JP2006116736A - Coated stainless steel sheet excellent in corrosion resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a coated stainless steel sheet which produces no red rust even in a severe corrosive environment and is useful as a facing material, an interior material, a surface decorative material and the like. <P>SOLUTION: The coated stainless steel sheet has an undercoating film containing a magnesium salt and a phosphate provided thereto through a chemical forming film wherein a titanium and/or zirconium compound and a fluoride coexist. As the titanium and/or zirconium compound, hexafluorotitanate, hexafluorozirconate or a metal salt of each of them is used. At least one of magnesium hydrogen phosphate, magnesium phosphate and magnesium tripolyphosphate is used as the magnesium salt and at least one of zinc phosphate, aluminum dihydrogen tripolyphosphate, aluminum phosphate and calcium phosphate is used as the phosphate. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a coated stainless steel sheet that maintains excellent corrosion resistance even at a bent portion or a cut end face where a coating film defect is likely to occur and does not contain a chromium compound having 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 recently. 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.

In order to obtain corrosion resistance superior to that of hot-dip galvanized steel sheets, extensive studies are being conducted on the coating original sheet, pre-coating treatment, coating composition, and the like.
With regard to the coating original sheet, hot-dip Zn-Al alloy-plated steel sheet, hot-dip Zn-Al-Mg alloy-plated steel sheet, hot-dip aluminum-plated steel sheet, etc., which have excellent corrosion resistance at the flat part and the coating wrinkled part, have begun to be used. The corrosion resistance of the 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. Even in the hot-dip aluminum-plated steel sheet, cracks generated in the Fe—Al—Si alloy layer having poor ductility tend to propagate to the Al—Si plated layer and become a starting point of corrosion.

Corrosion starting from a defective portion of the plating layer is suppressed by a coating pretreatment that forms 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 chromium-free chemical conversion film. The present applicant has developed a chromium-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.
JP 2002-30458 A JP 2002-38280 A

In particular, when a hot-dip Zn—Al—Mg alloy-plated steel sheet is used as a coating base plate and chromium-free coating is applied, excellent corrosion resistance is exhibited even in the processed part and the cut end face. The excellent corrosion resistance is considered to be derived from dense and poorly soluble Mg-based corrosion products that cover the exposed underlying parts such as plating layer defects and cut end faces, and Mg required for the production of corrosion products having a corrosion-inhibiting function. Even if it replenishes from the undercoat coating film, the corrosion resistance of the processed part and the cut end face is similarly improved. The present inventors have proposed a coated Zn—Al-based alloy-plated steel sheet (Patent Document 3) and a painted aluminum-plated steel sheet (Patent Document 4) in which a magnesium salt is blended in the undercoat film from such a viewpoint.
Japanese Patent Application No. 2003-411501 Japanese Patent Application No. 2004-75704

  The present inventors further investigated and examined the phenomenon in which excellent corrosion resistance was exhibited in a system in which the titanium and / or zirconium compound of the chemical conversion film and the magnesium salt of the undercoat film coexisted. As a result, it was found that the corrosion resistance improvement effect derived from the coexistence of titanium and / or zirconium compound and magnesium salt is effective not only for zinc-based and aluminum-based plated steel plates, but also for stainless steel plates as the coating base plate. It was. In addition, it has been found that when stainless steel plate is used as a coating original plate, crevice corrosion peculiar to stainless steel is also prevented.

  The present invention has been completed on the basis of such knowledge, and by combining a chemical conversion film containing titanium and / or a zirconium compound with an undercoat film containing magnesium salt and phosphate, a processed part or a cut end face can be obtained. It aims at providing the coated stainless steel plate which improved corrosion resistance and also suppressed crevice corrosion.

The coated stainless steel sheet of the present invention uses a stainless steel sheet as a coating original sheet, and is provided with an undercoat film in which magnesium salt and phosphate are blended via a chemical conversion film containing a titanium compound and / or a zirconium compound and fluoride. It has been.
As the titanium compound and / or the zirconium compound, one or more selected from hexafluorotitanic acid, hexafluorozirconic acid, and metal salts thereof are used.

As the magnesium salt, one or more of magnesium hydrogen phosphate, magnesium phosphate, and magnesium tripolyphosphate are used. As the phosphate, one or more of zinc phosphate, aluminum trihydrogenphosphate, aluminum phosphate, and calcium phosphate are used.
The chemical conversion film may be an organic / inorganic composite film. As the organic resin of the composite film, one or more selected from phenol resin, acrylic resin, acrylic olefin resin, and polyurethane resin are used.

