US2306143A - Preparing metal surfaces to resist staining or corrosion - Google Patents

Description

Patented Dec. 22, 1942 PREPARING METAL SURFACES TO RESIST .STAINING OR CORROSION Arthur E. Stevenson, Chicago, 111., assignor to Continental Can Company, Inc., New York, N. Y,, a corporation of New York No Drawing. Application May 9, 1939, Serial No. 272,713

1 Claim.

The invention relates generally to metal treatment and primarily seeks to provide a novel method of treating metallic surfaces, preferably the tinned surfaces of containers or plate to be formed into containers,'so as to form a protective film thereon which will prevent sulphur staining internally of said containers caused by reaction of the tin with sulphur products formed by the decomposition of sulphur containing proteins during the sterilizing or processing of the products packaged in such containers.

In the packaging of foodstuffs, of which vegetables, meats, meat products and fish constitute good examples, much difliculty is experienced due to the discoloration or staining of the containers. This discoloration occurs, over the entire interior of the can but is likely to be particularly troublesome in the headspace when the products are slack filled in the customary liquor or brine and subjected to prolonged processing. This staining is caused by the production of tin and iron sulphides, chiefly tin sulphide, which form a film on the interior of the can.

The invention therefore seeks to provide a novel method of chemically treating the tin plate containers, or the tin plate from which the containers are to be made, whereby a protective film will be formed on the tinned surfaces Which is effective to definitely prevent the so-called internal sulphur staining or discoloration.

A broad purpose of the invention is to provide a protective film on tin plate or containers formed of tin plate by means of a simple chemicaltreatment thereof with an oxidizing agent in an alkaline media,v and to provide a novel chemical treating material or solution of that character.

The tinned surface discoloration or staining herein referred to appears to result chiefly from the formation of tin sulphide by the sulphur compounds in the foodstufis, and the references herein made to such discoloration or staining are not to be confused with any actual etching or corrosive attack upon the tinplate sheet itself. It has been found that by practicing the method herein disclosed, that is by treating the tin plate or containers with the treating solution herein described, a protective film will be formed thereon which will prevent the reaction of the tin with sulphur products formed by the decomposition of sulphur containing proteins during the sterilizing or processing'of the packaged products.

Examples of various alkalies which may be included as alkaline media are tri-sodium phosphate or other alkaline phosphates or any of the caustic alkalies, and as the oxidizing agent, a chromate or some modification of a chromate such as a (ii-chromate or per-chromate, or salts of chromium in an alkaline solution which also contains some oxidizing material of sufficient oxidizing power to convert the chromium salts to chromates, may be employed.

In treating the tin plate or the containers formed therefrom according to this method, the plates or containers are immersed in the solution containing the alkali and chromate, or they may be sprayed with the solution, or, in the case of finished containers, they may be filled with the solution and then emptied. In each case the plates or containers would have to be thoroughly washed in order to remove the adhering solution. The treatment should be conducted in a hot solution. It is preferred that the solution' be maintained at a temperature of approximately 212 F., but temperatures well below 212 F.

as well as above 212 F. will give satisfactory results although the time of treatment will necessarily be longer when lower temperatures are employed.

When treating the containers or tin plate with a solution containing tri-sodium phosphate and sodium chromate, the prevention of staining may be accomplished in 15 seconds or less at a temperature of 212 F. Fairly good results have been obtained with,5 seconds treatment and even in some cases with 2 seconds. Different products vary in their staining effect on the container, this depending on the character of the product and the time and temperature of the sterilization process. The preferable time for covering all conditions would be 2 minutes. There does not seem to be any length of time beyond which the treatment is ineffective.

The film produced on the tin plate or conficient results varies widely depending on the concentration of phosphates in the solution. If we have 5 grams -of crystalline tri-sodium phosphate '(Na3PO4JZI-I2O, this contains approximately 57 percent of water and is the form in which tri-sodium phosphate is ordinarily sold) per liter of solution, it requires 0.5 gram of crystalline sodium chromate (NazCRO-rAHzO, containing approximately 31 percent of water) necessary is less and may drop somewhat lower than 2.5 percent figured on the crystalline. basis or 3.5 percent figured on the anhydrous basis.

So far as the use of chromate compound together with phosphates are concerned, a solution 4 containing a minimum of .034 percent of chromate compound and .215 percent of the phosphate compound (both figured on-the anhydrous basis) with a preferable concentration in the solution used of .86 percent of the phosphate compound and .084 percent of the chromate compound (both figures on the anhydrous basis) may be stressed asimportant, but it is .considered that the invention is not limited in scope to any particular maximum. To accomplish the desired purpose the proportion of phosphate to chromate need not be fixed but for each percentage of phosphate in solution there is a range of percentages of chromate in solution over which the treatment is efficient. For in stance with a solutioncontaining 2 percent of Na3PO4.12Hz0 a percentage of N 8.2C1O4.4H2O from .10 to .30 gives satisfactory results and with a solution containing 8.0 percent of NaaP04.12H:O-, a percentage of Na=CrO4.4HzO from .20 to 1.0gives satisfactory results. With increasing percentage of phosphate in the solution, there is an increase in the percentage range of chromate which will give satisfactory results.

It has been found also that caustic alkalies together with chromates are efiicient infproducing the desired stain or discoloration preventing film on tinplate or tin cans. A solution of this nature should include a minimum of .10 percent of caustic a all and .06 percent o'f chromate compound (calc ated on the anhydrous basis) or a proportion of approximately 37.5 percent of chromate comd to hydroxide. The proportion of the chromate compound to the caustic alkali may .be varied, but the lowest proportion of chromate found desirable would be such that it would make up 20 percent of the total chemicals (chromates and alkalies) in the solution. The preferred combination would be .2 percent caustic alkali and .10 percent chromate compound in solution. The invention is considered to be not limited in scope to the use of any particular maximum of caustic alkali or chromate.

In the foregoing description relating to phosphates and caustic alkalies, reference is made to chemicals containing water of crystallization. Both chromates and phosphates may be purchased with differing amounts of water of crystallization. and caustic alkalies with diflering percentages of moisture. Naturally the percentage of water present in the chemicals will influence the amount of each chemical used and the proportion of one to the other.

The phosphates may be used in the form of either potassium tri-phosphate or sodium triphosphate. The latter is very much cheaper than the former. Chromates could be used as ammonium chromate, potassium chromate or sodium chromate, or as .di-chromates or per-chromates of any of these alkalies.

. Extensive experiments have shown that when tin'plate or the containers formed therefrom are Patent is:

basis and substantially 0.084 percent of sodium chromate on the anhydrous basis for a time not essentially exceedingntwo minutes, and then thoroughly rinsing the tin plate before contact with the said products.

, ARTHUR E. STEVENSON.