JP6739048B2 - Device for visualizing potential distribution on metal surface - Google Patents

Description

The present invention relates to a device for visualizing a potential distribution on a metal surface.

The potential distribution on the surface of a metallic material in an aqueous solution is an important parameter that governs a reaction in the field of electrochemistry. For example, metal pitting or crevice corrosion, which is a typical electrochemical reaction, reduces the potential of a partial area on the surface of a metal in the passive state, which exhibits a relatively high potential, and locally increases the corrosion rate. It occurs by doing. Further, it is considered that when crevice corrosion occurs, the potential of the metal surface sharply drops in a part of the crevice and a potential gradient occurs on the metal surface in the crevice.

In the conventional measurement of the potential distribution on the surface of a metal material in an aqueous solution, the surface of the test piece is operated with a reference electrode. However, since scanning with a probe usually takes time, it has been difficult to suppress the potential distribution of the metal material and its change over time, which generally occur within 1 second, such as a passive-active state transition. Therefore, in order to analyze the electrochemical phenomenon, it is necessary to develop a method for simultaneously measuring the potential distribution on the surface of the metal material in the aqueous solution and the corrosion phenomenon in real time.

By the way, as a method of observing a potential distribution in a substance having an electrolytic solution, there is an electron beam holography technique (for example, refer to Patent Document 1). This is to analyze the electric potential from a holography produced by interfering an electron wave transmitted through a sample with an electron wave transmitted through a place where the sample is not present. In this method, since the sample is placed inside the holography-electron microscope, it is not possible to simultaneously measure the potential distribution on the metal material surface and the corrosion phenomenon. Further, since the electron beam cannot penetrate inside the metal, there is a technical problem that the potential measured by this method is the potential of the electrolytic solution near the metal, not the potential of the metal surface itself.

By the way, a method of visualizing the membrane potential of cells with a fluorescent substance is used in the field of biology. For example, a potential-sensitive fluorescent dye having silicon rhodamine in the mother skeleton is localized in the cell membrane as a dye whose fluorescence intensity largely changes when a voltage is applied, and the potential distribution is determined from the change in fluorescence intensity that occurs when light is irradiated. A method of visualization (see, for example, Non-Patent Document 1) or genetic manipulation of a potential measurement target causes a specific cell to express a proteinaceous membrane potential probe in which a fluorescent protein is linked to a potential sensor protein, and There is a method of visualizing the membrane potential from the change in color tone (for example, see Non-Patent Document 2). However, these methods are not suitable for visualizing the potential distribution on the metal surface because it is necessary to include an indicator in the potential measurement target or change the quality of the measurement target itself. Further, generally, the corrosion phenomenon of metal materials occurs in an aqueous solution having a pH value of 3 or less from weakly acidic to strongly acidic, but the aqueous solution targeted in the field of biology is in the neutral range of about pH 7. The above-mentioned fluorescent reagent has a problem that it quenches in an acid solution and does not function as a fluorescent reagent.

Further, as a device whose color tone changes according to a change in potential, there is an electrochromism module (for example, see Patent Document 2). This is because an electrochromism layer containing an indicator whose color tone changes due to an oxidation-reduction reaction is sandwiched between two transparent substrates provided with transparent conductive elements, and between the transparent conductive elements on the two transparent substrates. It is said that the color tone of the electrochromic layer changes according to the change in potential. In this method, since the electrochromism layer sensitive to electric potential is sandwiched between the transparent substrates, it is not suitable for visualizing the electric potential distribution on the metal surface as it is. That is, a specific method for producing a substance whose color tone changes according to the potential of the metal surface as a transparent or semi-transparent matrix and bringing it into proximity to the metal surface has not yet been disclosed.

By the way, as a method for measuring the potential distribution on the surface of a metal material in an aqueous solution, a method using polyaniline, which is a conductive polymer, has been disclosed (see, for example, Patent Document 3). There is a drawback that the change of color tone with respect to is gentle and the measurement accuracy is low. Regarding the improvement of the potential measurement accuracy when the conductive polymer is used as the potential measurement element, no specific method has been disclosed yet.

