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2,401,543V f Patented Sept. 10, 1946l UNITED _*sTATEs" PATENT olfl'ilclë:¥ METHOD 0F ANODICALLY POLISHIÑG BRASS ‘ Charles L. Faust, Columbus, Ohio, assignor to Battelle Memorial Institute, Columbus, Ohio, a corporation of Ohio _ Application November? .6, 1939, serial No. 302,952 11 claims. (c1. coi-14o) ` 1 'I'his invention relates to a method of and an , In order toget the best polishing results, how ever, I have found that the range of proportions of phosphoric acid, chromic acid and water electrolyte for anodically polishing brass. More particularly the invention relates to an aqueous electrolyte comprising chromic acid» and water, or chromic acid, phosphoric acid and water, and should be kept within certain narrowe;` limits, to the use of such an electrolyte in the anodic accompanying diagram by the separate, lesser polishing of brass. areas defined by the solid line AE, dot and dash line ELF, solid line FM, dot and dash lines MG and GH, and the solid line HA, and by the solid and these narrower limits are represented on th'e f I4 have now found that an aqueous electrolyte comprising chromic acid and water, or prefer ably chromic and phosphoric acids and water, 10 line IK and dot and dash line KJI. The pre ferred compositions of electrolyte, with respect can be very satisfactorily used in the anodic to the relative proportions of phosphoric acid, polishing of brass. If the relative proportions of the principal ingredients of the bath be keptl chromic` acid and water, lie within the two dis- connected areas just described, both of which are within certain limits, which I have determined, surfaces can be produced that have relativelyl wh'olly enclosed within the broader area ñrst superior characteristics to those obtainable by defined. mechanical polishing or bufñng operations. The reading of a trìaxial diagram such as the It is therefore an important object of my in- i accompanying one is well understood but the vention to provide an electrolyte containing phos following will be given for purposes of illustra phoric acid, chromic acid and water within cer 20 tion. The point on the diagram represented by tain limits as to their relative proportions, for the letter A, for instance, indicates a composition use in the anodic polishing `of brass to impart comprising 0% of phosphoric acid, 65% of chro thereto a highly lustrous surface. mic acid and 35% of water; the point indicated It is a further important object of this inven by the reference letter B a composition compris tion to provide a method of anodically polishing ing 85% of phosphoric acid, a small but signifi brass and using an electrolyte of novel composi cant proportion, say 0.1% of chromic acid and tion that'is operative through a wide range of the balance, somewhat less than 15%, water; the anode current densities and temperatures. point represented by the reference letter C a. com» Other and further important objects of this position comprising 5% of phosphoric acid, about invention will become apparent from the dis 30 0.1% of chro-mic acid and the balance, somewhat closures in the specification and the accompany less than 95%, Water; and the point indicated by ing drawing. ` the reference letter D a composition comprising This invention (in its preferred form) is illus l0% of phosphoric acid, 5% of chromic acid and trated in the drawing and hereinafter more fully 95% of water. described. It will be seen that the maximum phosphoric -On the drawing:acid content is 85% and the minimum 0%; the 'I'he figure represents a triaxial diagram show maximum chromic -acid content about 65% and ingfthe relative proportions of phosphoric acid, the minimum about 0.1%; and the maximum chromic acid and water for compositions of elec - water content 95% and the minimum about 13%. trolytes coming within the scope of my invention. The preferred composition limits are from 0 to In the accompanying diagram, the respective v85% phosphoric acid, from 2 to' 65% CrOa, and sides of the >triangle indicate the percentages o_f from 13 to 92% water. These relative percent phosphoric acid (HsPOt), of water (H2O), and ages of the ingredients of my bath, however, are of chromic acid (CrOs), from 0 to 100%. On interdependent, so that in order to insure an the basis of experimental data, I have deter 45 operative bath. its composition should be selected mined the relative proportions of phosphoric acid, by reference to- the triaxial diagram and the chromic acid and water that give compositions areas defined thereon. of electrolyte that are operative for the anodic The triaxial diagram thus shows the relative polishing of brass. The area representing opera proportions of chromic acid and water in a two tive compositions of electrolyte is defined on the 50 component system (the phosphoric acid being accompanying diagram by the solid lines AB, 0%), or the relative proportions of chromic acid, BC, CD and DA. Within the area so deñned, phosphoric acid and water in a three component any composition selected will be found to be op system. Suitable baths of these two or three component systems may nevertheless also include erative inthe method hereinafter described for the anodic polishing of brass. > 55 other ingredients, such as other acids and/or 2,407,543 . 