JP5225466B2 - Chromate-free precoated steel plate with excellent red rust corrosion resistance - Google Patents

Description

  The present invention relates to a chromate-free precoated steel sheet having excellent red rust corrosion resistance on an end face. More specifically, the present invention is a chromate-free precoated steel sheet that can be used in fields such as electrical appliances and architecture, which can be suitably used in an environment that gets wet with water (especially, an outdoor environment exposed to rainwater, etc.). Further, the present invention relates to a chromate-free precoated steel sheet having excellent red rust corrosion resistance on the end face.

  Pre-coated steel sheets (also referred to as “PCM”) are steel sheets that are pre-formed and shipped by customers for the products they require for their products, which can save them from painting and related work. In addition to being able to do so, facilities for such work are no longer needed, and its use is expanding in various fields. In early pre-coated steel sheets, chromate-treated steel sheets subjected to rust-proofing with chromate were used as the base steel sheets to be coated. After that, due to the toxicity problem of hexavalent chromium that may be eluted from the chromate-treated film, a chromate-free pre-coated steel sheet that uses a base steel sheet that has been treated with a non-chromium anti-rust treatment instead of a chromate-treated pre-coated steel sheet will attract attention. In recent years, its use has increased particularly. In conventional chromate-free precoated steel sheets, rust preventive pigments (generally, calcium exchange silica (sometimes called calcium silicate), aluminum tripolyphosphate, etc.) contained in the coating film are responsible for rust prevention. .

  As the mainstream of pre-coated steel sheets shifts from chromate-treated pre-coated steel sheets to chromate-free pre-coated steel sheets, pre-coated steel sheets used in outdoor environments exposed to rainwater, etc., generate red rust on the end faces, deteriorating the aesthetics of products using pre-coated steel sheets. Such problems have emerged suddenly. This problem does not occur in chromate-treated pre-coated steel sheets, and there is a need for a solution as a problem peculiar to chromate-free pre-coated steel sheets.

Several conventional techniques are known for improving the red rust corrosion resistance of the end face of the chromate-free precoated steel sheet.
For example, the coating film of the outermost layer formed on the plated steel sheet having the Zn-containing plating layer has a pH of 7 or more and 12 or less when dissolved in (A) ion-exchanged water at a concentration of 0.1% by mass, (B) Non-chromium compound that satisfies the condition that the electric conductivity when dissolved in ion exchange water at a concentration of 0.1% by mass is 500 μS / cm or more and (C) does not cause thermal decomposition by 200 ° C. Chromate-free pre-coated steel sheet that can prevent the occurrence of red rust caused by the influence of water with low electrical conductivity without using chromate-based anti-rust pigments by containing sodium tripolyphosphate) Known (Patent Document 1).

  Moreover, the water when the outermost coating film formed on the plated steel sheet having the Zn-containing plating layer is dissolved in (A) ion-exchanged water (electric conductivity: 4 μS / cm or less) at a concentration of 0.1% by mass. A non-chromium compound (for example, alkali metal phosphate, chloride, alkaline earth metal hypochlorous acid) satisfying the condition that the electric conductivity of the material is 500 μS / cm or more and (B) no thermal decomposition occurs until 200 ° C. A chromate-free pre-coated steel sheet capable of suppressing the occurrence of red rust on the end face by containing a chlorate) is known (Patent Document 2).

  Furthermore, the undercoat and / or the topcoat on the non-chromium-based undercoat of the metal plate are made of porous inorganic fine particles (for example, porous silica fine particles) and metal compound-based anticorrosive pigment (for example, Mg or Rust prevention of paint film containing 100 squares of 1 mm × 1 mm when immersed in a 40 ° C. carbonate pH standard solution so as to contain sustained release rust preventive fine particles combined with Ca compound) There is known a chromate-free pre-coated metal plate that defines the elution rate of metal ions eluted from a pigment and has excellent end face corrosion resistance (Patent Document 3).

  In addition, it has a coating film containing alkaline earth metal compound particles, zinc compound particles, and phosphate particles, and alkaline earth metal, zinc, and elution from the coating film when immersed in pure water at room temperature There is also known a chromate-free pre-coated metal plate that can suppress the occurrence of red rust on the end face by defining the molar ratio of the elution amount of the phosphorus component (Patent Document 4).

