chemical conversion film having a self-repairing action on the surface of the plating layer. A chemical conversion film with a self-repairing action is typically formed by chromate treatment using the oxidation-reduction reaction of hexavalent Cr → trivalent Cr, but there is a concern about the elution of hexavalent Cr harmful to the environment in the chromate treatment. Therefore, it tends to be replaced with a Cr-free chemical film. The present applicant has developed a Cr-free chemical conversion film containing a fluoride of valve metal such as Ti and Zr (Patent Documents 1 and 2). The valve metal fluoride contained in the chemical conversion film elutes into a corrosive atmosphere and then re-deposits as a poorly soluble compound, thereby self-repairing the defective portion.

As a plated steel sheet having excellent corrosion resistance, a Zn-6% Al-3% Mg alloy plated steel sheet is also used. The coated steel sheet obtained by applying Cr-free coating to the plated steel sheet exhibits excellent corrosion resistance even in the processed part and the cut end face (Patent Documents 3 and 4). As a result of investigation and research by the present inventors, it has been found that excellent corrosion resistance is derived from a dense and poorly soluble Zn—Mg based corrosion product that covers the underlying exposed portion such as a plating layer defect portion or a cut end surface.
JP 2002-30458 A JP 2002-38280 A JP 2002-69668 Gazette JP 2002-187234 A

  The present invention is based on the premise that a Zn-Al based alloy plating layer is covered with a Cr-free chemical conversion film, and has developed a knowledge obtained with a coated Zn-6% Al-3% Mg alloy plated steel sheet and has a corrosion inhibiting function. By replenishing Mg, which is necessary for the production of Zn-Mg corrosion products, from the undercoat, high corrosion resistance comparable to that of the coated Zn-6% Al-3% Mg alloy-plated steel sheet even when the plating layer does not contain Mg An object of the present invention is to provide a coated Zn—Al-based alloy-plated steel sheet to which is added.

The coated Zn-Al alloy-plated steel sheet of the present invention contains magnesium salt and phosphate on the Zn-Al alloy plating layer with a chemical film containing a titanium compound and / or a zirconium compound and fluoride. The prepared undercoat film is laminated.
As the titanium compound and the zirconium compound, one or more selected from hexafluorotitanic acid, hexafluorozirconic acid, and metal acid salts thereof are used. The chemical conversion film may be an organic-inorganic composite film containing an organic resin such as a phenol resin, an acrylic resin, an acrylic olefin resin, or a polyurethane resin. The magnesium salt contained in the undercoat film is one or more of magnesium hydrogen phosphate, magnesium phosphate, and tripolymagnesium phosphate, and the phosphate is zinc phosphate, aluminum trihydrogen phosphate, aluminum phosphate, calcium phosphate 1 type (s) or 2 or more types are used.

  The coated Zn—Al alloy-plated steel sheet according to the present invention has an undercoat film 4 and a topcoat film 5 formed on the surface of the Zn—Al-based alloy plated layer 2 on the base steel 1 with a chemical conversion film 3 interposed therebetween. It has a laminated surface structure (FIG. 1). The chemical conversion film 3 includes a titanium compound and / or a zirconium compound and a fluoride, and the undercoat film 4 includes a magnesium salt and a phosphate. Although the surface structure improves the corrosion resistance of the processed part and the adhesion of the coating film, the influence of the surface structure on the improvement of properties is presumed as follows, and is supported by the examples described later.

  The titanium compound and the zirconium compound contained in the chemical conversion film 3 react with the phosphate in the undercoat film 4 to improve the adhesion of the undercoat film 4 to the chemical film 3. The fluoride contained in the chemical conversion film 3 exhibits the action of promoting the reaction between the titanium compound, zirconium compound and phosphate. A part of the phosphate is also deposited on the surface of the plating layer 2 exposed from the defect portion of the chemical conversion film 3 to passivate the surface of the plating layer 2. As a result, an undercoat coating film 4 excellent in adhesion and water resistance is formed, and selective corrosion of coating film swelling and Zn-rich phase is suppressed.

