US2521106A

US2521106A - Method and bath for electropolishing

Info

Publication number: US2521106A
Application number: US642435A
Authority: US
Inventors: Harold J Wiesner
Original assignee: C G COUN Ltd
Current assignee: C G COUN Ltd
Priority date: 1946-01-19
Filing date: 1946-01-19
Publication date: 1950-09-05
Anticipated expiration: 1967-09-05

Description

Patented Sept. 5, 1950 UNITED STATES P' METHOD AND BATH FOR ELECTROPOLISHING Haroldd. W'iesner, Elkhar Gonn, Lt'dz, Elkhart,

diana t, Ind., assignor to G. G..

Ind., a corporation of In- No Drawing. Application January 19,- 1946, Serial'No. 642,435

16 Claims.

This invention relates to a methodof andbath for electropolishing, and more particularly to the electropolishing of copperand copper alloys.

One of the objects of the invention isto. provide a method of and bath for electropolishing in which all of the exposed surfaces of an article to be polished will be'given a uniform bright finish free from discoloration.

Another object is to provide a method. of and bath for electropolishing copper alloys containing lead which will produce a high. luster free from smut.

Still another object is to provide a method of and bath for electropolishing in which the bath is mildly alkaline.

A further object is to provide an electropolishing bath which is non-corrosive so that relatively inexpensive equipment can be used, which is harmless to operators. either from contact or from fumes and'which requiresno venting equipment.

According to the present invention, articles to be polished, which may be. any shape-or configuration of metal articles formedofcopper or copper containing alloys, areconnected'as an anode in the. polishingbath and have acurrent passed through them to a cathode. in the bath which is preferably copper or plated with copper. With the bath of the present invention current densities substantially inexcess. of. 100" amperes per square foot can be used to produce a uniform high luster over the entire surface of the articles, including recessedportions. The time required to produce a brilliantsurface is dependent upon the finish of the article prior to the electropolishing operation. It is apparent that for this reason no specific time for accomplishing the electropolishing to a high luster can be given,.but in general the time willyary fromfi to 30 minutes, andon anaverage will beabout 15to2'0 minutes.

The bath of this invention consists essentially of pyrophosphates of the alkali metals andeither borates or carbonates mixed therewith in an aqueous solution. The most satisfactory pyrophosphates are sodium pyrophosphate, which is preferably used in the anhydrous or deca-hydrate, and potassium pyrophosphate, which is preferably the trihydrate. These salts function in the solution to form. soluble compounds with the copper, zinc, and lead andother metals commonly found in the brass alloys during electropolishing. In the operation of the bath the copper plates out. as a sludge at the cathode so that the net loss of pyrophosphate is that which combines with the-zinc, lead and other metals. The

bath can therefore be used for relatively long periods of time with the addition of only very small amounts, if. any, of pyrophosphates to maintain its strength. I havefound that a satisfactory-concentration is on the order of 16 ounces brightening in therecesses of the work and other low current density areas, and etching mayoccur. Also, discolorations may occur in various areas of the work pieces. Among the specific com.- poundswhich I- have'found to be most satisfactory are boric acid, sodiumtetraborate, potassium tetraborate, ammoniumcarbonate, sodium bicarbonate; and potassium bicarbonate, which are: preferably used in concentrations offrom 2 to' 12 ounces per gallon. If ammoniumrcarbonate is used, it is. necessary to: keep the temperature of the bath'below about 135 F., as above this temperature it decomposes rapidly. Ammonium carbonate and sodium and potassium'bi carbonate alsoreact with acid, which may be added to the bath to lower its: pHto give off some carbon dioxide. Boric acid and sodium or potas sium tetraborate are preferred since they are thermally stable, do not decomposewhen. acid is added, and function to buffer the solution.

In addition to these basic constituents of the bath there may be added either an acid --or base in quantities sufficient to bring the pI-I-vof the bath in the range from 7.5 to 9.5. For this purpose sodium, potassium or ammonium hydroxide may be convenientl used to raise the pH, while sulfuric or meta phosphoric, and in some cases ortho phosphoric, acid may be used to lower the pH. If the pH of the bath is below about 7.5, the satisfactory current density range is substantially narrowed, while if the pI-f ist'oo high, a highly lustrous surface cannot be obtained. More specifically, if the pH drops below the aforementioned lower limit and electropolishing' is attempted on a complicated piece of work, it has been found. that brown to redbr'own discolorations will be apparent on the high current density areas.- As the pH of the solution is raised (with sodium hydroxide, for example) the current density range is broadened, and at a pH of 9.0 (for example) extremely high current densities may. be used without burning.

