US2775535A - Treatment of tinplate - Google Patents

Description

United States Patent TREATMENT OF TINPLATE William David Poole, Baltimore, Md., assignor to Bethlehem Steel Company, a corporation of Pennsylvania No Drawing. Application March 14, 1952, Serial No. 276,693

5 Claims.- (Cl. 148-63.)

This invention relates to an improved method of treating electrolytic tinplate surfaces, and more particularly to a method of cleaning and modified passivating of electrolytic tinplate produced in continuous strip form Which may, before treating, have upon its surface deleterious products'formed during, and in some cases prior to, the

enamels, and particularly phenolic type enamels, will satisfactorily adhere, not only at the time of formation, but after extended storage periods as Well.

Another object is to prevent yellow discoloration'of the tinplated surface during baking operations.

'In the manufacture of continuous electrolytic tinplate in strip form it is common practice to pass the tinplated strip thru a tin-fusing furnace. Fusing of the coating has a manifold purpose, chief of which is the production of a smooth surface having a bright lustre, and one which is less susceptible to abrasion marring. When the furnace atmosphere surrounding the plated strip is of a non-protective nature, objectionable surface products may develop thru contact of the furnace atmosphere with the strip. In some cases fluxes such as ammonium, zinc or hydrogen chlorides are used prior to, or during the fusing step. When a flux is used so as to enhance the surface lustre of the tinplate after fusion, the flux itself may produce a film or minute spots on the surface of the tin plate which are objectionable if they are not thoroughly removed.

In addition to the foregoing means by which deleterious substances may form on tinplate, oxides or metal salts formed during the pickling or electroplating operations, and present as minute surface residues on the steel surface prior to plating, have a tendency to come to the surface of the plated metal during the fusing operation.

Much of the tinplate used for cans requires a coating of enamel, and one of the commonly used enamels is the thermo-setting phenolic type. The phenolic type of enamel is very sensitive, and will not adhere satisfactorily to a tinplate surface which has a significant quantity of extraneous substances such as oxidation products upon it. n

I In some cases, especially when the tinplating and tinfusing are performed as separate non-continuous steps, it is desirable to remove from the plated strip, before fusing, products of oxidation orhydrolysis formed in the interval between plating and fusing, as well as traces of difficultly waterrinsable substances from theelectroplating bath. Such cleaning may be accomplished by treating the tinplatedstrip in a cathodic alkaline cleaning bath of a solution such as trisodium phosphate or sodium carbonate or 2,775,535 Patented Dec. 25, 1956 ice other suitable solutions of sodium or potassium salts, prior topassing the strip thru a fusion furnace.

In processing the strip, as has been previously explained, certain detrimental impurities may accumulate on the strip. In order to insure an adequately clean and an adequately passivated tinplated strip, it is desirable, and for most purposes necessary, to apply to the tinplate a suitable treatment after fusing. To obtain good enamel adherence, and other desirable surface qualities, after a fusing operation in which deleterious products may occur on the tinplate, it is required that a suitable surface treatment be employed. It is necessary not only to remove the deleterious products from the tinplate after fusing, but it is essential that some passivating treatment be applied also, so that oxidation and its concomitant,- oxide staining, are prevented, or sufficiently retarded, while the tinplate is in storage prior to enameling.

Various methods of treating tinplate after fusion to insure good enamel adherence have been tried with indifferent success. I have found that when electrolytic tinplated strip is cleaned cathodically in a dilute alkaline solution just prior to fusing, and the strip is then passed thru a fusing furnace, if the strip is given a sequential treatment of cathodic cleaning in a dilute alkaline solution and rinsed in a dilute chromic acid solution, good adherence of enamels, including the thermo-setting phenolic type, is obtained, and surface oxidation is adequately retarded, even when the strip has been in storage for relatively long periods of time prior to enameling.

Because of the sensitivity of the phenolic enamels it is necessary not only to select the proper solutions for postfusion treatment, but I find it mandatory to use the two stage post-fusing treatment as herein described to obtain the optimum surface conditions. As a practical matter, when treating continuous strip, it is obvious that treating time must be reduced to a minimum so as not to impede the progress of the strip thru the entire sequence of finishing operations. Temperature and concentration of the solutions must be controlled within reasonable limits.

dilute alkaline solution such as sodium carbonate, rinsed with Water and then passed thru a dilute chromic acid solution. The strip may then be rinsed in water and dried and oiled in any conventional manner.

