JP3651601B2 - Electrochemical measurement cell and electrochemical measurement method using the same - Google Patents

Description

で表される。ここで、ｉ_Oは交換電流密度、αは遷移係数、ｎは反応電子数、Ｆはファラデー定数、Ｒは気体定数、Ｔは絶対温度、ηは過電圧である。
【００２０】
ただし式（２）は電解開始直後とか、電位移動反応抵抗が極めて大きい場合に適用できるもので、一般には電解が進むにつれて、電極界面に反応種の浸度勾配ができるので電流と電位の関係は

で表される。ここでＣ_O及びＣ_Cは電極面上のＯ_X及びＲ_edの濃度である。本発明においては、低電流密度を短時間印加することにより、電極界面の反応種の濃度勾配の発生を極力除去したこと、また式の単純化を図り、その取り扱いを容易にする意図から、式（２）をもって論じることができる。
【００２１】
この電極系を電気的に等価な回路に置き換えると図７のようになる。この場合の反応抵抗Ｒ_eは反応速度に反比例し、塗膜下での低腐食反応速度の場合の腐食電流密度ｉ_corrを１０^-8ＡとするとＲ_eは約１０⁶Ω程度となる。また、Ｃ_dtは通常１０^-5Ｆ程度である。これから時定数は１０ｓｅｃ程度となって、塗膜の時定数との差が大きいため、明瞭に分離できることになる。
【００２２】
定電流印加の際

【００２３】
ｉ）ｎＦη／ＲＴ≪１のとき（η≦１０ｍＶ）
式（３）の指数関数を展開して近似すると、０＜α＜１であるから

この微分方程式を解いて
η＝（ｉ／ｉ_O）（ＲＴ／ｎＦ）｛１−ｅｘｐ（−ｔ／τ）｝ （４）
ここに
τ＝Ｃ_dtＲＴ／ｎＦｉ_O＝ＣＲ_e
Ｒｅ＝ＲＴ／ｎＦｉ_O
ｉ_O＝ＲＴ／Ｒ_eｎＦ
Ｃ_dt＝τ／Ｒ_e
ｔが非常に小さい場合はｉ_dtに比してｉ_fは無視できるので
ｉ＝Ｃ_dt［ｄη／ｄｔ］_t=o
また、式（４）は式（１）と全く同様に取扱うことができるので図６のグラフからτ、Ｒ_e、Ｃ_dtが求まりＲＴ／ｎＦが既知であればｉ_Oすなわち腐食速度も求まる。
【００２４】
ｉｉ）ｍＦη／ＲＴ≫１のとき
アノード分極の場合はカソード電流が無視できるので
ｉ＝Ｃ_dt（ｄη／ｄｔ）＋ｉ_Oｅｘｐ（αｎＦη／ＲＴ）
電流切断の際の減衰時はｉ_dt＝−ｉ_f、ｉ＝０とおけるので
Ｃ_dt（ｄη／ｄｔ）＝−ｉ_Oｅｘｐ（αｎＦη／ＲＴ）
この微分方程式を解くと
ｔ＝（ＣＲＴ／ｉ_OαｎＦ）ｅｘｐ（−αｎＦη／ＲＴ）
従って
η＝（−ＲＴ／αｎＦ）Ｉｎｔ＋（ＲＴ／αｎＦ）Ｉｎ（ＣＲＴ／ｉ_OαｎＦ）
となり、図８のη−１ｏｇｔの関係から、ターフェル係数が求まる。
【００２５】
得られたターフェル係数と分極抵抗Ｒ_eとから、スターンの式（６）より腐食速度が求まる；
ｉ_corr＝ｂ_a／ｂ_c／２．３Ｒ_e（ｂ_a＋ｂ_c） （５）
ただし式（５）はη＜１０ｍＶの範囲で成立する。ここに
ｂ_a＝２．３ＲＴ／αｎＦ （６）
ｂ_c＝２．３ＲＴ／（１−α）ｎＦ （７）
【００２６】
従って、前述のように分極抵抗Ｒ_eを求めることによって塗装鋼板における塗膜下の腐食を測定することができる。
【００２７】
【実施例】
（実施例１）
屋外に設置された外径１５ｃｍの円筒状の鉄鋼構造物の垂直部、鉛系さび止とフタル酸樹脂塗料が合計約１００μｍ塗装されて１０年経過した塗膜面に、図１に示される厚さ２ｍｍのマグネットシート及びカバーよりなるセル本体を設置し、マグネットシートの開口部（５×４ｃｍ）に３質量％食塩水１００質量部にカルボキシメチルセルロース５質量部を添加して作製したペースト状の電解質溶液を充填し、その上に４×３ｃｍの白金の対極を置き、白金対極と素地鉄鋼からリード線を出し、塗膜下金属腐食診断装置ＨＬ２０１（北斗電工（株）製）に結線した。次に、セル部分全体を網目の大きさ５ｍｍの金網で覆い、周囲を永久磁石でアングル材に固定しアースした。５時間放置し、電解質が塗膜内に浸透後、測定を行った。結果を表１に示す。
【００２８】
測定結果は良好で、塗膜抵抗、塗膜容量、分極抵抗及び分極容量の数値から塗膜は未だに健全であり、かつ塗膜下で腐食が開始していないと判断される。
【００２９】
更に、目視観察でも塗膜表面にさびは発生しておらず、塗膜を剥がして素地面を目視観察してもさびの発生は認められなかった。
【００３０】
（比較例１）
実施例１と同様のセルを用い、導電性遮蔽材である金網を用いず測定したところ、外部の電気ノイズを拾って正確な測定が不可能であった。
（比較例２）
図４に示すような円筒形の固形磁石を用いたセルで測定を試みたが、測定セルがうまく円筒状の構造体に固定できず測定不能であった。
【００３１】
【表１】