In baths containing ammonium carbonate, sodium hydroxide may be used satisfactorily to raise the pH; but in those baths containing boric acid or sodium tetraborate, ammonium hydroxide or an ammonium compound was found to give'better results. To lower the pH sulphuric, meta-phosphoric or ortho-phosphoric acids may be used satisfactorily but certain other acids sodium, di potassium and di ammonium phosphates, when put into solution, have pH values nearer to those in the operating range of the bath and for this reason are preferred. These salts, when added to the bath in concentrations of from 12 to ounces per gallon extend the current range and greatly increase brightness.

It has further been found that the addition of oxalic acid or one of its salts such as sodium oxalate, ammonium oxalate or potassium oxalate in concentrations of about 4 to 6 ounces per gallon enables the temperature of the bath to be increased up to as much as 200 F. with good results. Without the addition of these materials, it has been found that the surface tends to become irregular at temperatures substantially in excess of 150 F.

In general, I have found that agitation of the work or of the solution during electropolishing is desirable. As a result of using agitation, higher current densities are permitted, with less chance of burning the protruding sections of a complicated piece of work. The commonly employed methods of agitation used in the electroplating industry, such as oscillation of the work rod, air agitation, or solution agitation b means of pumps or paddles are satisfactory. The use of work rod oscillation is perhaps to be preferred.

As examples of several baths which I have found to be satisfactory, the following may be iven:

Example #1 Sodium pyrophosphate (anhydrous) 16 oz./gal. Boric acid 5 oz./gal. Ammonium hydroxide (28 B.) 2 ll. oz./gal. pH 8.5-9.0 Temperature 140-160 F. Current density Above 100 amps/sq. ft.

Example #2 Potassium pyrophosphate 16 oz./gal. Boric acid 4-6 oz./gal. pH 8.5-9.0 Temperature 140-160" F. Current density Above 100 amps/sq. ft.

Example #3 Potassium pyrophosphate 16 oz./gal. Di potassium hydrogen phosphate 5-12 oz./gal. Boric acid 4-6 oz./gal. pH 8.0-8.5 Temperature 140-160 F. Current density Above 100 amps/sq. ft.

Example #4 Sodium pyrophosphate (anhydrous) l6 oz./gal. Disodium hydrogen phosphate 12-20 oz./gal. Boric acid 5 oz./gal. pH 8.0-8.5 Temperature 130-150 F.

Current density Above 150 amps/sqf alloys of copper 4 Example #5 Sodium pyrophosphate 16 oz./gal. Boric acid 5 oz./gal. Sodium oxalate or oxalic acid 4-6 oz./gal. pH 8.0-8.5 Temperature 180-200 F. Current density Above 150 amps/sq. ft.

While the invention has been particularly described in connection with. several specific bath compositions, it will be understood that various combinations and concentrations of the constituents mentioned, as well as various operating conditions might be employed within the scope of the invention. The examples given are to be taken as illustrative only, reference being had to the appended claims for a determination of the scope of the invention.

What is claimed is:

l. The method of electropolishing copper and alloys of copper comprising connecting the part to be polished as the anode in a bath consisting essentially of water as the solvent, an alkali metal salt present in an amount ranging from 3 ounces per gallon of solution to a saturated solution of the salt, and from 2-12 ounces per gallon of solution of a water soluble salt selected from the group consisting of boric acid, water soluble salts of boric acid and water soluble salts of carbonic acid.

2. The method of electropolishing copper and alloys of copper comprising connecting the part to be polished as the anode in a bath consisting essentially of water as the solvent, an alkali metal salt present in an amount ranging from 8 ounces per gallon of solution to a saturated solution of the salt, and from 2-12 ounces per gallon of solution of a water soluble salt selected from the group consisting of boric acid, water soluble salts of boric acid and water soluble salts of carbonic acid, and maintaining the bath between a pH of 7.5 to 9.5.

3. The method of electropolishing copper and comprising connecting the part to be polished as the anode in a bath consisting essentially of water as the solvent, an alkali metal salt present in an amount ranging from 8 ounces per gallon of solution to a saturated solution of the salt, and from 2-12 ounces per gallon of solution of a water soluble salt selected from the group consisting of boric acid, water soluble salts of boric acid and water soluble salts of carbonic acid, and operating the bath at a temperature of between -212 F. and at a current density of in excess of 100 amperes per square foot.

