INDUSTRIAL A N D ENGINEERING CHEMISTRY
514
Vol. 19, K O . 4
The Tin-Iron Alloy in Tin Plate' By E. F. Kohman and N. H. Sanborn RESEARCH
LABORATORY. NATIONAL
A series of,lots of tin:plate were m a d e by varying the time a n d temperature that the plate was held in molten tin. These lots of tin plate were then made into enameled cans, whose service value for canning fruits was determined. By increasing the time a n d temperature the amount of tin which alloyed with the base plate was increased. The free tin coating was kept reasonably constant by passing the several lots successively through the same tin pot after the preliminary holding period. Cans were also m a d e from the untinned base plate by electrically welding the side seams. For comparison,
HERE is a scarcity of published data on the structure
T
and composition of the metal a t the junction of the tin and iron in commercial tin plate and no correlation of such data with service value for fruit cans. Even within recent years doubt as to the existence of a tin-iron alloy has been expressed. Mayer2 has shown that under certain conditions the two metals do alloy when iron is coated with tin by immersion in molten tin. He held steel of varying carbon content (electrolytic carbon-free iron and two steels containing, respectively, 0.06 and 0.41 per cent carbon) for 30 minutes in molten tin, respectively, a t 300", 500°, 750", and 950" C., and removed the specimens without subjecting them to rolls as is done in commercial tinning. Under these conditions photomicrographs disclosed an intermediate layer of crystals between the tin and iron, the thickness of which increased with the temperature of the tin pot, but which evidently was independent of the carbon content of the iron. The appearance of this intermediate layer of crystals was accompanied by a recession of the pearlite away from the tin coat, except in the specimen held a t 950" C., in which the pearlite was again in evidence adjacent to the tin layer. I n the carbonfree iron small crystals were in evidence within the large ferrite crystals adjacent to the tin coat. Mayer thus believes he has shown the diffusion of tin into the iron by a microscopic examination of the pearlite and ferrite crystals. From a patent granted to DavisS it is evident that the existence of tin-iron alloy between the tin and iron is recognized by manufacturers of tin plate. In this patent the following picture is presented: When the ferrous base and molten tin first come into contact. a layer of an iron-tin alloy forms on the surfaces of the base, this alloy having a melting point higher than that of the molten tin itself. The result of this action is that fine needle-like crystals of this iron-tin alloy are formed which protrude from the surfaces of the iron base and form a fine spongy or porous network of crystals, the interstices of which are filled with the molten tin. When the iron base and adherent alloy layer of crystals later pass between the exit rolls of the tin pot, which are located within a body of palm oil, the network of crystals is crushed and flattened against the iron base, and a large portion of the molten, interstitial tin is squeezed out and flows off the plate. The body of molten tin within the pot in reality soon becomes a saturated solution of iron-tin alloy in molten tin, this being shown by the large amount of scruff or iron-tin alloy which becomes deposited on the bottom of the pot, in the oil or exit end of the pot. 1 Received November 27, 1926. To be presented before the Division of Industrial and Engineering Chemistry a t the 73rd Meeting of the American Chemical Society, Richmond, Va., April 11 to 16, 1927. * Stahl Eisen, 38, 960 (1918). 8 U. S.Patent 1,528,407(March 3, 1925).
CANNERS AZSOC., WASHINGTON,
D.c.
