US2274963A - Process for plating tin and tin alloys - Google Patents

Description

March 3, 1942. w HOPPER PROCESS FOR PLATING TIN AND TIN ALLOYS Filed Aug. 10,.1958

, INVENTOR BY [Did 4R0 WfiOPfif/R ATTORN Ys Patented Mar. 3, 1942 PATENT" OFFICE PROCESS FOR PLATING TIN AND TIN ALLOYS Edward William Hopper, Midland, Pa., assignor,

by mesne assignments, to Crucible Steel Company of America, a corporation of New Jersey Application August 10, 1938, Serial No. 224,058

9 Claims.

This invention relates to method and apparatus for the production of smooth, bright, impervious and adherent tin or tin alloy coatings of uniform thickness on iron or steel sheetor strip by a process involving,electrodeposition of the tin or tin containing coating directly on the iron or steel base metal.

While the manufacture of tinplate by electrodeposition of the tin possesses advantages over the earlier practice of coating by hot dipping the base metal in a molten bath of tin, the electroplating process as hitherto practised has the disadvantage that the coating obtained is of nonuniform thickness and is lacking in brilliance and in adhesion to the iron or steel base. It has.

been sought to overcome these defects by heating the electrically coated article above the melting point of the coating, for example by immersion in a hot oil bath, of appropriate characteristics, such as palm oil, with the object of rendering the-coating bright and adherent by fusion. It has been found, however, that the molten metal contracts about numerous focal points forming raised beads or drops of the coating metal thereat separated by thinly coated or bare areas, the latter exposing the base metal. The surface thus assumes a grainy appearance like that of an orange peel. This phenomenon is not confined to fused, electroplated coatings but applies as well to coatings produced by hot dipping. The explanation of this effect as well as the variation in coating thickness, is to be found in the nonuniform character of the base metal. Although strip steel, such as is used for tinplate, is low in carbon, phosphorus, sulfur, silicon, etc., nevertheless such elements are present in sufficient amount as, in conjunction with the other impurities unavoidably present, such as slag inclusions, rolled-in scale, seams, pipes, etc., to render the surface thereof quite non-uniform from point to point as regards chemical composition whereby variations in surface tension and consequent wetting action with respect to the molten coating metal are present causing the metal to contract into beads as aforesaid, and whereby variations in surface potential are set up to render the thickness of the coating non-uniform.

In an attempt to overcome these defects various intermediate coatings of othermetals, such as copper, nickel, etc. and alloys thereof, have been applied between the base metal and the final coating of tin or tin alloy. Also it has been sequently brightened by fusion. All of these expedients are, however, unnecessarily expensive, because they involve procedures, equipment, labor and material which I have found to be unneccessary.

, With my invention coatings of tin and alloys thereof may be electrically deposited directly on iron or steel and thereafter brightened by fusion without necessity for deposition of intermediate coatings of other metals or fusion of successive coatings, such as is required by practices heretofore known.

According to my invention the base metal strip, suchas the low carbon strip steel'used for tinplate, is especially prepared and cleaned for plating by subjection to a pretreatment consisting in the successive steps of electrolytic cleaning in an alkaline bath, scrubbing in water, acid pickling, scrubbing in water, annealing or normalizing followed by a slight pinch roll on cooling, electrolytic recleaning in an alkaline cleaner, scrubbing in water, acid pickling and. a final scrubbing in water. This procedure has for its object the imparting of a clean surface to the strip containing as little foreign matter and inclusions as possible. The steps above recited preceding the annealing operation may be omitted, but the results are not so satisfactory as when these steps are included.

The strip as above prepared is now passed into an alkaline tin plating bath and plated uniformly over its entire surface. Inasmuch as an alkaline bath has better throwing power than the more commonly employed acid baths, a more uniform coating of finer particle size is thus obtained, including the surface areas aforesaidoffering greater resistance to electrolytic deposition. I have found in fact, that steel strip when cleaned and otherwise prepared in the manner above outlined, may be coated to commercial thicknesses in an alkaline bath, and thereafter brightened by fusion without formation of the grainy surface proposed to first fuse an extremely thinly deposited coating of tin alloy to the base metal, followed by deposition of a heavier coating subor orange peel" effect above referred to. If, for example, the strip so coated is passed through a bath of Hot oil, such as palm oil, maintained at a sufficiently high temperature to fuse the coating, and then through a bath of cool oil, that the hardened coating so obtained will be smooth, adherent and of high lustre approaching mirror brightness.

