US2304069A - Metal coating process - Google Patents

Description

De@ 8,. 1942- G. J. BEcKwrrH 2,304,069

METAL COATING PROCESS Filed DeC. 14, 1938 l HOT- (Quer/c I `\G 4L D" e 4M? (00m-E PatentedtDec. 8, 1942 l y UNITED STATE METAL COATING PROCESS Glenwood J. Beckwith, Brecksville, Ohio, assignor to The American Steel and Wire Company 4of New Jersey, a corporation of New Jersey Application Deember 14, 193s, serial No. 245,155

2 Claims. (Cl

The invention relates primarily to the production of strongly adherent coatings upon articles.

An object of the present invention relates to the bonding of hot-tinned coatings to ferrous steel strip, wire and like material.

There are many industrial applications for tin-4 coated steel articles that require that the tin be tenaciously bonded to the steel. For instance, there is a need in the bearing'industry for steel strip having tightly adherent -tin coatings. Upon the tin there is applied a layer of Babbitt metal. The babbitt is usually poured or extruded onto the tinned strip in a fused condition, and, upon freezing, is found to have become coalesced with the tin-coat, at least at the adjoining surfaces there- Under the best of circumstances, it is a di-iiicult job to establish a strong bond between steel and` tin, whether the latter be a hot-dipped coating, or an electro-deposited coating. Even more difficult -does this job become in the case of tinning hard, cold-rolled steel strip, which is what is required by the bearing manufacturers for the babbitting operation described above.

' The present invention relates to a method of producing a highly tenacious, tight bond between a tin coating and ferrous stock by means of an etch, in combination with several other steps,

which not only completely and consistently produces stock capable of meeting the bearing manufacturers standards and tests but which also may be practiced continuously and inexpensively.

In the drawing, there is illustrated conventional equipment capable of being used for carrying out the method of the present invention.

Generally, the present invention contemplates continuously cleaning, etching, iiuxing, hot-tinnlng, and tightly wiping under a pressure sufficient to stimulate a bondingl action between the coating and the base metal, in rapid succession, wire. strip, or other elongated metal stock, to produce a tightly adherent continuous coat of tin thereon. The cleaning I prefer to carry out in `a hot caustic solution, which is preferably electrolyzed by the application of outside current so thence on to the take-up that the work represents the cathode and the cell the anode. This may take the form of any conventional, caustic cleaning baths commonly referred to as degreasers. The etching is carried out preferably by the use of a nitric acid solution, which for continued, undiminished effectiveness is energized electrically with the work on the anodic side, and the cell on the cathodic side of the circuit. The fluxing is a strongly acidulated wash of muriatic acid and water approximating a pickling bath in strength and function.' It is necessary that this bath be more acid than is usual' in such fluxes in order to remove the oxide film that remains on the work after immersion in the nitric bath. The stock' is then absolutely In the drawing, Fig. 1 diagrammatically illusl trates a portion of a continuous coating apparatus employed in the practice of my invention, and Fig. 2 is a continuation thereof from the line :zt-rn, the -passage of the material being coated being from'right to left as indicated by the arrows.

Referring more particularly to the accompanyferrous metal stock,

ing drawing, the elongated such as strip steel, is fed continuously from reels X and successively introduced to the hot caustic bath A, the cold-'water rinse B, the electro-etching cell C, the water rinse D, the acid iiux or pickle E, and vthe molten -tin bath F. Upon leaving the bath' F, the coated material passes through pressure wipers maintaining a suiiicient pressure to effect a bonding between the tin coating and the ferrous base metal, aslat G, from whence it may pass through a cooling zone H, a final rinse J, blocks M through the compensating pull-out device L.

A metal container is provided to retain the caustic bath, which is usually composed of a. solution of such caustic salts as sodium hydroxide (NaOH), -trisodium phosphate (Naai-04), combinations of these, or many others of like nature.- A hot solution of this kind may be used effectively, However, l. have found it to be advantageous to electrolyzel the solution with the introduction of outside current by attaching the container to the anodic lead, and the work a D. C. generator. The ensuing electrolysis causes the deposition of free alkali on the strip, and

' greatly accelerates the evolution of hydrogen, and

is thought, for these reasons, to cleanse and scrub the work effectually. At any rate the work leaves the hot caustic bath thoroughly cleansed, and, of no less importance is the fact that the cleansing is accomplished by an immersion of but a few seconds which renders such a method of, cleansing ideally adapted to a continuous process.

In passing on to the electro-etching bath C, the Work is wiped and rinsed with water to prevent the pollution of succeeding baths by the liquid of preceding baths that becomes entrained on the moving work.

The. electro-etching step not only thoroughly and deeply etches the work passing therethrough but does it uniformly and continuously, within the space and time .restrictions growing from contin uous operations. Also, the etch must be effected' to the cathodic leadof between the coating and base metals, winch characteristics cannot be readily and quickly dispensed with by some treatment ensuing in the continuous coating train.

