Electrochemical cell, use of the electrochemical cell, and method for electrolytically contacting and electrochemically influencing a surface

Abstract

An electrochemical cell for electrolytically contacting and electrochemically inspecting surfaces which makes electrolytic contact with the surface through a body utilizing capillary action. The capillary force between surface and body utilizing capillary action prevents the electrolyte from escaping from the cell without the use of a sealing ring. The body utilizing capillary action allows the electrolyte to flow from an open porous container and wet the surface when the electrochemical cell contacts the surface. Escape of material from the open cell is prevented by the capillary action of the container and of the tip when the electrochemical cell is lifted from the surface. The electrochemical cell is independent of the force of gravity and enables measurements to be made on surfaces of any orientation. The electrochemical cell may be used to perform a multiplicity of electrochemical investigations and processes. The electrochemical cell may be moved over the surface continuously, thereby allowing electrochemical investigations or processes to be performed with lateral resolution. Due to the use of a maintenance-free reference electrode, the electrochemical cell may prefabricated commercially in complete form and stored for extended periods of time after closing.

Description

TECHNICAL FIELD

[0001] The invention relates to an electrochemical cell for electrolytically contacting and electrochemically inspecting surfaces according to the preamble of the first claim.

[0002] The invention further relates to an application of the electrochemical cell according to the preamble of the independent method claim.

[0003] The invention further relates to a method for electrolytically contacting and electrochemically modifying a surface according to the preamble of the independent method claim.

PRIOR ART

[0004] Essentially, there are two types of electrochemical cells which are used for electrochemically inspecting surfaces. With one cell, the surface to be examined is immersed in an electrolyte. The advantage here is that even rough or uneven surfaces may be electrochemically examined. The disadvantage is that only the surfaces of small components may be examined, since larger ones would require a large quantity of electrolyte. Selective examination of specific regions of a surface is possible only if the rest of the surface is covered by a lacquer. In the second type of cell, such as that disclosed by German Patent 197 49 111 A1, the examination surface is pressed against a hole in the outer wall of an electrolytic container. To prevent the electrolyte from escaping, a sealing ring is utilized which delimits the area wetted by the electrolyte. This method allows specific areas of the surface to be chosen selectively; however, the surface must be flat, and the size of the examined component is usually also restricted. A significant problem with this type of cell is the gap created between the sealing ring and the surface. In this gap, electrochemical inspection is possible only to a limited degree. In addition, this gap is the site in which crevice corrosion tends to occur. The sealing ring therefore results in undesirably uneven performance. The use of silicon-coated glass capillaries with a diameter in the range of 1 mm and smaller allows even rough or uneven surfaces to be examined, since at this scale even curved surfaces appear flat. The silicon coating here also acts as a sealing ring. However, since a microscope is used here to apply the capillary, here as well the size of the examined component is limited. The fabrication and handling of capillaries is also expensive and ill-suited for industrial use. In addition, the problem of the sealing ring remains unsolved.

[0005] For purposes of electrolytically inspecting a surface, a counter-electrode is required. An additional reference electrode is often employed as well. These reference Electrodes consist of a container containing a saturated solution. In most commercial systems, these are potassium chloride solutions. This container of the reference electrode is usually enclosed by a porous glass body. The result is that the saturated potassium chloride solution is able to remain in contact with the electrolyte, and any diffusion between the two bodies is limited. Over time, however, the potassium chloride will escape from the reference electrode and contaminate the electrolyte. This occurrence is especially undesirable during corrosion investigations since chlorides are extremely aggressive. As a result, in currently used cells, the electrolyte must be refilled before each measurement. The reference electrodes must also undergo regular maintenance. This procedure necessitates in part the handling of aggressive substances. This preparation of the cell requires considerable care since even very small air bubbles may prevent contact between the reference electrode and electrolyte. Faulty measurements, or even destruction of the examined surface, may result from this.

