US3839099A - Iron-phosphate coating for tin-plated ferrous metal surfaces - Google Patents

Abstract

A composition and method for treating tin-plated ferrous metal surfaces. The coating produced on the metal surface inhibits corrosion and acts as a bonding agent for subsequently applied organic finishing coats of sanitary lacquer or the like. More specifically, this invention relates to a composition for treating drawn and ironed tin-plated ferrous metal containers. The composition includes an aqueous solution of monobasic or dibasic monophosphates and mixtures thereof, alkali or ammonium fluoride or bifluoride and a hydroxylamine.

Description

United States Patent Jones Oct. 1, 1974 [75] Inventor: Thomas C. Jones, Philadelphia, Pa.

[73] Assignee: Amchem Products, Inc., Ambler, Pa.

[22] Filed: July 27, 1972 [21] Appl. No.: 275,620

[52] US. Cl. l48/6.15 R, 106/14 [51] Int. Cl. C23f 7/12 [58] Field of Search 148/615 R, 6.16; 106/14 [56] References Cited UNITED STATES PATENTS 2,160,061. 5/1939 2,298,280 l/1942 2,327,885 8/1943 2,665,231 l/l954 2,702,768 2/1955 Hyams et al. 148/615 R Primary Examiner-Ralph S. Kendall Attorney, Agent, or FirmErnest G. Szoke; Howard S. Katzoff 5 7 ABSTRACT A composition and method for treating tin-plated ferrous metal surfaces. The coating produced on the metal surface inhibits corrosion and acts as a bonding agent for subsequently applied organic finishing coats of sanitary lacquer or the like. More specifically, this invention relates to a composition for treating drawn and ironed tin-plated ferrous metal containers. The composition includes an aqueous solution of monobasic or dibasic monophosphates and mixtures thereof, alkali or ammonium fluoride or bifluoride and a hydroxylamine.

2 Claims, No Drawings IRON-PHOSPHATE COATING FOR TIN-PLATED FERROUS METAL SURFACES BACKGROUND OF THE INVENTION Containers used in the foodstuff industry can be made of tin-plated ferrous metal sheeting. These containers are ordinarily formed through a process referred to as Drawing and Ironing, that is the tinplated metal sheeting is drawn and intentionally thinned to form a drawn sheet providing uniform wall thickness and producing a thin walled and thick bottomed container. Because tin plate is expensive, only a thin tin-plating is employed. A serious problem with drawn and ironed tin-plated containers is that the drawing operation stretches the interior tin-plate surface thereby producing pores which leave the underlying steel base exposed. This allows the contents in the container to react with the underlying steel base surface thereby providing contaminants which give the contents an unpleasant taste. Moreover, as these containers are processed during manufacture, after the drawing and ironing step, conditions on the processing line can lead to corrosion of the exposed steel surface, leaving unsightly rust marks and blemishes rendering the containers unacceptable for use. Any corrosion and blemishes formed on the surface will adversely affect the adhesion of any subsequent sanitary lacquer coat which is applied thereafter.

It has become customary to provide containers with a conversion coating immediately after the drawing and ironing step, prior to applying the sanitary lacquer, so as to prevent corrosion and improve the adhesion of the sanitary lacquer. Chromate passivation treatments have been employed by the art to provide the conversion coating, and are ordinarily effected by cleaning the surface with a conventional alkaline cleaner, thereafter applying to the surface a solution having chromic acid or salts thereof. The deposition of chromates has proven to be detrimental, especially in the foodstuffs industry due to the high toxicity associated with hexavalent chromium. The deposition of hexavalent chromium to the container has also become unsuitable because of the difficulties inherent in removing the residues of the chromate solution from the surface prior to deposition of a siccative finish or sanitary lacquer. In addition, an ancillary problem associated with the use of chromate treating solutions is that the bath effluent cannot be discarded without treatment to reduce or eliminate the chromate toxicity.

It is an object of the present invention to provide a composition for the treatment of drawn and ironed tinplated ferrous metal containers which will enhance the corrosion resistance of container surfaces and improve the adhesion properties of a later applied siccative finish, such as sanitary lacquer.

An additional object of this invention is to provide a high-speed coating process for tin-plated ferrous metals.

A further object of this invention is to provide a one package coating composition for use on tin-plated ferrous metal surfaces, thereby facilitating easy handling and offering high stability.

A concomitant object of this invention is to provide a coating process for tin-plated ferrous metal containers to deposit a conversion coating of the type known in the art as an iron phosphate coating.