By the way, in order to understand the influence of the potential of the metal surface on the electrochemical reaction in the aqueous solution, it is necessary to carry out optical microscope observation as well as the potential distribution. For that purpose, it is preferable that the solid matrix is transparent or translucent to visible light, but a specific method capable of clearly observing the reaction state of the metal surface while measuring the potential with high accuracy is It has not been revealed.

JP, 2016-080604, A Japanese Unexamined Patent Publication No. 2012-037859 JP, 2018-009834, A

Y. L. Huang, A. S. Walker, and E. W. Miller, “A Photostable Silicon Rhodamine Platform for Optical Voltage Sensing”, J. Am. Chem. Soc., 2015, 137, 33, p.10767-10776 H. Tsutsui, S. Higashijima, A. Miyawaki, and Y. Okamura, “Visualizing voltage dynamics in zebrafish heart”, J. Physiol., 2010, 588, 12, p.2017-2021

The present invention has been made in view of the above circumstances, and an object thereof is to analyze a local corrosion phenomenon of a metal surface in pitting corrosion or crevice corrosion of a metal that occurs in an acidic environment, in order to analyze a metal surface in an aqueous solution. An object of the present invention is to provide a device for visualizing a potential distribution on a metal surface, for measuring the potential distribution of and its change over time as an image or a moving image with high potential measurement accuracy and high time resolution.

The present inventor has completed the present invention by overcoming such limitations of the conventional techniques and conducting various test studies in order to solve unsolved problems. The gist of the present invention is as follows.

A device for visualizing a potential distribution on a metal surface according to the present invention comprises a solid matrix composed of an organic polymer having conductivity, containing an indicator that discolors or develops a color by an oxidation-reduction reaction, and is used as a measurement target in an aqueous solution. It is configured to change the color tone in response to the potential of a certain metal surface, and contains sulfur and nitrogen as constituents of the indicator, or contains a phenothiazine compound, or one or both of methylene blue and indigo carmine. It is characterized by including .

In the device for visualizing a potential distribution on a metal surface according to the present invention, it is preferable that the organic polymer has a structure in which a π-electron conjugated system is continuous along a molecular chain. Further, the organic polymer may contain an imine nitrogen atom and an amine nitrogen atom between the six-membered ring, or may be a conductive polyaniline.

In the device for visualizing a potential distribution on a metal surface according to the present invention, the solid matrix may be a surface layer surface on an insulating substrate.

The present invention, in order to analyze the local corrosion phenomenon of the metal surface in pitting corrosion or crevice corrosion of the metal that occurs in an acidic environment, the potential distribution of the metal surface in the aqueous solution and its change with time, high potential as a still image or a moving image. It is intended to provide a device for visualizing potential distribution on a metal surface for recording/measuring with high measurement accuracy and high time resolution. According to the present invention, it is possible to measure an electric potential distribution on a metal surface and its change with time as an image in an acidic aqueous solution. In addition to the potential distribution, changes in the appearance of the material surface can be photographed with a camera using a normal visible light as a light source, an actual microscope, an optical microscope, or the like.

FIG. 3 is a side view showing an apparatus for measuring a change in color tone of a solid matrix due to a potential change on a metal surface by the potential distribution visualization device on the metal surface according to the embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described.
A typical example of the method of measuring the potential (electrode potential) on the metal surface is to use a reference electrode such as a standard hydrogen electrode. When the reference electrode and the metal surface (test piece) are both immersed in an aqueous solution and both electrodes are connected by a lead wire via a potentiometer, the electrode potential of the metal surface can be measured with the electrode potential of the reference electrode as a reference. The "potential" in the present specification is the electrode potential of the metal immersed in the aqueous solution as described above with respect to the reference electrode. It is possible to measure the potential distribution on the metal surface by thinning the reference electrode, devising it so that only its tip is sensitive to the potential, and scanning the metal surface. However, it takes a very long time to scan the reference electrode. Therefore, in order to measure the potential distribution on the metal surface in a short time, an indicator that changes color or develops color by a redox reaction was used as a potential sensitive substance. Thereby, the potential distribution can be measured in real time from the color tone distribution.

At this time, in order to accurately measure the potential of the metal surface, it is desirable that the indicator is in close contact with or close to the metal surface via the aqueous solution. For this reason, the indicator was included in the solid matrix rather than dispersed in the aqueous solution. When observing and analyzing a color tone change using visible light or the like as a light source, in order to achieve this with high accuracy, the solid matrix is preferably transparent or translucent.

