US3461058A - Method of producing a composite electrode - Google Patents

Description

United States Patent 3,461,658 METHOD 0F PRODUCKNG A COMPOSITE ELECTRODE Alfred J. Haley, .ln, Westfield, and Carl D. Keith, Summit, NJ., and James E. May, Dorchester, Mass, assignors to Engelhard Industries, IIHL, Newark, Ni, a corporation of Delaware No Drawing. Filed June 7, 1966, Ser. No. 555,676 Int. Cl. Btllk 3/06, 3/04; C2313 5/52 US. Cl. 204-290 3 Claims ABSTRACT OF THE DISCLOSURE A composite electrode comprising a refractory metal support to which is bonded a non-porous ductile platinum group metal coating, the coating being from 0.02 to 1 mil in thickness and having a surface roughness of less than microinches (RMS), and a method for producing the same are disclosed. Such anodes exhibit chlorine overvoltages at 500 amps/ft. of between 0.5 and 1.0 volt and are particularly useful for electrochemical processes.

The electrode is made by electrodeposition of the platinum coating on the support followed by heat-treating and mechanical working.

This invention relates to metallic electrodes for use in electrolysis processes and, in particular, to improved anodes having a refractory metal core coated with a platinum group metal coating, and to a method of producing such electrodes.

In electrochemical processes, the proper selection of material which can be employed in the fabrication of electrodes is very important, and influenced by various technical requirements. In these processes, it is conventional to use electrodes of iron, steel and the like in the cathode position, but there are only a few materials which may be used in the fabrication of insoluble anodes, because most materials, when made anodic, are susceptible to rapid corrosion. Metals of the platinum group have desirable characteristics when utilized as insoluble anodes, but because of the high cost of these metals, it is desirable to use substitute materials which are less costly, and graphite has heretofore been Widely employed for this purpose.

The selection of a suitable anode is particularly critical in highly-corrosive electrochemical baths, such as the brine solutions used in the manufacture of chlorine. Graphite has a number of disadvantages, in that it under goes continual disintegration and must be replaced frequently, thereby causing interruption of the electrochemical process. Further, disintegration of the graphite causes deposition of fine grain of graphite in the diaphragms which surround the electrode necessitating frequently cleaning or replacement of the diaphragms along with the electrodes.

The difiiculties encountered with graphite electrodes, particularly in brine electrolysis, are substantially eliminated when platinum metal anodes are employed. Such anodes can be of pure platinum or, as more recently taught in the literature, can comprise a corrosion-resistant core metal such as titanium or titanium alloys partially or completely clad with a platinum metal sheath or covering.

In addition to the requirement of substantial noncorrosivity, anodes which are employed in electrolytic processes are required to have certain favorable currentcarrying characteristics for economic operation. Thus, the anode should be highly conductive and capable of carrying a high current load, i.e. operable at the highest possible current density, without undue polarization. In

practice, polarization of the anode with resultant higher overvoltage characteristics, reduces the efficiency of the electrolysis and adversely affects the economics of the electrolytic process.

Electrodes having a refractory metal core, especially a titanium or titanium alloy core partially or completely coated with a platinum group metal have uniquely provided many of the physical characteristics necessary for optimum utilization in electrolytic reactions, especially brine electrolysis. In such electrodes, the chemical resistance of titanium oxide which forms on the unplated surface, or Within pores of the platinum plated area, prevents corrosive attack by the electrolyte, while the platinum metal surface acts as the effective conductor. It has been found, however, that while oxidation of the core material prevents corrosion thereof, the formation of oxides, when the platinum coating is not sufficiently adherent, results in erosion of the platinum metal plating normally used to provide the conducting surface. Since only the platinum plated area is an effective conductor in such anodes, the voltage requirements increase as the platinum plated area diminishes due to erosion. These anodes must be replated therefore, with the loss not only of precious metal values but of operating time and materials.

In the fabrication of coated electrodes of the afore mentioned type, it has heretofore been recognized that the platinum surface must be of such nature as to provide low overvoltage characteristics in brine electrolysis. Thus it is known that solid platinum has high chlorine overvoltage characteristics, and it has been proposed to provide a smooth electroplated coating of platinum on a titanium core with a platinum surface layer such as platinum black which provides the desired anode overvoltage characteristics.