[0006] East German Patent 263 829 A1 makes known an electrochemical measuring cell. The cell body consists of an electrically conductive, chemically resistant material, contains the reference electrode and counterelectrode, and accepts a small volume of aqueous electrolyte solution. The test sample is connected as the working electrode and is located completely outside the electrode. The material and the charge carrier exchange occurs through a semi-permeable sensitive wall. The reference electrode and test electrolyte here must be reintroduced into the cell before each use. The closed cell also entails problems with atmospheric pressure since the electrolyte is forced back through the porous wall in response to fluctuations in pressure, or is drawn out of the cell. If the cell is open, however, gravity causes the electrolyte to escape, unless extremely fine pores are used to prevent this. However, the fine pores cause an ohmic voltage drop which affects the measurement.

[0007] It is not possible, or is possible only with great difficulty, to apply cells used previously to vertically oriented surfaces. The reason relates to gravity which causes air bubbles to move upward, thereby preventing electrolytic contact to the monitored surface.

DESCRIPTION OF THE INVENTION

[0008] The goal of the invention is to bring a defined electrolyte into contact with a surface without the use of a sealing ring, thereby creating an electrolytic connection to a counterelectrode, and thus electrochemically inspecting the surface locally by the application of an electric current.

[0009] This goal is achieved according to the invention by the characteristic features of the first claim.

[0010] The essential feature of the invention is that a body utilizing capillary action is applied to a surface. The body utilizing capillary action is hereafter called the tip. This allows an electrolytic contact to be created between the surface and a container. The capillary action of the tip may be achieved in a variety of ways. For example, the entire body may consist of a porous material such as nylon felt. Another variant involves the use of a body with one or more capillaries. The cross-section of the tip is tapered toward the end. This feature allows for the minimum possible ohmic voltage drop. When the tip is applied, the electrolyte flows from the container through the tip and wets the surface. The container consists of a porous material. The porosity is designed so that the capillary action prevents the electrolyte from escaping, while at the same time enabling the maximum volume of electrolyte to be taken up. As a result, the greatest possible useful life is achieved for the electrochemical cell. Any escape of the electrolyte from the container and from the tip is prevented by their capillary forces. The container is open during use so as not to prevent the electrolyte from flowing due to the build-up of any partial vacuum. Ideally, the container is closed off from the environment by a casing to prevent evaporation of the electrolyte. The casing is designed to remain open during use to prevent any differences in pressure. The electrochemical cell may be easily closed by placing a cap onto it. Placing the tip onto a surface creates an electrolytic connection between the surface and a counterelectrode which is immersed in the container. The electrochemical potential of the surface may be modified by the flow of electrical current between the surface and counterelectrode. In the case of chloride-containing electrolytes, a silver surface, ideally coated with silver chloride, is used as the reference electrode. A tungsten surface may also be used. By using a maintenance-free reference electrode which employs the measurement electrolyte as the reference electrolyte, and by providing for complete closure through the placement of a single cap, a maintenance-free electrochemical cell is created which is extremely easy to manipulate and may be prefabricated in ready-to-use form.

[0011] The advantage of the invention is that the any escape of material from the electrochemical cell is prevented, with the use of a sealing ring, by the capillary action of the tip and of the container, and that the surface is wetted with a defined electrolyte simply by removing the cap and applying the tip. Since both the subsequent flow of the electrolyte to the surface and from the container to the tip is achieved exclusively by capillary action, the function of the electrochemical cell is completely independent of the effect of gravity. The result is that measurements can also be made on surfaces vertically oriented surfaces. In addition, it is easily possible to perform measurements in zero gravity. Since the container is open, atmospheric pressure also has no significant effect on the performance of measurements.

[0012] Since the cross-section of the tip is tapered toward the end and the container has maximum porosity, a large volume is available to conduct the electrolytic current in the container and in the tip. The ohmic voltage drop thus occurs only at the very end of the tip. As a result, any distortion of the measurement by the ohmic voltage drop can be minimized.

[0013] The cell can be easily applied to the examination surface since whenever the tip is lifted the electrolyte is prevented from escaping by the capillary action of the tip. As long as the tip is not significantly deformed when applied to the surface, the interface with the surface is only at a single point. The gap area is thus smaller than when a sealing ring is used where the entire periphery of the wetted surface constitutes a gap. If, however, the tip is deformed by the pressure of application, it conforms to the surface geometry and the entire wetted surface then constitutes a gap. The uneven performance observed during the use of a sealing ring therefore does not occur at all, or only to a small extent.

