US2516105A - Electrolytic polishing of metals - Google Patents

Description

July 25, 1950 E. DER MATEOS'IAN ELECTROLYTIC POLISHING 0F METALS Filed June 20, 1945 FREEZING MIXTURE SIZE-E FREEZING MIXTURE EDWARD DER MATEOSIAN Patented July 25, 1950 UNITED STATES PATENT OFFICE 7 Claims.

(Granted under the act of March 3, 1883, as amended April .30, 1928,; 370 .0. .G. 1757) My invention relates to a method and anwelectrolytic bath for the .anodic or electrolytic polishing of metals.

Electrolytic polishing methods usedlin the finishing of metal surfaces have remained highly empirical practices in which specific electrolytic solutions having polishing properties only for certain metals are effective. In general, the art shows that the solution used as the polishing electrolyte is strongly acidic and .quite specific for the metal being polished in order to obtain the result usually described. Accordingly, it is an object of my invention to provide a composition for an electrolyte and a method of polishing which do not have such specificity and can be used to polish many metals with only minor adjustments in the composition of the electrolyte.

Polishing solutions frequently contain heavy concentrations of strong oxidizing agents, such as chlorates, and conditions in the electrolyte not infrequently develop whichresult in the occurrence of explosive reactions within the electrolyte. Hence, it is another object of my invention to provide an electrolyticpolishing solution and method which can be carriedout-conveniently and in a manner such that the solution at all times will be free of high concentrations of oxidizing agents and, consequently, the danger of the occurrence of an explosion is eliminated.

It is another object of my invention'to provide a method of electrolytic polishing whereby a solution of electrolyte prepared according to'the principles of myinvention can be causedto -polish many metals.

It is a further object of my invention toprovide an electrolyte and method of polishing and treating metals and alloys to produce 'finished surfaces of a degree .of polish sufiicient for reproduction for examination under the electron microscope.

It is another object of my invention to provide a method of polishing alloys having "more than one phase present whereby one of the phases of the alloy can be selectively polished so that'the surface of the alloy can beexamined for the presence of that phase, the other phases remaining unpolished or etched.

Other objects and advantages of my invention will in part be obvious and in part appear hereinafter in the .following detailed description of its principles and several of its embodiments.

My invention, accordingly, "consists .of..;ele.c-

trolytic bath and the method of polishing-metals including the relationship of .one onmoreaofrthe constituents of the bath to each .of1the .others thereof and the relationship of one or more of '.cording to my invention.

Inorder to polish, an electrolytic bath should cause polarization of the anode and it should have the capacity for carrying a heavy current, for a necessary-condition for thelproduction of polarization within the cell is the employment of a highcurrent density. The bath should also cause the anode metal, or the metal being polished, to go into solution and, at the same time, liberate oxygen at the anode. It should also produce concentration polarization within the cell.

Ihave discovered a composition for an electrolyte which permits the control of the abovenamed eifects and includes simply a solution of .a substantially .neutral salt, such as an alkali metal salt of ,a mineral acid in water and an organic .solvent such asalcohol. The concentration and pH of the solutionare determine'd'by the polishing problem onlhand. The pH of the solution is adjusted until polishing takes place at the anode, which value of pH 'I call the polishing pH of the solution.

'It is necessary that'thesalt used have a solubility high enough in the water-organic solvent mixture to carry asufficiently heavy current to cause polarization of the anode. In general salts such as alkali metal salts of the mineral acids, .1. e., sodium, potassium and lithium salts of nitric, hydrochloric, hydrobromic, 'hydroiodic, hy-

thus permitting the applicationof current densities to specimens sufficient to accomplish polishing. It is' a -matter of common knowledge that some salts of the mineral acids particularly the phosphates are not neutral in solution.

However, .in such cases, .solutions of the salts scan .belmadelup .with isuilicientgfree acid to: :counteract the inherentbufifer nature of the salt-:thus

providing a substantially-neutral salt solution, as a starting point.

