EP0032960B1

EP0032960B1 - Method of electroplating a porous body

Info

Publication number: EP0032960B1
Application number: EP80100351A
Authority: EP
Inventors: James Arthur Mcintyre; Robert Floyd Phillips; Joseph Donald Lefevre
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1980-01-23
Filing date: 1980-01-23
Publication date: 1983-10-19
Also published as: EP0032960A1; DE3065335D1

Description

Porous bodies are known to be difficult to electroplate interiorly. The problem is intensified with an increasing diminishment of the voids interiorly of a porous body where the plating deposit is desired to be made on the enclosing wall surfaces thereof. This is particularly so in cases where the body to be interiorly plated is a porous electrode intended for electrochemical usage, and which contains an abundance of exceedingly fine, internal pores many of which are of miniscule size of less than 10 micrometres to as small as 0.1 micrometre.
Frequently, particularly with electrodes, a heavy plating deposit is neither needed nor wanted (and may even be deleterious) on the exposed exterior faces of the porous body to be plated and therefore represents a waste of the expensive plating material. Also, standard electroplating techniques tend to cause a buildup of plating deposit on the exterior surfaces of the porous bodies, especially around and about the egress sites of the pores. This is often substantial enough to cause blocking of the pores on the exterior surface resulting in a serious disadvantageous if not inoperative condition, particularly where electrodes are involved. Furthermore, the usual means employed to plate porous bodies interiorly thereof are not always effective in leaving a conservatively thin, but adequate deposition on the interior surfaces of the pores to be plated. This is economically undesirable when plating is performed with expensive coating materials such as silver. Silver or other relatively expensive noble or non- noble metals are often used for their enhanced catalytic effect on the less costly base metal bodies of such porous electrodes.
Attempts to overcome these difficulties have not met with success and have often required resorting to complicated and expensive procedures to avoid or minimize the indicated problems. For example, it has been proposed to pump a plating bath through a porous body to improve the application of internal coatings to the porous body. Such procedure is difficult, however, and not entirely reliable for realizing the desired results. Illustrative of such previous efforts are U.S. Patent Nos. 3,359,469 and 3,787,244 and Canadian Patent No. 921,111.
Accordingly, the present invention essentially resides in a process for electroplating porous bodies. More particularly, the present invention resides in a process for electroplating porous electrodes for use in electro-chemical processes, particularly for the use in chlor-alkali cells. Electroplating is performed in such a way as to preclude or minimize the deposition of a coating on the exterior surfaces of the porous body so as to cause a substantial and excessive blockage of the pores on the outer surface of the porous body. The process of the present invention provides for improved economy and deposition of an adequate and effective quantity of a uniformly thin plating layer on the interior surfaces of the porous body.
The present invention also resides in a process for applying a coating on the interior wall surfaces of at least a portion of a porous electroconductive body having a multiplicity of void spaces comprising the steps of at least substantially filling the void spaces in the porous body with a plating solution having dispersed therein an electrodepositable substance which forms said coating when the porous body is subjected to passage therethrough of an electrical current; immersing the porous body in an electroductive non-plating liquid medium, and applying a direct current electrical potential to said porous body to cause current to flow through said electroconductive liquid medium and porous body to apply said coating on the internal wall surfaces of said porous body.

FIGURE 1 is a flow-diagram and schematically illustrates one procedure for implementing the invention; and
FIGURES 2 and 3 are graphical presentations showing plots of experimental results obtained to demonstrate the invention.

