US1795079A - Method of electroplating zinc on iron or steel - Google Patents

Description

March 3, 1931. s. H. DAVIS 'r AL 1,795,079

METHOD OF ELECTROPLATING ZINC ON IRON OR STEEL Filed Sept. l2, 1928 ZINC CHLC//VE MENGE/V555 wax/0E (fram cef/j To @xm/ZE EEE/vous fao/v w soLaT/a/v SLI/ME LSTEEL fnv@ /Ho/v PHaauc-rl LAL frfu. 1 c1. EmvE/-GREHSE REMO ym. l

WASH WRTEP [nc/n P/c/rL/NG oF STEEL n/va UPON PRODUCTS] WHS H WHTEI? A,

CELL FR 5TH/KE C0197' OFZ/NG- m INSOLUBLE .NGDES-LW HCI T/ITE' CORTEZ STEEL /V /IYGN POD UCTS OPERA-UNG CELL -lrvso/.ueLEH/VODES-J -d'TE EL O77 /RoA/Ffoucrs 4s cnn/0065 -H/GH Hc/D SOLUTION I/ z/Nc conTE STEEL. nwo /no/v Pn'oaucrs l WITNESS Br FITTOH/YEYS Patented Mar. 3,I 1931 UNITED STATES PATENT OFFICE I 4 SIDNEY H. DAVIS AND CARL O. ANDERSON, OF BAXTER SPRINGS, KANSAS, AND WIL- LIAM N. SMITH,' 0F PLATTEVILLE, WISCONSIN, AND HERBERT R. HANLEY, 0F ROLLA, MISSOURI, ASSIGNORS T THE CENTURY ZINC COMPANY, A CORPORA- rIoN or onio METHOD oF ELECTEOPLATING zINc oN IRON on STEEL Application filed September 12, 1928. Serial No. 305,429.

The object of our invention, considered .iii its broadest aspect, is to provide a cyclic method by which zinc from a commercial raw vsourcevmay be directly plated on an iron or 'on the article is effected by submerging; the latter in a neutral or substantially neutralV solution of zinc sulfate or an alkaline solu tion of cyanide of zincand then passing'an electric current of relatively low density from a soluble zinc anode, likewise submerged in the bath, to the article actingv as a cathode. Both of these processes necessarily require initial production from the zinc ore of metal-1 lic Zinc, either for the preparation of the molten bath or to serve as the soluble anode in the plating bath. When the zinc or spelter is to be used for hot dipping, the most satisfactory results are obtained when'the metallic zinc isQ recovered from the ore by the well known retort process, experiments having shown that the substantially pure metallic zinc derived from the ore by the electrolytic processes now in use is unsatisfactory for hot dipping. In like manner the metallic zinc to be used as the soluble anode in the electroplating process is ordinarily produced by the retort process but irresp ,ctive of whether it be produced by the retortprocess or electrolytically, initial productionofthe metallic zinc a's such is required asa preliminary to its use in the electroplating process, so that in either of the methods hitherto employed forl coating iron or steel articles *withl zinc, .the latter must irst be produced by one proc- 'ess andithen utilized in an'entirely separate and different process, i. e., hot dipping or electroplating -for coating the article. In accordance with the preferred practice of our method however, we are able in a single cycle comprising a series ofvinter-rezinkiferous material to the article to' be plated, thus entirely avoiding the necessity for rst producing metallic zinc by one series of operations and then utilizing that zinc to form the coating upon the article in another and separate series of operations.

The preferred practice of our method is thus advantageousl in numerous respects,

among Which'may be mentioned marked economy in operating costs; a higher percentage of recovery of zinc from the concentrate than has been usual in the methodsof recovery heretofore practised, the loss of zinc by our method being less than one-third of that occurring in the combination of retort recovery and hot dipping now in use; permissible recover of ley-product metals such as cadmium and lead contained in the ore which are now wasted jor lost when metallic zinc is recovered by the retort method but which have a present value of several dollars lper tonof ore. Moreover, we are able in accordance with our invention. to produce upon the article a; coating of markedly different and superior: ;characteristics to that which it has'beenl-:p' vQ'ble to produce either by the hot dipping"relectroplating process as heretofore practised.