The organic solvent can ,be any completely water-miscible organic liquid or solvent such as ethyl alcohol, methyl-alcohol, glycerine, ethylene glycol, acetone, isopropyl alcohol, Cellosolves, water-miscible esters and like organic compounds. Apparently the function of the organic material in the ele rolyte is to reduce the ion velocity and therby to aid in the establishment of concentration polarization at the cell anode.

The pH f the solution of the salt in the waterorganic so vent mixture is adjusted by the addition of a small amount of astrong base or acid to adjust it to the polishing pH. Sodium hydroxide can be used as the alkali and, for acidification of the solution, such acids as, acetic, nitric, oxalic, citric, sulfuric, or hydrochloric acids can be used. In general, it is preferable, although not essential, that the acid used to acidify the solution be one that will supply the same acid radical as that in the solution as part of the salt. This is particularly to beobserved where there is possibility that ingredients of the electrolyte will react with each other.

When the electrolytic baths are freshly prepared, it is not immediately certain that they will polish the metal it is desired to polish. Simple experimental observation will tell the operator after a few seconds of electrolysis of his specimen whether or not the polishing is taking place for,

doing. In the drawing, represents a vessel for holding electrolyte 2|. Battery 22 supplies current to the cell by way of the cathode 23 and anode 24, the specimen to be polished. Electrical connection to the anode is made through spring which is adapted to hold specimen 24 closely if the polishing condition, that is, polishing pI-I, prevails in the solution, the outer corners and edges of the specimen will very quickly begin to appear bright. If inspection shows that the specimen is not being polished but is being etched,

the solution is made slightly more alkaline by the, addition of a few drops of strong alkali until observation shows that polishing is taking place. If inspection of the specimen after a few seconds of preliminary electrolysis shows it is being oxidized, the solution is made slightly more acidic,

It is a relatively" proceed smoothly and a specimen, with little pre- V liminary mechanical preparation or finishing,

can usually be polished within a matter of 15 to 60 seconds at a current density of about 0.3 to 3.0 amperes per square-centimeter.

The polishing pH for the bath depends upon the temperature of the solution,the temperature of the anode and the current density as well as upon the anode material. I have found that best polishing conditions are attained by maintaining the anode as cold as possible without causing precipitation of electrolyte. For example, in carrying out the method, cooling of the anode by contacting it with a freezing mixture or solid carbon dioxide, either continuously or intermittently, will maintain the anode sufficiently cold to promote the polishing.

Referring to the drawing, Figure 1 illustrates a convenient polishing apparatus in which I!) represents a vessel for electrolyte H. Battery [2 represents a source of current, I3 the cathode and I4 the anode which, for convenience in polishing small specimens, can be made from a piece of metal tubing one end of which has a slit IE to provide a sort of spring grip for specimen 16. Since it is desired to chill the specimen to a temperature well below that of the solution, a freezing mixture 11, conveniently Dry Ice, is placed in the specimen holder.

As has been mentioned, adjustment of the solution to polishing pH is done empirically by obagainst the bottom of vessel 20. The electrolyte makes contact with the anode through the hole 26 in the bottom of the vessel. The anode can be cooled by placing it in contact with a freezing mixture 21 as shown. By using such an apparatus it is possible to adjust the solution to polishing pH readily for the anode is at all times visible. Also, no interruption in its cooling or electrolysis is necessary to determine its state. By making the hole in the bottom of the vessel a definite area, such as an integral number of square centimeters, control of the current density employed is facilitated.

The nature of the electrolytic bath and the method of polishing according to the principles of my invention will be more clearly understood by reference to the following specific examples which comprise brief descriptions of a few embodiments of my invention.

Example I To 10 milliliters of a saturated aqueous sodium nitrate solution there was added milliliters of ethyl alcohol. The solution containing the sodium nitrate and the alcohol was cooled to about 10 C. and was used to polish a specimen of relatively pure iron after adjustment to polishing pH of a value slightly more than 7 by the addition thereto of a few drops of 1.0 normal sodium hydroxide solution.

Cells like those shown in the drawings, consisting of the above electrolytic bath, a, stainless steel cathode and the specimen as the anode were used. The anode was chilled to a temperature of about minus 30 C. by placing it in contact with a freezing mixture, which conveniently can be a cake of Dry Ice, and then dipped into the solution and subjected to electrolysis at a current density of 0.3 ampere per square centimeter.