  Moreover, since the chemical conversion film 3 is formed by application | coating of the fluoride containing chemical conversion treatment liquid which has an etching effect | action, Al concentrated on the surface layer of the plating layer 2 is ionized and eluted by the acid component of a chemical conversion treatment liquid. When an organic-inorganic composite film is formed as the chemical conversion film 3, Al ions eluted from the plating layer 2 are combined with an organic resin together with the titanium compound and the zirconium compound in the chemical conversion treatment liquid. As a result, an organic-inorganic composite film excellent in poor solubility and adhesion is formed.

  When the coated Zn—Al alloy-plated steel sheet is subjected to processing such as bending and deep drawing, the Zn—Al alloy plating layer 2 having poor ductility cannot follow the plastic deformation of the base steel 1 and is likely to crack. Although the base steel 1 is exposed and easily corroded in the cracked portion, the corrosion of the base steel exposed portion is also suppressed by the coating composition of the chemical conversion coating 3 and the undercoat coating 4. The corrosion suppression of the exposed portion of the base steel is presumed as follows, and the corrosion of the cut end face and the like is also suppressed by the same mechanism.

  When a Zn—Al-based alloy-plated steel sheet in which a crack 6 is generated by bending or the like is exposed to a corrosive atmosphere, corrosive ions 7 enter the crack 6 toward the exposed surface of the base steel 1 (FIG. 2). ). Since Mg contained in the undercoat film 4 has a high ionization tendency, it preferentially reacts with the corrosive ions 7 that have entered the cracks 6 and is eluted as Mg ions. The titanium compound and the zirconium compound contained in the chemical conversion film 3 promote the elution reaction of Mg. With the elution reaction of Mg, elution of phosphate ions and Zn ions proceeds, and the exposed surface of the base steel 1 is covered with a Zn-rich hardly soluble corrosion product 8 containing Mg and P.

The fracture surface of the plating layer 2 facing the crack 6 also acts similarly to Mg ions and phosphate ions coupled with the selective corrosion of Zn, and Al, Zn from the plating layer 2 and Mg, P from the undercoat film 4. It is covered with a hardly soluble corrosion product 9 containing
Since the hardly soluble corrosion products 8 and 9 are formed as a dense film, it functions as a barrier layer for suppressing subsequent corrosion. Even after the barrier layer is formed, Mg ions and phosphate ions, which are effective in inhibiting corrosion of the exposed surface of the base steel 1 facing the crack 6 and the fracture surface of the plating layer 2, are constantly supplied, and the defect is self-repaired. Is done.

  Reaction of the phosphate in the undercoat film 4 with the titanium compound and zirconium compound in the chemical conversion film can also be expected to suppress corrosion. The compound formed by the reaction of the phosphate with the titanium compound and the zirconium compound improves the adhesion of the undercoat film 4 to the chemical conversion film 3, and corrosive ions penetrate into the stretched part during bending. As a result, the lifting of the coating film caused by the coating film swelling and the corrosion product of the Zn-rich phase is suppressed.

Examples of the coating original plate include Zn-Al alloy-plated steel plates such as Zn-5% Al and Zn-55% Al. The Zn-Al-Mg alloy-plated steel sheet can also be expected to suppress corrosion by a similar mechanism, but there is a Zn-Al-Mg-based alloy plating layer as a source of Mg-containing corrosion products that function as a barrier layer. It is not necessary to add a magnesium salt to the undercoat coating 4.
Prior to the formation of the chemical conversion film 3, the coating original plate is cleaned by degreasing, pickling, and surface adjustment. When a chemical conversion treatment liquid is applied to the cleaned plating layer 2 and dried without washing with water, a chemical conversion film 3 having a self-repairing action is formed.

The Cr-free chemical conversion treatment liquid includes a treatment liquid containing an etching action titanium compound, fluoride, and organic resin developed by the present applicant (Japanese Patent Laid-Open No. 2002-38280), as well as an etching action titanium compound, zirconium. There are treatment liquids containing compounds and fluorides (JP 2002-30458 A). For example, an aqueous solution of aminomethyl-substituted polyvinylphenol in which fluorides such as hexafluorotitanic acid (H ₂ TiF ₆ ), hexafluorozirconic acid (H ₂ ZrF ₆ ), and metal acid salts thereof are dissolved in propoxypropanol (organic resin) Prepared as.