【００３２】
【発明の効果】
上述したように、本発明によれば、被測定面の形状に追随するフレキシブルなセル構成材と外部ノイズを遮断する金網等のシールド材を組み合わせることにより、被測定面が平面でなく、かつ垂直部であっても被塗面の状態を問わず測定セルを固定でき、精度の高い電気化学的測定をすることのできるセル及び該方法を提供することが可能となった。
【図面の簡単な説明】
【図１】本発明の電気化学的測定セル本体の概略構成図である。
【図２】セルの開口部に対極材を設置した様子である。
【図３】本発明の電気化学的測定方法の概略図である。
【図４】比較例に用いた測定セルの概略図である。
【図５】本発明に用いた測定原理のカレントインタラプター法を説明するための図である。
【図６】本発明に用いた測定原理のカレントインタラプター法を説明するための図である。
【図７】本発明に用いた測定原理のカレントインタラプター法を説明するための図である。
【図８】本発明に用いた測定原理のカレントインタラプター法を説明するための図である。
【符号の説明】
１ マグネットシート
２ カバー
３ 対極材
４ 導電性遮蔽材
５ 塗膜下金属腐食診断装置
６ 塗膜
７ 鋼材
８ マグネット
９ ペースト状の電解質溶液
１０ 高絶縁材[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a measurement cell portion in a case where an undercoat paint corrosion diagnosis of a coated metal is performed by electrochemical means.
[0002]
[Prior art]
Conventionally, methods for applying non-destructive corrosion diagnosis of metal under the coating include applying a current interrupter method that applies a minute direct current, and an AC impedance method that applies an alternating voltage with different frequencies. ing. The above method is a method for obtaining reliable data in a suitable case where the measurement state performed in the test room is in place, and many experimental data have been reported.
[0003]
However, when a coating film diagnosis is performed on an on-site structure in which the measurement state is not constant, the surface to be measured is not necessarily a flat surface, and may be a curved surface such as a pipe or uneven. Moreover, not only a horizontal surface but a vertical surface, a downward surface, etc. may be object. When the curvature of the measurement part is large or the unevenness is severe, the solid cell of the prior art (for example, Japanese Patent Laid-Open No. 2-285250) cannot be installed in a good condition on the measurement surface, or it can be applied even if it can be installed. Since it does not adhere to the film, there is a disadvantage that the measurement area is not constant and the accuracy of the measurement value is lowered.
[0004]
[Problems to be solved by the invention]
The object of the present invention is that it can be measured regardless of the state of the surface to be coated, such as a curved surface, a highly uneven surface, a vertical surface, a downward surface, etc., and the influence of external noise can be avoided even in the case of a high resistance coating film. It is an object of the present invention to provide an electrochemical measurement cell capable of performing accurate measurement and an electrochemical measurement method using the same.
[0005]
[Means for Solving the Problems]
According to the present invention, the cell main body is a magnet sheet having an opening of a certain size for holding the paste-like electrolyte solution and a cover for preventing the electrolyte solution from drying, and the opening is polarized. Electricity characterized in that a small counter electrode material is installed, the entire cell is further covered with a conductive shielding material, and the conductive shielding material is detachably held on the coating film via a high insulating material and grounded. A chemical measurement cell and an electrochemical measurement method using the same are provided.
[0006]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail.
[0007]
The electrochemical measurement cell of the present invention and the electrochemical measurement method using the same will be described with reference to FIGS.
[0008]
As shown in FIG. 1, the electrochemical measurement cell portion of the present invention has a flexible magnet sheet 1 as a cell main body, and is filled with a paste-like solution containing an electrolyte in an opening having a constant area. The electrolyte solution can be brought into close contact with the measurement surface, and the cover 2 can be covered to prevent evaporation of the solution water, and the coating film to be measured should be highly durable and immersed for a long time for measurement. If there is, we can cope. Also, since it is simple and inexpensive, it can be installed in many structures on site and measured after several days. In the measurement of a highly durable coating film, it takes time for the electrolyte to permeate and cannot be measured immediately after installation, and such usage is essential.