  In the coated stainless steel plate of the present invention, a chemical conversion film containing a titanium compound and / or a zirconium compound and a fluoride is formed on the surface of the underlying stainless steel plate. The chemical conversion film adheres firmly to the underlying stainless steel plate and is difficult to peel off from the stainless steel plate even by bending. Even in the case of peeling, the film defect portion is repaired by the self-repairing action, and the environmental barrier ability to protect the underlying stainless steel plate from the corrosive atmosphere is maintained.

And since magnesium salt and phosphate are mix | blended with the undercoat coating film, Mg and phosphoric acid are supplied to the chemical conversion film or stainless steel plate surface exposed as a result of a coating-film crack, and a protective film is reproduced | regenerated. Therefore, the invasion of corrosive ions to the chemical conversion film or the stainless steel plate surface through the cracked portion of the coating film is eliminated, and corrosion under the coating film and crevice corrosion are prevented.
By the way, when a coating film is provided on the surface of a stainless steel plate via a silica-based film, the adhesion of the silica-based film is inferior, so the coating film tends to float from the surface of the stainless steel plate in the processed part, and in some cases a phenomenon that is close to peeling or dropping off May occur. When corrosive ions enter the bent portion in this state, crevice corrosion of the stainless steel plate proceeds at the floating portion of the chemical conversion film. Since the corrosive ions once invaded hardly flow out of the system from the floating portion of the chemical conversion film, the concentration of the corrosive ions is increased in the gap portion and the crevice corrosion is accelerated.

  A combination of a chemical conversion film containing a titanium compound and / or a zirconium compound and a fluoride and an undercoat film containing a magnesium salt or a phosphate is also effective in protecting the cut end surface of the coated stainless steel sheet from corrosion. End face corrosion is caused by the fact that the cut end face not covered with the chemical conversion film or the undercoat film is exposed to a corrosive atmosphere. In the coated stainless steel sheet of the present invention, magnesium salt and phosphoric acid eluted from the undercoat film. By reprecipitation of the salt on the cut end face, a protective film having a corrosion inhibiting function is formed on the cut end face, and the corrosion resistance of the cut end face is improved. Moreover, since the adhesion of the chemical conversion film to the stainless steel plate is high, the invasion of corrosive ions to the stainless steel plate / chemical conversion film interface is surely suppressed, and there is no lifting of the undercoat film starting from the cut end surface. Crevice corrosion near the end face is also prevented.

There are no particular restrictions on the type of steel used for coating, and various stainless steels such as ferritic, austenitic, martensitic, and two-phase systems that have been used as exterior building materials, interior building materials, and various structural materials. Steel plate can be used.
The stainless steel plate is subjected to degreasing, pickling, surface adjustment, cleaning, etc. prior to chemical conversion treatment. When a chemical conversion treatment liquid is applied to the surface of the cleaned stainless steel plate and dried without washing with water, a chemical conversion film having a self-repairing action and excellent adhesion is formed.

As the chemical conversion treatment solution, a chromium-free chemical conversion treatment solution that does not contain chromium with a large environmental load is used. As this type of chemical conversion treatment liquid, a treatment liquid containing an etching titanium compound, a fluoride, and an organic resin (Patent Document 2), and similarly a treatment liquid containing an etching action titanium compound, a zirconium compound, and a fluoride ( Patent Document 1) and the like. 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 chromium-free film, the titanium compound is 1-100 mg / m ^{2 in} terms of Ti, and the 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 Cr, Ni, and Mo eluted from the surface of the stainless steel plate 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 titanium compound, zirconium compound, Cr, Ni, and Mo react with the organic resin to form a hardly soluble organic-inorganic composite film. Hereinafter, an organic-inorganic composite coating will be described as an example. However, even with a chemical conversion coating not containing an organic resin, the corrosion resistance, coating adhesion, etc. are improved 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. Conversely, an excessive amount of titanium compound saturates the performance improvement effect of the organic-inorganic composite film, and on the contrary, the workability and paint film adhesion after coating are also lowered. Too much titanium compound is not preferable from the chemical conversion treatment cost. Fluoride dissociates into fluorine ions in the chemical conversion solution, and when in contact with the surface of the stainless steel surface, the surface of the stainless steel is etched together with the acid component in the chemical conversion solution to improve the passive film to high corrosion resistance. Present. When there are few fluorine ions, etching will be insufficient, and the adhesion of the organic-inorganic composite film to the stainless steel surface will decrease. On the other hand, if too much fluorine ions are used, 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 stainless steel surface 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 an organic resin together with metal ions such as Cr, Ni, and Mo eluted from the stainless steel surface 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 film is provided on the chromium-free chemical conversion film. The coating composition for undercoating is prepared by blending non-chromium rust preventive pigments such as magnesium salts and phosphates with epoxy, epoxy-urethane, polyester, acrylic, epoxy-modified polyester, phenoxy, etc. as the base resin. The Magnesium salt and phosphate react with the titanium compound and / or zirconium compound in the chemical conversion film at the coating defects, forming a sparingly soluble film, and the cutting end face is removed from the corrosive atmosphere by elution and reprecipitation. A protective film for blocking is formed.

