CA1069459A - Method of producing metal strip having a galvanized coating on one side - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A method fro producing galvanized metal sheets or strip material having a zinc coating on one side only. The method includes immersing a strip (zinc-coating on both sides) in an electrolyte and passing it between anode means and separate cathode means so as to remove a zinc coating from one side of the strip while simultaneously depositing a substantially equivalent amount of zinc on the opposite side of the strip. The cathode means is immersed in a separate caustic catholyte solution which is kept separate from the main electrolyte by an anion exchange membrane, supported at least partly within the electrolyte, so as to prevent migration of zinc from the main electrolyte to the catholyte and cathode means and the formation of a deposit on the cathode means. The method is most economically performed with a steel strip having a differential coating of zinc.

Description

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Thi~ invention relates to a method for producing galvanized metal shee~ or strip materlal having a zinc coating on one side only.
More particularly, this invention relates to a method for treating zinc-coated metal strip or sheet material so as to remove the zlnc coating from one side thereof while simul~aneously depositing a substantially equivalent amount of zinc on the oppo~ite side whilP at the same time preventing the formation of a ~inc deposit on cathode means utilized in ~he proceR~.
The use of galvaniæed metal sheet or strip material ls conven-tional in many appllcation~ where corroslon re~istance ls important. How-ever, ln some case~, particularly when used in automobile ~ody con~truction and the like~ lt ic undeRirable to have a zinc coating on both sides of the metal sheet or s~rip since ~uch a coating has an undesirable effect on the weldabllity and surface finishing of the metal. In such instances lt is important to provide a material having a galvanized surface on one ~ide of the metal sheet, which side ia generally unprotected otherwlse, and a clean surface on the opposite side ~orefficient weldability and cosmetically acc~ptable surface fini~hing, such as pain~ing.
Effor~s to provide such galvanized sheet metal in the past have taken many form~ including, for example, that of U.S. Patent 2,894,850 is~ued 14 July 1959 to Green. Green describe6 a process which, in effect, includes ua~king one ~ide of sheet ~etal with an aluminate coating to prevent the adherence of zinc thereto. V.S. Patent 3,178,305 isRued 13 April lg65 ~ to Ward in part descr~bes a proce~s for electrolytically stripping one ~ide of a zinc coated ~teel strip to provide a galvanized product having a zinc coating on one side only, but the strip it~elf i9 made anodic and ~he ^~tripped zinc beco~es depo~lted on the cathode. No provislon i9 made for simultaneously depositlng an equal a~ount of ~inc onto the opposite side of the treated str1p.
It has now been discovered that metal sheet or strip material having a zinc coating on one ~ide may be electrolytically produced, preferably wi~hout a concurrent depo~ition of zinc on ca~hode mean~
utilized in the process, by an electrolytlc treatment which includes pasR- ~