[0009]
The magnet sheet 1 of the present invention is preferably flexible so that the measurement surface can be in close contact with a curved surface such as a pipe or an uneven surface, and a magnetic material such as a magnet is made of butadiene rubber, styrene rubber or nitrile rubber as a binder material. It can be formed by kneading with synthetic rubber or natural rubber. Since the sheet requires an opening of a certain area, it can be produced by opening a necessary area in the opening after forming a planar sheet. The thickness of the sheet is preferably 0.5 mm to 50 mm.
[0010]
The cover 2 is for preventing the paste-like electrolyte solution to be held in the opening from drying even if it is placed for a long time as described above, and may be anything as long as it prevents evaporation of the solution moisture. Absent. As the material, organic resins such as polyethylene and polypropylene are suitable.
[0011]
The counter electrode material 3 serving as an electrode installed in the opening of the sheet is a material with small polarization, specifically, platinum, silver / silver chloride, stainless steel, iron, carbon, aluminum, titanium, molybdenum, tungsten, or the like. An iron alloy material or the like can be used, and platinum, carbon, stainless steel and silver / silver chloride are preferable, and platinum is particularly preferable. The counter electrode material 3 is in contact with the ground surface of the coated metal steel material 7 to be measured through the undercoat metal corrosion diagnostic device 5.
[0012]
In order to prevent intrusion of surrounding electric noise during measurement, the conductive shielding material 4 is made of a conductive flexible material such as a metal net or aluminum foil via a high insulating material 10 so as to cover the cell body. It is detachably held on the coating film 6 of the steel material 7 with a magnet 8 or an adhesive tape. In the case where the noise shielding material is a metal mesh, if the mesh is too large, the effect of preventing noise intrusion is reduced, so at least the mesh size is preferably 1 cm or less. Further, the conductive shielding material needs to be grounded. The shielding material 4 may be anything as long as it has conductivity, and may be a metal net, an aluminum foil, a cloth woven with metal, or a metal-deposited material. The high insulating material 10 is inserted in order to electrically insulate the insulating material 4 and the coating film 6 and to block a leakage current at the time of measurement from the coating film 4 to the high insulating material 10. The material of the high insulating material 10 is preferably a material having an electrical resistance value of 10 ¹² Ω · cm or more, such as polytetrafluoroethylene.
[0013]
Since the paste-like electrolyte solution 9 can be installed on a vertical surface or a downward surface, the paste-like electrolyte solution 9 is preferably a material having a viscosity that does not leak from the opening of the cell. In particular, an aqueous solution having a high viscosity is carboxymethylcellulose. A water-absorbing polymer material or the like can also be used. In consideration of viscosity, the content of carboxymethyl cellulose is preferably 1% by mass to 20% by mass with respect to the total mass of the electrolyte solution. As the electrolyte, any electrolyte that is usually used for electrochemical measurements can be used, and examples thereof include sodium chloride, potassium chloride, ammonium chloride, sodium sulfate, potassium sulfate, ammonium sulfate, ammonium nitrate, sodium nitrate, and potassium nitrate. The amount of the electrolyte solution is 1 kΩ · cm or less, preferably 100 Ω · cm or less.
[0014]
The undercoat metal corrosion diagnostic apparatus 5 measures the film resistance, the film capacity, the polarization resistance, and the polarization capacity by the current interrupter method, and these effects are as follows.
-Coating film resistance ( _Rf ): The coating film resistance of a coated steel material is effective in knowing the soundness of the coating film.
