US2424674A - Electrolytic bright polishing - Google Patents

Description

Patented July 29, 1947 ELECTROLYTIC BRIGHT POLISHING Harold I. White, Middletown, Ohio, assignor to The American Rolling-Mill Company, Middletown, Ohio, a corporation of Ohio No Drawing. Application April 29, 1940, Serial No. 332,238

claims. (01. 204-140) i This invention relates to a process of electrolytic bright polishing and to a polishing bath which has been found advantageous in the bright polishing of various metals and alloys. In the past the most commonly used methods of polishing metals involved mechanical abrasion and bufling. These methods are expensive due to the cost of the equipment used, the hand labor involved, and, in the case of grinding, the high metal loss. They are dimcult to apply to intricate shapes, and often cause a dust hazard.

One worker has proposed an electrochemical smoothing and polishing of metallic surfaces in which the metal is made an anode in an electrolyte of concentrated phosphoric acid. While this process satisfactorily cleans most metal surfaces, it fails to produce a degree of brightness or luster, particularly on certain of the so-called stainless steels, which would enable it to replace entirely mechanical polishing and bufling.

It is an object of the present invention to provide an electrolytic polishing method involving a maximum brightening of the surfaces of a large variety of metals including all types of stainless steels, and in particular that type of stainless steel containing approximately 17% of chromium. It is a further object to accomplish this brightening with a lower weight loss, and to provide a bath which has superior throwing power. For example, I have polished intricate shapes of 17-chrome stainless steel to a uniform finish comparable to that obtained by buffing, with a weight loss of only approximately 0.068 ounce per square foot which corresponds to about 0.0001 inch removed from the thickness of the pieces.

It is an object-of my invention to provide a process by which a high polish can be produced without the necessity of mechanical buiiing, and on objects which cannot readily be buifed on account of their complexity of design or lack of strength. Another object of my invention is the (provision of a method of producing a polish on stainless steels comparable or equal to that produced by a buffing operation, without the disadvantage of bufling, lying in the production of a cold-worked surface layer containing some ferritic iron with its inferior corrosion resistance.

It is'an object of my invention to provide a novel electrolyte which will have very good throwing power and which will have a long life. Another object of my invention is the provision of a method of bright polishing which is not only advantageous for all of the various types of stainless steels, but which also is advantageous 2 steels, nickel, copper, brass, Monel metal, canmium and other metals and alloys which are low in non-metallics, and which together with their reaction products are soluble in the electrolyte.

These and other objects of my invention which will be pointed out in more detail hereinafter, or which will be apparent to one skilled in the art upon reading these specifications, I accomplish by that series of method steps and by that composition of matter of which I shall now describe certain exemplary embodiments.

Briefly, in the practice of my invention I provide an electrolytic polishing bath composed of concentrated phosphoric acid diluted by one or more soluble organic compounds having properties similar to those of normal butyl alcohol.

The work piece to be polished is made an electrode in this bath, the anode in the case of direct current, though alternating current can be used, and an electric current passed through it until the desired brightening is obtained.

While not wishing to be limited by theory I believe the action of the organic compound is to reduce the ion concentration of the electrolyte. Tests seem to indicate that for best results the polishing efiect should be brought about largely by the action of the electric current and that direct chemical attack by the electrolyte itself should be minimized. Dilution of the electrolyte with water would increase the ionization and result in a pitted surface having markedly decreased luster. Suitable organic compounds reduce the ion concentration and result in improved luster.

All organic compounds are not equally efiective in producing this result, and I have determined the properties which the compounds must possess to be suitable. Compounds which are too volatile, such as alcohols with fewer than three carbon atoms per molecule, should not be used, since they are very rapidly 10st from the bath by evaporation. While in some cases their addition produces a better result than phosphoric acid alone, the luster is definitely inferior to that produced by the higher alcohols. On the other hand it, is necessary that the organic compounds be soluble in phosphoric acid, and compounds such as are typified bythe long-chain alcohols cannot be used. Octyl alcohol, for example, has a sufiiciently long chain of carbon atoms to be low enough in solubility in phosphoric acid to decrease its effectiveness in producing brightening to a considerable extent.

