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The Secret Life of the Aluminum Can, a Feat of Engineering

Stashed in: Design!, Engineers!, Coca Cola, How It's Made, Diet Soda

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Today I learned how corrosive diet soda is.

Before coating the insides of their cans, Ball needs to know how corrosive the beverage within will be. The epoxy coating, after all, costs about a half penny per can and Ball doesn’t want to waste it. Also, some beverages are so corrosive that no amount of coating will protect their cans. (Roughly one in seven new energy drinks are too corrosive to put in cans.) Ball is not in the business of sending cans out into the world to be slaughtered by overaggressive liquids. The coating must perform. Otherwise, cans explode, and legal costs climb. Empirically, Ball has figured out how to determine the corrosivity of potential products. Corrosion-wise, sodium benzoate is bad. Copper is bad. Sugar is good. It absorbs carbon dioxide, decreasing the pressure within a can, and it also inhibits other corrosion reactions, because sugar tends to deposit onto pores in the coating. Thus, Diet Coke underperforms regular Coke on at least two counts. Citric acid and phosphoric acid are equally bad. Red #40 is pretty bad, and high chlorides are really bad, and together, they’re really, really bad. (Ball has developed a carefully-guarded formula quantifying all of this corrosion badness, but it was not displayed on any graph at Can School.)

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