JPH08278245A - Method for evaluation of stability of rust of steel material - Google Patents

Abstract

PURPOSE: To obtain a method in which the degradation of the capability of weather-resistant steel is detected at an early stage when stable rust is destroyted by a method wherein the particle size distribution of crystal particles constituting rust is measured and the stability of the rust is evaluated on the basis of the particle size distribution. CONSTITUTION: In a method, the particle size distribution of crystal particles constituting rust which is generated on the surface of a steel material and which is stuck closely to ground iron is measured, and the stability of rust on the surface of the steel material is evaluated on the basis of the particle size distribution. That is to say, a marking line is formed on the surface of the steel material by a sharp edge, and organic film is pasted on the part of the marking line, and the organic film is then stripped so as to be dried on a glass or the like. After that, carbon is vapor-deposited on the film, and the film is cut to a size which can be mounted on, and attached to, an stage at an electron microscope. Then, its surface is protected by molten paraffin or the like, the film is then immersed in methyl acetate, the organic film and the paraffin are dissolved, and rust particles are scooped up with a mesh so as to be used as a sample for the electron microscope. The sample is photographed, and the particle size distribution of the crystal particles is found by using an image processing program.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention is a technique for evaluating the durability of weather-resistant steel used unpainted in rural areas, in mountainous areas, and in environments where the corrosion of metals is promoted by the presence of sea salt particles. The present invention relates to a method for evaluating the generated rust layer.

[0002]

2. Description of the Related Art A weather-resistant steel used unpainted is said to be used as an almost permanent structure because the corrosion rate is remarkably reduced by forming a stable rust layer on the surface under the use environment. With characteristics. In conventional anticorrosion by painting, there is a problem that it is necessary to change the coating regularly and the cost is high, and it may be difficult to change the coating due to the structure.
In recent years, the demand for weathering steel has increased. However, in an environment where the amount of flying sea salt particles is high or an environment where proper drying / wetting cannot be obtained, stable rust with excellent weather resistance does not grow sufficiently, and the landscape as a structure is damaged or damaged. Since it may cause serious damage such as partial drop, it is unavoidable that the weathering steel currently has a limit of application.

Therefore, a performance evaluation technique regarding the corrosion rate becomes very important. As an evaluation method of the generated rust layer, “Research Report on Application of Weatherproof Steel to Bridges” by the Institute of Civil Engineering, Ministry of Construction, Steel Club, Japan Bridge Construction Association, etc. in March 1984. (II) ”describes the method by visual inspection. Although it is a method of using the ferroxil test to evaluate using blue spots generated by the reaction with the ground iron through the pinholes in the rust layer, it has a drawback that it is a subjective rating and quantitative evaluation is difficult.

Further, as a method of nondestructively and relatively quantitatively evaluating the steel material protective ability of the rust layer on site, the steel material protective ability of the rust layer described in "Corrosion" Vol. 45, page 347, published in 1989. Tester (RST: Rus
(known as t Availability Tester) is known. This evaluation method, which is based on the principle of measuring the ion permeation resistance in the rust layer, is a good method with excellent rapidity, but it is used to evaluate the characteristics of weathering steel that has begun to be applied even in more severe environments in recent years. In order to achieve this, it is necessary to consider evaluation methods that are more precise and have different viewpoints. There is a strong demand from this point of view, because an excellent evaluation method can optimize and reduce the maintenance cost of unpainted structures using weathering steel.

[0005]

DISCLOSURE OF THE INVENTION The present invention provides a method for periodically inspecting whether or not the desired stable rust is successfully growing on the surface of weather resistant steel after construction, and various reasons for the stable rust. The present invention intends to provide a method for early discovering that the ability of the steel as a weather resistant steel has deteriorated due to the destruction by the above.