Of the many organic compounds which I have in connection with aluminum, silicon steel, mild tried, the following have been found particularly stainless steels.

quantities of other compounds-will sometimes be permissible; for example, those which might occur in using impure mixtures of organic lay-products.

While for simplicity we prefer to use the pure compounds, it may be desirable for the sake of economy to use them in the impure or mixed state, and it can readily be determined by the worker whether the addition of a given amount of one or more impurities to the preferred composition will be detrimental to the production of my result.

The most desirable proportions of phosphoric acid and the organic compound may vary somewhat with the material being treated. As examples of particular compositions which have proved satisfactory for certain types of metals,

and without limitation, I cite the following: On'

ll-chrome steel, 1. e., a stainless steel containing in the neighborhood of 17% chromium, I have found that above of normal butyl alcohol (commercial) by volume with 95% of 85% phosacid with 25 parts of butyl alcohol gave nearly as bright a polish as the 50-50 composition of 85% phosphoric acid. These compositions work equally well on the 18-8 stainless steel, 1. e., a grade of stainless steel containing in the neighborhood of 18% chromium and 8% nickel; With this grade, however, as well as with other chromium-nickel stainless steels such as 25-12, it is not essential that the alcohol content be so high to obtain the optimum polish, as is the case with l7-chrome steel. I have found that with this type of stainless steel a mixture of 10 parts of normal butyl alcohol with 90 parts of 85% phospl oric acid produces a satisfactory brightness and results in an extremely low metal loss. -'I'he proportions of phosphoric acid and organic compound are generally not critical and can readily be determined by the operator to produce the maximum' luster with the minimum current and metal loss. It should be noted that it is preferable to use a grade of acid low in fluorides. The mixture should be well stirred after pouring together to insure uniform composition.

The temperature of the bath does not appear to be very critical, and satisfactory brightening temperatures. The optimum temperature can be temperatures for long periods of time some of the organic compound will evaporate.

The current density does not appear to be highly critical. I have employed direct current of densities from one eighth to ten amperes per square inch of surface brightened, and good results were obtained at current densities 'over this entire range with the 50-50 phosphoric acid, butyl alcohol bath. Generally speaking, high current densities will produce brightening in somewhat less time than the lower densities. I generally prefer to use from one-half to five amperesp square inch.

Although I have described my process as being applicable to the use of direct current, the work piece being made the anode, this is not necessarily so and in many instances a satisfactory brightening can be obtained by using alternating current. In this case both electrodes can be the work pieces being polished. In polishing some metals, for example ild steel. a very bright polish can be obtained using alternating current. With others, for example l'l-chrome steel, the surface is brightened but is not as smooth and mirror-like; and the solution tends to heat up, necessitating artificial cooling. For the most uniform results on various materials and to produce the highest polish I prefer to use direct current.

The time required to produce a given brightness, of course, depends upon the current density and the original surface of the metal. On certain refrigerator door parts I have found that a good bright sample was obtained in two minutes. using a current of one ampere per square inch of surface brightened. Considerable brightness was obtained in one-half minute and in one minute, although the brightness was not as great as that produced in two minutes. On the other hand, samples polished for three to five minutes did not appear much brighter than those polished for two minutes.

There is considerable range as to the choice of cathodes and they may be made of stainless steel, copper or any conductive material, metallic or otherwise, which will not contaminate the bath. Electrodes of carbon, for example, may be employed, but are not necessary.

For best results, but without limitation, it is preferred that the cathode have an area comparable to that of the surface being polished and that the two surfaces bdsituat'ed roughly parallel to each other and not too far apart. To polish deeply recessed objects, best results are obtained when a cathode is located within the recess.

The phosphoric acid-alcohol mixture may be held in a tank of some non-conducting material determined readily for each metal being polished such as crock or glassware, or acid brick. Lead and iron tanks have been in use for a considerable period of time, although the electrolyte attacks these materials slightly.