When an organic-inorganic composite film containing a titanium compound, fluoride, and phenol resin is formed as a Cr-free film, the titanium compound is 1-100 mg / m ^{2 in} terms of Ti and fluoride is 1-200 mg in terms of F. / M ² is preferably adjusted in the range. The titanium compound reacts with metal ions such as Al and Zn eluted from the surface of the hot-plated layer of the coating original plate to form a chemical conversion film having excellent corrosion resistance. In a system including an organic resin, metal ions such as a titanium compound, a zirconium compound, Al, and Zn react with the organic resin to form a hardly soluble organic-inorganic composite film. Hereinafter, although an organic-inorganic composite film will be described as an example, a chemical film containing no organic resin can improve the corrosion resistance, coating film adhesion and the like by the same mechanism.

  If the amount of the titanium compound is too small, the performance of the organic-inorganic composite film is inferior, and excellent coating adhesion and corrosion resistance cannot be obtained. On the other hand, an excess amount of the titanium compound saturates the performance improvement effect of the organic-inorganic composite film, and on the contrary, the workability after coating and the coating film adhesion also deteriorate. Too much titanium compound is not preferable from the chemical conversion treatment cost. Fluoride dissociates into fluorine ions in the chemical conversion treatment solution, and exhibits an action of etching the surface of the hot-dip plating layer together with the acid component in the chemical conversion treatment solution in a state where it is in contact with the surface of the coating original plate. When the amount of fluorine ions is small, etching on the surface of the hot-dip plating layer is insufficient, and the adhesion of the organic-inorganic composite film to the surface of the hot-dip plating layer is lowered. On the other hand, if the amount of fluorine ions is too large, an excessive amount of eluted metal is taken into the coating, the organic-inorganic composite coating becomes brittle, and the adhesion of the organic-inorganic composite coating to the hot-dip coating layer decreases.

The organic-inorganic composite film may contain a zirconium compound in an amount of 0.1 to 30 mg / m ^{2 in} terms of the amount of Zr conversion instead of the titanium compound. The zirconium compound exhibits the same action as the titanium compound, and reacts with metal ions such as Al and Zn eluted from the hot dipped layer together with the organic resin to form a hardly soluble organic-inorganic composite film. If the adhesion amount of the zirconium compound is small, the effect on adhesion and corrosion resistance is not sufficient. However, if the zirconium compound is excessive, the workability after coating and the adhesion of the coating film are lowered, and the chemical conversion treatment cost is increased.

  After the chemical conversion treatment, an undercoat coating film is provided on the Cr-free chemical conversion film. The coating composition for undercoating is prepared by blending non-chromium rust preventive pigments such as magnesium salts and phosphates based on epoxy, epoxy-urethane, polyester, acrylic, epoxy-modified polyester, phenoxy, etc. The Magnesium salt rust preventive pigments include magnesium hydrogen phosphate, magnesium phosphate, and magnesium tripolyphosphate. Phosphate rust preventive pigments include zinc phosphate, aluminum trihydrogen phosphate, aluminum phosphate, and calcium phosphate. is there.

  The non-chromium rust preventive pigment is preferably added at a ratio of 2 to 50% by mass (preferably 5 to 40% by mass) with respect to the non-volatile content of the paint. Although the rust preventive effect is seen at an addition amount of 2% by mass or more, when an excessive amount of non-chromium rust preventive pigment exceeding 50% by mass is added, the workability after coating and coating film adhesion may be lowered. In addition to non-chromium anticorrosive pigments, the undercoat paint requires various additives such as colored pigments such as titanium oxide, extender pigments such as silica, calcium carbonate, and barium sulfate, organic beads, organic resin powder, and inorganic aggregates. Depending on the formulation. The molecular weight of the base resin, the glass transition temperature, the pigment, the blending amount of the aggregate, and the like are appropriately adjusted according to the application of the coated steel sheet.