In the fabrication of such composite electrodes, it has been found that the ordinary electrochemical techniques for depositing the initial platinum layer on the core material, while providing a smooth platinum surface, do not provide a sufiiciently nonporous platinum coating and the coatings are not metallurgically bonded to the core material. As a result, while such composite electrodes are generally suitable for brine electrolysis, loss of the platinum coating due to oxide formation is still encountered.

Electrodes having a titanium or titanium alloy core partially or completely coated with a platinum group metal have also been suggested for use in electrolytic cells for the production of percompounds such as perchlorates. It is known that the electrical characteristics of anodes used in such cells should be as close to bulk platinum as possible. That is, the percompounds are produced at a high anode over-voltage. Therefore, the electrode material, in this case the platinum group metal coating, should have a high overvoltage. As in the case of the low overvoltage coatings, it is extremely desirable to have the expensive platinum group metal coatings as thin as possible, and such coatings must be very adherent and nonporous in order to avoid the problem of the base metal oxide formation causing the coating to flake off with resultant deterioration of the electrode. Heretofore, it has been difficult to fabri cate suitable composites for the production of percom pounds. Cladding does not lend itself to the formation of sufiiciently nonporous films at the low thicknesses desired by the industry, i.e. below about 1 mil and particularly below 0.5 mil. Thin platinum coatings formed by electrodeposition are also too porous and deteriorate rapidly and, in addition do not have the optimum high overvoltage characteristics.

In accordance with this invention, a composite electrode is comprised of a refractory metal base having a smooth,

substantially nonporous ductile coating of platinum group metal of from about 0.02 to about 1 mil thickness having a surface roughness of less than about 10 microinches and having a chlorine overvoltage of between 0.5 to 1.2 volts at a current density of 500 amps/ sq. ft. Such composites are especially useful as anodes in electrolytic cells for the production of percompounds and are also particularly useful as base structures for low overvoltage platinum group metal coatings. Thus, in an embodiment of this invention, such composites when overcoated with a low overvoltage coating are used in electrolytic cells in which gases are produced at low overvoltages such as in the production of chlorine in the electrolysis of brine. Anodes of this structure are particularly suited for chlorine electrolysis because, in use, the extent or degree of erosion of the low overvoltage layer can readily be determined by cell overvoltage measurements, and the utilization of the anode terminated when the overvoltage indicates that the anode has lost all or substantially all of the low overvoltage surface coating. At such time, the smooth, impervious protective layer of platinum group metal will still be intact, and is readily recoated with low overvoltage surface coating for reuse. By insuring that a substantially impervious platinum metal layer is present at all times, formation of oxidic coatings on the base metal which would result in peeling or erosion of the platinum group metal surface layer is avoided.

In manufacturing the composite anode in accordance with this invention, the base metal substrate is first provided with an adherent, ductile, and smooth precious metal coating of high purity and a thickness of from about 0.03 to about 1 mil. The base metal substrate comprises a refractory metal selected from the group consisting of titanium, tantalum, niobium, zirconium, tungsten, molybdenum and alloys thereof. Titanium and tantalum, and alloys thereof having anodic polarization properties comparable to those of the pure metals are preferred. The ductile precious metal coating is a metal or alloy of the platinum group metals. By a platinum group metal is meant one of the following: ruthenium, rhodium, palladium, osmium, iridium and platinum. As examples of the base metal substrate, commercially pure titanium or tantalum and a titanium-zirconium alloy containing 5% by weight zirconium may be mentioned. The platinum group metal may be, for example, pure platinum, alloys of platinum and palladium containing or 20% of the latter, pure rhodium, platinum-rhodium and platinum-iridium alloys or osmium-iridium alloys.

