TWI431158B - Non-chromate-type pre-coated steel plate with excellent red rust and corrosion resistance - Google Patents

Description

End mask with excellent red rust and corrosion-resistant chromate-free precoated steel sheet Technical field

The present invention relates to a chromate-free precoated steel sheet having excellent red rust and corrosion resistance of an end mask. More specifically, the present invention relates to a chromate-free precoated steel sheet having excellent red rust and corrosion resistance of an end mask, which is used in an environment which is wetted by water, particularly in an outdoor environment such as rainwater. A chromate-free precoated steel sheet used in the field of electrical appliances, construction, and the like.

Background technique

Pre-coated steel sheet (also referred to as "PCM") refers to a steel sheet that is pre-formed by the consumer to re-send the coating film required for its products, since it can be omitted by the consumer for finishing or related work and is no longer needed. The equipment used for such operations expands its use in various fields. In the initial precoated steel sheet, a chromate-treated steel sheet which has been subjected to rust-preventing treatment by chromate has been used as a bottom steel sheet to be applied.

In the initial precoated steel sheet, as the base steel sheet to be coated, a chromate-treated steel sheet subjected to rust-preventing treatment using chromate has been used. Thereafter, the problem of toxicity of hexavalent chromium having the possibility of elution from the chromate-treated film is such that instead of the chromate-treated precoated steel sheet, the use of a chrome-free rust-preventing bottom steel plate has been attracting attention, in recent years, Its use is more particularly increased. In the chromate-free precoated steel sheet to date, the antirust pigment (usually calcium ion exchange tannin (commonly known as calcium citrate), aluminum tripolyphosphate, etc.) contained in the coating film is responsible for prevention. Rust effect.

As the pre-coated steel sheet is transferred from the chromate-treated precoated steel sheet to the chromate-free precoated steel sheet, the precoated steel sheet used in the outdoor environment exposed to rain or the like will immediately have red on the end surface. Rust and damage have emerged using problems such as the aesthetics of pre-coated steel sheets. This problem does not occur on chromate-treated precoated steel sheets, but is a problem unique to chromate-free precoated steel sheets and is currently being sought.

Regarding the improvement of the corrosion resistance of the end face red rust of the chromate-free precoated steel sheet, several conventional techniques are generally known.

For example, the following chromate-free precoated steel sheet is known (Patent Document 1): the coating film having the outermost layer formed of the plated steel sheet having the Zn-containing plating layer satisfies (A) and is dissolved at a concentration of 0.1% by mass. The pH in the ion-exchanged water is 7 or more and 12 or less, and (B) the conductivity when dissolved in ion-exchanged water at a concentration of 0.1% by mass is 500 μS/cm or more, and neither occurs at (C) to 200 ° C. A non-chromium compound (for example, sodium tripolyphosphate) under conditions such as thermal decomposition is used as a rust preventive pigment, thereby preventing end faces caused by the influence of water having low conductivity without using a chromate-based rust preventive pigment. The production of red rust.

In addition, the following chromate-free precoated steel sheet is known (Patent Document 2): the coating film of the outermost layer formed of the plated steel sheet having a Zn plating layer satisfies (A) and is dissolved in a concentration of 0.1% by mass. Non-chromium compound (for example, alkali metal phosphate) in which the water conductivity in ion-exchanged water (conductivity: 4 μS/cm or less) is 500 μS/cm or more and (B) to 200 ° C does not cause thermal decomposition or the like (for example, alkali metal phosphate) , a halide, an alkaline earth metal hypochlorite, whereby the generation of red rust on the end face can be suppressed.

Further, it is generally known that there is no chromate-type precoated metal sheet having excellent end face corrosion resistance (Patent Document 3): in order to apply a primer coating film and/or a surface coating on a non-chromium base treatment film of a metal plate The film contains a sustained release rust-preventing fine particle obtained by combining porous inorganic fine particles (for example, porous vermiculite fine particles) with a metal compound-based rust preventive pigment (for example, Mg or Ca compound), and is immersed in a carbonate at 40 ° C. In the case of the pH standard solution, the elution rate of the metal ions eluted from the rust preventive pigment having a coating film of a square of 1 mm × 1 mm was specified. Further, the following chromate-free precoated metal sheet (Patent Document 4) is known: a coating film containing an alkaline earth metal compound particle, a zinc compound particle, and a phosphate particle, and is immersed in a normal temperature pure water. The amount of elution of the alkaline earth metal, zinc, and phosphorus components eluted from the coating film is a molar ratio, whereby generation of red rust on the end surface can be suppressed.

