JPH06322549A - Surface treated steel material excellent in weather resistance - Google Patents

Abstract

PURPOSE:To obtain a steel material which is capable of freely coloring and has long-term weather resistance even under an air corrosive atmosphere by applying an alpha-FeOOH coating film on a lower layer and an org. resin coating film on a upper layer. CONSTITUTION:The alpha-FeOOH being chemically stable corrosion product is formed previously on the surface of the steel material, and the org. resin coating film is formed thereon. In this way, long-term weather resistance is imparted without occurring of reaction such as corrosion at an interface. Because the alpha-FeOOH is chemically stable, and does not react or transform further even if moisture or oxygen, etc., is present, the stable interface between the alpha-FeOOH and the org. resin coating film being the upper layer is formed. Moreover, the alpha-FeOOH is strong in cohesive force and excellent in adhesion property with the steel surface.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a new treated steel material, and more particularly to a steel material having excellent weather resistance in an atmospheric corrosive environment.

[0002]

2. Description of the Related Art The addition of elements such as P, Cu, Cr and Ni to steel improves the corrosion resistance of the steel in the atmosphere. These low-alloy steels are generally called weather-resistant steels and form rust that protects against corrosion (hereinafter referred to as weather-resistant rust) in a few years outdoors, eliminating the need for subsequent corrosion-resistant treatment such as painting. This is so-called maintenance-free steel.

However, it takes several years to form weather-resistant rust.
Since it takes 10 years or more, floating rust such as red rust or yellow rust or flow rust occurs during that time, which is not only visually unfavorable but also causes pollution of the surrounding environment, and the color tone is dark brown. Since it is limited to (chocolate color), there is a problem that it is difficult to design the landscape with free coloring.

On the other hand, as a coloring method, coating with an organic resin has been conventionally known.

However, the barrier property of the organic resin for permeation of corrosive substances such as water and oxygen is limited, and it is not possible to completely prevent the corrosive substances from entering the steel surface.
When the corrosive substance reaches the steel surface, the surface of the steel material is corroded, so that the adhesive force between the organic resin coating and the steel material is lost, and the coating film floats or peels off.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a surface-treated steel material which can be freely colored and has long-term weather resistance.

[0007]

As a result of intensive studies, the inventors of the present invention have found that the deterioration of the adhesion of the organic resin is caused by the corrosion of the steel material at the interface between the organic resin film and the steel material. If α-FeOOH, which is a chemically stable corrosion product, is formed in advance on the surface and an organic resin film is formed on this, a reaction such as corrosion does not occur at the interface,
The present invention has been accomplished by finding that long-term weather resistance can be imparted.

The present invention has as its constitution the surface-treated steel material characterized in that the lower layer is coated with an α-FeOOH coating and the upper layer is coated with an organic resin coating.

[0009]

[Action]

(1) Effect of α-FeOOH coating Generally, when iron corrodes, iron oxide or hydroxide is produced. Under normal atmospheric corrosive environment, the formed corrosion products (rust) are α-FeOOH and γ-FeOOH.
It is roughly divided into.

By the way, if the structure of the formed rust film is dense, it is easy to physically block the atmospheric corrosive environment, and the subsequent corrosion reaction is reduced. α-FeOOH is easy to form a fine and dense structure, and has a higher environmental barrier ability than γ-FeOOH which forms a coarse coating film with large gaps.
In particular, the effect is high when the film is formed with α-FeOOH having a crystal grain size of 200 μm or less.

Further, α-FeOOH is chemically stable, and does not react or transform further in the presence of water, oxygen, etc., so that it should form a stable interface with the upper organic resin film. You can On the other hand, γ-FeOOH lacks chemical stability and, when coexisting with iron, oxidizes iron to reduce itself, or transforms into α-FeOOH to change the crystal size, resulting in a stable interface. Cannot be formed.

