US3190822A

US3190822A - Process for electrolytically etching valve metal surfaces

Info

Publication number: US3190822A
Application number: US81396A
Authority: US
Inventors: Burnham John
Original assignee: Individual
Current assignee: Individual
Priority date: 1961-01-09
Filing date: 1961-01-09
Publication date: 1965-06-22
Anticipated expiration: 1982-06-22

Description

June 22, 1965 J. BURNHAM 3,190,822

PROCESS FOR ELECTROLYTICALLY ETCHING VALVE METAL SURFACES Filed Jan. 9. 1961 v 2 sheets-sheet 1 J. BURNHAM 3,190,822

PROCESS FOR ELEGTROLYTICALLY ETCHING VALVE METAL SURFACES June 22, 1965 2 Sheets-Sheet 2 Filed Jan. 9, 1961 United States Patent O ice 3,190,822 p PROCESS FR ELECTRLYTICALLY ETCHING VALVE METAL SURFACES lohn Burnham, 10960 Verano Road, West Los Angeles, Calif. Filed Jan. 9, 1961, Ser. No. 81,396 Claims. (Cl. 20d-141) This invention pertains to etched surfaces of inert valve metal electrodes and to processes for manufacturing the same.

In electrolytic capacitors valve metal elect-rodes are used because of the asymmetrical electrolytic characteristics of adherent oxide coated surfaces of such metals. 4It is established p-ractice to etch such electrodes in order to increase their surface areas so as to make it possible to use relatively small electrodes in order to obtain relatively high capacitance ratings. The term etch ratio is commonly used in order to designate the proportionate amount by which the etfectivesurface areas of an electrode is increased by etching.

It is n-ot considered necessary to set forth in this specicaiton t-he reasons why relatively inert valve met-al electrodes, such as tantalum, niobium, and titanium electrodes are used in manufacturing electrolytic capacitors instead :of electrodes formed of the relatively more reactive metal aluminum. Unfortunately inert valve metals as indicated above cannot beetched satisfactorily using the established types of processes for etching commonly employed with aluminum. As a consequence of this a great deal of research and development activity has been devoted to the etching of inert valve metals and in particular tantalum.

Such -work has resulted in processes for electrolytic etching tantalum in various types of electrolytes such as, for example, aqueous hydroiluor-ic acid electrolytes and nonaqueous electrolytes in which solvents such as methanol or formamide contain inorganic ionogens. Such processes are considered to require critical concentration control within the electrolytes used in them. Frequently such concentrations are relatively difiicult to achieve and maintain. It isV considered that prior etching processes of these types vfor use with inert valve metals such as tantalum have not been successful in producing etched surfaces of a uniform, satisfactory character having high etch ratios.

An object of this invention is to provide processes for etching inert valve metals which overcome many of the disadvantages and limitations of prior processes for the same purposes as briefly indicated in the preceding discussion. Another object of this invention is to provide etched -surfaces on inert valve metals which are different from the etched surfaces provided on such metals by prior processes and which have comparatively high effective sur- Vface areas as indicated by etch ratio measurements.

Further objects of this invention are Ito provide processes as indicated which may be easily carried out, which are relatively inexpensive to perform and which may be satisfactorily used with commercially available inert Valve met-al foils and the like. Related objects of this invention are to provide etched inert valve metal electrodes which are physically strong, which have comparatively high etch ratios and which may be easily created.

These and other objects of this invention `as well as many specific advantages of it will be apparent to those skilled in the field of etching capacitor electrodes from a detailed consideration of the remainder of this specification including the appended claims and the accompanying drawings in which:

FIG. 1 is a curve indicating the effect of an electrolyte composition changes in preparing an etched surface in accordance with this invention;

3,190,822 Patented June 22, 1965 FIG. 2 is a photomicrograph of an etched tantalum foil surface created in accordance with this invention at 200 times its normal dimension; and

t process at 200 times its normal dimension.

A s an faid to understanding this invention it may be summarily explained as involving etched, inert valve metal surfaces as indicated in FIG. 2 of the drawings, these surfaces havinghigh effective areas as opposed to initial surface areas and having .a grooved type of comparatively uniform -surface configuration. Such etched foils are, in -accordance with this invention, created by electro-chemically etching these surfaces using an elect-rolyte composition containing at least one additive which is considered to facilitate the removal of metal so as to achieve a desired type of etched surface configuration Ias indicated in FIG. 2 of the drawings.

