JPH06264256A - Steel excellent in weather resistance - Google Patents

Abstract

PURPOSE:To obtain high weather resistant steel excellent in sea salt resistance by coating the surface of steel with a rust layer or alpha-FeOOH having a specified average grain size. CONSTITUTION:The surface of the steel of low alloy steel or carbon steel such as weather resistant steel is coated with a rust layer having <=200nm average grain size or alpha-FeOOH having <=200nm average grain size. In this place, rust denotes a corrosion product in which alpha-FeOOH, gamma-FeOOH, Fe3O4 or the like caused by the atmospheric corrosion of general iron are coexistent unless the antibiotics are not specified, and if only the grain size is <=1200nm, the corrosion preventing properties of rust is good not only in an industrial area and a country area but also in a seaside area which is a severest environment in which sea salt grains are flying. In this way, the steel having high corrosion preventing properties in which the generation of the loose scale and flowing rust of red rust, yellow rust or the like in an atomospheric corrosion environment is eliminated can be obtd.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a steel material having excellent weather resistance,
Particularly, the present invention relates to a steel material with excellent weather resistance whose surface is covered with rust that is stable and has high anticorrosiveness even in the most severe atmospheric corrosion environment where sea salt particles that adversely affect the weather resistance of steel come in. .

[0002]

2. Description of the Related Art Generally, by adding elements such as P, Cu, Cr and Ni to steel, corrosion resistance in the atmosphere can be improved. These low alloy steels are called weather resistant steels,
It is a so-called maintenance-free steel that forms rust that protects against corrosion (hereinafter referred to as weather-resistant rust) in a few years outdoors and does not require subsequent corrosion-resistant treatment such as painting.

However, since it takes several years until the weather-resistant rust is formed, floating rust such as red rust and yellow rust and flow rust are generated during the period until then, and not only it is not preferable in appearance. The problem remains that it also causes pollution of the surrounding environment.

Regarding this problem, for example, Japanese Patent Laid-Open No. 1-14
As in 2088, a surface treatment method for forming a phosphate film has been proposed.

[0005]

However, the content of the treatment is complicated, such as the necessity of performing an appropriate pretreatment before forming the phosphate film, and when the welding of steel materials is required, It is not easy to apply the treatment and there is a problem that it is difficult to apply it to a building structure.

Conventionally, a drunk surface treatment method such as applying a coating film on the surface of weather resistant steel or applying a coating after forming a phosphate film has been used. There are problems that the generation of rust becomes slower, and the coating film itself deteriorates and the appearance is significantly impaired.

Further, there is a problem that weather resistant rust is not formed in a severe environment such as a coastal area where sea salt particles fly.

Therefore, the object of the present invention is to prevent sea rust such as red rust or yellow rust or flow rust, and to cover the sea salt particle resistance covered by a rust layer which is artificially grown early and has excellent corrosion resistance. An object of the present invention is to provide a steel material excellent in weather resistance and used for civil engineering and construction.

[0009]

Means for Solving the Problems The present inventors have found that the surface of a steel material is covered with a rust layer having an average crystal grain size of a certain size or less, and particularly the average grain size of a certain size or less is It was found that if the rust contained is α-FeOOH, the weather resistance of the steel material is extremely high.

That is, the present application includes the following inventions. 1. Steel material with excellent weather resistance, characterized in that the surface of the steel material is covered with a rust layer having an average crystal grain size of 200 nm or less.

2. Steel material with excellent weather resistance, characterized in that the surface of the steel material is covered with α-FeOOH having an average crystal grain size of 200 nm or less.

[0012]

The present invention will be described in more detail below.
If the rust has few physical defects such as cracks and voids in the atmospheric corrosive environment, it will prevent the ingress of oxygen, water, and corrosive substances in the atmosphere, and as a result shut off the atmospheric corrosive environment. In addition, it reduces the elution of Fe ions, which is the fundamental cause of floating rust and flow rust.

The present inventors have found that if the average crystal grain size of the particles forming the rust covering the surface of the steel material is not more than a certain size, the rust becomes dense and has a high anticorrosion function. If the average crystal grain size is specifically 200 nm or less,
The corrosion resistance of rust is good not only in industrial areas and rural areas but also in coastal areas where sea salt particles fly in and are the most severe environment.

