US2314818A - Surface treatment of tinned material - Google Patents

Description

Patented Mar; 23, 1943 UNITED STATES PATENT OFFICE Willard 0. Cook and Hugh E. Romine, Pittsburgh,

Pa, assignors to Carnegie-Illinois Steel Corporation, a corporation of New Jersey No Drawing. Application April 29, 1939,

' Serial No. 270,912

3 Claims, (01. 204-140).

The present invention relates to' the surface treatment of tinned material to produce a tin-' tected immediately after the tinning operation,

but appears after the sheets have been in storage for a period of time or upon baking during lithographing or enameling. In brief, the shortcoming is a discoloration of the surface, generally golden yellow in color, and has been termed yellow stain. Yellow stain does not render the plate unsuitable for food containers, but is objectionable from an appearance standpoint especially after an enameling or lithographing operation, and interferes with the soldering operation in the manufacture of tinned containers.

An object of the present invention is to provide a uniform impermeable superficial oxide film upon'the surfaces of tinned material so as thereafter to render the surfaces substantially. free from yellow stain during storage or subsequent treatment, such as baking during lacquering and lithographing, etc.

Another related object of the present invention is to produce a tin-coated article of manufacture whose surface is substantially immune to yellow stain.

It has been broadly accepted scientifically that yellow stain is caused by the presence on the surface of the tinned material of an oxide film and since the defect is not detectable immediately after tinning, the development of the film is progressive. In other words, oxidation of the tinned surface takes place following tinning operations, and an increasingly thick oxide film is developed on the surface which film becomes visible, i. e. results in a yellow stain, when 1 its thickness is sufiicient to impart color to light reflected by the'surface.

The first appearance of yellow stain is therenot only on the chemical composition of the film, but also onits thickness and uniformity and its physical structure (porosity, continuity, integrity, etc.)

Further, the microscopic structural nature of a film of given chemical composition is known to vary with the conditions existing during the formation of the film, for example, oxide films formed at high temperature, are in general more porous and permeable than those formed at a low temperature (for example, room temperature). The conditions existing during the formation of the initial film onv the surface of the tin plate, which film must be formed and is presumably an oxide film if the accepted version of the cause of yellow stain is correct, are therefore, of primary importance, and it is our opinion that owing to the temperature at which tin plate is produced, a more or less discontinuous and porous oxide film is formed on the tin surface as the plate leaves the tin pot and that the character of this film permits oxidation to proceed relatively unimpeded and is therefore responsible for the subsequent appearance of yellow stain.

Under the present invention, this surface condition is remedied and the product of the tinning operation is materially improved. The invention comprises treating tinned material, whose surface has been substantially freed from oil and grease by the wet cleaning step conventionally used in the tinning operation, so as to remove any initial oxide film present on the surface as a result of the tinning operation and produce a virgin" metallic tin surface which upon exposure, at a relatively low temperature, to air, preferably pure air relatively free from obnoxious fumes or gases, spontaneously oxidizes to produce a superficialv oxide film of such compactness, uniformity and impermeability that further oxidation is retarded. In other words, the tin surface is rendered passive in that it is provided with a protective film which acts as a barrier to retard further oxidation and such a surface,.we have found, will not developyellow stain" under normal storage,

conditions nor under normal baking temperatures.

Under .the present invention, theremoval of the initial oxide film present on the surfaces of the tin plate may be accomplished by any one of several treatments; for example, by subjecting the tinned material to the action of a solution in which the oxide is soluble, or by subjecting the tinned material as a cathode to an electrolytic action in a suitable solution. The latter method is preferred as its action is more readily conmetallic tin itself by the solution during the removal treatment. i

In practicing the present invention, in its preferred form, the primary motion of the electrolyte is to enable current to pass through the treating solution under a reasonable impressed voltage. The solution in which the tinned material is treated must contain inherent properties capable of producing solely hydrogen at'the surface of thetinned material when supplied with the necessary amount of current needed toeffect the electrolysis. Any one of a number of com,- poundsare suitable electrolytes for the present urposes. For example, a desirable solution may contain one or more-of the following alkaline electrolytes; soda ash, caustic soda, borax, sodium phosphates, any of the alkaline silicates or many similar materials. It is also possible to use solutions of various neutral salts. For example, sodium chloride, sulphate, nitrate, chromate, and dichromate solutions have been found to give satisfactory results, In general, acid substances are unsatisfactory since they etch the tin.