Y Although this invention primarily pertains to etched tantalum foil electrodes such as electrodes created from cold-rolled tantalum foil, it i-s not limited to this type of electrode structure. The invention within its broader scope encompasses inert valve metal electrodes of any desired shape or configuration formed out of the valve metals tantalum, niobium, and titanium .and various allloys of these metals which are capable of being used in the s-ame manner as these metals themselves in electrolytic capacitors as electrodes. Thus, the present invention encompasses not only foil electrodes but etched electrodes having a cup-shaped appearance or etched wire electrodes. With electrodes of various shapes as indicated considerable variations in electrode thicknesses will be encountered. As a general rule, it is preferred to utilize the present in- Vention with cold-rolled metal foils such as tantalum foils having a thickness of from about one-half mil t-o about one mil.

Any such electrodes have micr-o-crystalline surface structures which :are at least to a limited extent influenced by 4manufacturing procedures and methods. It is presently lbelieved that many such electrodes, such as inert valve metal foils, contain comparatively minute amounts of surface impurities which tend to affect the ability of such electrodes to -be satisfactorily etched to high etch ratios. The presence of these surface impurities is normally detected when efforts are made to form or oxidize these foils to comparatively high voltages. While the precise nature of such impu-rities is not known, it is believed that they may consist of various surface oxides and compounds such as tantalum carbide, gases such as hydrogen, oxygen or the like located within the crystalline structures of electrode surfaces, and that the concentrations of such impurities will vary depending upon variations in such crystalline structures. lFurther, the amounts of such impurities areipresently considered to be dependent upon not only manufacturing methods, but upon conditions to which such surfaces have been subjected after their manufacture.

In carrying out this invention an unetched foil of such Yan inert valve metal may be `first subjected to an initial cleaning stepV if its surface is not alreadyv substantially free from surface impurities. This cleaning step is consideredto remove from its surface some of the valve metal itself, and more important than this, various surface impurities of the type indicated above. In carrying out this cleaning step a valve metal, such as tantalum foil is made the anode in an electrolytic cell containing an inert cathode and a non-aqueous electrolyte which includes either a solvent or a solvent mixture and a nontilm forming solute or solute mixture.

Suitable solvent-s for use in such an electrolyte are methanol, ethanol, and the like. Such solvents are of a polar character and are capable of placing in solution various types of ionic solutes. If desired, mixtures of such solvents may be employed. A solvent or solvent mixture as herein specified may contain trace quantities of various impurities such as up to about 0.10 percent by weight of water on the basis of the weight of the solvent without causing any significant effect on the cleaning operation performed in accordance with this invention. However, the presence of trace quantities of water in such solvents is considered to tend to cause a deeply pitted type of etching to occur during this initial cleaning step. A deeply pitted type of etched surface of the general variety referred to in this paragraph is indicated in FIG. 3 of the drawings. In accordance with the invention such pitted type etching is not normally desired. Instead it is preferred to uniformly clean an electrode surface in order to facilitate the subsequent etching so as to enable the production of the type of etched surface configuration indicated in FIG. 2 of the drawings.

The solutes used in the electrolyte employed for the cleaning step herein specified should, as indicated in the preceding, be of a non-film forming ionogen type of and, of course, they should be soluble in the solvent or solvent mixture employed. Suitable solutes are cornpounds furnishing halide ions in the solvents specified. Thus solutes such as ammonium chloride, fluoride, bromide, iodide, or bifluoride, or the corresponding halide salts of lithium, sodium, potassium, ferrie iron, copper, lead, calcium, beryllium, aluminum or the like may be used where such salts are soluble in the solvent or solvent mixture employed. Such solubility may be easily determined by reference to appropriate technical literature or by routine experimentations. Organic type ionogens such as tetramethyl ammonium bromide, fluoride, chloride or iodide or similar tetra-substituted ammonium halides or mixtures of these ionogens and or other ionogens as previously indicated may also be used as solutes in an electrolyte employed during the cleaning step carried out in accordance with this invention.