Further, although the reason is not clear, α-F
Although eOOH has a strong cohesive force and excellent adhesion to the steel surface, γ-FeOOH has a weak cohesive force and poor adhesion, and easily falls off the steel surface. This is because γ-FeOOH is α-
The crystal density is lower than that of FeOOH. In other words, Fe
It is considered that one of the reasons is that the volume expansion when reacting with FeOOH is large.

What is referred to as an α-FeOOH coating here does not necessarily need to be pure α-FeOOH.
Impurity elements such as r, Cu, P, and Al or oxides thereof may be included. Further, although a small amount of γ-FeOOH may be mixed, the mixture amount of γ-FeOOH causes a deterioration in performance, so that the mixed amount is preferably 10% or less.

The method for forming this α-FeOOH coating is not particularly limited. For example, the method described in Japanese Patent Application No. 4-244796 proposed by the inventors can be used.
That is, chromium (III) ions and copper (II) are present on the steel surface.
After applying an aqueous solution containing at least one of the ions, the rust layer formed on the surface is made to have an environment of PH7 or higher with an aqueous solution of sodium hydroxide, etc.
A film of OOH can be formed.

Further, an α-FeOOH coating formed by exposing so-called weather resistant steel to an outdoor environment can be used as the lower layer. At this time, the rust layer on the surface of the weather resistant steel has an α-FeOOH coating formed on the lower layer and a γ-FeOOH layer on the upper layer.
It has a two-layer structure in which OH is formed. Therefore, the surface γ-
The FeOOH coating needs to be removed. γ-FeOOH
The coating has a weak cohesive force and lacks adhesion, so it can be easily removed by shaving with a wire brush. On the other hand, since the lower α-FeOOH coating has both strong α-FeOOH cohesion and adhesion, it does not peel off even if it is scraped off and remains as it is. After removing the γ-FeOOH powder generated during keren by washing with water or the like, the surface moisture is evaporated with warm air or the like to obtain a relatively clean α-FeOOH coating surface.

The thickness of the α-FeOOH coating in the present invention is preferably 1 μm or more, more preferably 5 μm or more. When the α-FeOOH coating is less than 1 μm, α-F
The steel under the eOOH coating is likely to corrode, which may shorten the life of the upper organic resin coating.

(2) Effect of organic resin coating Although the above-mentioned α-FeOOH coating itself exerts anticorrosion property on steel materials, this effect is not perfect, and by combining it with an organic resin coating. Corrosion resistance is further improved. Further, although only a small amount of α-FeOOH coating is observed to cause flow rust, it is possible to completely eliminate flow rust by forming an organic resin coating on the upper layer.

Further, the organic resin film can be colored freely by adding a coloring pigment, so that the color tone can be adjusted according to the surrounding landscape.

The organic resin film as the top coat has a dry film thickness of 5 μm or more, more preferably 50 μm. 5 μm
In the case of a thinner organic resin film, the lower layer α-FeOO
Although it is affected by the film thickness of H, the life may be slightly reduced.

The organic resin used in the present invention is not particularly limited. Epoxy resin, urethane resin, vinyl resin,
A polyester resin, an acrylic resin, an alkyd resin, a butyral resin, a phthalic acid resin, etc., a silicone resin, a fluororesin, or a combination thereof or a laminate thereof can be used. In view of the object of the present invention, it is to provide a surface-treated steel material having long-term weather resistance, and in that sense, it is more preferable to form an organic resin coating film that is less discolored by ultraviolet rays and has good light resistance. Specific examples of suitable resins include acrylic resins, acrylic urethane resins, acrylic silicon resins, and fluororesins.

Further, the above resin is made into a coating material for coating.
In addition to solvent-based paints, there is no particular problem as water-based paints. Furthermore, in the organic resin coating, red iron oxide, titanium dioxide, carbon black, titanium yellow, phthalocyanine green, color pigments such as phthalocyanine blue, talc, silica, mica, barium sulfate, calcium carbonate and other extender pigments, chromium oxide, Rust preventive pigments such as zinc chromate and lead chromate, and other conventional additives such as thixotropic agents, dispersants, and antioxidants may be contained. This organic resin film can be applied by a method such as air spraying, airless spraying or brush coating, as in the usual coating method.