4. The method of electropolishing copper and alloys of copper comprising connecting the part to be polished as the anode in a bath consisting essentially of water as the solvent, an alkali metal salt present in an amount ranging from 8 ounces per gallon of solution to a saturated solution of the salt, and from 2-12 ounces per gallon of solution of a water soluble salt selected from the group consisting of boric acid, water soluble salts of boric acid and water soluble salts of carbonic acid, and operating the bath ata pH of 7.5-9.5, at a temperature between 100 F.2-12 F., and at a current density in excess of 100 amperes per square foot.

5. A composition for providing an electropolishing bath when dissolved in water, the ingredients being present in the following proportions per each gallon of solution to be made, said com- 2 52 1, roe

position consisting essentially of an: alkali metal pyrophosphate present in an amount ranging from 8"ounces tothe Saturation pointper gallon of solution, 2-12*'ounces of a water soluble compound selectedifrom the group consisting of borie acid, water soluble salts ofbori'c acid and water soluble salts of carbonic acid; and from 12-20 Ounces of a salt of" ortho phosphoric acid in which the metal componentis-sel'ected from the group consisting ofa-mmonia, sodium: and potas slum;

6. An electropolishing bath consisting essentially of water as the solvent, an alkali metal pyrophosphate' present in an amount ranging from 8 ounces per gallon of solution to an amount saturatingthe bath, from 2-12 ounces per gallon of solution of a water soluble compound selected from the group consisting of boric acid, water soluble salts ofboric acid, and water soluble salts ofcarbonic acid' and from 132 -20 ounces of asalt of orthophosphoriaacid' in which the metallic component is selected from the group consisting of sodium, potassium and ammonium.

'7. Acomposition for providing an electropolishing bath when dissolved in water; the ingredients of which are present in the following proportions for each gallon of solution, said composition consisting essentially of an alkali metal pyrophosphate present in an amount ranging from 8 ounces to an amount to maintain saturated solution, 2-12 ounces of a water soluble compound selected from the group consisting of boric acid and water soluble salts of carbonic acid, a neutralizing agent in an amount to adjust the pH of the solution to between 7.5 and 9.5, and from 12-20 ounces of a salt of ortho phosphoric acid in which the metallic component is selected from the group consisting of ammonium, sodium and potassium.

8. An electropolishing bath consisting essential- 1y of water as the solvent, an alkali metal pyrophosphate present in an amount ranging from 8 ounces per gallon of solution to an amount saturating the bath, from 2-12 ounces per gallon of solution of a water soluble compound selected from the group consisting of boric acid, water soluble salts of boric acid and water soluble salts of carbonic acid, a neutralizing agent present in an amount to adjust the pH of the bath between 7.5-9.5, and from 12-20 ounces of a salt of ortho phosphoric acid in which the metallic component is selected from the group consisting of ammonium, sodium, and potassium.

9. An electropolishing bath consisting essentially of water as the diluent, an alkali metal pyrophosphate present in an amount ranging from 8 ounces per gallon of solution to a saturated solution of the salt, and from 2-12 ounces per gallon of solution of a water soluble compound selected from the group consisting of boric acid, salts of boric acid, salts of carbonic acid, and from 4-6 ounces per gallon of a water soluble material selected from the group consisting of oxalic acid and water soluble salts of oxalic acid.

10. A composition for providing an electropolishing bath when dissolved in water, the ingredients being present in the following proportions per each gallon of solution to be made, said composition consisting essentially of an alkali metal pyrophosphate present in an amount ranging from 8 ounces to an amount to maintain a saturated solution of the salt in water, and from 2-12 ounces of a water soluble compound selected from the grou consisting of boric acid, water 6 soluble salts of boric acid, water soluble" salts of carbonic acid, and from 4-6 ouncesper gallon 01" a water soluble materialselected fromthe group consisting of oxalic acid and water soluble salts of oxalic'acid.