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1~ 59 ing the material throu~h an electrolyte solutioll and between anode means and separate cathode means so that the coating on the metal sheet or strip material opposite the cathode means is removed from the metal base and an amount of zinc equal to that removed is simultaneously deposited on the oppo~iteside of the material. Thu~, according to this invention, the sheet or strip material functions as a "bipolar" electrode. Specific-ally, the cathode mean6 is lmmersed in a ~eparate, caustic catholyte solution which i9 isolated from the main electrolyte by a perm-selective anion exchange membrane, so that zinc ion~ in the main electrolyte are prevented by the membrane from mlgrating to the vicinity of the cathode means.
This method is particularly applicable in treating differenti- ;
ally æinc-coated material wherein the ~hickne6s of the zinc coating on one side ls le~ than the thicknes~ of the zlnc coating on the opposite side.
In treating such material the strip will be passed through the electrolyte so that the side havlng the relatiyely thinner, i.e., lighter, coating faces the cathode means and the opposite side faces the anode means.
In the accompanying drawings, Figure 1 i8 a scbematic view of a tank of the present invention;
Figure 2 is a schematic view of an alternate tank of the invention in which the anodes and cathodes are arranged in a diagonal configuration;
Figure 3 is a schematic vlew o a further alternative tank i~ .
specifically adapted to carry out a preferred embodiment of this invention;
Figure 4 i8 a schematic vlew of yet another tank to carry out the embodiment of the inventlon described with reference to Figure 3; and Figure 5 is a graph showing theoretical current requlrement ~in amperes) ver~us coa~ing weight to be removed (in ounce/ftZ).
According to this invention, zinc~coated metal strip or sheet material is electrolytlcally treated to produce a galvani~ed product h~ving a zinc coating o= one slde ouly, the other side being uncoated for ma~imum weldability and surface fi~i~hing. As used hereinafter the term "strip"
should be construed as~including sheet material. A particular feature of .
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the invention i8 the isolation of cathode means from the main electrolyte solution so as to prevent the formation of a zinc deposit thereon. The isolation of the cathode means is accompliAhed through the use of a perm-select~ve anion exchange membrane which separates the main electrolyte from a catholyte solution in which the cathode means ls im~er~ed.
Thus, this invention comprises i~mersing a zinc-coated metal strip (coated on both sides) in an electrolyte solution so that it passe~
between separate cathode mean6 and anode means within the ~olution, the result of which i8 the removal of the ~inc coating from the side of the metal strip ad~acent the cathode means while a 6ubstantially equivalent amount of ~inc is simultaneously plated onto the opposite side of ~he strip, i.e., that ad~acent the anode means, and when it is desired to prevent formation of zinc deposits on the cathode means, the strip is pas~ed between anode means immersed in the main elec~rolyte and cathode means immersed in a catholyte solution which is separated from the main electro-lyte by a perm-selective membrane. In any arrangement according to this invention the metal strip functions as a bipolar electrode.
The me~al strip to be treated, preferably a steel strip 6 to 72 inches wide, may be hot dipped or coated in any desired manner but in any event, it i9 coated on both ~ides with a zinc layer. In order to limit the time and current required to remove the zinc coating from one side of the strip lt i9 preferred to have a difEerentially coated strip, i.e., having the lowest coating weight economically practicable on one ~ide. Thiq differential coating may be accomplished by any conventional method but the most convenient and preferred method is that disclosed in United States Pa~ent 3?499,418 issued lO March 1970 to John T. Mayhew. Such a product will generally have a zinc coating on one side whi~h is about 0.1 ounce/ft2 or leas, in general 0.01 - 0.15 o~nce/ft2 and a thicker coating on the opposite slde, normally 0.2 - 0.7 ounce/ft2. Of ~ourse where desired coated strip having a very light coatlng on both ~ides can be produced by the ~ -2pparatus of the afvre~aid United State~ Patent ~o. 3~499,418. The strip material may be provided in lts commercially acceptable coil form or it may be introduced into the 6ystem adapted to carry out the present invention ~6~
directly from a metal coating line.
According to a basic embodiment of this lnvention, as Lllustr-ated in Figures 1 and 2, the strip is passed into a tank of conventional design and immersed in an electrolyte formed of a relatively low acidic solution generally having a pH within the range of 1.0 - 4.0, preferably

2.8 - 3Ø In the preferred case, the electrolyte compri~es an aqueous solution of zinc sulfate and ~ulfuric acid and may contaln conventional additives such as minor amounts of aluminum ~ulfate, magnesium sulfate and sodium sulfate, the latter compounds providing improved conductivity and a "whiter" deposit. U~ually, zinc ~ulfate will be added in an amount which provides between about 10 - 20 ounces of zinc metal per gallon of electrolyte at a pH range of from about 1.0 - 4Ø The electrolyte w$11 generally be maintained at a te~perature within the range of about 120F
to 150F with a preferred temperature being 135F.
When i~mersed in the electrolytlc solution the strip will be passed between a cathode means and anode means 80 that there is a ~pacing of about 1 ~o 3 inches, preferably about 2 inches, between the strip and each electrode. For maximum effectiveness, average current densities on the strip, i.e., bipolar electrode, within the ~Ishadow~ of the external electrodes, i.e., cathode means and anode means, should be between about 200 to ljOOO ampsift2. A current denslty oE about 500 amps/ft is preferred.
The strip is preferably passed between the electrodes at a line speed of from 100 to 500 feet per minute or higher depending on the length of the tank. The electrolyte i8 circulated within the tank and, prefer-ably, directed towards the strip 80 as to minimi~e turbulence within the tank.
In some instances it will become necessary to increase voltage .
in order to off~et a polarization type effect presumably caused by a lack ~:
of movement in the ions emanatlng from the strip being treated. Under such circumstances there is a need for a greater clrculation to redistribute the ions in solution and this increasP in circulation may be accomplished '.'--4- ;