According to the present invention porous bodies are interiorly electroplated by providing substantially the entire volume of a plating bath from which the coating is to be deposited within all or at least a substantial portion of the porous body during the time that an electroplating current is applied to thereby effect the desired plate deposition on the wall surfaces of the internal voids in the porous body. According to the invention, an efficient, effective and sparing application of the plating is made on only the desired wall surfaces within the porous body such that the plating exhibits good uniformity and quality. At the same time, exterior surface plating is minimized to substantially reduce pore blockage which is normally associated with prior electroplating procedures. The latter problem can be a serious detriment to porous bodies, particularly where such porous bodies are used as electrodes in electrochemical applications.
Thus, in conventional electroplating the article to be plated is placed in a solution which contains the ion of the metal to be plated. Often, the anode is made of the same metal as the metal to be deposited as a coating on the porous body by electroplating such as a silver plating one a silver anode. This materially helps to keep a constant concentration of metal ions in solution through anode dissolution as metal ions plate out on the cathode. Since metal ion migration into the interstices of a porous body is relatively slow and is retarded by pores of a decreasingly smaller size, the concentration of metal ions within the body voids decreases with time during the plating as compared to the concentration of metal ions of the plating bath. Unavoidably, the plating rate is much faster on the exterior body surfaces where metal ions in the bath, per se, are in proportionally greater abundance to cause a much heavier plating on the exterior surface compared to the internal voids.
With particular reference to FIGURE 1 of the Drawing, there is shown one way of carrying out the procedure of the present invention. An electroplatable metallic porous body (5) (such as an electrode) is suitable pretreated, if necessary, to ready it for the plating operation. This may include chemical treatments, for example, degreasing, washing and cleaning or drying. Body (5) contains a plurality of internal voids (6) to give it a somewhat sponge-like structure. According to the present invention, it is intended to plate the wall surfaces of the voids within the porous body without sealing of the open pores on the exterior surface of the porous body. To this end, the electrode is immersed at a filling station (4) in a plating bath solution (7) within container (8).
The body (5) is preferably kept in the bath until the void spaces in the porous body are saturated with the plating solution. Accordingly, enough time should be allowed for immersion of the porous body in the solution to permit adequate penetration of and filling of the pores by the solution. Although the body is shown in a vertical position, penetration of the solution into the pores is also facilitated by having the body tilted in any position other than the vertical to minimize or avoid air entrapment in the porous body. The physical positioning of the body and/or vigorous circulation of the plating solution may also help to achieve more effective and quicker penetration of the solution into the porous body. Incomplete penetration and saturation of the solution into the pores of the body would result in less than total plating of the pore surfaces within the body.
Other methods of obtaining satisfactory saturation of the plating solution into the pores may also be resorted to, such as by spraying or forced-filling manipulations.
Though not shown in the drawings, it should be understood that one may also use the procedure of the present invention to plate only a portion of the internal pores of a porous body. For example, an outer portion of the porous body may be saturated with the plating solution by immersion of the body into the solution for a predetermined limited time period without obtaining complete saturation of the plating solution into the pores of the body. Alternatively, only a portion of the porous body may be immersed into the plating solution, such as one side or the lower portion of the body to saturate only that portion of the porous body before proceeding with the electrolytic plating procedure in accordance with the process of the present invention.
There are several techniques which may be used to fill the internal pores in only a portion of the porous body. For example, one may apply a gas or a non-plating solution to one side of the porous body while applying the plating solution to the other side of the porous body. By varying the pressure of each, it is possible to selectively control the proportion of voids which are saturated with the plating solution.
The porous body can be formed of any desired electroplatable material depending substantially on the particular use to which the body is subjected. Porous bodies for use as electrodes are frequently fabricated from metals such as iron, steel alloys (particularly the corrosion-resisting or so-called "stainless steel" types) copper, titanium or alloys of these metals, although there obviously is no limitation on the metals used for electrodes or any other porous bodies to be plated. Likewise, depending on the substrate of the porous body to be plated, any suitable and compatible plating solution may be employed.
After the porous body has been immersed into the plating solution for a predetermined period of time, it is transferred from the filling station (4) to a plating station (9). Care should be taken in this transfer to prevent or minimize the loss of plating solution due to leakage or spillage of the solution from the pores of the body (6). This can be accomplished by holding the body in a position to minimize such loss when taking the body out of the plating solution. Alternatively, a covering member may be held closely against the pore openings on the exposed surfaces of the body to prevent or minimize the leakage of the plating solution from the pores of the body. Bodies, such as electrodes, having extremely small pores are not too troublesome and can be manipulated without leakage or spilling of the plating solution when they are outside of the plating bath.
At plating station (9), a non-plating, electroconductive liquid (10) is provided in a vessel or container (11). It is generally satisfactory for the current-carrying liquid (10) to be an appropriately formulated aqueous saline solution, i.e., one containing a sufficient amount of a suitable and compatible ionizable salt that does not react with the plating solution and is adapted to adequately transport and conduct the electrical current necessary for plating. In other words, the saline solution (10) is intended to more or less function as a fluid electrical brush for the porous body to be plated and it should be substantially if not entirely free from reducible ions that would tend to interfere with the desired internal plating procedure.
