JPS629678B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Porous objects are known to be difficult to internally electroplat. The problem is that when the plating deposits adhere to the enclosed wall surfaces of the porous body, the voids of the porous body are rapidly reduced. The object to be internally plated is a porous electrode used for electrochemical applications, and a large number of fine particles, from less than 10 microns
If it contains internal holes as small as 0.1 micron,
This reduction in voids is noticeable.

Often, especially in the case of electrodes, heavy plating deposits are not needed on the exposed external surfaces of the porous object to be plated, and (even if they can be removed) are undesirable and therefore expensive plating deposits. waste material. Standard electroplating techniques also produce a build-up of plating deposits on the outer surface of the porous body, particularly around the pore exits. This is often enough to cause clogging of the pores on the outer surface, resulting in a non-working condition around the electrode, a serious drawback. Furthermore, the usual means used to plate the interior of porous objects are to deposit a preservative, thin, and
However, it is not always effective in leaving sufficient precipitate. This is economically undesirable when plating is done with expensive coating materials such as silver. Silver or other relatively expensive noble or non-precious metals are often used on the cheap base metal bodies of such porous electrodes to enhance their catalytic effectiveness.

Attempts to overcome these difficulties have been unsuccessful and often require complex and expensive processes to avoid or alleviate the problems identified above. For example, it has been proposed to pump plating baths through porous bodies to improve the application of internal coatings to the bodies. However, such procedures are difficult and completely unreliable for achieving the desired results. Such prior efforts are known in USP 3359469 and 3787244 and Canadian Patent 921111.

The invention therefore relates essentially to a method of electroplating porous objects. More particularly, the present invention relates to a method for electroplating porous electrodes for use in electro-chemical processes, in particular for use in chlor-alkali batteries. Electric plating is
It is carried out in such a manner as to eliminate or minimize deposition of coating on the outer surface of the porous body so as to result in substantial and excessive sealing of the pores on the outer surface of the porous body. The method of the invention results in the deposition of a sufficient and effective amount of a uniformly thin plating layer on the inner surface of a porous body.

The present invention provides a method for coating the inner wall surface of at least a portion of a porous electrically conductive object having a large number of voids, which includes a coating in which a substance capable of electrodeposition is dispersed in the voids of the porous material. filling the solution at least substantially, the electrodepositable substance comprising:
forming the coating when an electrical current is passed through the porous body; immersing the porous body in a conductive non-plating liquid medium; and applying a direct current potential to the porous body, passing an electric current through the conductive liquid medium and the porous object;
The present invention relates to a method characterized in that the coating is applied to the surface of the porous inner wall.

In accordance with the present invention, the porous body provides substantially the entire volume of the plating bath, from which the coating is deposited within all or at least a majority of the body during the time that the electroplating stream is flowing. and internally electroplated by forming the desired plating deposit on the wall surfaces of the internal cavities within the porous body. According to the invention, efficient, effective and preliminary application of electroplating is made within the porous body only to the desired wall surfaces so that the plating exhibits good uniformity and quality. At the same time, external surface plating is minimized to substantially reduce pore blockage commonly associated with previous electroplating methods. The latter problem is a serious detriment to porous bodies, especially when such porous bodies are used as electrodes in electrochemical applications.

In conventional electroplating, the product to be plated is placed in a solution containing the iodine content of the metal to be plated. Often, the anode is made from the same metal that is to be deposited as a coating on the porous body by electroplating, such as silver plating on a silver anode. This helps maintain a constant concentration of metal ions in solution throughout the anodic dissolution, substantially as metal ions plate on the cathode. The migration of metal ions into the interstices of a porous body is relatively slow and retarded by the smaller sized pores, so that the concentration of metal ions within the body space is equal to the concentration of metal ions in the plating bath. decreases over time during its plating compared to . Unavoidably, the plating rate will be faster on the external object surface if in the bath the metal ions themselves will be proportionately larger to give rise to more heavy precipitates on the external surface compared to the internal space. Become.

Referring to FIG. 1, one method of carrying out the method of the present invention is shown.

Electroplatable porous object such as an electrode 5
is suitably pretreated to prepare it for the plating operation, if necessary. This includes, for example, chemical treatments such as degreasing, washing, and cleaning or drying. The object 5 includes a number of interior spaces 6 giving it a somewhat spongy structure. According to the invention, it was envisaged to plate the wall surfaces of spaces within a porous body without blocking the open pores on the outer surface of the porous body. At this end, the electrode is immersed with a filling part 4 in a plating bath solution 7 in a container 8 .

Preferably, the object 5 is held in the bath until the spaces within the porous object are saturated with the plating solution. Therefore, the porous object should be soaked in the solution for a sufficient period of time to allow for sufficient penetration and filling of the pores by the solution. The object is shown in a vertical position and penetration of the solution into the pores is facilitated by tilting it in a position other than vertical to minimize or avoid air entrapment in the porous object. Ru. The physical location of the body and/or vigorous circulation of the plating solution helps achieve effective and fast penetration of the solution into the porous body. Incomplete penetration and saturation of the solution into the pores of the object occurs without the entire plating of the pore surface into the object.

