A previous paper2 we reported data which show that in such electrolytes as canned fruits tin may be less noble than the steel base plate used in making tin plate.Subsequently, similar results were reported by Lueck and Blair3 and by Culpepper and Moon.4 We pointed out that this relationship of the two metals explains many observations in connection with the corrosion in canned fruits which were unexplainable by the commonly held view that tin is the more noble. While reporting our results we had data that indicated some of the reasons for this relationship of tin and iron. In the present paper we submit data which probably indicate the most influential factors in determining the relative single potentials of tin and iron in various electrolytes. In the meantime, however, several views have been published which are so at variance with our data that a brief résumé is deemed advisable. Review of LiteratureLueck and Blair5 state that when tin and iron electrodes are first introduced into electrolytes such as fruit juice, immediately after contact "tin and iron exhibit their normal electrochemical relations-i. e., iron is the anodic metal. However, this condition is quickly reversed and the iron ceases to be corroded at an appreciable rate. This reversal of the polarity in the tin-iron cell under suitable conditions is the direct result of the large difference in the hydrogen overvoltage on the two metals." As we have already pointed out6 the high overvoltage on tin no doubt is an influential factor in determining the extent of corrosion wherever tin is involved as the cathode. Cathodic overvoltage in simple acid media is the result of hydrogen deposition. As long as the tin remains the cathode, the hydrogen overvoltage is an impediment to the progress of corrosion. It is conceivable that, when tin is in the beginning only mildly cathodic, the overvoltage on the cathode may temporarily cause a reversal of the current, particularly if the potential of the iron electrode should happen to fluctuate in a favor-1
A series of,lots of tin:plate were made 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 made from the untinned base plate by electrically welding the side seams. For comparison, HERE is a scarcity of published data on the structure and composition of the metal a t the junction of the T 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 ...
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