Certain Aspects of Internal Corrosion in Tin Plate Containers

BY R . R . HARTWELL

General Research Laboratory. American Can Company. Yaywood. Illinois

CONTENTS

I . Introduction . . . . . . . . . . . 2 . Academic Aspects . . . . . . .

11 . Tin Plate . . . . . . . . . . . . 1 . Manufaclure . . . . . . . . . 2 . Mechanism of Corrosion . . : . .

111 . Corrosion Characteristics of Foods . . . 1 . Classification of Foods . . . . . 2 . Types of Corrosion by Foods . . .

I V . Effect of Food Components . . . . . 1 . Acidity . . . . . . . . . . . 2 . Reactivity with Tin . . . . . .

1 . Scope of Consideration . . . . .

Page . . . . . . . . . . . 328 . . . . . . . . . . . 328 . . . . . . . . . . . 329 . . . . . . . . . . . 329 . . . . . . . . . . . 329 . . . . . . . . . . . 334 . . . . . . . . . . . 336 . . . . . . . . . . . 336 . . . . . . . . . . . 337 . . . . . . . . . . . 337 . . . . . . . . . . . 337 . . . . . . . . . . . 340

3 . Miscellaneous Factors . . . . . . . . . . . . . . . . . 342 4 . Inhibitors . . . . . . . . . . . . . . . . . . . . . 342 5 . Measurements of Corrosiveness . . . . . . . . . . . . . 344

344 V I . Variations Produced by Canning Operations . . . . . . . . . . 345

1 . O x y g e n . . . . . . . . . . . . . . . . . . . . . . 345 2 .Vacuum. . . . . . . . . . . . . . . . . . . . . . 347 3 . Headspace . . . . . . . . . . . . . . . . . . . . . 347 4 . Processing and Cooling . . . . . . . . . . . . . . . . 348 5 . Storage Temperatures . . . . . . . . . . . . . . . . . 349

V I I . Materials Added in Canning . . . . . . . . . . . . . . . . 350 1 . Sugar and Sirups . . . . . . . . . . . . . . . . . . 350 2 .Salt . . . . . . . . . . . . . . . . . . . . . . 351

V I I I . Trace Elements . . . . . . . . . . . . . . . . . . . . . 352 1 .Su l fu r . . . . . . . . . . . . . . . . . . . . . . 352 2.Copper . . . . . . . . . . . . . . . . . . . . . . 355

IX . Relation of the Container to Corrosion . . . . . . . . . . . . 356 X . The Tin Coating . . . . . . . . . . . . . . . . . . . . 356

1 . Tin Coating Weight . . . . . . . . . . . . . . . . . 356 2 . Porosity . . . . . . . . . . . . . . . . . . . . . . 360

X I . The Steel Base . . . . . . . . . . . . . . . . . . . . . 363 XI1 . Tinplate Testing . . . . . . . . . . . . . . . . . . . . 366

XI11 . Enameled Cans . . . . . . . . . . . . . . . . . . . . . 371

V . Packaging and Storage . . . . . . . . . . . . . . . . . .

X I V . Electrolytic 'Pinplatc . . . . . . . . . . . . . . . . . . . 374 X V . Summary . . . . . . . . . . . . . . . . . . . . . . . 377

References . . . . . . . . . . . . . . . . . . . . . . 378

327

328 R. R. HARTWELL

I. INTRODUCTION

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 country’s 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.

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.

1. Scope of Consideration

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-

INTERNAL CORROSION I N TIN PLATE CONTAINERS 329

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.

2, Academic Aspects

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.

11. TIN PLATE

1. Manufacture

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

These terms refer to steels which are not fully deoxidized and consequently tend Rimmed steel is made in an open

For mechanically capped steel a bottle top mold, The pressure built up soon stops gas evolution,

to evolve large volumes of gas duriiig solidification.

stantial thickness is built up. closed with a plug or cap is used. resulting in an ingot with a thinner “rim.”

top mold and the evolution of gas allowed to proceed until a skin or < < r i m ” of sub-

330 R. R. HARTWELL

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.

FIQ. 1. Cross section of hot dipped tin plate.

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.

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

INTERNAL CORROSION I N T I N PLATE CONTAINERS 331

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

FIQ. 2. Cold reduction mill (Courtesy of Jones & Laughlin Steel Corp.).

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 Crombie’s (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.

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.

332 R . R. HARTWELL

FIG. 3. Cold reduction mill (Courtesy of Jones & Laughlin Steel Corp.).

FIG. 4. Electrolytic plating line (Courtesy of Carnegie-Illinois Steel Corp.).

INTERNAL CORROSION I N TIN PLATE CONTAINERS 333

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

FIG. 5. Factory view of caii making operatioiiu.

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.

334 R. R. HARTWELL

2. Mechanism of Corrosion

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.

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

There is a curious aspect to the modern appreciation of the protective ability of tin in food containers. Judged by its practical effect on the study of container corro- sion this observation is not 25 years old, but none of the workers in the field seems to have realized that anodic behavior of tin to iron in acid solutions was noted one hundred years previously. In 1828 De La Rive pointed out that many metals could be either anodic or cathodic to each other depending upm the electrolyte used and he commented on exposure of an electrode to air causing a reversal in polarity as well as noting that reversals often took place shortly after the electrodes had been immersed in the test solution. Both these observations have been more recently made with respect to tin-iron couples and De La Rive specifically shows tin as anodic t o iron in two concentrations of nitric acid. In 1840, Faraday also reported similar work which confirmed the relative positions of the two metals in nitric acid and showed the same relationship in hydrochloric and sulfuric. It seems likely that prac- tical progress on tin plate Corrosion would have been much more rapid had it not first been necessary to rediscover the forgotten observations of Faraday '8 time.

INTERNAL CORROSION I N T I N PLATE CONTAINERS 335

proposed this as the principal mechanism. While there has been no exact agreement on cause, there is no doubt of the ability of tin on a tin plate container to extend electrochemical protection to the iron base. This property emerges as the most important single fact about internal can corrosion, not only because it is responsible for the success of fruit can- ning, but also, as earlier mentioned, because it is the foundation on which the modern concept of corrosion inside a tin plate container rests.

Due to these relationships of iron and tin, the present theory of the corrosion mechanism is that while tin itself is relatively inert, in contact with the steel it corrodes, depositing hydrogen to polarize the ever-pres- ent exposed areas of the base metal, thus inhibiting their natural ten- dency to dissolve in acids. With the proper conditions, it appears that the reaction at the cathodic (steel) areas is slow, limiting the deposition of hydrogen and thus extending the influence of the tin over a consid- erable period of time. Because the hydrogen equivalent of the t in present in a plain hot-dipped container is more than enough to produce a swell, it would seem that the vacuum loss should occur a t a fairly uniform and noticeable rate as a consequence of the protective effect of tin. Fortunately, for one of several reasons, i t appears that very little hydrogen from this source accumulates in the can. Typically, if all con- ditions are satisfactory, vacuum losses during storage of cans tend to be relatively small until the point is reached where a great part of the pro- tective influence of tin is lost, whereupon hydrogen evolution from the steel commences at an accelerating rate. This tendency can be strongly marked, such as in one case cited Vaurio e t al., (1938) where some plain cans packed with peaches required about two years at 37.78”C (100°F.) to produce 1520% loss of vacuum, but two additional months brought about complete loss of vacuum. Because the major part of the vacuum loss occurred after virtually all the tin had been removed it seems ap- parent that such observations mean that solution of the tin in a container does not contribute much to the formation of a hydrogen springer. Re- gardless of the internal appearance, reaction between steel base and contents is the principal source of hydrogen and vacuum losses correlate well with dissolved iron. Corrosion failures are characterized by rela- tively high iron contents, and their tin content may be high o r low, depending on conditions.

While all observed facts are capable of fitting into this concept, it should still be regarded as the ideal case. The commercial side of the corrosion problem arises from the fact that while the protection given the steel base is powerful, it is not necessarily perfect. In some instances this can be reflected in an excessive solution of iron with consequent hydrogen evolution and a tendency toward a “pitting” type of attack,

336 R. R. HARTWELL

even though the steel is protected by the tin. I n other instances, corro- sion may take the form of a n accelerated removal of tin, presumably by hydrogen being discharged a t the steel too rapidly, and thus bring closer the time when the steel can be subjected to general attack. I n some cases both phenomena seem to occur. Actually, many individual factors appear to be capable of influencing the effectiveness with which steel is either protected o r attacked. Fundamentally these reduce to two types; (a) factors affecting the corrosiveness of the food product, and (b) factors influencing the corrosion resistance of the container. The corrosiveness of foods and the mechanics of the corrosion process attracted a great proportion of the initial attention to the problem and so probably provide the best point to begin a review.

111. CORROSION CHARACTERISTICS OF FOODS

1. Classification of Foods

Factors influencing the corrosiveness of food products may be divided fairly well into two groups: (1) intensity and type of corrosive attack inherent in the food itself, and ( 2 ) corrosiveness due to packing and storage conditions.

Probably one of the most frequent comments concerning the shelf life of canned foods concerns the widely varying intensities of corrosive attack exhibited by different foods. This is actually one of the most important phases of the comniercial problem and Clark and Brighton (1946a) mention that classification of foods fo r this property was a necessary preliminary to the development of tin plate specifications. They divided foods into 3 classes: the first, or highly corrosive class, comprising such products as berries, cherries, prunes, pickles, and sauer- krau t ; the second class, or moderately corrosive, included products such as peaches, pears, pineapple, and grapefruit ; peas, corn, green beans, tomatoes, meats, and fish were typical of the third, or mildly corrosive class of foods. The marked difference in intensity of corrosion is a prominent feature of many investigations and interesting information on this point will be found in the work of Culpepper and Moon (1929a), Morris and Bryan (1936), Hirst and Adam (1937), Hoar, Morris, and Adam (1939), Jalrobsen and Rlathiesen (1946) and others.

It is less commonly appreciated that different lots or varieties of the same food can show about as much variation in intensity of attack as may exist between different types of foods, h u t a number of reports which show this tendency are available (Vaurio e t al., 1938; Culpepper and Moon, 1929; Morris and Bryan, 1936; IIirst and Adam, 1937; Adam, 1944; Kohman, 1928). As early as 1915 (National Canners ASSOC.,

INTERNAL CORROSION I N T I N PLATE CONTAINERS 337

1917), investigation disclosed such an effect with apples and pumpkin, and it has since been found in some vegetables (Adam and Dickenson, 1943; 1945). The same fruit from different areas may vary severalfold in corrosiveness (Bohart e t a.l., 1934; Kohman, 1925), there may also be differences with variations in color and size of fruit (Kohman, 1925) ; ripeness of fruit may or may not be a factor (Hirst and Adam, 1937; Dickenson 1945 ; Kohman 1926) , apparently depending on the particular product. I n fact, it is probably fair to say that finding a twofold or greater variation between different packs of the same food in identical containers is the usual experience of investigators in this field.

2. Types of Corrosim by Fwds

While from the commercial viewpoint intensity of corrosion is the major interest, the type of attack produced may also be important be- cause of its possible influence on the choice of container materials or constructions. Various foods may exhibit considerably differing ten- dencies in this direction. While one thinks of a case, such as cling peaches, where the effect of the tin-iron couple is well marked, as the typical example, it is important to realize that some foods have a greater tendency to react with tin than others. Actually, there appear to be few foods which are entirely free of such properties. In others there is a marked tendency toward attack on the steel member, this usually being present in the more corrosive products, and this is very likely related t o the anodic influence which tin can exert under the particular conditions. This leaves a picture somewhat different from that presented by the idealized concept of the tin-iron couple alone and while little is written directly bearing on the subject, it is a well-recognized fact in commercial experience. These effects are well shown in such work as that of Cul- pepper and Moon (1929a) and Morris and Bryan (1936) with tin-iron couples or partially detinned tin plate. These studies make clear that the product influences the extent of the reaction with tin and, more important, the degree to which the steel is protected.

IV. EFFECTS OF FOOD COMPONENTS

1. Acidity

The reasons for the varied behavior of foods have been the subject of considerable speculation and quite apparently, considering the nature of the problem and the fact that nearly all canned foods have pH values under 7, food acidity should play an important role. The classification by Clark and Brighton reflects this belief since the products of highest acidity generally appear in the most corrosive class. Unfortunately,

338 R. R. HARTWELL

while the intensity is thus at least associated loosely with acidity, it is not proportional to it, nor is acidity alone a satisfactory measure (Morris and Bryan, 1936 ; Hirst and Adam, 1937 ; Culpepper and Caldwell, 1927 ; Kohman and Sanborn, 1924 ; GOSS, 1917 ; Adam and Dickenson, 1944). Leach (1899) seems to have been one of the first to notice this point, certain fruits with lower acidity being found more corrosive than some with higher total acidity. In further experiments with citric, malic, and tartaric acids of these strengths, he found the acids alone were even less corrosive than the fruit. Similarly, Hirst and Adam (1937) point out that in England white cherries are much more corrosive than the more acid gooseberries. Such comments are frequent in the literature and it appears well agreed that two products of the same p H or acidity will not necessarily be equally corrosive.