The undercoat coating film is preferably formed with a film thickness of 3 μm or more for concealing the base, adhesion of the coating film, and corrosion resistance. However, in the case of a thick film exceeding 20 μm, not only the paint consumption is increased, but coating film peeling is likely to occur during the processing of the coated steel sheet.
A top coat film of preferably 10 to 300 μm is provided on the undercoat paint. For top coating, it is based on thermosetting or thermoplastic resin such as polyester, urethane, acrylic, silicone-modified polyester, silicone acrylic, polyvinyl chloride, polyvinylidene fluoride-acrylic, and as necessary, colored pigment, extender pigment, organic Various additives such as mineral aggregates, inorganic aggregates, metallic powders, lubricants, antifouling agents, fungicides, UV absorbers, light stabilizers (antioxidants), photocatalyst particles, matting agents, etc. A coating composition is used. One or more of magnesium salts and phosphate-based anticorrosive pigments may be added to the coating composition in the same manner as the undercoat coating.

When the top coat is applied and baked with a roll coater or the like, the top coat is laminated on the undercoat. Instead of the top coat film, an upper resin film may be formed by pasting a resin film.
In this way, when a hot-dip plated steel sheet with a Zn-Al alloy plating layer is used as a coating original plate and a non-chromium undercoat is formed on the surface of the coating original plate via an organic-inorganic composite coating, Coat adhesion and workability comparable to a coated steel sheet with a chromium-based primer coating on the chromate coating. And since it does not contain chromium, it is used in various fields as an environmentally friendly material.

The present invention will be described in detail with reference to an example in which a Zn-55% Al alloy-plated steel sheet having a thickness of 0.5 mm and a coating amount per side of 73 g / m ² is used as a coating original sheet. Similar results are obtained even when other Zn-Al alloy-plated steel sheets such as Zn-6% Al-3% Mg, etc. or Zn-Al-Mg alloy-plated steel sheets are used.
[Surface adjustment]
After spraying an alkaline degreasing aqueous solution (Surf Cleaner 155: manufactured by Nippon Paint Co., Ltd.) adjusted to a liquid temperature of 65 ° C. on a Zn-55% A1 alloy-plated steel sheet for 5 seconds, it is washed with hot water and water. Dried.

[Chemical conversion treatment]
Application of a coating-type Cr-free chemical conversion treatment solution containing hexafluorotitanic acid: 55 g / l, hexafluorozirconic acid: 10 g / l, aminomethyl-substituted polyvinylphenol: 72 g / l to a plated layer surface after surface adjustment And dried at 100 ° C. without washing with water. When the surface of the plated layer after drying is analyzed, the amount of Ti converted deposit: 10 mg / m ² of titanium compound, the amount of Zr converted deposit: 2.5 mg / m ² of zirconium compound, the amount of F converted deposit: 20 mg / m ² fluoride , Polyvinylphenol equivalent adhesion amount: An organic-inorganic composite film containing an organic component of 40 mg / m ² was formed.

For comparison, a coating-type Cr-free chemical conversion treatment solution containing a titanium compound, a water-dispersible silica not containing a zirconium compound, and a phenol resin and having a temperature of 20 ° C. is applied to the surface of the plated layer after the surface adjustment, and 100 ° C. without washing with water. When dried, an organic-inorganic composite film containing a compound having an Si equivalent adhesion amount of 70 mg / m ² was formed. Moreover, when a coating type chromate treatment liquid (Surfcoat NR-C300NS: manufactured by Nippon Paint Co., Ltd.) was applied with a roll coater and dried at 100 ° C. without washing with water, the total Cr equivalent adhesion amount was 40 mg / m ² . A chromate film was formed.

[Undercoating]
Based on epoxy resin, in addition to rust preventive pigment, undercoat paint containing titanium oxide (colored pigment), barium sulfate (external pigment), silica powder (external pigment) is applied to the coated original plate after chemical conversion treatment, and 215 ° C An undercoat film having a dry film thickness of 5 μm was formed by drying and baking.
A Cr-free undercoating compound containing magnesium hydrogen phosphate, magnesium phosphate, zinc phosphate, and aluminum tripolyphosphate was applied to the original coating provided with an organic-inorganic composite coating containing a titanium compound, zirconium compound and organic components. .