:, , . . : . ' :~ ' ` ' ~6~59 by any conven~ional means. Also, if there is insufficient clrculatlon, burned areas form on the strlp inltially at the edges since the current densities are higher there.
The cathode means utilized ln the basic embodiment of this invention is generally a good conductor which does not react with the electrolyte. Such cathode means may be made of a materlal which will not plate out zinc on its surface or at least resists the plating of metal ions tending to di~charge on its s~rface. In either ca~e, the cathode means will discharge hydrogen lons. Alternatively and less preferably but more practically, the cathode means is made of a material from whlch the ~inc lons discharge to form a depo~it which can be easily stripped therefrom. Specific examples of ~uch materials, which may also be used as anodes in this in~ention are lead and lead alloys, carbon, platin~m plated titaDium and aluminum~ ~inc may also be removed from the cathode by reversing the polarity and turning the strip ~providing the electrodes are of the same materlal) whereupon the zinc plating will dissolve in the electrolyte.
It is preferred9 although not necessary to the invention, to remove dissolved iron periodically from the electroly~e. This iB prefer-ably carried out by an oxidatlon procesR, for example wlth zinc peroxide ;
or manganese dloxide to ferric and precipitation as a hydroxide at a pH
of 3, for example, using calcite. An alternative method would be precipi-tating the iron as ferric sulfate using, for example, ammonium sulfate as a reactant. If the iron i8 not periodically removed it will eventually plate ou~ as iron-zinc alloy. This will have no effect on the satisfactory performance of the ~ethod of thls invention in respec~ to mo~ us~s of the product but it will be important lf recovery of free zinc from the cathcde - is intended.
It has been noted tha~ the conventlonal hot-dipped zi~c coating on steel comprises three layer~; a Cop zlnc layer, an intermediate iron-zinc alloy layer, and a metalllc6 layer~ pre6umed ~o be an iron-zinc-aluminum alloy, which contacts ~he substrate. At higher current densities, ' ~S

