US2470689A - Process for tin-coating the interiors of copper tubes - Google Patents

Description

'cRAMP'roN ETAL PROCESS FORVTIN-CQATING THE INTERIORS OF COPPER TUBES May' 17, 1949.

4 Sheets-Sheet 1 Filed Feb. 16,. 1943 bs N3 W E W3 6 S. 2. i 3

May 17, 1949. D. K. CR-AMPTON :rm. 2,470,589

PROCESS FOR TIN-COATING THE INTERIORS OF COPPER TUBES Filed Feb. 16, 1943 4 Sheets-Sheet 2 L. h'ood May 17', 1949. n. K. CRAMPTON ETAL 2,470,689 PROCESS FOR TIN-COATING THE INTERIORS OF COPPER TUBES Filed Fob.- 16, 1943 4 Sheets-Sheet 3 lllllij llill May 17, 1949. n. K. CRAMPTON ETAL 2,470,639

PROCESS FOR TIN-COATING THE INTERIORS OF COPPER TUBES Filed Feb. 16. 1943 4 Sheets-Sheet 4 72 ,1 74 wmnr/m "I 71 51 1% l :l 73 52 73 Patented May 17, 1949 PROCESS FOR TIN-COATING THE INTERIORS OF COPPER TUBES Donald K. Crampton, Marion, and Maurice L. Wood, Waterbury, Conn, assignors to Chase Brass & Copper Co. Incorporated, Waterbury,

Conn, a corporation Application February 16, 1943, Serial No. 476,046

' 2 Claims. 1

The present invention relates to improvements in processes for coating the interiors of tubes, and relates more particularly to improvements in processes whereby coating of tin or tin alloys may be applied to the interiors of tubes formed of copper or copper alloys.

One of the objects of the present invention is to provide a superior process whereby tubes of copper-base materials may be provided interiorly with a continuous and effective coating of tinbase materials in a rapid and economical manner.

Another object of the present invention is to provide a superior process whereby relativelythick coatings of the character referred to may be applied and solidified on the interior surfaces of copper-base tubes with a minimum of dross loss and without deleteriously affecting the tubes.

A further object of the present invention is to provide a superior process whereby hard-drawn tubes of copper-base materials may have their interiors provided with a continuous and effective coating of tin-base materials without causing such hard-drawn tubes to become appreciably annealed and thereby deprived of their desired hardness and rigidity.

A still further object of the present invention is to provide a superior process of the character referred to whereby the coating of tin-base material may be applied to the interior surfaces of copper-base tube without also coating the exterior surfaces of such tubes and without causing the appreciable discoloration thereof.

With the above and other objects in view, as will appear to those skilled in the art from the present disclosure, this invention includes all features in the said disclosure which are novel over the prior art.

In the accompanying drawings, in which certain modes of carrying out the present invention are shown for illustrative purposes:

Fi 1 is a broken view in side elevation of one form of apparatus suitable for carrying out the process of the present invention;

Fig. 2 is a broken top or plan view thereof;

Fig. 3 is a broken detail view mainly in vertical central-longitudinal section and partly in side elevation of the left-hand portion of the apparatus shown in Figs. 1 and 2;

Fig. 4 is a transverse sectional view taken on the line 44 of Fig. 3;

Fig. 5 is a broken detail sectional view taken on the line 5-5 of Fig. 1;

Fig. 6 is a transverse sectional view taken on the line 6-6 oflFig. 5;

Fig. 7 is a broken transverse sectional view taken on the line 'l'l of Fig. 1;

Fig. 8 is a similar view taken on the line 8-43 of Fig. 1;

Fig. 9 is a broken perspective view of the tubedriving pulley, together with a portion of a tube;

and

Fig. 10 is a perspective view partly in section schematically showing a coated tube produced in accordance with the present invention.