Some (Carrasco, 1934) suspect a tendency for a maximum corrosion rate in the neighborhood of p H 4, at least until considerably lower pH values are encountered. It is interesting to note that dried prunes, one of the most corrosive fruit products, have a pH in this range and that there is at least one recorded instance where an unusually corrosive pack of a product had a relatively low acidity. Although it is not necessarily the cause, a 1915 Committee (National Canners Association, 1917) formed to study tin plate noted that of 3 packs of apples, one had about two-thirds the acidity of the other two and had a much shorter shelf life.

Because of these indications, attempts have been made to reduce corro- sion by deliberately adding fruit acids. Kohman and Sanborn (1930) added citric, malic, tartaric, and maleic acids to dried prunes, reporting that they made the tin more anodic to the iron, thus presumably furnish- ing more effective protection to the steel. In the same article citric acid is referred to as inhibiting corrosion of black cherry cans. I n a later paper Kohman and Sanborn (1933) not only are data given showing the extent of improvement by adding citric acid to pureed prunes, but refer- ence is also made to the two-year successful use of lemon juice for this purpose. There is also a reference by Kohman and Sanborn (1934) to the use of strained cranberries and pineapple juice for this purpose. Morris and Bryan (1936) found this effect with cherries, but while Hirst and Adam (1937) thus inhibited corrosion by cherries with .2-.3% citric acid, they also indicate that larger additions accelerate the reaction. It is further pointed out that citric acid helped reduce corrosion only in dried prunes blanched before canning and accelerated reaction in those receiving an overnight preliminary soak, Furthermore, acid additions accelerated loss of canned strawberries and blackberries instead of in- hibiting it. For these reasons they regard inhibition due to acid addi- tions as not arising from a change in pH but instead, as resulting from

INTERNAL CORROSION IN TIN PLATE CONTAINERS 339

the inactivation of some naturally occurring accelerator present in cer- tain fruits. Dickenson (1946) supports this view.

Since the lack of correlation between quantity of acid or pH and corrosion had been realized €or many years, the discovery of the now accepted tin-iron relationship seems to have stimulated investigation of the possibility that the kind of acid might be important. Lueck and Blair (1928a), using .2N solutions of 13 acids, showed, by measuring the volume of hydrogen evolved from specimens of steel alone of coupled with tin, that the amount of protection exerted by tin depended on the kind of acid used. This varied from no inhibition, or even acceleration, to about 95% effective protection under the conditions of the test. Since the acids varied in strength, the same normalities produced solutions of various pH values but the degree of protection showed no particular relation to pH. It is worth noting that in these tests no protection was given to iron when coupled with tin in acetic acid, but in citric acid it was protected very effectively. The latter will be recognized as a consti- tuent of some of the more mildly corrosive fruits, while the severe corro- sion problems often attending products containing acetic acid are well known.

Along similar lines are findings of IZohman and Sanborn (1928~) that in .75% malic and citric acids, tin was anodic to iron, but cathodic in acetic, mdonic and succinic acids. Following this further, they showed by potential measurements in several acids that either tin or iron could be anodic or cathodic dependent on the type and concentration of the acids, This appears to be an important point to consider in regard to the effect of any specific acid on corrosion, although in the light of later work, it will be appreciated that the tendency of the steel itself to react induences the potential relationships between tin and iron in such in- stances. The quality of the steel used is not exactly known, but it was definitely poorer than that applied for fruits today and for this reason the range of conditions under which iron would be anodic to tin was probably greater. Hoar’s (1934) experiment fits into this picture also, for he showed that the potential relationships of tin and iron, both singly and coupled, depended on the acid used, and included a convincing dem- onstration that some acids had much stronger tendencies to form complex ions with tin than others.

All these papers seem to establish principles which fit the observed facts well and there are other references to the action of specific acids which are of interest. Kohman and Sanborn (1930) mention that sugar sirups containing citric acid permit more effective protection of steel by tin than when the sirup contains malic acid. Later Kohman and Sanborn (1935), added enough of various acids to dried prunes to lower the pH

340 R. R. HARTWELL

from 4.05 to 3.84 and report citric acid extended the shelf life to the greatest extent. Bohart (1935) in a similar experiment but apparently for the opposite reason added 6 different acids to Royal Anne cherries, adjusting to a pH level of 3.5, and found the greatest vacuum loss re- sulted from the addition of phosphoric acid. He also commented that cherries contained considerable natural phosphate. There are several references to the tendency of oxalic acid to react with tin (Kohman and Sanborn, 1928c ; Morris and Bryan, 1936 ; Jakobsen and Mathiesen, 1946 ; Culpepper and Caldwell, 1927 j Kohman and Sanborn, 1924 ; Goss, 1917) , and Morris and Bryan (1936) point out that the hydroxy acids differ in their behavior from the nonhydroxy acetic and succinic acids, a point which agrees well with Kohman and Sanborn, particularly as malonic acid is also a nonhydroxy acid.

From all this work it thus appears that internal corrosion of tinned containers is not only related in some way to the amount of acid, but to its nature as well. Some food products, however, particularly fruits, exhibit more intense corrosive characteristics than canned solutions of the acids themselves which makes it apparent that naturally occurring accelerators are perhaps of equal importance. That natural inhibitors may also coexist with them appears more than possible from experiments like those of Kohman and Sanborn (1928~) wherein the anodic relation of iron to tin in acetic acid was changed to cathodic by adding a miscel- lany of protein containing substances.

2. Reactivity with Tin

A few foods apparently possess a marked ability to react with tin. This reaction is distinct from the solution of tin brought about by elec- trochemical protection of the steel base and from the interaction of oxygen, food, and tin. Jnstead it is due to a specific capacity for the food to react with tin regardless of external influences and is naturally a factor in the corrosion process because tin lost in this way is not avail- able for electrochemical protection of the steel. While to some degree the tendency is exhibited by a wide variety of food products, many con- sider the subject more of academic interest because where marked, con- trol of corrosion is readily effected by the use of enameled cans.

Pumpkin seems to have been among the products where this effect was first noted (Leach, 1899), and the 1915 Committee commented (N.C.A., 1917) on a roughly three-fold variation in reactive power between their New York and Illinois packs. Huenink (1921a) recognized that some unknown corrosion accelerator was responsible and Cruess (1921) sug- gested this might be an amino compound. Culpepper and nloon (1928b) , however, believed the effect was due to nitrates because of the detin-

INTERNAL CORROSION I N T I N PLATE CONTAINERS 341

ning powers of citric acid-potassium nitrate mixtures and demonstrated amounts equivalent to 56.6 and 81.2 mg. KN03/100 g. in two varieties. They also report 638 mg./100 g. in a sample of beets, and, as might be expected, there is a reference (Jakobsen and Mathiesen, 1946) to the same substance in mangold stems. Recent unreported experiments also indi- cate that nitrates and possibly nitrites may be related to this reactivity.

Rhubarb has also drawn comment because of its reactive nature and Clough and Clark (1925) related this to the presence of oxalic acid, which has already been referred to as unusually corrosive in this respect. A measure of the reactive nature may be gained from the experiments of Culpepper and Moon (1929a).

Spinach has also been studied and there is one report (Jakobsen and Mathiesen, 1946) on aluminum containers relating corrosiveness to water- soluble oxalate content, more of which was found in the more corrosive smaller leaves. I n addition, the early summer crop had more water-solu- ble oxalates than the crop harvested in the fall. Adam and Horner (1936) have also suggested that amino compound may be important in spinach.

This last class of substances has often been thought of as involved in the reactive tendencies of fish and meats and there are reports (Jakobsen and Mathiesen, 1946; Jakobsen e t aZ., 1946) which relate the varying degree of this effect with several varieties of fish to the amoupt of trimethylamine oxide present, trimethylamine and a number of other substances being unable to bring about such an effect.

Possibly the anthocyan pigments, which have received considerable attention Kohman (1925) , Culpepper and Caldwell (1927) , Carrasco (1934), and Morris and Bryan (1931) should also be included as factors for they appear to have been involved in Leach’s (1899) observation on fruit. Interest in pigments originally arose from the fact they were a constituent of brightly colored fruits which at that time presented a highly critical commercial problem. Kohman (1925) felt that these sub- stances functioned as depolarizers, which would permit corrosion of iron to proceed with the formation of negligible amounts of gaseous hydrogen and demonstrated that addition of this type of substance to soaked apples accelerated corrosion. On the other hand, Culpepper and Caldwell (1927) report a detailed study of the coloring substances in a large number of foods and believed that the anthocyans were responsible for reactivity with tin and further commented that the effectiveness of these pigments was inversely related to acidity. Regardless of which is the more accurate explanation, with the better steel now available, the chief type of container failure formerly attributed to anthocyans (perfora- tions) is no longer common and shelf-life values for this type of product have been extended threefold or more. The anthocyan pigments, how-

342 R. R. HARTWELL

ever, are undoubtedly a factor in corrosion although probably their role is a less critical one than it once seemed.

3. Miscellaneozts Factors

There are several other comments regarding factors which can influ- ence the inherent corrosiveness of foods, such as a reputed accelerating effect of tannins (Culpepper and Caldwell, 1927) and spices (Hallman, 1941). Kohman and Sanborn (1930) mentioned an increased corro- siveness of dried prunes as compared to fresh, with the difference in potential between iron and t in becoming less. Many other empirical observations are mentioned, which although interesting in themselves, do not contribute greatly to an understanding of the basic factors.

I n short, from all reported work, i t appears that while the amount and nature of the acids, and certain known accelerating influences are promi- nent in determining inherent corrosiveness of a food, there are still unknown ingredients capable of influencing this factor. The limited knowledge about the most obvious factor influencing the service life of containers may well seem strange until it is realized that even if the identity of the various constituents were known, the knowledge would not furnish the most effective means of dealing with the problem. I n effect, i t has seemed that most investigators have decided that it was more important to attempt to arrest corrosion or a t least measure the net effect of the mixture of the ingredients in various foods than to deter- mine its exact nature.

4. Znhibitors

To many, inhibitors have presented attractive prospects as a practical solution to the corrosion problem, and a large number of substances has been tried. The limited possibilities of acid additions have already been mentioned in part, and Adam (1938) also recommends citric acid for cherries, blueberries, and green gage plums. Gelatin (Hirst and Adam, 1937 ; Jakobsen and Mathiesen, 1946 ; Morris and Bryan, 1931 ; Dawson, 1938) has been tried with some slight success and there appear to be sharply limited potential uses for thiosulphate (Hirst and Adam, 1937; Clough and Shostrom, 1930). Under certain experimental conditions such substances as disodium phosphate (Morris and Bryan, 1936; Pellerin and Lasausse, 1931) may be inhibitive, as may be sodium citrate (Adam, 1946). Agar-agar may apparently also inhibit (Anon., 1948) or be with- out effect Hirst and Adam (1937), depending on conditions.

Possibly in a class by themselves as inhibitors are stannous ions which strongly inhibit the acid corrosion of steel (Kohman and Sanborn, 1928c ; Hirst and Adam, 1937 ; Hoar and Havenhand, 1936 ; Morris, 1930 ; Koh-

INTERNAL CORROSION IN T I N PLATE CONTAINERS 343

man, 1929; Watts, 1912). There is some disagreement about the way they exact their effect, Kohman and Sanborn ( 1 9 2 8 ~ ) indicating them capable of raising hydrogen overvoltage on steel a t low current densities and Hoar and Havenhand (1936), studying the corrosion of steel, stating they were important in affecting polarization at the anodic areas. What- ever the reason, many have thought this property played an important part in the corrosion of tinned containers and it has been suggested that one of the most important functions of the tin coating is to furnish a supply of stannous ions (Kohman, 1929) which might well be in the amounts formed in the natural course of corrosion. This has led to the impression that additional amounts of tin compounds would further inhibit corrosion, but there appear to be no published data confirming such an effect in a tin plate container.