Inasmuch, however, as alkaline tin plating baths are difficult to maintain and are of relatively high electrical resistance and correspondingly low plating efllciency and deposition rate as compared to acid baths, I prefer to employ the alkaline bath only to apply a flash coating to the strip metal, whereupon the strip after being rinsed and possibly scrubbed, is passed into an acid plating bath for plating to the requisite thickness. In the drawing, Figs. 1, 2 and 3 show diagrammatically, progressive portions of the apparatus through which the strip to be plated is fed during the cleaning, pickling, annealing, recleaning and pickling, plating and brightening operations. Fig. 1 shows apparatus for the initial cleaning, pickling and annealing; Fig. 2 that for the recleaning, pickling and plating; and Fig. 3 the apparatus for brightening and cleaning the plated strip.

Referring now to the figures of the drawing, which show diagrammatically a preferred form of apparatus for practicing the invention, strip steel I, such as is used for tin plating, is fed from a roll thereof 2, over guide rolls, 3 to I inclusive, into an electrolytic alkaline cleaning bath 3, thence within the bath between a pair of metal plate electrodes 9, III, which may be of stainless steel or the like, and out of the bath over rolls ll, l2. The electrodes 3, II) are connected to the positive terminal of a direct current generator l3, the negative terminal of which is connected to the guide roll I, thereby to complete an electrical circuit through the bath between the steel strip l and the electrodes 9, III, whereby the strip is subjected to cathodic cleaning. I prefer to use for bath 8 a solution of sodium ortho-silicate, containing a small amount of a cresylic acid compound, the latter for improving the emulsifying properties of the bath. I may employ for example, 8 ounces of the silicate per gallon of water in the bath, and add thereto 1% by weight of the cresylic acid compound. I have found such compounds sold under the trade names of Dri-Sil and'Quaker 700-)! to be satisfactory.

After leaving the cleaning bath 8 the steel strip i is preferably fed through a scrubber [4 wherein it is scrubbed by .wire brushes l5. I3, between which and the cooperating rolls l1, la the strip is passed, in a water spray supplied from nozzles I9, 20.

On leaving the scrubber the strip is fed by rolls 2| to 24 inclusive through a pickling bath 25, in order to remove all possible rolled-in scale, rust, cake particles and other surface impurities. I preferably use in the bath 25, a hydrochloric acid or a sulfuric acid plus sodium nitrate pickle. Following this operation the strip is fed through a scrubber 26 similar to II, to remove the pickling solution and any loose and insoluble surface material. The strip is then rinsed by feeding over rolls-21 to 30 inclusive, through a water bath 3|, and is then dried by passage through a dryer 32 heated by steam pipes 33.

After drying the strip is preferably annealed or normalized in a reducing atmosphere by passage through a furnace 34, in order to relieve the eifects of cold rolling and to impart softness, deep drawing and forming properties thereto. In the event it is found desirable to apply this treatme'nt as a separate, isolated operation, the strip may be reeled up after passage through the dryer 32 and subsequently unreeled for passage through the furnace as indicated by the break in the strip at 35. Likewise the strip may be rereeled after passage through the furnace and th'ereafter unreeled for subsequent treatment as indicated by the break in the strip at 36.

After the strip has cooled following the far- 75 nace treatment, it is pinch rolled, i. e. rolled slightly thinner, by passage between polished rolls 31, to impart a fine, fiat surface finish and to improve the physical properties of the strip.

It is then cathodically recleaned by passage through an alkaline bath 38, identical in construction and arrangement with bath 8 above referred to. The strip is then scrubbed and washed in water by passage through a. scrubber 39 similar to l4, and is then fed through an acid pickling bath 40. In this bath I prefer to use a 17% hydrochloric acid solution maintained at about 190 F., the bath being of such dimensions in relation to the speed of the feed of the strip, that the strip is immersed therein for about 5 110.10 seconds. The pickled strip is then scrubbed and washed by passage through a scrubber 38 similar .strip treated by this method and followed by a brightening process in accordance with my invention, gives a better wetting or alloying action, resulting in more even coating and a smooth, mirror finish when brightened, in contrast to the orange peel and porous coatings generally secured.

The strip steel as thus treated is now passed into an alkaline tin plating bath 42, and preferably irnmersed th'erein only for a suflicient interval to impart a flash coating such, however, as to insure complete coverage and uniformity of coating over the entire steel surface. I have used the following bath, plating for 5 to 20 seconds at amperes per sq. ft. and at a temperature of about 160-180 F.:

Na-rSnOa oz./gal 33.5

NaOH.. do 2.80 NazCOa ..d0.. 0 to 10.0

The plating is effected in the usual manner by passing the strip within the bath between metal electrodes 43, 44, such as copper, having sheets 45 of the coating composition, 1. e., tin or alloy thereof aflixed thereto, these electrodes being connected to the positive terminal of a direct current generator 46, the negative terminal of which is connected to one or more of the feed rolls 4! over which the strip is fed while immersed in the bath.

