ROGER H. LUECK
AND
KENNETH W. BRIGHTON
American Can Company, Maywood, 111.
r\’ ANTICIPATION of the time when the country might be-
perience with electrolytic and Bonderized plates and outlined a program by which tin could be further conserved without extending restrictions on the volume of items packaged in metal containers. This program (10) hinges on the increasing use of electrolytic and Bonderized plate in food as well as nonfood containers. I n outlining the program each essential food product w m considered individually, and a projection was made covering the extent to which electrolytic or Bonderized steel can parts could be used satisfactorily for each item normally packed in can#. The projections were published in the so-called Blue Book, extensively used by WPB in the management of tin allocation. At that time relatively little electrolytic or Bonderized plate was available so the suggestion Eas made that the conservation plates 6rst be used for can ends only. Then as the production of electrolytic plate was increased, and the can manufacturers completed the changes in their equipment necessary to handle the plate, it was planned to extend the use of electrolytic plate to can bodies. Thus the program was outlined as three stages of increasing tin conservation. The first involved the use of 1.25-pound hotdipped plate. The second stage required electrolytic or Bonderized ends on 1.25-pound hot-dipped bodies, and the third stage consisted of electrolytic or Bonderized ends on electrolytic bodies. An example of the resulting tin conservation is seen in cans for a typical product such as peas. One thousand No. 2 cans, made of 1.50-pound plate which was the standard before the war, require 3.9 pounds of tin exclusive of that used in the solder. An equal number made of 1.25-pound plate in stage 1 require 3.25 pounds of tin, I n stage 2, 2.48 pounds of tin are required per lOQ0cans
I
come involved in war in the Pacific, can manufacturers had begun to investigate substitutes for hot-dipped plate, even before the invasion of Poland. Late in the fall of 1940 the principal can manufacturers cooperated with the National Canners Association and the U. S. Army Quartermaster Corps in a n experiment to determine whether the tin coating on tin plate could be reduced safely from 1.50 pounds per base box (pot yield) to 1.25 pounds per base box. I n 1941 the Can Manufacturers Institute formed a Committee for Tin Conservation which, in turn, named a Technical Subcommittee. At that time the Subcommittee visualized the problem of tin conservation as a series of retrenchment steps, the first of which involved the use of 1.35-pound plate for sterilized food products, and the use of tin plate substitutesnamely, electrolytic or Bonderized black plate-for such items rn coffee, oil, shortenings, paints, etc. However, the sequence of logical steps which was planned was completely upset when the Japanese bombed Pearl Harbor. Faced with the necessity of quickly reducing consumption of tin, and steel as well, the War Production Board took two stbps. I t reduced the tin coating weight on hot-dipped plate from 1.35pound coating earlier decreed by the Office of Production Management to 1.25 pounds per base box, and also drastically restricted the items for which tin plate could be used. These two steps were admittedly effective in reducing the consumption of tin. However, a fundamental program was needed for conserving the supply of tin without further restricting the volume of metal containers which are so essential in the preservation of food for the armed forces, our allies, and the civilian population. The Teohnical Subcommittee, therefore, drew on its limited ex-
532
lune, 1944
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
533
the characteristics of the two important substitutes for tin plate will be reported briefly. Electrolytic plate wm recently described in detail (9). ELECTqOLYTlC PLATE
if made with electrolytic ends on 1.25-pound hot-dipped bodies, and only 2 pounds of tin if the ends are made of Bonderized steel. In stage 3, 1.27 pounds of tin are required if both ends and bodiea are made of electrolytic plate, and 0.80 pound if the ends are made of Bonderized steel. With the program outlined, the various organizations involved moved into action. The steel mills began the construction of additional electrolytic and Bonderizing lines. The can manufacturers built additional organic coating equipment and modified existing can-making equipment in order t o solder electrolytic plate. A t the same time the research laboratories of the various can manufacturers inaugurated extensive experimental packs to serve as a check on the projections of the Technical Committee. The results of certain of these packs constitute the main section of this paper. Before the results obtained with food products packed in cans of electrolytic and Bonderiaed plates are discussed,