. Other advantages and novel features and characteristics of our invention are hereinafter more specifically mentioned or will be apparent to those skilled in the art as a more detailed comprehension thereof is acquired from the following description.

As stated, our method is preferably a cyclic one inwhich, among other things, zinc calcines derived from raw zinc concentrates and/or other zinkiferousV materials are continuously leached with an .acid solution supplied from the cell or cells in which the plating ofthe article is effected, and which4 for convenience may be termed the operating cells, so as to produce a zinc sulfate solution which, after suitable treatment, is 'fed inv regulated quantity to the plating bath so as to maintain the bath, and in which the article to be coated and acting as the cathode and an insoluble anode are suspended, in a relatively highly acid condition while a current of relatively highdensity is passed from the anode to the cathode to eiect the deposition thereon the bath, the process is a truly cyclic one, it`

being only necessary during its operation to continuously supply a sufficient quantity of the zinc calcine or other source of zinc to compensate for the amount of metallic zine deposited on the ferrous articles.

In vthe use of an insoluble anode resides an important and distinguishing difference between our method and the ordinary electroplating processes, enabling us as it does to achieve and satisfactorily utilize plating baths of relatively high acidity and currents of high density. In the ordinary electroplating practice, on the other hand, soluble anodes are employed Which prohibit the use of relatively high acid solutions or baths since the same by dissolvingV the anodes would destroy or materially lower their own acidity.

By the use of insoluble anodes, however, this result is obviated, and we are thereby enabled to provide a process in which the acid generated in the operating cell or cells is desirably utilized for dissolving the zinc ealcine or other zinc material serving as the source of the zine which is ultimately plated on the ferrous article by the utilization of high density currents with resulting production, through the use ofsuch currents in connection with high acid solutions, of a coating or plating markedly superior to those which it has been possible to heretofore produce by any method or process with which We are familiar.

The deposition of zinc froinliiglily acid p zinc sulfate solutions on iron or steel articles acting as cathodes isrfrst attended by the evolution and deposition of hydrogen at the artlcle or cathode. After a variable interval of. time, the exact length of which differs in accordance with voperating conditions and the like the major evolution of this hydrogen ceases andl the cathode is found to be incipiently coated with a thin deposit of zine but during the formation of this initial or incipient coating while the major evolution of the hydrogen is taking place the current efficiency is relatively lowl withI resultant diminution of operating economy., However, after the formation'of the initial coating and cessation of the major evolution ofv hydrogen, the current eiiciency increases and the zmc 1s deposlted more readily.

We have found the major evolution of hydrogen can be checked and an initial or strike coating of zinc formed on the article in a given interval of time after it has been rendered properly vzinc-receptive by pickling and brushing and/or other surface conditioning inthe usual or any suitable way, by the use of a current whose density .varies almost directly with the acidity of the solution or electrolyte; thus, for example, if'the evolution of hydrogen and initial deposit of zinc can be effected on an iron or steel article having a given type of surface in 30 seconds when using a current density of 100 amperes per sq. ft. of cathode surface and a solution containing 5% sul- -phuric acid, it is requisite to effect the same result when employing a solution containing 10% sulphuric acid, thatis, a solution having twice the acidity of the first solution, to employ a current having a density of substantially 200 amper-es per sq. ft. of cathode surface. From this it follows that f.if iron or steel articles are to be coated with zinc to a thickness sufficient forordinary cominercial uses in a reasonable length of time, it is necessary when utilizing relatively high acid solutions to employ 'currents of relatively Vgreat density necessarily requiring eX- tremely heavy busbars, wiring and other instrumentaliti s for their transportation with resultant enhincement of the cost of initial installation and maintenance as Well as of fixed charges during the life of the plant.

Under certain conditions this may not be found objectionable but in accordance with the preferred practice of our invention We consider it generally more advantageous to effect an initial or strike coating on the iron or steel article by means of a current of very high density and at high efficiency and to then form a subsequent coating by using a current of relatively lower density but with equal e''iciencywith the result that since the high density current is employed but momentarily it is unnecessary to provide means for transportingand handling it of the same capacity and size as would be the case were the said current employed for the longer interval necessary to form a thicker' deposit; additionally, as this practiceA materially shortens the period during which the evolution of hydrogen takes place in large quantities fromthe steel or iron article acting as a cathode, the production of an ultimate coating of great perfection and high quality is I derived, reference in this connection being l Madere.

had the accompanying drawing showing a flow sheet therecf, the method, however, `being capable of modification in many particulars without departingfron'i, the spirit and scope of the invention, for example as -will' hereinafter appear, by the employment of metallic zinc instead of raw zinc coricen trates as the source of zinc supply.

