Studies on Internal Corrosion of Cans

IV. Possible Mechanism of the Action of Nitrate in Canned Drinks

Abstract

Nitrate, when contained in canned acid products, is known to cause severe corrosion on the internal tin surface of container, accompanied by rapid dissolving of tin which migrates into the contents. The present paper dealt with results obtained by a test pack experiment and a model experiment, and mechanism of nitrate action to dissolve tin was discussed.
(1) Apositive logarithmic relationships were observed between initial nitrate amounts in the can and formation of ammonia. The tin-dissolving rate was found to be proportional to the former.
(2) Mechanism of the nitrate action was studied with a model pack experiment, in the event of which a model canned drink containing nitrite or nitrate was kept in the absence of oxygen. It was found that detinning by nitrite was rapid with the rapid decrease of nitrite, whereas, in the case of nitrate, both detimling and nitrate reduction were very slow.
From these results, it was concluded that the nitrate reduction to nitrite was a ratelimiting step in the over-all reaction, and nitrite acted in the canned acid products as a strong oxidizing agent against tin, while itself rapidly being further reduced to ammonia. It was assumed that when bivalent tin, Sn (II), a strong reducing agent, was formed by the action of oxygen, nitrate present was reduced to nitrite which readily attacks metallic tin to form bivalent tin, and, thus, the reaction proceeded in such a manner as a chain reaction. Oxygen must, therefore, be playing such a role as a “trigger” in the over-all reaction.