For the purposes of the present invention, it has been found that the platinum metal coating must be formed on the base metal in accordance with certain specific procedures. The base, preferably in cleaned and roughened condition, is first coated with a ductile precious metal coating of high purity and thickness of about 0.03 to 1 mil. The coated substrate is then heat treated at a temperature between about 700 and 1400 C., preferably 800 to 1100 C., in a non-oxidizing atmosphere to interdifiuse the precious metal and base metal at the interface. Thereafter, the composite material is mechanically treated, as by passing through a rolling mill, to provide a thin substantially nonporous lustrous precious metal surface having a thickness of 0.02 to 1 mil and a surface finish of less than ten microinches.

The initial platinum metal coating is desirably from about 0.03 to 1 mil in thickness. In order to form a coating which will not completely interdiliuse with the base metal substrate on heating, the initial coating should be at least about .03 mil in thickness. Below such thickness, the coating does not retain its integrity on being subjected to the heat treatment. The platinum group metal coating can, of course, be thicker than 1 mil, but, for economic considerations, such thicker coatings are not desirable.

In addition to the required thickness for good adherence and integrity of the surface, the platinum group metal coating must be ductile. A very ductile coating lends itself readily to the mechanical treatment step because this step compresses and spreads the platinum coating so that the pores therein are sealed.

Methods are known for the deposition of thin platinum group metal coatings of high ductility upon base metals. Generally, coatings of platinum on titanium produced by such methods as electrodeposition, thermal decomposition of an applied paint film, electroless plating, etc. are porous, and vary inductility, but desirable ductile coatings, e.g. of platinum, can be prepared by known methods, e.g. as taught in U.S. Patent No. 2,792,341. It has been found, for purposes of illustration only, that deposits obtained by electroplating platinum from a bath containing platinum as the dinitritodiamine compound (P-salt) in accordance with U.S. Patent 1,779,436 provide electroplated surfaces of suitable ductility and thickness for the purposes of the present invention.

Such coatings can, for example, be compared with platinum deposits electroplated from electrolytes containing platinum as the commercial alkali metal hexahydroxy platinate baths, which do not provide platinum coatings of suitable ductility and which, accordingly, cannot be mechanically treated to provide a platinum surface of the desired smoothness and imperviousness.

It will be understood, therefore, that the particular method of applying the platinum metal coating to the substrate is not critical, provided that the resultant platinum metal coating is ductile.

In order to obtain an adherent electrodeposit, it has been found desirable to roughen the surface of the titanium substrate prior to coating. Generally, the desired surface roughness of the titanium will vary with the thickness of the initial platinum group metal deposit. For thinner deposits, a smoother surface is required, but the surface must be rough enough for good keying. For example, with an initial coating of about .microinches thickness, a surface roughness of 30 to 60 microinches is preferred; for a coating of about 250 microinches thickness, the surface roughness may be about microinches; for a very thin coating of 20 to 30' microinches, the roughness may be about 15 microinches. In electrodeposition, the platinum metal layer will follow the contour of the substrate, and the plated product will have a corresponding, or slightly reduced, surface roughness.

In the fabrication of the anodes of the present invention, it is critical to heat-treat the platinum metal coated substrate at a temperature which will cause interditfusion at the platinum-titanium interface prior to the subsequent step of mechanical treatment to produce the desired ultrasmooth surfaced product. It is known, for example as taught in U.S. 3,066,042, to coat a titanium substrate with a relatively thick layer of platinum, and to mechanically treat the resultant composite, followed by heating at interdiffusion temperature. We have found that mechanical working of the composite prior to heat treatment results in a composite which is not suited for use in electrolytic cells. The poorer bond and poor coating are possibly due to oxide formation on the titanium substrate during the mechanical working, which generates substantial heat at the surface. Accordingly, anodes of the present invention are prepared by heating the coated substrate at a temperature of about 700 to 1400 C., preferably 800 to 1100 C. in a non-oxidizing atmosphere to interdiffuse the platinum group metal and base metal at the interface prior to mechanical treatment. An inert atmosphere is essential in the heating step, since at this stage the platinum metal layer is ordinarily quite porous. The time of heating is not critical, but should be sufficient to provide the desired degree of interdiffusion, for example about A to 4 hours, generally one hour sufiices. Reference is here made to U.S. Patent 2,719,797, which teaches similar diffusion treatment of platinum-coated tantalum.