Pursuant to the preferred practice offth'e present method, zinc calcines derived by roasting or other suitable treatment of the raw zinc concentrates arev continuously leached with an acid solution derived from the cells in which the articles are plated so as to produce a zinc sulfate solution which, after suitable treatment, is fed in regulated quantity to one or more of said cells so vas vto maintain the baths therein, in which the said articles acting as thecathodes and insoluble anodes are suspended, in arelatively highly acid condition while `a current isbeing passed therethrough from the anodes to the articles to plate thereon metallic zinc supplied from the baths as distinguished from a soluble zinc anode as in the ordinary electroplating process, atleast two'of such source from which the zinc is obtainedr tothereby form fresh zinc sulfate solution containin' ya chemically equivalentquanti-ty of zinc w ich in turn is fed back'to the cells, the process is a truly 'cyclic one, it being' onlynecessary during its operation to lcon-l tinuously supply a suiiicient quantity of the' zinc calcines or other zinkiferous material to make up foi' the metalliczinc ultimately abf stracted from vthe operation in the Yforni of coatings on the iron and/or steel articles. Thus, when employing the raw zinc concentrate,1the samemay first beroastedin the :usual lWay so as to oxidize thezinc sulphide into zinc oxide and, to some extent, into .zinc sulfate which compounds a'resolublein d1lute sulphuric acid and invv water' respectively.

i* i The zinc calcines produced by the ALroasting operation are then leached with a dilute solution of sulphuric acid which, as hereinafter 3 ,explained,.is.drawn :from the cells. Due to -the immersion of the iron and steel articles in the plating baths 'or from other causes, this solution may contain some iron which it is necessary to remove, conveniently by first oxidizing the iron sulfate from the ferrous to the erric state with some eiective and permissible oxidizer, for example, manganose dioxide (hydrated) which is producedl as a by-product `from the anodes in the strike and operating cells. Following the addition of the manganese dioxide to the solution, conveniently in the leaching tank, an excess of base, for exapmple, zinc oxide in the 'form of 'zinc calcine, is added to the solution, thus neutralizing any residual acid and then, byy

further addition, causing precipitation -of the ironas ferrichydrate or similar insoluble iron compound: should, arsenic and/0r antimony be present they will be precipitated with the iron: At this point all of the zinc which is soluble is in solutionaszinc sulfate in the leaching tank; additionally, relative ly -small amounts of copper and'cadmium and possibly other metals such as nickel and cobalt -are also in solution as sulfates, these metals having been'leached out of the concentrate.

The contents of the leach tank may then bewpassed to a suitable classifierwhich sepai rates the coars'e'r particles from the slime and the former may then be 'carried through a ball mill or other suitable device to grind them toa slimewvhich is returned to the classifier as shown, these slimes constituting the final state of division of the solids remaining from the leaching operation. Fromthe classifier the slimes may be directed to a Dorr or other suitablefthickener in which a large proportion of the liquid is separated from the solids; the` thickened "slimes may then be passed to a continuous suction filter where the final separationof the solids from the liquid is effected and the, filtrate then merged with the liquid coming* from the Dorr thickener. p p

This solution usually contains, as previous! ly noted, some'copper and; cadmium in addi- V tion` to the major metal zinc andthese, namely, the copper andl cadmium, are/next prel cipitated iroml the solution in any suitable way, desirably by the addition of zinc dust,

and together with yany excess of the z inc dust then' separated from the solution by means of a 'iilte or thelike and should any other impurities such as lnickelv 'and cobalt be present in the solution they are removed'y by any of i the standard methods of purification utilized in Vthelelectrolytic zinc industry. The solids Nfrom the ylter may thenbesubjectedto an treatment -suitable for recovering the ca miumand/ory copper as by-products aswell .as the zinc .dusttl 1isv treatment forming in o part of the present invention; in this permissible recovery o the cadmium and/or lcopper and other metals'as by-products, however, resides one ofthe' advantages'of our method, leading, as it does, to a larger monetary yield from each ton of ore treated than is possible when processes are employed which result in the loss of these by-products, such, for example, as the usual retort process.

