US2525603A - Method of making lead coated copper - Google Patents

Description

Oct. 10, 1950 R H, JENKS ETAL 2,525,603

METHD OF MAKING LEAD COATD COPPER Filed Nov. 21, 1945 .yuca yownwi #RAMP Lm m. E;

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UNITED STATES PAT ENT QF F ICE METHOD or MAnrNdLEADwATED cori-Ea ration of Maryland Application November 21, 1,945, Serial No. 630,106

1 Claim.

Our invention relates to articles having copper surfaces, such as sheet copper, coated with a thin layer of lead for improving the resistance of the copper to atmospheric and other forms of corrosion, and to methods of making sucharticles.

Heretofore, in coating copper surfaces with lead, it has been common practice to alloy the lead with a small amount of tin for insuring proper adherence of the coating to such surfaces. Applicants have found that in employing tin for this purpose pitting of the copper frequently occurs by corrosive action where pinholes exist in the lead coating as formed, or where the coatingfis locally accidentally removed by mechanical means such asscratches. Although Vthe reason for such pitting heretofore has not been exactly understood, it is believed by applicantsthat it occurs because when ,the` coating is applied some of the atin alloys with the surface portion .ofthe copper to form on the latter a lm of substantially binary copper-tin alloy, with the `result that when `,pnholes in the `lead coating exist, .or the coating is removed mechanically at localized points, there `.are `exposed both bare copperand this nlm, and, the bare copper being anodlc to the film, the bare copper is pitted .when the article is exposed to a corrosive medium.

Applicants havefound that in place .of tin `the lead may be alloyed with cadmium for insuring satisfactory adherence of the lead tothe copper, and that if the ,cadmium is ,properly proportioned to the llead such .aliilm either is .not formed or if formed` is anodic tothe bare copper, so that pitting of the bare copperdoes not occurasa resultof pinholes .or Vthe like in the coating.

Applicants have found, however, thatif itis attempted -to .apply according to prior methods` a coating of lead containing cadmium,` the `cad- Inum` in `the `lead bath rapidly oxidizes and is lost, forming a layer of cadmium oxide ldross .on the surface of the bath. This` dross adheres superncially to the coatingand mars `its .appearance. It furthermore tends to mix W-iththe-,material of the coating layer to form cadmiumloxide segregates therein which act seriously `to reduce the corrosion resisting properties .of the` coating itself.

According `to the present invention, .thedrossing of the lead-cadmium bath is effectively pre vented by causing the ,bathto contain aluminum. As little as 0.001 `aluminum will preventdrossi-ng of the cadmium and the presence of `cadmium oxides in the coating, the surface of the `bath with this amount of aluminum remainngtbllight .and clean. `The amount of aluminum however should not exceed its limit `of solubility in `eitherthe moltenorsolidifiedbath material, which limit is about 011%, as `otherwisemetallic aluminum segregates may-be introduced into `the coating causing deterioration thereoflby galvanic action. Preferably to Ainsure satisfactory results about 0,005 to 0.05% aluminumi-semployed.

The aluminum may be compounded with the lead by "melting the lead and raising -its temperature to atleast about 1300 whereupon the aluminum may be added and `the bath stirred. The cadmium may be added to the lead following the addi-tion thereto of the aluminum. Preferably,` however, the lead-aluminum alloy is cast i-nto billets, andthe cadmium-is added when `the billets are melted in the lead pot to form the coating bath which preferably is at a temperature between25 and 875 F., this tempera-ture however not being critical but being merely high enough to` cause the bath to have sufficient uidity properly to spread over lthe copper surface of `thearticjle when applied thereto to form thereon a thin coating of *the lead alloy of from about 0.001 -to0.01 of an inch in thickness.