4 3 . salts. From the diagram, however, th‘e relative proportions that chromic acid and water, or that chromic acid, phosphoric acid and water, should ily obtained. ’I‘he highly lustrous surface ob tainable by my method, using an electrolyte lof preferred composition, is an important feature bear to each other may be determined for opera of my invention and one that sharply distin guishes it from prior art ñnishes produced in the tive and preferred ranges of composition, re gardless. of what other non-essential ingredients may be present in the bath. ' - electrolytic cleaning of brass. The formation of highly polished and lustrous surfaces is undoubtedly associated with the pres Thus, for instance, in making up an electro lyte for the anodic polishing of brass, one might ence of la polarizing film over the surface of the select the point X on the triaxial diagram as 10 metal during-the process of anodic dissolution. being a bath of preferred composition. The com The nature ‘of this ñlm is such that selective position represented by the point X would be attack on the various phases present in the brass is minimized. Anodic dissolution apparently 58% of phosphoric acid, 12% of chromic acid and takes place _at a relatively high rate and at a 30% of water. During the continued use of suchk a bath in the electropolishing of brass, the bath 15 high anodic polarization value, with the result composition would necessarily change, owing to that anodic dissolution of the metal acts to level the anodic dissolution into the bath of copper the crystal surfaces thereof` and to vproduce a and zinc from the brass undergoing polishing. mirror-like finish. `These conditions do not pre There might also be some change in the water f vail in the simple electrolytic cleaning treatment content, such as an increase due to absorption 20 known to the prior art. . ` , by the bath of moisture from the air, or a de In order to obtain the best results in a reason crease due to evaporation of water from the sur able length of time, it is preferable to use rela face of the bath,Í to decomposition of water by tively high current densities, such as those of electrolytic action, or to loss of Water from the the order of magnitude of from 100 to 1000 arn bath by entrainment thereof in gases given off 25 peres per sq. ft. It will be understood, however, „ from the bath. that lower current densities, even as low as l0 Notwithstanding such changes in its composi amperes per sq. ft., may be employed with con tion as may occur during continued use, if the sequent prolongation of the time of treatment. relative percentages of phosphoric acid, chromic Higher current densities, up to as high as 2000 acid and water, expressed as percentages by 30 or 3000 amperes per sq. ft., may also be used, weight of the total weight of only these three but such high current densities imply, in gen ingredients in the bath composition, remain with eral, larger currents, which require more expen sive equipment. The length of time to eñ‘ect the desired results depends upon the magnitude of factorily. Preferably, however, the bath compo 35 the current densities employed and to some ex sition should be maintained within the preferred tent upon the particular analysis of the brass, areas of composition, which have been defined and also upon the character of the surface of the brass initially. Rough surfaces, of course, Consequently, where the relative percentages of require a longer time to polish than relatively phosphoric acid, chromic acid and water in a 40 smooth ones. ' given bath composition lie within either the pre With any of the compositions of electrolyte ferred or less preferred areas defined on the ac lying within the preferred areas defined by the companying triaxial diagram, such bath compo solid line AE, dot and‘dash line ELF, solid line Qsition is intended to come within the scope of FM, dot and dash lines MG and GH, and solid , my invention, even though it may contain othen 45. line HA, or by the solid line ZIK and dot and dash acids> than phosphoric and chromic and even line KJI on the accompanying triaxial diagram, though it may ,contain a substantial quantity of> excellent polishes are obtained on brasses of any in the area defined by the solid lines AB, BC, CD and DA, the bath will continue to operate satis ' above. metallic salts. ' ~ of the usual compositions. ‘ Instead of chromic acid, soluble chromates and The term “brass” is intended to cover‘copper bichromates may be substituted therefor and are 50 zinc alloys of varying compositions, some of which to be considered the equivalent of chromic acid may contain small percentages of lead, tin, on a stoichiometric basis. Thel term ‘.‘chromic and/or other metals. With the more ordinary acid equivalent,” as used in this specification and brasses, such as -those containing around '70% in the claims, is therefore intended to include copper, 29% zinc and up to 1% tin, I prefer to chromic acid itself (CrOs) and stoichiometrically 55 employ anode current densities of about 500 am equivalent weights of , soluble chromates and bi peres per sq. ft., a bath temperature lying within chromates. the range of 80 to 175° F., and a time of treat Similarly, in `place of orthophosphoric acid, ment varying from`11/2 to 12 minutes. In gen other phosphoric acids, such as meta-_ and pyro eral, the temperature may be maintained at any phosphoric acids, may be used and are to be con 60 point between room temperature and the boiling sidered as included within the term “phosphoric point of Water, but temperatures of around 100° acid.” F. are found very satisfactory. In the method of anodically- polishing brass, From the foregoing description of my inven using a bath of a composition indicated to be tion, it will be apparent that I have provided suitable by reference to the accompanying tri a novel composition of electrolyte and a method axial diagram, the brass, or an article having a whereby the same may be used to produce highly lustrous polishes on brass. My electrolyte and surface of brass, is made the anode in a bath of the selected composition and an electric cur method avoid the disadvantages of the old meth rent is passed therethrough of sufficient density ods of mechanically polishing brass and enable and for a suñicient length of time to produce the 70 the production of brass articles having surfaces desired high degree of luster, or polish, on the that are free from mechanical strain, Vdragging brass. By