Coating film capacity (C _f ): A numerical value related to the proportion of moisture in the coating film. This facilitates water absorption and is related to the ease of corrosion through the membrane. It is important to know the characteristics of the membrane.
Polarization resistance (R _e ): Polarization resistance is a resistance to the corrosion reaction that is an electrode reaction. A decrease in the polarization resistance can cause a moisture layer at the metal interface under the coating, This reflects that the adhesion of the metal has been impaired, and is effective in determining the start of corrosion under the coating film.
Polarization capacity (C _dt ): A layer having a thickness of about 10 ⁻⁸ m present at the contact interface between the electrode and the electrolyte, and there is a potential difference corresponding to the electrode potential on both sides. The layer has a capacitance specific to the electrode and the charge. Therefore, if the polarization capacity is known, the characteristics of the electrode interface under the coating film can be understood.
[0015]
From these results, it is possible to know the soundness of the coating film applied on the metal and / or the corrosion degree of the metal under the coating film, and particularly when the coating film resistance and the polarization resistance are 5.5 × 10 ⁶ cm ² or more. It is known that no wedge is generated (Sato, Hoshino, Tanabe: Anticorrosion technology, 28, 524-531 (1979)).
[0016]
The current interrupter method used in the undercoat metal corrosion diagnostic device 5 is based on a polarization transient phenomenon during application and cutting of current in constant current electrolysis, and can be applied to a coated steel material. FIG. 5 shows a simplified equivalent circuit of the painted steel material at the time of measurement. Here, R is a pure resistance, and in the present invention, the solution resistance corresponds to this. Coating the impedance equivalent to the coating film capacitance C _f, is represented by the coating film resistor R _f, as the impedance of the metal surface forms the polarization capacity C _dt, the polarization resistance R _e and the series circuit, as a result, If there is a significant difference in the time constant, it becomes possible to separate the respective polarization phenomena. The resistance of the conductive solution of the present invention is several tens of Ω or less, and in the coating film circuit, the capacitance C _f is about 10 ⁻⁹ to 10 ⁻¹⁰ F and the resistance R _f is about 10 ⁶ to 10 ⁷ Ω. The time constant is about 1 msec from the relationship between the product of capacitance and resistance. Impedance of the metal interface capacitance C _dt is about 10 ^-5 F, if the polarization resistance R _e is to be about 10 ⁶ Omega, the time constant is approximately 10 sec. While the time constant of the coating is about 1 msec, the time constant of the metal interface is 10 sec, and there is a significant difference between them. As a result, it is possible to examine the two separately. it can.
[0017]
Here, as shown in FIG. 5, when the impedance of the coating film is represented by a single C and R element, the polarization phenomenon when a constant current is applied is represented by the following equation:
i = i _R + i _C = V / R + C (dv / dt)
[0018]
Therefore, V = iR (1−e ^{−t /} τ) (τ = CR)
In addition, the potential decay at the time of current disconnection is V = iRe- ^{t /} τ
Therefore, logV = logiR−t / 2.3τ (1)
It becomes. If this is represented by a graph, it becomes as shown in FIG. 6, and from this, the time constant τ, the capacitance C, and the resistance R can be obtained.
[0019]
On the metal base under the coating film, an electrode reaction, that is, a corrosion reaction occurs through the immersion liquid when a constant current is applied. The relationship between the potential and current of the electrode reaction of the redox system O _x + ne⇔R _ed is that when the concentration of reactive species on the electrode surface is in equilibrium with the offshore concentration, the Faraday current _if is