[0014] The use of a silver surface coated with silver chloride, or a silver surface, or a tungsten surface creates a very simple reference electrode which remains stable and completely maintenance-free for months at a time. There is no possibility of contamination of the electrolyte, such as occurs with most reference electrodes, by the saturated potassium chloride solution. The result is that the complete cell along with the electrolyte may be prefabricated. Operation is extremely simple and does not require a technician. No manipulation of the electrolyte is required. The entire procedure consists in the removal of the cap from the electrochemical cell and application to the examination surface. After use, the cap is replaced and the electrochemical cell is ready for the next use.

[0015] Since any escape of material from the electrochemical cell is prevented by the capillary action of the tip even without the tip's contacting the surface, the electrochemical cell is extremely simple to manipulate. It may, for example, be held in the user's hand and applied to the highly curved surface of any large component (computer chip, automobile, pipeline, etc.). Since the interface of the tip is only at a single point, or the tip conforms elastically to the surface, any type of curved surface geometries may be examined. When a sealing ring is used, the surface must allow for a circular interface of the sealing ring, a fact which results in certain requirements in terms of flatness or surface radius. The surface of the component is wetted locally with electrolyte, thus allowing electrochemical investigations of local resolution. This type of simple operation was previously impossible using conventional electrochemical cells for large components with complex geometries and vertically oriented surfaces. With the invention, for example, the quality of a welding seam on a pipeline may be easily inspected without having to cut out the welding seam, grinding it flat, and inserting it into a classic cell. Thanks to the invention, it is therefore possible to conduct electrochemical investigations in a standardized routine fashion as a nondestructive test method in quality assurance, research, etc. Since no measures related to sealing, such as sealing rings, are required, the tip may be moved continuously along the surface, thereby allowing for simple electrochemical inspection of large surfaces with local resolution. Multiple sequential spot measurements are also easily performed. The electrochemical cell may be employed for a multiplicity of electrochemical investigations and processes. After completion of the electrochemical modification, the electrochemical cell is simply lifted from the surface and closed. When lifted, any escape of material from the cell is prevented by the capillary action of the tip. The electrochemical cell may be stored for long periods in a closed condition and used at any time without any preparatory efforts. This means a great savings in time when conducting electrochemical measurements. In addition, the electrochemical cell may be commercially prefabricated complete and ready to use, thus enabling its use in a standardized routine manner.

[0016] During prolonged measurements, the electrolyte is able to evaporate at the tip, thus leading to an increase in concentration. This may be prevented by surrounding the tip by a jacket. A high level of humidity is quickly established within the jacket which prevents any further evaporation. As a result, prolonged measurements may be performed with the electrochemical cell. The jacket may also be provided with additional functions. It may, for example, be composed of a conductive material, thereby creating an electromagnetic shield for the cell or the conductive contact with the surface. By additionally utilizing a spring, the jacket may also be used to ensure a constant application pressure. This feature enhances the reproducibility of measurements and the useful life of the tip.

[0017] Due to its porous structure, the tip prevents any convection of the electrolyte. For processes which are controlled by mass transfer, such convection produces poor reproducibility and prevents meaningful findings. In the tip, the subsequent transfer of the initial constituents for the electrochemical reactions is controlled almost exclusively by diffusion. This means that reproducible results are obtained. Use of the invention enables characterization of the mass transfer processes to be significantly improved.

[0018] **During certain types of electrochemical investigations, various reaction products are formed on the surface. Since the tip permits practically no convection, these reaction products are not discharged but are instead concentrated. This problem may be very simply remedied by stretching a porous sheet-like material such as a fabric over the tip. Hereafter this porous sheet-like material is identified as a porous sheet. The electrolyte emerges through the tip and the surface wetted as before. The electrochemical inspection of the surface is thus in no way restricted. However, the electrolyte is carried away laterally due to the capillary action of the porous sheet. Due to evaporation on the large surface of the sheet, a continuous electrolyte flow is created which carries the reaction products away laterally. As a result, a simple-to-manipulate flow-through cell is created. The addition of an electrode which contacts the sheet allows the composition of the electrolyte to be electrochemically characterized. An additional potentiostat, or ideally, a bipotentiostat, is required for this purpose. Ideally, the tip is mounted in the casing by force fit. The tip is inserted into a tube which is provided with a thread, the walls of which are elastically deformed in a radial direction. This elastic deformability is ideally created by longitudinally oriented slots. The opening in the casing is conical so that the diameter shrinks as the tube is screwed in. Screwing the tube with the tip into the casing thus causes a force fit. As a result, the tip may be easily replaced at any time.

