[Advances in Food Research] Advances in Food Research Volume 3 Volume 3 || Certain Aspects of Internal Corrosion in Tin Plate Containers

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<ul><li><p>Certain Aspects of Internal Corrosion in Tin Plate Containers </p><p>BY R . R . HARTWELL General Research Laboratory. American Can Company. Yaywood. Illinois </p><p>CONTENTS </p><p>I . Introduction . . . . . . . . . . . 2 . Academic Aspects . . . . . . . </p><p>11 . Tin Plate . . . . . . . . . . . . 1 . Manufaclure . . . . . . . . . 2 . Mechanism of Corrosion . . : . . </p><p>111 . Corrosion Characteristics of Foods . . . 1 . Classification of Foods . . . . . 2 . Types of Corrosion by Foods . . . </p><p>I V . Effect of Food Components . . . . . 1 . Acidity . . . . . . . . . . . 2 . Reactivity with Tin . . . . . . </p><p>1 . Scope of Consideration . . . . . Page </p><p>. . . . . . . . . . . 328 </p><p>. . . . . . . . . . . 328 </p><p>. . . . . . . . . . . 329 </p><p>. . . . . . . . . . . 329 </p><p>. . . . . . . . . . . 329 </p><p>. . . . . . . . . . . 334 </p><p>. . . . . . . . . . . 336 </p><p>. . . . . . . . . . . 336 </p><p>. . . . . . . . . . . 337 </p><p>. . . . . . . . . . . 337 </p><p>. . . . . . . . . . . 337 </p><p>. . . . . . . . . . . 340 3 . Miscellaneous Factors . . . . . . . . . . . . . . . . . 342 4 . Inhibitors . . . . . . . . . . . . . . . . . . . . . 342 5 . Measurements of Corrosiveness . . . . . . . . . . . . . 344 </p><p>344 V I . Variations Produced by Canning Operations . . . . . . . . . . 345 </p><p>1 . O x y g e n . . . . . . . . . . . . . . . . . . . . . . 345 2 .Vacuum. . . . . . . . . . . . . . . . . . . . . . 347 3 . Headspace . . . . . . . . . . . . . . . . . . . . . 347 4 . Processing and Cooling . . . . . . . . . . . . . . . . 348 5 . Storage Temperatures . . . . . . . . . . . . . . . . . 349 </p><p>V I I . Materials Added in Canning . . . . . . . . . . . . . . . . 350 1 . Sugar and Sirups . . . . . . . . . . . . . . . . . . 350 2 .Salt . . . . . . . . . . . . . . . . . . . . . . 351 </p><p>V I I I . Trace Elements . . . . . . . . . . . . . . . . . . . . . 352 1 .Su l fu r . . . . . . . . . . . . . . . . . . . . . . 352 2.Copper . . . . . . . . . . . . . . . . . . . . . . 355 </p><p>IX . Relation of the Container to Corrosion . . . . . . . . . . . . 356 X . The Tin Coating . . . . . . . . . . . . . . . . . . . . 356 </p><p>1 . Tin Coating Weight . . . . . . . . . . . . . . . . . 356 2 . Porosity . . . . . . . . . . . . . . . . . . . . . . 360 </p><p>X I . The Steel Base . . . . . . . . . . . . . . . . . . . . . 363 XI1 . Tinplate Testing . . . . . . . . . . . . . . . . . . . . 366 </p><p>XI11 . Enameled Cans . . . . . . . . . . . . . . . . . . . . . 371 </p><p>V . Packaging and Storage . . . . . . . . . . . . . . . . . . </p><p>X I V . Electrolytic 'Pinplatc . . . . . . . . . . . . . . . . . . . 374 X V . Summary . . . . . . . . . . . . . . . . . . . . . . . 377 </p><p>References . . . . . . . . . . . . . . . . . . . . . . 378 327 </p></li><li><p>328 R. R. HARTWELL </p><p>I. INTRODUCTION </p><p>Twenty years after commercial canning began in the United States in 1820, tinplate became the leading container material for food com- modities and has since preserved that position. The growth of the con- tainer and associated industries is well illustrated by the fact that they now require about 4% of the countrys finished steel capacity, or more than 3 million tons in 1948. Because of the age of these industries, the volume of container production, and the significance of their products in the national economy, the volume of past work designed to eliminate or control corrosion of tin plate containers may be readily appreciated. </p><p>Corrosion phenomena in tin plate containers have attracted the interest of many investigators over a long period of time. Corrosion has meant many things to different individuals and in some of the literature the term is applied exclusively to the formation of tin or iron sulfide, rust, dissolution of the tin coating, or attack on the steel base metal. To a certain extent, these are specialized uses of the term. As normally employed in the can manufacturing industry, internal corrosion is ordi- narily not considered to refer to any one of the above individual stages, but rather to their net effects as they may influence the time required to make the product unsalable, either through the production of enough hydrogen to cause distention of the can ends or by perforation of some part of the container. The length of time a can will withstand this attack is referred to as its corrosion shelf life and is one of its im- portant performance requirements. It is fortunate that although the tendencies for corrosion are present in varying degrees in all moist food products, the problem of obtaining an adequate corrosion shelf life has proven a critical one only for certain classes of foods. </p><p>1. Scope of Consideration </p><p>The universal importance of the corrosion problem and the fact that a great deal has been accomplished in a practical way toward alleviating commercial difficulties and furnishing better containers a t lower prices have led to a volume