An anode (12) is positioned inside of vessel (11) together with means (not shown) to receive and mount the porous body saturated with the plating solution. An electrical circuit is established between the anode and the porous body, which functions as the cathode through electric line (14) connected to a suitable direct current power source (1 3) which, in turn, is connected by electric line (15) to the anode (12). It is preferred to completely submerge the saturated body in the saline solution (10) for the plating procedure, although there are circumstances when only partial immersion will suffice such as while the porous body is saturated with the plating solution over a portion of its surface.
It is necessary that the electrode counter to the porous body being plated (which, as is primarily described, is anodic although that is not necessarily the case) be inert. In other words, the counter-electrode should not dissolve in the saline solution so as to yield platable ions in the solution. Instead, the counter-electrode material should be selected so as to be capable of allowing for gas evolution or some other non-interference-provoking reaction in the electrolysis process.
A plating current is applied to the porous body to cause a deposition of the metal ions from the plating solution within the void spaces or pores (6) of the saturated porous body onto the interior walls of the pores. The current is applied through the saline solution (10) for a period of time sufficient to accomplish plating and at a relatively lower current level as compared to the current level used in conventional electroplating. Reduction generally being such that the current rate in the practice of the present invention is from 5 to 40 percent, preferably less than 10 percent, of normally utilized electroplating currents in standard plating procedures utilizing the same metal substrate and plating materials. However, the current density should be sufficient to perform the plating quickly enough to minimize or avoid diffusion the plating solution of (7) into the saline solution (10) and vice versa, during the plating procedure.
While the optimum current density employed will vary from system-to-system and also depend upon the size and material, for example of such porous bodies to be plated and particularly upon the level of plate deposit desired, it is usually desirable to apply a current density of at least about 0.05 amp/in² (0.008 amp/cm²). A current density at a level much lower than 0.05 amp/in² (0.008 amp/cm²) may require too much time to allow for deleterious plating solution/saline solution diffusion and mixing. Upper acceptable current density levels are reached where excessive formation and evolution of hydrogen occurs. For most purposes, a current density of about 0.1 amp/in² (0.015 amp/cm²) is found satisfactory. It will be understood, however, that the precise current level to be employed for any given situation is readily determinable by persons skilled in the art.
After the plating procedure is completed, the plated porous body is removed from the saline solution (10) and given a washing, drying or other post-plating treatment such as may be needed to finish the porous body for final intended use.
In some instances, the quantity of the coating applied to the porous body, a single-pass plating procedure may not be adequate. In such a situation, the desired thickness of the plating deposit can easily be achieved by a repetition of the plating procedure for as many times as necessary to achieve the desired results.
It is obvious that a plurality of porous bodies can be plated simultaneously in following the procedure of the present invention.
In an example of the present invention, a flat, disc-like body of porous nickel having a 2t inch (6.35 cm) diameter was washed thoroughly with acetone and air dried at about 110°C. The porous body (made of a commercial, pressed and sintered powdered nickel electrode stock) had a thickness of 70 mils (0.178 cm) and an average diameter pore size of 10 micrometres. It had a porosity of 80 percent by volume.
The porous body was saturated in an aqueous plating solution containing 50 g/I AgCN (silver cyanide) and 100 g/I KCN (potassium cyanide). The porous body saturated with the solution was electrically connected as the cathode in an electrolytic cell containing as the current carrying medium a 1/10th molar (0.1 M) aqueous solution of sodium perchlorate (NaC10₄). A platinum (Pt) electrode was inserted into the cell to serve as the anode. An electric current of 0.1 amp/in2 (0.015 amp/cm²) was passed for 30 minutes through the cell to deposit a silver coating on the inner walls of the pores in the nickel body.
To demonstrate the efficacy of the obtained plating, the plated and thus catalyzed electrode was tested in an experimental cell along with an unplated electrode made of the same porous nickel stock.
Each of the electrodes was mounted for evaluation as a depolarized cathode in a standard electrolytic test cell having an expanded titanium mesh anode provided with a coating of titanium oxide and ruthenium oxide. Anode-to-cathode spacing was 9/32 inch (0.714cm) with an intermediate "Nafion" (registered Trade Mark) ion exchange membrane separator in the cell. The anolyte was 300 g/I NaCI and the catholyte 100 g/I NaOH; with the cell operated at a temperature of about 60°C and at a gas pressure on the back side of the cathode maintained at between 2 and
psig (13.8 and 17.2 Nm^-2). The applied current density was 0.5 amp/in² (0.075 amp/cm2)_.
The test was carried out to determine the performance of the electrode over increasing time periods and the voltage savings realized in comparing cell operation with both nitrogen and oxygen gases applied to the electrode. The differences (arrived at by subtraction of the voltage values obtained from nitrogen (i.e., inert gas) operation at any given point of measure and those from oxygen (i.e., active gas) operation at the same point of measure) provided a reliable indication of voltage savings obtained as well as a corresponding depolarization effect upon use of the electrode as a cathode.
The results obtained are graphically depicted in Figures 2 and 3. Figure 2 particularly shows the performance of uncatalyzed (uncoated) porous nickel as a depolarized cathode while Figure 3 illustrates the performance of a porous nickel body coated in accordance with the practice of the present invention.
Good results were obtained with other porous bodies plated in accordance with the procedure of the present invention when employed for other electrochemical and diverse purposes.
It will be understood that the plating procedure of the present invention is applicable to materials other than the metals specifically identified herein. Moreover, coating solutions other than the solutions specifically identified herein and which may even be organic in nature but which are electro-depositable from such appropriate solutions and suspensions are usable in the plating procedure, particularly in electroplating procedures, of the present invention.