While various electrolytes are hereinbefore set forth as capable of being used to prepare a suitable solution, ithas been found that the best results are obtained when using a soluble alkaline silicate. Since the composition of soluble silicates is not bound by the limits implied by the formula of definite chemical compounds which may be separated and studied, but are generally defined on the basis of a molecular ratio of alkali to silica and therefore, in a dilute solution, any ratio may exist. For practical purposes, we have set limiting ratios of Nero to $10: to define a group of these materials which will be suitable for preparing a solution for the treatment as follows:

1. Maximum silica contents:

As the ratio of NazO to S10: decreases, the conductivity of a silicate solution decreases, the solubility decreases and the tendency to precipitate silicious gels during electrolysis increases. Therefore, the ratio No.20 to S10: of 1:2 can be set as the upper limit of silica content. This ratio corresponds to the chemical formula NaaSia-Os, sodium di-silicate, commonly called water glass.

2. Maximum alkali content:

Since a dilute straight caustic soda, NaOH, solution gives satisfactory results, the specification of an upper limit of the NazO content of the silicate used is not necessary.

While any soluble silicate within the Naaozsioz ratio limits of 1:0 to 1:2 can be used, we prefer:

= 1. Sodium meta-silicate, molecular ratio Na20:SiOz 1:1

trolled and there is less danger ofattack of'the tin tin plate) the time of treatment used in the present method is so short that, provided adequate rinsing facilities are available, etching of the tinned surface will not be encountered. Since, however, concentrated solutions are not only costly to prepare but also to operate owing to high drag-out loss, it does not app al economical to exceed a 10 per cent. (by weight) concentration. v

Concerning minimum concentration of the electrolyte, it suiiicies to say that our treatment has been successfully carried out using 0.05 per cent. (by weight) solution of both sodium meta and ortho-silicate. Such dilute solutions, however, have high electrical resistivity and we prefer to use some intermediate concentration. As examples of preferred composition, any of the following water solutions, individually, are given:

Per cent. (byweight) Sodium ortho-silicate s. 0.5 Sodium meta-silicate 0.5 Sodium hydroinde (caustic soda) 1.0 Sodium carbonate (soda ash) 1.0 Tri-sodium phosphate 5.0 Sodium borate (borax) 10.0 Sodium di-silicate (water glass) ...1 10.0 Neutral salt, such as sodium chloride, sulphate, nitrate, chromate, etc 1.0

It is to be understood thatothersalts than those of sodium can be used and that various combinations of the previously specified materials can be made. For. example, a small amount of caustic soda might be added to phosphate,

borax or sodium di-silicat'e solutions to improve theirconductivity. As an example of a solution containing more than one ingredient, such may consist of an alkaline solution containing 150 grams of tri-sodium phosphate, 50 grams of sodium carbonate and 20 grams of caustic soda per liter of water. Such proportions will provide a solution having high conductivity and excellent characteristics as a treating vmedium. However, such a solutionhas been foimd to be more costly than is needed for the results desired and therefore, from an economical standpoint, is not desirable in the aforementioned solutions.

While we have hereinabove set forth specific solution concentrations, such concentrations are not a critical factor. It is to be expressly imderstood that the solutions hereinbefore mentioned are those which have been found to successfully produce the result desired by the present invention, but any non-etching solution which contains inherent properties capable of producing solely hydrogen at the surface of the tinned material, may be satisfactorily used and will produce a virgin surface upon the tinned material when it is treated therein so that upon withdrawal of the tinned material from the solution, and p n exposure to air, preferably pure air relatively free from obnoxious fumes or gases, there is spontaneously formed on the virgin surface an invisible dense uniform protective film which renders the so-treated tinned material resistant to yellow stain during normal storage conditions or Inasmuch as the tin plate is cathodic in the present treatment. concentration of the electrolyte has not been found to be critical. Even in those instances where the solution will be recognized as having definite tendencies toward chemical attack on tin (for example, 10 per cent. caustic soda (NaOH) solution can be used to desubsequent baking at normal temperatures, such as in lithographing and enameling, etc.

