JP4927233B2 - 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 pre-coated steel sheet used in the field of electrical products, architecture, etc., used in an environment wetted by water, particularly exposed to rainwater, etc. The present invention relates to a chromate-free pre-coated steel sheet having excellent resistance.

  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 Also known is 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 coating film of the outermost layer among the coating films of the back surface of the said precoat steel plate contains the tungstate or silicate with high end surface retention property, It is characterized by the above-mentioned. As used herein, “high end-face retention” refers to a property that is determined to be ◯ or Δ by an end-face retention evaluation test described below.

  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 surface contains tungstate. Contains.

  In addition to the upper coating layer on the back surface, the primer coating layer on the back surface may contain a tungstate or silicate with high end surface retention.

  Examples of preferred tungstates are sodium tungstate, calcium tungstate, ammonium tungstate, lithium tungstate, magnesium tungstate, with sodium tungstate and magnesium tungstate being particularly preferred. Two or more tungstates can be used in combination.

  The content of tungstate in the coating film is preferably 6 to 50 wt%.

  Examples of preferred silicates are sodium silicate, potassium silicate, lithium silicate, with sodium silicate and potassium silicate being particularly preferred. Two or more silicates can be used in combination.

  The content of silicate 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 tungstate or silicate having a high end surface retention property contained in the outermost coating film of at least the back surface of the chromate-free precoated steel sheet contributes to the expression of end face red rust corrosion resistance, Effectively suppresses the occurrence of red rust on the end face in outdoor environments exposed to rainwater. 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 chromate-treated pre-coated steel sheets, which were widely used before chromate-free pre-coated steel sheets, the term “face corrosion resistance” refers to the property of preventing blistering of the paint film in the vicinity of the end faces. In order to prevent the coating film from bulging from the end surface even when used in a salty area, the development target has been to prevent zinc from eluting from the zinc coating layer of the pre-coated steel sheet. 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. . On the other hand, rust preventive pigments (calcium-exchanged silica, aluminum tripolyphosphate, etc.) contained in the coating of ordinary chromate-free pre-coated steel sheets have low water solubility, and even when eluted in water, the rust preventive ability is better than chromate components Since it is inferior, it is insufficient for prevention of red rust generation in a wet environment where the sacrificial corrosion protection of zinc does not function effectively.

In view of the above points, the inventor has satisfied the following conditions (1) to (3) in order to obtain a sufficient end face red rust suppressing effect in developing a new type of chromate-free precoated steel sheet of the present invention. I thought it was necessary to be done.
(1) When the precoated steel plate end face is 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 generation.
(3) The eluted component continues to stay in the vicinity of the end face.

These conditions will be described in detail next.
About condition (1) The occurrence of red rust on the edge of the pre-coated steel sheet is caused by the fact that the sacrificial anticorrosive effect of zinc cannot be manifested in an environment where the edge face continues to get wet with water with low electrolyte content in high humidity such as during the rainy season. 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 contained in the coating film must have sufficient water solubility.

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

Condition (2) The component eluted from the coating film into water must be a substance capable of suppressing the occurrence of red rust or red rust and white 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 500 hour immersion time in these tests is an effective substance for suppressing the occurrence of red rust on the end face, assuming the actual use conditions of the pre-coated steel sheet, even if the immersion is continued for 500 hours, Is set as a necessary requirement.

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 classified according to the following criteria.
Substances classified as (i) Red rust was generated in Test 1, no red rust was generated in Test 2, and a large amount of white rust was 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 Condition (3) Even if the test substance satisfies the requirements of the above conditions (1) and (2) (that is, even if 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, it is expressed that the substance has "end surface 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 substance is dissolved in 40 ml of distilled water and a coating film containing calcium exchange silica or aluminum tripolyphosphate as an antirust pigment (inhibitor) is formed (with a tendency to cause red rust on the end face) ) Is cut into 50 × 50 mm and 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 above 2), the evaluation of end face retention was x when red rust was generated within 350 hours, red rust was not generated when immersed for 350 hours, and Δ when immersed for 500 hours. However, the case where no red rust occurred 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”. 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.

  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.

  As a result, it is unclear which of the above-described “residence” mechanisms develops high end face retention, but in the tungstate group, sodium tungstate, calcium tungstate, ammonium tungstate, lithium tungstate and tungstic acid. It was found that tungstate represented by magnesium acts as an iron dissolution inhibitor and is a useful substance that imparts end face red rust corrosion resistance to chromate-free precoated steel sheets.

  Within the tungstate group, sodium tungstate and magnesium tungstate have been found to be particularly preferred. From Table 1, it was also confirmed that the end face red rust corrosion resistance shown by these tungstates is equivalent to strontium chromate which is a rust preventive component in the chromate-treated precoated steel sheet. Moreover, although it is a tungstate, barium tungstate was determined to be “◯” in the evaluations of Test 1 and Test 2, but was determined to be “X” in the end face retention test. Sodium molybdate, which is conventionally known as a rust preventive substance, was also judged as “good” in the evaluations of Test 1 and Test 2, but was judged as “x” in the end face retention test.