~6~59 i.e~, usually greater than 500 ~mps~t2, usually all of the~e layers will be removed from the deplated side of the strip. ~o~ever, at lower ;
current densities, i.e., usually lower than 500 amps/ft2~ a metallics layer may remain as a 1008e black coating after the deplating treatment.
In that event, as the strip emer8es from the treatment apparatus the deplated side thereof may ,be ~ubJected to a light bru~hing to remove the residual loose blaok coating. The brush ~elected for this purpose should be one that will not cause scarring on the ~urface of the strip. A
brush sold by Minnesota Mining & Manufacturing Co. under the mark Scotch-brit ~ as been found to be useful in this regard.
Referring in detail to Figures 1-2 which illustrate the basic concept of this invention, a steel strip 10 i9 supplied from a coil or other source not Yhown. The strip is differentially coated wl~h ~inc so that the thickness of the coating on one side 11 is less than the coating on the opposite side 12. It is guided by ~uitable deflector rQlls such as roll 9, so that i~ passeæ into 8 tank 13 filled with a dilute aqueous solution, shown generally as 14, of sulfuric acid or a sulfate radical, . . .
and under sink roll6 15. The 8trip passes through the electrolytic ` solu~ion be~ween cathodes 16 and anodes 17 ~hich are connected respective-; 20 ly to a direct current electromotlve source, not shown. The cathodes, for example, may be connected to a source of direct current such as a battery or to a direct current generator. It i6 important that the side 11 of the strip, whlch is the side from whlch the zinc coating is removed, is facing the cathodes a~ it passes through the electrolyte. As the strip passes between the cathode and anode, zinc is removed from the strip 11 by the reaction indicated, and may be deposited on the cathodes, hydrogen also belng evolved at that point. At the anodes, æinc metal is reduced . . ,: .
to zinc ion which goes into solution and i~ plated on the immediately adjacent side 12 of the strlp;~ water dissocia~ion at the anode i8 also ~ -indicated. Thus, æinc is ~imultAneously re ved from and plated on . : ~
respectlve ~ides of the steel strip. A6 the 6trip emerge~ from the treat-ment apparatus it may be sub~ected to subsequent treatment such as non-.
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abrasive brushing oE the deplated surface, and the like.
Figure 2 i8 a diagrammatic lllustration of an alternate form of the invention wherein the cathodes 50 and anodes 51 are arranged in a diagonal configuration~ the zinc coated strlp 52 being supplied from a coil or other suitable source, not ~hown~ and pasged between the elec~rodes by deflector rolls 53. The operating condi~ions generally described above are also applicable in thi~ ~mbodiment.
It i8, of couræe, possible to place at least two of the electrolytic cells descrlbed above in ~eries whlch would per~it speeding up of the line or the utilization of a lower current density.
As a specific example of the basic process of this invention, a 8iX inch wide steel ~trip having a galvanised coatÇng of 0.1 ounce/ft2 on the light side and 0.5 ounce of zinc/ft2 on the heavy side was introduc-ed into an elec~rolytic solution, essentially in the manner ~llustrated in Figure 1. The tank utilized in this case was 52 inches in length, 30 inches wide and 4 feet in depth. It contained 180 gallons to overflow of an electrolytic solution which~was circulated through pipes connecting the treatment tank with a storage tank holding 200 to 300 gallons of the solution. The electrolyte was maintainad at a temperatur2 of about 135F
and was separately formulated as 530 gallons of an aqueous solutlon includ-lng 1,030 pounds of zinc sulfate and 25 pounds of concentrated sulfuric acid. The steel strip was passed between the anodes and cathodes, again as depicted in Figure 1, at a line speed of 10 ft/minute. The total anode area was 1.625 sq. ft., specific dlmensions being 39 inches long, 6 inches wide and 0.75 lnches thick. The cathode plate had the same dimensions. The total current input in the system was 872 amps, the current density being 540 amp5/ft2 and the voltage being 14.5 volts. The elec~rodes were each apaced about 2 inches from the strip. The treated strip emerging from the tank was found to have one surface, i.e., that previously having the lighter coating, stripped free of zinc while the opposite side of the strip had a zinc coating of about 0.6 ounce/ft2. ~ -The zinc surface was still spangl~d ~nd brighter than the original.
,, ~1369~59 Where the cathode means i6 lsolated by an anion membrane from the ~inc ions in the electrolyte, it is imposslble for a 2inc depo~it to form thereon. It i8 also important to note ~ha~ in such an ar~ange-ment the only substance depleted in this process is water which decomposes at the cathode means to yleld hydrogen gas and hydroxyl ions and at the anode means to yield oxygen gas and hydrogen ions. The hydroxyl ions formed at the cathode means carry the current through the anion membrane and into the electrolyte where they unlte with the hydrogen ions generated at the anode means to reform water.
T~ese reactions are illustrated In detail in ~igures 3~4 of the drawings. Referring specifically to Figure 3~ a steel strip 60 is supplied from a coil or other mean~ not ~hown. The strip is differenti-ally coated with zinc 80 that the thlckne~s of the coating on one side 61 is less than the coating on the opposlte side 62. It is guided by suit able deflector rolls, one of which ifi shown at 71, so that it passes ~-into a tank 63 filled with a dilute aqueous solution, shown generally at 64, of sulfuric acid and zinc sulfate, and under sink rolls 65. The strip passes through the electrolytic ~olution between a cathode 66 and anode 67 which are connected respa~eively to direct current connections not shown. It i8 important tha~ the side 61 of the strip, which is khe ~ :
slde from which the zinc coating is removed, i~ facing the cathode as it passes through the electrolyte. The cathode 66 i~ immersed in a 5% by weight aqueous sodium hydroxide solution, shown generally at 68, which is separated from the electrolyte 64 by a container structure inciuding wall , portions 69, a foraminous bottom portion 70~ and a perm-selective anion ;
membrane ~hich is sealed in any convenient manner to the wall portions 69 and supported by the foraminou~ bottom portion 70. Usually the membrane will be bonded to it~ supporting structure by any onvenient means~ After ; a per~od of US2 the ~embrane ~ay become ~pent whareupon it wlll bP removed a~d replaced with a new menbrane.
As the strip passes between the anode and the isolated cathode, the zinc coating on the side o~ ~he strip 61 ad~acent the cathode is -8~-- .