The solid matrix needs to be composed of a conductive organic polymer. This is because the indicator contained inside the solid matrix needs to change color or develop color in response to the potential of the metal surface. If the degree of conductivity is 0.01 mS/cm or more, it functions as a device for visualizing the potential distribution on the metal surface without any problem, but 1 mS/cm or more is desirable from the viewpoint of response speed to potential changes.

The organic polymer having conductivity generally has a property of slightly changing its color tone by being sensitive to an electric potential in an aqueous solution. Then, by incorporating an indicator that discolors or develops color due to a redox reaction from a conductive organic polymer into a solid matrix, a large discoloration/coloring phenomenon appears more than a simple summing effect, and a metal with extremely high sensitivity. It becomes possible to visualize the surface potential. Among organic polymers having conductivity, a conjugated polymer in which a double bond and a single bond are alternately linked has a π orbit and has a high intermolecular interaction, so that it is in an aqueous solution or ultraviolet visible light. In contrast, many of them show high durability for a long time, and moreover, the action of sensitizing the effect of the indicator to the potential is high. Furthermore, when the conductive organic polymer contains an imine nitrogen atom and an amine nitrogen atom between the six-membered rings, the effect of the indicator being more remarkably sensitive to the potential is sensitized. Above all, when the solid matrix is a conductive polyaniline, a very excellent sensitizing effect appears.

The method for producing the conductive polyaniline is not particularly limited, but when performing potential visualization with high accuracy, a method for producing aniline by a chemical polymerization method is preferable. Generally, a polymer having conductivity can be produced by an electrolytic polymerization method or a chemical polymerization method. However, a polymer produced by the electrolytic polymerization method is polymerized by artificially applying an electric current, and thus the chemical polymerization method is used. It is considered that a larger amount of electric charge remains or is charged as compared with the polymer produced by. Therefore, those produced by the electrolytic polymerization method affect the reactivity in an aqueous solution, and the potential cannot be accurately measured. Therefore, in order to accurately measure the potential distribution on the surface of a metal material in an aqueous solution with a resolution of approximately 0.01 V or higher, it is desirable to use an organic polymer having conductivity, which is polymerized using a chemical polymerization method. ..

Further, in order to measure the potential distribution on the metal surface, the potential distribution visualization device must be brought into contact with the metal surface. Therefore, the solid matrix needs to have shape designability and flexibility, and exhibit durability against a load when pressed against a metal surface. The polymer produced by the oxidative polymerization method using an aqueous solution of aniline sulfate and ammonium peroxodisulfate has not only a high degree of freedom in its geometrical shape and mechanical strength, but also a clear and highly accurate potential change on the metal surface. Since it is imaged by the method, it is suitable when it is necessary to measure the potential distribution and its change with time due to an electrochemical reaction such as metal corrosion with high accuracy and at the same time as observing the corrosion phenomenon.

By the way, as a generally known solid matrix to be used in a state of containing a color change/color development indicator, agar, gelatin, hydrous silicon dioxide in a gel form prepared by a sol-gel method, sodium polyacrylate in a gel form , Polyvinyl alcohol hydrogel, etc. However, these are non-conductive per se, and do not correspond to the solid matrix of the present invention composed of a conductive organic polymer. For example, agar containing an aqueous KCl solution is electrically conductive as a whole gel-like substance, but the substance agar itself is non-conductive, which is different from the solid matrix of the present invention. One of the features of the present invention is to prepare a solid matrix using an organic polymer having conductivity, and by incorporating therein an indicator that discolors or develops a color by an oxidation-reduction reaction. It is to exhibit extremely high potential sensitivity that cannot be obtained by the organic polymer alone.

Next, an indicator that changes color or develops color due to a redox reaction will be described. The indicator is not particularly limited, but in a solid matrix composed of an organic polymer having conductivity, a redox indicator containing a heterocyclic compound exhibits a high discoloring/coloring action. When higher potential visualization is required, an indicator containing sulfur and nitrogen is preferably used, and a phenothiazine-based compound is more preferable. In particular, when one or both of methylene blue and indigo carmine are contained, the potential can be visualized with particularly high accuracy.