' The purified zinc sulfate solution delivered from the filter isnow readyto be fedto the izo4 4 mesme.

strike andJoperating cells, and may for con- As heretofore stated, the bath in the strike veniencebe termed the primary-solution. cell is preferably maintained at a consider- While for convenience of description we tably lower acidity than the bathin the operhave herein referred to employmentvr of they ,'ating cells thus, for. example, good'results ,usual single leachprocess in the preparal areobtainediwlien tliebatli in the former is tion yof the primary zinc. sulfate solution, it i maintained at an acidity of 4% to 5%, i. e.,

vwill be understood that, if desired, the ,dou- 4()`.to"50 'grammes ofsulphuric acid per thouble'leachWprocess may be employed iisitead sand cubie'lceiitinietes and the bath in the and under certain conditionswill be prelatter atan aciditynofl 71/% to 10%, i. e.,

ferred, the primary object in either case, of p from 75 to l0() grammes of sulpliuric acid-per course, being to obtain at the lowest possible thousandcubicl centimeters of solution when cost and in the most convenient way a substrike and operatingcuirents having density,

stantially puife neutral zinc sulfate solution respectively of approximately 150 and 60y am- Which is to` be fed to the strike and operating'f peres per sq. ft. of cathode surface are to cells to maintain the desired acidity therein be employed. .The particular acidity of the fandl replace the metallic zinc plated out/onY ytwo baths, however, isfc'apable of considerthe ferrous articles. The manner of producable variation both aboveand below the pei'- -ing the neutral Zine sulfate heretofore de- 4ceiitages indicated and will usually be deter scribed does not in itself form any part of mined in vpractice by ytheispcritic operating relatively lower density. In this -manner, cells having thus been brought to the desired 4electrolyti'c zinc and lithopone industries.

the present invention as the same is .well conditions present,yitb'eiiig of course appreknown and at present in common use in the ciated that, within limits, the higher the density of the" strike current with relation In accordance with the preferred-practice to the acidity ofthe `bath the more rapid of the present invention the plating` on the will be the [deposition of the strike coating iron oiisteel article is effected in twobi' more and consequently the yshorter interval persteps `respectively performed in the strike mitted for theI evolution of hydrogen. The

' coating cell with a relatively'low'l acid solu- ,f addition to the baths of a small-amount of 'andthen in one,or more opeiatingcellswith usually found advantageous. a solution of higher acidity and a current of The solution in the strike and operating tion and acurrent of relatively high density some/suitable addition agent such as glue is l as will hereinafter more fully'appear, weare degreeof acidity theiron or steel article to able by the substantially momentary passage which the coating is to be applied, having of the high densityA current. through the been given a suitable-surface conditioning as 'strike cell to very rapidly effect air initial by immersion in an alkali cleaning solution to strikecoating of zinc upon thearticlevwitheffect grease removal, followed by rinsing out the evolution of an undue 'amount of hy- ,i and by pickling to remove scale, followed by drogen and with high current eiticiencyand, Vrinsing and brushing again followed by alafter the formation of this ycoating to thicken kali cleaningy orelectro-alkali cleaning if the sameY to any required'degree by transdeemed desirable, is suspended in the soluporting the initially coated article't'o the option inthe strike coating cell to act as a erating cell orcellsand passing therethrough cathode` and a direct current passed thereto for a longer period, adem-rent of relatively through the bath from an insoluble anode also lower 'current density, insoluble anodes, de-` suspendedfin the solution Vand desirably of linsoluble anodeto solution to a proper operating value. zinc sulfate solution with which the cellsl -apparent that because of the relatively short sirablyof lead, being employed in both/the lead. As stated, the density of the strike strike andoperating cells. y A current will be governed` more or less by VVhile'the method isin operation the) baths operating conditions and the like, it being, in thestrike and operating cells areirespechowever, desirab-le to use a current of as high tively automatically maintained in-properly density as possible within the bounds of ecoacidfconditionjas will hereinafter appear but nomic'goperation, for example, 150 amperes must be initially brought to" that condition per sql ft. ofk cathode surface, a current of by the addition of the proper amount ,ofsulthisdensity being capable with a solution phuric acid or by assing a current from an having an acidity of 4% to 5% of depositing build up the acidity'of the /a very-satisfactory strike coating in approxi- The mately one minute. It will therefore bey are initially filled may be obtained in any time this current is flowing, the busbars and 'desired way, for example, by forming a other yconnections and instrumentalities by sufficient quantity of the primary solution in which isY carried may, without becoming the manner described but utilizing for the unduly .overloaded and heated, be of" matelll) leaching operation necessary therefor a di- -rially smaller size and capacity than would lute sulphuricacid solution supplied from an be requiredvv were thecurrent directedthrough external source in suitable quantity instead themfor a materially longer interval, with of from the electrolytic cellslas in the cyclic consequent decrease in cost of initial 'installaoperative performance of the method. l 'dom/plant maintenance and fixed charges.