The particular apparatus chosen for purposes of illustrating one mode of carrying out the present invention comprises two tubular bus-bar sections l5 and I6 extending in parallelism with each other, though slightly laterally displaced with respect to each other so that the inner portion of each overlaps the adjacent inner end of the other bus-bar member. The said bus-bar members are, in the instance shown, of rectangular form in cross section and are preferably formed of copper or other high electro-conductive material.

Interposed between the respective overlapping inner portions of the bus-bar members l5 and I6, is an insulating-plate I! which serves to physically hold the inner portions of the said bus-bar members apart and to insulate the same one from the other. Embracing the overlapping inner portions of the bus-bar members l5 and [6, as well as embracing the insulating-plate H, are two tubular clamping-members l8 and [9. The said clamping-members l8 and I9 are each of rectangular form in cross section, as is shown particularly well with respect to the clamping-member I8 in Fig. 6.

The tubular clamping-member I8 is in electrical engagement with the bus-bar member [5 but is insulated from the complemental bus-bar member I6 by a substantially U-shaped insulatingplate 20, as is particularly well shown in Figs. 5 and 6. Conversely, the tubular clamping-member I9 is in electrical contact with the bus-bar member I6 but is insulated from the complemental bus-bar member [5 by an insulating-plate 2| corresponding to the insulating-plate 29 before described.

The outer wall of the bus-bar member If: is rigidly clamped in electrical engagement to the adjacent side wall of the tubular clamping-member I8 by means of a clamping-bolt 22 extending laterally through both thereof, as well as through the perforated upper end of a conventional terminal-lug 23. Threaded onto the inner end of the clamping-bolt 22 at a point within the bus-bar member I5 is a nut 24, as shown in Figs. 5 and 6.

The tubular clamping-member I8 is also rigidly connected to, though insulated from, the outer wall of the bus-bar member l6 by means of a clamping-bolt 25 and a complemental nut 25. The clamping-bolt 25-eXtends-' inwardly through the adjacent wall of the clamping-member l8, thence through the insulating-plate 20 and finally through the outer wall of the bus-bar member l6, and the nut 26 is applied to the inner end of the said clamping-bolt at a point within the said bus-bar member I6. The clamping-bolt 25 is insulated from both the clamping-member l8 and the bus-bar member l'fiiby meansof an insulating-bushing 21, while the nut 26 is insulated from the bus-bar member l6 by means of an insulating-washer 28, as shown'in Figs. and 6.

The bus-bar member [6 is both mechanically and electrically connected to the tubular clamping-member [9 by means of a clamping-bolt 29 which extends inwardly: through the perforated upper portion of av terminal-lug 30,, thence through the adjacent side wall of, the clampingmember 19 and finally through the outer wall of thebus-barmember [6. On the inner end of the clamping-bolt 29 atxa: point within the bus bar member I6, is a nut 3|, as shown in Fig. 5.

In a. manner similar to the connection of the clamping-member l8;to-t-heybus-bar member I6, the clamping-member I9 is mechanically connected' to but insulated from the bus-bar member [5. The connection is .accomplished by means of a clamping-bolt 32. extending. inwardly through the clamping-member l 9, thence through the insulating-plate 2| and finally through the outerwall of the bus-bar member l5. The inner end ofthe clamping-bolt 32 has tightl threaded thereon a nut 33 whichis insulated from the busgglrmember l5 by means of an insulating-washer from both the clamping-member Wand the busbar member [5 by means of an insulating-bushing 35, as is shown in Fig. 5.

As thus secured together, the busebar-members I5-and 16 together constitutea single rigid struc ture in which the respective members are insulated one from the other at their respective inner portions.

At its left end (as viewed in the drawings), the bus-bar member I5 is supported by the bifurcated upper end of a post 36 to which it is releasably coupled by a transverse pin 31. The post 36 may rest upon the upper surface of a platform 38 adjacent the left endof the apparatus. For convenience, steps 39 lead upwardly to the platform 38 from the floor 4D or other supporting surface, which latter is preferably of wood or other suitable material which will be effective as insulation at low potentials.