employing an electrolyte having a and “pilingf’ and which are superior to'those composition within the preferred areas, as de obtainable by mechanical polishing methods. fined on the accompanying triaxial diagram, a It Will, of course, be understood 4that various highly lustrous, mirror-like surface can be read 75 details of the process may be varied through a 2,407,543 7. The method of anodically 'polishing brass -wide range without departing from the principles which comprises making brass the Aanode in a of this invention and it is, therefore, not the purpose to limit the patent granted hereon other wise than necessitated by the scope of the ap pended claims. solution comprising essentially av composition lying within the area defined in the accompany ing diagram by the lines IJK and KI, and passing , an electric current therethrough at a current I claim as my invention: 1. The method of anodically polishing brass, density of from one hundred to three thousand 3. The method of anodically polishing brass ing diagram by the solid line AE, the dot-dash line ELF, the solid line FM, the straight dot-dash line MG, the dot-dash line GH, and the straight amperes per square foot at a temperature be which comprises making brass the anode in a tween 80° and_212° F. for a sufiicient period of solution comprising from 5 to 65% CrOa, the balance comprising essentially water, and at a 10 time to eiîect the polishing of said brass. 8. The'method of anodically polishing brass temperature below 212° F. and a current density which comprises making brass the anode in a not in excess of three thousand amperes per solution comprising essentially a composition. square foot passing an electric current there lying within the area deñned in the accompany through of sufficient density and for a suiiicient ing diagram by the line AB, the straight line BC, period of time 'to eifect the polishing of said the straight line CD and the straight line DA, brass. and at a temperature below 212° F. and a current 2. The method of anodically polishing brass, e density not in excess of three thousand amperes which comprises making brass the anode in a per square foot passing an electric current there solution comprising from 42% to 65% CrOa, the balance comprising essentially water, and at a 20 through of suiiicient density and for a suñlcient period of time to eiîect the polishing of said brass. temperature below 212° F. and a current density 9. The method of anodically polishing brass not in excess of three thousand amperes per which comprises making brass the anode in a square foot passing an electric current- there solution comprising essentiallyl a composition through of sufiicient density and for a sufficient 25 lying within the area deiined in the accompany period to effect the polishing of said brass. which comprises making brass the anode in a solution comprising from 24% to> 34% CrOa, the solid line HA, and at a temperature below 212° F. balance comprising essentially water, and at a temperature belowv 212° F. and a current density 30 and a current density not in excess of three thousand amperes per square foot passing an not in excess of three thousand amperes per electric current therethrough of sufllcient density square foot passing an electric current there-v and for a suiiicient period of time to effect the through of suiiicient density and for a. suiiicient polishing of said brass. « period of time to effect the polishing of said brass. 4. The method of anodically polishing brass 35 which comprises making brass the anode in a . solution comprising essentially a composition ~ 10. The method of anodically polishing brass which comprises making brass the anode in a , solution comprising essentially a composition lying within the area defined on the accompany lying within the area defined in the accompany ing diagram by the line AB, the straight line BC, ing diagram by thelines IJK and KI and at a temperature below 212° F. and a current density 40 the straight line CD, and the straight line DA, and passing an electric current therethrough at not in excess of three thousand amperes per a current density of from one hundred. to three square foot passing an electric current there thousand amperes per square foot at a tempera through of sufûcient density and for a suillcient ture between 80° and 212° F. for a suiiicient period period of time to effect the polishing of said brass. of time to eilîect the polishing of said brass. 45 5. The method of anodically polishing brass ‘ 11. The method of anodically polishing brass which comprises making brass the anode in a which comprises making brass the anode yin a ` solution comprising from 5% to 65% CrOa, the solution comprising essentially a composition balance comprising essentially water, and pass lying within the area deilned on the accompany ing an electric current therethrough at a current density of from one hundred to three thousand 50 ing diagram by the solid line AE, the dot-dash line ELF, the solid line FM, the straight dot-dash amperes per square foot at a temperature be line MG, the dot-dash line GH, and the straight tween 80° and- 212° F. for a sufficient period of solid line HA, and passing an electric current time to ei‘rect- the polishing of said brass. therethrough at a current density of from one 6. The method of anodically polishing brass which comprises making brass the anode in a 55 »hundred to three thousand amperes per square foot at a temperature between 80° and 212° F. for solution comprising from 42% to 65% CrOa, the a suiiicient period of time to eiîect the polishing balance comprising essentially water, and passing an electric current therethrough at a current density of from one hundred 'to three thousand amperes per square foot at a temperature between 80° and 212° F. for a sufñcient period of time to effect the polishing oi' said brass. ' l of said brass. CHARLES L. FAUST.