  Although the mechanism by which tungstates such as sodium tungstate have the effect of suppressing red rust on the end face has not been fully elucidated as described above, it forms a dense iron oxide passivation on the iron surface of the precoated steel sheet end face. It is assumed that it works as an iron dissolution inhibitor. Since the occurrence of some white rust is observed in the above test 2 for determining the type of substance, tungstate seems to have a low effect of suppressing the elution of zinc in the plating layer. However, since tungstate does not suppress zinc elution (self-dissolution), zinc elution occurs moderately and a sacrificial anti-corrosion effect is obtained. It is considered that an end face protective film is formed under the actual use environment of the final product including the pre-coated steel sheet, and works favorably by suppressing the occurrence of red rust. In addition, by combining with calcium-exchanged silica (trade name: Sealdex) used in current chromate-free pre-coated steel sheets and anti-corrosion pigments such as aluminum tripolyphosphate, it is possible to synergistically improve red rust corrosion resistance on the end face. It is expected to be possible.

  In the silicate group, silicates such as sodium silicate, potassium silicate, and lithium silicate are not only iron dissolution inhibitors but also zinc dissolution inhibitors. It was found that it is a useful substance that imparts red rust corrosion resistance to end faces. Of these, sodium silicate and potassium silicate were found to be particularly preferable. From Table 1, it was also confirmed that the end face red rust corrosion resistance of the silicate of the present invention is equivalent to strontium chromate, which is a rust preventive component in the chromate-treated precoated steel sheet. Moreover, although it is a silicate, it was determined that calcium silicate had no evaluation antirust effect in both Test 1 and Test 2. This is probably because calcium silicate is a sparingly soluble salt. Magnesium silicate was determined to be “good” in the evaluations of Test 1 and Test 2, but “×” in the end face retention test.

  Although the mechanism by which silicates such as sodium silicate have the effect of suppressing red rust on the end face has not been fully elucidated as described above, it remains on the iron surface and zinc surface of the precoated steel sheet end face, and both metals dissolve. It is thought to act as an inhibitor. In the tests 2) and 3) of the end face retention evaluation described above, it was recognized that red rust began to occur when the immersion time exceeded 500 hours. Therefore, even if the silicate was adsorbed on the end face, its strength Is weak, and is likely to be a retention due to the presence of a liquid film boundary layer.

  Since phosphate substances such as sodium phosphate and magnesium hydrogen phosphate are known to have a feature of adsorbing to the iron surface, they were expected to have "high end face retention". The result of the end face retention test was x. This example shows that a substance having a high adsorption ability to the iron surface does not necessarily have a high end face retention.

  In the present invention, it is preferable that the tungstate or silicate having high end face retention that imparts end face red rust corrosion resistance to the chromate-free precoated steel sheet is present in the coating film on the back surface side of the precoated steel sheet. When there are a plurality of coating films on the back side, the tungstate or silicate having high end face retention may be present in all coating films or only in some coating films. However, in order for the tungstate or silicate having a high end surface retention to effectively exhibit end face red rust corrosion resistance, the tungstate or silicate is present at least in the outermost coating film. Is preferred. In order for tungstate or silicate with high end face retention to exhibit end face red rust corrosion resistance, it is necessary to dissolve in water that wets the pre-coated steel sheet, and the outermost coating film exposed on the back surface of the pre-coated steel sheet is This is because it is possible to provide a necessary area (an area that gets wet with water). Since the coating film other than the outermost layer is only exposed at the end face portion and the exposed area is very small, only the coating film other than the outermost layer has an effective amount of tungstate or silica having a high end surface retention. It may be insufficient to ensure the elution of the acid salt. Although it is possible to include a tungstate or silicate with high end face retention in the front side coating film, in this case, from the customer required for the front side coating film The tungstate or silicate should be mixed so long as the requirements are not impaired.

  Tungstate with high end face retention is about 6 to 50 wt% in the outermost layer coating film on the back surface of the precoated steel sheet, and silicate with high end surface retention is 5 to 50 wt% in the outermost layer coating film on the back surface of the precoated steel sheet. It can be mixed to some extent. If it is less than these ranges, the end face red rust corrosion resistance is insufficient, and if it exceeds 50 wt%, the tungstate or silicate has high end face retention when the coated film is exposed to a wet environment. May be excessively eluted, and the coating film may be swollen or peeled off. The tungstate or silicate concentration with high end face retention in the back surface outermost layer coating is more preferably 7 to 40 wt%, and most preferably 10 to 30 wt%.

