December, 1928
I X D USTRIAL AND ENGINEERIhTG CHEMISTRY
1373
Factors Affecting- the Relative Potentials of Tin and Iron’ E. F. Kohman and N. H. Sanborn NATIOXAL CANNERSASSOCIATION, WASHIXGTON, D. C.
able direction for a reversal. Data here given show t h a t products similar t o those N A previous paper2we reIt is difficult to conceive that found in canned fruits, namely, apple pomace, prune ported data which show this reversal of the current can kernels, and other protein-bearing products, markedly that in such electrolytes be m a i n t a i n e d indefinitely change the relative potentials of iron and tin in various as canned fruits tin may be without exhausting the cause acid solutions. In acid solutions in which tin is anodic less noble than the steel base of the reversal-i. e., mont o iron, these products render it even more anodic. I n plate used in making tin plate. atomic hydrogen. This solutions in which tin is cathodic they may cause it t o Subsequently, similar results would have the result of again become anodic t o iron. were reported by Lueck and r e n d e r i n g the tin cathodic. The single potential of tin in various acids is shown Blair3 and by Culpepper and It appears therefore that the t o be markedly affected by t h e hydrogen-ion concenMoon.4 We pointed out that anodic relationship of tin to tration, while t h a t of iron is little affected, if a t all. this relationship of the two iron must be explained in some The higher t h e hydrogen-ion concentration the less metals explains many obserother manner. noble tin becomes. vations in connection with the Culpepper and Moon4 atIt is shown t h a t small quantities of tin in solution corrosion in canned f r u i t s tempt to explain our data in have a marked effect in inhibiting iron corrosion. I n which were unexplainable by which tin is anodic to iron by explanation of this it is shown t h a t such quantities of the commonly held view that the presence of trivalent iron. tin in solution have a marked effect in raising t h e catin is the more noble. While We specifically s t a t e d t h a t thodic polarization on iron a t low current densities. reporting our results we had e v e r y possible attempt was Apple pomace likewise is shown t o influence cathodic data that indicated some of made to exclude oxygen from polarization on iron in a similar manner. the reasons for this relationour corrosion media. It is The conductivity of the electrolyte i s found t o be a ship of tin and iron. I n the impossible for trivalent iron negligible factor in influencing corrosion in canned present paper we submit data to exist in the presence of fruits. which probably indicate the metallic iron and a hydrogenThese data are discussed in connection with viewmost influential f a c t o r s i n ion concentration as found in points of others and shown t o be in harmony with determining the relative single fruits. The reaction commercial experience i n the corrosion resulting in potentials of tin and iron in 2Fe++ 2H+ =e canned fruits. various electrolytes. I n the 2Fe+++ H2 meantime, however, several views have been published which are so a t variance with our is said by Clark7 to be in equilibrium when [H+] = 10-1 and when the hydrogen pressure is of the order of data that a brief resume is deemed advisable. atmosphere. Under the conditions of the corrosion in Review of Literature fruits, the hydrogen-ion concentration is much less than this figure. Detectable amounts of hydrogen gas may Lueck and Blair5 state that when tin and iron electrodes usually be found in any can of fruit showing even slight are first introduced into electrolytes such as fruit juice, corrosion. This amount would, of course, be enormously immediately after contact “tin and iron exhibit their normal in excess of the amount required by the above equation. electrochemical relations-i. e., iron is the anodic metal. I n view of these considerations, trivalent iron can be ruled However, this condition is quickly reversed and the iron out as a factor in determining the potential relationship ceases to be corroded at an appreciable rate. This reversal of tin and iron in canned fruits. of the polarity in the tin-iron cell under suitable conditions Culpepper and Caldwel18 suggest that in the more acid is the direct result of the large difference in the hydrogen fruits the tin is kept in solution because the tin salts of the overvoltage on the two metals.” As we have already pointed anthocyan pigments are not permitted to form. According out6 the high overvoltage on tin no doubt is an influential to their view, in the more acid fruits a higher concentration factor in determining the extent of corrosion wherever tin of tin ions is permitted to exist, thus tending to inhibit tin is involved as the cathode. Cathodic overvoltage in simple corrosion. This, however, would tend to make tin more acid media is the result of hydrogen deposition. As long noble with reference to the iron and thereby increase iron as the tin remains the cathode, the hydrogen overvoltage corrosion, which, in turn, would result in pitting of the base is an impediment to the progress of corrosion. It is con- plate and a greater number of perforations. But commercial ceivable that, when tin is in the beginning only mildly ca- experience is that perforations are more general in some of thodic, the overvoltage on the cathode may temporarily the least acid fruits. cause a reversal of the current, particularly if the potential Lueck and Blair5 make the following statement: of the iron electrode should happen to fluctuate in a favorStrictly speaking, the corrosion of the iron in an iron-tin * Received August 6, 1928. Presented before the Division of In- couple should be inappreciable as long as it bears a cathodic dustrial and Engineering Chemistry at the 76th Meeting of the American relation to the tin. Except in electrolytes possessing a high Chemical Society, Swampscott, Mass., September 10 to 14, 1928. conductivity, however, this condition is not attained unless all ~ I N DEND. . Crrsu., 90, 76 (1928). portions of the cathodic iron are in very close proximity to the ‘Canner, 66, 23 (1928). anodic tin. In case parts or all of the iron element are located
I
+
4
Canning Age, 9, 461 (1928).