JP 2008-229518 A JP 2009-045923 A JP 2009-077845 A JP 2008-189965 A

  The present invention promotes the expansion of the use of chromate-free pre-coated steel sheets that do not dissolve harmful chromium, so that it can be used in outdoor environments exposed to rainwater, etc., without being bothered by the occurrence of red rust that impairs aesthetics. The purpose is to provide a chromate-free pre-coated steel sheet with excellent resistance to red rust on the end face.

  The chromate-free pre-coated steel sheet having excellent red rust corrosion resistance according to the present invention is a chromate-free pre-coated steel sheet having a chromate-free chemical conversion treatment layer and one or more coating films formed thereon on both sides of a zinc-based plated steel sheet. And the outermost coating film of at least the back surface of the precoated steel sheet contains a water-soluble organic compound having a —C (COOH) (OH) atomic group in the molecular structure. Features.

  The typical chromate-free precoated steel sheet of the present invention has a primer coating layer and an upper coating layer on the chromate-free chemical conversion treatment layer on each side, and the upper coating layer on the back side contains the organic compound. Contains.

  In addition to the upper coating layer on the back surface, the primer coating layer on the back surface may contain the organic compound.

  Preferred organic compounds are glycolic acid, lactic acid, malic acid, tartaric acid, 2-hydroxybutyric acid, citric acid, isocitric acid lactone, citramalic acid, ascorbic acid, or mixtures thereof. More preferred organic compounds are tartaric acid, 2-hydroxybutyric acid, citric acid, or ascorbic acid.

  The content of the organic compound in the coating film is preferably 5 to 50 wt%.

  A pre-coated steel sheet (PCM) as a product is generally supplied to a customer in a form in which a coating film having a performance required by a customer who manufactures the final product by processing it is provided on one side. In the present invention, the surface provided with the coating film having the performance required by the consumer is regarded as the “front surface” of the precoated steel sheet, and the surface opposite to the surface is referred to as the “back surface”. The “front surface” is located on the outer side of the final product manufactured by the consumer and is the surface that is visible to the human eye, whereas the “back surface” is not visible to the human eye on the final product. The coating film on the back surface does not require the performance required for the coating film on the front surface. For this reason, the distinction between the “front surface” and the “back surface” of the pre-coated steel sheet is obvious from its appearance. Compared to the two, the “back surface” is particularly inferior from an aesthetic point of view and can be easily distinguished. Can do.

  According to the present invention, the organic compound contained in the outermost coating film of at least the back coating film of the chromate-free precoated steel sheet contributes to the development of end face red rust corrosion resistance, and the end face in an outdoor environment exposed to rainwater or the like. Effectively suppress the occurrence of red rust. This can greatly contribute to the expansion of the use of chromate-free pre-coated steel sheets that are free from harmful chromium elution.

  Here, first, a description will be given of how the inventor completed the invention of a new type of chromate-free precoated steel sheet having excellent end surface red rust corrosion resistance.

  In the chromate-treated precoated steel sheet that has been widely used before the chromate-free precoated steel sheet, end face corrosion resistance refers to the property of preventing blistering of the coating film in the vicinity of the end face. Therefore, how to prevent zinc from leaching out of the zinc coating layer of the pre-coated steel sheet, in order to prevent the paint film from swelling from the end face portion even when used in a salt damage area such as the area along the coastline, It has been the target of development. The reason for this is that in the era of chromate-treated precoated steel sheets, due to the excellent corrosion resistance of chromate, there was almost no red rust on the end face in a mere wet environment such as wetness with rainwater, so the corrosion resistance in a mere wet environment This is because attention was paid to the swelling of the coating film due to dissolution of zinc. Therefore, in developing new products of chromate-treated precoated steel sheets, the salt spray test (SST) in the laboratory was emphasized. In the electrolyte environment of the salt spray test, even if white rust is generated due to the sacrificial anticorrosive effect of galvanization of the precoated steel sheet, red rust is hardly generated until the zinc is consumed.

  However, as the use of chromate-free pre-coated steel sheets, whose corrosion resistance level is inferior to chromate-treated pre-coated steel sheets, became widespread, the problem of red rust generated on the end face in a short time in a non-electrolytic atmosphere became apparent. For example, the occurrence of red rust in air conditioner outdoor units has become a major problem. In particular, the occurrence of red rust was regarded as a serious problem because water was absorbed by capillary action, such as the fitting part with the side plate of the draw-formed top plate and the knockout part (structure part shifted by about 1 mm by cutting the steel plate). It was a part where the steel sheet end face was easy to collect and the steel plate end face kept wet for a long time (in some cases, several days). In addition, the area where red rust occurred was mainly in the mountainous area, not the salt damage area along the coast. This is because corrosion cells are not formed because the end face continues to get wet with water that does not contain electrolytes, so zinc is not dissolved sacrificially when exposed to a high humidity environment (that is, especially during the rainy season). This was because the end face of the iron ground would corrode.