DETAILED DESCRIPTION OF THE INVENTION It has been discovered that an aqueous solution comprising a primary and/or secondary phosphate, a hydroxylamine salt, and a fluoride salt when applied to drawn and ironed tin-plated ferrous metal surfaces will provide an excellent corrosion resistant coating which also enhances the adhesion of a later applied siccative finish.

It should be understood that the term ferrous metal used herein includes a wide variety of steels, iron, and iron alloys.

The solutions to be used in the present invention preferably are prepared by employing a one package aqueous concentrate consisting of the primary or secondary phosphate, hydroxylamine salt, and an alkali or ammonium fluoride or bifluoride. This one package aqueous concentrate can be added directly to water in order to prepare the aqueous solution for use. The one package aqueous concentrate offers the advantage of high stability for prolonged periods of time.

The aqueous concentrates to be employed to make up the aqueous coating solution may contain a primary or secondary phosphate, a fluoride or bifluoride salt, and a hydroxylamine salt in varying concentrations, since the concentrate is added to appropriate amounts of water to prepare a working solution having constituent concentrations within the operative ranges set forth hereinbelow.

The primary and secondary phosphates suitable for use herein are a monobasic or dibasic monophosphate or a mixture of both the monobasic and dibasic monophosphate. Condensed phosphates should be avoided, since they have a tendency to hydrolyze in acidic solutions thereby adversely affecting the operating parameters of the working bath. The monophosphate can be incorporated in the aqueous concentrate in any convenient form which is soluble. The monophosphate can thus be introduced, for instance, in the form of either sodium, potassium, or ammonium phosphate. It is preferred that monosodium phosphate be employed, as it is readily soluble and contributes to optimum stability of the coating bath. The alkali or ammonium phosphate can be present in the coating solution in an amount from about 1.0 to about 5.0 percent by weight. It is pre ferred that the phosphate salt be present in amounts from 1.3 to 1.6 percent by weight.

The hydroxylamine salt to be utilized to prepare the coating solution can be in the form of any soluble salt, such as hydroxylammonium acid sulphate, hydroxylammonium nitrate, and hydroxylamine hydrochloride. The hydroxylammonium salt should be present in the working coating solution in an amount from about 0.07 to about 0.5 percent by weight. Preferably from about 0.14 to about 0.17 percent by weight will be used.

The source for fluoride in the aqueous concentrate can be an alkali bifluoride, ammonium fluoride or bifluoride, such as sodium fluoride, sodium befluoride, ammonium fluoride, ammonium bifluoride, potassium fluoride, and potassium bifluoride. A complex fluoride can be employed in the present invention, providing that the quantity of complex fluoride present will be that which will yield or liberate free fluoride in amounts equivalent to the prescribed amount necessary when a fluoride or bifluoride salt is used. For example, the source of fluoride can be fluosilicic or fluotitanic acid.

A drawback in employing a complex fluoride is that as various salts build up in an operating solution during continuous use, hydrolysis will ordinarily not be sufficient to supply the necessary amounts of free fluoride. Accordingly, the preferred process of this invention utilizes the simple alkali fluoride or bifluoride salt. The fluoride or bifluoride salt should be present in the aqueous coating solution in an amount from about 0.03 to about 0.5 percent by weight. For optimum results, it is preferred that the fluoride salt be present in an amount from 0.07 to about 0.1 percent by weight.

The free fluoride in the coating solution aids in the attack and removal of residual oxides on the metal substrate which have formed thereon during or after the forming or drawing and ironing operation, and which would ordinarily impede deposition of a uniform coating. The removal of any oxides on the substrate facilitates more efficient and uniform coating information. A surprising aspect of this invention is that the coating process can be effected when the amount of free fluoride in the coating solution, having constituent concentrations as specified herein, is as low as 0.15 gms/liter.

Typical examples of concentrated aqueous formulae, suitable for dilution with additional water to make a coating solution of optimum quality having constituent concentrations and operating parameters within the ranges set forth herein, are as follows:

These makeup concentrates include all constituents which will be employed in the coating solution, thereby allowing for easy preparation of the coating solution by simple addition of the concentrate to water. Of course, it is desirable in certain instances to add each of the constituents to water separately in order to prepare the aqueous coating solution. In addition, as is specified herein, appropriate additions of acid or base may be necessary to adjust the operating pH of the coating solution during use.

In the preferred embodiment of this invention, an operating aqueous solution is employed comprising monosodium and disodium monophosphate, hydroxylammonium acid sulphate, and ammonium bifluoride. Optimum results will be obtained when the concentration of constituents in the solution, at the beginning of the operation, as well as upon replenishment, is maintained such that the disodium and monosodium monophosphate together are present in an amount from about 1.3

to about 1.6 percent by weight. the hydroxylammonium sulphate is present in an amount from about 0.14 to about 0.17 percent by weight, and the ammonium bifluoride is present in the amount from about 0.07 to about 0.1 percent by weight. When a coating solution is operated and maintained within these prescribed limits, it has been found that excellent corrosion resistance of the tin-plated ferrous metal surface will result. A most surprising result is that the surface will maintain its original appearance, that is a highly polished look. The coating produced will also improve the adhereance of a subsequently applied sanitary lacquer, such as an acrylic base coat.