Example II A sample of pearlitic steel was polished according to the method described at a current density of 0.3 ampere per square centimeter in a bath consisting of milliliters of ethyl alcohol and 10 milliliters of saturated aqueous sodium nitrate solution, which bath had been slightly alkalized with a few drops of 1.0 normal sodium hydroxide. Under such conditions twenty seconds were required to polish the specimen.

Example III A sample of Armco iron, more than 99 per cent iron, was polished according to the method outlined at a current density of 0.3 ampere per square centimeter in a bath consisting of 100 milliliters of ethyl alcohol and 15 milliliters of saturated aqueous potassium nitrate solution, which bath has been alkalized with a few drops of 1.0 normal potassium hydroxide,

Example IV A sample of austenitic steel was polished at a current density of 0.6 ampere per square centimeter in an electrolyte consisting of 10 milliliters of saturated aqueous sodium nitrate solution and 125 milliliters of methyl alcohol. The solution was alkalized with a few drops'of 1.0 normal soassurance :dium hydroxide and chilled to:0 .C. The anode :waschilled tosabout1minusr30 C. by contacting;:it "with solid carbon dioxide .during sthe 'el'ectro'lysis- About forty seconds were required forlc'om- .plete polishing.

Example V Samples of 0.45 carbon steel, air-,cooledffrom 1550 F., werep'olished at 0.43. ampere per square centimeter in difi'erent baths consisting of "15 milliliters 'of saturated aqueous-potassium Iiitra'te solution and 125 milliliters of methyl alcohol, ethyl alcohol and acetone slightly alkalized with a'few drops of 1.0 normalsodium' hydroxide. The solutions were chilled to temperatures vof about0 to l0 C. andthespecimens.chilledwith ,solid carbon dioxide during the electrolysis.

"Similar samples of steel tempered at 1300 F. werepolished in the same baths under'like temperature conditions at current densities of 0.2 ampere persquare centimeter.

Example VI "Asample of stainlesssteel of the'l8 8 variety "Was polished in an electrolytic 'bath consisting of 15 milliliters of saturated aqueous sodium ni- "trate solution, 75 milliliters of methyl alcohol and 0:1 milliliter of 1.0 normal nitric acid. The-polishing required 30 seconds in the aggregate and was accomplished-by chilling the sample .by con- "tacting it with a freezingmixture giving a tem- .perature of about minus 40 C., electroly'zing for about seconds, cleaning the electrolyzed surface by gentle brushing andrepeating the .steps.

Asimilar sample of"18-'8 stainless steel waspolished in a solution consisting of millilitersof saturated aqueous sodium nitrate .solution, "75 milliliters of acetone and"0'.2..m'ill'iliter of 110 :nornial sodium hydroxide.

A specimen of stainless steel was polished to a, high luster using the technique describedand tion was maintained at room temperature while the anode was chilled with a freezing mixture giving the temperature of about'minus e40 C.

Example VIII A sample of brass composed of about per cent copper and 30 per centrzinc :was polished in I a bath consisting of 5 milliliters of saturated aqueous sodium nitrate solution, milliliters of ethyl alcohol and 0.1 milliliter of 1.0 normalnitric .acidata current..densityiofl0.2 ampereper square centimeter in about 45seconds. "The anode-was chilled with a freezing mixture-during the polishing. The solution also polished without the addition of the nitric'acid but it did not give a polish of as high luster nor did it polish as rapidly as the acidified solution.

The solution was also used in substantially the same manner-.toapolish samples of phosphor bronze.

When polishing :a binary or "multi cornponent alloy such as "brassprbearing metal; it "is'possi- 1-:ble my slow adjustment of the pH toward the :ideal polishing value to .find a point at which .one phase is etched and the other polished. This wasdone with severalbrasses. bronzes and bearing metals.