The steel material used in the present invention may be of any steel type. It may be ordinary steel or weathering steel. However, high alloy steel such as stainless steel has α on the surface.
It is usually limited to low alloy steels because it does not form a FeOOH coating.

[0023]

EXAMPLES The effects of the present invention will be specifically described below with reference to examples. Table 1 shows the chemical components of the test pieces used in the examples of the present invention. Table 2 shows the method for forming the α-FeOOH coating, and Table 3 shows the composition of the topcoat paint. The dimensions of the test piece are 150 × 70 × 3.2 mm. Regarding the Cr and Cu treatments, an aqueous solution containing 10 mass% of chromium sulfate and 10 mass% of copper sulfate was applied once or several times, and then alkali treatment was performed with sodium hydroxide of PH9. Α-F having various film thicknesses by changing the number of Cr and Cu treatments
An eOOH coating was obtained. Further, regarding the exposed material, samples were obtained in which the proportion of α-FeOOH contained in the rust coating was changed by changing the degree of keren.

With respect to the urethane resin in the top coating, the total amount of the main component polyol and the isocyanate curing agent is shown, and the compounding ratio thereof is 3: 1. The pigment was 6% by mass of the coloring pigment, 13% by mass of the extender pigment, and 1% by mass of the thixotropic agent, and 20.0% by mass were added to the top coating material in total.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

[Table 3]

The top coat was coated by airless spray coating. This sample test piece was evaluated by the combined cycle test. The combined cycle conditions were a cycle of 35 ° C. salt spray test for 8 hours, 50 ° C., 3% saline immersion for 8 hours, 50 ° C., drying for 8 hours (irradiation with xenon lamp). It is considered that 20 cycles of this test correspond to about 1 year under the normal exposure environment in Japan. This test was performed for a maximum of 600 cycles (corresponding to 30 years), the deterioration state of the organic resin film was observed over time, and evaluation was made by the number of cycles in which deterioration such as floating and peeling occurred. The color difference was measured with a color difference meter, and the number of cycles in which the color difference exceeded 5 was evaluated.

Table 4 shows sample preparation conditions and results. Regarding the film thickness of α-FeOOH and the proportion in the rust film, the cross-section and the surface were observed by a polarization microscope and the extinction portion (α-FeOOH was extinction, γ-FeOOH was emission).
The thickness was determined and the surface photograph was subjected to image analysis processing to determine the thickness in percentage. The deterioration cycle number is 60
0 means that no deterioration was shown even after 600 cycles.

[0030]

[Table 4]

Looking at the results of the examples, test numbers 1 to
In No. 14, excellent weather resistance is obtained due to the synergistic effect of the lower α-FeOOH and the upper organic resin coating. If the upper organic resin film has the same film thickness, the life can be significantly extended as compared with the case where the conventional blast-treated steel material is directly coated. On the other hand, it can be seen that even if the same rust layer is coated on γ-FeOOH, the life is shortened. In addition, since a resin having excellent light resistance is generally expensive,
It is also possible to reduce the amount of expensive paint used by applying multiple layers as described above.

[0032]

As described above, according to the present invention, it is possible to inexpensively obtain a surface-treated steel material having extremely high long-term weather resistance in an atmospheric corrosive environment. If this steel material is widely applied to steel materials used for civil engineering or building structures, not only will maintenance of the steel materials be unnecessary or the period will be extended, but also free coloring will be possible, which will be useful for landscape conservation. is there.

Claims (1)

[Claims] 1. A surface-treated steel material having a lower layer coated with an α-FeOOH coating and an upper layer coated with an organic resin coating.