l As soon as the strike coating is formed, the strike current is shut off and the article being plated transferred to the operating cell containing a bath of zinc sulfate of considerably higher acidity than that in the strike-coating cell and after the article has been immersed therein a direct current of lower densit-y than employed in the strike cell is passed from an insoluble anode, desirably of lead, to the article acting as a cathode for an interval suiicient to form an ultimate plating of the 'dcsired thickness on the article. For satisfactory and economical operation, a current having a density of 60 amperes per sq. ft. of cathode surface may be employed with a bath having an acidity of substantially 10%, but

as in the case of the strike coating cell the acidity of the bath and the density of the current are capable of considerable variation from these figures, while, of course, the length of time during which the current is passed will be determined by the thickness of the 'plating it is desired to produce on the article.

As a guide, it may be stated that the application of60 amperes per sq. ft. of cathode surface with a solution containing 4% to 6% of sulphuric acid for a period of ten minutes will ordinarily deposit some 0.38 ounces'of zinc per sq. ft. while under similar conditions the applicationof the current for fifteen minutes will deposit a ,considerably greater amount as from 0.50 to 0.7 ounces per sq. ft. depending upon the efiiciency of the current, but these amounts will necessarily vary in accordance with different operative condi y tions.

soluble anodes to the iron or steel cathodes in the strike coating and operating cells results in the deposition on their respective cathodes, that is, the articles to be coated which act as such, of metallic zinc derived from the solution in the cells and the generation of a chem? solution being required to maintain a higher ferred practiceof our invention the efliuent acidity in the cell and a larger volume to maintain a lower acidity for the same rate of acid generation. In accordance with the presolution or overflow from the strike coating cell is directed into the operating cell and asit isof lower acidity lthan the .solution in the latter it to some extent tends to decrease the As the passage of the'current' from the inacidity thereof while simultaneously a suiiicient quantity of the primary solution is also fed to the operating cell so as to hold its acidity substantially constant, and the efliuent acid solution from this cell returned to the leaching tank or other point where the leaching operation is performed for the purposr` of leaching additional zinc calcine. As the amount of acid contained in this effluent solution is the substantial chemical equivalent of the Weight of Zinc which has been deposited, a given quantity of the solution when so returned to the leaching operation will, for practical purposes, dissolve from thecalcines or other source of zinc supply a weight of zinc equivalent to that which was deposited while that quantity of the eluent solution was being discharged from the operating cel?, so that the method is thus capable of being carried on uninterruptedly after being once putinto operation so long as sufficient zinc calcines or other zinkiferous materials are supplied to compensate for the zinc deposited on the iron or steel articles which, from time to time as they receive their coatings of metallic zinc to the required thickness, are removed from the operating cells and after rinsing and drying may be marketed without further treatment.