As the name implies, a coating of tin is electrodeposited on steel. By electrodeposition, uniform coatings as low as 0.1 pound per base box can be applied. This is in contrast to the hot d i p ping method where it is impractical to apply coatings lighter than 1.25 pounds per base box, pot yield. The first electrolytic plate bore a 0.5-pound coating of tin, and the concept of electrolytic plate bearing this weight has continued, even though lighter and heavier coatings are now available. In this paper the 0.6-pound coating will be indicated unless the coating weight is otherwise stated. I n the- electrolytic process the temper-rolled steel strip is pickled eontinuously to remove oxide and other surface impuriti-, washed, and then passed into the plating electrolyte. There are two general types of electrolytes, the acid bath based on bivalent tin compounds and the alkaline bath based on sodium or stannates. The operating advantages and disadvantages of the two types of baths have been discussed by Lippert (8) and by Lowenheim (9). After plating, the strip is thoroughly rinsed, and the tin coating dried and melted in situ. There are several methods of melting the tin deposit, each of which serves to improve the appearance of the plate. Melting also increases the corrosion resistance of the plate and provide8 better soIdering characteristics than the plate with a scratch brushed or matte finish, and is therefore favored by the can manufacturers. After the tin coating is melted, the strip is quenched in water or oil followed by treatment in a mild oxidant, usually a solution of chromic acid. The chemical treatment forms a thin protective film on the surface of the tin which reduces the tendency to oxidize further during the baking operation incident to the application of organic coatings. The treatment also improves the adhesion of organic coatings to the plate. Excellent adhesion of organic coatings is more essential with electrolytic than with hot-dipped plate; with the former the OFganic coating is relied upon largely to provide the necessary resistance t o subaqueous and atmospheric corrosion. Oleoresinous enamels generally adhere reasonably well to electrolytic plate.
Two Important Substitutes for Tin Plate Are Electrolytic and Bonderized Plates. Photo on Page 532 Shows an Electrolytic Tinning Unit (Front End)) Below Is Equipment for Black Plate Bonderizing.
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534
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 1. Surface OF Melted 0.5-Pound Electrolytic Plate (above) and of 1 .PS-Pound Hot-Dipped Plate (below), X 30
However, considerable difficulty has been encountered in obtaining adequate adhesion of heat-reactive phenolic enamels, particularly on alkaline plate. This type of enamel is now being extensively used on the inside of cans because of its excellent resistance to fats and oils. It is also being used on the outside of the cans because it is not thermoplastic and does not mar so readily as the oleoresinous types in cannery operations. The heat-reactive phenolic enamels are particularly sensitive to foreign materials on the surface of the plate; certain suppliers have engaged in considerable experimental work to develop a surface suitable for application of organic coatings. After drying, the plate is lubricated, either before or after shearing. The lubricant is applied to facilitate handling at the mill and at the can factory, to reduce rusting, and to retard the oxidation of the tin surface on storage. Dry electrolytic plate oxidizes rapidly, particularly in warm, humid climates. The mode of oxidation was described by Beynon and Leadbeater ( 1 ) in England. If heat-reactive phenolic enamels are applied to oxidized plate, severe flaking of the coating occurs when the plate is fabricated. The tin plate branner has been used in the past to control the amount of palm oil remaining on hot-dipped plate. However, it is difficult to control a branner so that it will apply palm oil within the limits that are necessary if heat-reactive phenolics are to be
Vol. 36, No. 6
applied. Therefore a new approach to the lubrication problem is now being attempted and plasticizers such as dibutyl sebacate, which are more compatible with the enamels than palm oil, are being tried as lubricants. An excess of these materials can be applied without promoting loss of adhesion. The plasticizers can be applied by branner or by spraying from an emulsion. The latter method has an advantage over branning in that it does not abrade the chemical film on the surface of the plate. One of the most important characteristics of electrolytic plate to the can maker is its solderability. The early plates, particularly the acid plate with a brushed finish, were difficult to solder. Changes in electrolyte and addirion agents were made to improve solderability, but the step that did most to make possible the soldering of electrolytic can bodies was the use of low-tin solder (less than 6% tin). With low-tin solder of the proper composition it is now possible to solder electrolytic plate and obtain stronger bonds than were previously obtained on hot-dipped plate Kith conventional 40-60 tin-lead solder. Another obvious advantage is the considerable conservation of tin effected by the use of low-tin alloys. The resistance of electrolytic plate to atmospheric corrosion is approximately proportional to the weight of tin coating it bears. The 0.5-pound electrolytic plate is considerably less resistant than the 1.25-pound hot-dipped plate. However, if the 0.5-pound plate is enameled, it becomes more rust resistant than plain 1.25pound plate. I n fact, the enameling of plates of various tin coating weight6 tends to decrease the spread in resistance to atmospheric corrosion. The difference between the performancw of enameled 0.5-pound electrolytic plate and