Other methods for obtaining satisfactory saturation of the plating solution into the pores are by spraying or force filling operations.

Although not shown in the drawing, the method of the invention can be used to plate only the internal pore portions of porous bodies. For example, the outer portion of the porous object may be prepared by immersing the object in the plating solution for a predetermined limited time without obtaining complete saturation of the plating solution within the pores of the object. Can be saturated with Metzki solution. Alternatively, only one part of the porous object, e.g. only one side or bottom side of the object, is immersed in the plating solution before electroplating according to the method of the invention. saturate.

There are several techniques that can be used to fill the internal pores in only one portion of the porous body.
For example, a gas or non-plating solution can be applied to one side of the porous body while the plating solution is applied to the other side of the porous body. By varying the respective pressures, it is possible to selectively suppress the proportion of the space that is saturated with the plating solution.

The porous body can be formed from any desired electroplatable material depending in substance on the particular application for which the body is served. Porous objects for use as electrodes are often composed of iron, steel alloys (particularly corrosion-resistant or so-called stainless steel types), copper, titanium or alloys of these metals, but the electrodes or plated There are no limitations on the metals used for other porous objects. Similarly, suitable and compatible plating solutions can be used depending on the substrate of the porous body to be plated.

After the porous body has been immersed in the plating solution for a predetermined time, it is transferred from the filling section 4 to the plating section 9. Care should be taken to prevent or minimize leakage or spillage of the solution from the holes in the object 6. This is done by holding the object in a position to minimize such losses when removing it from the plating solution. Alternatively, the covering member is held tightly against the hole opening onto the exposed surface of the object to prevent or minimize leakage of the plating solution from the hole in the object. Objects such as electrodes with extremely small holes are not too cumbersome and can be handled without leaking or spilling the plating solution when they are outside the plating bath.

In the plating part 9, the non-plating conductive liquid 10
is provided in container 11. In general, the current carrying liquid is a suitably composed aqueous salt solution, i.e., a solution containing compatible ionizable salts that do not react with the plating solution and are suitable for carrying and directing the current required for plating. should be satisfied. In other words, the salt solution is used to act as a fluid electric brush for the porous object to be somewhat plated, and it contains no reducible ions that would tend to interfere with the desired internal plating operation. It shouldn't be.

Anode 12 is located inside vessel 11 together with means (not shown) for receiving and attaching a porous body saturated with the plating solution. Electric current flows between the anode and the porous body, which acts as a cathode through a wire 14 connected to a suitable DC power source 13, which power source is connected to the anode 12 by a wire 15. There is. It is preferred to completely immerse the saturated object in the aqueous salt solution for the plating process, but only partially while the porous object is saturated with the plating solution over a portion of its surface. There are circumstances when dipping should be satisfied.

It is necessary that the electrode (mainly, but not necessarily an anode) for the porous body to be plated be inert. In other words, the opposite electrode must not be dissolved in the saline solution that would result in plateable ions in the solution. In fact, the counter electrode material should be selected in such a way that it can allow gas evolution or some other non-disturbing reaction in the electrolysis process.

A plating current is applied to the porous body to cause precipitation of metal ions from the plating solution within the saturated porous body spaces or pores 6 on the inner walls of the pores. The current is applied through the brine solution for a sufficient time to achieve plating and at a relatively low current level compared to the current normally used in electroplating. 5 to 5 of the electroplating current commonly utilized in standard plating methods utilizing the same metal substrate and plating material.
A current rate of 40% reduction, preferably 10% reduction is used. However, the current density should be sufficient to complete the plating quickly enough to minimize or avoid diffusion of the plating solution into the brine solution or vice versa during the plating operation. .

The optimum current density used will vary from system to system and will also depend, for example, on the size and material of the porous body to be plated and on the particular level of plating deposits desired, but at least about 0.05 amperes per sq. It is generally desirable to apply a current density of 0.008 amps per square inch (0.008 amps per square inch). Current densities below 0.05 amps per square inch require more time for diffusion and mixing of the hazardous plating solution/salt solution. Acceptance The upper limit of current density that should be applied is up to the point where excessive hydrogen formation and evolution occurs.For most purposes, approximately
A current density of 0.1 amps per square inch (0.015 amps per square centimeter) has been found to be satisfactory; however, the exact current level to be used for a given condition can be easily determined by one of ordinary skill in the art. be understood.

After the plating operation is completed, the plated porosity is removed from the brine solution 10 and washed.
Drying or other post-plating treatments are performed to finish the porous body for its final intended use.

In some cases, the amount of coating applied to the porous object may not be sufficient in one plating operation. In such conditions, the desired thickness of the plating deposit can be easily achieved by repeating the plating operation multiple times to achieve the desired result.

It is clear that a large number of porous objects can be plated simultaneously by the method of the invention.