  For comparison, a modified silica-blended Cr-free undercoating was applied to a coating original plate provided with the same organic-inorganic composite film. In addition, as a comparison of Cr-free systems, Cr containing magnesium hydrogen phosphate, magnesium phosphate, zinc phosphate, and aluminum tripolyphosphate in a coating original plate on which an organic-silica composite film containing no titanium compound or zirconium compound was formed. Free undercoat paint was applied. As a comparison with chromium, a chromate-based undercoating compound containing strontium chromate was applied to a coating original plate on which a chromate film was formed. Table 1 shows the types and amounts of the rust preventive pigments contained in the undercoat coating film.

[Top coating]
Next, a top coating material based on a polyester resin was applied, and a top coating film having a dry film thickness of 15 μm was laminated on the bottom coating film by drying and baking at 215 ° C.
Test pieces were cut out from each of the prepared coated steel sheets and subjected to a coating adhesion test, an accelerated corrosion test, and an outdoor exposure corrosion test.

[Coating adhesion test]
The surface of the coating film to be evaluated was set on the outside, 2t contact bending was performed in a room adjusted to 20 ° C., and the peeling state of the coating film was observed by applying / peeling the adhesive tape to the bent portion. The coating film which did not peel is ◎, the coating film where 5% or less peeling occurred is O, the coating film where 5-20% peeling occurred is Δ, the coating film where peeling occurs 21% or more is × Adhesion was evaluated.
In the as-processed state, cracks in the plating layer were propagated to the undercoat and topcoat films in any coating composition, and cracks in the coating film were visually recognized in the processed part. Did not occur.

[Accelerated corrosion test]
After holding the upper coating film surface of the coated steel sheet on the outside and bending it for 4t, a test piece was prepared in which the left and right, lower cut end surfaces and the back surface were repaired with paint. 150 cycles acid rain combined corrosion test [1 cycle: spraying with 0.1% NaC1 corrosive solution (35 ° C. × 1 hour, pH adjustment with sulfuric acid) → drying (50 ° C. × 4 hours) → wetting (50 ° C. × 3 hours, (Relative humidity 98%)], and the bent portion was observed to investigate the occurrence of white rust. Calculate the white rust occurrence rate (area%) with respect to the area of the test object, ◎ for the test piece without white rust, ◯ for white rust occurrence area ratio of 5 area% or less, △ 5-15 area% , 15 area% or more was evaluated as x, and the corrosion resistance of the bent portion was evaluated.
Moreover, the undercoat corrosion of the plating layer generated in the bent portion was observed with an optical microscope, and the ratio (area%) of the undercoat corrosion to the plating layer cross section of the test target portion was calculated. Corrosion resistance under coating is defined as ◎ for specimens where the plating layer of the bent part is not corroded, ○ for a corrosion rate under the coating film of 3 area% or less, △ for 3 to 5 area%, or × for 5 area% or more. evaluated.

[Outdoor exposure corrosion test]
Exposure test angle: In the 35 ° outdoor exposure corrosion test, contrary to the accelerated corrosion resistance test, the corrosion of the lower part where rainwater containing corrosive flying objects tends to accumulate is accelerated. Therefore, after holding the lower coating surface of the coated steel plate outside and bending it for 2 tons, a test piece was prepared in which the left and right and upper cut end surfaces and the back surface were repaired with paint. After an outdoor exposure test for 6 months at an exposure test site located about 5m inland from Ichikawa City, Chiba Prefecture, we observed the bending process and investigated the occurrence of white rust. Calculate the white rust occurrence rate (area%) with respect to the area of the test object, the white rust occurrence rate is 5 area% or less ◎, 6-10 area% ○, 11-20 area% △, 20 area% or more The corrosion resistance of the bent portion was evaluated with x.
Moreover, the ratio (area%) of undercoat corrosion was calculated from the undercoat corrosion of the plating layer generated in the bent portion, and the undercoat corrosion resistance was evaluated according to the same criteria as the accelerated corrosion test.