Presumably there have been conditions under which some of these substances have been found capable of inhibiting corrosion, for patents exist on many. Richardson’s (1931) and McConkie and Lueck’s (1932) covering casein coatings and gelatin, respectively, are typical. McConkie (1935) also patented an organic coating containing calcium sulphite, sodium thiosulphate, or other inorganic sulfur compound capable of inhibiting corrosion. The use of .25% or more pectin in the liquid part of the contents is the subject of claims by Whitfield (1940). Hirst and Adam (1937) found smaller amounts ineffective. Singleton (1944) states that salt in citrus juices impeded corrosion. The latter claim is interesting because others have indicated salt was an accelerator under their conditions. The discovery by Stevenson and Flugge (1939) that the use of substances such as thiourea in the end sealing compound in- hibited corrosion apparently aroused more interest. From the reports, it appears that in many cases it is a powerful inhibitor, but in others this compound had little effect o r even accelerated corrosion (Brighton, 1943; Dickenson, 1945; Hirst and Adam, 1945). The addition of salts formed by a corrosion process to inhibit the reaction is the general subject of study by Strauch (1937), SnO or Sn ( OH)z being suggested for tinned containers. Perkins (1934) has suggested use of aluminum or its alloys inside a container to inhibit corrosion by means of more active electro- chemical characteristics, although it would seem that hydrogen evolution from the aluminum was a possibility.

From the work reported so far, the prospects for inhibitors for any general class of products, such as fruits, do not look particularly bright. It seems clear that. most, if not all, are effective only under limited con- ditions and some, if applied to certain products, are capable of acting as violent accelerators. Other inhibitors may produce obvious changes in the characteristics of the product and the propriety of adding many of

344 R. R. HARTWELL

these to foods may be open to serious objection. In any event it is clear that since no universal solution is yet available, use of inhibitors is to be approached with caution and applied only after the most thorough in- vestigation and clearance with the proper agencies.

5. Measurements of Corrosiveness

Being unable to identify enough of the corrosive constituents of a product to predict its behavior accurately, and having no satisfactory inhibitor to fit all cases, it remains to devise some means of measuring the hazard. This seems particularly desirable in view of the large varia- tions in different lots of the same food product for knowing abnormal corrosion characteristics existed would permit early disposal of such lots.

Strangely enough, if the attempts to measure the weight loss of steel in various foods be excluded, there appears to have been little attention directed to the problem until Diekenson’s recent efforts along this line. He at first (1943) employed a “corrosivity index,” which is the per cent loss of a stccl sample tested by immersion in fruit syrup for 3 days, as related to the loss of the same sample in a preliminary two-minute pickle in boiling dilute hydrochloric acid, the latter test providing the necessary common basis fo r comparison. In later work (1944, 1945), the method was changed so as to obtain results a t several corrosion levels. The data were then plotted and the intercept with one axis used as the “index.” The most recent test (1946) consists of electrolyzing frui t extracts be- tween steel (tin plate base metal) electrodes a t various low current den- sities, measuring the hydrogen evolved, and using these data to derive constants indicating corrosivity. The methods have been used to study the effects of such variables as storage temperatures and periods, ripeness, and the like.

V. PACKAGING AND STORAGE

While the type and intensity of corrosion may be in a great part determined by the product itself, it has been realized for a long time that they may also be influenced profoundly by the operations in the cannery and during subsequent storage. Much of the work in this coun- try was done before 1932, when some degree of control over the corrosion problem began to be achieved by limiting the types of tinplate used. Considering that it was important to find some solution to the corrosion problem and that nothing effective had then been obtained from tinplate. i t is not surprising that many investigators became interested in the canning operations where there were factors which could be identified and controlled.

The exact practice employed in commercial canning depends on sev-

INTERNAL CORROSION I N TIN PLATE CONTAINERS 345

era1 factors, one of the most important naturally being the particular product being packed. Nevertheless, certain basic operations are com- mon to many foods, as follows:

Cleaning-While the purpose is obvious, it is of particular interest to the corrosion problem because it may influence the amounts of spray residues present.

Blanching-Food may be treated in hot water or live steam for a number of rea- sons. From the corrosion viewpoint, the more common role of blaiicliing in removing gases contained in the food is of greater interest, a s in blanching or scalding when employed to assist pecling.

Exhausting-After filling, some means are necessary to reduce the air and other gases in the container so that when sealed i t will have a satisfactory vacuum. This may be accomplished by exposure of the can exterior to hot water or steam, or scaling may be done under a partial vacuum. I n certain instances, gas flow or steam flow machines which sweep out the headspace gases with a n inert gas or steam at the time of closure may be used.

Sealin-Quite apparently necessary to prcvent reinfection of the contents after processing, this step is also of interest in corrosion because imperfectly formed or improperly treated seams can affect the corrosion shelf life through entrance of air.

Processiiig-This phase of packing has not been of broad interest to workers on corrosion.

Cooling-Water cooling is most widely used and commercially cans a re generally cooled to an average temperature of about 373°C. (100°F.) so tha t enough hcat remains to dry the exterior. The results of failure to cool to a reasonably low temperature have attracted some attention, particularly when subsequent storage has been under conditions where cans lose this licit very slowly.

Storage-Sincc the time cans a rc storcd before consumption is f a r longer than tha t involved in any of the previous operations, i t is reasonable to expect this t o be one of the most potentially important parts t o the corrosion problem. Good practice calls for cool, clean, dry conditions.

VI. VARIATIONS PRODUCED BY CANNING OPERATIONS

The majority of comments on this phase of the corrosion problem refer to a relatively small number of the specific variables, but Hirst and Adam (1937) discuss the entire subject a t some length. Bohart (1929, 1930) was interested in this general subject, and Esty (1928) refers to earlier work by Bohart and associates. Mrak (1929) and Mrak and Richert (1929a,b), have written on a study of hydrogen springer forma- tion in canned prunes, investigating nearly all possible variables associ- ated with canning which could conceivably influence the results.

1. Oxygen

This element has been recognized as playing a par t in corrosion pro- cesses for many years. Considering that blanching or some similar operation, where employed, exhausting, and sealing all could influence oxygen content, it is not surprising tha t it drew considerable attention

346 R. R. HARTWELL

at a rather early date. Baker (1912)) for example, mentions its part in the corrosion process, commenting on its rapid disappearance in the sealed can and the fact that no hydrogen appears until oxygen is con- sumed. Kohman (1923) and Kohman and Sanborn (1924) demonstrated the role of oxygen in canned apples by removing it through a. vacu- umizing process, replacing oxygen by nitrogen, and obtaining better shelf life. He also conclnded that a limited amount, trapped in the container, did not act as a catalytic agent in the sense of producing a permanently more corrosive product, but only accelerated the corrosion process until it has been consumed.

There are many accounts which indicate oxygen is a corrosion acceler- ator within the can (Hirst and Adam, 1937; Culpepper and Caldwell, 1927; Carrasco, 1934; Dickenson, 1946; Morris and Bryan, 1931; Pel- lerin and Lasausse, 1931 ; Mrak and Richert, 1929a, b ; Schmidt-Kielsen and BjZrgum, 1943 ; Bryan, 1930). A good proportion of these studies point out the very great increase in attack on the tin by fruit acids in the presence of this element. Apples appear to be a special case because conditions have frequently been encountered where the oxygen content helped lead to attack on the steel, causing perforations, and this effect conceivably could be related to the amount present or its distribution.

The effectiveness of certain special treatments such as the vacuumiza- tion (Kohman and Sanborn, 1929; Powell and McHenry, 1924) or soak- ing (Huenink, 1921b) of apples prior to canning operations is usually attributed to oxygen removal. Similar conclusions might be drawn from other work but i t is frequently difficult to decide how much of the effect can be ascribed to oxygen and how much to other factors. Clough and Clark (1925), for example, markedly reduced the corrosiveness of rhu- barb by prolonged soaking which undoubtedly removed gases from these tissues, but it is estimated 40% of the oxalic acid was removed as well. Similar possibilities exist in work with blanching Royal Anne cherries (Bohart e t aZ., 1934).

Normally, in foods canned under satisfactory commercial practice, small quantities of oxygen are present and consumed with little obvious effect except to dissolve some tin in plain containers. It will be recog- nized that where leakage occurs much more extensive damage can occur, depending on the amount of oxygen present. Many moderately corrosive foods, such as grapefruit or pineapple products, which do not usually present a serious problem, are very strongly corrosive in this case. A t its worst, leakage in plain cans may result either in complete detinning of can interior, perforation a t different spots, or most commonly severe detinning a t and near the liquid level accompanied by perforation a t this point.

INTERNAL CORROSION I N TIN PLATE CONTAINERS 347

2. Vacuum

It is usually difficult to separate in the literature the effects of the internal vacuum and oxygen because of their intimate relation to the effectiveness of exhausting, closing temperature, and other factors. These four interrelated variables have been the subject of comment by nearly every author on container corrosion, particularly in the early 1920’s when a larger than usual carry-over made perforations in cans for red pigmented fruits an even more serious problem than usual a t the time (Cruess, 1921 ; Huenink, 1921b ; Wiegand, 1921 ; Reynolds, 1921 ; Todd, 1921 ; Bigelow, 1922 ; McEwing, 1922).

There are few specific recommendations regarding these factors al- though there are some data, such as Bohart’s (1930), to show that increasing the effectiveness of the exhaust prolongs shelf life, particu- larly as compared to no exhaust. Hirst and Adam (1937) have similar information showing omission of exhausting resulted in one-third and nearly two-thirds reduction in shelf life for the two products tested, compared to their regular procedure. Both (Bohart, 1929; Hirst and Adam, 1937) have experimented with a clinched cover exhaust with the idea of preserving a larger proportion of water vapor in the headspace. The latter’s data show about 20% increase in shelf life.

It seems apparent that more effective exhausting and higher closing temperatures will tend to remove more oxygen, but that they will also produce higher vacuum readings which in themselves will furnish some increase in shelf life. I n any event, it is well agreed that it is advan- tageous to hold the vacuum as high as possible without encountering difficulties of other types, such as paneling.

3. H e a d s w e

Since larger headspaces presumably would tend to provide more oxy- gen, i t would be expected that this question was also related to air elimination. However, most of the many comments on the effect of head- space seem to emphasize the “hydrogen reservoir” theory ; that is, larger headspaces provide more room for the accumulation of hydrogen result- ing from corrosion and thus extend the time required to form a hydrogen swell. Bohart (1929), reports 40, 330 and 360 days required to produce 50% failure in a pack of Royal Anne cherries closed with gross head- spaces of 3, 5, and 7/16“ respectively, and suggests for #2$5 cans, regu- lation between the last two values. Hirst and Adam (1937) present calculations on this point, showing how a decrease in headspace reduces the amount of hydrogen required to destroy the vacuum. Adam and Dickenson, in dealing with methods of approach to problems involving

348 R. R. HARTWELL

corrosion loss (1944) consider gross headspaces of 10/32“ as the lowest safe limit. They suggest further that this factor be considered a con- tributory, and possibly the main cause for an outbreak of corrosion losses in a pack where the hydrogen swells have a headspace of 9-10/32 and the average of the normal cans is less than 11/32”.

I n spite of the agreement in published opinion there appears to be room for some doubt about the importance of headspace as a major influence on shelf life. It is frequently possible to obtain experimental data indicating that increased headspaces prolong shelf life, but it is also possible to obtain data, such as that shown in Table I wherein in- creased headspaces, even when made greater than normal, are associated with shorter shelf life.

TABLE I

Effect of Headspace on Shclf Life of #2 Plain Cans

Pineapple Juice at 70°F. Tomato Juice at 100°F. Can Lot 9/32” Hdsp. 20/32” Hdsp. Can Lot 9/32” Hdsp. 15/32” Hdsp.

39 75 mos. 70 mos. 56 16 mos. 14 mos. 59 72 59 126 13 8 40 19 19 15 20 17

4 Unpublished data of American Can Co. b Approx. number of months to produce failure in 50% of cans.

It is thought that all these facts suggest that headspace alone, within the limits of reasonable commercial practice, is not ordinarily a very significant factor in determining shelf life. The various data suggest that any conclusions regarding its effect should be considered not only in respect to its volume, but also in respect to the internal can vacuum and the effectiveness with which air is removed.

It is possible that the improvement in tin plate steels is partly respon- sible for the varying results reported. It will be seen that the more marked is the tendency for the greater part of the vacuum loss to occur near the end of the container life, the less important headspace volume will be.