Bythe. above method the strip surface is denuded of impurities accumulated from the plating and/or by oxidation products formed after plating and prior to fusing, impurities which might otherwise produce an unfavorable reaction'in-the elevated temperature conditions of the fusion furnace, the post-fusion cleaningsubsequently removing any deleterious products that may be produced on the strip while in-the furnace and due to the accumulation on the tin surface of impurities originally on the steel surface, orjfrom fluxes used in conjunction with fusing. By Water rinsing the strip, as it leaves the postfusioncleaner, the strip enters the chromic acid rinse with an adequately cleaned tin surface. The chromic acid, in the dilute concentration and short time intervals used, tends to render a partial passivation of the tin surface so that the tinplate may be stored, even in moist atmospheres, for commercially employed periods of time 3 without the development of an undesirable degree of surface oxidation. It has been found that complete passivation of the tinplate tends to render a poor bond between the tinplated surface and certain enamels, therefore, the partial, or modified, passivation of the tin surface is necessary in order to provide cleaned tinplate whichis receptive tophenolic enamels, and so that application of the enamel results in a good bond'with the tinned surface. Furthermore, the clean, partially passivated tinplate does not'develop, in subsequent baking operations,.the yellow oxide stain noticeable on improperly cleaned tinplate.

While sodium carbonate solution has produced good results in both the pre-fusion cleaner and the post-fusion cleaner, a solution of any alkaline compound may be used which functions primarily to release appreciable quantities of sodium or potassium ions at the cathode upon electrolysis. The pH of the solution immediately adjacent the cathodically treated metal surface must be above 7, in order to obtain a properly cleaned tin surface. However, I have found it desirable to maintain the pH of the solution above 9. Other compounds, or mixtures of the same, which may be used are sodium hydroxide, tri-sodium phosphate, potassium carbonate, potassium hydroxide, sodium bicarbonate, mono-sodium hydrogen phosphate, di-sodium hydrogen phosphate and potassium bicarbonate. The concentration of the alkaline solutions should be kept quite low, in the range of from 0.5% to 5.5%, preferably 1% to 3%. In the pre-cleaner, as in the post-cleaner, the current density may range between 3 and 30 amp./sq."ft., while the temperature should range between 160 F. and 210 F. Excellent results have been obtained when the current density has been maintained at approximately amp/sq. ft. at a temperature of around 180 F. The time required for cleaning in either the pre-cleaner or the post-cleaner should be, for practical operation, from 0.5 to 3 seconds, although longer periods of operating time are permissible.

In partially passivating the electrotinplated strip by immersing the strip in aqueous chromic acid solution 1 after the post-fusion cleaning operation, it is preferable to use a chromic acid (CrOs) concentration of from about 0.1% to 1.0%, if the strip is subsequently rinsed with water. Generally, the immersion time for the chromic acid treatment is substantially the same as for post-cleaning.

Another method of obtaining a tinplated strip surface having the proper degree of passivation is that of first immersing the strip in 0.1% to 1.0% chromic acid and then passing the strip thru a second, dilute, chromic acid bath of less than 0.1% concentration, with no further rinsing.

A third alternative for chromic acid immersion is that of a one-step immersion in chromic acid with no further rinsing, wherein chromic acid concentration is less than 0.1%.

The first method of partially passivating the electrotinplated strip does not employ a definite surface residual of chromic acid. This method permits optimum phenolic adhesion at some sacrifice in degree of surface passivity attained. The second and third methods employ minute surface residuals of chromic acid which accomplish an increased degree of surface passivity but at some sacrifice in phenolic enamel adhesion quality.

For certain types of tinplate products, such as milk cans, where final coating operations, i. e., enameling, etc., are not applied to the tinplate, and thus, the passivating chromatic acid dip is unnecessary, my invention can be used' as a three-step process, wherein the electroplated strip is cathodically cleaned in alkaline solution, passed thru the tin-fusing step and then cathodically cleaned in alkaline solution under the controlled conditions previously described. In a situation where pro-fusion cleaning is considered unnecessary, as may be the case when electroplating and fusion are performed as continuous in-line operations, my invention may be limited to a two-step process, i. e., the tinplate is fused and then cleaned cathodically in alkaline solution under controlled conditions.

I claim:

1. In a method for treating electrolytic tinplate in strip form the steps comprising heating strip to a temperature above the melting point of tin, cleaning the strip cathodically in an aqueous alkaline solution of sodium carbonate in which the concentration of alkaline compound is approximately 0.5% to 5.5% at a current density of from 3 to 30 .amp./ sq. ft. and at a temperature of from 160 F. to 210 F. for a period of time in excess of 0.5 second, rinsing the strip with water and then passing the strip thru a chromic acid solution having a CrO concentration of from 0.1% to 1.0% at a temperature of from F. to 200 F., and rinsing the strip.