. Applicants also have found, `that unless suitable precautions are: taken in respect to the amount of cadmium incorporated in Ythe lead, the coating .itself wil-l be subject to serious deterioration when subjected to the action of a corrosive medium. It has been found that Vthe amount of cadmium incorporated should be suchV as to result in the coating having `a substantially single alloy phase structure. A plural alloy phase structurabecause it in effect consists of an intimate mixture of different metal alloys, causes the occurrence of ugalvanic action on the coating itself when subjected to a corrosive medium, :resulting in corrosion of the coating. This `sing-le phase structure, it has been found, will exist when the coating contains up to about 3.3% cadmium Whether ornot the aluminum is present. provided 'if the aluminum is present it is, not ,substantially in` `excess. `of its limit .of solubility `in `the lead. With amounts of cadmiumnotinexcess .of about 3.3% the ,existence chasingle phase structureapparently, for all practical purposes, is independent` of the ,rate Aof coolingof the coating ,after it is .appliedyin liquid stateto ,the copper, the phase structure tof the `.thin .coating not .beingaffected by ra-pid .cooling ,of `the t coating as ,it Apasses ,from a temperature between the liquidus and solidi-ls .to `a .temperature below the -solidus, `which Lis not the `,case with lead-.cadmium alloys `1.0i CQnSidCI- able mass `.or thickness. :with `-elnfiourlts ,of nach -miumtin excess of about .3,3% ,the solid-ined coat"` 3 ing will have a plural phase structure, characteru istically a so-called alpha-beta structure, no matter at what rate it is cooled from any higher temperature to any lower temperature within the range of from any temperature at which it is liquid down to room temperatures. With amounts of cadmium less than 3.3% the coating has substantially wholly a so-called alpha structure.

To secure proper results from the cadmium at least about 0.5% thereof should be incorporated in the lead, and for the reasons above stated no more than about 3.3% thereof should be present. To insure satisfactory results preferably about 0.5 to 1% cadmium is employed.

Preferably the coating is applied by dipping the article into the bath, with the latter at about 850 F. so that it will be highly fluid, whereupon the article is removed vertically from the bath to permit the excess of the coating to drain therefrom and the coating to spread uniformly over the copper surface. After the article is removed from the bath the coating may be permitted to cool, say by exposing it to air at room temperature or by subjecting it to avdraft of air or to air jets.

The copper surface to be coated is, if necessary, first cleaned, and preferably fluxed before immersing the article in the bath. The cleaning of the copper may be effected by any suitable means, for example, by so-called scratch brushing. The flux may be applied in liquid form at room temperature or slightly above, a suitable flux consisting of the addition to water of ammonium chloride and hydrochloric acid in the proportion of two pounds ammonium chloride and one gallon technical hydrochloric acid to one gallon water, the hydrochloric acid being added after the ammonium chloride is dissolved. Other acid fluxes may be employed, for instance, either aluminum chloride or cadmium chloride may be substituted for the ammonium chloride in this example. Also a thin coating of oil or the like is preferably applied to the fluxed copper surface for insuring against oxidation of such surface prior to the article being immersed in the lead bath. Palm v oil, rapeseed oil, tallow, petroleum jelly and various petroleum oils may be employed for this purpose. It will be understood that cleaning the copper and fluxing and oiling it act to insure a continuous lead coating and good adherence of the same, but may not in all cases be necessary depending upon the condition of the copper surface to be treated and, if it is necessary to clean it, the time that must elapse between the cleaning operation and the emersion of the article in the bath.

Suitable apparatus for applying the coating to sheet copper is illustrated by the accompanying drawings in which:

Fig. l is a more or less diagrammatic vertical longitudinal section of the apparatus;

Fig. 2 is a plan of the iiux and oil applying instrumentalities;

Fig. 3 is a plan of the instrumentality for cooling the coated sheet; and

Fig. 4 is a plan of the lead tank with parts omitted.

As shown in the drawings, the copper sheet S successively passes through a body of liquid flux in a tank I, through a body of palm oil in a tank 3, through a bath of molten lead in a pot 5, and through an air box l.