It is represented by Here, i _O is the exchange current density, α is the transition coefficient, n is the number of reaction electrons, F is the Faraday constant, R is the gas constant, T is the absolute temperature, and η is the overvoltage.
[0020]
However, equation (2) can be applied immediately after the start of electrolysis or when the potential transfer reaction resistance is extremely large. In general, as the electrolysis progresses, there is a dip gradient of reactive species at the electrode interface, so the relationship between current and potential is

It is represented by Here, C _O and C _C are the concentrations of O _X and R _ed on the electrode surface. In the present invention, by applying a low current density for a short time, the generation of the concentration gradient of the reactive species at the electrode interface is eliminated as much as possible, and the intention is to simplify the expression and facilitate its handling. It can be discussed with (2).
[0021]
When this electrode system is replaced with an electrically equivalent circuit, the result is as shown in FIG. The reaction resistance R _e of the case is inversely proportional to the reaction rate, R _e corrosion current density i _corr and When 10 ^-8 A in the case of low corrosion rate of reaction under the coating film becomes approximately 10 ⁶ Omega. C _dt is usually about 10 ⁻⁵ F. From this, the time constant is about 10 seconds, and since the difference from the time constant of the coating film is large, it can be clearly separated.
[0022]
When applying constant current

[0023]
i) When nFη / RT << 1 (η ≦ 10 mV)
When the exponential function of Equation (3) is expanded and approximated, 0 <α <1.

Solving this differential equation, η = (i / i _O ) (RT / nF) {1-exp (−t / τ)} (4)
Where τ = C _dt RT / nFi _O = CR _e
Re = RT / nFi _O
i _O = RT / R _e nF
C _dt = τ / R _e
Since t is the i _f than the i _dt when very small negligible _{i = C dt [dη / dt} ] t = o
Further, since the expression (4) can be handled in the same manner as the expression (1), τ, R _e , and C _dt are obtained from the graph of FIG. 6, and if the RT / nF is known, i _O, that is, the corrosion rate can be obtained.
[0024]
ii) When mFη / RT >> 1, the cathode current can be ignored in the case of anodic polarization, so i = C _dt (dη / dt) + i _O exp (αnFη / RT)
When the current is attenuated, i _dt = −i _f and i = 0, so C _dt (dη / dt) = − i _O exp (αnFη / RT)
Solving this differential equation, t = (CRT / i _O αnF) exp (−αnFη / RT)
Accordingly, η = (− RT / αnF) Int + (RT / αnF) In (CRT / i _O αnF)
Thus, the Tafel coefficient is obtained from the relationship of η-1 ogt in FIG.
[0025]
From the obtained Tafel coefficient and polarization resistance R _e, than the corrosion rate is obtained stern of formula (6);
i _corr = b _a / b _c /2.3R _e (b _a + b _c ) (5)
However, Formula (5) is materialized in the range of η <10 mV. Where b _a = 2.3 RT / αnF (6)
b _c = 2.3 RT / (1-α) nF (7)
[0026]
Therefore, it is possible to measure the corrosion under a coating film in the coated steel plate by determining the polarization resistance R _e, as described above.
[0027]
【Example】
(Example 1)
The thickness shown in Fig. 1 is on the vertical part of a cylindrical steel structure with an outer diameter of 15 cm installed outdoors, the coating surface after 10 years have passed since a total of about 100 µm of lead-based rust inhibitor and phthalate resin paint is applied. A paste-like electrolyte prepared by installing a cell body consisting of a 2 mm thick magnetic sheet and a cover, and adding 5 parts by mass of carboxymethyl cellulose to 100 parts by mass of 3% by weight saline in the opening (5 × 4 cm) of the magnet sheet. The solution was filled, a 4 × 3 cm platinum counter electrode was placed thereon, lead wires were taken out from the platinum counter electrode and the base steel, and were connected to an undercoat metal corrosion diagnostic apparatus HL201 (manufactured by Hokuto Denko Co., Ltd.). Next, the entire cell portion was covered with a wire mesh having a mesh size of 5 mm, and the periphery was fixed to an angle member with a permanent magnet and grounded. The sample was allowed to stand for 5 hours, and the measurement was performed after the electrolyte had penetrated into the coating film. The results are shown in Table 1.
[0028]
The measurement results are good, and it is judged from the numerical values of the coating film resistance, the coating film capacity, the polarization resistance and the polarization capacity that the coating film is still healthy and that corrosion has not started under the coating film.
[0029]
Further, no rust was generated on the surface of the coating film by visual observation, and no rust was observed even when the coating was peeled off and the ground surface was visually observed.
[0030]
(Comparative Example 1)
When the same cell as in Example 1 was used and measurement was performed without using a wire mesh as a conductive shielding material, it was impossible to accurately measure by picking up external electric noise.
(Comparative Example 2)
Measurement was attempted with a cell using a cylindrical solid magnet as shown in FIG. 4, but the measurement cell could not be fixed to the cylindrical structure and measurement was impossible.
[0031]
[Table 1]