Conventional technical literature Patent literature

Patent Document 1 JP-A-2008-229518

Patent Document 2, JP-A-2009-045923

Patent Document 3, JP-A-2009-078450

Patent Document 4, JP-A-2008-189965

The present invention aims to promote the expansion and utilization of a chromate-free precoated steel sheet which does not dissolve harmless chromium, and aims to provide a novel type of end mask with excellent red rust and corrosion resistance without chromate type precoated steel sheet, even if It can be used in the outdoor environment exposed to rain, and it can also be troubled by the red rust that damages the appearance.

The chromate-free precoated steel sheet having excellent red rust and corrosion resistance of the end mask of the present invention has a chromate-free chemical conversion treatment layer and a coating film of one or more layers formed thereon on both sides of the galvanized steel sheet. Among the coating films on at least the back surface of the precoated steel sheet, the coating film of the outermost layer contains a tungstate or a niobate having a high end surface retention property. The term "high end surface retention" used in the present specification means a property of ○ or Δ determined by the end face retention evaluation test described below.

A typical chromate-free precoated steel sheet of the present invention has an undercoat layer and an overcoat layer on the chromate-free chemical conversion treatment layer of each side, and the upper coating layer on the back side contains tungstic acid. salt.

In addition to the upper coating layer on the back side, the undercoat layer on the back side may also contain a tungstate or a niobate having a high end retention.

Examples of preferred tungstates are: sodium tungstate, calcium tungstate, ammonium tungstate, lithium tungstate, magnesium tungstate, and sodium tungstate and magnesium tungstate are particularly preferred. Two or more kinds of tungstates may also be used in combination.

The content of the tungstate in the coating film is preferably from 6 to 50% by weight.

Preferred examples of citrate are sodium citrate, potassium citrate and lithium niobate, with sodium citrate and potassium citrate being particularly preferred. Two or more kinds of phthalates may also be used in combination.

The content of the citrate in the coating film is preferably from 5 to 50% by weight.

As a precoated steel sheet (PCM) of a product, in general, a coating film having a desired performance is supplied to a person who needs it in a single-sided manner, and the person who is required to process the pre-coated steel sheet to produce a final product. In the present invention, a surface on which a coating film having a desired performance is desired is referred to as a "surface", and a surface on the opposite side of the surface is referred to as a "back surface". The "surface" is located on the outer side of the final product. The "back" is not visible in the final product, so the coating on the back does not require the opposite surface. The degree of performance required for the film. Because of this, the difference between the "surface" and the "back surface" of the precoated steel sheet is clear from the appearance. When the two are compared, the "back surface" can be easily distinguished because it is dwarfed from the viewpoint of beauty.

According to the present invention, the tungstate or bismuth hydride having a high end surface retention property contained in the coating film on the outermost layer of the coating film on the back surface of the chromate-free precoated steel sheet contributes to the red rust resistance of the end surface. The red rust on the end face can be effectively suppressed in the outdoor environment exposed to rain. Thereby, it is possible to contribute greatly to the expansion and utilization of the chromate-free precoated steel sheet which is free from harmful chromium elution.

The best form for implementing the invention

Here, first, the inventors have explained the ins and outs of the invention in which the novel end face red rust corrosion resistance of the present application is completed without the chromate type precoated steel sheet.

Compared with the chromate-free precoated steel sheet, in the chromate-treated precoated steel sheet which has been widely used in the past, when the end surface corrosion resistance is mentioned, it means that the swelling of the coating film near the end surface is prevented (bubble) Therefore, in order to prevent the swelling of the coating film from the end face portion even in the salt-damage region such as the coastal area, how to prevent the zinc from being eluted from the zinc coating layer of the precoated steel sheet As a development goal. The reason is that in the era of chromate-treated precoated steel sheets, it is hard to have red rust on the end faces due to the excellent corrosion resistance of chromate and the simple wet environment such as rain and dampness. Attention is paid to the corrosion resistance in a simple damp environment, but the swelling of the coating film due to the dissolution of zinc attracts attention. Therefore, the salt spray test (SST) in the laboratory has been paid attention to in the development of new products for chromate-treated precoated steel sheets. In the electrolyte environment of the salt spray test, even if white rust is generated due to the sacrificial anticorrosive effect of the zinc coating of the precoated steel sheet, red rust does not occur until the zinc is consumed.