The acid bath comprises a nitric acid solution diluted to about a to 10% concentration in water. Such an electrolyte may be made by acidulating water directly by the admixing of HNOa therewith, or may be made by dissolving any of the salts of HNOa in water, such, for example, as ammonius nitrate. Ammonium persulphate has also been employed satisfactorily, as have combinations of these salts, and other combinations; e. g., H2SO4+NH03; HCl-i-NHOs, etc., so long as the nitric content is present. However, these latter combinations, when electrolyzed, though satisfactory from the standpoint of the results produced on the work, give oiI so much nitrous oxide (NO2) gas as to render their use quite objectionablefrom the operators standpoint due to the poisonous fumes.

A properly regulated bath of nitric acid solution (preferably a 7% concentration), if either energized by the application of outside current with the work in circuit as the anode, or else maintained hot (175 F. more or less) at slightly movement be interrupted. If desired, an acidulated zinc chloride solution may be used in the pickling bath and when such a solution is used, the acidulation is effected by the muriatic acid of a concentration of to 25%.

'I'he work. now having been brought to the state of greatest affinity for the coating metal,

higher concentrations when not electrically energized, will perform the requisite etching in seven (7) seconds. This means that the stock may be passed through a nine foot (9') eighty feet (80') a minute.

I have found that the use of the etching solution without the application of electrolyzing current is unsatisfactory in that after it has been in use a short while, the acidity drops off to a point where the solution is too weak to etch effectually. I havev also found that the application of electrical energy to such a bath, with the work in the anodic circuit, will render it effective as an etching medium even after the concentration pan at about has fallen to approximately 3%, where it would be inefiicacious without the application of outside current. .'I'herei'ore, it is obviously essential, in the interests of uniformityof results and continuity of operation, that the nitric acid etch be electrically energized properly to fulll its functions in a continuous process.

As actually applied, a 7% nitric acid solution is provided with cathode terminals affixed to the container, if this be a conductor, or, otherwise, immersed in the bath. In thelatter eventuality, stainless steel plates, due to their resistivity to attack by the nitric acid, are advantageously employed as the .cathodes The work is introduced to the bath carrying the anodic side o1' the current (6 volts D. C.-50 to 100 amperes per square foot), and the electrolyte solution is carried at a temperature up to 175 F., to expedite the etching operation by accelerating the rate of oxidation of the work.

After leaving the electro-etching cell, through which-it passes in seven (7) seconds or less, the work is again wiped and washed, as by a water spray D, and continues on without pause to the flux or pickling bath E.

The pickling bath comprises a 10% to 20% solution of muriatic acid, which, by virtue of the heavy oxide lm remaining on the surface of the work as a result of the nitric acid etch, is of higher concentration than ordinary flux baths of this type in o rder that the oxide film may be quickly and completely removed. The action of this bath is such as to render the work bright and clean without necessitating that its continuous is immersed in a bath F, of molten tin, through which it is moved for the usual length of immersion to acquire a good coating of tin. It is then removed and passed through a pressure wipe G where all appreciable thickness of coating is removed.

'I'his pressure wipe is designated not only to.

remove all surplusage of tin, but, incident thereto, by exerting several hundred pounds a square inch of coated surface, to work and set the coating well into the surface interstices or pores of the base metal, to which it becomes irremovably bonded thereby. Under certain conditions, the pressure may vary from about 100 up to 1000 pounds per square inch. Under such a treatment, the material is provided -with a tin coating of .08 to .09 ounce of tin per square foot of the base metal.

Under the present method, a greater weight of coating for the same overall diameter of wire, or thickness of strip, is produced upon the surface of the material as well as forming a bond between the tin coating and the base metal. The nitric etch penetrates the steel in such a way as to expose thev iron crystals, and to leave them for presentation to the molten tin in this condition.I At any rate, a bond, be it physical, chemical, or physico-chemical, results which will meet not only the rigorous tests of the bearing manufacturers but other trade and laboratory tests del vised for this purpose.

While I have shown and described a specific embodiment of thepresent invention, it willv be seen that I do not wish to be limited exactly thereto since various modifications may be made without departing from the scope of this invention, as defined by the following claims.

1. In the herein described continuous process of hot tinning cold rolled steel strip, the steps including cathodically cleaning the strip in a hot caustic solution, rinsing the cleaned strip, anodically etching the strip ina nitric acid bath of 3% to 10% concentration at a temperature up to about 175 F. and at a current density of from about to 100 amperes per square foot,

deoxidizing the etched strip in a iiux containing muriatic acid, coating the deoxidized strip with molten tin, and thenY tightly frictionally wiping the coated strip under an applied pressure of' .from about 100 to 1000 pounds per square inch.

2. The herein described continuous process oi' hot tinning cold. rolled steel strip which comprises cathodically cleaning the strip in a bath of hot caustic, rinsing the cleaned strip, an-

odically etching the strip in 7% nitric acid at a temperature up to about 175 F. for approxicoated strip under an applied pressure of from about to 1000l pounds per square inch, air

cooling the wiped strip, quenching in water, and

then dryingA the finished strip.-

GLENWOOD J. BECKWITH.

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