Whil it has been found in practicing the present invention, that hot solutions are more eifec tive, if desired, the treatment can be performed at room temperature. The use of a hot solution is particularly desirable from an economic standpoint since the resulting decrease in electrical resistance lowers power consumption. So-

lutions of any.of the electrolytes previously listed may be operated up to their boiling point with the possible exception of caustic soda solutions containing more than 2.5 per cent. NaOH. In the latter case, it-is preferable not to exceed a temperature of 150 F., since such caustic soda solutions will react upon the surface of the tin when heated beyond this temperature.

In contrast to electrocleaning, which is a wellknown art, the preferred treatment of the present invention can be carried out at very low current densities. This is possible since the desired removal of the initial oxide film from the surface of the tin plate is electrochemical in nature whereas in electrocleaning, it is desired to remove grease, oil and/or dirt from a metallic surface, which removal is mechanical in nature, requir ing a scrubbing action obtained by a. vigorous evolution of hydrogen or other gas at the surface. The scrubbing action resulting from the evolution of gas not only blows" dirt from the surface, but aids in the saponification and/or emulsification of the grease and oil by the cleaning solution. For this reason, electrocleaning is' performed at relatively high current densities, at least 50 amperes per square foot being desired for practical cleaning; while the removal of the initial oxide film desired in the present invention can be successfully accomplished by subjecting tinned materialto electrolytic action as the cathode in a hot (122 F.) 0.5 per cent. (by weight) solution of-sodium; ortho-silicat at a current density of less than 1- ampere per square foot for a period of 40-120 seconds. Under such conditions, no visible evolution of hydrogen takes place.

While the present treatment can be carried out at current densities at which there is no Visible evolution of hydrogen or other gas at the surface of the material, it is to be understood that evolution of gas does not interfere with the successful treatment of the material unless such evolution becomes so vigorous as to affect the surface characteristics of the material being treat ed. In fact, we prefer to use a current densityof to amperes per square foot as this re-' duces the time oftreatment to 3 to 10 seconds which is considerably shorter than that necessary in the aforementioned example.

That the too vigorous evolution of hydrogen has an adverse effect, is shown in the following example. Tin plate treated in a 10 per cent. (by

weight) sodium carbonate solution at 100 'ammosphere will immediately oxidize uniformly to produce an impermeable superficial oxide film.

This film being formed at alow temperature is" of such compactness and impermeability that further oxidation is retarded and the tinned sur face is so renderedresistant to "yellow stain.

While it has been stated in the example that the solution is hot F.) it can, if desired, be successfully operated at room temperature. This electrolytic step is extremely flexible in regard to time of treatment and current density used.

In view of the foregoing, it is believed apparent to those skilled in the art that the practice of the present invention renders the surface of the tinned material passive or immune to the formation of "yellow stain.

This application is a continuation-in-part of an application filed jointly by us on February 11, 1937, Serial No. 125,352 on "Surface treatment of tinned material.

While we have described specific embodiments of the present invention, it will be understood that we do not wish to be limited exactly thereto, since various modifications may be made without departing from the scope thereof, as defined in the following claims.

We claim:

1. The method of treating tinned container stock having thereon a porous oxide film formed at the high temperature incidentlto the'hot tinning of said stock, consisting in removing all of said oxide from and baring a virgin'tin surface on said stock, and then subjecting said bared surface to substantially pure air at a temperature relatively low compared to the hot-dip temperature, thereby forming a dense uniform impermeable oxide fllm throughout said surfacerendering said surface substantially immune to peratures encountered in lithographing and enamelling.

2. The method of claim 1, in which the step of removing the porous oxide is conducted by cathodically treating the tinned container stock in a non-etching electrolyte at a cathode current density .and for a time insuflicient to etch the tin surface.

3. The method of claim 1, in which the step of removing the porous oxide is conducted by cathodically treating the tinned container stock in a non-etching electrolyte at a current density of from 10 to 20 amperes per square foot for from 3 to 10 seconds.

WILLARD 0. COOK. HUGH .E. ROMINE.