  When mixing tungstate or silicate with high end-face retention in the coating film other than the outermost layer coating film on the back surface of the pre-coated steel plate, from the viewpoint of facilitating elution into water, It is preferable to employ an amount close to the upper limit of the concentration range.

  The tungstate or silicate having a high end surface retention used in the present invention is a powder at room temperature, and the maximum particle size is 5 to 20 μm, more preferably 5 to 10 μm in consideration of the thickness of the mixed coating film. It is preferable to use it dispersed in the paint until it reaches a certain level.

  The tungstate or silicate powder having high end face retention can be mixed in the coating material for the target coating film by any method. For example, it may be added to a paint ready to be painted (including necessary ingredients and solvents other than tungstates or silicates with high edge retention) or combined with one or more ingredients of the paint in advance. The material may be mixed with other ingredients of the paint to prepare the paint.

  The coating film of the chromate-free type precoated steel sheet of the present invention is a normal chromate-free type precoated steel sheet except for adding tungstate or silicate having a high end surface retention necessary for imparting end face red rust corrosion resistance. It can form with the coating material used for. As an example, on the front surface, a polyester resin, melamine resin, isocyanate, calcium exchange silica, aluminum tripolyphosphate, titanium oxide, silica, other additives, etc., primer coating, polyester resin, melamine A surface coating film composed of resin, isocyanate, calcium exchange silica, aluminum tripolyphosphate, titanium oxide, silica, various color pigments, wax, other additives, and the like can be formed. On the back side is a primer-based coating composed of polyester resin, melamine resin, isocyanate, calcium exchange silica, aluminum tripolyphosphate, titanium oxide, silica, and other additives, polyester resin, melamine resin, isocyanate, calcium exchange A surface coating film composed of 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 tungstate or silicate having high end face retention 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 a chromate-free pre-coated steel sheet except that a coating containing a tungstate or silicate having a high end surface retention property is used to form at least the outermost coating film on the back surface. It can be manufactured by a normal manufacturing facility and manufacturing method, and these need not be described 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: Sodium tungstate D: Calcium tungstate E: Ammonium tungstate F: Lithium tungstate G: Sodium phosphotungstate (comparison)
H: Sodium bicarbonate (comparison)
I: Potassium chloride (comparison)
J: Magnesium tungstate K: Barium tungstate (comparison)
L: Sodium molybdate (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 (greater than 10%) 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 the 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 small to many (less than 8F level) was evaluated as ×.

  Table 3 shows the results of the above tests.

  Furthermore, each said test was done similarly, replacing the antirust pigment component with the antirust pigment component shown next. Table 4 shows the components blended and Table 5 shows the test results.

The antirust pigment components blended in the paint of Table 4 were as follows.
a: calcium exchange silica b: aluminum tripolyphosphate c: sodium silicate d: magnesium silicate (comparison)
e: Potassium silicate f: Calcium silicate (comparison)
g: Sodium bicarbonate (comparison)
h: Potassium chloride (comparison)
i: Lithium silicate

Claims (9)

  A chromate-free pre-coated steel sheet having a chromate-free chemical conversion treatment layer and at least one coating film formed thereon on both surfaces of a zinc-based plated steel sheet, the outermost coating film of at least the back surface of the pre-coated steel sheet. A chromate-free precoated steel sheet excellent in end face red rust corrosion resistance, wherein the outer layer coating film contains tungstate or silicate having high end face retention.   It 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 tungstate or silicate with high end surface retention. The chromate-free precoated steel sheet excellent in end face red rust corrosion resistance according to claim 1.   The chromate-free precoated steel sheet excellent in end face red rust corrosion resistance according to claim 2, wherein the primer coating layer on the back surface further comprises tungstate or silicate having high end face retention.   The tungstate with high end surface retention is sodium tungstate, calcium tungstate, ammonium tungstate, lithium tungstate, magnesium tungstate or a mixture thereof according to any one of claims 1 to 3. Chromate-free pre-coated steel plate with excellent red rust corrosion resistance.   The chromate-free excellent end face red rust corrosion resistance according to any one of claims 1 to 3, wherein the silicate having a high end face retention is sodium silicate, potassium silicate, lithium silicate, or a mixture thereof. Type pre-coated steel sheet.   The chromate-free precoated steel sheet having excellent end face red rust corrosion resistance according to claim 4, wherein the tungstate having high end face retention is sodium tungstate and magnesium tungstate.   The chromate-free precoated steel sheet having excellent end face red rust corrosion resistance according to claim 5, wherein the silicate having high end face retention is sodium silicate and potassium silicate.   The chromate-free precoated steel sheet having excellent end surface red rust corrosion resistance according to any one of claims 1 to 5, wherein the content of the tungstate having high end surface retention in the coating film is 6 to 50 wt%.   The chromate-free precoated steel sheet excellent in end face red rust corrosion resistance according to any one of claims 1 to 5, wherein the content of the silicate having high end face retention in the coating film is 5 to 50 wt%.