of literature on container corrosion. It is intended to summarize these reports in a logical order from the several viewpoints to be considered. It will be apparent, however, that these reports vary not only in scope, but in significance as well, because later work has in part been built on the foundation laid by earlier investigators who did not have the advantage of full understanding of the basic principles. Considering this and the volume of work, no attempt has been made t o indicate all subjects covered in each report. While i t is thought the most important points have been brought out and the major sources of infor- </p></li><li><p>INTERNAL CORROSION I N TIN PLATE CONTAINERS 329 </p><p>mation indicated, those wishing further details may find the items in the bibliography marked with an asterisk of particular interest. It will be apparent also that the reports on the various phases are hardly in complete agreement and this has introduced opinions of the author based on unpublished data and experience, particularly in instances where the literature does not appear to reflect all sides of some questions. </p><p>2, Academic Aspects </p><p>A review of this type may carry erroneous implications to persons not directly associated with the canning or allied industries. In discussing findings made in the various studies on corrosion, the reviewer neces- sarily must include some discussion of specific food products which prac- tically, under present conditions, offer little or no corrosion hazard. Means of control, such as alterations in packaging techniques, specifica- tion of the chemistry of the steel, and in some instances the application of enamel coatings have largely rendered corrosion by many products of academic interest rather than of practical concern. While f o r reasons which will become apparent, i t is not suggested corrosion losses in tin- plate containers will disappear, i t is reasonable t o expect that if advances in knowledge of tinplate over the next 18 years keep pace with those made since 1932 the role of this factor will have become more of academic than of commercial aspect. </p><p>11. TIN PLATE </p><p>1. Manufacture </p><p>Detailed information on tin plate malting practices is available else- where (Camp and Francis, 1940, IIoare and Hedges, 1945). I n general, low carbon, rimmed or mechanically capped open hearth o r Bessemer steel are used for this purpose. Typical analyses given by Camp and Francis (1940) and Nekervis and Gonser (1948) fall in the range .04-11% C, .25-.50% Mn, .05% Max. S, and . O l % Max. Si. Phos- phorus may be held low (.015% Max.) or allowed to range to amounts over .1% depending on the properties desired. For some purposes, restrictions are placed on certain residual metals. The range covered by these analyses is not a chance arrangement., but rather reflects the fact </p><p>These terms refer to steels which are not fully deoxidized and consequently tend Rimmed steel is made in an open </p><p>For mechanically capped steel a bottle top mold, The pressure built up soon stops gas evolution, </p><p>to evolve large volumes of gas duriiig solidification. </p><p>stantial thickness is built up. closed with a plug or cap is used. resulting in an ingot with a thinner rim. </p><p>top mold and the evolution of gas allowed to proceed until a skin or &lt; &lt; r i m of sub- </p></li><li><p>330 R. R. HARTWELL </p><p>that over a period of years several types of steel suitable for various purposes have been developed through cooperative research (Clark and Brighton, 1946a). A type L plate with characteristically low metal- loid and residual metal content, is recognized. A tin plate similar in metalloid content but less restricted in residual metals is known as MR plate. The MC type of designation indicates that stiffness is in part achieved by control of the metalloid content of the steel. </p><p>FIQ. 1. Cross section of hot dipped tin plate. </p><p>I n making tin plate, ingots of steel are hot rolled in a series of opera- tions to a thickness of about .06-.OW; cold reduced to tin plate gages (roughly . O l ) ; annealed ; temper rolled to impart some stiffness ; and subsequently tinned. This product is known as cold reduced plate to distinguish i t from the hot rolled variety which was the standard since the beginning of the can manufacturing industry ; reduction of this type of plate to final gage was entirely by hot rolling. Because of the shortcomings of the latter, and the economies arising from the former, cold reduced plate entirely displaced the older process 15 years after its introduction. Since 1943 no hot-rolled tin plate has been made in the United States and similar changes are taking place in other countries. </p><p>The coating on tin plate is one of the factors determining price and quality and it is customary to measure it in lbs. per base box, the latter term being the standard unit of area (31,360 sq. in.). For prac- tical purposes, a 1# per base box tin coating on tin plate is equivalent to about 60 millionths of an inch of tin on each side of the individual sheet of tin plate. Commercially, two methods for tinning exist, the older </p></li><li><p>INTERNAL