4. Processing and Cooling

The search for factors capable of accelerating the corrosion process has even on occasion been extended to the processing and cooling of canned fruits which presumably were investigated because i t would seem that the longer foods are held at ‘elevated temperatures, the more extensive corrosion should be. There is little comment on severity of processing possibly because it obviously cannot be reduced below the point necessary for sterilization. Culpepper and Moon (1928~) mention that increased

IN‘I‘ERNAL CORROSION IN T I N PTJATE C O N T A I h E R S 349

spvrrity of process incwases the rvactivity of pumpkin towards t in on p la i~ i can4 which scrnis in liiic with wli;if might be cspccteil. I t 1)cw)nit.s inor(> (Iifficrilt to explain I h h a r t ’4 (1 930) results for I hrcc products (1iog;il Aiiiie cherries;, st r:iwlwrri~~s, antl 1og:inberries) where loiigcr pro- cvs,ing tiriirs clec.rc~:iseil corrosion wmcwhat.

illiiny authors (Vanr io ct al., 1!)38; l t i r s t and Adam, 1937 ; Ada111 and I)icltenviri, I!) $4 ; (hrrwsco, 1934 ; Uohart, 1930 ; Mrak, 7929 ; ‘J’0dc1, 1!)21 ; Jl(.Ii3wing, l!W; Md’onkic, 11137; Ilirst and Adani, 3 930) incxntion thc irtiportanc.c~ of adequate cooling or state that poor control of this factor is 0111’ of the caiisrs of conimcrcial corrosion loses. Specific recoinincrida- tions tire few, b u t l l i rs t and Rtlani (1937) suggest 37.8”C. (l0OoF.) or lower, a r i d i t appears r.cason;il)lc to rn;~intain this figure a t as low a valuc as possible, although it will l x rccogriizcd increasingly lower te1nperil- turcs reqrijre better drying of can9 to avoid rusting.

It seems quite possible that tlrcse gcneral sobjects might wcll bear additional investigation. Thcrc lravc Iwcn occasions where stacking at exccssive teml)erature~ has not only l m n respoiisiblc f o r an incrensed corrosion r a t e hiit a different type of corrosion as wcll. This has h e m notcd in conjiinction with the forination of a pink tliscoloratiori in cer- tain fruits together with a pitting typc of attack on the plain coniuiners, Icatling to perforation in sonic1 instiirlcacs, which was quite in cwntrast i n t h c usual niotlt) oE at t;ic.k. Al01ig this linc, Dickerison (l!IKl), incritions a n increiisrd ( ~ o r r o ~ i w i i r s ~ ol)sc.rvctl in fruit juiccl containiiig sugar w h m heated to a high but unspccifictl temperature fo r a short time. IJater, Aclain and 1)icltenson (1914) indi(*:ii(d that rorrosion accelerators de- velop on storage above 110°F. ancl that these arc ii cause of loss d r i r t o 11 o t s t aclc ing .

5 . St (I rag c l ’ e m p c m t ures

It will tw apparent thiit froin t hc service life standpoint, low storage tcmperatares should prodnce bettcr rcsiilts. Some of the earlicr data on this point canie froin Kohman (1929) who found coke cans packed with fruits and storcd a t 0°C. (32°F.) were equally as corrosion resistant as vans made. fiwm 1A or 3A charcoal coatings on the same stcrl, but storrd at ortlinary ttmpcratnrrs, presnnmbly about 21.1”C. (70°F.) I l i rs t arid Adani (1937) found the difference hctwecn 35 and 222°C. (95 and 72°F.) c;i~iwd a thrpcfold eliangc i n thcir p:tclrs of 10 fruits. Vi111ri0, Clark, and 1 ~ ~ l c (1938) indicatcd thiit the effect of storage temperaturcs varied with thr prodilct. These workers, and Hirst antl Adam (1937) indicate t in plate quality as a factor, the ratio of shelf life at low tempera- tiires to that at high, tending to brcorrre lowcr with the less corrosion resistant plate. Adam and Diekenson (1943) mention that vegetables

350 R. R. HARTWELL

stored a t 35OC. (95°F.) lose vacuum at 3 to 4 times the rate of those stored under normal conditions (England). Brighton (1943) , Pilcher (1944), Lueck and Brighton (1944), and Stewart and Pilcher (1944) exhibit data in connection with experimental work on tin plate substi- tutes which indicate that the ratio of shelf life a t 37.8”C. (70°F. t o 100°F.) varies with the product, a value of 3 to 4 being a common rela- tionship, but greater or smaller values occurring. Because the effects of temperature on increasing or shortening corrosion shelf life are compar- able to anything that can be accomplished by packaging or choice of container material, i t is difficult to over-emphasize its importance to the commercial problem.

One other phase of storage conditions has been commented on by Carrasco (1934) who suggests prolonged agitation such as occurring during sea voyages may be responsible for corrosion although the reason for this comment is not made clear. On the other hand, Kohman’s (1930) earlier experiment wherein the cans were rotated 12 times each day had failed to produce any indication of such an effect. Hirst and Adam (1937) agitating cans for 5 to 10 minute periods twice a day, similarly found no indication of acceleration of corrosion, but commented that continuous agitation might be different.

VII. MATERIALS ADDED IN CANNING

1. Sugar and Sirups

The materials common to many canning operations have also been investigated to some extent, sugar and sirups drawing considerable atten- tion. McEwing (1922) apparently made one of the earliest references to this in claiming a greater incidence of perforations in colored fruits packed in sirups than those canned in water. Similar comments were made by Kohman (1929, 1930), and Bohart (1929) mentions an experi- ment with glucose and levulose in which the rate of failure was greatest where the latter was present, glucose appearing to have no effect and invert sugars furnishing intermediate results. Later, however, Kohman and Sanborn (1935) conclude that sugars more generally tended to in- hibit corrosion, and Mrak (1929) found that increasing sirup concen- trations inhibited corrosion in dried prunes. Morris (1936) considered sucrose a weak inhibitor of the acid corrosion of iron, but a fairly power- ful one of tin, at least in air. H e concluded that in citric acid solutions sucrose increased the evolution of hydrogen from partly detinned speci- mens of tinplate made from a “slow corroding” steel. In spite of in- creased volume of hydrogen, the weight loss of the specimens was smaller which suggests that in this experiment sucrose functioned as an inhibitor

INTERNAL CORROSION IN TIN PLATE CONTAINERS 351

for the tinned portion of the samples and as an accelerator for the exposed iron areas. This work is also mentioned in a collaborative report with Bryan (1936).

From these and many similar reports it will be seen that there is poor agreement on the effect of sugars. This suggests that their true effect on the corrosion process is probably fairly small under ordinary circum- stances and that the contradictory nature of the reports is more likely due to unrecognized impurities. Various investigators have been quick to point out (Morris and Bryan, 1931; Clough and Shostrom, 1930) that mgars, especially those containing sulfur dioxide, could be a source of corrosion accelerators, and this may account for some of the disagreement. Morris and Bryan (1931) also showed that some sugars, particularly beet sugar, contained substances strongly inhibiting the acid corrosion of iron. This could account for further discrepancies and beet sugar (Morris and Bryan, 1936; Hirst and Adam, 1937) has been suggested as a practical measure for dealing with the hydrogen swell problem. Taken as a group, these reports suggest that inhibitors and accelerators occurring in sugars are probably much more important from the prac- tical viewpoint than the pure chemical substances themselves.

2. Salt

Sodium chloride is a constituent of many canned foods but it is not frequently found among those in the strongly corrosive class. Probably for this reason there is little information on its effect, although there is Clark’s observation (1923) that an increased salt content in canned ap- ples also increased the rate of container perforation. This observation was made during a series of experiments on the use of a brine soak to eliminate air, and from the perforation data for apples grouped according to 0 to .15%, .16 to .45% and more than .45% salt, it was suggested that an excessive salt content might well nullify the beneficial effects of oxy- gen removal. Kohman and Sanborn (1924) felt their data did not con- firm Clark’s conclusion on the effect of salt, although from an inspection of their results, it appears that there was some tendency in the same direction. It should be pointed out that i t is possible the difference in results arose from the tinplate. It was at a somewhat later date that the effect of this factor began to be properly appreciated and methods devised to prevent variations in its corrosion resistant properties from obscuring the results of such experiments as those above. I n the absence of a method of controlling the tinplate factor, there is some question of the significance of differences smaller than could have been caused by the variation in the tinplate of that time.

There are also two reports (Morris and Bryan, 1936; Pellerin and

352 R. R. HARTWELL

Lasausse, 1931) of experiments with acid solutions to which salt was added. In the first, it was considered that salt inhibited the corrosion of tin, and accelerated iron. Morris and Bryan agreed with this observa- tion in respect to tin in citric acid solutions and it seemed that with tinplate there might have been an accelerating influence. From these reports it seems quite possible that salt may have an accelerating effect on the corrosion process, although the points quite apparently need con- firmation with respect to canned foods under present conditions.

VIII, TRACE ELEMENTS

1. Sulfur

As may be inferred by the work reported on sugars, accidental conta- mination by various substances can be potentially important in the course of corrosion. Many possibilities exist, but there are few accounts, and of these the effect of sulfur has been the chief subject. Sulfur could be introduced in a number of ways, but spray residues appear to have been the leading cause and were responsible for the initial attention given the problem by Clough, Shostrom, and Clark (1924). Here as little as 2 p.p.m. of sulfur in the form of lime-sulfur spray markedly reduced the shelf life of plain cans for gooseberries, enameled cantainers faring some- what better. Culpepper and Moon (1928a, c) state that hydrogen sulfide, sodium sulfide, sodium thiosulfate, stannous sulfide, ferrous sulfide, and sulfurous acid in addition to the lime-sulfur spray all accelerate the formation of hydrogen springers in peach cans. They also present data obtained from immersion of tin and iron specimens in citric acid solutions containing flowers of sulfur, demonstrating an increased rate of attack on the iron. Morris (1930) found .5% citrate buffers containing 8 p.p.m. SO2 accelerated corrosion of iron only on the acid side of pH 4-4.5 and that between this and neutrality, SO2 actually inhibited corrosion.

Clough and Shostrom (1930) experimented with gooseberries, straw- berries, Royal Anne and Bing cherries packed with sirups containing 5 p.p:m. of sulfur in the form of SO2 or thiosulfate, finding acceleration in plain cans of all products but marked inhibition in enameled cans for cherries. There are many further comments concerning the role of sulfur in the corrosion process (Morris and Bryan, 1936; Carrasco, 1934; Hoar and Havenhand, 1936), and additional data, such as those of Hirst and Adam (1937) show that it most frequently functions as an accelerator for fruits, but may also be an inhibitor for white cherries in plain or enameled cans. However, Hirst and Adam later (1945) reported on a n experiment in which sodium sulfite in amounts corresponding to 2, 8, and 20 p.p.m. SO2 were added to the sirup used for packing gooseberries

INTERNAL CORROSION I N T I N PLATE CONTAINERS 353

and Pershore plums in both plain and enameled cans and loganberries and blackberries in enameled cans. From these studies it was concluded that sulfur was without effect in the concentrations used, a result which seems somewhat a t variance with other data reported. I n the same jour- nal, however, Adam, Dickenson, and Marsh (1945) concluded that ferric dimethyldithiocarbamate (employed f o r spraying) was capable of ac- crlerating corrosion in plain cans of black currants by decomposing t o form sulfide sulfur. I n a later report (1947) , two similar substances had the same effect and 7 other spray materials tested were considered harm- less from the corrosion viewpoint.

Horner (1939), added 5-10 p.p.m. of sodium sulfite, sodium sulfide, and sulfur to several fruits before canning and determined the amount remaining a t various times up to 6 months after packing. Gradual dis- appearance of sulfite and sulfide was noted (in part, a t least, going to form metal sulfides) although significant amounts remained after 6 months and in the former case small, but undetermined, amounts of sulfide sulfur were also found. Elemental sulfur produced traces of hydrogen sulfide in enameled cans and larger quantities (.5-.8 p.p.m.) in plain containers.

The work of Wiegand and associates (Wiegand, 1936; Wiegand et al,, 1936) presents a somewhat different type of information. Here it was intended to study the amounts of spray residues retained on fresh Italian prunes and their effects on shelf life. A maximum of .3 p.p.m. of reduci- ble sulfur was found on washed fruit, considerably less than is employed in most investigations. These seemed to have relatively small effects although there may be some question of unintentional container variation since the type of spray residue in some cases appeared to be an accelerator in charcoal tinplate cans and inhibitor in coke plate cans. Parallel experiments, adding t o prunes, 10 p.p.m. sulfur in the form of different substances show varying behavior ; sodium thiosulfate being noticeably worse than the sulfide or the element and cans with sodium sulfite ap- proximating the shelf life of the control. Similar information was obtained with Royal Anne cherries.

Since some of the references to the effect of sulfur date back to a time when the corrosion resistant quality of tinplate was poorer than at the present time, the following previously unpublished data in Table I1 is of interest.