Example IX EA sampleof aluminum was'polished in a solu- "tionconsisting or 15 milliliters of saturated sodi- "rentdensity of 025 ampere p'er'square centimeter.

nitrate, "'15 milliliters of methyl alcohol and 0.1 milliliter of Lo'normal nitric acid at a cur- It was found, also, that the following solution polishedaluminum quite satisfactorily: '75 milliliters ofethyl Cellosolve, 15 milliliters of saturated aqueous potassium 'riitrate solution and "5 "milliliters of water, which solution was made acid to indicator paper using nitric acid. The samples werepolished at temperatures below that .of the solution. .It was found that 1.0 ampere per square centimeter as a current density was i necessary to produce a satisfactory polish in a reasonably short time when the anode was not cooled to a'temperature substantially lower than that of the bath.

The aluminum was also polished with thefollowing solution: '75 milliliters of acetone, 15 milliliters of saturated aqueous sodium nitrate solution, 10milliliters of waterand a few drops of 1.0 normalnitricacid to acidify the solution.

The aboveexamples have been given to illus- "trate the steps of'the method and the general proportions of constituents used 'to make up the baths. In a similar'manner, as mentioned above, and in like proportions, the alkalimetal salts of the "mineral acids such as'hydrochloric, hydrobromic, hydroiodic, hydrofluoric, phosphoric,

chromicand sulphuric can be used as the electrolytes in'baths for polishing purposes.

In general, in the polishing of any of the :meta1s, the best: conditions for' obtaining the most lustrous polish seem to prevail when the solutions are adjusted topl-I values such that uniform "gelatinous layers of .precipitate areobserved on the surfaces being electrolyzed.

From these .dataan'd descriptions illustrating my invention, its features will' be apparent. 'The method ofpolishing is generally applicable to all metals and consisting in employing a high enough current density .to induce polishing, maintaining the sample being polished at alow temperature,

preferably substantially. below that of thepOlishingv solutionand adjusting the pl-I of the bath to .a value which I have denominated the polishing pH. Thisterm is descriptive and definesthepl-I at which polishing occurs which, in general, is close to the neutral in the range of 6"to 8 and could also be described as that value of pI-l at "which a gelatinous precipitate of metal oxide or hydroxide forms substantially uniformly overthe surfacefbeingpolished. The composition of'tlie .bath, aswill be noted from the data given above, isnot specific for the metal being polishedas has been true of. prior art electrolytic polishing baths.

"be'use'd asithe' electrolyte in apolishing solution "for a'metalrequiring a polishing pH of substan- "tiallythe' value to which such salt, bufiers itself.

This class of materials is well "exemplified bywhe borates, some phosphates, acetates and citrates and other organic and inorganic salts as described in greater detail in my copending. application Serial Number 600,631, filed of even date herewith, (now Patent Number 2,506,582). A second ingredient of the solutions is a completely watermiscible organic or non-ionizing liquid or solvent which apparently functions to reduce the ion velocity in the solution to aid in the establishment of concentration polarization at the anode. The final ingredient of the solutions is merely acid or base to adjust the pH to the polishing value.

The polished surfaces appear to develop a high degree of resistance to corrosion for I have found that polished spots on samples remain bright when left exposed to the atmosphere, whereas adjacent polished areas corrode rapidly.

All of the metals polished according to the examples given above were finished to a Very high luster and the surfaces showed no signs of scratches or defects when reproduced and examined under the electron microscope at 50,000X.

Since certain changes in carrying out the electrolytic polishing method can be made in the individual steps thereof and wide modifications in the composition of the electrolytic baths which embody the invention can be made without departing from the scope of the invention, it is intended that all matter contained in the above discussion, description and examples shall be interpreted as illustrative and not in a limiting sense,

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes Without the payment of any royalties thereon or therefor.

Having described my invention, what I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of electrolytically polishing metals of the group consisting of ferrous metals, nickel, brass and aluminum comprising making a specimen of the metal to be polished an anode in an electrolytic cell in which the electrolyte consists essentially of a mixture of a concentrated aqueous solution of an alkali metal salt of an acid selected from the group consisting of nitric, sulfuric, chromic, hydrochloric, hydrobromic, hydroiodic and hydrofluoric acids and a completely water miscible alcohol, adjusting the pH of the mixture to between about 6 and 8, chilling the anode to a temperature below that of the electrolyte, and passing an electric current through said cell.