coke a n d charcoal cans were m a d e from the same lot of
base plate. There is no consistent variation evident in service value in comparing t h e coke cans and the various lots of cans with more tin alloy, while the charcoal cans show appreciably better service. All lots of tin cans, even t h e charcoal cans, showed a much greater tendency t o perforate than the untinned cans, while the latter showed a m u c h greater tendency to hydrogen formation. The reason for this, as well as t h e better service value of t h e charcoal cans, is discussed. The essential feature of the patent is the introduction of an extra set of rolls in the tin pot, between which the sheets pass shortly after entering the molten tin. These rolls are in a compartment more or less separate from the main body of the molten tin from which the sheets finally emerge. The purpose of the extra set of rolls can best be given by again quoting from the patent: In passing the plates between the pressure rolls, the porous mass of needle-like crystals of iron-tin alloy formed on the plate as it passes into the molten tin bath is rolled down and flattened and smoothed, and the surplus, interstitial tin, which is still molten, is squeezed out and removed from the alloy coating now on the plate. Some tin alloy coating also is removed, as is shown by the deposit of scruff which collects on the bottom of the pot below these rolls. The iron-tin alloy, formed as a porous, spongy network of very fine crystals upon the iron sheet, when the plate enters the tin bath, is flattened out and pressed closely upon the plate by the rolls I. There being but little agitation of the molten metal in the pot, the flattening and smoothing operation and the removal of the excess iron-tin alloy while the plate is in the compartment B and before reaching the compartment C of the tin pot, the tin in the compartment Cremains substantially pure and does not become saturated with iron-tin alloy as with the old methods. Any excess alloy not tightly adhering to the sheet also is removed and is forced into the tin bath, finally settling to the bottom of the pot directly below the rolls. For a number of years this laboratory has used a boiling solution of sodium plumbite for detinning purposes. Continued boiling in such a solution no longer removes any tin after about 30 seconds, nor does it remove appreciable amounts of iron. I n ordinary tin plate about 0.2 pound tin per base box (plus or minus a few hundredths pounds for different lots) is not removed, however, by such treatment. This amount can readily be increased by holding tin plate in an oil bath abdve the melting point of tin or in molten tin, and presumably it represents the amount of tin alloyed with the iron. I n view of this possibility of varying the tin which thus alloys with the iron, it seemed desirable to study the effect of this alloy on the service value of tin plate for fruit cans. Tin is commonly regarded as having a lower solution tension than iron, and when this is true the tin would tend to accelerate the corrosion of any exposed iron according to the electrolytic theory of corrosion. This may not be true for all foods, however, for some attack the tin more readily than iron. I n so far as it is true an iron can might be expected to be less subject to corrosion than a tin can. From the standpoint of the electrolytic theory of corrosion, there is no available information concerning the tin-iron alloy in tin plate, nor has it been
INDUSTRIAL A N D ENGINEERILVG CHEMISTRY
April, 1927
definitely shown that the tin coating on tin cans actually does accelerate corrosion in canned fruit. Although the alloying of the two metals might be assumed to result in a more intimate contact between the two metals and a consequent lessened exposure of iron, the brittleness of tin-iron alloys might, on the contrary, cause a greater exposure because of a greater rupture of the tin coat in the forming processes involved in can manufacture. The writers4 have already called attention to the complicated nature of corrosion in canned fruits. There is no correlation between the acidity of fruits and their tendency to corrode the can. I n fact, it so happens that some of the least acid fruits are the most corrosive. The degree as well as the nature of corrosion is determined largely by depolarizing agents or hydrogen acceptors in the fruits. Among such substances probably the coloring matter, the anthocyanin pigments, are the most important. Effect of Varying Amounts of Alloy
I n the manufacture of tin plate the time of the plate in the tin pot is probably already reduced to a minimum because of the demands of economic production. A study of varying amounts of alloy would thus naturally take the course of increasing it progressively above that found in commercial plate. Consequently, a series of eight lots of tin plate was made, varying the time and temperature of holding the plate in molten tin previous to passing through the type of tin pot used in making commercial coke plate. All the sheets were made from the same ingot of steel and subjected to as uniform milling operations throughout as possible up to the tinning stage. For comparison, coke plate was made from the same lot of base plate, a t the same time and in the same pot that the experimental sheets received their “coke” coating after the preliminary holding period in molten tin. Likewise, charcoal plate was made from the same lot of base plate in a regular charcoal pot. The time of all these sheets in the tin pot during the tinning operation after the preliminary holding period of the experimental sheets was a matter of about 11 seconds.