However, in the case of a non-electrolyte atmosphere in which the degree of corrosion resistance is inferior to that of the chromate-treated precoated steel sheet, the problem of red rust on the end face in a short time suddenly becomes apparent. For example, the generation of red rust in an outdoor unit of an air conditioner is a big problem. The generation of red rust is strongly regarded as a problem that cannot be ignored, especially in the fitting portion or the knockout portion of the top plate and the side plate which are drawn and formed (the structure portion in which the steel plate is inserted into the slit and staggered by about 1 mm) The water is easily stagnated by capillary action, and the end face of the steel sheet continues to be wetted for a long period of time (depending on the occasion, for several days). In addition, the main red rust-producing area is not a salt-damaged area such as the coast, but a mountainous area. This is because the end face is continuously wetted by the electrolyte-free water and does not form a corroded battery. When it is in a high-humidity environment (that is, especially during the rainy season), the zinc does not sacrificially dissolve and causes the exposed iron end face. Caused by corrosion.

Such red rust is less likely to occur on chromate-treated precoated steel sheets because the chromate component (bismuth chromate) is eluted into the water from the end face (the nearby chromate treated film), and a protective layer is formed on the end face of the iron. The reason. On the other hand, the anti-rust pigment (calcium ion exchange tannin or aluminum tripolyphosphate) contained in the general chromate-free precoated steel sheet has low water solubility, and is inferior to the chromate composition even if it is eluted into water. Therefore, it is not possible to sufficiently prevent the occurrence of red rust in a wet environment in which the sacrificial corrosion prevention of zinc does not function effectively.

In view of the above, when the inventors developed the novel chromate-free precoated steel sheet of the present invention, it is considered that the following conditions (1) to (3) must be satisfied in order to obtain a sufficient red rust suppressing effect on the end surface.

(1) When the end surface of the precoated steel sheet is wetted by water for a long time, a sufficient amount of the component is eluted from the coating film.

(2) The dissolved component is effective for suppressing red rust or red rust and white rust.

The dissolved components are continuously retained in the vicinity of the end faces.

These conditions will be described in detail below.

About conditions (1)

The generation of red rust on the end surface of the precoated steel sheet is due to the high humidity period such as the plum rain period, and the sacrificial anticorrosive effect of zinc cannot be expressed in the environment where the end surface is continuously wetted by water having a small electrolyte content, and the iron is eluted into the water. Red rust is generated from the end face in a very short time. In order to suppress the occurrence of such red rust, the coating film contains a component effective for rust prevention, and when the end surface is wetted by water, the active component is eluted from the coating film, and the occurrence of red rust on the end surface is effectively suppressed. If the water solubility of the substance contained in the coating film is insufficient, even if the substance has rust preventing ability, the effect of suppressing red rust cannot be obtained because it cannot be eluted into water. Therefore, the substance contained in the coating film does not have sufficient water solubility.

Incidentally, an anti-rust pigment such as calcium ion exchange tantalum (trade name: SHIELDEX) and aluminum tripolyphosphate which is used in the current chromate-free precoated steel sheet is an example which does not have sufficient water solubility.

About conditions (2)

The component which is eluted into the water from the coating film must be a substance which suppresses the generation of red rust or red rust and white rust. Even for substances having red rust suppressing ability, the rust preventing mechanism is various, and according to the rust preventing mechanism, substances having red rust suppressing ability can be classified as follows.

(i) A substance which is eluted into water to form an electrolyte to increase the electric power, and exhibits a sacrificial anticorrosive effect of zinc to suppress generation of red rust.

(ii) A substance that is adsorbed to the surface of the iron to form a passive state or the like, and which inhibits the elution of iron itself (a substance that functions as a dissolution inhibitor of iron).

(iii) A substance which exerts an effect as a dissolution inhibitor of zinc in addition to the effect of the above (ii).

If it is a water-soluble salt, it is ionized to become an electrolyte once dissolved in water, and therefore it is considered to be a substance which is classified as the above (i) only if it does not have the characteristics of the substance of the above (ii) or (iii). However, the red rust suppressing mechanism of the substance (i) above is premised on the corrosion of zinc, so that generation of white rust and swelling of the coating film (foaming) cannot be avoided. If the corrosion of zinc continues, the sacrificial anti-corrosion effect will disappear, resulting in the beginning of red rust.

On the other hand, since the substance of the above (ii) does not depend on the sacrificial anticorrosive effect of zinc, but prevents the dissolution of iron itself, the dissolution of zinc is suppressed by the degree of self-dissolution in water, white rust and foaming. Less production. In the substance of the above (iii), since the self-dissolution of zinc is suppressed, generation of white rust and generation of foaming become less. From the above viewpoints, in the present invention, as the substance effective for suppressing the generation of red rust on the end face of the chromate-free precoated steel sheet, the substance of the above (ii) or (iii) is used instead of the above ( i) substance.