  The reason why such red rust is unlikely to occur in the chromate-treated precoated steel sheet is that the chromate component (strontium chromate) elutes into the water from the end face (neighboring chromate-treated film) and forms a protective layer on the iron end face. . In contrast, rust-preventing pigments (calcium-exchanged silica, aluminum tripolyphosphate, etc.) contained in the coating of ordinary chromate-free pre-coated steel sheets have low water solubility and are inferior to chromate components even when eluted in water. Therefore, it is insufficient to prevent the occurrence of red rust in a wet environment where the sacrificial corrosion protection of zinc does not function effectively.

In view of the above points, the present inventor has developed the new type of chromate-free precoated steel sheet of the present invention, and in order to obtain a sufficient end face red rust suppressing effect, the following conditions (1) to (3) are all I thought it was necessary to be satisfied.
(1) When the pre-coated steel plate end face gets wet with water for a long time, a sufficient amount of components should be eluted from the coating film.
(2) The eluted component is effective in suppressing red rust or red rust and white rust.
(3) The eluted component continues to stay in the vicinity of the end face.

These conditions will be described in detail next.
Regarding condition (1): The occurrence of red rust on the end surface of the pre-coated steel sheet occurs when the end surface continues to get wet with water with low electrolyte content at high humidity during the rainy season, etc. This is a phenomenon in which red rust is generated from the end face in a very short period of time. In order to suppress the occurrence of red rust, an effective component for rust prevention is contained in the coating film, and when the end face gets wet with water, the active ingredient is eluted from the coating film, thereby suppressing the occurrence of red rust on the end face. Is effective. If the substance contained in the coating film is not sufficiently water-soluble, even if the substance has rust prevention ability, it cannot be eluted in water, so that the effect of suppressing red rust cannot be obtained. Therefore, the substance to be included in the coating film must have sufficient water solubility.

  Incidentally, rust preventive pigments such as calcium exchanged silica (trade name Sealdex) and aluminum tripolyphosphate used in current chromate-free precoated steel sheets are examples of those that do not have sufficient water solubility.

Regarding the condition (2), the component eluted from the coating film into water must be a substance capable of suppressing the occurrence of red rust. Even if it is a substance which has red rust inhibitory ability, the rust prevention mechanism is various, and the substance which has red rust inhibitory ability can be classified according to the rust prevention mechanism as follows.

(I) A substance that suppresses the occurrence of red rust by eluting into water to become an electrolyte, increasing the electrical conductivity, and exhibiting the sacrificial anticorrosive effect of zinc.
(Ii) A substance that suppresses the elution of iron itself by adsorbing or forming a passive state on the iron surface (a substance that acts as an iron dissolution inhibitor).
(Iii) A substance that acts as a zinc dissolution inhibitor in addition to the effect of (ii) above.

  If it is a water-soluble salt, it will be ionized when dissolved in water to become an electrolyte. Therefore, unless it has the characteristics of the substance (ii) or (iii), it is classified as the substance (i) above. It is thought. However, since the mechanism of suppressing red rust of the substance (i) is premised on corroding zinc, the occurrence of white rust and the occurrence of blistering under the coating film cannot be avoided. As the corrosion of zinc progresses, the sacrificial anticorrosive effect disappears, and eventually red rust begins to occur.

  On the other hand, since the substance (ii) prevents the elution of iron itself without relying on the sacrificial anticorrosive effect of zinc, the dissolution of zinc is suppressed to the level of self-dissolution in water, and white rust and blister The occurrence of is reduced. In the substance (iii), even the self-dissolution of zinc is suppressed, so that the occurrence of white rust and blistering is further reduced. From the above viewpoint, in the present invention, the substance (ii) or (iii) is used instead of the substance (i) as a substance effective for suppressing the occurrence of red rust on the end face of the chromate-free precoated steel sheet. To do.

  Whether the specific substance is any of the above (i) to (iii) can be generally determined by the following method.

The following two tests are performed on the substance to be tested.
(Test 1) A cold-rolled steel sheet is immersed in a solution obtained by dissolving 100 mg of a test target substance in 40 ml of distilled water (500 hours).
(Test 2) A normal chromate-free precoated steel sheet (with a tendency to easily cause red rust on the end face) in which a coating film containing calcium exchange silica or aluminum tripolyphosphate as a rust preventive pigment is formed in the same aqueous solution (500) time).