1
IND. ENO.CHEM., 90, 443 (1928).
8
Ibid., 19, 614 (1927).
8
U. S. Pub. Health Service Hyg. Lab., Bull. 111, 6 (1928). 3. Agr. Research, 81, 107 (1927).
+
1374
INDUSTRIAL A N D ENGINEERING CHEMISTRY
at appreciable distances from the anodic tin, the protective influence of the tin is lost and corrosion of the cathodic iron occurs as a result of “local action.”
Vol. 20, No. 12
The difference in potential of tin and iron is not great in most fruits in comparison with the difference between either one and hydrogen. It is not unreasonable, therefore, to expect so-called “local action” in an acid medium like the fruits, even on very minute areas, in spite of any galvanic action between the two metals. Gundersong has shown experimentally that local couples are possible on a metallic cathode. Whitman and Russell’O state that in water from Cambridge, Mass., the galvanic action of a copper and steel couple was effective over a distance of 0.5 cm., while in 0.001 N sodium chloride it was effective over the longest distance studied, 6.35 cm. SpellerL1 states that galvanic action is effective for a fraction of an inch in pure natural waters to one foot in sea water. Our evidence presented below indicates that for the products we have tested the galvanic effect is influenced little by distances ranging from 0.5 to 8 cm.
cause of its amphoteric nature, the tin ion is able to do this in both alkaline and acid solutions. As practically all food products are acid in reaction, we may limit our considerations to acid media. In pointing out various factors which affect corrosion in canned fruitsL4we have called attention to the fact that the amount of corrosion is not proportionate to the acidity of the fruit. In fact, the general indication is that among the fruits packed in enameled cans the less acid give the greatest trouble from corrosion considered from the commercial standpoint, that is, they result in a greater number of perforations and hydrogen swells. An exact correlation is not to be expected since other factors are not constant in all fruits. The data which we will present on the effect of hydrogen-ion concentration on the single potential offer a logical interpretation of this observation in commercial experience. Our observations lead us to believe that it is only because tin is anodic t o iron that the tin can is a suitable container for fruits. Without this condition there would be excessive pitting of the can. The acidity of the fruit exerts its effect by influencing the relative potential of the two metals.