Whether or not a specific substance is any of the above (i) to (iii) can be roughly determined by the following method.

Regarding the substance of the subject, the following two tests were carried out.

(Test 1) A cold-rolled steel sheet (500 hours) was immersed in a solution obtained by dissolving 100 mg of the test substance in 40 ml of distilled water.

(Test 2) A general chromate-free precoated steel sheet having a coating film containing calcium ion exchange tannin or aluminum tripolyphosphate as an antirust pigment in the same aqueous solution (there is a tendency to easily form red rust on the end surface) () 500 hours).

In the immersion time of 500 hours in these experiments, it is assumed that the actual use conditions of the precoated steel sheet can suppress the occurrence of red rust on the end surface even if it is immersed for 500 hours continuously, and it is considered to be effective for suppressing the red rust on the end surface. Set by the necessary conditions for the substance.

The occurrence of red rust and white rust in each of the steel sheets taken out from the aqueous solution was examined, and which of the above-mentioned (i) to (iii) substances was determined based on the following criteria.

Substance classified as (i)

In the test 1, red rust was generated, and in the test 2, red rust was not generated, and white rust was generated in a large amount.

Substances classified as (ii)

In the test 1, red rust was not produced, and in the test 2, red rust was not generated and white rust was generated minutely.

Substances classified as (iii)

In the test 1, red rust did not occur, and in the test 2, red rust and white rust did not occur.

About conditions (3)

Even if the test substance satisfies the requirements of the above conditions (1) and (2) (that is, even if the coating film is sufficiently eluted into water and acts as a dissolution inhibitor of iron), if the substance is immediately washed off, it cannot be Acts as an inhibitor. Therefore, in order to ensure the corrosion resistance of the end face red rust, it is necessary that the substance belonging to the inhibitor component is continuously retained at or near the end face.

The term "stagnation" as used herein refers to a person who has a variety of meanings. For example, (a) the inhibitor component occludes or forms a passive film to cover the end face, (b) the physical solution of the inhibitory solution persists at the end face due to physical factors such as viscosity or surface tension, (c) (fluid flowing through the solid surface) In the case where the flow rate of the contact with the solid surface is 0, the liquid film interface layer region where the concentration of the inhibitor is high and which cannot be moved is present on the end surface, and any of them conforms to the "stagnation" as referred to herein. In addition, the "stagnation" caused by any phenomenon exhibits "end-end retention" when the effect of the inhibitor of the test substance is exerted.

The evaluation of the end surface retention of a specific substance can be carried out by the following method.

1) Dissolving 10 mg of the substance in 40 ml of distilled water to form a general chromate-free precoated steel sheet containing a coating film containing calcium ion exchange tannin or aluminum tripolyphosphate as an antirust pigment (inhibitor) (having an end face) A person who tends to generate red rust is cut into 50 × 50 mm and immersed in the aqueous solution for 72 hours.

2) The precoated steel sheet (50 × 50 mm) was taken out from the aqueous solution of the above 1) (at this time, red rust was not produced), and after gently washing with distilled water, it was quickly immersed in distilled water for 350 hours and 500 hours.

In the above 2), in the evaluation of the end surface retention property, the case where red rust was generated within 350 hours was set to ×, and red rust was not generated in the 350-hour immersion, but red rust was generated in the 500-hour immersion. In the case of Δ, it was set to ○ even if red rust did not occur at all for 500 hours. In the case where red rust did not occur for 350 hours (in the case of △ or ○), it was judged that the substance was "high in end surface retention". The immersion time of the inhibitor aqueous solution for 72 hours in the precoated steel sheet of the above 1) is a standard condition set to make the difference in the end surface retention of the substance remarkable. This is a condition that is consistent with the conditions of the top plate fitting portion of the air conditioner outdoor unit in which a large amount of red rust is generated in the plum rain period. The immersion time of 350 hours and 500 hours in the above 2) was set for the same reason as the immersion time of the test for the type of the substance having the red rust suppressing ability.

Under the premise of the above, the inventors have investigated the effects of various substances on the dissolution inhibitor of iron, and found several promising substances. Further, the end surface retention properties of the materials were tested. A part of the evaluation results is consolidated in Table 1.

As a result, although it is still unknown whether or not it exhibits high end-end retention depending on any of the above-mentioned "stagnation" mechanisms, in the tungstate group, sodium tungstate, calcium tungstate, ammonium tungstate, tungstic acid The tungstate represented by lithium and magnesium tungstate acts as a dissolution inhibitor of iron, and is determined to be a useful substance for imparting red rust resistance to the end face of the chromate-free precoated steel sheet.