  The immersion time of 500 hours in these tests is effective in suppressing the occurrence of red rust on the end face, assuming that the pre-coated steel sheet is actually used, and the occurrence of red rust on the end face can be suppressed even when immersed for 500 hours continuously. It is set as a necessary requirement for the substance.

  The state of occurrence of red rust and white rust is examined for each steel plate taken out from the aqueous solution, and it is determined which of the substances (i) to (iii) is based on the following criteria.

Substances classified as (i) Red rust was generated in Test 1, no red rust was generated in Test 2, and large amounts of white rust were generated.
Substance classified as (ii) No red rust was generated in Test 1 and no red rust was generated in Test 2, and a small amount of white rust was generated.
Substances classified as (iii) Test 1 does not generate red rust, and test 2 does not generate red or white rust.

Regarding the condition (3), even if the test substance satisfies the requirements of the above conditions (1) and (2) (that is, the test substance is sufficiently dissolved in water from the coating film and acts as an iron dissolution inhibitor) ) If the substance is washed away immediately, it cannot act as an inhibitor. Therefore, in order to ensure the end face red rust corrosion resistance, it is necessary that the substance as the inhibitor component is continuously “stayed” at or near the end face.

  The term “residence” here includes various meanings. For example, (a) the inhibitor component forms an adsorption or passive film to cover the end surface, (b) the inhibitor solution stays on the end surface due to physical factors such as viscosity and surface tension, (c) ( Any of the fluid film boundary layer region having a high inhibitor concentration is present on the end face because the flow rate of the fluid flowing on the solid surface in contact with the solid surface is 0). It corresponds to. And even if it is "residence" by any phenomenon, when it exists in the state which can exhibit the inhibitor effect of a test substance, the substance is expressed as "having end face retention."

  The evaluation of the end face retention for a specific substance can be performed by the following method.

1) Normal chromate-free pre-coated steel sheet in which 10 mg of the test substance is dissolved in 40 ml of distilled water and a coating film containing calcium exchange silica or aluminum tripolyphosphate as an anticorrosive pigment (inhibitor) is formed (red rust tends to occur on the end face) What is cut into 50 × 50 mm is immersed in the aqueous solution for 72 hours.
2) Take out the pre-coated steel plate (50 × 50 mm) from the aqueous solution of 1) above (no red rust is generated at this time), rinse lightly with distilled water, and immediately immerse in distilled water for 350 hours and 500 hours. put.

  In the following 2), as the evaluation of end face retention, the case where red rust was generated within 350 hours was x, red rust was not generated when immersed for 350 hours, but the case where it was generated after immersion for 500 hours was Δ, 500 hours A case where no red rust was generated even after immersion was marked as ◯.

  When red rust did not occur after standing for 350 hours (in the case of Δ or ○), the substance was judged to be “high in end face retention”. In the present invention, it is preferable to use the organic compound having such “high end-face retention” from the viewpoint of effectively acting as an inhibitor without the substance being immediately removed from the end face.

  The 72-hour inhibitor aqueous solution immersion time of the precoated steel sheet in 1) is set as a model condition for revealing the difference in the end surface retention of the substance. This is a condition in which the result is almost the same as the condition at the top plate fitting part of the air conditioner outdoor unit where a large amount of red rust was actually generated during the rainy season. The immersion time of 350 hours and 500 hours in the above 2) is set for the same reason as the immersion time in the test for determining the type of the substance having red rust suppressing ability.

インヒビター水溶液浸漬は、赤錆の全く発生していないものを○、少しでも発生したものを×と評価した。
上記のインヒビター水溶液浸漬は、赤錆の全く発生していないものを○、少しでも発生したものを×と評価した。
「滞留性」試験は、５００時間でも赤錆の全く発生していないものを○、３５０時間では発生せず５００時間で発生したものを△、３５０時間で発生したものを×と評価した。Based on the above assumptions, the inventor investigated whether various substances act as iron dissolution inhibitors and found some promising substances. In addition, the end face retention of these materials was tested. Some of the evaluation results are summarized in Table 1.