[0006]

[Means for Solving the Problems] As a method for evaluating a rust layer formed on the surface of a weather-resistant steel, it is not based on an appearance inspection or an electrochemical method, but a method focusing on the structure itself. By observing, it was found that the crystal grain size to be contained can be classified and evaluated as a parameter, and as an evaluation method therefor, it was found that observation at an electron microscope level is particularly suitable, leading to the present invention.

That is, the gist of the present invention is as follows. (1) A method of measuring the particle size distribution of the crystal particles that form rust that are generated on the surface of a steel material and that are in close contact with the base iron, and evaluate the stability of rust on the surface of the steel material based on the particle size distribution. (2) It is characterized in that after sticking an organic film on a portion of a steel material where a scribing line is attached with a sharp blade, the organic film is peeled off and the particle size of the transferred rust is measured by an electron microscope. The method for evaluating the stability of rust on the surface of a steel material according to (1).

Next, a detailed description of the present invention will be given. The stable rust formed on the surface of the weather resistant steel has various compositions and structures, but it is known that, for example, a rust layer mainly composed of crystal grains of α-FeOOH phase is relatively stable. From the viewpoint of the structure of rust, it can be expected that stable rust can be formed if a dense, glassy rust is formed on the steel plate to prevent erosion by incoming sea salt particles, but the weather resistance of the current technology The rust layer in the heat-resistant steel has a form containing crystal grains, and the evaluation method of the present invention is intended for a weather-resistant steel having a rust layer containing such crystal grains.

Generally, the thickness of the rust layer adhered to the surface of the weather resistant steel is about 30 to 100 μm, and in order to evaluate the morphology of the constituent particles of the rust without scratching the steel material itself, the thickness of about 10 μm from the surface. I decided to collect the part up to the thickness. In recent years, a weather resistant steel is known in which a rust having a two-layer structure in which the inner layer portion close to the steel sheet is amorphous and the outer layer portion is crystalline is known, but in this case, rust from the inner layer portion is removed. It is necessary to collect.

In the present invention, if the generated rust is destroyed when the rust is collected, the effect of the evaluation method is halved, so a sharp blade is used for collecting the rust and the length is about 10 mm. It is better to collect by using a few marking lines. A few marking lines are sufficient. By this method, the internal structure of the rust layer can be examined very effectively. According to such a sampling method, damage that would lead to partial destruction of the stable rust is not given to the weathering steel, but other sampling methods may be used as long as they do not lead to damage of the stable rust.

The method for measuring the particle size of the collected rust is to disperse it in a solvent that does not react with rust, and measure the average crystal grain size using a sedimentation velocity detection type or laser light projection type particle size distribution measuring instrument. However, it has been found that the use of electron microscopy is superior for obtaining more accurate information on grain morphology. It is possible to observe with an electron microscope by placing the collected rust on a collodion film or the like as it is. As a method of observing the rust morphology more easily and with good reproducibility, a method of collecting using an organic film as described below was devised.

First, using a sharp blade on the surface of the weather resistant steel, several scribe lines having a length of about 10 mm are inserted. A few marking lines are sufficient. Next, after dipping an organic film such as acetyl cellulose in methyl acetate, it is attached to the injured part. After the film is once attached to the surface of the weather resistant steel, the film is immediately peeled off, and the organic film is dried on glass or the like. In this way, the rust on the surface of the steel material can be transferred onto the film and collected. After that, carbon vapor deposition is applied to this film, and the film is cut into a size that can be mounted on an electron microscope stage. Since the electron microscope stage usually has a diameter of 3 mm, it is cut into small pieces of about 2 mm square. After protecting the surface with molten paraffin, etc., soak it in methyl acetate again to dissolve the organic film such as acetyl cellulose and the paraffin, and scoop up the carbon film and the rust particles adhering to it to a 3 mmφ mesh. Use as a sample for electron microscope observation. This method is based on the well-known replica method as a sample preparation method for electron microscope observation, but the major purpose of the conventional replica method is to transfer surface irregularities for observation. It was At the same time, it was possible to observe the precipitates by electron microscopy by transferring the precipitates attached to the surface at the same time, but there was no idea to see the internal state by the replica method.