In the tungstate group, it is known that sodium tungstate and magnesium tungstate are particularly preferred. It can also be confirmed from Table 1 that the end face red rust corrosion resistance exhibited by the tungstate is equivalent to the strontium chromate which is a rust preventive component of the chromate-treated precoated steel sheet. Further, although tungstate was used, the evaluation of Test 1 and Test 2 was judged as ○, but the end face retention test was judged as ×. The sodium molybdate which has been widely known as a rust preventive substance has been judged as ○ in the evaluation of Test 1 and Test 2, and was judged as × in the end face retention test.

The mechanism for obtaining the red rust suppressing effect of the end face by the tungstate such as sodium tungstate is not completely clarified as described above, but it is presumed that it forms a dense iron oxide on the iron surface of the end surface of the precoated steel sheet. Passive state, by which it acts as a dissolution inhibitor of iron.

From the point of view that the generation of some white rust is recognized in the above test 2 for determining the substance type, the tungstate is considered to have a low effect of suppressing the dissolution of zinc of the plating layer. However, on the contrary, since tungstate does not inhibit the dissolution of zinc (self-dissolving), it is considered that the dissolution of zinc is moderately obtained to obtain a sacrificial anticorrosive effect, and the actual exposed environment (including the condition of repetitive dryness and wetness) Under the actual use environment of the final product of the processed precoated steel sheet, an end face protective film is formed to suppress the generation of red rust, thereby advantageously exerting an effect. also. By using the calcium ion exchange tantalum (trade name SHIELDEX) or the antirust pigment such as aluminum tripolyphosphate used in the current chromate-free precoated steel sheet, it is also possible to synergistically match the end face red rust and corrosion resistance. Sexual improvement is highly anticipated.

In the citrate group, the citrate represented by sodium citrate, potassium citrate, and lithium ruthenate acts as a dissolution inhibitor of iron and also acts as a dissolution inhibitor of zinc, confirming that it is A useful substance for imparting red rust resistance to the end face of a chromate-free precoated steel sheet. Among them, sodium citrate and potassium citrate were found to be particularly excellent. It is also confirmed from Table 1 that the tantalum rust corrosion resistance exhibited by the bismuth salt of the present invention is equivalent to that of the chromic acid strontium which is a rust-preventing component of the chromate-treated precoated steel sheet. Further, although it was a citrate, calcium citrate was judged to have no rust prevention effect in either of Tests 1 and 2. This is generally considered to be because calcium citrate is a poorly soluble salt. Further, although magnesium citrate was judged as ○ in the evaluations of Test 1 and Test 2, it was judged as × in the end face retention test.

The mechanism for obtaining the red rust suppressing effect of the end face by the citrate of sodium citrate or the like is not completely clarified as described above, but it is considered to be retained on the iron surface and the zinc surface of the end surface of the precoated steel sheet as two metals. Those who dissolve the inhibitor and exert their effects. It is recognized that the red rust starts to be generated when the immersion time exceeds 500 hours in the tests of 2) and 3) of the above-mentioned end face retention evaluation. It is considered that the bismuth salt is weak even if it is adsorbed to the end face. The presence of a membrane interface layer is highly likely to cause retention.

Though it is known that the phosphate-based substance such as sodium phosphate or magnesium hydrogen phosphate has a characteristic of adsorbing to the surface of iron, "the end surface retention property is high", but the result of the above-mentioned end surface retention test is ×. In this example, a substance having a high adsorption capacity to the surface of iron is not necessarily a high end retention property.

In the present invention, the tungstate or bismuth salt having a high end surface retention property imparting red rust corrosion resistance to the chromate-free precoated steel sheet is preferably used in the coating film on the back side of the precoated steel sheet. In the case where the coating film on the back side is plural, tungstate or citrate having a high end surface retention property may be present in all of the coating films, or may be present only in a part of the coating film. In this case, in order to effectively express the end face red rust corrosion resistance, the tungstate or bismuth salt having a high end surface retention property is preferably present in at least the outermost coating film. In order to exhibit the red rust resistance of the end face, the tungstate or bismuth salt having a high end retention property must be dissolved in the water which damps the precoated steel sheet because the outermost coating film exposed on the back surface of the precoated steel sheet can provide the necessary The area (area that is wet by water). The coating film other than the outermost layer is exposed only to the end surface portion, and since the exposed area is only a little, it is considered that only the coating film other than the outermost layer cannot sufficiently ensure the dissolution of the tungstate or bismuthate having an effective amount of end surface retention. . In the case where the coating film on the surface side may contain a tungstate or a bismuth salt having a high end surface retention property, in this case, it should be mixed in a range which does not impair the need for the coating film on the surface side. Tungstate or citrate.