Magnesium salt rust preventive pigments include magnesium hydrogen phosphate, magnesium phosphate, magnesium tripolyphosphate, etc., and phosphate rust preventive pigments include zinc phosphate, aluminum trihydrogen phosphate, aluminum phosphate, calcium phosphate, etc. 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 an antirust effect is obtained with an addition amount of 2% by mass or more, if an excessive amount of non-chromium anticorrosion pigment exceeding 50% by mass is added, workability and coating film adhesion after coating may be deteriorated. 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 non-chromium undercoat is formed on the surface of a stainless steel plate via an organic-inorganic composite film, it is comparable to a coated stainless steel plate provided with a chromium-based undercoat on a conventional chromate film. Film adhesion and processability are obtained. And since it does not contain chromium, it is used in various fields as an environmentally friendly material.

Thickness: 0.4 mm SUS304 stainless steel 2D finish material is used to illustrate the present invention in an example, but the same applies to stainless steel plates of other steel types and different finishes. Of course, it applies.
[Surface adjustment]
The stainless steel plate was sprayed with an alkaline degreasing aqueous solution (Surf Cleaner 1089N-1: manufactured by Nippon Paint Co., Ltd.) at a liquid temperature of 65 ° C. for 5 seconds, washed with hot water and washed with water and dried. Further, a phosphate aqueous solution (Surfdyne ZS9100: manufactured by Nippon Paint Co., Ltd.) having a liquid temperature of 60 ° C. was sprayed and contacted for 5 seconds, followed by washing with hot water and washing with water and drying.

[Chemical conversion treatment]
Applying a coating-type chromium-free chemical conversion treatment solution containing hexafluorotitanic acid: 55 g / l, hexafluorozirconic acid: 10 g / l, aminomethyl-substituted polyvinylphenol: 72 g / l to the surface of the stainless steel after the surface adjustment. And dried at 100 ° C. without washing with water. Analyzing the dried stainless steel surface, Ti equivalent deposition amount: 8 mg / m ² titanium compound, Zr equivalent deposition amount: 2 mg / m ² zirconium compound, F equivalent deposition amount: 16 mg / m ² fluoride, polyvinyl Phenol equivalent adhesion amount: An organic-inorganic composite film containing an organic component of 32 mg / m ² was formed.

For comparison, an application-type chromium-free chemical conversion treatment solution containing a titanium compound, a water-dispersible silica not containing a zirconium compound, and a phenol resin at a temperature of 20 ° C. is applied to the surface of the stainless steel after the surface adjustment and washed at 100 ° C. without washing. 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 NRC300NS: 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 coating amount: 25 mg / m ² 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.
Chromium-free undercoat paint containing magnesium hydrogen phosphate, magnesium phosphate, zinc phosphate, and aluminum tripolyphosphate on the coating original plate provided with an organic-inorganic composite coating containing titanium compounds, zirconium compounds, fluorides, and organic components Was applied.

  For comparison, a chromium-free undercoat paint containing modified silica was applied to a coating original plate provided with the same organic-inorganic composite film. In addition, as a comparison of chromium-free systems, chromium containing magnesium hydrogen phosphate, magnesium phosphate, zinc phosphate and aluminum tripolyphosphate is added to the original coating on which an organic-silica composite film that does not contain titanium and zirconium compounds is 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, an overcoat paint based on a silicone-modified polyester resin was applied, and an overcoat film having a dry film thickness of 15 μm was laminated on the undercoat film by drying and baking at 230 ° C.
A test piece is cut out from each of the prepared coated steel sheets, the coating surface to be evaluated is set to the outside and bent for 2 tons in a room at 20 ° C, and then subjected to coating adhesion test, accelerated corrosion test, outdoor exposure corrosion test. Provided.