Theoretical Consideration
Experimental
The underlying factors responsible for the single potentials of metals are the solution pressure of the metal and the concentration of the metal ions in the electrolyte. The striking influence of the concentration of the metal ions may be recalled by the textbook experiment in which zinc is made more noble than copper by addition to the electrolyte of potassium cyanide, which maintains a low copper-ion concentration. The concentration of the ions of the metals therefore suggests itself as a possible predominating influence in establishing the potential of tin and iron in canned fruits. I n our previous paper,2 to illustrate the approximate values that may be encountered, we gave as single potentials for iron - 0.039 volt in strawberries and -0.038 volt in rhubarb. For tin we gave -0,081 volt in strawberries and -0.141 volt in rhubarb. It is known for bivalent metals that, as the metal-ion concentration changes tenfold, the single potential changes 0.029 volt. It follows that in the case of the values given for strawberries if either of the metal ions should undergo two successive tenfold changes in concentration in the right direction, the relative potentials would be reversed. *There is ample evidence that the concentration of the tin ion in canned fruits is kept a t a relatively low figure. Bigelow12 and GossI3 have pointed out that most of the tin in canned foods is not in soluble form. Goss has shown that in berries this tin is largely precipitated in the seeds. Seeds have a relatively high protein content, and it is known that tin salts tend to be adsorbed from their solutions by protein materials. This removal of the tin from solution favors continuous corrosion of the tin and increases its negative potential as measured against the solution, thus making it less noble. Since most of the iron which corrodes remains in solution in canned fruits, the increasing concentration of ferrous ions renders the base plate increasingly more cathodic. It is not to be inferred that the removal of tin from solution is limited to the protein material, although it is probable that protein material is the most effective constituent in this respect Tin ions are characterized by their ability to enter into combination with other ions to form complex ions. Be-
The corrosion experiments which me here present were performed in a similar manner to those described in our previous papere2 The corrosion medium was boiled and while boiling poured into bottles of approximately 150 cc. capacity. A single-holed rubber stopper was inserted and a glass rod then inserted in the rubber stopper. After cooling the bottles under water they were opened, one a t a time, and the corrosion specimens inserted in the bottles, which were then immediately closed. So that no bubble of air could be trapped in the bottle, it was completely filled with air-free electrolyte and the excess was forced out when the rubber stopper was inserted. Contact between tin and iron was made by fusion of the tin on the iron as described in the earlier paper. As a number of organic acids are met in canned fruits, it was deemed advisable to study the effect of various organic acids upon the relative potential of tin and iron. In Table I are given the results of corrosion tests with a number of acids of a strength equivalent to 0.75 per cent malic acid. This strength of malic acid was chosen because it represents approximately the percentage of acid found in apples. In the same table is given the effect of added apple pomace which had been extracted with cold running water for several days. It will be noticed that the corrosion of iron when not in electrical contact with tin is greatly reduced by the presence of apple pomace and by the presence of a specimen of sheet tin in the same bottle. Contact between iron and tin was prevented by a glass tube of approximately 1.25 cm. diameter. Naturally it was not the presence of the tin specimen which prevented the corrosion of the iron, but a small amount of the tin which had gone into solution. When both apple pomace and a tin specimen are present the iron corrosion is not markedly less than when either alone is present. This is adequately explained on the basis that the apple pomace would tend to adsorb the tin that goes into solution and thus nullify its effect. I n a search for an explanation for the striking effect of small amounts of tin in solution in rnhibiting iron corrosion, both in solutions in which tin is more noble than iron and in solutions in which it is less noble than iron, under widely varying conditions of temperature and hydrogen-ion concentration, a possible effect on cathodic polarization sug-
IND. ENO. CHEM.,20, 866 (1928). Zbid., 16, 278 (1924). ‘Corrosion,Causes and Prevention,” p. 30 (1926). I* J. IND. ENO.CHEM.,8, 813 (1916). 1aZbid., 9, 144 (1917). 9
10
11
1 4 IND. END.CHEM.,15, 527 (1923); 16, 290 (1924); C ~ n n i n gAge, 6, 308, 310, 311, 314, 385, 389 (1924), 6, 191 (1925); 7, 187 (1926); Canner, 60 (11, pt. 2), 151 (1925).