The inhibitor aqueous solution immersion was evaluated as “◯” when no red rust was generated and “×” when it was generated even a little.
In the case of the above-mentioned immersion in an aqueous solution of the inhibitor, the case where no red rust was generated was evaluated as ◯, and the case where any rust was generated was evaluated as ×.
In the “Residence” test, a case where no red rust was generated even after 500 hours was evaluated as “◯”, a case where it did not occur after 350 hours was evaluated as Δ, and a case where it was generated after 350 hours was evaluated as “X”.

  As a result, a water-soluble organic compound having a -C (COOH) (OH) atomic group in the molecular structure is a useful substance that imparts end face red rust corrosion resistance to chromate-free pre-coated steel sheets, although the exact mechanism is unknown. I found out. Examples of this type of organic compound include glycolic acid, lactic acid, malic acid, tartaric acid, 2-hydroxybutyric acid, citric acid, isocitrate lactone, citramalic acid, and ascorbic acid. Each of these organic compounds is represented by the following structural formula.

これらの化合物のうち、イソクエン酸ラクトンとアスコルビン酸は脱水されており、水に溶解した状態（インヒビターとして実際に機能する状態）で、−Ｃ（ＣＯＯＨ）（ＯＨ）原子団を持つ。

Of these compounds, isocitrate lactone and ascorbic acid are dehydrated and have a —C (COOH) (OH) atomic group in a state dissolved in water (actually functioning as an inhibitor).

  Among the organic compounds imparting end face red rust corrosion resistance shown in Table 1, tartaric acid, 2-hydroxybutyric acid, citric acid and ascorbic acid have end face red rust corrosion resistance equivalent to strontium chromate, which is a rust preventive component in chromate-treated precoated steel sheets. Which proved to be a particularly preferred organic compound.

  Although the mechanism by which a water-soluble organic compound having a —C (COOH) (OH) atomic group in the molecular structure exhibits an effect of suppressing red rust on the end face has not been fully elucidated as described above, The group O (which is not carbonyl-bonded) and the OH group O are coordinated to the metal atom M on the bare iron surface and zinc surface by the chelate effect as shown below to form a stable five-membered ring. By forming, organic compound molecules are presumed to be adsorbed on the iron surface and zinc surface of the end surface of the pre-coated steel sheet and exhibit red rust corrosion resistance on the end surface. Moreover, even if the test piece of the precoated steel sheet is immersed again in distilled water in the test of 2), red rust does not occur, so it is assumed that the organic compound is adsorbed and stays on the iron surface and zinc surface. .

  The expression mechanism of the end face red rust inhibitory effect by the organic compound is presumed in this way, but the end face red rust inhibitory effect was different depending on the compound. In the test of 2), no red rust was observed in any compound at 350 hours of immersion, but at 500 hours of immersion, tartaric acid, 2-hydroxybutyric acid, citric acid, and ascorbic acid exhibited an effect of suppressing red rust on the end face. Other compounds produced red rust.

  There is a difference in the effect of suppressing the red rust on the end face depending on the compound. The effect of suppressing the red rust on the end face of a specific compound depends on the balance between the adsorption strength of the molecule on the metal surface and the solubility in water, This is considered to be because it depends on the balance of the degree of hydrophobicity of the portion not forming the five-membered ring involved in the adsorption (the portion indicated by A in the above formula).

  For example, compared with glycolic acid and lactic acid, 2-hydroxybutyric acid having a similar structure to that of red has good red rust resistance (no red rust is generated even after 500 hours of immersion). It can be interpreted that it increases due to its long length and is highly effective in keeping water away from the metal surface. As another interpretation, since glycolic acid and lactic acid have a relatively high water solubility due to a large number of OH and COOH per molecular weight equivalent, there is a possibility that the adsorption is partially eliminated.

  Although tartaric acid has high water solubility, it has two adsorption sites in one molecule and can be strongly adsorbed, so it is considered that the tartaric acid exhibits good end face red rust corrosion resistance.

  Despite the fact that ascorbic acid and citric acid are considered to be highly water-soluble in terms of molecular structure, the reason for showing good end face red rust corrosion resistance is not clear, but it does not constitute a five-membered ring involved in adsorption. It is conceivable that the reactive substituent part of the part A is bonded to form a strong network structure and exhibits a barrier effect.

  Other than the compounds listed in Table 1, for example, phosphate substances such as sodium phosphate and magnesium hydrogen phosphate are known to have a feature of adsorbing to the iron surface, so “high retention” Although these were expected, they were not particularly good in end face red rust corrosion resistance evaluated by the wet test (HCT). This example shows that a substance having a high adsorption ability to the iron surface cannot always be said to have a high retention.