~6~45~i oxidized to 7.inc ions which 8 into solutLQnJ ~hile a substantially equivalent Amount of zinc ions are reduced to zinc metal and deposits on the side of the strip facing the anode. Water dissociates at the anode and cathode as indicated, the hydroxyl ions which are generated at the cathode functioning to carry the current through the membrane into the electrolyte. The membrane, of course, prevent~ zinc ions fro~ migrating to the cathode. Thus, zinc i8 8imultaneouBly removed from and plated on respectlve sides of the ~teel strip without the fon~ation of zinc on the cathode.
The apparatus schematically illu~trated in E'igure 4 operates essentially the same as tha~ depicted in Pigure 3 except that the cathodes 80 and anodes 81 are arranged in ~ dlagonal configuration, the strip 52 being supplied from a coil or other suitable source, not shown, and passed between ~he electrodes by deflector rolls 83. As in the embodiment of Figure 3, the cathodes are immersed in a catholyte, shown generally as 84, which is separated from the electrolyte, shown generally as 85, by a perm-selective anion membrane 86. A shield 87 of inert material, such as polypropylene or polyvinyl chloride, supported in any convenient manner, is positioned so that it initially insulates or masks the side of the s~rlp having the hea~ier zinc coating from the anode to promote etching of that coating thus facilitating a greater adhesion thereto of the subsequen~ly plated ~inc layer. The arrangement of Figure 4 will in some cases be preferred since it facllitates removal of gas formed at the electrodes and provides a treatmlt substantially equivalent to that of the horizontal design but in a much smaller floor space.
Of course3 it is po~sible to place at least two of the electrolytic cells described above in ~eries and this will per~it a speeding up of the line or the utili~a~ion of ~ lower current dsnslty.
The electrolyte in whlch the strip iB immersed (identified here-in as the "main electrolyte") i~ for~ed of a relatively low acidic - solution consis~ing e~sentially of dlvalent ions of zinc, generally having a pH within the range of 1.0 4.0, but preferably less than 2 for reasons _g_ .
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which w~ll be desc~ibed hereina~ter~ In the preferred case~ the electro-lytc comprise~ an aqueous sclution of zinc sulfate and sulfuric acid and may contain conventlonal additive~ such as minor amount~ of aluminum sulfate, magnesium sulfate and sodium sulfate; the lat~er compounds pro-viding improved conductivity and a "wh~ter" deposit. Usually, zinc sulfate will be added in an amount which provide~ between about 10 - 20 ounces of ~inc metal per gallon of electrolyte at a pH range of from about 1.0 - 4Ø Usually the electrolyte will be maintained at a temperature withln the range of about 120F to 150~F with a preferred temparsture being 135F. Aa previously indicated ? the electrolyte i~
isolated from the cathode means by a container ~tructure including a perm-selec~ive anion m~mbrane which holds a caustic catholyte solution in which the cathode means is imDer~ed.
The catholyte may be any suitable caustlc solution but it is important that it contain no metallic ions which can be deposited on the cathode. Preferred catholytea included aqueous sodium hydroxide and potsssium hydroxide solution~ the alkali content belng within the range 5 - 15~ by weight. It should be empha~ized, however, that a low . ~, .
concentration of hydroxide 18 de~lrable and a 5% aqueous solution has 0 been found to be very ~atisfactory. ~-The anion exchange membrane separating the electrolyte from the catholyte includes an anion exchanger held onto a foraminous support by a binder. It contains a fixed poait~ve charge and a mobile negative charge, the fixed positive charge functioning to repel positively charged ions. Thus, the anion membrane repels positively charged zinc ions and prevents them from getting through to the cathode while allowing nega-tively charged hydroxyl ions to pa~s through its structure. The membrane acts as a sieve snd is perm-selective. An equilibrium is eventually set up 80 far as migration of nega~ive ions are concerned. Any of the conventional anion exchange me~branes may be utilized in thia inven~ion, thelr selec~ion being de~enmined by desired temperature and chemlcal stability. Included among the an~on exchange membranes which have been . "
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~L~369a~9 found ~o be use~ul are those manu~actured by Ionac Chemical Company and sold under their mark Ionac MA 3475 and Ionac~ 475 R. Each of these membranes contain strong ba~e type anion exchange resins. They are generally supplied in dry form but they may be prepared for u~e accord-ing to a method described in a bulletin entitled "Ionac Ion Exchange Membranes" which is publicly distributed by Ionac Chemical Company.
The membranes utllized in thi~ invention will generally have a thickness of from 7 to 15 mils but in any evsnt ~hould be thick enough to pre~ent tearing during their use. The above mentioned Ionac membranes have been shown to have a chemical stabillty up to 125C and problem free operation utilizing a current density aa high as 1,000 amps/ft2. In some cases, usually during use of high current den~ities, a precipitate of zinc hydroxide will form on the side of the membrane facing the anode, a problem which can be cured elther by increaslng the acid content of ~he electrolyte to a pH of 2 or less, for example down to about 1.0, or by a preferred method whlch lncludes introducing air agitation around the membrane by any conventional means9 such as tubing9 æo as to eliminate local areas of higher pH near ~he mLmbrane and move the hydroxyl ions into a low pH area where they may be neutralized by hydrogen ions to form water before they can react with the 7.inç ions.
The maintenance of a low pH, i.e., less than 2 in the electro- ;
lyte around the anode is also useful in treatlng hot-dipped zinc-coated metal strip where it ic sometimes difflcult to obtain a strong bond between the zinc surface and the plated layer. Thisproblem will be eliminated by ma$ntaining a low current density near the surface of the electrolyte where the strip is entering the solutlon 80 that in the low pH environ-ment a slight etching of the spangled surfaee will occur thus promoting ;~
greater adhesion bet~een the base zinc layer and the electroplated layer.
The mO6~ convenient manner for maintaining such a ~ow current density near the surface of the electrolyte i~ ~o employ a shield or ~asking element as depicted ln Figure 4. Of course o~her means may be utilized to achieve this slight etching but the acidic low current density treatment .~' ' --11-- , , , is preferred since it simply requlre~ ~niti~l st~ip contact wlth the acid electrolyte prior to its pa_sage between the anode and cathode. However, it is Lmportant to no~e in this regard that the acid concentratlon of - the electrolyte should not be so hlgh a6 to adversely affect the smooth surface of the deplated ~ide of the Rtrip, which for use in the auto-motive indus~ry ~hould have a finlshed 6moothness of 40 - 50 microinche6 as measured on a Bendix Profilometer.
A~ previou~ly mentloned in relation to th~ ~bodiments illu-strated in Flgures 1-2, the strip shown in Figure6 3-4 when i~nersed in the electrolyte ~olution will similarly pas~ between a cathode means and anode mean8 90 that thare i5 a ~pacing of about 0.5 to 3 inches, prefer-ably 1 - 2 inche~, be~ween ths strip and the an~de and a spacing of about 1 - 4 inches, preferably about 2 inches, between the strip and the cathode.
For ma~imum effectiveness, current den6ities on the strip, i.e., the bipolar electrode, within the "~hadow" of the external electrodes l.e., cathode and anode, should be between about 200 to 1,000 amps/ft2.
A current density of about 500 ampa/ft2 is preferred. Of course the current density ~elected will be determined in part by the thickness or weight of the ~inc coatlng to be removed. The theoretical current require-ments may be conveniently determlned by reference to Flgure 5 of the drawings or by calculation using the following formula:
ft~ x (mgslft2) x mg8 The strip is preferably pas~ed between the electrodes at a line speed of from 100 to 400 ~eet per minute. The electrolyte is generally circulated within the tank, preferably dlrected towards the strip so a~ to minlmize turbulence within the tank. ~n ~ome instances it will become neces~ary to increase voltage in order to offset a polari-zatlon type effect pse~umably csused by a lack of movement in the lons e~anating from the ~trlp being treated. Under such clrcumstances there is a need for a grea~er circulation to redistribute the ions in solutlon and this lncrease $n circulatlon may be accompllshed by any conventional , .