December, 1928
INDUSTRIAL AND ENGINEERING CHEMISTRY
gested itself. T o determine such an effect the data in Figure 1 were obtained by the use of a cell similar to that described by Haring.'s The necessity was early realized, as outby ~l~~ and Rawdon,16 of the use of a low Current density. Our first apparatus was so designed as to current densities from o.ooo8 to o,04 amp, per sq, dm., but no significant effect in the way of cathodic polarization manifested itself. By enlarging the electrodes sixfold, to cover correspondingly lower current densities, a marked cathodic polarization was demonstrable. .I, From Figure 1it is apparent t h a t both apple pomace and the presence of 3 small quantities of f tin in solution raise 3 the c a t h o d i c po$I larization on iron. In addition to this 5 .2 factor, which would tend to de$ creB se the corrosion of the iron, 6 , a p p l e pomace no doubt has amechanical effect in decreasing the area ICurmnf Dens&-m/homp/s~ ~m 7 of iron e x p o s e d . It should be stated Figure 1 that the hot acid solutions used undoubtedly extracted some material from the apple pomace which cold water would not extract. We believe the major effect of the apple pomace is to increase hydrogen overvoltage on the iron and to decrease tin-ion concentration. It is also apparent from Table I that for the strength of acid used tin is distinctly cathodic in acetic, malonic, and succinic acids, in that the corrosion of iron is increased by virtue of contact with tin, whereas the corrosion of tin is decreased. Measured by the same basis, tin is very little, if a t all, cathodic t o iron in malic acid, and distinctly anodic to iron in citric acid. Addition of apple pomace to acetic, malonic, and succinic acids reverses the condition and the tin becomes anodic. I n media in which tin is anodic to iron any tin in solution exerts a protective effect on the iron by raising the cathodic polarization on iron, while metallic tin in contact with the iron exerts an additional protective effect by virtue of the electrochemical action of the galvanic couple. I n solutions in which tin is cathodic, small amounts of tin in solution likewise manifest a protective effect on the iron by raising the cathodic polarization, whereas metallic tin in contact with the iron, being cathodic t o the iron, increases the corrosion of the iron. I n all the experiments discussed in this paper there was no visible evidence of tin being deposited on iron, even in solutions in which tin was cathodic to iron. Tin deposition, however, could be brought about by having sufficient tin in solution and imposing an adequate current density. I n Table I1 is given the effect of various other substances when added to a solution of acetic acid in which tin is distinctly cathodic t o iron. It is apparent that the presence of casein, egg white, gelatin, crushed peanuts, crushed almonds, and crushed prune kernels cause tin to become anodic t o iron. An electrometric determination of the potentials of the metals in these various media substantiated theLcorrosion experiments in demonstrating that the tin
-j
*
15;Trans. A m . Electrochem. Soc.. 49, 417 (1926). 16.Ibid., 68, 463 (1927).
1375
was in fact cathodic to the iron in acetic acid alone, but anodic to iron when the various additions were made. Table I-Loss in Weight of Specimens of Base P l a t e a n d Sheet Tin 2.5 X 4.0 c m Exposed 210 Hours at Room Temperature to Various Acids of Str6;gthe Equivalent to 0.75 Per C e n t Malic Acid with a n d w i t h o u t Added Extracted Apple Pomace (10 G r a m s Air-Dried per 140 C C . ) ~ IRONIN TININ IRON TIN ALONE ALONE
ACID
IN
IN
BOTTLE BOTTLE
Citric acid: Alone With apple pomace Malic acid: Alone With apple pomace Acehc acid: Alone With apple pomace Malonic acid: Alone With apple pomace Succinic acid: Alone With apple pomace
IRONIN TININ CONTACT CONTACT
"T:',"
WITH
B U T I ~ T TIN CONTACT CONTACT
WITH
IRON
Mg.
Mg.
Mg.
Mg.
Mg.
45.6
0.9
6.8
1.0
3.9
1.5
4.2
2.2
5.0
1.4
1.1
8.4
ME.
21.8
1.2
4.0
1.1
4.3
1.0
3.8
1.2
4.0
1.1
1.7
3.9 0.1
18.5
1.7
9.7
1.1
13.3
5.1
0.2
5.2
0.4
10.2
1.2
60.3
1.3
11.2
1.1
21.9
0.9
5.1
2.3
5.7
1.9
5.1
2.9
27.1
1.1
10.3
0.9
18.6
0.2
3.8
0.4
3.9
0.3
2.8
1.0
In all corrosion experiments the figures represent the average of a t least duplicates which check satisfactorily. The figures in Tables I11 and IV for sulluric and hydrochloric acid are the average of triplicate experiments. 0
Table 11-Loss in Weight of S ecimens of Base Plate a n d Sheet Tin 2 5 x 4.0 c m Exposed 210 Ifours a t Room Temperature in Acetic Acid Solutiox?E uivalent t o 0.75 Per C e n t Malic Acid, w i t h o u t a n d w i t h Various A%ditions a s Noted per 140 CC. Bottle Displacing an Equivalent Volume of Solution ~~
ADDITION
IRONIN
TIN IN
"T':,"
2:;
IRON TIN ALONE ALONEBUT
IRONIN T I N I N CONTACT CONTACT
jgOT BUT jgoT ",W :; T N
T N
WITH
IRON
COGACT CONTACT None Casein (2grams) Hard-boiled egg white(25grams) Gelatin (2 erams) Crushed' peanuts (5 grams) Crushed almonds (5 grams) Crushed prune kernels (5grams)
MR.