  In this invention, it is preferable that the organic compound which provides end face red rust corrosion resistance to a chromate free type | mold precoat steel plate exists in the coating film of the back surface side of a precoat steel plate. When there are a plurality of coating films on the back side, the organic compound may be present in all coating films or only in some coating films. However, in order for the organic compound to effectively exhibit end face red rust corrosion resistance, the organic compound is preferably present in at least the outermost coating film.

  In order for the organic compound to exhibit end face red rust corrosion resistance, it must be dissolved in water that wets the precoated steel sheet, and the outermost coating film exposed on the back surface of the precoated steel sheet is required for that area (area wetted by water). ) Can be provided. Since the coating film other than the outermost layer is only exposed at the end face and the exposed area is very small, the coating film other than the outermost layer is insufficient to guarantee the elution of an effective amount of organic compounds. It is possible to do. It is possible to include an organic compound in the front side coating film, but in this case, the organic compound is mixed within a range that does not impair the requirements from the customers required for the front side coating film. Should be allowed.

  In some cases, organic compounds can be used in combination of two or more.

  The organic compound can be mixed in the outermost layer coating film on the back surface of the precoated steel sheet by about 5 to 50 wt%. If the amount is less than 5 wt%, the end face red rust corrosion resistance is insufficient, and if it exceeds 50 wt%, the organic compound is excessively eluted when the coated film is exposed to a wet environment, and the coated film is swollen or peeled off. May occur. The organic compound concentration in the back outermost layer coating film is more preferably 7 to 40 wt%, and most preferably 10 to 30 wt%.

  When mixing an organic compound into a coating film other than the outermost layer coating film on the back surface of the precoated steel sheet, from the viewpoint of facilitating elution into water, an amount close to the upper limit of the concentration range in the outermost layer coating film described above is used. It is preferable to adopt.

  Any of the organic compounds used in the present invention is solid at room temperature, and can be mixed in the form of a powder with a coating material for a desired coating film. Considering the thickness of the coating film to be mixed, it is preferable to use it dispersed in the coating material until the maximum particle size becomes 5 to 20 μm, more preferably about 5 to 10 μm.

  An organic compound can be mixed in the coating material for the target coating film by an arbitrary method. For example, it may be added to paints that are ready to be painted (including necessary components and solvents other than organic compounds), or a mixture that is pre-mixed with one or more ingredients of the paint is mixed with the other ingredients of the paint. A paint may be prepared.

  The coating film of the chromate-free pre-coated steel sheet of the present invention can be formed by a paint used for the production of a normal chromate-free pre-coated steel sheet except that an organic compound necessary for imparting end face red rust corrosion resistance is added. .

  As an example, on the front side, a polyester resin, melamine resin, isocyanate, calcium silicate (calcium exchanged silica), aluminum tripolyphosphate, titanium oxide, silica, other additives, etc., primer coating film, polyester A surface coating film composed of a resin, a melamine resin, an isocyanate, calcium silicate (calcium-exchanged silica), aluminum tripolyphosphate, titanium oxide, silica, various color pigments, wax, and other additives can be formed. .

  On the back side, a primer coating composed of polyester resin, melamine resin, isocyanate, calcium silicate (calcium exchanged silica), aluminum tripolyphosphate, titanium oxide, silica, and other additives, polyester resin, melamine resin, A surface coating film composed of isocyanate, calcium silicate (calcium exchanged silica), aluminum tripolyphosphate, titanium oxide, silica, various color pigments, wax, other additives, and the like can be formed. Among these coating films, at least the outermost coating film on the back surface contains an organic compound according to the present invention.

  As the steel plate that is the base of these coating films, a zinc-based plated steel plate can be used. As the galvanized steel sheet, for example, any one such as a hot dip galvanized steel sheet, an electrogalvanized steel sheet, an Al-galvanized steel sheet, an Al-Mg-Si-galvanized steel sheet can be used. When steel plates are used, the improvement in red rust resistance is particularly remarkable.

  A normal chromate-free chemical conversion treatment layer can be provided on the surface of the galvanized steel sheet as a pretreatment layer for applying the above-mentioned paint for coating film. As an example, the chemical conversion treatment layer can use a film containing two or more of silica, a silane coupling agent, tannin or tannic acid, a zirconium compound, and a titanium compound and a resin. The chemical conversion treatment layer can be formed by dip coating, roll coater coating, ringer roll coating, brush coating, spray coating or the like.