2. In a method for treating electrolytic tinplate in strip form the steps comprising heating strip to a temperature above the melting point of tin, cleaning the strip cathodically in an aqueous alkaline solution of trisodium phosphate in which the concentration of alkaline compound is approximately 0.5% to 5.5% at a current density of from 3 to 30 amp./ sq. ft. and at a temperature of from F. to 210 F. for a period of time in excess of 0.5 second, rinsing the strip with water and then passing the strip thru a chromic acid solution having a C10,; concentration of from 0.1% to 1.0% at a temperature of from 150 F. to 200 F., and rinsing the strip.

3. In a method for treating electrolytic tinplate in strip form the steps comprising heating the strip to a temperature above the melting point of tin, cleaning the strip cathodically in an aqueous alkaline solution formed from an alkali metal compound of the group consisting of sodium carbonate, sodium hydroxide, trisodium phosphate, sodium bicarbonate, monosodium hydrogen phosphate, di-sodium hydrogen phosphate, potassium bicarbonate, potassium hydroxide, potassium carbonate and mixtures thereof in which the concentration of alkali metal compound is approximately 0.5% to 5.5% at a current density of from 3 to 30 amp/sq. ft. and at a temperature of from 160 F. to 210 F. for a period of time in excess of 0.5 second, rinsing the strip with water and then passing the strip through a chromic acid solution having a C1O3 concentration of from 0.1% to 1.0% and rinsing the strip.

4. In a method for treating steel having thereon a fused coating of electrolytically deposited tin, the steps comprising cleaning the material cathodically in a bath consisting essentially of an aqueous solution of a compound of the group consisting of sodium carbonate, sodium hydroxide, trisodium phosphate, sodium bicarbonate, mono-sodium hydrogen phosphate, di-sodium hydrogen phosphate, potassium bicarbonate, potassium hydroxide, potassium carbonate and mixtures thereof, in which the concentration of such compound is between 0.5% and 5.5 rinsing the material with water, passing the strip thru a chromic acid solution having a CrOa concentration between 0.1% and 1.0% and then rinsing the strip with Water.

5. The method of treating electrolytic tinplate in strip form wherein the tin coating of the tinplate has been fused, which comprises cleaning the strip cathodically in an aqueous alkaline solution formed from an alkali metal compound of the group consisting of sodium carbonate, sodium hydroxide, trisodium phosphate, sodium bicarbonate, monosodium hydrogen phosphate, di-sodium hydrogen phosphate, potassium bicarbonate, potassium hydroxide, potassium carbonate and mixtures thereof in which the concentration of alkali metal compound is from approximately 0.5% to 5.5% at a current density of from 3 to 30 amp./ sq. ft. and at a temperature of from 160 F. to 210 F. for a period of time not less than 0.5 second, rinsing the strip with water and then passing the strip thru a chromic acid solution having a CrOs concentration of from 0.1% to 1.0% and rinsing the strip.

References Cited in the file of this patent UNITED STATES PATENTS 2,240,265 Nachtman Apr. 29, 1941 2,314,818 Cook et a1 Mar. 23, 1943 2,410,213 Herro et a1. Oct. 29, 1946 2,450,509 Glock Oct. 5, 1948 6 OTHER REFERENCES 5 through 265, only pages 259 and 260 relied on.

Improved Chromating Process, by S. E. Maxon, published Dec. 13, 1943, in Steel, pages 134-136.

Claims (1)

1. IN A METHOD FOR TREATING ELECTROLYTIC TINPLATE IN STRIP FORM THE STEPS COMPRISING HEATING STRIP TO A TEMPERATURE ABOVE THE MELTING POINT OF TIN, CLEANING THE STRIP CATHODICALLY IN AN AQUEOUS ALKALINE SOLUTION OF SODIUM CARBONATE IN WHICH THE CONCENTRATION OF ALKALINE COMPOUND IS APPROXIMATELY 0.5% TO 5.5% AT A CURRENT DENSITY OF FROM 3 TO 30 AMP./SQ. FT. AND AT A TEMPERATURE OF FROM 160* F. TO 210* F. FOR A PERIOD OF TIME IN EXCESS OF 0.5 SECOND, RINSING THE STRIP WITH WATER AND THEN PASSING THE STRIP THRU A CHROMIC ACID SOLUTION HAVING A CRO3 CONCENTRATION OF FROM 0.1% TO 1.0% AT A TEMPERATURE OF FROM 150* F. TO 200* F., AND RINSING THE STRIP.