~L~6~S~3 mean~. A180, if there i~ insuf~lcient circulatlon in the electrolyte, burned areas form on the strip inltlally at the edges since the current densities are higher there.
It may be necefisary to cool the catholyte (using any conventional means, such as a steel cooling coil) since there is resistive heating in the catholyte which may raise the tenperature to a point where it will adversely affect the useful life of the anlon membrane. In general, the temperature of the catholyte and electrolyte should be maintained at about the same point, plus or minus ten degree6.
The cathode means utillzed in the invention is preferably a good conductor and i8 of a material which doeq not react with the caustic catholyte solution. Included among those materials found to be useful ln this regard are lead and lead alloys, carbon, platinum plated titanlum9 and steel. Materlals useful for anode means include lead or lead alloys, carbon and platlnum coated titanium. The anode means should also be of a material which is unreactive and insoluble in the electrolyte.
As a specific example of the process of this invention illustrated in Figures 3 and 4, a six inch wide steel strip having a galvanized coating of 0.1 ounce/ft on the light slde and 0.5 ounce of zinc/ft2 on the heavy side was introduced into an electrolytic solution, essentially in the ~anner illustrated in Figure 3. The prototype tank utili~ed in this case was 52 inches in length, 30 inches wide and 4 feet in depth. It contained about 178 gallons to overfluw of an electrolytic solution which was circulated through pipes connecting the treatment tank with a storage tank holding 200 to 300 gallons of the solution. The electrolyte was maintained at a temperature of about 135F and was separately formulated as 500 gallons of an aqueous solution having a pH
of 1.5 and including 1,030 pounds of zinc sulfate along w$th 75 pounds of concentrated sulfuric acid. The steel strip was passed between the lead ~ -anode and a æteel cathode, agaln as depicted in Figure 3, at a line speed of lO ft/minute. The ~otal anode area was 1.625 sq. Et., specific dimensions being 39 inches long9 6 lnches wide and 0.75 inches thick.