The reduction in shelf life appears larger proportionally than pre- viously reported, possibly because the tinplate employed was better and the control lots thus had a longer life. Practically, it is estimated that these figures mean a reduction in shelf life from about 6 years to one year or less under normal warehouse conditions, due to contamination

354 R. R. HARTWELL

TABLE I1

Effect of Various Forms of Sulfur on Corrosion Shelf Life' of Plain Freestone Peach Cans

Amount S added

(p.p.m.) None .2

1.0 2.0 5.0

10.0 20.0

(Sulfur added immediately before closing)

As S As Na,S As Na,SO,

Serv. % Decrea.se in Serv. % Decrease in Serv. % Decrease in life serv. life life serv. life life serv. life

- 573 - 603 - 717 - - 561 22 409 29 287 52 - - 124 83 462 19 75 87 81 89 133 77 - - 76 89 42 93 55 91 76 89 31 94

- - - -

'Days at 37.8'C. (100'F.) to produce failure in 50% of cans. b Unpublished data of American Can Co.

with 2 p.p.m. of sulfur in some forms. They also illustrate that there is as yet no tinplate which will be corrosion resistant to any and all condi- tions found in the food product. For this reason it is equally as impor- tant to avoid abnormal conditions in the food as i t is to obtain suitable tinplate.

Failed containers packed with fruits where sulfur compounds have accelerated corrosion quite of ten lack signs of obvious corrosive attack. If contamination is severe for the product, usually the tin appears nearly intact, although some may be removed particularly around profile rings or similar areas and the tin often bears a visible brownish to bluish film. Removal of the tin coating will make it apparent that, in spite of the lack of superficial signs of attack, the steel has been partly or completely deprived of the electrochemical protection normally afforded by tin and is noticeably corroded. Some doubt exists concerning the mechanism by which this happens. Hoar and Havenhand (1936) mention the role of sulfur in reducing anodic polarization in the acid corrosion of iron and there is the comment that sulfur renders iron permanently anodic to tin (Morris and Bryan, 1931). On the other hand it would seem that the formation of a stable film on the tin could alone account for the observed facts (Adam et al., 1945).

While there is thus no uniform agreement on all phases of the subject, it seems plain that under certain conditions minute amounts of some sulfur compounds which can be changed to sulfide sulfur are capable of very greatly influencing and generally accelerating the corrosion of the container. Commercial experience has shown this to be a major source of trouble in the moderately corrosive fruit products such as

INTERNAL CORROSION IN TIN PLATE CONTAINERS 355

peaches. On the other hand, it is widely recognized that there are many foods, such as fish, meats, and some vegetables which are capable of forming sulfide discoloration in the container and yet have never been considered to present an unusual hazard. It may well be that observa- tions such as those of Morris (1930) regarding the failure of sulfur com- pounds to accelerate or even to inhibit corrosion at high p H provide the clue to this behavior.

The practical importance of this subject may be judged by the number of comments in the literature and it may seem strange that sulfur conta- mination of fruit continues to be one of the leading causes of early corro- sion failure. Actually i t is thought probable that sulfur compounds have played even more of a part than usually suspected in corrosion losses since experimentally i t is usually possible to recover only a portion of the sulfur known to be present immediately before closing. Thus it often becomes difficult to demonstrate significant amounts of sulfur contami- nation in containers which have failed from this cause.

It may be noted that although several spray materials have been inves- tigated for their possible effects on the corrosion shelf life, there are many for which no report appears. Until they are definitely known to be harmless from the corrosion viewpoint, i t would be well to bear in mind that some might have as great an effect as sulfur.

2. Copper

The only other trace element studied seems to be copper, although there are surprisingly few reports, considering the trouble this element has caused from time to time. Donauer points out (1923)) as would be expected, that foods containing dissolved copper will deposit it when put in a metallic container. It is apparent that this effect in acid prod- ucts could lead to accelerated corrosion, either by tending to strip the tin or by producing local attack on the iron, possibly leading to perfora- tion, depending on circumstances. There are two reports (Nehring and Nehring, 1931 ; Heuser and Krapohl, 1937) of German investigations of these effects, both subsequent to the use of copper compounds to color gooseberries and leafy vegetables green. A Norwegian report (Jakobsen and Mathiesen, 1946) also mentions copper salts in the blanch water as increasing corrosion in bean cans. Actually there have been many unre- ported investigations of trouble due to this source usually occurring when acid foods such as tomato products are handled in copper equipment, without first removing the oxide film built up while idle. Like sulfur, the control of contamination by copper deserves more attention than it has received.

356 R. R. HARTWELL

IX. RELATION OF THE CONTAINER TO CORROSION

Regardless of whether product, packing procedure, or container is the real cause for any specific example of early corrosion failure, the obvious signs of attack appear on the tinplate. For this reason tinplate has drawn the major share of attention and there have been two groups of factors considered as influencing the corrosion shelf life of containers. Of these, the amount and continuity of the tin coating attracted interest before much consideration was given to the steel, possibly because these factors are frequently the only index of performance for other clad metals and hence it is natural to assume the same considerations extended to tinplate.

X. THE TIN COATING

1. Tin Coding Weight

As pointed out by Hoare and Hedges (1945), the history of tin coat- ings over the past hundred years has been one of gradually decreasing weights presumably due in part to the relatively high cost of tin as compared to iron. At various times there has been a tendency to attribute whatever faults may arise in the tinplate container .to this trend and the first systematic inquiry into the effect of tin coating weights seems to have been made as a result of a considerable force of similar opinion in 1915. The committee in charge of this work (N. C. A., 1917) studied the problem by making 7 different lots of cans, each in 7 different tin coating weights ranging from about .8# to 3#/BB. (Base Box). Con- tainers made from these lots of t inplate were packed with various products considered to be representative of those presenting hydrogen springer or perforation, sulfide discoloration, and detinning problems. I n addition, the rust resistance of the various tin coating weights was evaluated.

The results of their work are worth considering in some detail because in spite of the more than 30 years elapsing since that time, substantially the same conclusions are true today. In brief, they found heavy tin coatings no cure-all. There tended to be less sulfide discoloration, e.g. , in cans coated with more tin but in no coating weight was complete elimination approached. Similar small variations in external rusting were also thought insignificant beside the fact that under poor storage conditions all plate rusted quickly and under good conditions, even the lightest were satisfactory.

For resistance to internal corrosion, more definite results were obtained, and the following data summarize the condition of one pack of apples after approximately one year’s storage.

INTERNAL CORROSION IN TIN PLATE CONTAINERS 357

TABLE I11

Per cent of Perforated Caus Occurring in Lots of Indicated Coating Weight

(Includes all steels)

Coating Weight, Ib/BB .83 1.03 1.19 1.40 1.70 1.96 2.94 Perforations, % 37.5 27.7 17.0 13.7 14.0 7.7 2.1

0 National Canners Association, 1917.

From these data it is apparent that increased amounts of tin deferred the incidence of perforations somewhat, but excessive amounts were still found even with the highest coating weights. Of greater significance is the following summary of their data showing the extent to which the 7 different groups of cans contributed to the total number of corrosion failures, irrespective of coating weight.

TABLE I V

Effect of Steel Source"

Proportion of total perforations due to each group of cans made from the same steels

(Includes all coating weights)

Lot w-1 w - 2 x - 1 x - 3 Y-1 Y-4 z - 1 Perforations, % 4.7 7.7 11.4 2.0 13.9 30.8 27.4

Rased on National Canners Association. 1917.

From the two summaries, it is plain that while tin coating was an important factor, some other variable associated with the steel had a t least as large an effect. Actually the most lightly coated part (approx. Bibs.) of lot X-3 was equal in performance to the heaviest coatings (approx. 2.91bs.) on lot Y-4. I n other words, the unknown variation in this experiment was equivalent to the effect of some two pounds of tin, or on a different basis of comparison, the tin used was 3 to 4 times as effective in the first case.

Since that time there have been references to the fact that heavier coatings are valuable in extending the shelf life of cans for corrosive foods (Kohman, 1929 j Huenink, 1921b ; Todd, 1921) and more data on the relative value of coke and charcoal plate from the 1917 and 1919 (N. C. A., 1923) extensions of the 1915 experiment. There was also an attempt to determine the role of the tin-iron alloy in the corrosion prob- lem (Kohman and Sanborn, 1927) by making cans from plate which had been held in the tin pot for various periods to thicken the alloy beyond the point normally encountered. It was concluded that this was certainly not helpful, and very possibly detrimental.

Carrasco (1934) recognized the value of heavier coatings but com-

358 R. R. HARTWELL

mented that corrosion resistance depended more on steel quality than tin coating thickness. Hirst and Adam (1937) express a similar view, illus- trating it very well by a series of 3 experiments in consecutive years with different lots of plate where charcoal plate (a) outperformed coke two- to fivefold, depending on the product, (b) was about equal to coke, and (c) was much inferior to coke, in this case the latter lasting 2 to 4 times as long. Later data from other work are available for various electrolytic coatings on the same steel used for plain peach and prune cans ( h e c k , 1942) and for many other products (Stewart and Pilcher, 1944, Brighton, 1943, 1944a, b) . Additional interesting data are reported by Hirst and Adam (1945) for an experiment where 3 different coating weights of tin were applied to two steels. With one steel, increased tin coating pro- longed shelf life markedly. For the other, pack results showed the coating applied (18, 21 and 25 oz./BB) made little practical difference to the results, although it is mentioned that statistical treatment of a portion of the data revealed a correlation not obvious from pack data. Presum- ably the difference in results shown by the two lots of plate arises from the variation in corrosion resistant properties of the steel.

An additional item of interest in the latter report is a comparison of 9 lots of #50 electrolytic plate with a 23 oz. hot-dipped coating for plums with some of the electrolytic plate nearly approaching the performance of the control, having roughly three times the coating.

Another type of comment concerns the coating weight variation nor- mally encountered in hot-dipped plate and seems to have as its basis the opinion that the lowest coated spot in a can determines its shelf life. No data to substantiate this view appear to have been published. I n plain containers a t least, if conditions are such that tin can provide the usual electrochemical protection it is difficult to see how there can be commer- cially significant differences between two cans with the same average coating but differing only in evenness of distribution. Perhaps one of the strongest indications of the minor effect of such a difference is the fact that under normal conditions corrosion failures in plain cans of fruit products do not occur until nearly all the coating exposed to the contents has disappeared. If the lowest spot were the controlling factor, failures should occur fa r short of this point. Variations in coating weight from the sheet average occurring over areas large enough to include one of the components of the can appear to be of more importance and un- doubtedly are one of the causes for the spread in results found in any experiment. It is probable that this is less important than it would a t first seem because there is less chance of having all 3 parts from the more lightly coated parts of the 3 sheets involved than if one sheet only were involved.

INTERNAL CORROSION IN TIN PLATE CONTAINERS 359

One early reference (Powell and McHenry, 1924) mentions variation in tin coating thickness as of potential importance, although no data to support this are given. Adam and Dickenson (1944) suggest that be- cause of tin coating variation, the average preferably should remain above 22 oz., and certainly should not fall below 20. This view seems to be based on three observations ; An expected 3- to 4-02. standard deviation of individual spots from the sheet average (Adam and Dickenson, 1944) which would mean much 16 oz. coated stock existed in a 20-oz. delivery ; a reported (Hirst and Adam, 1945) corrosion loss in plums (England) in cans bearing a 16-18 ounce coating ; and the statement that below l#/BB, the corrosion performance of tinplate is thought to fall off rapidly (1944). The basis for the latter statement is not very clear and it does not appear to agree very well with the 1915 work (N. C. A., 1917) nor with commercial experience since 1942 with common coke plates which have a substantial proportion of sheets with less than 1# coating.

From these and similar reports it seems reasonable to conclude that tin coating weight is important to the shelf life of containers for moder- ately and strongly corrosive products, but that as a determining influence it is outranked by the steel. So far as moderately and strongly corrosive foods are concerned, it seems probable that i t is most accurate to consider that when used on the same steels, two different coating weights will influence the shelf life by amounts fairly closely proportional to the quantities of tin present, if these are the only variables involved. It should be noted that the proportionality between corrosion shelf life and tin coating is expected only under conditions where the steel is suitable for the particular application and the corrosive characteristics of the food are normal. Departure from these conditions may be expected to decrease the degree of electrolytic protection afforded by t in and at the worst reduce it to a point were the tin acts as hardly more than a me- chanical barrier. In such circumstances it is not reasonable to expect tin coating will be as effective in extending shelf life as indicated above.

Because the relationship of tin to the corrosion resisting properties of the container appears to be generally misunderstood the subject deserves further comment. Nearly all the reports in the literature stress the im- portance of tin on the extension of shelf life resulting from heavier coatings and it, will be apparent that tin is indeed a major factor. Never- theless, a survey of the literature necessarily presents a distorted view because there does not seem to be any report which mentions an important fact known to many in the container industry; that is, there are almost never commercial losses due to reasonable variations in tin coating. This latter fact, and the previous comments on the importance of tin seem

360 R. R. HARTWELL

contradictory and to the average person are the hardest part of the problem t o understand.