Furthermore, in the case of analyzing the change in liquidity due to the gap on the surface of the metal electrode, when the metal material for measuring the electric potential has a simple form such as a flat plate, the potential sensitive material is the surface layer on the insulating substrate. It is necessary to be. When the base is made of an insulating material, the electric potential can be measured with high accuracy, and it becomes easy to bring the electric potential sensitive material into close contact with or close to the metal surface. The term “proximity” in the present specification means that the surface of the solid matrix is brought closer to the metal material within 15 μm.

Hereinafter, the present invention will be described in detail based on examples, but the present invention is not limited to the description of the examples.

FIG. 1 is a side view of an apparatus for measuring a change in color tone of a solid matrix due to a change in potential on a metal surface. The solid matrix was formed as a film on one surface of glass (non-electrically conductive) by the method described later. The glass plate was placed in an electrochemical cell so that the surface on which the membrane-like solid matrix was present was on the lower side, and the solid matrix was immersed in an electrolyte solution of 0.01 M sodium chloride solution (pH 3.0). In addition, a platinum block was placed in the electrochemical cell at a distance of about 5 μm from the solid matrix. The size of the upper surface of the platinum block was 5 mm × 5 mm. The platinum block and the solid matrix were arranged to face each other with a gap of about 5 μm. Furthermore, a 0.01 M sodium chloride solution (pH 3.0) was introduced into this gap.

A platinum counter electrode and a silver-silver chloride reference electrode were placed in the electrochemical cell, and a potentiostat was used to control the potential of the platinum block facing the solid matrix. The silver-silver chloride reference electrode has a liquid junction at its tip, and the internal liquid and the electrolytic solution are in electrical contact with each other. Visible light was irradiated from the back surface of the glass having a solid matrix through an optical microscope using an objective lens, and the color tone of the solid matrix was measured by a spectrophotometer through the objective lens and the optical microscope. At this time, since the reflected light from the platinum block facing the solid matrix was also spectroscopically measured, the surface of the platinum block facing the solid matrix was mirror-polished. After setting the potential, the time until the measurement by the spectrophotometer was completed was set to be within 1 second. Since the color tone of the solid matrix was slightly different between the outer peripheral portion and the central portion, the outer peripheral portion of the solid matrix, which is more sensitive to potential, was evaluated.

A xenon lamp was used as the light source of the visible light to be irradiated, and the color tone of the solid matrix was analyzed as L ^* , a ^* , and b ^* values based on JIS Z 8781-4, and the color tone of the solid matrix at the natural immersion potential was analyzed. The value was evaluated by the color difference ΔE ^* _ab value between the value and the value of the color tone at the potential where the color tone change was most remarkable. And, those with color difference of 19 or more are "◎ ^★ ", those with 15 or more and less than 19 are "◎", those with 10 or more and less than 15 are "○", those with 3 or more and less than 10 are "△", less than 3 In Tables 1 and 2, evaluation results for combinations of various solid matrices and an indicator that changes color or develops color due to redox reaction are arranged. The reference of the potential in this example is based on the silver/silver chloride electrode standard in which the internal liquid is 3.33 M KCl.

Further, as the detection sensitivity of the potential change, in the potential region where the color tone change (color difference) with respect to the potential was most remarkable, the value of the potential change necessary for the color difference to change by 1.0 was calculated and described in Table 1 and Table 2. did. The smaller this value, the higher the detection sensitivity of the potential change.

Comparative Example 1 is an example in which 0.01 g of methylene blue was added to 30 mL of agar prepared using a saturated KCl aqueous solution as a solid matrix. Although the color difference changed with the change in the potential, the change was small.

Comparative Example 2 uses polythiophene as a solid matrix containing no indicator. A glass plate coated with acid-resistant tape on one side was immersed in a mixed solution (normal temperature) of 30 mL of 0.5 M sulfuric acid and 1 mL of thiophene, which was stirred at 300 rpm (revolutions per minute) with a stirrer, and then 0.1 M peroxo was added. A polythiophene film was prepared on one side of the conductive glass plate by adding 50 mL of ammonium disulfate and performing a dipping treatment for 60 minutes. The acid resistant tape was peeled off, washed with pure water, immersed in a 0.01 M sodium chloride solution (pH 3.0) and stored for 1 hour, and then the color tone was measured to evaluate the color difference. It can be seen that the polythiophene film, which is an organic polymer having conductivity, does not contain an indicator, but the color tone changes in response to the potential. However, the degree of change is small, and the potential of the metal surface cannot be accurately analyzed.