2. The method of electrolytically polishing metals of the group consisting of ferrous metals, nickel, brass and aluminum comprising making a specimen of the metal to be polished an anode in an electrolytic cell in which the electrolyte consists essentially of a mixture of a concentrated aqueous solution of an alkali metal salt of an acid selected from the group consisting of nitric,

sulfuric, chromic, hydrochloric, hydrobromic, hy-" droiodic and hydrofluoric acids and a completely water miscible liquid selected from the group consisting of methyl alcohol, ethyl alcohol, glycerine, ethylene glycol, acetone and isopropyl alcohol, adjusting the pH of the mixture to between about 6 and 8, chilling the anode to a temperature at least 10 C. below that of the through said cell.

i ,3. The method of electrolytically polishing stainless steel comprising, making a specimen of said metal the anod in an electrolytic cell in which the electrolyte consists essentially of a mixture of a concentrated aqueous solution of an alkali metal salt of nitric acid and ethyl alcohol, adjusting the pH of the mixture to between about 6 and 8, chilling the anode to a temperature at least 10 C. below that of the electrolyte, and passing an electric current through said cell. 4. The method of electrolytically polishing medium-carbon steel comprising making a specimen of said metal the anode in an electrolytic cell in which the electrolyte consists essentially of a mixture of a concentrated aqueous solution of an alkali metal salt of nitric acid and a mixture of methyl alcohol, ethyl alcohol and acetone, adjusting the pH of the mixture to between about 6 and 8, chilling the anode to a temperature at least 10 C. below that of the electrolyte, and passing an electric current through said cell. 5. The method of electrolytically polishing stainless steel comprising making a specimen of said metal the anode in an electrolytic cell in which the electrolyte consists essentially of a mixture of a concentrated aqueous solution of an alkali metal salt of nitric acid and glycerine, adjusting the pH of the mixture to between about 6 and 8, chilling the anode to a temperature at least 10 C. below that of the electrolyte, and passing an electric current through said cell.

6. The method of electrolytically polishing aluminum comprising making a specimen of said metal the anode in an electrolytic cell in which the electrolyte consists essentially of a mixture of a concentrated aqueous solution of an alkali metal salt of nitric acid and methyl alcohol, adjusting the pH of the mixture to between about 6 and 8, chilling the anode to a temperature at least 10 C. below that of the electrolyte, and passing an electric current through said cell.

'7. The method of electrolytically polishing brass comprising making a specimen of said metal the anode in an electrolytic cell in'which the lectrolyte consists essentially of a mixture of a concentrated aqueous solution of an alkali metal salt of nitric acid and ethyl alcohol, adjusting the pH of the mixture to between about 6 and 8, chilling the anode to a temperature at least 10 C. below that of the electrolyte, and passing an electric current through said cell.

EDWARD DER MATEOSIAN.

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

UNITED STATES PATENTS OTHER REFERENCES Proceedings of The American Electroplaters SocietyXf June 1946, pages and 64.

Claims (1)

1. THE METHOD OF ELECTROLYTICALLY POLISHING METALS OF THE GROUP CONSISTING OF FERROUS METALS, NICKEL, BRASS AND ALUMINUM COMPRISING MAKING A SPECIMEN OF THE METAL TO BE POLISHED AN ANODE IN AN ELECTROLYTIC CELL IN WHICH THE ELECTROLYTE CONSISTS ESSENTIALLY OF A MIXTURE OF A CONCENTRATED AQUEOUS SOLUTION OF AN ALKALI METAL SALT OF AN ACID SELECTED FROM THE GROUP CONSISTING OF NITRIC, SULFURIC, CHROMIC, HYDROCHLORIC, HYDROBROMIC, HYDROIODIC AND HYDROFLUORIC ACIDS AND A COMPLETELY WATER MISCIBLE ALCOHOL, ADJUSTING THE PH OF THE MIXTURE TO BETWEEN ABOUT 6 AND 8, CHILLING THE ANODE TO A TEMPERATURE BELOW THAT OF THE ELECTROLYTE, AND PASSING AN ELECTRIC CURRENT THROUGH SAID CELL.

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