It helps make the subject clearer to point out that the amounts of tin used for various corrosive products have been fairly well established by experience and that these furnish enough corrosion resistance to meet the usual marketing requirements conservatively. If the best that can be expected from tin is that i t influence shelf life in approximate pro- portion to the amount involved, i t will be seen that large reductions are needed to make serious changes in performance. Failure due to 207’0 or 30% reduction in tin coating should still occur after the relatively long time required to exhaust the protective powers of tin, but difficulties of this sort are rarely found in practice. Instead, typical commercial troubles are those where only a small fraction of the expected life is obtained and have as their true causes such factors as abnormalities in the product, packing procedure, trace elements, leakage, storage condi- tions, nature of the steel and the like. It is not frequent that average coating weight changes of as much as 30% are under consideration and it is reasonable these be a far less frequent cause of trouble than the factors listed above, each of which is capable of reducing performance as much as 80% or 90%.

2. Porosity

Porosity, the other factor associated with tin coating, also has been mentioned frequently as a highly detrimental influence, and some appear to have considered it as the chief factor determining corrosion-resistance quality. [Powell and McHenry (1924) ; Kohman and Sanborn (1927) ; Walker (1909) ; Serger (1922) ; Elwell (1924) ; Serger (1926) ; Mc- Naughton et d. (1932) ; Morris (1934)l.

Tests for porosity of tin plate have undergone many revisions and many methods have been suggested, the earliest being based on ferri- cyanide, employing this reagent under conditions which would cause small amounts of iron to dissolve and form the insoluble blue pigment a t points of base metal exposure. The test has been criticized frequently as misleading so far as the impression i t leaves regarding the size of the discontinuities. Furthermore, over the years since it was first introduced it has been found that the substances added with the ferricyanide cannot only influence the apparent porosity, but also be corrosive enough so that successive readings in the same location show successively greater num- bers of pores. For this reason, some of the results reported earlier may be open to question. McNaughton et al. (1932) investigated the factors influencing the test and improved i t to the point of giving reliable results. They also developed a method, for measuring porosity, dependent on

INTERNAL CORROSION IN T I N PLATE CONTAINERS 361

the development of rust a t pores when samples were immersed in hot water under controlled conditions. A most convenient method is re- ported by Kerr (1942) who exposed the tin plate to a solution of acetic acid, hydrogen peroxide, and ammonium thiocyanate. Where the base metal is exposed iron dissolves in the ferric state and the red complex formed makes a simple colorimetric determination possible. Pearson and Bullough (1948) point out further precautions which should be taken to secure reliable results from Kerr 's test. Later, however (Pearson, 1949) there is a comment that porosity measured in this way varied with the rate a t which the base metal corrodes. This worker, although giving no details, also mentions assessment of porosity by cathodic suppression of the solution of the exposed iron.

Information regarding the porosity of t in coatings appears in several places and some of the more recent data are well summarized in Hoare and Hedges (1945). Without going into detail it may be said that within limits, the porosity is an inverse function of tin coating weight and that while coke and electrolytic coatings contain pores in great numbers, the heaviest procurable charcoals are by no means pore-free. The average total area of iron exposed in a 1.5# coating has been variously estimated as .67 to .798 sq. mm./sq. meter (Hoare and Hedges, 1945) which is far smaller than usually supposed.

Initial interest in porosity arose when iron was thought to be anodic to tin, but even with this phase understood it would still seem that it should play some part in tin plate corrosion on the basis that without exposed steel, there could be no corrosion of the tin-iron couple type. Strangely enough, in spite of the many comments that porosity is a very important matter to internal corrosion resistance, there are no published data to prove it. On the contrary, the 1915 Committee (N. C. A., 1917) found no particular relationship between porosity and internal corrosion ; similar data were presented also in a later report (N. C. A., 1923). Lueck and Blair (1928a) concluded that the effect of porosity had probably been overstressed because of the anodic character of tin protecting ex- posed steel. More recently, a similar view has been expressed by Hoare (1939) who points out that in many cases more has been read into por- osity test results than is justified scientifically ; that it is a test for con- tinuity of coating only, that other factors influence corrosion resistance as well; and that no published correlation exists between porosity and performance. He concludes by stating that whether i t will eventually be possible to assign tin plate to various usages depending on the coating porosity, is a matter for argument although he entertains serious doubts about it. This also expresses very nicely the unpublished opinions of many American investigators in the container and tin plate industries

362 R. R. HARTWELL

who have arrived a t similar conclusions, not only for the reasons cited by Hoare but also because of data obtained from attempts to measure the effects of increasing porosity. The results of such an experiment, where five l/" shallow holes were made in the body stock with a hollow end mill, appear in Table V and are typical of the type of data obtained from such efforts. These depressions, extending one to two thousandths of an inch into the steel, have penetrated far beyond the tin coating and have a total area of .21 sq. in. or about .3% of the exposed inner surface. It will be noted that in addition to increasing the iron exposed, cold working of the steel is also introduced which means that if an error is introduced it is on the side of ascribing too much influence to exposed areas of iron.

TABLE V

Grapefruit Juice in #2 Plain Cans

Days at 100°F. to produce 50% failures

Tinplate A Tinplate B Control 197 days 413 days Increased porosity 188 377

0 Unpublished data of American Can Co.

While there is a change in service life due to the operations on the surface, it is of minor importance compared to the difference between the two lots of tinplate which had the same initial tin coating weight (.5#/BB) and porosity, and which were made from the same steel. I n brief, there is not much justification for attempting to improve porosity alone when it appears the time can be more profitably spent on other factors.

All this information, together with the fact that i t has not been neces- sary to attempt control of this factor in order to obtain more corrosion resistant tinplate, suggests that porosity usually plays a very much less important role in the corrosion process than was once thought. It seems probable that whatever correlation may exist between porosity and shelf life is more likely due to both being the result of the same factors, rather than one causing the other. In other words, it is the causes for porosity rather than the effect itself which is most likely to be of interest.

Comments in the literature regarding porosity usually refer to the tinplate as received and should be distinguished from the effect given the same name and arising from the operations required to form the can. It seems apparent that in forming an end one cannot very well avoid dis- rupting the thin tin coating and any method of measuring porosity will show some increase in profile ring or countersink areas. This fact alone may provide some basis for speculation on the possible relation of porosity

INTERNAL CORROSION IN TIN PLATE CONTAINERS 363

tests on flat specimens to the behavior of the completed can. Data such as those in Table V make it appear that corrosion effects actually due to increased porosity resulting from the forming of can parts are probably negligible and that any effect is more likely related to the deformation of the steel. Vaurio, Clark, and h e c k (1938) point out that forming of an end results in some loss in corrosion resistance as measured by the hydrogen evolution test, but that this loss was much larger in the case of samples giving very low test values. This effect was in part related to the mechanical qualities of the steel and so fa r as is known porosity tests do not furnish a satisfactory method of detecting it.

XI. THE STEEL BASE

I n view of the effect of the factors associated with the tin coating, it will be apparent that from the corrosion viewpoint, the most important part of tin plate is the steel. Several have commented on the influence of this factor on shelf life (Vaurio e t al., 1938; Hirst and Adam, 1937; Carrasco, 1934 ; Hoare and Hedges, 1945 ; Van Vleet, 1948) or as a cause of such troubles as sulfide black (Elwell, 1923) and in spite of the other variables entering into the problem it is through this medium that com- mercial control of the corrosion problem has been established in the United States (Van Vleet, 1948).

There could conceivably be many characteristics of steel which would influence its ability to be protected by tin, and of these the most obvious is the composition. The first actual work in this direction grew out of the 1915 investigation on the influence of tin coatings and involved two experiments, one in 1917 and in 1919 (N. C. A., 1923). From these it was concluded that increasing silicon, phosphorus, and sulfur contents adversely affected corrosion resistance, copper had no consistent effect for all products, and titanium treatment was generally beneficial. That they also felt other factors were involved is indicated by the statement “the outstanding feature of the investigation is that no tinplate made from any heat of steel, regardless of the method of manufacture, chem- ical composition, or weight of coating, gave service indicating a satis- factory solution of the problem of perforations.’’ Undoubtedly one important factor in reaching this conclusion was that none of the special lots of steel made for the work was as resistant as the commercially made control. This occurrence sharply points out the fact that there is more to a satisfactory tinplate steel than composition.

Additional data pertaining to this subject were not published for a number of years, although there are comments on the composition of steel said to be employed by tinplate makers (Bigelow, 1922; Serger, 1926). In the 1930’s, published interest again began and McConkie (1930)

364 R. R. HARTWELL

showed that copper in amounts from .5-2.0% was harmful to the shelf life of cherry cans and probably helpful f o r loganberries. Morris and Bryan (1931) mentioned Armco iron as having a superior corrosion re- sistance to citric acid than a tinplate steel base they employed and com- mented on a very large variation between different sheets of the latter. Carrasco (1934) also commented on this last fact and compared the chemical composition of 5 tinplate lots with the shelf life of cherry cans made from these materials, the total of carbon, manganese, sulfur and phosphorus in the steel apparently being considered important. Hoar and Havenhand (1936), measuring the rate of attack of mild steels in citric acid, concluded sulfur might be particularly important because i t reduced polarization of anodic areas and thus stimulated corrosion, whereas copper tended to counteract this and hence was also important. Their suggestions fo r tinplate were for a low sulfur rimming steel, con- taining twice as much copper as sulfur.

Hirst and Adam (1937) commented extensively on some of this work, presented additional information on the deleterious effects of phosphorus for some of the most corrosive products, and mentioned that of the common impurities carbon, silicon, manganese, chromium, nickel, and arsenic seemed to have little effect. Vaurio, Clark, and h e c k (1938) writing on the hydrogen evolution test, mentioned “low metalloid” or type L plate as being advantageous f o r the greatest corrosion hazards, which fit in fairly well with previous comments, but they also classified copper and possibly other alloying metals as harmful for certain pur- poses. Hoar, Morris, and Adam (1939) made extensive experimental packs in enameled cans and by statistical treatment of the results con- cluded that high copper-low phosphorus steels were better for 6 of 7 products, with the sulfur content being considered immaterial. I n a final report (Hoar e t al., 1941) on this subject involving the plain can portion of the work the effects of copper and phosphorus appeared incon- sequential. It is also said that for plain cans, steel base composition is not very important except as it affects “cathodic increment,” that is as it may permit more rapid discharge of hydrogen on :he cathodic areas of the steel and hence accelerate corrosion.

Another report (Hartwell, 1941) summarizes several years of experi- mental packs wherein phosphorus and silicon were found to have adverse influences and copper either detrimental for some products such as cherries, or helpful for other products, such as loganberries. Nickel, and possibly chromium were said to exhibit effects similar to phosphorus for certain fruits, whereas it was considered that neither carbon, manganese, nor sulfur content alone determined corrosion resistance, although in- got iron was outstandingly good in some instances. For certain of the

INTERNAL CORROSION I N T I N PLATE CONTAINERS 365

strongly corrosive products such as red sour cherries, relatively small changes in composition such as an increase in copper content to a value slightly above .06% and an increase in phosphorus much over .01570 reduced the shelf life by 75% which indicates the importance of steel composition for some uses.

The history of much of this work, so far as the United States is con- cerned, is summarized by Clark and Brighton (1946a) who traee the commercial development cjf better steels for tin plate. From this it is apparent that the process of obtaining more corrosion-resistant tinplate has been one of elimination where one factor has been found, removed, and the problem restudied. Thus silicon, phosphorus, coppeu', nickel, chromium, molybdenum, and other unspecified elements are suacessively mentioned as coming under control to make more resistant plate for the strongly corrosive products.

It will also be apparent that variables other than composition should be capable of influencing the corrosion performance of steel. IIoar and Havenhand (1936) considered massive cementite undesirable, which could mean that either the form in which the substance occnrs o r the steel processing necessary to secure i t is important. Stwl making prac- tice (BigeIow, 1922; N. C. A., 1923), heat treatment (IIirst and Adam, 1937 ; Carrasco, 1934 j Bigelow, 1922 ; Hartwell, 1941 ), various other points in the tinplate malting process (Morris and Bryan, 1931 ; Bigelow, 1922; Hartwell, 1941) as well as grain size and other microstructural details (Hirst and Adam, 1937; Carrasco, 1934; Hoar and Havenhand, 1936) have all been suggested a t various times and it has been indicated that commercially important, but unspecified, factors exist (Van Vleet, 1948). There is, however, nothing published which appears t o serve as the basis of a sound standard for any 3f these factors, and it is plain that room for more work exists in this field in spite of the frequent belief that composition and tin coatings constitute the only variables.

At first it will appear that much of the information on steel, and for composition in particular, is quite contradictory. Hoare and Hedges (1945) point out this is not really true with copper, for example, since the helpful results in certain cases and harmful in others really reflect the marked difference in the corrosive character of the various foods, which was previously mentioned. This factor and the unreported effects of other variables than composition, coupled with the large number of foods used in the researches, very likely account for the disagreement on the effect of various elements.