This cleaning step is in the general nature of a polishing step and, as indicated in the preceding, is primarily intended so as to remove surface metal containing impurities and not to accomplish any material etching of an electrode surface. Because of this the conditions under which it may be carried out are primarily of a type wellknown in the electrolytic field. Thus, the temperature of this initial cleaning step may be varied in practice so as to encompass a range of temperatures from about 20 C. to about the boiling point of an electrolyte used. In general the higher the temperature the more rapidly metal is removed from an electrode surface. Satisfactory results can be achieved at room temperature.

Similarly the current and voltage used during this cleaning step may be varied with comparatively wide limits. In general sufficient total charge should be used so as to remove substantially all surface impurities. The amount of such total charge will depend upon the amount of such impurities present upon an electrode surface being cleaned or polished. For this reason it is not considered feasible to set forth in this specification any range of value indicating this variable in carrying out the process herein described. In practice it is presently preferred to carry out cleaning steps as herein specified to a number of different total current values with electrodes resulting from identical processing or manufacturing conditions, and then to etch the electrodes so cleaned as hereinafter described, and next to measure the etch ratios of the resultant electrodes in order to determine the amount of cleaning which is best used with electrodes resulting from such a single manufacturing batch Using this procedure the particular electrode having a high etch ratio cleaned using less total current than other electrodes having substantially the same etch ratio indicates the total amount of current which should be used during this cleaning operation `or step. If an excess of total current is passed through an electrolytic cell during the cleaning step, obviously metal is wasted and the strength of an electrode is decreased unnecessarily. This becomes important when the procedures set forth herein are used with thin electrode foils such as one-half mil tantalum foils.

The current form used during a cleaning step as herein specified may be varied over comparatively wide limits. It is preferred to etch with a combined A.C. and D.C. voltage where the R.M.S. value of the A.C. voltage component is approximately equal to the D.C. voltage irnpressed. However, it is possible to utilize during this cleaning or polishing step square wave form currents and to periodically reverse the polarity in the electrolytic cell employed.

The voltages used during a cleaning or polishing operation as herein described will vary depending upon the specific resistance of the electrolyte employed as well as the spacing of the anode and the cathode in any cell used. In general, the peak voltages should be held below a value at which scintillation will occur. Similarly the current densities used may be varied within comparatively wide limits. In general the greater the currentpdensity the greater the amount of metal removed and the greater the amount of heating within an electrolytic oell. Satisfactory results have been achieved using current densities of from about 20 to about 60 milliamperes per square inch of electrode surface and voltages consisting of an A.C. half wave superimposed upon a D.C. voltage of equal magnitude to the magnitude of the R.M.S. value of the A.C. wave so that either the D.C. voltage or the R.M.S. value of the A.C. voltage used is from about 2.5 to about volts.

Following the cleaning step indicated in the preceding discussion an electrode in accordance with this invention is electrochemically etched as the anode in a cell containing an inert cathode and an electrolyte composition separating these two electrodes. The preceding cleaning step may, of course, be omitted when foils capable of being directly etched are to be processed. As pointed out in the preceding such foils are considered to be substantially free from surface impurities.

The electrolyte composition used during this etching step is considered to be extremely important in obtaining satisfactory etch ratios. Basically such an electrolyte is the same as the electrolyte as used during the cleaning or polishing step previously described except for the addition of an additive which is capable of facilitating the removal of an inert valve metal from the surface of such a metal during the electro-etching step. Because of this it is possible to carry out the invention by adding to an electrolyte as previously described in a cell desired quan titles of one or more additive compounds. It is also possible to carry out the process of this invention by physically transferring electrodes from one electrolytic cell to another so that the cleaning and polishing steps are carried out in separate cells instead of in the same cell.

Preferred additives used with the present invention are sulfoxide complexing agents which are soluble in anhydrous methanol or similar solvents, which additions fall within the following general formula R1-|S-R2 in which the radicals R1 and R2 are of an inert character. Particularly suitable compounds falling within this general formula are dimethyl sulfoxide, diethyl sulfoxide, methylethyl sulfoxide and diphenyl sulfoxide. Other complexing yagents such as pyridine and similar heterocyclic organic compounds containing nitrogen in the ring structure or substituted derivatives of such compounds in which the ring substituted substituents are of an inert character or such as carbon disulfoxide are considered to be capable of being used in achieving at least some advantages in etching inert valve metals during this step of the present invention. A mixture of complexing agents as well as a single such agent can be employed in the etching electrolyte. Solubility of such an agent -or agents in the solvent or solvent mixture employed is, of course, required.