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The cathode plate~s dimensiona ware 36 ~ 5 x ~.75 lnche~ The catholyte, comprising about 3 gallons of a 5~ by weight aqueous sodium hydroxlde solution i8 contained in a box-shaped structure meaauring 37 x 6 x 3 inches, including a foraminous polyvinyl chloride bottom portion to which a perm-selective anion exchange membrane identified as Ionac's MA-3475 membrane is bonded. The bot~om portion and membrane were secured to the side portions of the structure in a liquid-tight seal. The temper-ature of the catholyte i~ ~aintained ae a point about the same as the electroly~e through the u~e of a water-cooled steel coil which is im-mersed therein. The total curren~ input in th~ system is 872 amps, the current density being 540 amp~/ft2 and the voltage being 14.5 volts. The electrode~ are each spaced about 2 lnches from the strip. The treated strip emerging from the tank i~ ound to have one surface, i.e., that previousl~ ha~ing the lighter coatlng, stripped free of ~inc while the opposite side of the strlp had a zinc c03ting of about 0.6 ounce/ft2. The zlnc surface was still spangled and brighter than the original and the steel cathode remained free of a zinc deposit.
The above embodime~s are to be considered in all respects as illustrative and not restrictive since the invention may be embodied in other specific forms without departlng from its spirit or essential characterlstics. ~herefore, the scope of tbe invention is indicated by the claims rather than by the foregoing description, and all changes which come within the meanin8 and range of the equivalents of the claims are intended to be embraced therein.