  The chromate-free pre-coated steel sheet of the present invention is manufactured by the usual manufacturing equipment and manufacturing method for chromate-free pre-coated steel sheets, except that at least the paint on which an organic compound is added is used to form the outermost coating film on the back surface. They are not discussed in detail here.

  The invention will be further described by way of examples. It goes without saying that the invention is not limited to the examples presented here.

A. Sample Material Plate A sample material was prepared using the following plate materials GI and SD.
(1) GI: Hot-dip galvanized steel sheet (0.6 mm thickness, plating adhesion 80 g / m ^{2 on} one side)
(2) SD: 11% Al-3% Mg-0.2% Si-galvanized steel sheet (0.6 mm thickness, plating adhesion 80 g / m ^{2 on} one side)

B. Chemical conversion treatment of the original plate The chromate-free chemical conversion treatment (100 mg / m 2) of the original plate was performed using a silane coupling agent, tannic acid, silica, and a polyester resin mixed treatment agent.

C. Coating of Sample Material Using a paint in which the anticorrosive pigment component shown in Table 2 was blended with the following paint resin, primer coating and top coat coating were performed on each surface of the original plate subjected to the chemical conversion treatment.

(1) Front / back primer paint resin Polyester / melamine + isocyanate combined curing type (FLC687 paint resin made by Nippon Fine Coatings)
(2) Front surface top coat paint resin High molecular polyester / melamine curable type (FLC7000 paint resin manufactured by Nippon Fine Coatings)
(3) Backside topcoat paint resin Polyester / melamine curing type (FLC100HQ paint resin manufactured by Nippon Fine Coatings)

The rust preventive pigment components blended in the paint of Table 2 were as follows.
A: Calcium exchange silica B: Aluminum tripolyphosphate C: Tartaric acid D: 2-Hydroxy enantioic acid E: Citric acid F: Ascorbic acid G: Glycolic acid H: Malic acid I: Lactic acid J: Citramalic acid K: Isocitrate lactone L: 3-Hydroxy tangle acid (comparative)
M: Sodium bicarbonate (comparison)
N: Potassium chloride (comparison)
O: Fumaric acid (comparison)
P: Maleic acid (Comparison)

  In Table 2, the numerical value in parentheses following the symbol of the rust preventive pigment indicates the weight concentration percentage of the rust preventive pigment with respect to the total solid content of the paint (for example, “A (20)” indicates that the paint contains 20% by weight of the pigment A. It indicates that it contains). In Table 2, only the rust preventive pigment component blended in the paint is shown, and the colored pigment component is not shown.

  The following performance test was performed on the prepared sample material.

1. Rural Exposure Test The rural exposure test was intended to directly evaluate the resistance to red rust that occurs in a short time in a non-electrolyte moist environment.

The processed material which processed the sample material and reproduced the shape of the top plate fitting part of an air conditioner outdoor unit, and the unprocessed flat plate were used as a test material.
Immediately after entering the rainy season, each test material was installed in the countryside (land in Kimitsu City, Chiba Prefecture) and visually observed after one month.

  The red rust evaluation of the top plate mating part is ◎ when red rust is not generated at all, ○ when the amount of red rust generated is small (less than 1% red rust), and the amount of red rust generated is small (1 to 5% red rust). A thing with a large amount of red rust (over 5% red rust) was evaluated as x.

  The blister evaluation of the flat part of the flat surface of the flat plate is ◎ when no blisters are generated, ○ is fine (over 8F level according to ASTM D 714), × is low to many (less than 8F level) × It was evaluated.

2. Salt Damage Area Exposure Test The salt damage area exposure test was intended to evaluate the corrosion resistance from the conventional viewpoint of long-term tolerance in an electrolyte environment.

  The processed material which processed the sample material and reproduced the shape of the top plate fitting part of an air conditioner outdoor unit, and the unprocessed flat plate were used as a test material.

  Each test material was installed in a salt damage area (Okinawa coastal area) and visually observed after 2 years.

  The red rust evaluation of the top plate mating part is ◎ when red rust is not generated at all, ○ when the amount of red rust generated is small (less than 1% red rust), and the amount of red rust generated is small (1 to 5% red rust). A thing with a large amount of red rust (over 5% red rust) was evaluated as x.

  In the evaluation of the end face swelling of the front surface of the flat plate, the case where the maximum value of the swelling from the end face was 2 mm or less was evaluated as ◯, and the case where it exceeded 2 mm was evaluated as x.