After the flash coating has thus been imparted,

issimilar to that of the alkaline bath 42. For

the acid bath I prefer to use the following:

SnSO4 lr oz. /gal 20.0 H2804 ..--d0- 3 Cresylic acid do 0.3 .HzSO4-i-C16SY11C acid approx 0.8N

Hydroquinone oz./gal 0.1 Sulfonated alcohol by volume 3 The plating of this bath is carried out for about 20 to 60 seconds at about 30 to amps/sq. ft., depending on the speed of the strip, coating thickness desired, etc., and at a temperature of about 70 to 100 F.

The thus coated strip is now scrubbed and washed by passage through a scrubber 50 similar to H, and therafter rinsed in a water bath following which it is dried by passage through a steam-heated dryer 52 similar to 32.

The strip is now ready for the brightening process which is effected by passing the strip over a feed roll 53, and thence in a straight line through a suitable hot oil bath 54, for example palm oil, and through a duct 55 interconnecting the hot oil bath with a cool oil bath 56 to a feed roll 51 immersed in a cool oil bath, under which the strip is passed and fed out of this bath between a pair of squeegee rolls 58 for removing the excess oil adhering to the strip.

The hot oil bath 54, is maintained at a temper ature suiilciently high, for example, 475 to 500 F., to fuse the tin coating to a smooth, bright adherent surface; while the cool oil bath 56 is maintained'at a temperature sufiiciently low, for example 135'to 150 F., to quickly harden and solidify the coating, thereby permanently to retain the finish imparted in the hot oil bath. I have I 'found palm oil to be suitable for these baths.

The strip of course must not be permitted to pass over feed rolls or into contact with other hard objects whilethe coating is in the plastic or molten condition resulting from immersion in the hot oil bath, because this would produce imprints of the rolls, etc. in the coating and thus mar the finish. Likewise the coating must be cooled rapidly in a non-oxidizing atmosphere to retain its brilliance and freedom from oxidation and crystallization or spangling of the surface such as would otherwise occur, This necessitates maintaining the hot and cool oil baths in liquid contact so that the strip may be passed directly from one to the other without passage over rolls or being exposed to the atmosphere. This is ac-.

complished-by means of the duct 55 which extends downwardly from the hot oil to the cool oil bath to such adegree that its entry 59 into the cool oil bath is situated entirely below its entry .60 into the hot oil bath. As is more fully dethe strip then passing out of bath 63 and between a pair of squeegee rolls 1 l, for removing the excess solvent, and between steam-heated plates 12 of an evaporator 13 for drying the strip is reeled'up as indicated at 14 after passage over guide rolls 15.

The baths are so arranged as to provide a flow of the cleaning liquid between them in the direction opposite to that in which the strip is fed. This is accomplished by feeding purified cleaner, preferably continuously, into bath 63 from a supply line 16, a corresponding portion of the liquid in bath 63 in'turn flowing into bath 62 through and-around the felt members 59. A like portion of the liquid in bath 62 in turn flows into bath 6| over the low barrier 11 separating these baths, while a corresponding portion of the liquid in bath 6| flows into bath 60 through and around the felt members 65. Bath 60 in turn is provided with an outlet pipe 18 by means of which a corresponding portion of the liquid in this bath is drawn oif,- preferably continuously. As a result of this arrangement, the strip is given an exceptionally efiicient cleaning in passing through the baths, inasmuch as the'most contaminated cleaning' liquid accumulates in bath 60 which the uncleaned strip first enters, whereby the principal portion of the grease, etc., thereon is removed, the strip thereafter passing into progressively cleaner baths as the stripitself becomes progressively cleaner. The novel features of these baths and the associated evaporator 13 are described and claimed in copending application Serial No.

scribed Patents 2,141,382 and 2,192,303, an inter- I connecting duct so arranged, provides an effective oil seal between .the hot and cool oil baths 224,100, filed August 10, 1938.

What I claim is: v

1. The method of producing smooth, bright adherent coatings of tin and alloys predominantly of tin, on iron and steel strip, which comprises: electrolytically depositing a thin coating of the coating material on said strip from an alkaline plating bath, superimposing thereon a thicker coatingof said material electrolytically deposited from an acid plating bath, immersing the so-coated strip in a hot bath of fatty acid containing, non-oxidizing oil to fuse and brighten said coating, and thereafter immersing said strip in a bath of cool non-oxidizing oil to harden said 4 coating in said brightened condition.

such as substantially to prevent thermal .ex-'-" from nozzles 55a at its entrance into the coql oil=-"" baths, and drawing oil oil at an intermediate point between the baths, asby means of the outlet pipe 55b.