[0032]
【The invention's effect】
As described above, according to the present invention, by combining a flexible cell constituent material that follows the shape of the surface to be measured and a shielding material such as a wire mesh that blocks external noise, the surface to be measured is not flat and vertical. Even if it is a part, the measurement cell can be fixed regardless of the state of the coated surface, and it is possible to provide a cell capable of performing electrochemical measurement with high accuracy and the method.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of an electrochemical measurement cell body of the present invention.
FIG. 2 shows a state in which a counter electrode material is installed in an opening of a cell.
FIG. 3 is a schematic view of the electrochemical measurement method of the present invention.
FIG. 4 is a schematic view of a measurement cell used in a comparative example.
FIG. 5 is a diagram for explaining a current interrupter method of a measurement principle used in the present invention.
FIG. 6 is a diagram for explaining the current interrupter method of the measurement principle used in the present invention.
FIG. 7 is a diagram for explaining a current interrupter method of a measurement principle used in the present invention.
FIG. 8 is a diagram for explaining a current interrupter method of a measurement principle used in the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Magnet sheet 2 Cover 3 Counter electrode material 4 Conductive shielding material 5 Under-coating metal corrosion diagnostic device 6 Coating film 7 Steel material 8 Magnet 9 Paste electrolyte solution 10 High insulation material

Claims (8)

The cell body is a magnet sheet having an opening having a certain size for holding the paste-like electrolyte solution and a cover for preventing the electrolyte solution from drying, and a counter electrode material having a small polarization is provided in the opening. An electrochemical measurement cell that is installed and further covered with a conductive shielding material, the conductive shielding material is detachably held on the coating film via a high insulating material, and is grounded . The electrochemical measurement cell according to claim 1, wherein the counter electrode material is selected from the group consisting of platinum, carbon, stainless steel, and silver / silver chloride. The electrochemical measurement cell according to claim 1 or 2, wherein the paste-like electrolyte solution contains carboxymethyl cellulose. The cell body is a magnet sheet having an opening having a certain size for holding the paste-like electrolyte solution and a cover for preventing the electrolyte solution from drying, and a counter electrode material having a small polarization is provided in the opening. It was installed, and the entire cell was covered with a conductive shielding material, and the conductive shielding material was detachably held on the coating film through a high insulating material, and a grounded electrochemical measurement cell was used. A characteristic electrochemical measurement method. The electrochemical measurement method according to claim 4, wherein the counter electrode material is selected from the group consisting of platinum, carbon, stainless steel, and silver / silver chloride. The electrochemical measurement method according to claim 4 or 5, wherein the paste-like electrolyte solution contains carboxymethylcellulose. The electrochemical measurement method according to claim 4, wherein the measurement principle is a current interrupter method. The electrochemical measurement method according to claim 7, wherein one or both of a soundness level of a coating film applied on a metal by the measurement method and a corrosion level of a metal under the coating film are measured.

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