Following the interdifiusion step, the anode is cooled in an inert atmosphere, and thereafter mechanically treated to obtain the desired smooth surface having high chlorine overvoltage characteristics. The smooth, lustrous, nonporous surface may be obtained by such techniques as burnishing, rolling, drawing or swaging. Suitable composite anodes have a surface finish of less than microinches (root mean square) as determined by Profilo meter or other comparable surface roughness measuring instruments. In order to obtain the necessary smooth surface, rolls or dies smoother than 10 microinch surface roughness are employed for the mechanical treatment step.

The composite material prepared in accordance with the foregoing detailed procedure, and having a smooth, lustrous surface of platinum metal displays a chlorine overvoltage of between 0.5 and 1.2 volts at anode current densities of about 500 amps/ft. similar to smooth platinum sheet, and is itself suitable as an anode in electrolytic cells for the production of percompounds. In order to be economically useful as an anode for the production of chlorine by electrolysis, the platinized titanium composite is further coated with a layer of fine-divided or porous platinum which exhibits chlorine overvoltage of 0.025 to 0.2 volt at comparable anode current densitities. Such finely-divided platinum surface layer can be in the form of platinum black, or a porous metallic platinum layer which is deposited by electroplating, decomposition of platinum-containing paints, etc., in a manner known to the art. The resultant anode is particularly suited for chlorine electrolysis since the low overvoltage surface coating is worn off through use, and the condition of the anode is readily determined by simple observation of the voltage increase necessary for the continued operation of the chlorine electrolysis cell. When the overvoltage rises to about 50% of the difference between the theoretical reversible voltage and the voltage of bulk platinum, the anode is removed for recoating. At such time, the smooth, lustrous platinum coating is undamaged, and readily re coated with the desired low overvoltage surface coating.

EXAMPLE 1 A sample of 0.040 in. thick titanium, approximately 6 x 12 in. in size was etched and cleaned by soaking in 30% muriatic acid for a period of 18 hours. The surface was etched to about 50 microinch (RMS) surface roughness.

The sample was then scrubbed with pumice, rinsed thoroughly and then plated with a platinum coating employing a P salt bath of the following composition:

Platinum (as Pt(NH (NO g./l 6.5 Disodium phosphate g./l 100 Diammonium phosphate g./l 20 Ammonium hydroxide ml./l 10 Plating was effected at 80 C. with a current density of 45 ampere/ft. for 90 minutes, with moderate agitation, using a platinum anode. After 90 minutes, the titanium sample was coated with a platinum layer 0.120 mil thick as determined by beta ray black-scattering.

The electroplated sample was then heat treated in flowing, pure argon for one hour at 800 C. and finally cooled in argon atmosphere before removal from the furnace. The sample was then roll-milled using 12 inch wide rolls, 10 inch in diameter with a surface roughness of less than 2 microinches. The sample was passed through the mill about six times which resulted in a thickness reduction from 0.040 to 0.033 inch. The platinum finish was bright and mirror-like, and exhibited surface roughness less than five microinches (RMS) The chlorine overvoltage characteristics of the coated sample were determined in a brine solution using a conventional Luggin capillary and saturated calomel reference electrode system. At 500 amps/ftfl, the chlorine overvoltage of this sample was 0.69, substantially identical with that of a polished sheet of platinum, which has an overvoltage of 0.68 at the same current density.

6 EXAMPLE 2 The coated titanium sheet prepared in Example 1 was painted with a coating of a thermally reducible platinumbearing compound in an organic vehicle, and fired in air at 500 C. for 30 minutes. The surface roughness before application of the paint was about two microinches, and after firing about 50 microinches. Chlorine overvoltage measurements showed a decrease from about 0.6 volt for the smooth surface to about 0.1 volt for the rough surface.

EXAMPLE 3 A piece of titanium 2" x 6" x 0.040" was etched in concentrated hydrochloric acid for a period of 18 hours and then rinsed with water and scrubbed to remove surface film and fine particles lodged in pores. After rinsing in distilled water, the piece was coated with 0.15 mil of platinum by electrodeposition in a P salt bath of the composition described in Example 1.