The fundamental requirement for tinplate steel appears to be the property of being easily protected electrochemically by the tin and the facts suggest this condition is easily enough satisfied with mildly corro-

366 R. R. HARTWELL

sive products so that differences due to Composition are very small or nonexistent. In fact, Clark and Brighton (1946a) point out that most of the tin plate produced before the effects of steel composition were known would have been satisfactory for the mildly corrosive foods. Un- der these conditions, a wide variety of steels will be adequate for the purpose and no advantage is to be gained by unnecessary restrictions on the producer. With increasingly more corrosive products, experience indicates that steels become increasingly difficult to protect until a point is reached a t which substantial variations in many elements or in other details of tin plate manufacture may be capable of causing noticeable differences in performance. Considering these facts it is not surprising that one investigator may report a marked effect of one element, and a sec- ond find none. Because the best composition of steel thus depends on the intended use there are few generalizations which can be made, unless it can be said that as corrosiveness increases steels with minimum contents of metalloids and residual metals tend to give better performance. Cop- per, having a dual role, can be partially excluded from this generaliza- tion. In any event, it is apparent that selection of such materials must be based on the results of considerable experience, particularly when it is considered that physical and economic factors also play a role and that in the lack of full knowledge i t is possible to make gains in one direction at the expense of the others.

XII. TIN PLATE TESTINQ

Among the many corrosion studies reported in the literature have been frequent observations on the variation in corrosion-resisting quality shown by various samples of t in plate. While these led to the investiga- tions just outlined, they also brought about an interest in the possibility of devising some test to predict tin plate quality. Basically, if corrosion losses are due to the interaction of the container with its contents it would not seem difficult to reproduce the process in the laboratory, but there are more limitations than at first appear. To have maximum value any corrosion test must furnish results in a relatively shcA time. Further- more, as Hirst and Adam (1937) point out, it is difficult to devise a test with “all factors present” which also implies in their proper proportions as well. The problem of adequate testing is made no easier by the dem- onstrated dependance of tin plate quality on several factors, and it has been confused by the fact that what is satisfactory for one food may be unsatisfactory for another. In addition, although it seems self-evident that no tin plate test can be of value unless there is a well established background of correlation with performance for the particular use con- sidered, in some instances this point has received little or no attention.

INTERNAL CORROSION IN TIN PLATE CONTAINERS 367

If the porosity and coating weight tests already mentioned be excepted, practically no work was reported on testing until after 1930. Since that time several suggestions have appeared ; these fall into two general types. One type of test attempts to measure simultaneously the effects of all tin plate properties on the corrosion process, setting up conditions thought to simulate those encountered in canning. The second type of test may be described as more arbitrary in nature, for the reagents used may produce effects not very closely resembling those brought about by foods and the

Bakelite Plate

FIQ. 6. Complete hydrogen evolution test unit.

A-Open bottom, glass bell shaped unit with ground-glass flange %Bronze base C-Threaded disk countersunk to take sample D-Reservoir for acid displaced by evolved gas E-Tin plate specimen G-Gas endiometer X-Special rubber gaskets Z-Spanner wrench, fastened to table a-Threaded ring which screws onto C, clamping unit together &Floating ring

368 R. R. HARTWELL

test conditions may bear no obvious resemblance to those existing in a can. Apparently the most successful of the latter type are those which attempt to measure a single important factor.

Of the two, the first type would appear preferable, if it could be developed, and that mentioned by the only report of this class has been successfully used in control of commercial production. This is the “hy- drogen evolution” test, described in some detail by Vaurio, Clark, and Lueck (1938), in which an attempt is made to duplicate the corrosive action of fruit by subjecting die-formed tinplate samples to the action of hydrochloric acid under standardized conditions. Under test condi- tions, evolution of hydrogen is a t first slow until nearly all of the tin is removed, a t which time less than 5 cc. of gas will have been collected. As the protective influence of tin disappears, hydrogen evolution acceler- ates, paralleling vacuum loss behavior in plain cans for fruits. Because arrival a t this “break point” in both test and pack foretells early com- plete failure, this is the logical end point for the test, but for practical reasons the time to produce 5 cc. is employed. The hydrogen evolution test was an outgrowth of an investigation of a large corrosion loss in the early 1930’s (Clark and Brighton, 1946a). Test results are influenced by the factors associated with tin coating (amount and continuity) and the steel base (composition, mill processing, mechanical properties) and it is the only test known to be in commercial use which gives some simul- taneous measure of both sets of factors. Properly applied, test values have been demonstrated by Vaurio, Clark, and Lueck (1938) to correlate satisfactorily with the shelf life of cans for moderately or strongly cor- rcsive foods and the test consequently became of practical value by per- mitting detection and diversion of tinplate which would have caused commercial losses in fruit packs.

Experience since the intraduction of the hydrogen evolution test indi- cates its application and significance are frcquently misunderstood so that some further comment appears desirable. Common misconceptions are: (1) Good test results mean the tinplate has superior performance for any purpose (actually, the test was devised for tinplate employed for moderately and strongly corrosive foods. There is no reason to believe its use is necessary, desirable or even has any significance when applied to tinplate used for other purposes). (2 ) It is a type of porosity test. While porosity is one factor influencing test results it was not devised for this purpose and porosity affects test results no more than i t influences the corrosion shelf life of containers. ( 3 ) Good test results alone guarantee satisfactory performance for moderately or strongly corrosive foods. Vaurio, Clark, and Lueck (1938) stress the point that satisfactory correlation between test values and pack results are obtained

INTERNAL CORROSION IN TIN PLATE CONTAINERS 369

only where “. . . correlating data have shown that variations in alloy content of the steel . . . do not have a specific effect on service life with the specific f rui t tested.” This thought can be extended to cover other steel conditions as well. (4) The test is a method of measuring t in coat- ing. The t in coating weight is one factor prominently affecting test results, but a high tin coating weight no mow guarantees good test results than good container performance. One aim of testing is to obtain plate of the performance level thought to be satisfactory, but probably a n even more important one is to obtain maximum test values per unit of tin coating and this is done by employing the best steel.

Among the weak points of the hydrogcn evolution test are those gen- erally found in any test of an empirical nature, including the importance of the gasket material. More fundamental limitations arise from the fact that no fruit has exactly the same corrosive characteristics as hydro- chloric acid, and many differ substantially therefrom. The copper con- tent of steel, for example, does not seem to reduce test results or the shelf life of peach cans, but can be a potent influence 011 the corrosion process for cherry cans. Because there is no universal electrolyte, fur- ther development of the “all-inclusive ” type of test appears to have discouraging prospects. Furthermore, while it is important to be able to predict corrosion behavior, in the long run i t is much more important to both producer and consumer to be able to recognize which of the many factors are responsible for corrosion. While the hydrogen evolution test is the only recorded attempt of the “all-inclusive” type of test, there are a relatively large number in the other category. Hoare (1939), for example, mentions a test devised by Gire to determine how susceptible tinplate was to sulfide formation. I n the same article it is mentioned that the same investigator also developed a test in which tinplate is exposed to the action of acetic acid for 10 days, the amounts of t in and iron dissolved being related to corrosion-resistant quality. A similar idea, but of a more qualitative nature, is mentioned by Carrasco (1934) who immersed tinplate samples in acetic acid-hydrogen peroxide mix- tures. The results were obtained by visual examination of the corroded specimens and greater tendencies to undermine the t in coating by corro- sion of the underlying steel were said to indicate poorer quality tinplate. The same author was also interested in measurements of ductility as a possible index of quality, and thought i t likely the more ductile steels tended to be more corrosion resistant. However, with none of these reports are there enough correlating pack data to base a n estimate of the probable value of the tests proposed.

The most interesting tests of this general type are the studies of the rate of acid attack on the steel base of tinplate, undertaken because the

370 R. R. HARTWELL

nature of the steel has appeared to be the determining factor for many of the corrosive products, particularly in enameled cans. The most detailed studies are those of Hoar, Morris, and Adam (1939, 1941) where significant correlations between shelf life and corrosion rate of the steel in citric acid were found for several products. Significant correlations between potential measurements during acid corrosion of the steel and pack life were also found. It would be expected that an extract of the fruit itself would permit for a better test but in the three instances tried, it proved to be worse. It seems to have been concluded that boiling 11% hydrochloric acid was a better test medium. The latter test is suggested by Adam and Dickenson (1944) as a tool to be used in the investigation of corrosion losses but the wide range of values given suggests it was not considered to be a very precise measurement a t that time. There is the later comment by Hirst and Adam (1945) of some indication that this method was a less reliable measure than had previously been thought. Subsequently, Dickenson mentions the possibility of employing an elec- trolytic method (1946).

Indicating the discrepancies which are encountered in this type of work is the work of Rhodes (1938) who published the results of tests wherein tinplate steel was corroded in citric acid solutions and fruit extracts. He concluded that citric acid tests bore some relation to the results of packs but that the actual fruit was better, although the tests are described as providing only “ a fair measure of correlation.”

Judging from the disappointments implied from the changes in test methods, i t seems likely that measurements of rate of acid attack of the steel or similar tests of the steel alone as a measure of container quality are subject to more limitations than might at first be thought. Basically such tests seem to involve the assumption that what happens with steel alone will parallel tinplate performance. While this tendency is not only expected but also obtained in some instances it would seem that what is really needed is the behavior of steel under the protection of tin. The reports suggest these need not be the same, particularly as finer degrees of distinction are sought and substances other than the actual product used €or a test medium.

I n brief, from what has been learned to date in regard to testing, the fact is outstanding that one is not likely to achieve much success without considering the ultimate use of the material. Even when attempting to consider groups of similar products, such as fruits, in relation to hydro- gen evolution test results, enough discrepancies appear to make it plain that there are not promising chances of developing any single test which will faithfully reflect in their proper proportion all tinplate factors im- portant to corrosion shelf life. On the other hand, the test for a single

INTERNAL CORROSION IN TIN PLATE CONTAINERS 371

tinplate factor offers the advantage of being able to determine the cause for varying behavior. I n view of the number of variables influencing corrosion performance, however, it seems plain that the degree of corre- lation between pack behavior and a test for any one factor must neces- sarily be somewhat less than many interested in testing tinplate would desire. This handicap could be overcome by employing a series of short, simple tests for the various special properties of demonstrated importance to the particular use, and experience so far suggests that tinplate testing eventually may best be done i n this manner. I n the sense that the various elements going to make up the composition of steel are special properties, it will be recognized this is essentially the technique men- tioned by Clark and Brighton (1946a).

XIII. ENAMELED CANS

Enameled cans are used for such purposes as preventing sulfide dis- coloration or the bleaching of certain brightly colored products, par- ticularly fruits. Many of the latter are among the group of strongly corrosive foods and from an early date constituted an important problem to the industry. For this reason frequent reference has been made in the literature to the relative corrosion resistance of enameled as compared to plain cans and the usual comment indicates shorter service life is to be expected. One of the first and most interesting of these is Walker and Lewis’s (1909) investigation wherein i t was found that enamel coat- ings intensified corrosion in strawberry cans. They were able to show that the coating used was capable of functioning as a depolarizer. The impression apparently persists today in some quarters that such tenden- cies still exist, so it is worth noting that the coatings they investigated were cured at very low temperatures and tha t with more effective bakes this effect became negligible. For a great many years, coatings have not been cured commercially in the manner which Walker and Lewis found to accelerate corrosion.

I n subsequent years many other comments appear, not based on the above concept, bu t with the general observation that in enameled cans corrosion hazards are greater than in plain (Kohman and Sanborn, 1928b; Hirst and Adam, 1937; Carrasco, 1934; Kohman and Sanborn, 1930; Adam and IIorner, 1936 ; Morris and Bryan, 1931 ; Pellerin and Lasausse, 1931 ; Kohman, 1929 ; Bigelow, 1922 ; Wiegand, 1936 ; Wiegand et al., 1936; Morris, 1934; Bigelow, 1928; Mrak and Richert, 1931). Some are more explicit, citing scratches in the organic coating as of particular importance. Kohman (1929) pointed out that damage to the body wall due to double seaming could affect the results and Hirst and Adam (1937) discuss damage due to stamping ends. Because of these

372 R. R. HARTWELL

observations, there axe comments on the implied necessity of perfectly continuous organic coatings, Morris (1934), for example, discussing a can given two roller coats of enamel followed by a spray coat. Hirst and Adam (1937, 1939) go into this in some detail, stating that the chief practical problem connected with the manufacture of enameled fruit cans is to find some means of making cans with a perfectly continuous coating. They present data showing large increases in shelf life for spray lac- quered cans (after forming) and for enameled cans which had been recoated by hand after forming on the countersink and side seam areas. Brighton (1943) and Lueck and Brighton (1944) also present data on side seam striping which also show large increases in shelf life.