The right end of the-unit comprising the busbar members [5 and l6, as well as the parts carried thereby, is supported by means of complemental vertical posts 4l4-|- which extend vertically upwardly from a carriage 42'. The right end of the bus-bar member I5 is coupled to the respective upper ends of the posts 4l--4'I by means of a transverse pin 43 extending laterally through any desired oneof a plurality of apertures 44 formed in a bus-bar member l6 at longitudinally-spaced intervals along the length thereof, as is shown in Fig. 1=. To provide for bringing the upper ends of its posts 4-l4l into registration with any desired one of the apertures: 44-44 in the bus-bar member IS, the carriage 42. upon which the said posts are mounted, is. provided? with four (more or less) wheels 45. The wheels In turn, the clamping-bolt 32 is insulated- '4 45 are disposed one at each of the respective four corners of the carriage 42, as is shown in Fig. 2. Two of the parallel wheels on one side of the carriage 42 ride on a track 46, while the opposite pair of wheels ride on a track 41 extending parallel with the track 46;

At its left end, the bus-bar member l5 has rigidly attached thereto a head-fixture 48 having a portion extending upwardly beyond the upper surface of the bus-bar member l5 and also having a portion extending into the open left end of the said member and secured thereto by plurality of rivets 49. As will be seen especially well by reference to Fig. 4., the upper portion of the head-fixture 48 is provided in its upper face with anarcuate recess 50 extending transversely of the head-fixture and opening upwardly. In the recess 50 is mounted a lower contact-shoe 51 which is preferably formed of graphite to provide both conductivity and lubrication. The upper face of the contactshoe 5| is curved in a direction transverseof the structure to conform to a portion of the periphery of a tube 52, the interior of which it is desiredtotcoat.

Arranged over the lower. contact-shoe 5| and in oppositon thereto is an upper, contact-shoe 53having itslower face conforming to and contacting the tube 52, asis particularly well shown in' Fig. 4. The upper-contacteshoe 53 is mounted in a downwardly-opening transverse arcuate recess 54 formedin the under face of a verticallymovable carrier-plate .55. The said upper contact-shoe 53, like its, complemental. contact-shoe 5|, ispreferably formedof graphite or other suitable electro-conductive. and self-lubricating material.

Thecarrier-plate 55. above referred. to is adaptedto reciprocate vertically on two vertical and laterally-spaced-apart guide-studs 55a--55a, passing through the carrier-plate 55 with a smooth sliding. fit and having their respective lower ends threaded. into the head-fixture 48 before referred to, all as is particularly well shown in Fig. 4. Encircling eachv 0f the studs 55a55a. at a point. intermediatethe under face of the carrier-plate 55 and the upper face ofv the head-fixture 48 is one of two helical retiringsprings 55-55 exerting a constantv effort to lift the carrier-plate 55 upwardly into a retired position.

At their respective upper ends, the guide-studs 55a55a have. rigidly secured thereto a mounting-plate 57, in the central portion of which is screwed the open lower end .ofaninverted. cupshaped cylinder 58'. Reciprocating. within the cylinder 58 just referred to is a, piston 59 having a depending-stem 60. threaded into thecentral portion. of the carrier-plate 5.5., As shown, the upper end of they cylinder 5.8. has connected to it a pressure-pipe 5| leading tov any suitable source of compressed air or other fluidwhich willserve, when desired, to forcibly lower the,v carrier-plate 55 against the tension of the springs -5656' to engage theupper contact-shoe 53' with the tube 52.