The-strip on emerging from the squeegee rolls adhering thereto and must, therefore, be given a careful cleaning before it is ready for use. This 'is accomplished by passing the strip through a succession of baths 60 to 63 inclusive, containing a liquid solvent, such as carbon tetrachloride. The strip enters bath 60, passing under a feed roll 64 2. The method of producing smooth, bright,

"adherent coatings of tin and alloys predominantly of tin, on iron and.steel strip, which comprises: electrolytically, depositing a thin coating of the coating material on said strip from an alkaline plating bath, superimposing thereon a thicker coating of said material electrolytically deposited from an acid plating bath, and immersing the so-coated strip in a hot bath of a fatty acid containing, non-oxidizing oil to fuse and brighten said coating.

3. The continuous method .of producing smooth, bright, adherent coatings of tin and al- 58, still has considerable oil and other impurities loys predominantly of tin, on iron and steel strip,

which comprises: progressively feeding said strip and between a pair of felt or like members 65 dis- "posed in an orifice of a partition 66 separating baths B0 and 6|, the strip thus passing by means of the feed rolls 68 out of bath BI and into bath 62, thence from bath 62 into bath 63 between a pair of felt or like members 59 disposed in an orifice in a partition I0 separating baths 62 and 63,

through an alkaline plating bath to deposit a thin coating of the coating material thereon, feeding the strip thence through an acid plating bath to superimpose thereon a thicker coating of the coating material, and feeding the strip thence through a hot bath of fatty acid containing nonoxidizing oil to fuse and brighten said coating.

4. The continuous method of producing smooth, bright, adherent coatings of tin and alloys predominantly of tin, on iron and steel strip, which comprises: progressively feeding said strip through an alkaline plating bath to deposit a thin coating of the coating material thereon, feeding the strip thence through an acid plating bath to superimpose thereon a thicker coating of the coating material, feeding the strip thence through a hot bath of fatty acid containing, nonoxidizing oil, to fuse and brighten the coating, and feeding the strip thence, under non-oxidizing conditions and free from mechanical contact, into a bath of a cool, non-oxidizing oil to harden said'coating in said brightened condition.

5. The. continuous method of producing smooth, bright, adherent coatings of tin and alloys predominantly of tin, on iron and steel strip, which comprises: progressively feeding a single thicknessof said strip through a normalizing furnace to anneal the same, thence cleaning the strip electrolytically in alkaline solution followed by acid pickling, feeding the strip thence through an alkaline plating bath to deposit a thin coating of the coating material thereon, feeding the strip thence through an acid plating bath to superimpose thereon a thicker coating of the coating material, feeding the strip thence through a hot bath of fatty acid containing non-oxidizing oil to fuse and brighten the coating, and feeding the strip thence, under non-oxidizing conditions and free from mechanical contact, into a nonoxidizing bath of a cool oil, thereby to harden said coating in said brightened condition.

6. The method of producing smooth, bright, adherent coatings of tin and alloys predominantly of tin,'on iron and steel strip, which comprises: annealing the strip and electrolytically cleaning inlalkaline solution, acid pickling the strip, clectrolytlcally depositing thereon a thin coating of the coating material from an alkaline platlngbath, superimposing thereon a thicker coating of said material electrolytically deposited from an acid plating bath, immersing the socoated strip in a hot bath of fatty acid containing non-oxidizing oil to fuse and brighten said coating and thereafter immersing said strip in a bath of cool, non-oxidizing oil to harden saidv immersing said strip in a cool bath of non-oxielectroplated from an acid plating bath directly onto the coating plated from said alkaline plating bath.

9. The method of producing smooth, adherent coatings of tin and alloys predominantly of tin,

on a ferrous base metal, which comprises: electrolytically depositing a thin coating of the coating material on said ferrous base metal from an alkaline plating bath, and superimposing thereon an overcoating of said coating material electrolytically deposited from an acid plating bath.

EDWARD WILLIAM HOPPER.

- Patent No; 23714365- CERTIFICATE-OF CORRECTION.

March 5, 1914.2.

EDJARD WILLIAM HOPPER..

"It is hereby certified that error appears in the prin sp cific' tion of the above. numberedpatent requiring correction as follows: Page 3, sec 0nd column, line 1;, e fterfstrip'f insert following which the strip;

' and that the acid. Letters Patent shouldbe read with this correction therein that tbe same may conform to the record of the case in the Patent Office.

Signed and sealed this 21st day of April, A. n. 19h2.

. Henry Von Airedale,-

xs I Actingcommissioner of Patents.

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