Ma.
Ma.
0.5 0.8
7.4 3.0
0.3 0.4
bln. 11.4 1.9
0.1
2.1 1.5
0.2 0.5
2.1 1.6
0.1 0.3
1.3 1.4
0.8 1.0 2.4
Me. 13.2 2.6
Mg. 1.0
1.5
0.5
1.6
0.1
0.2
1.8
0.4
1.7
0.1
0.0
3.4
2.6
0.3
3.2
0.3
1.a
2.8
The question arises as to the reason for a difference in the relative polarity in the organic acid solutions to which no addition has been made. Two possible explanations suggest themselves. I n the first place, although equivalent solutions of the different acids were used, they would not yield equal hydrogen-ion concentrations. I n the second place, the possibility of the cation having some effect cannot be ignored. Tables 111, IV, and V throw some light on the question. I n these tables it is clearly brought out that in hydrochloric and sulfuric acids of strengths ranging from 0.05 to 20 per cent, tin becomes more and more anodic to iron with increased concentration of the acid. I n considering Tables I11 and IV, it must be borne in mind that when the iron and tin were not in electrical contact they were in separate bottles. It will be noted that the corrosion of the iron specimen in contact with tin wag enormously reduced, particularly in the higher concentrations of the acids. As is brought out in Tables I and 11, the corrosion of iron is greatly inhibited by the mere presence of a specimen of sheet tin in the same bottle, even though no electrical contact existed between the two metals. This is the effect of a small amount of tin in the solution. Therefore the less extensive corrosion of the iron in contact with tin can be
INDUSTRIAL AND ENGINEERING CHEMISTRY
1376
ascribed only in part to galvanic effect. I t mill be observed in Tables 111,IV, and V that the corrosion of the tin specimen is very greatly increased when in electrical contact with iron, particularly in the more concentrated acid solutions. This is to be ascribed in large measure, if not wholly, to galvanic action. The tin is progressively more anodic t o iron as the strength of t h e a c i d solution i n creases. As a matter of fact, in the very weakest acid solutions tin is actually cathodic to iron. This is the result of the hydrogenion concentration, as is brought out in Figures 2 and 3. The graphs in these figures demonstrate that the single potential of iron H-,on Concenfmfmn - p H Figure 2 is little, if a t all, influenced by the hydrogen-ion concentration, whereas the single potential of tin is very markedly influenced thereby.
PESTRENGTH OF ACID
TOTAL Loss
RIOD OF
Ex-
Vol. 20, No. 12
Loss
K O CONTACT
CONTACT
Iron Tin
Iron Tin
PER
DAY
NO CONTACT
CONT4CT
PO-
SURE
% Hours M2. 0.05 0.10 0.25 1.0 5.0 20.0
308 235 139 1 1 1
11.6 23.2 17.9 0.4 0.8 8.2
M2. 2.7 3.2 0.4 0.1 0.3 2.3
Iron
Tin
Mg. M2. 12.9 47.6 9.2 0.4 0.6 0.8
Mg. Mg. 0.6 0.904 0.211 0.7 2.369 0.326 0.5 3.089 0 . 0 6 7 0.5 9.60 2.40 1 . 7 19.2 7.2 3 0 . 4 196.8 55.2
M2. M2. 1.005 0.048 4.862 0.072 1 . 6 2 2 0.080 9.60 12.0 14.4 40.8 19.2 729.6
Table 111-Loss in Weight of Specimens of Base Plate and Sheet Tin 2.5 X 4.0 c m . , Exposedin Sulfuric Acidof Varying Strengthsat 2O0 C.=
Loss IN WEIGHT
STRENGTHPERIOD OF
ACID
OF
EXPOSURE
NO CONTACT
Jron
Tin
1
ELECTRICAL COSTACT
Iron
Tin
RASPBERRIES
CONTACT a
Specimens not in contact were in separate bottles.