Toprovide for the rotation of the tube- 52 which: is to be interiorly coated; acircumferentially-grooved tube-driving ipulley 62 is mounted at the left end of'the said: tube: at: a point outwardly with respect to the -headrfixture, 4B. and associated parts. 011- its outer face, the pulley 52 is formed with arr outwardly-extending web 63 formed integral therewith. Upstanding; from and formed integral at their lower ends with the web 63, are a pair of complemental flexible clamping-arms 64-65 adapted to be drawn together into clamping engagement with the periphery of the outer end of the tube 52 by means of a clamping-screw 65. The tube-driving pulley 62 may be driven by means of a V-belt from any suitable source of power,

At its extreme right end the bus-bar member 96 has rigidly bolted thereto a head-fixture 66 complementing the head-fixture 48 before described, at the upper or left end of the assembly comprising the bus-bar members I and I6. Like the head-fixture 48 before described, the headfixture 65 is provided in its upper face with an arcuate transverse and upwardly-opening recess El receiving a lower contact-shoe 68 having its upper surface transversely curved to conform to the curvature of the tube 52, as is shown particularly well in Fig. '7. Located above and arranged in opposition to the lower contact-shoe 68 is an upper contact-shoe 59 having its under face contoured to conform to the curvature of the tube 52 and preferably, as is also the contact-shoe 68, formed of graphite.

The upper contact-shoe 69 just above referred to is mounted in a transverse downwardly-opening arcuate recess in formed in the under face of a vertically-movable carrier-plate H. The carrier-plate ll has extending therethrough with a free sliding fit on each of the respective opposite sides of its contact-shoe 69, one of two vertically-extending guide-studs 1212 which are threaded at their lower ends into the head-fixture 65. Encircling each of the guide-studs 2 2-4? at a point intermediate the under face of the carrier-plate ll and the upper face of the head-fixture 65, is one of two helical retiringsprings l3--l3. The said retiring-springs 13-13 exert a constant effort to raise the carrier-plate ll upwardly to disengage the upper contact-shoe 69 from the tube 52.

Threaded at its lower end into the central portion of the carrier-plate ll, is the stem 14 of a vertically-reciprocating piston 15. The said piston i5 reciprocates in an inverted cup-shaped cylinder it threaded at its lower end into the central portion of a mounting-plate H. The said mounting-plate ll extends transversely between and is rigidly mounted upon the respective upper ends of the guide-studs 12-12 as is shown in Fig. 7. The cylinder 16 has connected to its upper end a pressure-pipe 18 leading to any convenient source of compressed air or the like.

Adjustably mounted along the length of the unit comprising the bus-bar members I5 and I6 respectively at spaced-apart intervals thereon, is a plurality of corresponding tube-supporting fixtures 79, Each tube-supporting fixture includes an inverted U-shaped clip 80 formed of spring material and fitting over the adjacent upper edge of one or the other of the bus-bar members l5 and It, as is especially well shown in Fig. 8. Brazed or otherwise secured to the upper face of the clip 85 of each of the fixtures 19, is a transversely-extending bracket 8| to which is riveted a vertical plate 82 extending transversely of the bus-bar structure in a direction substantially perpendicular to the longitudinal axis of the tube 52 and formed of insulating material.

Bolted against one face of the plate 82 is a tube-supporting plate 83 which is preferably formed of wear-resistant material and which has its upper edge notched to support and laterally stabilize the adjacent portion of the tube 52. The tube-supporting plate 83 is, as shown in Fig. 8, secured to the plate 82 by means of bolts 8l84 heating current.-

which respectively extend through vertical clearance-slots -85 formed in the plate 83 to permit the latter to be vertically adjusted.

Located adjacent the right-hand or lower end of the tube 52 is a receptacle 86 having the upper portion of its face which is directed toward the adjacent end of the tube 52, open to receive excess tin or tin alloy in a manner as will hereinafter appear.

Operation A clean copper-base tube is selected and while the respective left and right carrier-plates 55 and H are in their elevated positions as indicated by broken lines in Figs. 4 and 7 respectively, the said tube is positioned so that its respective opposite ends rest upon the lower contact-shoes 5i and 68. The tube 52 or its equivalent is of copper-base material, i. e., commercially-pure copper or an alloy predominantly of copper such, for instance, as brass, commercial bronze, cupro-nickel, etc.

