Some cool details on corrosion:
Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially to another when both metals are in electrical contact, in the presence of an electrolyte. This same galvanic reaction is exploited in primary batteries to generate an electrical voltage.
A “lasagna cell” is accidentally produced when salty moist food such as lasagna is stored in a steel baking pan and is covered with aluminum foil. After a few hours the foil develops small holes where it touches the lasagna, and the food surface becomes covered with small spots composed of corroded aluminum.
In this example, the salty food (lasagna) is the electrolyte, the aluminum foil is the anode, and the steel pan is the cathode. If the aluminum foil only touches the electrolyte in small areas, the galvanic corrosion is concentrated, and corrosion can occur fairly rapidly.
Beware The “Lasagna Cell”: How Some Metals Can Ruin Your Meal And When Even A Non-Reactive Pan Can Be A Hazard
“It happened when marinating meat overnight. I take the foil off and everywhere the meat touched the foil there is a hole in the foil, and silver liquid on the meat. I’m stumped.”
Potato Battery and Lasagna Cell
Why does ketchup dissolve aluminum foil?
June 25, 1982
I have recently experienced a phenomenon that a friend of mine declares has also happened to her. It’s rather ghastly. I covered a meatloaf with a ketchup glaze and stored the thing in the refrigerator covered with aluminum foil. Where the foil touched the meat I found that it was eaten away, dissolved somehow, leaving a gray aluminum puddle deposit on the glaze. Thinking it was a fluke I re-covered the meat loaf with another piece of foil and the same thing happened. What happened?
— Ms. T., Dallas
Aluminum has what we scientists call a “highly negative standard reduction potential,” which means, if I may be permitted to bowdlerize a few pertinent scientific concepts, that it readily loses electrons and oxidizes. Ketchup, on the other hand, is highly acidic, having a pH of 3.85 (7.0 is neutral), and like all acids likes to oxidize obliging metals. The result, therefore, of a conjunction of foil and ketchup is, as you can attest, a grayish-black mush of aluminum oxide.
Ketchup is by no means the most potent product in your pantry in this respect. I note on my list of food acid levels that Coca-Cola, the all-American beverage, has a pH of 2.7. I guess if you spill a Coke aboard one of those aluminum naval vessels so popular these days, you’d better hope you can swim.
But this is no time for idle speculation. Standard reduction potentials also explain why it’s painful for people with silver tooth fillings to chew aluminum spitballs. Silver, it turns out, has a highly positive standard reduction potential, which means it has a craving for electrons. In the presence of an appropriate catalyst, such as your mildly acidic saliva, we have what amounts to a crude electric battery, in which electrons flow from the aluminum to the silver. This current is transmitted to the nerves of your teeth, producing the unpleasant sensation familiar to all.