The tungstate with high endurance retention can be mixed into the outermost coating film on the back side of the precoated steel sheet by about 6 to 50% by weight, and the tantalum salt having high end surface retention can be mixed into the outermost layer of the back surface of the precoated steel sheet by about 5 to 50% by weight. In the film. If the range is less than the above, the red rust resistance of the end face is insufficient, and when it is more than 50% by weight, the tungstate or bismuth salt having a high end surface retention when the film after film formation is exposed to a wet environment will be Excessive dissolution, but there is a case where the coating film is swollen or peeled off. The concentration of the tungstate or bismuth salt having a high end surface retention property in the outermost coating film on the back surface is preferably from 7 to 40% by weight, and most preferably from 10 to 30% by weight.

When a coating film other than the outermost coating film on the back surface of the precoated steel sheet is mixed with a tungstate or a bismuth salt having a high end surface retention property, it is preferable to use a concentration close to the outermost coating film from the viewpoint of being easily eluted into water. The amount of the upper limit in the range.

The tungstate or bismuth salt having a high end surface retention property used in the present invention is a powder at a normal temperature, and it is preferable that the maximum particle diameter is 5 to 20 μm, and more preferably 5 to 10 μm, in consideration of the thickness of the coating film to be mixed. Disperse in the paint for use.

The tungstate or bismuth silicate powder having a high end-end retention property can be mixed into the coating material for the intended coating film (including a necessary component other than the tungate or bismuth salt having a high end retention property and a solvent) by any method. For example, it may be added to the coating material for which the preparation for finishing is completed, or a mixture obtained by mixing one or more components of the coating material in advance may be mixed with other components of the coating material to prepare a coating material.

The coating film of the chromate-free precoated steel sheet of the present invention can be used for general chromate-free if it is removed by adding a tungstate or a bismuth salt having a high end surface retentivity necessary for imparting red rust resistance to the end surface. A coating of a pre-coated steel sheet is formed. For example, an undercoat film and a surface coating film can be formed on the surface; the undercoat film is composed of a polyester resin, a melamine resin, an isocyanate, a calcium ion exchange tantalum, an aluminum tripolyphosphate, a titanium oxide, a vermiculite. (or cerium oxide), other additives, etc.; and the surface coating film is composed of the following: polyester resin, melamine resin, isocyanate, calcium ion exchange silicone, aluminum tripolyphosphate, titanium oxide, vermiculite (or two Cerium oxide), various coloring pigments, waxes, other additives, and the like. A primer film and a surface coating film are formed on the back surface; the primer film is composed of a polyester resin, a melamine resin, an isocyanate, a calcium ion exchange silicone, an aluminum tripolyphosphate, a titanium oxide, a vermiculite (or two). Cerium oxide), other additives, etc.; and the surface coating film is composed of the following: polyester resin, melamine resin, isocyanate, calcium ion exchange silicone, aluminum tripolyphosphate, titanium oxide, vermiculite (or cerium oxide) , various color pigments, waxes, other additives, etc. In the coating film according to the present invention, at least the coating film on the outermost side of the back surface contains a tungstate or a bismuth salt having a high end surface retention property.

As the steel sheet which is the base of the coating film, a galvanized steel sheet can be used. For the galvanized steel sheet, for example, a molten zinc plated steel sheet, an electrogalvanized steel sheet, an Al-zinc plated steel sheet, an Al-Mg-Si-zinc plated steel sheet, or the like can be used, but Al-Mg-Si-zinc is used. In the case of a plated steel sheet, the improvement in red rust resistance is particularly remarkable.

On the surface of the galvanized steel sheet, a general chromate-free chemical conversion treatment layer can be provided as a pretreatment layer for applying the coating material for a coating film. For example, a chemical conversion treatment layer may be a film containing two or more kinds of vermiculite (or cerium oxide), a decane 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 coating, wrench roll coating, brush coating, spray coating, or the like of the chemical conversion treatment liquid.

The chromate-free precoated steel sheet of the present invention can be formed by using a chromate-free coating if the coating of the outermost coating film on at least the back surface is removed by using a coating having a tungstate or a bismuth salt having a high end surface retention property. The pre-coated steel sheet is generally manufactured by a manufacturing apparatus and a manufacturing method, and this point is not necessarily described in detail herein.

Example

The invention is further illustrated by the examples. Needless to say, the invention is not limited to the embodiments presented herein.

A. Original material of sample material

The sample material was produced using the following original plates GI and SD.