CORROSION I N T I N PLATE CONTAINERS 331 </p><p>being hot dipping wherein the sheets after pickling are passed through a layer of flux into molten tin and then into palm oil where pairs of rolls control coating distribution while the sheet is being cooled to the point where i t will not oxidize excessively. A wide range of coating weights can be produced in this manner but most plate is in the common1 or standard coke grades. These have been referred to in the past as 1.25 or 1.50#/BB Cokes, respectively, the figures meaning the amount </p><p>FIQ. 2. Cold reduction mill (Courtesy of Jones &amp; Laughlin Steel Corp.). </p><p>of tin added to the tin pot for each base box produced, rather than that on the sheet. The coating weight on individual sheeti of hot dipped plate may vary considerably from the average of the delivery, and various portions of any one sheet also show similar variations. The extent to which hot dipped tin coatings vary has not been reported widely and Crombies (1949) paper is thus of particular interest for it not only gives the most detailed information on what may be expected, but also deals with the causes of the variations. </p><p>The nonuniformity of tin coating on hot dipped plate, inherent in the method of manufacture, has long been recognized as an undesirable characteristic and provided one reason for the development of electrolytic plate, made commercially possible by continuous cold reduction of the steel base plate. Basic operations for the electrolytic plate consist of cleaning and/or pickling, electroplating, melting of the tin coating, chemical treatment, and oiling. It will be seen tha.t there are not only differences in the method of coating the sheet, as compared to the hot dipping process, but in the treatment of the steel and deposit, as well. </p></li><li><p>332 R . R. HARTWELL </p><p>FIG. 3. Cold reduction mill (Courtesy of Jones &amp; Laughlin Steel Corp.). </p><p>FIG. 4. Electrolytic plating line (Courtesy of Carnegie-Illinois Steel Corp.). </p></li><li><p>INTERNAL CORROSION I N TIN PLATE CONTAINERS 333 </p><p>Production of this material began in 1937, but it was not until World War I1 that electrolytic plate became a substantial facttor in the industry and i t then played a major part in t in conservation. Tts LISP liar grown until the amount produced in 1947 and 1948 was nearly as great as that of hot-dipped tin plate and many consider lliat it will evenlually rep1:ice </p><p>FIG. 5. Factory view of caii making operatioiiu. </p><p>the older product. For economic reasons, coating weights on electrolytic plate have so f a r been lighter than on hot dipped, most production being in the .25# and .50# grades with some .75#/BC. Details of its develop- ment and applications, together with some hint of the difficulties en- countered, appear in several places [ Clark and Brighton (1946b), Stew- art and Pilcher (1944), h e c k and Brighton (1944), Pilcher (1944) and Brighton (1943) 1. Those interested in the processes and the prospects for electrolytic plate will find the paper by Hoare (1948) particularly informative. </p></li><li><p>334 R. R. HARTWELL </p><p>2. Mechanism of Corrosion </p><p>Although it was not discovered until work had been under way for many years, the most important fact about the internal corrosion of cans is that tin is capable of furnishing electrochemical protection to the underlying steel base. In early investigations it was presumed the oppo- site was true because in the usual electrochemical series of the elements tin is shown as being more noble, or cathodic to iron. In effect, this erroneous concept dictated that any protection offered by tin could only be mechanical in nature and that so fa r as any electrolytic effects were concerned, tin should actually help the steel base metal corrode. This possibility naturally led to some question (Kohman, 1927) as to whether tin plate was actually a suitable material for food containers. </p><p>The truth began to become evident between 1926 and 1928 from the investigations of several workers. Mantell and associates (Mantell and Lincoln, 1926; Mantell and King, 1927; Mantell and Lincoln, 1927) found tin was sometimes anodic to iron and postulated passive films as the cause, but considered that this effect occurred only in strcng electro- lytes. Thus tin would be anodic only in such mildly corrosive canned products as corn, to which is added a substantial amount of salt during canning. Lueck and Blair (1928a, b) together with Kohman and San- born (1928a, b, c) showed that, on the contrary, the anodic character of tin was the normal state of affairs, advancing relatise hydrogen over- voltages and low ionic tin concentrat,ions respectively as their reasons. In the latter concept, adsorption of dissolved tin by the food and forma- tion of complex ions were both cited as means of maintaining stannous ions a t a minimum. Culpepper and Moon (1928a) had a somewhat dif- ferent theory and Hoar (1934), in an interesting series of experiments, demonstrated the part that formation of complex tin ions could play and </p><p>There is a curious aspect to the modern appreciation of the protective...</p></li></ul>