JP2000054172A - Method for evaluating stability of rust in steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To swiftly evaluate the stability of a rust layer in steel non- destructively, to estimate the corrosion rate of steel in an actual structure on the spot, and to evaluate the performance thereof by measuring the anode current density of a rust layer under certain electric potential of steel composed of weather resistant steel or the like. SOLUTION: By using a steel 2 composed of weather resistant steel contg. Cu, Cr and P in a bare state over a long period, a rust layer 1 having high corrosion preventability is formed. A connecting wire 3 of Cu or the like freed from a part of the rust layer 1 and fitted and sponge 4 impregnated with an electrolytic soln. of Na2SO4 or the like on the rust layer 1 are arranged, and a reference electrode 5 connected with this and a counter electrode 6 of Pt or the like are connected to a potentiostat 7. In this way, the anode current density of the rust layer 1 is measured under certain electric potential, e.g. of about -0.3 to +0.5 V. This current density is made small as the stability of the rust layer 1 increases, and the corrosion rate reduces. Thus, the stability of the rust layer 1 is evaluated from this measured value, and the forecast of the service life of the steel 2, the repairing period thereof or the like can precisely be judged.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for evaluating rust stability of a steel material.

[0002]

2. Description of the Related Art Weather-resistant steel containing Cu, Cr, P, etc., in the steel, produces rust with high anticorrosion properties (hereinafter referred to as stable rust) due to long-term exposure to the atmosphere when used in the naked state. Is significantly reduced, and is used as a maintenance-free material for land structures such as bridges.

However, the time required for forming a stable rust layer varies depending on the use environment of the steel material, and stable rust may not be obtained when there are many flying sea salt particles. Need to be evaluated.

[0004] The stability of the rust layer can be determined by (1) a method of measuring the change over time of the corrosion loss of steel and measuring the corrosion rate, and (2)
The amounts of α-FeOOH and γ-FeOOH in the rust were measured by X-ray diffraction, and their weight percentages (α-FeOOH / γ-FeOOH) (hereinafter abbreviated as ratio (α-FeOOH / γ-FeOOH)) Method) has already been used.

[0005]

SUMMARY OF THE INVENTION
In the method (1), the rust must be removed and the thickness must be measured on site, resulting in inferior workability, and no information on stabilization of the rust can be obtained.

The method (2) has a problem in that it is necessary to collect rust from a steel material, take it back to the laboratory and measure and evaluate it, and it is not possible to immediately know the stability. Therefore, these methods are not only unsuitable for the performance evaluation of steel materials in actual structures, but the information obtained by these measurement methods is macroscopic information, and cracks and defects existing locally in steel materials There is also a problem that information on the like cannot be obtained.

An object of the present invention is to provide a method for non-destructively and quickly evaluating the rust stability of a steel material on a real structure in situ.

[0008]

Means for Solving the Problems The present inventors have conducted research using various electrochemical techniques, and as a result, the following (A)
~ (B) were obtained.

(A) FIG. 1 is a graph showing anodic polarization curves of weatherable steel materials having different air exposure periods. As shown in the figure, there is a saturated current density value for each of the weathering steels with different exposure periods, and the saturated current density value increases as the air exposure period becomes longer as shown by the arrow in the figure. It was newly observed that the steel became smaller as the layer stability increased, and that the steel behaved much like a passivated state.
From this result, the stability of the rust layer of the steel material can be evaluated by measuring the anode current when the potential is set to a certain constant potential.

(B) FIG. 2 shows a potential +0.3 V when a saturated luster electrode is used as a reference electrode and a platinum electrode is used as a counter electrode.
5 is a graph showing the relationship between the anode current density in (vs. SCE) and the corrosion rate obtained from the actual measurement of the corrosion weight loss of each steel material. The test piece used was a bare material of weather-resistant steel exposed to air in an industrial zone in Amagasaki City, Hyogo Prefecture, and a surface-treated material obtained by subjecting weather-resistant steel to a process of promoting stable rust formation. As shown in FIG. 2, it can be seen that there is a highly accurate linear relationship between the current density and the corrosion rate regardless of the presence or absence of the surface treatment of the weathering steel. Here, the anode current density is 100 μA / cm
² corresponds to a corrosion rate of 10 μm / year.

In general, in actual structures, the corrosion margin after 50 years of exposure is set to 0.3 mm. If the corrosion rate is 6 μm / year or less, the rust layer has sufficient protection.
It can be determined that it has stabilized.

The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) A method for evaluating rust stability of a steel material in which the stability of the rust layer of the steel material is evaluated by measuring the anode current density of the rust layer at a constant potential.

(2) A method for evaluating rust stability of a steel material, wherein the method according to claim 1 is applied to a rust layer of an actual structure in a non-destructive manner.