515
digesting the disk in hydrochloric acid, after detinning with sodium plumbite, and titrating with iodine. The five different determinations represent five different sheets taken a t random from each l0t.5 Table I-Time
a n d Temperature of Holding Base Plate in Molten Tin Previous to Passing through Coke Pot
TIXEIN TI^ POT
LOT
TEMPERATURE
Minutes E
E-1
O
Commercial coke plate Commercial charcoal plate
1
E-2 E-3 E-4 E-5 E-6 E-7 E-8 E-9
5 to 6 13 32 63 33 35 32
10 to 30 to 60 t o 30 t o 30 t o 30 t o
’ F.
c.0
610 610 608 616 610 515 558 650
320 320 320 320 320 270 295 350
to to to to to to
620
620 fil5 623 615 538 to 57.5 to 706
a Approximate temperatures
It will be noted that the amount of tin in the alloy is quite constant for each lot, while the coating of free tin varies considerably, as is the case in regular commercial plate. The amount of tin in the alloy increases very rapidly during the first few seconds, having reached 0.2 pound per base box in about 11 seconds (E and E-1). Thereafter the amount of tin in the alloy increases with time but at a gradually decreasing rate. If the four lots held for 30 minutes in the tin pot a t varying temperatures are considered, and if from each is deducted the 0.2 pound of tin per base box which alloys within the first few seconds, it will be noted that the tin alloys a t a slightly increasing rate as the temperature rises. These considerations would be of significance if it were desirable to increase the alloying which now occurs in commercial plate. Since the alloying occurring during the first few seconds is far out of proportion with the time, which in commercial practice is already reduced to a minimum for economy in production, and since the temperature of the tin pot is determined by other important considerations, it seems impractical to lower the extent of alloying by a variation in the time and temperature of the sheets in the molten
/CU
BO Y
P 4 J
60
E L 0
I 0
8
24
16
TINE
IN
32
40
I
1
48
56
I
64
4
THISJ O L R S A L , 15, 527 (1923), 16, 290 (1924).
48
1
I
SZ
56
TIME
WEZKS
Chart I-Combined Loss w i t h Blueberries f r o m Hydrogen Formation (Springers a n d Swells) a n d Perforations; Represents t h e Data in Column 3 of Table 111
Table I gives the schedule of the actual time and temperature of the preliminary holding period. Table I1 gives the free tin (the loss in weight in boiling sodium plumbite solution to constant weight) and the tin in the alloy (the tin remaining after treatment with boiling sodium plumbite solution). These determinations were made on disks containing 4 square inches (25 sq. cm.). The tin in the alloy was determined by
44
Ilr
I
I
60
64
I 68
WEEKS
Chart 11-Combined Loss w i t h Sour Pitted Cherries from H drogen Formation (Springers and Swells) and P e r z r a t i o n s ; Represents t h e Data in Column 3 of Table IV
tin. Whether lowering the extent of alloying would be beneficial to service value is still undetermined. That an increase in alloying over present commercial practice is of no benefit, or is even detrimental, is evidenced by Charts I and I1 which are constructed from column 3 of Tables I11 and IV. 5 The accompanying photomicrographs with notes were prepared by G. F. Cornstock, Niagara Falls, N. Y .
INDUSllIUA 1' ;1 .\:I) I"INBLlfL1h'U CHI3.VISI'R Y
5lli
111 l':*iiIc~111 rmd I V are given tlie exteut of cornision for each lot of tin plate as measured in column 1 by the percentage of perforated cans; iii column 2 by the per cent of cans in which enoudi llvdroceil has formed to bulee out one end (springer) or bzth ends &well) of the can, and:n column 3 bv boil1 factors. Column 3 is not a sum of 1 and 2. because some cans represented in coluinii 2 were perforated at a later date and tlieil iverc inr~ludnlin colurmr 1
(Pound
1 Tin 88 Tin-Iron Alloy r bare box)
Fnm
TIN
TINZN
ALLOY
Tmn~
Tm
E-7. 40 Miii., 500- F.*
1.17
ILRa 0.71 I.il 0.96
0.27 0.28
0.29
1.44 1.31 1.00
0.27 0.28
1.48 1.24
..
0.85 0.X4 1.20 1.23 A". 1.0s I:.H 0.96
0.77 0.90
0.77 1.02
A".0.90
* The r c t u i i l time
and tempe:
IC
19,
so.4
limited quantity enables tlmn to augrirezit tile corrusion until the can is perforated in the case of enameled e m s with limited areas of metal exposed, hut which are soon spent on the entire surface of a nlain can. Comparison of Tinned and Untinned Cans
In order to determine to what extent the tit1 coat induced corrosion by setting up an electric conple, untinned cans were
snmgte E-4
Sample E-2
Table IC---Free Tin
\-