Claims

1. A process for applying a coating on the interior wall surfaces of at least a portion of a porous electroconductive body having a multiplicity of void spaces by applying a direct current, electrical potential to said porous body, comprising the steps of at least substantially filling the void spaces in the porous body with a plating solution having dispersed therein an electrodepositable substance which forms said coating when the porous body is subjected to passage therethrough of the electrical current, characterized by immersing the porous body substantially filled with the plating solution in an electroconductive, non-plating liquid medium and causing the electric current to flow through said electroconductive liquid medium and porous body to apply said coating on the internal wall surfaces of said porous body.

2. The process of Claim 1, wherein said porous body has internal, body-traversing pores of an average diameter of less than about 10 micrometres.

3. The process of Claim 1 or 2, wherein the porous body comprises a metal selected from nickel, iron, corrosion-resisting steels, copper, titanium and its alloys.

4. The process of Claim 1, 2 or 3, including the step of at least partially filling the porous body with an electroplating silver solution.

5. The process of Claim 1, 2 or 3, wherein said plating solution is an aqueous solution of silver and potassium cyanides and said electroconductive liquid medium is an aqueous solution of potassium perchlorate.

6. The process of any one of the preceding claims, including the step of passing said current through said electroconductive liquid medium at a density that is between 0.05 amp/in² (0.008 amp/cm²) and that causing formation and evolution of hydrogen at and from the porous body being plated.

7. The process of Claim 6, wherein said current density is about 0.1 amp/in² (0.015 amp/cm2)_.

8. The process of any one of the preceding claims, wherein said procedure is repeated.