[Coating adhesion test]
A test piece of 50 mm × 50 mm was bent, and the peeling state of the coating film was observed by sticking / peeling the adhesive tape on the bent part. 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, the cracks in the conversion coating were propagated to the top coat and the cracks were observed in the processed part in any coating composition, but abnormalities such as peeling occurred in the coating itself. Did not occur.

[Salt spray test: CASS]
In the salt spray test, a corrosive solution prepared by adding 0.26 g / l of copper chloride to 5% NaCl aqueous solution and adjusting the pH to 3.1 to 3.3 with acetic acid was used, and the corrosive solution was 70 mm × 170 mm in an atmosphere of 49 ° C. After continuously spraying the test piece, the bent part and the cut end face were observed after 480 hours.
[Composite cycle corrosion test: CCT]
90 cycles of corrosion test with [5% NaC1 corrosion solution spray (35 ° C x 1 hour, pH 7) → drying (50 ° C x 4 hours) → wet (50 ° C x 3 hours, relative humidity 98%)]] After repeating, the bent part and the cut end face were observed.

[Outdoor exposure corrosion test]
In the outdoor exposure corrosion test, contrary to the accelerated corrosion test, the corrosion of the lower part where rainwater containing corrosive flying objects easily collects is accelerated. Therefore, a test piece of 100 mm × 200 mm prepared by bending the 2t bend while holding the lower coating film surface of the coated stainless steel plate on the outside was prepared. After an outdoor exposure test for one month at an exposure test site located about 5 m inland from Ichikawa City, Chiba Prefecture, the bent part and cut end face were observed.

  In any of the salt spray test, the combined cycle corrosion test, and the outdoor exposure test, the test piece in which red rust did not occur on the surface of the test piece was marked with ◎, the area ratio of the generated red rust was 10% or less, and 10 to 20%. The test piece in which red rust occurred at an area ratio exceeding Δ and 20% was evaluated as x, and the corrosion resistance of the bent portion and the cut end surface was evaluated.

  As can be seen from the test results in Table 2, a chromium-free chemical conversion film containing a titanium compound, a zirconium compound and a fluoride is combined with an undercoat film containing a chromium-free rust preventive pigment such as magnesium salt or phosphate. In Nos. 1 to 5 (examples of the present invention), processed part corrosion resistance and end face corrosion resistance comparable to conventional chromate test number 8 (comparative example) are obtained in any of the salt spray test, combined cycle corrosion test, and outdoor exposure test. It was.

  In test number 6 (comparative example) provided with an undercoat film in which modified silica was blended as a rust preventive pigment even when the same chromium-free chemical conversion treatment solution was used, the corrosion resistance was insufficient and red rust generation on the cut end face was remarkable. there were. Moreover, in test number 7 (comparative example) which provided the silica and the organic resin type | system | group 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 in place of the organic-inorganic composite film, a coated stainless steel sheet excellent in corrosion resistance and coating film adhesion was produced.

  As described 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, Mg, Ti having a corrosion inhibiting function , Zr-based corrosion products become a dense barrier layer, and coat defects and cut end surfaces are covered. As a result, even if cracks or delamination occur in the coating film when processed into a product shape, the exposed surface and cut end surface of the base steel are suppressed from becoming a starting point of corrosion, and the excellent corrosion resistance inherent to the stainless steel sheet can be maintained for a long time. The crevice corrosion peculiar to stainless steel is also suppressed. Since the adhesion of the coating film is also improved by the chemical conversion film, 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.

Claims (5)

  Painted stainless steel sheet with excellent corrosion resistance, characterized in that an undercoat film containing magnesium salt and phosphate is formed on a stainless steel surface through a chemical conversion film containing a titanium compound and / or a zirconium compound and fluoride. .   The coated stainless 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 salts thereof.   The coated stainless 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 a phenol resin, an acrylic resin, an acrylic olefin resin, and a polyurethane resin.   The coated stainless 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 stainless steel sheet according to claim 1, wherein the phosphate is one or more of zinc phosphate, aluminum trihydrogenphosphate, aluminum phosphate, and calcium phosphate.