The electrolyte used during this etching step may contain comparatively small or trace quantities of water. It is presently believed that up to about 1,000 parts per million of Water on the basis of the total solvent or solvent mixture present does not detrimentally affect the character of theV etch obtained in accordance with this invention. Greater proportions of the water than this are preferably -avoided in practicing this invention inasmuch as such greater proportions of this contaminant tend to cause perforation of Ian electrode surface during electro-etching, and as a general rule this is to be avoided in achieving the preferred type of etched surface as indicated in FIG. 2 of the drawings. Similarly the presence of ketones such as acetone should be avoided in an electrolyte used during this electro-etching step for `substantially the same reasons. Also, various contaminants of this type tend to decrease to a substantial extent the etch ratios which can be achieved in accordance with this invention for various reasons which are at the present time not completely understood or known. Many salt-type contaminants such as anhydrous magnesium perchlorate also detrimentally affect the etching process achieved inaccordance with this invention.

The amount of an additive as herein .specified which 4 this electrolyte are shown on the basis of parts by weight.

From this curve it will be seen that comparatively high etch ratios can be achieved by the addition of from about 0.5% by weight of this additive to about 70% by weight of this additive. The effectiveness of the use of an additive of the type indicated over this entire range is ing reactions occurring directly upon an electrodesurface, but that it indirectly affects such reactions by tending to facilitate the removal of inert valve metal from such a surface. It is believed that in -all probability an additive as herein described operates in the process of this invention so as to form a solvated complex with a valve metal such as tantalum which is attacked during the etching operation. It is also believed that such -a complex places such a valve metal in to a form where it is removed from the innerfacial areas where electrode reactions take place, thus facilitating the etching operation. It is also believed that water interferes with the etching reaction by hydrolyzing the complex ion and forming an insoluble hydrated oxide which may coat the surface and interfere with the flow of current to specific sites on the metal surface. It is presently 'believed that comparatively small trace-quantities of such contaminants effectively inhibit or prevent this type of action. This is shown in the point A in FIG. l indicating an electrolyte having a composition as shown by the curve and containing 4.2 parts by Weight of water Ion the basis of the weight of the solvent employed. This Water caused slightly in excess of a drop of etch ratio which could be achieved with this invention.

In general the temperature and electrical conditions under which an electro-etching step as herein specified was carried ,out are substantially the same as the conditions used in carrying out an electro-cleaning step as vary the etch ratio obtained in a final electrode.

previously specified. For this reason it is not considered necessary to repeat the preceding discussion as to temperature, voltages, currents and the like. The amount of total current used during the etching will, of course, affect the amount of etching achieved and, hence, will As a general rule it is desired to carry out the electro-etching so as to achieve as high an etch ratio as possible and to use no more current than is necessary in order to achieve such an etch ratio since etching beyond a maximum etch ratio only serves to remove metal and, hence, serves to weaken the mechanical structure of an electrode. The amount of total current which should be used in etching will vary somewhat depending upon the final results achieved with this invention. In general, however, it can be stated that satisfactory etch ratios can be achieved by passing from about to about 200 coulombs of current per square inch of electrode surface etched.

It will, of course, be realized that the final etch ratio achieved on any electrode surface is normally determined by means of capacitance measurements taken after such a surface is anodized in accordance with conventional practice. Depending upon the conditions of such anodization the etch ratio which is measured for any electrode will vary over comparatively wide limits. Thus, for example, cold rolled tantalum foils which have been cleaned and etched in an identical manner in accordance with this invention have been determined to have an etch ratio of about 14 when formed in accordance with con-l ventional practice at 25 volts and of about 4 when formed at volts under identical conditions except as to voltage.

From an examination of FIG. 2 of the drawings showing an etched surface created in accordance with this invention it will be realized that these etched surfaces as herein described are of a comparatively uniformly etched character, and that when viewed under a microscope they have the essential appearance of a series of ridges and valleys, these ridges and valleys being substantially regularly located parallel to one another and all having an uneven surface configuration. This regularity and the distance between successive ridges is determined by the fineness of the metallographic grain yand the direction of the Valleys and ridges is determined by the direction of rolling of the metal. This results from the fact that the rolling operation breaks down crystals into smaller ones and the fragments are laid out along lines parallel to the rolling direction. The fineness of the grain is of particular importance therefore to the ratios of surface developed on etching and metal having the maximum degree of cold working by rolling, extruding or drawing is much to be preferred.