In the preferredA practice of our method as above described raw zinc concentrates are employed as the source of the zinc to be ultimately plated on the ferrous articles and, as heretofore pointed out, it is thus possible by the use of the method to effect a direct transference of the zinc in the ore to the articles and thereby avoid the disadvantages necessarily vinherent in first recovering the zinc as a metal by the retort or electrolytic process and then utilizing this metal for hot dipping or electroplating. However, under some conditionsit may be preferred to utilize metallic zinc derived in any suitable way instead of zinc calcines in the leaching tank and to there subject this metallic zinc, in a suitable state of division to facilitate rapid dissolving, to the action of the eflluent acid solution drawn from the operating cell as hitherto described and in this manner provide the primary neuv tral Zinc sulfate solution for the strike and tinuous section filter shown on the flow sheet would ordinarily be unnecessary; equally so,

-if the metallic zinc supplied is devoid of cadmium, copper and/or other impurities, the addition of the zinc dust or other step designed'to effect precipitation thereof may be omitted.

at the same current deisitare employed,

While under mostconditions but a single y.

a series of operating cells containing solui tions of different degrees of acidity and/or operated with currents of varying density. Additionally, while we prefer to gradually and continuously feed to the strike coating cell sufficient zinc sulfate solution to maintain the solution in the cell at a constant degree Aof acidity and to conduct the eliiuent acid solution displaced by the neutral zinc sulfate solution either to the operating cell or directly back to the leaching tank, it is also within the scope of our invention to refrain from feeding any zinc sulfate solution to the strike coating cell and to permit the acidity of its solution to gradually increase as the strike current is intermittently carried'therethrough and to vary the densitybf this current in accordance with the increasing acidity of the cell instead of operating with a strike current of constant density as hitherto described.

Under certain circumstances it may be deemed advantageous to utilize but a single plating cell instead of separate strike and operating cells and to initially employ in this cell a relatively low acid solution as, for example, one containing4% or 5% of sulphuric acid so that a strike or initial coat- -ing can be formed on the ferrous article in that-cell with high current eiiiciency by momentarily passing a current of relatively high density as, for example, 150 amperes per sq. ft. of cathode surface, .the operation in this case being similar to that employed in forming the strike coating in accordance with the preferred practice of the invention. Following the formation of the strike coating, however, instead of transferring the article to another cell having a bath of higher acidity and then continuing the formation of the coating therein Aby the use of a current of relatively lower density, the article may be allowed to remain in the bath and the passage of the current continued, its amperfar as the character of the ultimate coating is concerned, the results contemplated by our" invention, but We dov not consider this method of operation as advantageous as the preferred practice of the invention to which We have hitherto referred for the reasons, among others, it is ordinarily less economical and does not lend itself as readily to the desirable cyclic method of operation.

It will of course be understood that the instrumentalities .employed in the practice of our improved method may be of any desired form suitable for the performance of their various functions.

In the light of the foregoing disclosure of our invention the advantages arising therefrom and to some of which brief reference has already been made can now more readily be appreciated and may in part be summarized as (a) reduction in plant installation and operating costs and in reduction of fixed charges because of ability to utilize current carrying appliances of relatively small size and capacity; (b) increased operating eliiciency of the currents employed due largely to the shortening of the period of hydrogen evolution during the format-ion of the primary or initial coating' of zinc; (c) almost instantaneous coating of the ferrous article with a strike coating of zinc so that but a negligible quantity of the iron or steel is dissolved in the electrolyte before the article is protected from t-he action of the latter; (d) the strike or incipient coating formed in the strike coating cell upon the article greatly facilitates subsequent coating thereof in the operating cell at economical current density and solution acidity; (e) the ability to employ a lrelatively low acid solution for the formation of the initial coating which in itself tends to diminish the evolution of hydrogen and decreases tendency of the .desired thick-ness on an iron or 'steel article by means of relatively high acid solutions of zinc sulfate and currents of such character as may be handled economically within time intervals satisfactory for commercial work; (g) the character of the plating produced, exhibiting as it does substantial chemi cal purity, au extremely fine crystalline structure, great density, good color and appearance, lack of porosity or sponginess. and part-icularly markedly greater resista-nce to corrosion in the presence of weak acids than the zinc coatings which it has hitherto been possible to produce upon iron or steel articles of commerce by any process or method known to us.

Having thus described our invention, we

'claim and desire to protect by Letters Patent of the United States: A

1. In a method of clcctroplating zinc on ironor steel, the steps of forming a thin initial coating of zinc on the article to be plated by passing thereto from an insoluble anode through a bath of a. solution of zinc sulfate of relatively low acidity a. direct current of relatively high current density and then afterv the formation of said initial coating continuing the formation of the plating by the passage of a. current of lower density tothe article from an insoluble anode throughv a bath of higher acidity than the first mentioned bath.