The coating process is normally employed after cleaning of the metal surface has been accomplished. The cleaning steps are ordinarily carried out after the tin-plated metal has been drawn and ironed to form the container. The cleaning step can be carried out by conventional methods which form no part of the present invention. A conventional alkaline cleaner can be employed followed by a water rinse. Should the surface be heavily soiled, a detergent cleaner additive may be employed in the cleaning step.

1n the preferred embodiment of the present invention, the metal surface should be cleaned employing techniques that obtain a completely water-break-free surface in a short period of time. An alkaline cleaner having the following formula can be employed:

7: by Weight Borax Pentahydrate 60 0 During the coating operation, depletion of the constituents in the coating bath will occur at about the same rate. These losses must be replaced to maintain the bath within its optimum operating limits. The coating bath is maintained within its prescribed limits with suitable additions of a replenishing concentrate having the constituents therein in the same proportions in which these constituents exist in the operating bath.

In the process of the present invention, the tin-plated ferrous metal substrate is brought into contact with the aqueous coating solution under suitable conditions of pH, temperature, and contact time.

The time of treatment of the metal surface with the coating solution need only be long enough to insure complete wetting of the surface and can be as long as 5 minutes. Preferably, contact time between metal substrate and coating solution should be from about one second to about one minute. One of the distinct advantages of the present invention is that a suitable protective coating can be formed on tin-plated ferrous metal surfaces utilizing a treating time of as little as one second.

The coating process can be effected by employing any of the contacting techniques known to the art. Contact can be effected by spraying or immersing the workpiece to produce the desired surface coating. Preferably, the coating solution will be applied to the metal surface by conventional spray methods. When spray techniques are employed, low pressure application of the coating solution has proven effective in depositing satisfactory coatings on the workpiece. Nozzle pressures as low as 10 20 psi have been employed with excellent results.

The pH of the treating solution should be maintained at a level within the range of from about 4.5 to about 5.5. Preferably, the solution is operated at a pH of from about 5.0 to about 5.4. It should be understood that a suitable acid, such as phosphoric acid, or base, such as sodium hydroxide, can be added to the coating solution to adjust the pH of the bath within the operating limits prescribed herein. The use of the coating solution having a pH above 5.5 is to be avoided, as little or no coating will be produced. It should be noted that operation of the coating solution below a pH of about 4.5 will result in too vigorous attack of the tin-plated ferrous metal surface.

The coating process can be operated at a temperature from about 75F. to about 150F. It is preferred to operate the coating bath at a temperature from about 120F. to about 140F. Generally, a slight change in the temperature will not necessitate substantial alteration of treating time, concentration parameters, or pH adjustment.

Following the application of the coating solution, the coated surface can be rinsed with water or a conventional final rinse and then dried. The water rinse is necessary to remove any remaining residues which may have remained after thecoating step.

For optimum results, after the coating solution has been applied to the surface, a water rinse followed by a deionized water rinse can be utilized.

The method of the present invention can be effected by treating the tin-plated ferrous metal surface after the drawing and ironing step. Generally, the coating solution will be applied directly after the forming operation or a short period of time thereafter.

Once the coating and rinse steps are accomplished, the surface can be dried by conventional means and a siccative finish, such as a sanitary lacquer, can be applied to the surface with good adhesion results. After the drying step and application of the siccative finish, the workpiece is ready for use and will not suffer corrosive attack from any liquid or foodstuff placed in the container.

A particular advantage of the present invention is that after the coating and water rinse stages have been accomplished, the workpiece will not suffer corrosive attack, even when subjected to prolonged exposure to air due to processing line stoppage prior to application of the siccative finish.

The examples presented below are illustrative of this invention and are not considered as limiting for other materials and operating conditions falling within the scope of this invention that might be substituted.

EXAMPLE 1 Tin-plated drawn and ironed steel containers were employed in this procedure. The containers were cleaned by wiping with a solvent cleaner consisting of butyl cellosolve and neutral spirits so that any oil layer remaining from the drawing operation was removed. The containers were then cleaned with an alkaline cleaner for about one minute at about 175F. so that the metal surfaces were water-break-free. The containers were then rinsed with water.