[0019] Additional advantageous embodiments of the invention are found in the subclaims.

BRIEF DESCRIPTION OF DRAWINGS

[0020] FIGS. 1, 3, 4 and 5 show sample applications of the electrochemical cell. Identical and analogous elements are identified by the same reference symbols.

[0021] FIG. 2 illustrates electrochemical measurements at different points on a component made of stainless steel (DIN 1.4529) including a welding seam. The horizontal axis shows the electrochemical potential, recalculated for a saturated calomel electrode (SCE), while the vertical axis records the current density.

[0022] FIG. 6 illustrates an electrochemical measurement of a heterogeneous surface made by automatic scanning using the electrochemical cell. The measurement surface is 18×18 mm. The value shown is the current density at a constant electrochemical potential of 0.16 V SCE. Maximum current density is 5 &mgr;A.

METHODS OF IMPLEMENTING THE INVENTION

[0023] FIGS. 1, 3 and 4 show the electrochemical cell 1 consisting of elements 2 through 6. A tip 6 is in contact with an open container consisting of a porous material 3 containing the electrolyte. Container 3 is surrounded by an open casing 4 including an opening 18 in order to reduce evaporation of the electrolyte. The tip consists of a body utilizing capillary action, the cross-section of which is tapered toward the end, thereby reducing the ohmic voltage drop to a minimum level. The capillary action may be achieved, for example, by a body consisting of a porous material or by a body having one or multiple capillaries of a given cross-sectional area. The tip may, for example, consist of pressed nylon felt, a fiber bundle, or two concentric plastic cylinders with a cylindrical gap. It is also possible to fabricate the container and tip out of the same body. For example, a nylon felt used as a container may be compressed on one side by hot-press molding to form a tapered tip. The capillary action is such that the electrolyte is prevented from escaping when the tip is lifted. When the tip is applied to surface 7, the electrolyte flows from the container through the tip and wets the surface locally. Any escape of the electrolyte 15 from the container is prevented by the capillary action between surface 7 and tip 2, as the enlargement of the tip in FIG. 1 shows. The enlargement shows the case of a tip which is not deformed upon application. As a result, only a small gap is created. Preferably, the tip has a smaller diameter at the end than at the connection point to container 3. The container consists of a porous material or another material with capillary action such as felt. A counterelectrode 5 is installed in this container and is in contact with the electrolyte. This counterelectrode consists of an electrically conductive, preferably inert, material such as platinum, graphite, gold, silver, titanium or stainless steel. The counterelectrode is connected in an electrically conductive manner with a current source 8 such as a battery, a potentiostat, or a galvanostat. This current source is also is also connected in an electrically conductive manner to the examination surface. The flow of current between surface 7 and counterelectrode 5 causes the surface in the wetted region to be electrochemically modified. The tip may be moved over the surface continuously or in steps, thereby electrochemically modifying different regions of the surface.

[0024] Both the wetting of the surface and the flow of electrolyte 15 is effected by the capillary action of the tip and container. As a result, the operation of the electrochemical cell is independent of the effect of gravity. Use on vertically oriented surface and in zero gravity is easily possible.

[0025] Installation of a reference electrode 6 additionally allows for measurement of the electrochemical potential of surface 7. With chloride-containing electrolytes, this reference electrode may consist of a silver surface which is ideally coated with silver chloride.

[0026] Using the described electrochemical cell 1, it is possible to automatically scan larger surface areas in order to determine their electrochemical properties. In this manner, local weak points and inhomogeneities on materials may be detected quickly and simply. As a result, the electrochemical cell is suited for routine investigations in surface engineering.