Although the present invention is based on the replica method, it succeeds in extracting internal information of rust such as weather resistant steel by inserting a characteristic scribe line on the surface of a steel material. Is just a different point of view. After observing the constituent particles with an electron microscope, the constituent particles are photographed, and the grain size distribution is obtained using an image processing program represented by a particle analysis program.

By utilizing the above-described method for measuring the average grain size of the crystal grains constituting rust, the morphology of rust grains formed on the surface of weather resistant steel and ordinary steel exposed under various conditions was investigated. It was Generally, the form of rust that occurs when the stable rust of ordinary steel or weathering steel is destabilized for some reason is called delaminated rust. It is often seen on the seashore and easily peels off with thin flaky rust. On the other hand, it has been found that the stable rust formed in the weather-resistant steel has no delamination, and the corrosion hardly progresses after a certain period of time.

Although the evaluation method according to the present invention is not a technique for limiting the type of weathering steel, the rust morphology of various weathering steels is observed by an electron microscope, and as a result, it is found in a region with many sea salt flying particles. When the distribution of the particle diameter of 0.2 μm or more forming the rust was measured with respect to the exposed weather-resistant steel, it was 0.5.
It has been found that the characteristics do not satisfy the target values when the particle size is 1% or more. This is an example of an evaluation standard, and this standard value is set according to the usage environment.

[0016]

EXAMPLES The present invention will be described below with reference to examples. In establishing the evaluation method, the morphology of rust formed on various weathering steels was observed. Taking a 1-year exposure test in Nagano as an example, a representative example of the particle size distribution when a slight amount of delaminated rust is generated and the characteristics start to deteriorate is shown in Fig. 1, and a typical example when stable rust is produced. Is shown in FIG. The particle shape of rust is not necessarily spherical, but may be elliptical or rectangular parallelepiped. Therefore, when measuring the particle size, the length of the longest part was defined as the particle size. For the measured particles, select coarse particles having a particle size of 0.2 μm or more, and the number of measurements is 50.
More than one As shown in FIG. 2, when stable rust is generated, the particle size of 0.6 μm or less occupies most. In this case, whether or not the proportion of particles having a particle diameter of 1 μm or more is 1% or more may be used as a characteristic evaluation standard.

Next, the weathering steels shown in Table 1 were used to examine the rust morphology with an electron microscope when an exposure test was conducted in a beach area, which is a severe environment for the weathering steels. Tables 2 and 3 show the results of examining the change in the amount of corrosion with each year, the average particle size in each period, and the ratio of the particles having a particle size of 1 μm or more through the exposure test for 5 years. At the same time, the results of evaluation as to whether stable rust is generated are also shown from the viewpoint of rust releasability. In this case, the existence ratio of particles having a particle size of 1 μm or more is 0.
When it exceeds 5%, delaminated rust is generated, and it is understood that stable rust is not sufficiently formed.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

[Table 3]

[0021]

By applying the evaluation method of the present invention to the weather-resistant steel being applied, the life of the unpainted steel sheet can be easily examined without breaking. Even if the start of deterioration is found, it is possible to prevent the fatal damage as a building by immediately performing partial coating or the like.

[Brief description of drawings]

FIG. 1 shows a typical example of a coarse particle size distribution when delaminated rust is generated.

FIG. 2 shows a representative example of a coarse particle size distribution when stable rust is generated.

Claims (2)

[Claims] 1. A method for measuring the particle size distribution of crystal particles that form rust that is produced on the surface of a steel material and that adheres to the base steel, and evaluates the stability of rust on the surface of the steel material based on the particle size distribution. 2. A method of sticking an organic film to a portion of a steel material where a scribe line is attached with a sharp blade, peeling off the organic film, and measuring the particle diameter of the transferred rust with an electron microscope. The method for evaluating the stability of rust on the surface of a steel material according to claim 1.