An aqueous concentrate of Formula 4 was prepared by adding a 50 percent solution of caustic soda to a 75 percent solution of phosphoric acid, thereby neutralizing the acid and forming the mixture of disodium and sodium monophosphate in situ. The hydroxylammonium acid sulphate was then added followed by additions of ammonium bifluoride and water to make 1 liter ane complete dissolution of the constituents was achieved.

The use of phosphoric acid and sodium hydroxide in an aqueous medium, results effectively in the formation of monosodium and disodium monophosphate and has the same effect as if the alkali monophosphates alone were used. The choice of ingredients in this case is made for practical economical reasons.

An aqueous coating solution was prepared by adding the aqueous concentrate of Formula 4 to water at the rate of ml. of concentrate for each 4 liters of water. The pH of the resulting solution was adjusted to 5.0 by addition of phosphoric acid. The test containers were immersed in the coating solution, for a period of one minute at a temperature of about F.

After treatment, the containers were allowed to stand for 24 hours and were observed to have retained a bright, rust-free appearance.

Control containers were employed and were cleaned and rinsed and given no further treatment. The control exhibited badly rusted surfaces, both inside and out, after a period of 24 hours.

EXAMPLE 2 Tin-plated steel containers were cleaned as in Example 1, first with a solvent cleaner, then with an alkaline cleaner. The test containers were then sprayed with an aqueous solution having the following constituents:

7: by Weight Monosodium Monophosphate 0.53 Disodium Monophosphatc 0.47 Hydroxylammonium Acid Sulphate 0.12 Ammonium Bifluoridc 0.06 Water 98.82

The pH of the above solution was measured at 5.3. The solution was applied to the containers at a pressure of about 20 psi and at a temperature of about 125F. A spray time of one minute was utilized.

The containers were then rinsed with water, followed by a deionized water rinse, and dried at ambient temperature. The containers had retained a rust-free appearance after treatment. I

One set of control tin-plated steel containers was also employed in this procedure. The controls were cleaned with a solvent cleaner, then with an alkaline cleaner, and were given no further treatment.

The test containers were permitted to stand for a period of 72 hours and were then subjected to humidity testing and rust testing. In the humidity test, 10 ml. of tap water was placed in each test container, which was then covered with a watch glass and allowed to stand for one hour in an oven at 100F. The containers were then examined and evaluated for any rusting. They showed little rusting and still retained their original untamished, bright appearance. The control containers showed bad rusting.

In the rust test, a group of test containers were evaluated for exposed iron pores. Strips of filter paper were immersed in a solution of K Fe(CN) and then placed in contact with the interior of each container for one minute. The appearance of blue stains indicated the presence of exposed iron pores which are susceptible to rusting. All test containers exhibited no staining. The

controls portrayed staining over much of their surfaces.

ent in an amount from about 0.07 to about 0.5 percent by weight, said solution having a pH from about 4.5 to about 5.5; thereafter rinsing the surface with water; and thereafter applying a siccative finish to the surface.

2. The method of claim 1 wherein the monophosphate is present in an amount from about 1.3 to about 1.6 percent by weight, the fluoride salt is present in an amount from about 0.7 to about 0.1 percent by weight, and the hydroxylamine salt is present in an amount from about 0.14 to about 0.17 percent by weight.

Claims (2)

1. A METHOD FOR COATING A TIN-PLATED FERROUS METAL SURFACE COMPRISING THE STEP OF: CLEANING THE METAL SURFACE; APPLYING OT THE SURFACE AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF A MONOPHOSPHATE SELECTED FROM THE GROUP CONSISTING OF MONOBASIC AND DIBASIC MONOPHOSPHATE AND MIXTURES THEREOF PRESENT IN AN AMOUNT FROM ABOUT 1.0 TO ABOUT 5.0 PERCENT BY WEIGHT, A FLURIDE SALT PERCENT BY WEIGHT, AND A HYDROXYLAMINE SAL TO ABOUT 0.5 PERCENT BY WEIGHT, AND A HYDROXYLAMINE SALT PRESENT IN AN AMOUNT FROM ABOUT 0.07 TO ABOUT 0.5 PERCENT BY WEIGHT, AND SOLUTION HAVING A PH FROM ABOUT 4.5 TO ABOUT 5.5; THEREAFTER RINSING THE SURFACE WITH WATER; AND THEREAFTER APPLYING A SICCATIVE FINISH TO THE SURFACE.

2. The method of claim 1 wherein the monophosphate is present in an amount from about 1.3 to about 1.6 percent by weight, the fluoride salt is present in an amount from about 0.7 to about 0.1 percent by weight, and the hydroxylamine salt is present in an amount from about 0.14 to about 0.17 percent by weight.