3. Lab test Lab test (distilled water immersion test, 40 ° C HCT test, boiling water 2 hour immersion test) is intended to simply evaluate the resistance to red rust that occurs in a short time in a non-electrolyte moist environment. It was something to do. However, regarding the present invention, it is the primary meaning that the result of the exposure test in the countryside is good, and the laboratory test is a reference test.

  The time in which the aqueous solution from which the substance has eluted from the coating film stays in the vicinity of the end face is extremely long in the distilled water immersion test, and conversely short in the 40 ° C. HCT test. Real environmental conditions are considered to be intermediate between these laboratory tests. Therefore, it is considered that a good result obtained in these laboratory tests results in a good result in an actual environment.

  In the distilled water immersion test, a 50 × 50 mm sample material was immersed in 40 ml of distilled water for 500 hours, and then the red rust on the end face was evaluated. The case where no red rust was generated was evaluated as 、, the fine (less than 1% of red rust) was evaluated as ◯, the low (1-10% of red rust) was evaluated as △, and the high (over 10% of red rust) was evaluated as ×. .

  In the 40 ° C. HCT test, a 50 × 100 mm sample material was installed in a vertically-long high-humidity tank (40 ° C., 98% RH) in a vertically long manner, and the red rust on the end face after 240 hours was evaluated. The case where no red rust was generated was evaluated as ◎, the fine (less than 5% red rust) was evaluated as ○, the small (red rust 5-10%) was evaluated as △, and the high (over 10% red rust) was evaluated as ×. .

  In the boiling water 2-hour immersion test, the flat plate was immersed in boiling water for 2 hours, and blisters on the front and back plane portions were evaluated. The case where no blister was generated was evaluated as ◎, the case where it was fine (8F level or higher by ASTM D 714) was evaluated as ○, and the case where it was low to high (less than 8F level) was evaluated as ×.

  The results of each test are shown in Table 3.

Claims (6)

A chromate-free precoated steel sheet having a chromate-free chemical conversion treatment layer and one or more coating layers formed thereon on both sides of a zinc-based plated steel sheet;
The outermost coating film of at least the back surface of the precoated steel sheet is a water-soluble organic compound having a —C (COOH) (OH) atomic group in the molecular structure , and is glycolic acid, lactic acid, apple Containing an organic compound that is acid, tartaric acid, 2-hydroxybutyric acid, citric acid, isocitrate lactone, citramalic acid, ascorbic acid, or mixtures thereof ;
It has a primer coating layer and an upper coating layer on the chromate-free chemical conversion treatment layer on each side, the upper coating layer on the back contains the organic compound,
The primer coating layer on the back further comprises the organic compound;
The O atom of the COOH group of the atomic group that is not carbonyl-bonded and the O atom of the OH group of the atomic group are coordinated to the metal atoms on the exposed iron surface and zinc surface by a chelate effect and are stable. Chromate-free type precoat with excellent end face red rust corrosion resistance , wherein the organic compound molecules are adsorbed on the iron surface and zinc surface of the precoated copper plate scene and exhibit end face red rust corrosion resistance. steel sheet. The chromate-free precoated steel sheet excellent in end face red rust corrosion resistance according to claim 1 , wherein the organic compound is tartaric acid, 2-hydroxybutyric acid, citric acid, or ascorbic acid. The chromate-free precoated steel sheet excellent in end face red rust corrosion resistance according to claim 1 or 2 , wherein the content of the organic compound in the coating film is 5 to 50 wt%. When the cold-rolled steel sheet is immersed in a solution of 100 mg of the organic compound in 40 ml of distilled water for 500 hours, the organic compound has an “iron dissolution inhibitor” function that does not generate red rust on the cold-rolled steel sheet. The chromate-free precoated steel sheet according to claim 1, which is an organic compound. When the normal chromate-free precoated steel sheet is immersed in a solution in which 100 mg of the organic compound is dissolved in 40 ml of distilled water for 500 hours, the organic compound does not generate red rust on the precoated steel sheet. The chromate-free precoated steel sheet according to any one of claims 1 to 4 , which is an organic compound having a function. The organic compound is immersed in an aqueous solution in which 10 mg of the organic compound is dissolved in 40 ml of distilled water, and an ordinary chromate-free pre-coated steel plate (50 × 50 mm) is immersed for 72 hours, lightly rinsed with distilled water, and then immediately in distilled water. The chromate-free precoated steel sheet according to any one of claims 1 to 5 , which is an organic compound having "high end face retention" so as not to generate red rust on the precoated steel sheet when immersed in a steel plate for 350 hours. .