Table IV-Loss i n Weight of Specimens of Base Plate and Sheet Tin 2.5 X 4.0 c m . , Exposed in Hydrochloric Acid of Varying Strengths at Z O O
Loss 1.v WEIGHT
STRENGTHPERIOD of ACID
P e r cent 20 10 5 1 1 0.5 0.1 0.05 a
OF
EXPOSURE Hours 1 1 1 1 10 10
10 10
N O CONTACT
Iron
Tin
M2. 6.6 1.2 0.6 0.5 2.0 1.5 0.9 1.3
1 1
ELECTRICAL CONTACT
Iron
Tin
M2.
M2.
1.6 0.6 0.3 0.3 0.5 0.9 0.6 1.2
0.3 0.1 0.1 0.1 0.0 0.2 0.4 1.1
Mg. 31.4 6.5 2.6 0.7 2.2 2.6 0.8 0.3
Specimens not in contact were in separate bottles.
The data given in Figures 2 and 3 were obtained in the and sheet tin were made for each strength of acid tested. The specimens were 2.5 by 4 cm., with a narrow projection sufficiently long to extend through the rubber stopper used in the corrosion bottles. To mount the specimens in this rubber stopper a large hole was first cut in it. A small oneholed rubber stopper was then inserted in this hole, with the narrow projections of the two specimens extending through the large hole of the large rubber stopper. Instead of inserting a glass rod in the hole to close the glass bottle, a U-shaped tube filled with the acid solution used was inserted. The outer end of the U-tube was placed in a solution of the same acid, with which the standard cell used for making the single potential measurement was also connected by a U-tube. I n this manner single potential measurements of a
None At one corner. soeciniens in same plane, distanc; center to center, 8 cm. A t diagonal corners, specimens parallel, 0.5 cm. apart
Iron
Tin
Mg. 4.0
0.9
3.5
3.0
2.4
3.5
Mg.
1
RHUBARE
Iron
Tin
Mg.
Mg. 1.9
4.0 0.4
13.2
0.5
15.2
Note-For these experiments products were used which had undergone considerable corrosion in the can and therefore had a high tin content. It is likely, therefore, that the reduction in iron corrosion noted when in contact with tin is due largely to galvanic action rather than to tin in solution supplied by the tin specimen. The galvanic action in rhubarb is more pronounced than in any other product we have encountered. The increased tin corrosion when in electrical contact with iron is probably very largely, if not wholly, the result of galvanic action.
Table V I is pre,800 s e n t e d t o show the effect of distance on $B the galvanic couple in f media representative 2 .'O0 of canned fruits. In this table couples are 2 c o m p a r e d in which the iron and tin speci- >.60c mens, 2.5 by 4 c m . , were in one case Darallel to each other a t .5 a distance of 0.5 cm. and electrically joined #-ion Co?centrot/on--pN a t two diagonal Figure 3 c o r n e r s , and in the other case parallel in the same plane and electrically joined by a projection of the iron specimen 6 cm. long, which was in-
q-j--jJ
INDUSTRIAL AND ENGINEERING CHEMISTRY
December, 1928
sulated with sealing wax. Although there may be a slight effect of this distance of 8 cm. from center to center, as compared with 0.5 em. in the first case described, yet the effect is so small as to be fairly negligible. As the current density representing the extent of corrosion that occurs in a tin can must be exceedingly small, it is not likely that the conductivity of the electrolyte and hence the distance of two electrodes that might be set up within a can of fruit are pronounced factors in determining the rate or nature of corrosion. Application of Results t o Commercial Experience
The effect, from the standpoint of corrosion in canned fruits, in plain tin cans, as compared with that in enameled cans, is very striking. The purpose of enameled cans should be borne in mind. Enameled cans are used for colored fruits whose color is bleached by their action on plain cans. The bleaching action in plain cans is evidence that the coloring matter enters into the corrosion. The fruits which are generally packed in plain cans, as well as those which are ordinarily canned in enameled cans, do not tend to perforate unenameled cans, although the colored fruits generally have a tendency to perforate enameled cans. The action of the fruit on an unenameled can is generally distributed over the entire area of the can. In plain cans the area of tin exposed always enormously exceeds the area of iron exposed. The effect of the anodic tin in inhibiting iron corrosion is therefore pronounced, while the effect of the small area of cathodic iron exposed is almost negligible in increasing the amount of tin corrosion. In an enameled can a different condition exists. A vast proportion of the area of tin is eliminated from the corrosion picture by the enamel coating. It is true that portions of the iron which the tin does not coat may also be covered by the enamel coating. However, it is a t that portion of the can which is adjacent to the seam and other points where the plate undergoes a strain in forming the can that the bulk of the corrosion occurs. At such points both the enamel coat and the tin coat tend to be more or less fractured. At such points, therefore, the relative areas of tin and iron