Similarly, Comparative Examples 3 and 4 are examples using polypyrrole and polyaniline as the solid matrix, respectively. In any case, the indicator is not included. In each case, 1 mL of pyrrole and aniline were used as raw material reagents, and a film was prepared by the chemical polymerization method as in Comparative Example 2. Although it is a solid matrix film containing no indicator, it can be seen that the color tone changes in response to the potential. However, the degree of change is small, and the potential of the metal surface cannot be accurately analyzed.

Examples 1 to 42 are examples of the present invention in which, as a solid matrix, an organic polymer having conductivity is used, and an indicator that changes color or develops color by a redox reaction is contained. 30 mL of 0.5 M sulfuric acid stirred with a stirrer at 300 rpm (revolutions per minute), 1 mL of a conductive organic polymer raw material reagent, and 0.01 g of an indicator mixed solution (room temperature) with an acid resistant tape Soak a glass plate coated on one side with, add 50 mL of 0.1 M ammonium peroxodisulfate, and perform a dipping treatment for 60 minutes to produce a conductive organic polymer film on one side of the conductive glass plate. did. The acid resistant tape was peeled off, washed with pure water, immersed in a 0.01 M sodium chloride solution (pH 3.0) and stored for 1 hour, and then the color tone was measured to evaluate the color difference. When polyaniline was prepared as a conductive organic polymer, aniline was used as a raw material reagent, thiophene was used as polythiophene, and pyrrole was used as polypyrrole. In the case of using multiple indicators, 0.01 g was added as the total mass. All the solid matrices containing the indicator were translucent, and the metal surface could be observed with an optical microscope through the solid matrix.

From the comparison of Comparative Examples 1 to 4 and Examples 1 to 42 in Table 1 and Table 2, a substance containing an organic polymer having conductivity as a solid matrix and containing an indicator that discolors or develops color by a redox reaction. It can be seen that by using, the color tone change in response to the potential of the metal surface to be measured in the aqueous solution can be visualized with high sensitivity.

Further, from Tables 1 and 2, it is understood that the potential distribution visualization device manufactured according to the present invention can visualize the potential of the metal surface to be measured in an aqueous solution with high sensitivity.

INDUSTRIAL APPLICABILITY The present invention can be used as an imaging device for measuring, as an image or a moving image, the potential distribution on the surface of a material in the initial stage of corrosion of a metal such as stainless steel and its change over time.

Claims (7)

Consisting of a solid matrix made of an organic polymer with conductivity that contains an indicator that changes color or develops color by a redox reaction, and is configured so that in an aqueous solution, the color tone changes in response to the potential of the metal surface to be measured. A device for visualizing a potential distribution on a metal surface, which contains sulfur and nitrogen as constituent components of the indicator . Consisting of a solid matrix made of a conductive organic polymer containing an indicator that discolors or develops color by a redox reaction, and is configured so that in an aqueous solution, the color tone changes in response to the potential of the metal surface to be measured. A device for visualizing a potential distribution on a metal surface, comprising a phenothiazine compound as a constituent of the indicator. Consisting of a solid matrix made of a conductive organic polymer containing an indicator that discolors or develops color by a redox reaction, and is configured so that in an aqueous solution, the color tone changes in response to the potential of the metal surface to be measured. A device for visualizing a potential distribution on a metal surface, which comprises one or both of methylene blue and indigo carmine as a constituent component of the indicator. The organic polymer is, the device for potential distribution visualizing the metal surface according to any one of claims 1 to 3, characterized by having a structure π electron conjugated system is contiguous along the molecular chain. The device for visualizing a potential distribution on a metal surface according to any one of claims 1 to 4, wherein the organic polymer contains an imine nitrogen atom and an amine nitrogen atom between six-membered rings. The device for visualizing potential distribution on a metal surface according to any one of claims 1 to 5 , wherein the organic polymer is a conductive polyaniline. Wherein the solid matrix, the device for potential distribution visualizing the metal surface according to any one of claims 1 to 6, characterized in that the surface layer on the insulating substrate.