Following the positioning of the tube as above described, air or other fluid pressure is supplied to the cylinders 58 and 16 through their respective pressure-pipes 6| and 18. The application of the pressure just referred to will serve to depress both of the carrier-plates 55 and ll to thereby cause the upper or left end of the tube to be gripped (with capacity for rotation, however) between the lower contact-shoe 5i and the upper contact-shoe 53. At the same time, the lower or right end of the tube will be gripped (with freedom for rotation) between the lower contact-shoe G8 and its complemental upper contact-shoe 69. In this manner, the upper or left end of the tube 52 is electrically connected to the left bus-bar member [5, and the lower or right end of the tube is electrically connected to the right bus-bar member It, which bus-bars, as previously noted, are insulated one from the other at the center of the bus-bar structure where the respective members overlap.

The tube-driving pulley 62 may now be slipped over the projecting upper end of the tube 52 and the clamping-arms 6 1-64 thereof tightened into gripping engagement with the tube by means of the clamping-screw B5.

The tube-driving pulley 62 may now be driven to correspondingly rotate the tube 52 from any convenient source of power and at a predetermined rate. A relatively-high heating current is now supplied to cables 81 and 88, respectively extending to the terminal-lugs 23 and 35'. As previously noted, the terminal-lug 23 is electrically connected to the inner end of the left bus-bar l5, while the terminal-lug 30 is similarly connected to the inner end of the right bus-bar member 16.

A suitable flux may now be poured into the upper or left end of the tube 52 preferably while a relatively-high heating current is applied to the tube 52 through the cables Bl and 88 and while the said tube is being rotated. When the tube 52, or its equivalent, reaches the desired temperature, i. e., a temperature above the melting-point of the tin-base material to be applied to the interior of the tube, the current-supply to the cables 81 and. 88 and hence to the respective opposite ends of the tube 52, is cut down in value to what may be aptly termed a sustaining current, which has a heating-value substantially such as will maintain the previously-heated tube at the elevated temperature which it has reached by virtue of the initial and relatively-high Tin-base material. in. a; molten. condition (tin or-tina11oyssuch for instance, as? tin-lead-alloys or tin-lead-antimony, alloys) may now: be introducedinto the left or upper end of. the tube 52. while; the sustaining'current is on. The said 'molten. tin-base material will flow downwardly through the interior of the. tube and coat the: interior thereof, which interior has. .been fluxed, as previously described. Thequa-ntityof tin-base material poured into the upper end of the tube is preferably in an amount in excess of what by test has been determined to be required for the coating, so that excess: tins-base material will drip from the lower'or right-hand end ofthe tube.

When the tin-base material pouredinto." the upper end of the tube 52 fully shows at the lower-endthereof, a blast-of carbon: dioxide, or other suitable gas, is blown into the upper: end ofthe'tube 52 while the latter is still rotating and still under the influence of the sustaining current, so that excesstin-base material will be blown out of the lower end of the tube and: into the receptacle 86.

The current-supply to the cables 81 and 88 and hence to the" tube 52; may now be. com.- pletely shut off, whereupon pressureto therespective cylinders 53 and Hi-may be released, to thereby permit the upper contact-shoes 53' and elite be retired out of engagement with the upper and lower ends of the tube, respectively. The tube-driving pulley 62 may now be removed from the upper end of the tube and-the saidtube completely removed-from the apparatus; with its now-solidified interior coating of tin-basematerial.

By a procedure such as above outlined, a tube of copper-base material may be readily supplied with an interior coating of tin-base materialof very appreciable thickness and of asmooth continuous and non-defective character. The thickness of the coating may be varied-within relatively-wide limits by varying the inclination of the tube, its speed of rotation, the=temperature to which the tube is heated; and the timeduring which the elevated temperature is maintained following the introduction of the tin-base-material. In addition to the foregoing factors;- the temperature of the molten tin-base material, when the sameis introduced into the tube, will also affect thethickness of the coating when other factors are equal.