This type of etched surface is considered to be differ- -ent in kind from etched surfaces of the general Variety indicated in FIG. 3 of the drawings created in accordance with known techniques. Such etched surfaces on tantalum and other inert valve metals are of essentially a pook-marked configuration and they appear to be essentially smooth except where they are intersected by what may be termed holes extending into the metal surface. The smooth areas in prior art electrodes as indicated in FIG. 3 are considered'to be areas where no significant increase in effective surface area has been achieved by the etching process used in creating them. The smooth areas seen on the prior art electrode surface represent areas which have been essentially left untouched by the electrochemical reaction and it is believed that these may be caused by very fine films of compounds of tantalum which are resistant to etching and that with prior processes the etching action achieved primarily follows boundary lines between crystalline regions in the foil being treated. The reduced etch ratios obtained in the prior art thus appear to be due to the fact that not all the metal surface cornes into play. The amount of tantalum dissolved in forming the electrode surfaces shown in FIGS. 2 and 3 was exactly the same. Another advantage of the present invention is that it allows very thin foils, as low as 0.2 mil, to be etched successfully Without perforation.

If desired, it is possible to utilize the procedures as herein specified in conjunction with various types of known etching steps for inert valve metals such as tantalum so as to etch metal surfaces which have previously been etched so as to have surface congurations of the type indicated 4in FIG. 3 of the drawings. When this is done the beneficial results of this invention -appareutly not only affect flat surface areas as indicated in FIG. 3 but affect an increase of effective surface areas within the pock-marked regions indicated in this figure. If desired, such electrodes can be completely perforated by prior etching processes and then treated in accordance with this invention. Such perforated and etched electrodes then can be rolled into a type of capacitor anode having characteristics which are similar to the characteristics of the so-called sintered type of capacitor anodes made from small particles of valve metals. It is not generally preferred, however, to utilize the present invention with prior types of etching processes serving to perforate or deeply pitmetal surfaces because of the effect of such perforations or pits when further treated in accordance with this invention to lower the mechanical strength of the electrode member.

As an aid to understanding this invention the preceding specific examples of carrying out processes as herein described in creating etched surfaces of the type indicated in FIG. 2 of the drawings are given. These examples are given by way of illustration only and are not considered as limiting this invention. For convenience of presentation these examples are -set forth in tabular form. All of the examples pertain to the treatment of commercially available one-half inch cold-rolled tantalum foil. In all of these examples identical electrochemical cells were employed.

In all examples the etch ratioswere determined after forming (oxidizing) the etched electrodes indicated to 25 volts at an electrolyte temperature of about 90 C. using 20 milliamp. current. The same current was used in all of the forming operations regardless of electrode size or the degree to which an electrode had been etched. In the forming the electrodes in Examples I to XII the electrolyte employed contained: 60 parts by weight ethylene glycol; 40 parts by weight water; and 10 parts by weight oxalic acid. In all other examples the electrolyte contained l part by weight ammonium phosphate and 1000 parts by weight water.

I claim: v

1. A process for electrolytically etching an inert valve metal selected from the group consisting of tantalum, niobium and titanium which comprises:

electrochemically etching an inert valve metal member comprising an inert valve metal selected from the group consisting of tantalum, niobium and titanium in an electrolyte including a non-aqueous solvent and an ionogen solute dissolved in said solvent, said solute providing halogen radicals in said electrolyte, said electrolyte also including an additive having the general formula R1g-Rs in which R1 and R2 are lower alkyl or aryl radicals, said additive being soluble in said electrolyte, said additive being Vcapable of forming a solvated complex with ions of said inert valve metal removed from said inert valve metal member during the electrochemical etching of said inert valve metal mem ber, said electrolyte being substantially ketone free and containing no more than 1,000 ppm. of water,

said additive being present in an amount suiicient to form such a complex.