[0027] In order to prevent any uncontrolled evaporation of the electrolyte, any exchange of air with the environment is prevented by a jacket 9, as shown in FIG. 3,. This jacket 9 may, for example, consist of a solid plastic cylinder or a soft rubber sleeve. The essential requirement is that it contact the surface and prevent any exchange of air with the environment, or at least strongly inhibit this. The additional use of a spring 10 allows a constant application pressure of tip 2 on surface 7 to be obtained. If the jacket consists of an electrically conductive material, it may be also employed as shielding against electromagnetic fields and/or for the purpose of electrically contacting surface 7.

[0028] As shown in FIG. 4, the use of a porous sheet 11 such as a nylon fabric allows for the flow of electrolyte, and thus for a continuous renewal of the electrolyte on the examined surface.

[0029] The design described makes possible a multiplicity of electrochemical investigations and methods, while the lateral resolution roughly matches that of the surface wetted by the electrolyte. The following discussion illustrates a few of these investigations and methods.

[0030] The use of current density potential measurements allows for electrochemical characterization of the surface and for determination of the resistance of a material against local corrosion.

[0031] By recording impedance measurements, the rate of corrosion may be determined with minimal effect on the surface. In addition, semiconductor properties such as flat band potential and doping concentration may be determined for semiconductors. In the case of coatings, charge transfer resistance and capacitance may be analyzed.

[0032] Coatings may be applied locally by electrolytic deposition. For example, copper, polypyrrole or other substances may be applied locally without any masking of the surface using photolithography. Conversely, a local etching process may be implemented by electrolytic dissolution.

[0033] Since the tip is moved continuously or in stepwise fashion over the surface, material parameters (such as capacitance or charge transfer resistance) may be determined with lateral resolution. In addition, structures such as conductive tracks may be drawn on surfaces by electrolytic deposition. The electrochemical cell may be commercially prefabricated, both inexpensively and complete, or at least in part. Since the cell may be used immediately without any preparation, there is a significant savings in time. After measurement has been completed, the electrochemical cell may be closed until the next measurement is performed. Thanks to the stability of the reference electrode, long-term storage is possible, and no kind of preparation is required for subsequent measurements.

[0034] FIG. 1 illustrates an sample application of the invention. Tip 2 is connected to container 3 which contains 1 M NaCl solution. The solution is prevented from evaporating by casing 4. A platinum wire is immersed in the electrolyte forming counterelectrode 5, as is a silver wire coated with silver chloride forming reference electrode 6. These electrodes are connected through terminals with a potentiostat 8 which is also connected to surface 7 of a welded component, consisting in this example of stainless steel (DIN 1.4529). When the tip is applied to the surface, the 1M NaCl flows from the open container through the tip and wets the surface in the area in contact with the tip. This creates an electrolytic connection which enables electrochemical inspection of the surface by passing a current between counterelectrode and surface. The tip is applied at different locations on the component. Using the potentiostat to measure current density potential curves, the resistance of the materials is analyzed at these locations. The results are shown in FIG. 2. Over the entire potential range, there is no occurrence of pitting either in the base material (a) or in the welding seam (c). In contrast, a significant current rise indicating the poor resistance of this region against local corrosive attack is found in the heat affected zone (b) at even low potential values. This shows clearly that the welding parameters must be improved to achieve a higher corrosion resistance in all regions. After measurement is complete, the electrochemical cell may be re-closed until the next measurement. Storage over long periods of time is possible, and no kind of preparation is required for later measurements.

[0035] During prolonged investigations, it is critical that any evaporation of the electrolyte through jacket 9 be prevented. This is possible by using a jacket 9 such as that shown in FIG. 3. Any exchange of air with the environment is significantly reduced by jacket 9 so that the air around the tip becomes saturated very quickly by evaporation of the electrolyte. Any further evaporation is halted as a result. If jacket 9 is also composed of a conductive material, it may be employed simultaneously as a shield and to contact surface 7. By employing a spring 10, jacket 9 may also be used to set a reproducible pressure of application.

[0036] An sample application for measurements during electrolyte flow is shown in FIG. 4. By using a porous sheet 11, such as a nylon fabric, the electrolyte is drawn away by the capillary action of the sheet from the contact area on the surface. The continuous evaporation on the relatively large surface of the porous body allows a continuous flow of electrolyte to be obtained. As a result, the reaction products are readily removed. If the sheet is in contact with an electrically conductive, preferably inert, material, the composition of the removed electrolyte may also be electrochemically analyzed. The function of the electrode may then be taken over, for example, by jacket 9. In the case of this configuration, a bipotentiostat 12 is employed.