enameled hot-dipped plate is slight, as far as atmospheric corrosion is concerned, and the choice of enamels is more important than the choice of plate. The subaqueous corrosion resistance of electrolytic plate on the inside of the can, like the external corrosion resistance, is a function of the amount of tin on the plate, to a considerable degree. K'ithout an organic coating the 0.5-pound plate is far less resistant to corrosion than hot-dipped plate, and an organic coating must be applied to provide adequate corrosion resistance Kith most food products of high moisture content. I n their present stage of development the electrolytic coatings in the thicknesses available can be considered only as war time substitutes in cans for all but items containing less than 20'% moisture. The uniformity of electrolytically deposited coatings has been emphasized repeatedly. As normally determined, the coating is more uniform than that applied by the hot-dipping process. However, if we consider variations from one microscopic area to another, the coating is actually less uniform than might be supposed. This is shown in Figure 1 where the numerous craters or pits in the electrolytic coating in its present stage of development are apparent BONDERIZED STEEL PLATE
The Bonderite K process is a special phosphating treatment d e veloped early in 1941 by the Parker Rustproof Company a t the request of, and in cooperation with, the principal can companies. Unlike the well-known Bonderizing process applied to automobile fenders and other prefabricated parts, plate treated by the K process can be formed without disrupting the crystalline phosphate deposit. The cut sheets are first passed through rubber rollers which activate the surface. They are then slushed with a solution of zinc dihydrogen phosphate containing an oxidizing agentandacceleratorswhichreacts with the activated steel surface to form a codeposit of zinc and ferrous phosphates. The crystalline deposit is washed free of residual salts in the next section. The sheets then pass through a dilute solution of calcium dichromate, through squeegee rolls, and into a final drying section. The Bonderite K process was tailored to meet the specifications for a chemical treatment which would provide good adhesion for can enamels, retard rusting of the plate during transit from the mills to the can factories, and p r b e n t underfilm corrosion. The
INDUSTRIAL AND ENGINEERING CHEMISTRY
lune, 1944 Table I.
535
the time the experimental cans were made, it was not possible to melt wide ------DaysStoredat7O0F.: ------strip a t the mill from 156 lQ1 230 286 317 348 which the acid plate was 0 f o .01 +O0 21 10 2 10. 7 20. 5 obtained; therefore the f0.2 +O 2 0.2 2.9 3.5 4.5 strip was sheared after plating into narrow widths and melted in a pilot unit. There are indications that this plate would h a v e given better performance in the corrosion packs if it had been melted on a commercial line. To obtain plates from an alkaline electrolyte, coils of temperrolled plate from the two test heats were shipped t o another mill and coated with a 0.5-pound tin coating. Half of the plate was melted and the remainder left in the matte condition. This alkaline electrolytic plate was not chemically oxidized. The various lots of plate were fabricated into cans on a standard factory line, part of each lot having been previously enameled in the flat. At the cannery, cans from each lot were alternated through the line t o minimize variations in packing house procedures. The processed cans were returned from the cannery, and fifty cans from each lot stored a t 100" F. in a constant-temperature room. Another fift cans from each lot were stored a t 70" F. Additional cans from t%e same pack were also held a t each terpperature for eriodic opening t o observe the interior of the can and the conztion of the product, . and to assay the tin and iron pickup in the food. To detcrmine the rate a t which the cans in the 100" and 70" F. storage were failing before actual failure occurred, periodic flip vacuum readings were made on each can, and the average loss of flip vacuum was taken as the criterion of failure. The flip vacuum device (Figure 3) is a means for applying an external vacuum to one end of the can to offset the vacuum on the inside of the can and cause the end to flip out. During storage the vacuum in the can is gradually dissipated by the hydrogen formed through corrosion processes, and progressively less external vacuum is needed to flip the end. By determining the vacuum required to flip the ends in a particular lot of cans, and then subtracting the vacuum required to flip the ends of the same cans a t succeeding monthly intervals, it is possible t o determine the rate of failure without destroying them. Table I is an example of the type of data obtained. There is a slow but steady loss of vacuum in the cans made of hot-di ped plate, and a more rapid drop in cans b a d e from 0.75- anf0.5-pound melted electrolytic plate. The accelerating effect of increased temperature on the corrosion process causes the vacuum in the can to drop more rapidly a t 100" than a t 70" F. The acid type electrolytic late was included in more experimental packs than the alkagne plate. Unless otherwise indicated, the melted acid plate is referred to when the term "electrolytic plate" is used in discussing the experime?tal results.