The thickness of the coatingon the interior of thetube may be varied from about 0.0005 to about 0.002", whereas, with prior practice, it

has been economically practical to achieve a smooth coating'of only about 0.0001 in thickness.

By heating the tube by means ofits own-resistance, rather than by flame 'orby induction, the said tube at substantially all points between the respectivepairs of contact-shoes'at its respective opposite ends, achievesa' substantiallyuniform temperature and thus contributes materially to the uniformity of the coating, in com:-

bination with the'other' essentials of the process as set forth'in theappendedclaims; One of the very troublesome factors in the coating of tubes ofcopper-ib'ase material has beentovercome by the presentzmethod, namely, that of having the tin-base material erode, scour, or. dissolve. portions of the copper-base material at pointsdnthetube where the temperatureis excessively high, as has occurrediniconnection with the; usesof prior processes. Ordinarily and after coating, the: very small portions ofthe tube 52,- or its equivalent, which are gripped between aairof contact-shoes, as-well as the portionsof the tube extending. outwardly therebeyond, should be severed from the finished-tubainasmuclrasthe heating and coating; of :these-parts is: ordinarily not as 1 uniform asarexthe portions oftthetube. extending between the respective pairsofcontact-shoes.

Angle of inclination of the tube Theoptimum anglewith reference to-the horizontal .has been found to vary. somewhat. with thelength of the tubes being coated. Suitable anglesofinclination areas followsfor tubes of commercially-pure. hard-drawn copper tubing:

Angle Preferred Length of 'lubc Range Angle Degrees Degrees l5 20 to 30 v 30 25 to 35 35 30 to 40 40 Speed of rotation of'the tubes The. optimum speed'of. rotation ofr a given tube isnoti markedly criticaland for. purposes of descriptionnlet it be assumedthat the tubetin question has-anv internal diameter of. .75 with a walk-thickness of about' .05". With such a tube itshas'beenfound that R. P. M. gave very satisfactory results and insuredthe uniformdistributionof the'tin-base coatingwithinthe tube, as=wel1f as. the drainage of the excesscoating from the lower. end thereof, though it may be noted that-aspeed of about R; P'. M; produces substantially the same satisfactory result.

Flaming Temperature of tubesd'lLTiflg' process The temperatures" to which the tubes are heated should'besuchasto beat or above the melting-point of the. tin-base'materiahbut not sufficiently high to anneal the tube norto materially oxidize or discolor the. outside thereof.

When employing .acommercially-puretin coating,,. it .has been-found that if the molten tin is 'maintainedat about 600 F; whenlpour'ed'into the interior'of the-tube; andrif the temperature of thetube itself is maintained withinthe ranges of about 500 =E; to about-575 Fi, .excel-lent results are achieved without occasioningthe scouring or erosion-of theinterior. of'thetubes: It may be here-5' noted that in 1 pouring the molten tin-base material into-*the-tube; such: pouring shouldbe done :under' circumstances= (well' known in the art) which will serve: toprecludethe: entrybf dross: into the tube Resistance heating of the tubes The most satisfactory and economical results have been achieved by first applying to the given tube a relatively-high current which, as before noted, may be aptly termed the heating current, followed by the application of a substantiallylower current-density which may be described as the sustaining current.

By way of example, it has been found that in coating a 21-long tube of phosphor copper having an internal diameter of about 0.75" and a wall-thickness of about 0.05, the initial heating current should amount to about 2500 amperes applied at from about 6 to 10 volts. This initial high amperage will very rapidly drop to about 2000 amperes, due to the rise in temperature and hence the increase in resistance of the tube. This initial heating current, under the conditions just described, should be maintained for about to seconds.