  As can be seen from the test results in Table 2, in test numbers 1 to 5 (examples of the present invention), Cr-free conversion coatings containing titanium compounds, zirconium compounds and fluorides were made of Cr-free rust prevention such as magnesium salts and phosphates. When combined with an undercoat film containing a pigment, processed part corrosion resistance comparable to that of the conventional chromate test number 8 (Comparative Example) was obtained in both the accelerated corrosion test and the outdoor exposure corrosion test. On the other hand, even when the same Cr-free chemical conversion treatment solution was used, Test No. 6 (Comparative Example) provided with an undercoat film in which modified silica was blended as a rust preventive pigment had insufficient white rust resistance. Moreover, in test number 7 (comparative example) which provided the silica and the organic resin type chemical conversion film, it was inferior to both coating-film adhesiveness and a process part corrosion resistance.

  In the above example, the case where an undercoat paint is provided through a chemical conversion film containing both a titanium compound and a zirconium compound together with fluoride has been described. However, a chemical conversion in which either a titanium compound or a zirconium compound coexists with fluoride. In the same manner, excellent corrosion resistance and adhesion to the coating film were obtained. Moreover, even when a chemical conversion film not containing an organic resin was formed instead of the organic-inorganic composite film, a coated Zn—Al-based alloy-plated steel sheet excellent in corrosion resistance and coating film adhesion was produced.

  As explained above, by combining a chemical conversion film containing a titanium compound and / or a zirconium compound and a fluoride with an undercoat film containing a magnesium salt and a phosphate, Zn-Mg having a corrosion-inhibiting action. The corrosion product becomes a dense barrier layer, and the exposed surface of the base steel and the fracture surface of the plating layer are covered. As a result, even if cracks occur in the plating layer when processing into a product shape, the exposed surface of the base steel is suppressed from becoming a starting point of corrosion, and the original excellent corrosion resistance of the Zn-Al alloy plating layer is maintained for a long time. Since it is maintained and the adhesion of the coating film is improved, it is used in a wide range of fields such as exterior materials, interior materials, and cover materials that are exposed to severe corrosive environments. Moreover, since neither the chemical conversion film nor the coating film contains a chromium compound, it is a material suitable for a tendency where environmental conservation is important.

The schematic diagram which shows the layer structure of the coating Zn-Al type alloy plating steel plate surface layer according to this invention Diagram explaining that corrosion of the exposed surface of the base steel facing the crack and the fracture surface of the plating layer is suppressed

Explanation of symbols

1: Base steel 2: Plating layer 3: Chemical conversion film 4: Undercoat film 5: Topcoat film 6: Crack 7: Corrosive ions 8, 9: Slightly soluble corrosion product

Claims (5)

  An undercoat film containing magnesium salt and phosphate is laminated on a Zn-Al alloy plating layer through a chemical film containing a titanium compound and / or a zirconium compound and a fluoride. A coated Zn-Al alloy-plated steel sheet with excellent corrosion resistance.   The coated Zn-Al-based alloy-plated steel sheet according to claim 1, wherein the titanium compound and / or the zirconium compound is one or more selected from hexafluorotitanic acid, hexafluorozirconic acid and metal acid salts thereof.   The coated Zn-Al alloy-plated steel sheet according to claim 1, wherein the chemical conversion film is an organic-inorganic composite film containing one or more organic resins selected from phenol resin, acrylic resin, acrylic olefin resin, and polyurethane resin. .   The coated Zn-Al alloy-plated steel sheet according to claim 1, wherein the magnesium salt is one or more of magnesium hydrogen phosphate, magnesium phosphate, and magnesium tripolyphosphate.   The coated Zn-Al alloy-plated steel sheet according to claim 1, wherein the phosphate is one or more of zinc phosphate, aluminum trihydrogenphosphate, aluminum phosphate, and calcium phosphate.

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