In particular, it has been found that when the steel surface is covered with α-FeOOH, which is a final corrosion product that is stable in a normal atmospheric corrosion environment, the weather resistance of the steel is further improved. It is considered that this is because α-FeOOH is thermodynamically stable and suppresses the electrochemical reaction associated with the phase transformation and dissolution of rust.

However, the steel referred to in the present invention does not include stainless steel, Ni-based alloys and the like, which do not form so-called rust which is usually a precipitation type film, and mainly refers to low alloy steel such as weathering steel and carbon steel. .

The term "rust" as used in the present invention means that, unless the names of its constituents are specified, the corrosion produced by the atmospheric corrosion of general iron is a mixture of α-FeOOH, γ-FeOOH, Fe ₃ O _{4 and the} like. Refers to something.

[0017]

EXAMPLES Next, the effects of the present invention will be clarified by examples. Table 1 shows the chemical composition of the test steel used in the examples. In order to cover the steel surface with a rust layer having an average crystal grain size of 200 nm or less, for example, by the method disclosed in JP-A-3-320646, an appropriate amount of an aqueous chromium (III) sulfate solution is applied while being exposed to the atmosphere. Well, α-FeOOH less than 200nm
In order to cover the above, for example, by the method described in Japanese Patent Application No. 4-199701, an appropriate concentration of chromium (III) sulfate aqueous solution and an appropriate concentration of sodium hydroxide aqueous solution are applied and exposed to the atmosphere. Further, the average crystal grain size of rust can be controlled by controlling the concentration of the applied aqueous solution.

The test was carried out by exposing the steel surface to a 0.05 to 40 mass.% Aqueous solution of chromium (III) sulfate once a week for 3 months while exposing it to the atmosphere, or after the above treatment, an aqueous sodium hydroxide solution of pH 9 was used. Was applied to the steel surface once a week and was exposed to the atmosphere for another month to obtain rust with various average grain sizes or α-FeOO with various average grain sizes.
An exposure test piece composed of H 2 and covered with rust was prepared and performed.

About 100 mg of the rust layer was collected with a cutter knife until a steel surface could be confirmed and pulverized into a powder sample, which was used to measure the average crystal grain size of rust by transmission electron microscopy.

At this time, the constituent iron compounds were identified by taking an electron beam diffraction pattern.

The test piece had a size of 150 × 60 × 3 mm ^3, and the surface before treatment was polished with emery paper and buffed and used as a mirror surface.

Each test piece was exposed to the coastal area of Kitakyushu for 10 years under the same conditions.

The average corrosion rate (amount of reduction in plate thickness per unit time) during the exposure period was evaluated by measuring the weight of the steel plate. A concrete plate was placed directly below the exposed test piece, and the amount of flow rust was evaluated based on the degree of dirt on the concrete. The results are shown in Tables 2 and 3 and FIGS. 1 and 2.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

[Table 3]

FIG. 1 shows the relationship between the average crystal grain size of the rust layer and the corrosion rate. When the average crystal grain size of rust is 200 nm or less, the corrosion rate is low, and it is clear that the rust is dense and has high anticorrosion properties. Further, as shown in FIG. 2, when the rust layer is composed of α-FeOOH, it can be seen that the corrosion resistance is particularly large when the average crystal grain size is 200 nm or less.

Further, as shown in Tables 2 and 3, when the average crystal grain size exceeds 200 nm, the corrosion rate rapidly increases and the amount of flow rust increases particularly. On the contrary, when the thickness is 200 nm or less, almost no flow rust was observed.

[0029]

According to the present invention, the surface of the steel material has an average crystal grain size of 200
Due to the fact that it is covered with a rust layer with a thickness of nm or less or the surface of the steel material is covered with α-FeOOH with an average crystal grain size of 200 nm or less, floating rust and flow rust such as red rust and yellow rust in the atmospheric corrosive environment A steel material having rust with high corrosion resistance that hardly occurs is provided. Therefore, it greatly contributes to the enhancement of social capital such as buildings and structures.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the corrosion rate of a steel sheet and the average crystal grain size of surface rust.

FIG. 2 is a graph showing the relationship between the corrosion rate of a steel sheet and the average crystal grain size of surface rust composed of α-FeOOH.

Claims (2)

[Claims] 1. A steel material having excellent weather resistance, characterized in that the surface of the steel material is covered with a rust layer having an average crystal grain size of 200 nm or less. 2. A steel material having an average crystal grain size of 200 nm or less α
-A steel material with excellent weather resistance, which is characterized by being covered with FeOOH.