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Claims (17)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for treating zinc-coated metal strip material comprising:
a) immersing the strip in an electrolyte solution, b) passing the strip through the electrolyte solution and between anode means and cathode means so that the strip functions as a bipolar electrode, c) electrolytically removing a zinc coating from the side of the strip facing the cathode means while simul-taneously depositing a substantially equivalent amount of inc on the opposite side of the strip, and d) preventing the formation of a zinc deposit on the cathode means.

2. A method as defined in claim 1 wherein the formation of a zinc deposit on the cathode means is prevented by immersing the cathode means in a separate caustic catholyte solution which is contained within means including an anion exchange membrane, the container means being supported at least partly within the electrolyte solution so that said membrane is in a spaced relationship with the strip and the anode means, said membrane preventing the migration of zinc ions to said cathode means.

3. A method as recited in claim 2 wherein the anion exchange membrane comprises a strong, base-type anion exchange resin.

4. A method as recited in claim 3 in which the electrolyte solution is an aqueous solution of zinc sulfate and sulfuric acid and contains from about 10 to 20 ounces of zinc metal per gallon of solution.

5. A method as recited in claim 4 wherein the electrolyte solution has a pH of from about 1 to 4.

6. A method as recited in claim 5 wherein the caustic catholyte solution is an aqueous alkali metal hydroxide solution.

7. A method as recited in claim 6 wherein the caustic catholyte solution is a 5% by weight aqueous sodium hydroxide solution.

8. A method as recited in claim 6 in which the electrolyte and catholyte solutions are maintained at a temperature within the range of from about 120°F to 150°F.

9. A method as recited in claim 8 in which air is introduced into the electrolyte solution so as to provide agitation and minimize formation of a precipitate on the anion exchange membrane.

10. A method as recited in claim 2 in which the spacing between the strip and the anode means is about 0.5 - 3 inches, and the spacing between the strip and the cathode means is about 1 - 4 inches.

11. A method as recited in claim 2 wherein the anode means and cathode means are arranged in a substantially diagonal configuration and the strip is passed between the anode means and cathode means also along a substantially diagonal line.

12. A method as recited in claim 11 in which the anode means and cathode means comprise two sets of electrodes.

13. A method as recited in claim 2 in which subse-quent to the electrolytic removal of the zinc coating from the first side of said strip said first side is subjected to a brushing treatment which does not scar the surface of the strip, so as to remove therefrom any residual loose coating.

14, A method as recited in claim 2 in which prior to electrolytic treatment, the zinc coated strip is subjected to an etching treatment so as to improve the adherence thereto of a zinc deposit subsequently applied by said electrolytic treatment.

15. A method as recited in claim 1 wherein the zinc-coated steel strip has a zinc coating on a first side which is less than a zinc coating on the opposite side.

16. A method as recited in claim 15 in which the zinc coating on the first side of the strip is about 0.01 -0.15 ounce/ft2 and the zinc coating on the opposite side of the strip is within the range of about 0.2 - 0.7 ounce/ft2.

17. A method as recited in claim 15 wherein the apparent current density (calculated average current density) on the strip opposite the cathode means ranges from about 200 to about 1,000 amps/ft2.