2. A process for electrolytically etching an inert valve metal selected from the group consisting of tantalum, niobium and titanium which comprises:

electrochemically etching an inert valve metal member comprising an inert valve metal selected from the group consisting of tantalum, niobium and titanium in an electrolyte including a non-aqueous solvent and an ionogen solute dissolved in said solvent, said solute providing halogen radicals in said electrolyte, said electrolyte also including an additive selected from the group consisting of dimethyl sulfoxide, diethyl sulfoxide, methylethyl sulfoxide and diphenyl sulfoxide, said additive being capable of forming a solvated complex with ions of said inert valve metal removed from said inert valve metal member during the electrochemical etching of said valve metal member in said electrolyte, said electrolyte being substantially ketone Ifree and containing no more than 1,000 p.p.m. of water, said additive being present in an amount suiiicient to form such a complex.

3. A process for electrolytically etching an inert valve metal selected from the group consisting of tantalum, niobium and titanium which comprises:

electrochemically etching an inert valve metal member .comprising an inert valve metal selected Vfrom the group consisting of tantalum, niobium and titanium in an electrolyte including a non-aqueous solvent and an ionogen solute dissolved in said solvent, said solute providing halogen radicals in said electrolyte, said electrolyte also including a heterocyclic saturated organic compound of the pyridine family, said additive being soluble in said electrolyte, said compound serving as an additive capable of forming a solvated complex with ions of said inert valve metal removed from said inert valve metal member during the electrochemical etching of said valve metal member in said electrolyte, said electrolyte being substantially ketone free and containing no more than 1,000

p.p.m. of Water, said additive being present in anV amount suflicient to form such a complex.

4. A process for electrolytically etching an inert valve metal selected from the group consisting of ltantalum, niobium and titanium which comprises:

electrochemically etching an inert valve metal member comprising an inert valve metal selected from the group consisting of tantalum, niobium and titanium in an electrolyte including a non-aqueous solvent and an ionogen solute dissolved in said solvent, said solute providing halogen radicals in said electrolyte, said electrolyte also including pyridine, said pyridine serving as an additive capable of forming a solvated complex with ions of said inert valve metal removed from said inert valve metal member during the electrochemical etching of said valve metal member in said electrolyte, said electrolyte being substantially ketone free and containing no more than 1,000 p.p.m. of water, said additive being present in an amount suiiicient to form such a complex.

5. A process for electrolytically etching an inert valve metal selected from the group consisting of tantalum, niobium and titanium which comprises:

electrochemically etching an inert valve metal member comprising an inert valve metal selected from the group consisting of tantalum, niobium and titanium in an electrolyte including a non-aqueous solvent and an ionogen solute dissolved in said, solvent, said solute providing halogen radicals in said electrolyte, said electrolyte also including carbon disulfide, said caribon disulfide serving as an additive capable of forming a solvated complex with ions of said inert valve metal removed from said inert valve metal member during the electrochemical etching of said valve metal member in said electrolyte, said electroinert valve metal electrode member comprising an inert valve metal selected from the group consisting of tantalum, niobium and titanium in a first electrolyte, said first electrolyte consisting of a non- 6. A process for electrolytically etching an inert valve 5 metal selected from the group consisting of tantaluni, niobium and titanium which comprises:

electrolytically cleaning surface impurities from an aqueous solvent containing in solution a non-film forming ionogen capable of providing in said first electrolyte a halogen radical; and

electrochemically etching said member in a second elecinert valve metal electrode member comprising an said additive being soluble in said electrolyte, said additive being capable of forming a solvated compleX with ions of said inert valve metal removed from said inert valve metal member during the electrochemical etching of said inert valve metal member in said second electrolyte, said second electrolyte being substantially ketone free and containing no more than 1,000 p.p.m. of water, said additive being present in an amount sufficient to form such a complex.

trolyte, said second electrolyte including a non-aqueinert valve metal selected from the group consisting ous solvent and an ionogen solute dissolved in said of tantalum, niobium and titanium in a first elec- Solvent, said solute providing halogen radicals in trolyte, said first electrolyte consisting of a nonsaid second electrolyte, said second electrolyte also aqueous solvent containing in solution a non-film including a heterocyclic saturated organic compound forming ionogen capable of providing in said first of the pyridine family, said additive being soluble electrolyte a halogen radical; and in said electrolyte serving as an additive, said addielectrochemically etching said member in a second elective being capable of forming a solvated complex trolyte, said second electrolyte including a non-aquewith ions of said inert valve metal removed from ous solvent and an ionogen solute dissolved in said Said inert valve metal member during the electrosolvent, said solute providing halogen radicals in said chemical etching of said inert valve metal member second electrolyte, said second electrolyte also inin Said Second electrolyte, said second electrolyte cluding an additive having the general lformula being substantially ketone free and containing no O more than 1,000 p.p.m. of Water, said additive being 1| present in an amount sufiicient to form such a com- R1`S-R pleX. lll Wlllcll R1 and R2 are lower alkyl 0f aryl radicals, 25 9. A process as defined in claim 8 wherein said heterocyclic organic compound is pyridine, said additive being present in an amount suiiicient to form such a complex. 10. A process for electrolytically etching an inert valve metal selected from the group consisting of tantalum, niobium and titanium which comprises:

electrolytically cleaning surface impurities from an inert valve metal electrode member comprising an inert valve metal selected from the group consisting of tantalum, niobium and titanium in a first electrolyte, said first electrolyte consisting of a non- 7. A process for electrolytically etching an inert valve metal selected from the group consisting of tantalum, niobium and titanium which comprises:

electrolytically cleaning surface impurities from an inert valve metal electrode member 'comprising an 40 inert valve metal selected from the group consisting of tantalum, niobium and titanium in a first elecaqueous solvent containing in solution a non-film forming ionogen capable of providing in said first electrolyte a halogen radical; and

electrochemically etching said member in a second electrolyte, said second electrolyte including a non-aqueous solvent and an ionogen solute dissolved in said solvent, said solute providing halogen radicals in said trolyte, said first electrolyte consisting of a nonaqueous solvent containing in solution a non-lm forming ionogen capable of providing in said first second electrolyte, said electrolyte also including carbon disulfide serving as an additive, said additive being capable of forming a solvated complex with electrolyte a halogen radical; and

electrochemically etching an inert valve metal member comprising an inert valve metal selected from the group consisting of tantalum, niobium and titanium in a second electrolyte including a non-aqueous solvent and an ionogen solute dissolved in said solvent,

ions of said inert valve metal member during the electrochemical etching of said inert valve metal member in said second electrolyte, said electrolyte being substantially ketone free and containing no more than 1,000 p.p.m. of water, said additive being present in an amount sufficient to form such a cornsaid solute providing halogen radicals in said second p1eX electrolyte, said second electrolyte also including an additive selected from the group consisting of di- References Cited bythe Examiner methyl sulfox'ide, diethyl sulioxide, methyl ethyl sulf- 55 UNITED STATES PATENTS oxide and diphenyl sulfoxide, said additive being capable of forming a solvated complex with ions 4431170 5/48 Smlth 29-180 of said inert valve metal removed from said inert 216991599 l/ Potcllell 29-180 valve metal member during the electrochemical etch- 2:742416 4/56 Jenny 204--141 ing of said valve metal member in said second elec- 217751553 12/56 Kahan 204-l4l trolyte, said second electrolyte being substantially 2186?8ll 12/58 Rusetla 204-141 ketone free and containing no more than 1,000 10701522 12/62 Robll'lson et al 204-141 ppm. of water, said additive being present in an FOREIGN PATENTS 1 amount sufficient to form such a compex. 593,403 3/60 Canada.

8. A process for electrolytically etching an inert valve metal selected from the group consisting of tantalum, niobium and titanium which comprises:

electrolytically cleaning surface impurities from an WINSTON A. DOUGLAS, Primary Examiner. JOHN H. MACK, JOHN R. SPECK, Examiners.

Claims

1. A PROCESS FOR ELECTROLYTICALLY ETCHING AN INERT VALVE METAL SELECTED FROM THE GROUP CONSISTING OF TANTALUM, NIOBIUM AND TITANIUM WHICH COMPRISES: ELECTROCHEMICALLY ETCHING AN INERT VALVE METAL MEMBER COMPRISING AN INERT VALVE METAL SELECTED FROM THE GROUP CONSISTING OF TANTALUM, NIOBIUM AND TITANIUM IN AN ELECTROLYTE INCLUDING A NON-AQUEOUS SOLVENT AND AN IONOGEN SOLUTE DISSOLVED IN SAID SOLVENT, SAID SOLUTE PROVIDING HALOGEN RADICALS IN SAID ELECTROLYTE, SAID ELECTROLYTE ALSO INCLUDING AN ADDITIVE HAVING THE GENERAL FORMULA