Following the application of the heating current to the tube, it has been found under the conditions above outlined that only about 170 amperes are required as a sustaining current to permit the full coating of the interior of the tube, and to maintain the same at a temperature above the melting-point of tin, despite heat losses due to radiation, convection, etc.

The blowing-out of the tube The gas employed for ejecting excess tin-base material from the interior of the tube and smoothing the remaining coating, is preferably in the form of a non-oxidizing gas or vapor such, for instance, as carbon dioxide, steam, or nitrogen, though, in most instances, an air blast will serve the purpose without causing material oxidation of the coating.

Time cycle It has been found that in most instances and with a properly-trained crew that tubes may be heated and coated at the rate of about one each 30 to 40 seconds without any sacrifice of quality of the coating and without appreciably oxidizing or discoloring the uncoated exterior of the tubes.

The invention may be carried out in other specific ways than those herein set forth without departing from the spirit and essential characteristics of the invention, and the present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.

We claim:

1. The process of coatin the interior of a substantially straight copper-base metal tube, with tin-base metal, comprising: heating a substantially straight copper-base metal tube to a substantially-uniform temperature substantially throughout its length between its opposite end portions, which temperature is at least as high as the melting-point of the tin-base metal with which the interior of the tube is to be coated but not in excess of about 600 F., by means of the inherent resistance of the tube to the passage of electric current longitudinally through the tube, by applying electric current to the said opposite end portions of said tube and passing said electric current longitudinally through said tube; rotating the said heated tube about its longitudinal axis with said axis in a position inclined to the horizontal with one end of said tube higher than the other end with both of said ends open; and then introducing said tinbase metal in molten condition into the said open higher end of said rotating inclined heated tube into molten alloying coating contact with the interior surface of said tube, any excess of said molten tin-base metal draining out through said open lower end.

2. The process of coating the interior of a substantially straight copper-base metal tube, with tin-base metal, comprising: heating a substantially straight copper-base metal tube to a substantially-uniform temperature substantially throughout its length between its opposite end portions, which temperature is at least as high as the melting-point of the tin-base metal with which the interior of the tube is to be coated but not in excess of about 600 F., by means of the inherent resistance of the tube to the passage of electric current longitudinally through the tube, by applying electric current to the said opposite end portions of said tube and passing said electric current longitudinally through said tube; rotatin the said heated tube about its longitudinal axis with said axis in a position inclined to the horizontal with one end of said tube higher than the other end and with both of said ends open; and then introducing said tin-base metal in molten condition into the said open higher end of said rotating inclined heated tube into molten alloying coating contact with the interior surface of said tube, any excess of said molten tin-base metal draining out through said open lower end; the electric current employed to heat said tube including an initial heating-current and a subsequent sustaining current; the said initial heating-current being more than twice as great as said subsequent sustaining current, and being sufficient and employed to heat the said tube rapidly to temperature substantially at least as high as the melting-point of the said tin-base metal; and the said subsequent sustaining current which is less than half as great as said initial heating-current being suiiicient and employed to maintain the said tube substantially at said temperature at least as high as the melting-point of said tin-base metal during the coatin process subsequent to the said initial heating operation.

DONALD K. CRAMPTON. MAURICE L. WOOD.

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

UNITED STATES PATENTS Number Name Date 811,288 Greenthaler Jan. 30, 1906 1,146,791 Greenfield July 20, 1915 1,660,597 Conkle Feb. 28, 1928 1,695,791 Yunck Dec. 18, 1928 1,818,008 Bitter et al. Aug. 11, 1931 1,997,761 Bailey et al Apr. 16, 1935 2,040,767 Dudley May 12, 1936 2,121,393 Braun June 21, 1938 2,286,194 Bradley June 16, 1942 2,319,657 Brown May 18, 1943 FOREIGN PATENTS Number Country Date 504,793 Great Britain Apr. 28, 1939

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