This process and polishing bath will work on a number of metals although they will not work on certain metals such as, for example, tin bronze. It is believed that the reason it will not work on tin bronze is that the tin forms an insoluble phosphate which interferes with the brightening reaction. It is not successful on cast iron in some cases because of the high amount of graphite which is insoluble. In general it has proved successful on all metals and alloys which under the influence of the electric current are not completely insoluble in phosphoric acid, which are lowin metalloids, and which do not form temperature, since if the bath is held at high in oluble reaction products with phosphoric acid.

The process has produced satisfactory brightening on the various stainless steels, silicon steels, ordinary carbon steels, aluminum, nickel, copper, brass, Monel metal, cadmium, and alloys thereof.

Because of its cheapness and convenience I prefer to use ortho-phosphoric acid, although in many cases satisfactory results are obtained by the use of metaor pyro-phosphoric acid. Therefore, whenever I speak of phosphoric acid I intend to include not only the preferred orthophosphoric acid but also the other two forms insofar as they are equivalents.

While I have pointed out certain specific examples of the composition of the bath and certain specific examples of current densities, temperatures and polishing times, it is to be understood that these are exemplary only, and that modifications will present themselves to those skilled in the art. I therefore do not intend to limit myself except as pointed out in the claims which follow.

Having now fully described my invention, what I claim as new and desire to secure by Letters Patent is:

1. An electrolyte for electrolytically bright polishing stainless steel which, as initially produced, consists of from substantially 57.4% to substantially 82.9% by weight of the substance phosphoric acid, from substantially 2.5% to substantially 3 .5% by weight of normal butyl alcohol, and from substantially 10.1% to substantially 21.4% by weight of water.

2. A process of anodically polishing metals and alloys which under the influence of the electric current are not completely insoluble in phosphoric acid, which are low in metalloids, and which do not form bath-insoluble reaction prodnets with phosphoric acid, said process comprising preparing an electrolyte consisting of from substantially 57.4% to substantially 82.9% by weight of, the substance phosphoric acid, from substantially 10.1% to substantially 21.4% of water by weight, and from substantially 2.5% to substantially 32.5% by weight of an organic substance selected from the group consisting of normal butyl alcohol, secondary butyl alcohol, butyl acetate, normal amyl alcohol, secondaryamyl alcohol, and amyl acetate, and electrolyzing therein such metals or alloys.

3. The process of claim 2' wherein the current density is from substantially 1 to substantially 5 amperes per treated square inch of the said article and wherein the electrolyte is maintained at a temperature of substantially F. to substantially F.

4. In a process of anodically polishing stainless steels, the steps of preparing an electrolyte consisting of from substantially 57.4% to substantially 82.9% by weight of the substance phosphoric acid, from substantially 10.1% to substantially 21.4% of water by weight, and from substantially 2.5% to substantially 32.5% by weight of an organic substance selected from the group consisting of normal butyl alcohol, secondary I butyl alcohol, butyl acetate, normal amyl alcohol, secondary amyl alcohol, and amyl acetate, and electrolyzing therein a stainless steel article.

5. The process of claim 4 wherein the current density is from substantially 1 to substantially 5 amperes per treated square inch of the said article and wherein the electrolyte is maintained at a temperature of substantially 120 F. to substantially 180 F.

HAROLD I. WHITE.

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

UNITED STATES PATENTS Number Name Date 571,531 Langhans Nov. 17, 1896 2,315,695 Faust Apr. 6, 1943 2,294,227 Delaplace et al Aug. 25, 1942 2,040,618 Mason ct a1 May 12,, 1936 FOREIGN PATENTS Number Country Date 707,526 France Apr. 14, 1931 686,131 Germany Jan. 3, 1940 682,248 Germany Oct. 20, 1939 404,819 Great Britain Jan. 25, 1934 OTHER REFERENCES "Transactions of the Electrochemical Society, volume 78 (1940), pages 265 thru 272; article by Uhlig, entitled "Electrolytic Polishing of Stainless Steel.