The opposite suggestion was also made by McEwing (1922) j that is, if enameled cans present a worse corrosion hazard than plain and per- forations tend to appear in certain areas, perhaps it would be possible to leave relatively small unenameled portions neax those points and obtain a good share of the benefits of each type of container. No pub- lished record of such an attempt apparently appears, but i t was tried in this laboratory and did not provide a solution to the corrosion problem existing at that time.

The reason for the behavior of enameled cans in these instances was given considerable thought and there were many who appeared to feel it only natural that since smaller amounts of metal were exposed, corro- sion at those areas should be intensified. Some of these opinions appear to be based on the concept that the entire corrosive capacity of the con- tents was directed a t the exposed areas, although there does not seem to be any very logical reason for this, Kohman and Sanborn, however, who also devoted considerable thought t o the problem, associated all these observations with the results of experiments (Kohman and Sanborn, 1928a, c) showing that the amount of electrochemical protection given iron by tin could become less as the ratio of iron to tin increased, and pointed out that this condition existed in enameled, as compared to plain cans. For this reason, they state (1930), the iron which may be fairly well protected a t the beginning, although less than in a plain can, also loses the protection more rapidly because the tin remaining becomes progressively less able to cope with local couples on the iron. Presumably unusually efficient cathodic areas on the steel would also produce the same net result. It will be recognized that this concept is basically the same as the consensus of many workers mentioned at the beginning of this paper ; that is, the difference is a matter of efficiency of protection.

While there was no reason to consider i t a t the time in view of the quality of tinplate materials available, it will be apparent that in addi- tion to the obvious remedy of complete protection of the metal by organic

INTERNAL CORROSION I N TIN PLATE CONTAINERS 373

coatings, tlie shelf life of enameled cans could be improved by employing more easily protectible steels because this would permit t in to be effec- tive for a longer period. That this is one of the benefits resulting from

FIG. 7. Architecture of the enanieled sanitaiy tin can.

A. TIIE SIDE SEAN. Tlic edges of the c a n l~ody are first hooked and thrn bumped or flattened together. Then final sealing is accomplished by solder- ing the outside of tlie side scam.

B. TIIE NOTCH. I f side seam were extended to can end, four folds of mctal would have to be inclnrlrd in the double seam. Body blank is notched, Iiowcrer, so that only a double layer of metal extends into the double seam. This permits tighter sealing.

C. THE TIN PLATE. This cross-section shows tlie relative thicknesses of component layers of tin plate. Steel is large segment; first layer on eitlier surface is tin-iron alloy, second is tin. Insidc surface is enamel coating.

n. THE DOUBLE SEAN. Tlic curl nn the can end containing sealing com- pound and tlie flange on the can body are indexed and rolled flat, forming fise folds of metttl. Sealing compciriiici between folds gives an air-tight seal.

the conimercial changes outlined by Clark and Brighton (1946a) is per- haps best indicated by the almost complete disappearance from American literature of references indicating perforations as the major hazard asso- ciated with canning strongly corrosive fruits in enameled cans. Without detracting from the benefits to be secured from complete enamel coverage, it would seem tha t improved steel is a necessary preliminary factor, for

374 R. R. HARTWELL

perfect enamel coverage would be difficult to maintain through all opera- tions to which cans and canned foods are subject, and in the end would provide mechanical protection only.

While the majority of comments in the literature have stressed the increased corrosion hazards associated with enameled cans, there have been an increasing number of reports where enamel coatings have been used to minimize corrosion. Huenink (1921a), Cruess (1921), and Cul- pepper and Moon ( 1 9 2 8 ~ ) mentioned that enameling cans reduced the rate of attack on the t in coating by pumpkin; this is the standard con- tainer for the product. The latter worker also tried enameled black iron containers, finding that they formed hydrogen springers quite rapidly. Similar results for blueberries were obtained by Kohman and Sanborn (1927), showing that even if enameled, the t in was still a factor. There are other comments (Clough and Shostrom, 1924, 1930) to the point that enameled containers tended to be better than plain for certain fruits contaminated with sulfur bearing substances. Clough and Clark (1925) also mentioned that enameled cans gave better service life for rhubarb. Since electrolytic plate appeared there have been a number of instances reported ( h e c k and Brighton, 1944 ; Brighton, 1943) where enameled cans not only outperformed plain with certain foods, bu t in some cases the enameled can a t least equaled the shelf life of plain cans with double the coating. It will be noted that in many of the reported instances the foods involved are those in which the capacity of the product to react with the t in plays a major role i n the corrosion process in plain containers.

From all these reports, it seems difficult t o justify any general state- ment from the corrosion standpoint regarding the superiority of either the plain or enameled can to the other. The facts suggest tha t the relative behavior of the two must depend on a t least three factors; the corrosive characteristics of the food, the amount of t in coating, and the corrosion- resisting character of the steel. There is no way reported to apply this information to a specific case except by experience based on trial packs.

XIV. ELECTROLYTIC TINPLATE

Since 1937, when electrolytic plate first began to be available, results of work with this material have gradually appeared, especially in con- nection with t in conservation during and subsequent to the war (Clark and Brighton, 1946b; Stewart and Pilcher, 1944; h e c k and Brighton, 1944; Pilcher, 1944; IIirst and Adam, 1945; Lueck, 1942; Brighton, 1944a; Maier and Flugge, 1944). “Electrolytic plate” has so far auto- matically signified lighter coatings than on hot dipped so i t is clear that the problems surrounding its use have been somewhat different than those reported prior to that time. Previous efforts to improve corrosion

INTERNAL CORROSION I N TIN PLATE CONTAINERS 375

shelf life tended to be along the lines of providing adequate shelf life by any means, whether by work on the product, packing or storage con- ditions, or the container itself, and in the latter case, no restrictions were placed on coating weight in order to meet this aim. With electrolytic plate for processed food products, the problem has been more how t o employ the lightly coated stock so as to obtain either the same service life as with hot dipped, or a shelf life long enough to leave no doubt of a successful practical solution.

In view of the fact that shelf life is about proportional to t in coating weight, all other things being equal, corrosion researches during the war were centered on the mildly and moderately corrosive foods, as well as on various details of container construction. From the references cited above three factors concerning the application of electrolytic plate appear to be outstanding. The first of these is the previously mentioned fact that for many foods enameled electrolytic cans have better corrosion resistance than plain hot dipped. Enameled containers are not generally used, however, for such products a s peaches and pears because they become darker than in plain cans. F o r this reason the fact that the service life of hot-dipped cans could be approximated and sometimes sur- passed by using cans with plain hot-dipped bodies and enameled electro- lytic ends fitted in well with the tin conservation work and deserves to rank as a second important factor.

The third, and more unfortunate point, appearing in published work is that to date electrolytic plate has left something to be desired so f a r as plain packers cans are concerned. To be helpful in the t in conserva- tions program it was necessary that one be able to rely on a shelf life reduced in direct proportion to the coating weight change involved. Brighton (1943) pointed out that in 1942 much electrolytic plate came far from meeting this requirement, even when the hot dipped and elec- trolytic were made from the same steel. Later, with Lueck (1944), this same subject is mentioned and the variation between different lots of electrolytic plate illustrated. It seems apparent that until electrolytic plate has both reasonably good and consistent corrosion resistance propcr- ties, its potentialities for plain packers cans will be sharply limited. These considerations of course, do not apply to those cases where it has been employed in the enameled form.

Since the end of the war there has been no published information on plain electrolytic plate, but from 1942 the reason for its eccentric corro- sion behavior has been under investigation. It is expected that in the near future an article dealing with this work will appear, indicating steel surface conditions play an important role.

Factor identified as influencing corrosion shelf life

(1) Nature of product

( 2 ) Contaminating substances

Steel quality

Storage tern- perature

Tin coating weight

Oxygen, ex- haust. vacuum, fill, cooling

TABLE VI

Advances in Study of Corrosion Phenomena in Tinplate Containers

Factor affects

Shelf life and type of corrosion

Shelf life and type of corrosion

Shelf life and type of corrosion

Shelf life, some- times type of cor- rosion as well

Shelf life

Usually shelf life, sometifnes type of corrosion

(7) Plain or Shelf life enameled cans

Degree of potential influence

Up to tenfold or more for entire range of products packed.

Up to fourfold for individ- uals of same product.

Up to fivefold or more, de- pending on circumstances.

Up to fourfold, if various steel grades a re applied in- discriminately. With selec- tion, variation is much less, becoming most noticeable in itrongly corrosive foods.

Up to fourfold, and more in exceptional cases, with tem- perature ranges usually en- countered.

For corrosive products in hot- dipped plate, usually about in proportion to coating weight; a.e a drop in average coat- in; weight from 1.50# to 1.35#/BB should lower shelf life about 10%. assuming a normal corrosive character of the food and satisfactory steel.

No accurate estimate avail- able for range of recognized good commercial practice.

May be as large as twofold or more in either direction, depending on (l), (3), ( 5 ) .

Practical progress

None directly affecting product except for use of inhibitors and addition of acids in certain instances. Corrosive products are dealt with by selection of steel, tin coating, and type of container construction to pro- vide satisfactory level of performance. In any container, however, variation due to product persists.

Avoid sources of reducible sulfur com- pounds and copper salts.

The method by which commercial control of the corrosion problem has been established. Satisfactory level of performance is secured by choice of proper steel type and accom- panying coating, selection being based on the results of experience and trial packs.

Keep cool as practicable without inducing other troubles such as sweat rusting.

Without sacrifice of shelf life, and in many cases with a n improvement in this respect it has become possible to use lower tin coat- ings on containers of many foods with the accompanying lower costs. Improved steels made this possible and also furnished the basis for effective tin conservation during and subsequent to the war.

Neither overfill nor underfill. Without bringing about troubles of other types, use most effective exhaust or other means of reducing oxygen and maintaining good vacuum, cool to lowest practicable tempera- tures.

0 4 Q,

Future needs and trends

Means for measuring cor- rosiveness of foods with aim of detecting abnormal properties in raw or canned stock.

Evaluation of new sub- stances such as spray resi- dues, which offer potential source of contamination.

Removal, to a reasonable degree, of remaining causes ? of corrosion resistance vari-

F

ation. E z F

Depending o n p rogres s made on (3) still lower tin coatings will be usable for a considerable number of products.

INTERNAL CORROSION I N T I N PLATE CONTAINERS 377

xv. SUMMARY

One method of expressing the results of the many years of study given to corrosion in tinplate containers is shown in Table V I where an effort has been made to summarize what appear to be the most important factors.

In brief, it appears one should think of the corrosion problem in tinplate containers as affected by two groups of factors, one associated with what goes into the can and how it is handled, and the container itself. Each is of equal importance not only to the final result, but also to the type of attack produced as well. Thus short service life coupled with unusually rapid loss of the tin coating is most frequently due t o the first set of variables, but can also be due to tinplate or leakage. Similarly, a marked tendency toward pitting of the steel base is con- sidered by many t o indicate unsatisfactory tinplate, but actually its true cause as often as not lies elsewhere and it is apparent that more than visual inspection is required to distinguish between them.

Of the two controlling sets of factors, those associated with the product are distinguished by the fact that they were recognized early, but with the exception of what might be termed external influences, such as stor- age temperatures and contaminants, failed to provide a widely applicable solution to the problem. On the other hand, research on the factors associated with tinplate, within relatively recent years, have provided commercial control of the problem, formed the sound basis €or tin con- servation during war years, and is expected eventually to make possible still less expensive containers by permitting use of smaller quantities of tin. The latter trend began in prewar years when steel improvements made it possible to employ coke coatings €or some uses where once even charcoals were inadequate.

While the basic practical advances have been achieved through the tinplate phase and more will be forthcoming, it will be apparent that this factor alone cannot be depended upon to withstand all influences arising in the product OY its handling. For this reason, it is obvious that the full benefits from better tinplate cannot be realized until more atten- tion is paid to the corrosive characteristics of the contents of the con- tainer. The method of providing control over this factor may be open to some question, although it would seem that as a first step a test reliably indicating corrosiveness would assist either in diverting raw materials or making possible early disposition of unusually corrosive lots. What- ever the method, i t seems axiomatic that the more tinplate quality is improved and relied upon to provide longer shelf life, the more important the role of product and associated variables become.

378 R. R. HARTWELL

ACKNOWLEDQMENT

The author is indebted to the American Can Company for permission to publish this material, to many of his associates for their assistance and, in particular, t o Dr. R. W. Pilcher for his helpful suggestions.

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380 R. R. HARTWELL

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1941.

See also Sheet Metal Inds. 15, 1017 (1941). 1937.

1930.

1939.

Hoar, T. P., Morris, T. N., and Adam, W. B. 1941.

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1940.

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