IVX

2. In a method of electroplating zinc on iron or steel, the steps of forming aI thin initial coating of zinc on the article to be plated by passing thereto from an insoluble anode through a bath of a solution of zinc sulfate containing at least 4% of sulphuric acid a direct current of relatively high current density, transferring the initially coated article to another bath containing a solution of zinc sulfate of greater acidity, and then continuing the plating of zinc on the article therein by passing through the second bath from an insoluble anode to the article a direct current of lower density than the first mentioned current.

3. In a method of electroplating zinc on iron or steel articles, the steps of electrolyzing a bath of zinc sulfate containing a relatively small amount of sulphuric acid by passing therethrough from an insoluble anode to the article acting as a. cathode a current of relatively high density to thereby form on the article an initial relatively thin coating of zinc without excessive hydrogen evolution and then continuing the formation of the coating by electrolyzing a bath of relatively greater acidity by passing therethrough for a longer period a current of relatively lower density from an insoluble anode to the article acting as a cathode.

4. In a method of electroplating zinc on iron or steel articles, the steps of electrolyzing a bath of zinc sulfate containing a relatively small amount of sulphuric acid by passing therethrough from an insoluble anode to the article acting as a cathode a direct current of relatively high density to thereby form on the article an initial relatively thin coating of zinc without excessive hydrogen evolution, then continuing the formation of the coating by electrolyzing a bath of relatively greater acidity by passing therethrough for a longer period a direct current of relatively lower density from an insoluble anode to the article acting as a cathode, and maintaining the acidity of said baths substantially constant by feeding thereto regulated quantities of a neutral solution of zinc sulfate.

5. In a method of electroplating zinc on iron or steel articles, the steps of forming a relatively thin initial coating of zinc on the article by electrolyzing for a short interval a solution of zinc sulfate relatively low in acid by passing a current of relatively high density through the solution from an insoluble anode to the article acting as a cathode, transferring the article after the formation of said initial coating to a second solution of maintaining the acidity of said second solution constant by feeding thereto a predetermined regulated quantity of a neutral solution of zijne sulfate.

6. In a method of electroplatin zinc on iron or steel articles, the steps o forming an initial relatively thin coating of zinc on the article by electrolyzing a lsolutionof zinc sulfate containing substantially 5% of sulphuric acid by momentarily passing therethrough from an insoluble anode to the article acting as a cathode a direct current having a density of substantially 150 amperes per sq. ft. of cathode surface, then transferring the article to a second solution of zinc sulfate containing substantially 10% of sulphuricacid, continuing the formation of the zinc 0n the article therein by electrolyzing said solution by passing therethrough from an insoluble anode to the article acting as a cathode a direct current having a density ofV substantially amperes per sq. ft. of cathode surface and maintaining the acidity of the said solutions substantially constant by feeding thereto sufficient neutral solution of zinc. sulfate to compensate for the sulphurie acid generated therein by the passage of the currents.

7. In a. method of electroplating zinc on iron or steel articles, the steps of forming an initial relatively thin coating of zinc on the article by electrolyzing a solution of'zinc sulfate containing substantially 5% of sul-l phuric acid by momentarily passing therethrough from an insoluble anode to the article `acting as a cathode a direct current having a density of substantially 150 amperes per sq. ft. of cathode surface, then transferring the article to a second solution of zinc sulfate containing substantially 10% of sulphuric acid, and continuing the formation of the zinc on the article therein by electrolyzing said solution by passing therethrough from an insoluble anode tothe article acting as a cathode a direct current having a density of substantially 60 amperes per sq. ft. of cathode surface.

In witness whereof we have aiiixed our signatures.

SIDNEY H. DAVIS. CARL O. ANDERSON. WILLIAM N. SMITH. HERBERT R. HANLEY.

zinc sulfate of greater acidity and continuing the formation of the coating on the article therein by passing through said solution from an insoluble anode to the article acting asl a cathode a current of less density for so long a time as may be required to form a coating ofthe desired thickness on the article and

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