Loss of Flip Vacuum as a Measure of Corrosion (Grapefruit Juice in Plain No. 9 Cans) -Vacuum Loss in Inohes I
r
Plate,Lb. 111 159 Hot-dipped, 1 . 2 5 0 0.2 Electrolytia,0.75 2 . 4 3 . 9 Electrolytic,0.5 3 . 4 6 . 0
Days Stored at 100' F.: 195 226 269 800 1.0 1.0 3.7 4.5 10 2 12.5 5.4 7 2 17 7 19.2 10.2 14.1
.
343 5.0 13.1 100% (swells)
latter phenomenon (Figure 2) is a wormlike formation of tubules containing rust which develops under organic coatings applied to untreated steel when exposed to unfavorable atmospheric conditions. The primary function of the Bonderizing treatment is to provide a suitable substrate for organic coatings. It is from the latter alone that the necessary corrosion resistance is obtained. If can enamels are applied to untreated steel, considerable loss of adhesion occurs when the cans are thermally processed in steam or water. When exposed t o the same conditions, enameled Bonderized steel shows practically no loss of adhesion of the enamel film. The codeposit of zinc and iron phosphates on the surface of Bonderized steel acts as a retardant to the rusting normally encountered with untreated steel in transit to, and storage at, can factories. However, the plate in itself does not possess sufficient resistance to either atmospheric or subaqueous corrosion to be used as produced, and for every application must be enameled both inside and out. In the use of Bonderized steel the organic coating is relied upon solely to provide the necessary resistance to corrosion. I n this respect it differs from tin plate or even electrolytic tin plate. As shown by Kohman and Sanborn (7), Lueck and Blair (fl), and Hoar ( J ) , under the conditions existing in a sealed can, tin will anodically protect steel from subaqueous corrosive action even a t discontinuities in the tin coating. Since this protection is missing in Bonderized steel, reliance must be placed completely on the organic coating. Consequently, some can manufacturers employ a two-coat system of enamels on this type of plate. Even so, there are bound to be discontinuities in the coating after fabrication of the can; therefore the use of Bonderized steel has been limited to the less corrosive foods such as meats, marine products, and some vegetables (peas, corn, lima beans, etc.). Bonderized steel was projected primarily for can ends only, although it was learned later that the plate could be soldered satisfactorily if extensive equipment changes were made. However, as a result of the shortage of critical materials required for changes in equipment, only experimental food cans have been made completely of Bonderized plate. It is possible that Bonderized plate cans will be used for many items not now permitted by the War Production Board, if the steel situation improves while that of tin is still critical.
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RESULTS W I T H P L A I N C A N S
The experimental work with electrolytic plate in 1937 indicated that plain cans (not enameled) were unsatisfactory for foods of high moisture content, because of the rapid formation of hydrogen gas; in comparison, the results with enameled electrolytic
EXPERIMENTAL TECHNIQUE
In making the corrosion studies t o check the projections of the Technical Subcommittee, care was taken t o control all variables as closely as possible. The plate was obtained from one mill; from one heat of low-metalloid steel (phosphorus