As illustrated, the flux and palm oil are maintained in the tanks I and 3 at levels 9, the sheet being guided through the body of flux and palm oil by spaced curved guide bars I I below the sheet and idle guide rolls I3 above the sheet. Steam coils I5 may be submerged in the flux and palm oil beneath the guide bars I I for slightly heating these substances if necessary or desired. At the exit ends of the flux and palm oil tanks I and 3 the sheet passes between pairs of driven wiper rolls Il for removing excess flux and oil therefrom and advancing the sheet, which excess drains back into the tanks. At the entrance end of the palm oil tank 3 the sheet passes between a pair of driven offset rolls I8 which deflect the sheet into the palm oil tank 3.

As shown, the lead pot 5 at a pair of opposite sides thereof is supported by beams I9 resting on the opposite walls ZI of a furnace chamber into which the lead pot extends. Below the lead pot are provided gas burners 23 for heating the contents of the pot. As shown, the bath is maintained in the lead pot at a level 25, the sheet being deflected into the bath by driven offset rolls 2l. The sheet is guided through the bath in the lead pot by the spaced curved guide bars '29 positioned above the sheet and the spaced curved guide bars 3l positioned below the sheet. At the exit side of the bath is provided an idle roll 33 adapted to engage the upper side of the sheet for assisting in guiding it in a vertical direction in which it is drawn by driven guide rolls 35 at the top of the air box l, between which rolls 35 it passes after traveling 'through said box, these rolls 35, like the rolls lll, I8 and 2l, acting to advance the sheet.

In the air box I the sheet is cooled for hardening the coating sufciently to prevent the rolls 35 from injuring it. To this end the sheet in the air box passes between vertical banks of spaced horizontally disposed compressed air conducting pipes 31 each provided with a row of spaced jet nozzles 39 for directing air toward each of opposite sides of the sheet, thus subjecting the latter to an air draft for cooling the coating. Each of the horizontal pipes 3l is connected to and supplied with compressed air from a vertically arranged header 4l. These headers are severally connected through a regulating valve 43 to a common pipe 45 supplied with compressed air from a suitable source. By use of the valves 43 the force of the air jets on opposite sides of the sheet may be so regulated that the sheet will not be forced laterally in either direction. Preferably however, to insure against the sheet striking the jet nozzles under untoward conditions, spaced curved vertically extending guard bars 47 are arranged in the air box at opposite sides of the sheet between it and the nozzles, these bars being in offset relation to the nozzles so as not to interfere with the air jets discharging therefrom.

It will be understood that within the scope of the appended claim wide deviations may be made from the herein described embodiments of the invention without departing from the spirit of the invention.

We claim:

The method of protecting the copper surface of an article against corrosion comprising coating it with an adhering alloy of single phase structure substantially free from tin and containing a material which forms with copper a copper alloy anodic to copper, that material being cadmium, by forming a bath of molten alloy substantially free from tin and consisting essentially of lead having dissolved therein not less than about 0.5% cadmium to insure such adherence, or more than about 3.3% cadmium to insure against the 5 6 coating having other than a single phase struc REFERENCES CITED ture, and also having dissolved therein not less The following references are of record in the than about 0.001% aluminum to prevent drossfue of this patent: ing of the cadmium and inclusion of cadmlum oxide segregates in the coating, or more than 5 UNITED STATESP,A'1"1i|"1\1"1`5l about 0.1% aluminum also to insure against the Number Name ,Date coating having other than a single phase struc- 243,766 Feehy `1111315, 1381 ture, immersing the article in said bath to cause 1,126,434 Kirby Jam 26, 1915 the bath alloy to adhere thereto, and Withdraw- 1,760,603 Milbring May 27, 1930 ing from the bath the article coated with such 10 2,052,363 Snyder Aug 25, 1936 allOY- 2,196,855 Dietloff Apr. 9, 1940 RICHARD H, JENKS, FOREIGN PATENTS WILSON LYNES- Number Country f Date n 528,558 Great Britain Oct. 31, 1940

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