1377
are of a vastly different proportion than in an unenameled can. At such points in an enameled can the area of the exposed iron apparently is sufficient to materially influence corrosion of the anodic tin. It is a common observation that such areas are, relatively early in the life of the can, completely detinned. The result is that, comparatively early in the life of many enameled cans, the area of iron exposed actually exceeds the area of tin exposed. The exposed iron, therefore, loses to a considerable extent the protective effect of the anodic tin. Local couples no doubt are set up on the iron, augmented by its non-homogeneous nature, whereby hydrogen is formed, and eventually pitting and perforation result. At any rate, it is common commercial experience that enameled cans give rise to hydrogen formation and perforations to a much greater extent than plain tin cans, even when the same fruit is considered. If, as pointed out by Walker and Lewis,I7 and to whom Lueck and Blair5 refer, the enamel film may act as a cathode to the iron, this would be an additional cause for perforation of the iron base plate in enameled cans. We have already stated that tin is anodic to iron, even in the less acid fruits in which hydrogen formation and perforations are the most common. It is, however, not so strongly anodic as in certain more acid fruits in which there is less tendency to hydrogen formation and perforation of the can. Our data on the effect of hydrogen-ion concentration on the single potential of tin offer an explanation for the relative nobility of the two metals under these varying conditions of acidity. The less pronounced tendency to hydrogen formation and perforations in the more acid fruits may be explained on the basis of the more effective protection of the iron by the tin, which is more markedly anodic. Tin is only mildly anodic to iron in certain varieties of black cherries, among the less acid fruits. These cannot be successfully canned because of the hazards of corrosion. It is believed that if tin were actually cathodic in any fruit, as has long been believed by many, the modern tin can would not be a practical container for such a fruit. 17
J. IND. END.CHEM.,1, 754 (1909).
Specific Gravity of Glycerol' L. W. Bosart and A. 0. Snoddy CHEMICAL DIVISION, THEPROCTER A N D GAMBLE COXPANY, IVORYDALE, Oruo
UR paper entitled ''Yew Glycerol Tables"2 was published to provide practical working tables based upon glycerol of the utmost possible purity, which, it was believed, could be relied upon for accuracy to the fourth decimal place for the percentages of glycerol. This work was done because none of the tables then in use seemed sufficiently accurate for either scientific or technical purposes. I n the case of some of the data published, no special precautions had been taken to insure a 100 per cent glycerol from which to start; in some, the determination of the specific gravity was only incidental t o other work; in others, no attempt was made a t accuracy beyond the third decimal place, although calculations showing the specific gravity in the fourth place were made; sometimes it was not clear whether apparent or true specific gravities were intended; and some of the tables were incomplete. I n short, none of the tables were entirely satisfactory. R e believed that
0
1 Received
* IND.
July 2, 1928.
ENG. CHEM., 19, 506 (1927).
our glycerol tables would be thoroughly satisfactory and would give results that could be relied upon for complete accuracy in the fourth decimal place a t least. This was the sole purpose of the work and therefore its only justification. Since the publication of our tables the third volume of the International Critical Tables has been published, and it contains, on page 121, tables for the absolute density of glycerol a t 15", 15.5", 20°, 25O, and 30' C. As the International Critical Tables are generally considered authoritative and as they show certain wide disagreements with ours, the whole question of the correct specific gravity of glycerol and its water dilutions is again opened. The International Critical Tables necessarily take into consideration the work of various investigators and probably reflect something of a composite of such work as is given the greatest credibility. Hence they are in fair agreement with ours for 15", 15.5', and 25" C., but in very poor agreement for 20" C. This is shown in Table I, where our values, originally given in terms of specific gravity a t 15'/15", 15.5O/15.5", 2O0/2Oo,