1 1 A composition for providing an electropolishing bath when dissolved in water; the ingredients being present in thefollowing proportionsper each gallon of solution tobe made, said composition consisting essentially of an alkali metal pyrophosphate present in an amount ranging from 8 ounces to the" saturation point per gallonof solution, 242 ounces of a water soluble compound selected from thegroupconsisting of boric acid, water soluble" salts of boric acid and water soluble salts of carbonic acid; a neutralizing: agent in an amount to adjust thepI-I to between" 715-95, and from 4-6 ounces per gallon of a water soluble material selected from the group consisting of oxalic acid and water soluble salts of oxalic acid;

'2. A composition for providing anelectrospolishing bath when dissolved in water, the ingredi'ents being present inthe following proper": t'i'onsper each gallon ofsol'ution to-be made; said composition consisting essentially of analkali metal pyrophosphate present in an amount ranging from 8 ounces to the saturation point per gallon of solution, 2-12 ounces of a water soluble compound selected from the group consisting of boric acid, water soluble salts of boric acid and water soluble salts of carbonic acid, from 12 to 20 ounces of a salt of ortho phosphoric acid in which the metal component is selected from the group consisting of ammonia, sodium, and potassium, and from 1-6 ounces per gallon of a water soluble material selected from the group consisting of oxalic acid and water soluble salts of oxalic acid.

13. An electropolishing bath consisting essentially of water as the solvent, an alkali metal pyrophosphate present in an amount ranging from 8 ounces per gallon of solution to an amount saturating the bath, from 2-12 ounces per gallon of solution of a water soluble compound selected from the group consisting of boric acid, water soluble salts of boric acid, and water soluble salts of carbonic acid, from 12-20 ounces of a salt of ortho phosphoric acid in which the metallic component is selected from the group consisting of sodium, potassium and ammonium, and from 4-6 ounces per gallon of a water soluble material selected from the group consisting of oxalic acid and water soluble salts of oxalic acid.

14. A composition for providing an electropolishing bath when dissolved in water, the ingredients of which are present in the following proportions for each gallon of solution, said composition consisting essentially of an alkali metal pyrophosphate present in an amount ranging from 8 ounces to an amount to maintain saturated solution, 2-12 ounces of a water soluble compound selected from the group consisting of boric acid and water soluble salts of carbonic acid, a neutralizing agent in an amount to adjust the pH of the solution to between 7.5 and 9.5, from 12-20 ounces of a salt of ortho phosphoric acid in which the metallic component is selected from the group consisting of ammonium, sodium and potassium, and from 4-6 ounces per gallon of a water soluble material selected from the group consisting of oxalic acid and water soluble salts of oxalic acid.

15. An electropolishing bath consisting essentially of water as the solvent, an alkali metal pyrophosphate present in an amount ranging from 8 ounces per gallon of solution to an amount saturating the bath, from 2-12 ounces per gallon of solution of a water soluble compound selected from the group consisting of boric acid, water soluble salts of boric acid and water soluble salts of carbonic acid, a neutralizing agent present in an amount to adjust the pH of the bath between 7.5-9.5, from 12-20 ounces of a salt of ortho phosphoric acid in which the metallic component is selected from the group consisting of ammonium, sodium, and potassium, and from 4-6 ounces per gallon of a water soluble material selected from the group consisting of oxalic acid and water soluble salts of oxalic acid.

16. The method of electropolishing copper and alloys of copper comprising connecting the part to be polished as the anode in a bath consisting essentially of water as the solvent, an alkali metal pyrophosphate present in an amount ranging from 8 ounces per gallon of solution to an amount saturating the bath, from 2-12 ounces per gallon of a water soluble salt selected from the group consisting of boric acid, Water soluble salts of boric acid and water soluble salts of carbonic acid, 12-20 ounces per gallon of solution of a salt of ortho phosphoric acid in which the metallic component is selected from the group consisting of ammonium, sodium and potassium, from 4-6 ounces per gallon of solution of a water soluble material selected from the group consisting of oxalic acid and water soluble salts of oxalic acid, maintaining the bath at a pH between 7.5 and 9.5 and at a temperature between 100-212 F. and operating the bath at a current density in excess of 100 amperes per square foot.

HAROLD J. WIE'SNER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS

Claims

1. THE METHOD OF ELECTROPOLISHING COPPER AND ALLOYS OF COPPER COMPRISING CONNECTING THE PART TO BE POLISHED AS THE ANODE IN A BATH CONSISTING ESSENTIALLY OF WATER AS THE SOLVENT, AN ALKALI METAL SALT PRESENT IN AN AMOUNT RANGING FROM 8 OUNCES PER GALLON OF SOLUTION TO A SATURATED SOLUTION OF THE SALT, AND FROM 2-12 OUNCES PER GALLON OF SOLUTION OF A WATER SOLUBLE SALT SELECTED FROM THE GROUP CONSISTING OF BORIC ACID, WATER SOLUBLE SALTS OF BORIC ACID AND WATER SOLUBLE SALTS OF CARBONIC ACID.