[0037] FIG. 5 is a constructional diagram of the electrochemical cell. Tip 2 along with tube 13 is screwed into open casing 4. This achieves the force fit of tip 2 which allows for simple replacement of tip 2. Mounting cap 14 closes off opening 18 and tip 2. The electrochemical cell 1 is thus maintenance-free and ready to use over extended periods of time.

[0038] FIG. 6 illustrates a measurement on a thermally sprayed coating. The electrochemical cell here was moved continuously over the examination surface. The examined surface measured 18×18 mm. The vertical axis shows the current density at a constant electrochemical potential of 0.16 V recalculated against a calomel electrode. The maximum value for the current density is 5 &mgr;A for a measurement surface of 0.25 mm2. It is evident in the sample application shown that there are two types of defect present in the coating. One involves isolated pinholes 16, the other two-dimensional defects 17 which may be attributable to cracks and poor adhesion of the coating. Based on the measurement, it is clear that parameters of fabrication have to be optimized. Of course, the invention is not restricted to the sample applications shown and described here. For example, the electrochemical cell may be also used to conduct potentionstatic and galvanostatic retention tests and crack tests. In addition, impedance measurments may be performed; and electrodeposition and electrodissolution are also possible. The potentiostat may, for example, be replaced by a simpler voltage source such as a battery.

[0039] The essential point is that by applying the tip the surface is locally wetted by a defined electrolyte flowing from an open porous container, and that the surface is electrochemically modified by an electrical current flowing between the surface and the counterelectrode.

Claims

1. Electrochemical cell (1) for electrolytically contacting a surface with a defined electrolyte (15) and for electrochemically modifying the surface (7) by supplying current (8) between the surface (7) and a counterelectrode (5), with a body utilizing capillary action (2) being applied to the surface (7), with the electrolyte flowing from a container (3) through the body utilizing capillary action (2) onto the surface (7), and with the electrolyte locally wetting the surface,

characterized in

that the container (3) consists of a porous material.

2. Electrochemical cell (1) according to claim 1,

characterized in,

that the container (3) is surrounded by an open casing which reduces evaporation of the electrolyte, the open casing being closable.

3. Electrochemical cell (1) according to claims 1 or 2,

characterized in,

that the counterelectrode (5) is immersed in the container.

4. Electrochemical cell (1) according to one of claims 1 through 3,

characterized in

that the body utilizing capillary action (2) has a cross-sectional area tapered toward the front.

5. Electrochemical cell (1) according to one of claims 1 through 4,

characterized in

that a reference electrode (6) is immersed in the container (3).

6. Electrochemical cell (1) according to claim 5,

characterized in

that the reference electrode (6) consists of a silver surface coated with silver chloride or a tungsten surface.

7. Electrochemical cell (1) according to one of claims 1 through 6,

characterized in

that evaporation of the electrolyte from the body utilizing capillary action (2) is prevented by a jacket (9).

8. Electrochemical cell (1) according to one of claims 1 through 7,

characterized in

that a porous sheet (11) located between the body utilizing capillary action (2) and the surface (7) causes a flow of electrolyte at the surface (7).

9. Electrochemical cell (1) according to claim 8,

characterized in

that the porous sheet (11) is in contact with an electrically conductive electrode (9).

10. Use of the electrochemical cell (1) according to one of claims 1 through 9 to modify the electrochemical potential of the surface (7).

11. Method for electrolytically contacting a surface with a defined electrolyte (15) and for electrochemical modifying the surface (7) by supplying current (8) between the surface (7) and a counterelectrode (5) of an electrochemical cell, with a body utilizing capillary action (2) being applied to the surface (7),

characterized in

that the electrolyte flows from a container (3) made of a porous material through the body utilizing capillary action (2) onto the surface (7), and that the electrolyte locally wets the surface (7).

12. Method for electrolytically contacting a surface with a defined electrolyte (15) and for electrochemically modifying the surface (7) according to claim 11,

characterized in

that the electrochemical cell (1) is moved continuously over the surface (7) to be characterized.