[0015]

FIG. 3 is a conceptual diagram showing a method for measuring an anode current density. As shown in the drawing, a part of the rust layer 1 is removed and a connection wire 3 is attached in order to establish conduction with the steel material 2. Although a Cu wire is used as the connection line, any conductive wire may be used. Next, the rust layer is wet with a sponge 4 impregnated with an electrolyte solution, and a reference electrode 5 and a counter electrode 6 are set thereon. The electrolyte solution may be a conductive aqueous solution. For example, 0.1M-Na ₂ SO
An aqueous solution of ₄ (M = mol / l) can be used, and the concentration of the solution may be 0.01 M to a saturated solution. If the concentration is less than 0.01 M, the solution resistance may increase, and it may be impossible to measure an accurate potential. A saturated electrode (SCE) is used as a reference electrode and a platinum electrode is used as a counter electrode, but other reference electrodes and counter electrodes can also be used.

The electric potential is set by the potentiostat 7 to a value in the range of -0.3 V to +0.5 V with respect to the saturated gingival electrode (SCE). The reason is described below.

The stabilized rust layer has a potential (corrosion potential) in a natural state in contact with the solution of -0.3 V to +0.
Since it is about 2 V, measurement cannot be performed at a potential lower than -0.3 V. If it exceeds +0.5 V, an excessive potential difference is added to the corrosion potential, and a correct measurement value may not be obtained. The preferred set potential is +0.2
V to + 0.3V.

The graph shown in FIG. 2 is obtained by measuring the current density in a state where the set potential is maintained at +0.3 V, as described above. If the potential is within the above range, a graph having the same tendency is obtained. And the stability can be evaluated.

Since the measurement method of the present invention is information at the contact point of the reference electrode, it is possible to evaluate local rust stability and obtain detailed information, not macro information as in a method based on chemical analysis. There are features.

Further, since the measuring method of the present invention can be performed in a non-destructive manner, it is suitable for evaluation of an actual structure. For example, the life expectancy from the corrosion resistance of the actual structure or the time for repair can be accurately determined. can do.

[0021]

Example: Potential was measured by potentiostat on a test piece of a low-alloy weather-resistant steel material exposed to the real bridge in the industrial area of Amagasaki City, Hyogo Prefecture for several months to 32 years, and a surface-treated steel material subjected to stable rust generation promotion treatment. The saturated ginger electrode (SCE)
The current density was measured in the same manner as described above using an aqueous solution of 0.1 M-Na ₂ SO ₄ (M = mol / l) while maintaining the current density at +0.2 V and +0.3 V.

Table 1 shows +0.2 for low alloy weathering steel.
It shows the value of the saturated current density [μA / cm ² ] for each period of exposure to the air while maintaining V and +0.3 V.

[0023]

[Table 1]

Table 2 shows the values of the current density [μA / cm ² ] of each of the low-alloy weather-resistant steel materials subjected to the stable rust formation accelerating treatment at +0.2 V and +0.3 V for each atmospheric exposure period. .

[0025]

[Table 2]

As shown in Tables 1 and 2, the saturated current density [μA /
It was found that the stability of the rust of various steel materials could be evaluated at any potential of +0.2 V or +0.3 V with the value of [cm ² ] as the air exposure period became longer.

[0027]

According to the present invention, the following effects can be obtained. (1) The evaluation of rust stability in steel can be measured nondestructively and quickly. (2) It is possible to evaluate the stability of local rust, not macro information of the measurement site, and obtain detailed information. (3) Since the measurement can be performed nondestructively, it is suitable for evaluation of actual structures. Thus, for example, it is possible to accurately judge the life expectancy or the repair time from the viewpoint of the corrosion resistance of the actual structure.

[Brief description of the drawings]

FIG. 1 is a graph showing anodic polarization curves of weathering steel materials having different exposure periods to the atmosphere.

FIG. 2 is a graph showing a relationship between an anode current density and a corrosion rate obtained from corrosion loss of each steel material.

FIG. 3 is a conceptual diagram showing a method for measuring an anode current density.

[Explanation of symbols]

 1: rust layer 2: steel material 3: connection wire 4: sponge 5: reference electrode 6: counter electrode 7: potentiostat

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Hiroshi Kishikawa 4-33 Kitahama, Chuo-ku, Osaka City F-term in Sumitomo Metal Industries Co., Ltd. 4K060 AA10 CA06 CA12 CA15 EA08 FA07

Claims (2)

[Claims] 1. A method for evaluating rust stability of a steel material, wherein the stability of the rust layer of the steel material is evaluated by measuring the anode current density of the rust layer at a constant potential. 2. A method for evaluating rust stability of a steel material, wherein the method according to claim 1 is applied non-destructively to a rust layer of an actual structure.