(1) GI: molten zinc plated steel plate (0.6 mm thick, single-sided plating adhesion amount 80 g/m ² )

(2) SD: 11% Al-3% Mg-0.2% Si-zinc plated steel plate (0.6 mm thick, single-sided coating adhesion amount 80 g/m ² )

B. Chemical conversion treatment of the original board

The original plate was subjected to a chromate-free chemical conversion treatment (100 mg/m2) by a decane coupling agent, tannic acid, vermiculite (or cerium oxide), and a polyester resin mixed treatment agent.

C. Finishing of the sample material

A primer obtained by blending the rust-preventive pigment component shown in Table 2 into the coating resin described below was used to apply primer and top finish to the original chemically converted original sheet.

(1) Surface ‧ back primer resin

Polyester/melamine + isocyanate and hardened type (FLC687 coating resin manufactured by FINE COATINGS, Japan)

(2) Surface top coating resin

Polymer polyester/melamine hardening type (FLC7000 coating resin manufactured by FINE COATINGS, Japan)

(3) Back top coating resin

Polyester/melamine hardening type (FLC100HQ coating resin manufactured by FINE COATINGS, Japan)

The anti-rust pigment components blended into the coating of Table 2 are as follows.

A: Calcium ion exchange silicone

B: aluminum tripolyphosphate

C: sodium tungstate

D: calcium tungstate

E: ammonium tungstate

F: lithium tungstate

G: sodium phosphotungstate (comparative)

H: sodium bicarbonate (comparative)

I: potassium halide (comparative)

J: Magnesium tungstate

K: strontium tungstate (comparative)

L: sodium molybdate (comparative)

In Table 2, the value in the bracket after the mark of the rust preventive pigment indicates the percentage by weight of the rust preventive pigment of the coating relative to the total solid content (for example, "A(20)" indicates that the coating contains 20% by weight of the pigment A. ). Only the anti-rust pigment components formulated into the paint are shown in Table 2, and the color pigment components are not shown.

The following performance tests were performed on the produced sample materials.

1. Pastoral zone exposure test

The purpose of the idyllic exposure test is to directly assess the resistance to red rust generated in a short period of time in a non-electrolyte wet environment.

The processed material which is processed into a shape of the ceiling portion fitting portion of the air conditioner outdoor unit after processing the sample material, and the unprocessed flat plate are used as test materials.

Upon entering the Mei period, the test materials were placed in the idyllic zone (inland of Junjin City, Chiba Prefecture) and visually observed one month later.

The red rust evaluation of the top plate fitting portion was evaluated in the following manner: the amount of red rust generated was ◎, the amount of red rust generated was small (red rust less than 1%) was ○, and the amount of red rust was small (red rust) 1 to 5%) is △, and the amount of red rust is large (red rust is more than 5%) is ×.

The foaming evaluation of the flat surface portion of the flat plate was evaluated in the following manner: those who did not produce foam at all were ◎, and the trace amount (above the ASTM D 714 standard was 8F or more) was ○, less to more (less than 8F) ) is ×.

2. Salt damage exposure test

The purpose of the salt damage exposure test is to evaluate the corrosion resistance from the viewpoint of the long-term resistance in an electrolyte environment.

The sample material is processed and reproduced as a processing material in the shape of the top plate fitting portion of the air conditioner outdoor unit, and an unprocessed flat plate is used as a test material. Each test material was placed in a salt-damaged area (the coastal zone of Okinawa) and visually observed after 2 years.

The red rust evaluation of the top plate fitting portion was evaluated in the following manner: the amount of red rust generated was ◎, the amount of red rust generated was small (red rust less than 1%) was ○, and the amount of red rust was small (red rust) 1 to 5%) is △, and the amount of red rust is large (red rust is more than 5%) is ×.

The evaluation of the end surface swelling of the flat surface portion of the flat plate was evaluated in such a manner that the maximum value of the swelling from the end surface was ○ or less than 2 mm, and the value was more than 2 mm.

3. Laboratory test

The purpose of the laboratory test (distilled water immersion test, 40 ° C HCT test, boiling water 2 hour immersion test) was to simply evaluate the resistance to red rust generated in a short period of time in a non-electrolyte wet environment. However, with regard to the present invention, the results of the exposure test in the pastoral zone are the first of the best, and the laboratory test is only used as a reference test.

The time during which the aqueous solution of the substance eluted from the coating film stayed near the end face was very long in the distilled water immersion test, but was extremely short in the 40 ° C HCT test. The actual environmental conditions are generally considered to be intermediate conditions in such laboratory tests. Therefore, it is believed that good results can be obtained from these laboratory test conditions, which will lead to good results in the actual environment.