In an embodiment of the invention, 2 1/2 inches (6.35
A flat, disc-shaped porous nickel with a diameter of 1 cm) is thoroughly cleaned with acetone and air-dried at approximately 110 °C. The porous body made from commercially available compressed and sintered powdered nickel electrodes had a thickness of 70 mils (0.178 cm) and a pore size of 10 microns average diameter. It had a porosity of 80% by volume.

The porous object is AgCN (silver cyanide) 50g/
and an aqueous plating solution containing 100 g/KCN (potassium cyanide). The porous body saturated with the solution is electrically connected as a cathode in an electrolytic cell containing a 1/10 molar aqueous solution of sodium perchlorate (NaClO ₄ ) as the current conducting medium.
A platinum (Pt) electrode is added into the bath to act as the anode. A current of 0.1 ampere per square inch is passed through the bath for 30 minutes to deposit a silver coating on the inner walls of the pores in the porous body.

To demonstrate the efficacy of the resulting plating, the plated and catalyzed electrode was tested in an experimental chamber with an unplated electrode made from the same porous nickel.

Each electrode was mounted for evaluation as a depolarized cathode in a standard electrical test chamber with an elongated titanium mesh anode with titanium oxide and ruthenium oxide coatings. The anode-to-cathode spacing was 9/32 inch (0.714 cm) with a "nafion" separating ion exchange membrane in the middle in the bath. The anolyte is NaCl300g/, and the catholyte is
NaOH 100g/, the electrolyzer is at a temperature of about 60°C and behind the cathode 2~21/2
Operated with gas pressure held at psig. The applied current density was 0.5 amps/in².

The tests were conducted to determine the performance of the electrode over an increased period of time and the voltage savings realized compared to bath operation with nitrogen and oxygen gases applied to the electrode.

The difference between the voltage value obtained from nitrogen gas (i.e., inert gas) operation at a given measurement point and the voltage value obtained from oxygen (i.e., active gas) operation at the same measurement point is the resulting voltage savings. A corresponding depolarization effect was shown when using the electrode as a reliable pointing point and cathode.

The results obtained are shown graphically in FIGS. 2 and 3. FIG. 2 specifically illustrates the performance of uncatalyzed (uncoated) porous nickel as a depolarized cathode, while FIG. 3 illustrates the performance of a coated porous nickel body in accordance with the practice of the present invention.

Good results have been obtained with other porous bodies plated according to the method of the invention when used for other electrochemical and different purposes.

The plating method of the present invention can also be applied to materials other than the metals shown herein. Furthermore, solutions other than those shown herein and which may be organic,
However, coating solutions which can be electrodeposited from such solutions and suspensions can be used in the plating process of the invention, especially in the electroplating process.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram illustrating one method of the invention, and FIGS. 2 and 3 are graphs plotting the invention. 5... Porous object, 10... Salt aqueous solution, 6...
Internal void, 11... Container, 7... Plating bath solution, 12... Anode, 8... Container, 13... Power source, 9... Plating tank, 14... Electric wire, 15... Electric wire.

Claims (1)

[Scope of Claims] 1. A method of coating at least one inner wall surface of a porous conductive object having a plurality of spaces, wherein the spaces of the porous object contain a substance that can be electrodeposited. filling the porous object at least substantially with a plating solution, the electrodepositable material forming the coating when an electrical current is applied to the porous object, in a conductive non-plating liquid medium; applying a direct current potential to the porous body and passing an electric current through the conductive liquid medium and the porous body;
A method characterized in that the coating is applied to an inner wall surface of the porous material. 2. The method of claim 1, wherein the porous body has pores internally and across the body with an average diameter of less than or equal to 10 microns. 3. A method according to claim 1 or 2, wherein the porous body is of a metal selected from nickel, iron, corrosion-resistant steel, copper, titanium and alloys thereof. 4. A method according to claim 1, 2 or 3, comprising the step of at least partially filling the porous body with an electroplating silver solution. 5. The method according to claim 1, 2 or 3, wherein the plating solution is in the form of an aqueous solution of silver cyanide and potassium cyanide, and the conductive liquid medium is an aqueous solution of potassium perchlorate. 6. Applying the electrical current through the conductive liquid medium at a density ranging from 0.05 amperes per square inch to forming and generating hydrogen from the porous object to be plated. Method described. 7. The method of claim 6, wherein said current density is 0.1 amps per square inch. 8 A method for coating at least one inner wall surface of a porous conductive object having a large number of voids, including: (a) applying a plating solution containing an electrodepositable substance to the voids of the porous object; at least substantially filling the porous body with the electrodepositable material that forms said coating when an electric current is applied to the porous body; (b) filling the porous body in an electrically conductive non-plating liquid medium; (c) applying a direct current potential to the porous body and passing an electric current through the conductive liquid medium and the porous body to apply the coating to the inner wall surface of the porous body; and (d) ) A method characterized in that it consists of repeating the steps (a) to (c).