The coated sheet was cut in half to make two 1" x 6" x 0.040" samples and the samples were numbered 1 and 2 respectively.

Sample No. 1 was heated in argon at 800 C. for 1 hour and after cooling was rolled to approximately 1" x 7" in size.

Sample No. 2 was first rolled to approximately 1" x 7" in size, and then heated in argon at 800 C. for 1 hour and cooled.

The platinum coating of Sample No. 2 lifted or peeled in the area about A in from the edges of the sample. The coating of Sample No. l was adherent all over. Furthermore, soaking the samples in concentrated hydrochloric acid for three hours showed the platinum was removed almost entirely from Sample No. 2 while Sample No. 1 showed no apparent sign of attack.

EXAMPLE 4 A piece of titanium 2" x 6" x 0.040" was etched in concentrated hydrochloric acid and coated with 0.15 mils platinum as described in Example 1.

The coated sheet was cut in half making two 1 x 6" x 0.040" sample and these samples were numbered 3 and 4.

Sample No. 3 was heated in argon to 800 C. for 1 hour and after cooling rolled to approximately 1" x 7" in size. The rolled sample had a surface finish roughness of less than 5 microinches.

Sample No. 4 was not treated further.

Chlorine overvoltage measurements were made on both Samples 3 and 4 as described in Example 1. Sample No. 3 showed a chlorine overvoltage of 0.68 volt at 500 amps/sq. ft. Sample No. 4 showed a chlorine overvoltage of 0.45 volt at the same current density. The chlorine overvoltage for Sample No. 3 was 200 to 250 millivolts greater than that for Sample No. 4 at 500 amps/sq. ft. Sample No. 4 showed an overvoltage considerably below that for bulk platinum.

EXAMPLE 5 A piece of titanium was etched, coated with platinum halved, and one half treated in argon and rolled as described in Examples 3 and 4. The treated half is referred to as Sample No. 5 and the untreated half as Sample No. 6.

Sample No. 5 (treated) and Sample No. 6 (untreated) were soaked in concentrated hydrochloric acid for 24 hours, and then boiled in concentrated sulfuric acid for 6 hours, and afterwards inspected for platinum adherence.

Results of inspection showed the platinum coating on Sample No. 6 (untreated) was peeling at the edges whereas Sample No. 5 (treated) showed no apparent sign of attack. Furthermore, treatment of the samples with 48% hydrofluoric acid showed the coating on Sample No. 6 lifted almost immediately while the coating on Sample No. 5 showed no sign of attack for 30 minutes.

What is claimed is:

1. A method of producing a composite electrode which comprises the steps of:

(a) electrodeposition upon a titanium support a ductile platinum coating from about 0.1 to 1 mil in thickness;

(b) heat-treating the coated support at a temperature between about 700 and 1400 C. in an inert atmosphere to eflect interdifiusion of the coating and support at the interface thereof, and

(c)mechanically treating the heat-treated coated support to produce a thin non-porous platinum metal surface having a thickness from 0.02 to 1 mil and surface roughness less than 10 microinch (RMS), and exhibiting a chlorine overvoltage of from 0.5 to 1.0 volt at 500 amps/ftP.

2. The method of claim 1 wherein the coating is deposited by electroplating from an electrolyte containing platinum diamino dinitrite.

3. A method of producing a composite electrode which comprises the steps of:

(a) electrodepositing upon a titanium support a ductile platinum coating from about 0.1 to 1 mil in thickness;

(b) heat-treating the coated support at a temperature between about 700 and 1400 C. in an inert atmosphere to effect interdiffusion of the coating and support at the interface thereof;

References Cited UNITED STATES PATENTS 7 2,719,797 10/ 1955 Rosenblatt et a1. 3,102,086 8/ 1963 Cotton. 3,115,702 12/1963 Scutt et a1. 3,177,131 4/1965 Angeli et a1. 3,287,250 11/1966 Brown et a1.

20 JOHN H. MACK, Primary Examiner D. R. JORDAN, Assistant Examiner U .8. Cl. X.R.