In the distilled water test, after immersing 50 × 50 mm of the sample material in 40 ml of distilled water for 500 hours, the red rust of the end face was evaluated. Those who did not produce red rust at all were evaluated as ◎, those with a small amount (red rust less than 1%) were ○, a small amount (red rust 1 to 10%) was Δ, and a large amount (red rust was more than 10%) was ×.

In the 40 ° C HCT test, 50 × 100 mm of the sample material was vertically suspended in a constant temperature and high humidity bath (40 ° C, 98% RH), and the red rust of the end surface after 240 hours was evaluated. Those who did not produce red rust at all were evaluated as ◎, those with a small amount (red rust less than 5%) were ○, a small amount (red rust 5-10%) was Δ, and a large amount (red rust exceeding 10%) was ×.

In the boiling water for 2 hours immersion test, the flat plate was immersed in boiling water for 2 hours, and foaming of the flat surface portion and the flat surface portion of the back surface was evaluated. Those who did not produce foam at all were evaluated as ◎, and a small amount (above 8 degrees of ASTM D 714) was ○, and less (more than 8F) was ×.

The results of the above tests are shown in Table 3.

Further, the rust preventive pigment component was replaced with the rust preventive pigment component shown below, and each of the above tests was carried out in the same manner. The ingredients to be formulated are shown in Table 4, and the test results are shown in Table 5.

The anti-corrosive pigment components of the coatings formulated to Table 4 are as follows.

a: calcium ion exchange silicone

b: aluminum tripolyphosphate

c: sodium citrate

d: magnesium citrate (comparative)

e: potassium citrate

f: calcium citrate (comparative)

g: sodium bicarbonate (comparative)

h: potassium halide (comparative)

i: lithium niobate

Claims (10)

A chromate-free precoated steel sheet having excellent red rust and corrosion resistance of an end mask, which is on both sides of a galvanized steel sheet, and has a chromate-free chemical conversion treatment layer and a coating film of one or more layers formed thereon. And an exposed iron end face; wherein, in the coating film on at least the back surface of the precoated steel sheet, the coating film of the outermost layer contains a tungstate or a niobate having a high end surface retention property, and the exposed iron end face In the environment in which the water in the non-electrolyte gas environment is continuously wetted, the tungstate or the foregoing niobate is eluted from the coating film, and the dissolved tungstate or the foregoing niobate is in an environment in which the water is continuously wetted. Continued to remain on the exposed end face of the iron. The chromate-free precoated steel sheet having excellent red rust and corrosion resistance, such as the end face of the patent application, has a primer coating layer and an upper coating layer on the chromate-free chemical conversion treatment layer on each side. Further, the upper coating layer on the back surface contains a tungstate or a cerium salt having a high end surface retention property. For example, the chromate-free precoated steel sheet having excellent red rust and corrosion resistance of the end face of the second application of the patent application, the undercoat layer on the back side further contains a tungstate or a bismuth salt having a high end retention property. The chromate-free precoated steel sheet having excellent red rust and corrosion resistance of the end mask according to any one of claims 1 to 3, wherein the tungstate-based sodium tungstate and calcium tungstate having high end surface retention property are used. A mixture of ammonium tungstate, lithium tungstate, magnesium tungstate or the like. The chromate-free precoated steel sheet having excellent red rust and corrosion resistance of the end mask of any one of the first to third aspects of the patent application, wherein the bismuth citrate having high end surface retention property is sodium citrate, potassium citrate, Lithium niobate or a mixture thereof. The chromate-free precoated steel sheet having excellent red rust and corrosion resistance, such as the end face of the fourth aspect of the patent application, wherein the tungstate-based sodium tungstate and magnesium tungstate having high end surface retention properties are used. The chromate-free precoated steel sheet having excellent red rust and corrosion resistance as the end face of the fifth application of the patent application, wherein the bismuth citrate having high end surface retention property is sodium citrate and potassium citrate. The chromate-free precoated steel sheet having excellent red rust and corrosion resistance of the end mask of any one of claims 1 to 3, wherein the coating having a high endurance retention of the end face is 6 to 50% by weight. . The chromate-free precoated steel sheet having excellent red rust and corrosion resistance of the end mask of any one of claims 1 to 3, wherein the sulfonate content of the coating having high end surface retention is 5 to 50% by weight. . The chromate-free precoated steel sheet having excellent red rust and corrosion resistance, such as the end face of the patent application, has a tungstate content of 6 to 50% by weight in the coating film.