Yup, your lasagne is actually eating holes in the metal. (No reflection on your cooking.)
Aluminum is what chemists call an active metal, easily attacked by acids such as the citric and other organic acids in tomatoes.
In fact, you shouldn’t cook tomato sauce or other acidic foods in aluminum pots because they can dissolve enough metal to make them taste metallic. Stomach linings, on the other hand, contain a much stronger acid (hydrochloric) than the acids found in any foods, and are even immune to office coffee.
But in your case, something else was going on besides the simple dissolving of a metal by an acid. It turns out that tomato sauce can eat holes in the aluminum foil covering a leftover container only if the container is made of metal, not glass or plastic. So without even asking you, I know that your leftover lasagne must have been in a stainless steel pan or bowl, right? (Elementary, my dear Watson.)
When aluminum metal is in simultaneous contact with a different metal and an electrical conductor such as tomato sauce (you knew, of course, that tomato sauce conducts electricity, didn’t you?), the combination of the three materials actually constitutes an electric battery.
Yes, an honest-to-goodness electric battery. An electrical (more accurately an electrolytic) process, not a simple chemical one, is what chews up the foil. While it would be difficult, not to mention messy, to run your Walkman on lasagne power, it could in principle be done.
Here’s what was going on.
Your stainless steel bowl is, of course, mostly iron. Now, iron atoms hold onto their electrons much more tightly than aluminum atoms hold onto theirs. So if given an opportunity, the iron atoms in the bowl will steal electrons away from the aluminum atoms in the foil.
The sauce provides that opportunity by offering a conductive path through which the electrons can get from the aluminum to the iron. But an aluminum atom that has lost electrons is no longer an atom of metallic aluminum; it is an atom of an aluminum compound that is capable of dissolving in the sauce. (Techspeak: The aluminum has been oxidized to an acid-soluble compound.) So what you see is that the aluminum foil has been dissolved only where the sauce makes the aluminum-to-iron transfer of electrons possible.
If the lasagne had been put into a nonmetallic bowl, none of this would have happened because glass and plastics have no desire to suck electrons away from other substances. You’ll have to either take my word for that or sign up for Chemistry 202.
You can test this for yourself. Put a tablespoon or so of tomato sauce (ketchup will do) in each of three bowls, stainless steel, plastic, and glass. Lay a strip of aluminum foil on each blob of sauce, making sure that the foil also makes good contact with the bowl. After a couple of days, you’ll see that the foil in the stainless-steel bowl has been eaten away wherever it touched the sauce, while the foil in the other two bowls will be unchanged.
There are a few practical morals to this story.
First of all, your leftover sauce, and it doesn’t have to be tomato sauce; it can be any acidic sauce such as a wine reduction or one containing lemon juice or vinegar, can be kept in any kind of container and covered with anything you wish. But if it’s in a metal bowl covered with aluminum foil, just make sure that the foil isn’t in contact with the sauce.
Second, don’t hesitate to use those aluminum lasagne pans sold in supermarkets. They’re inexpensive, and disposable, and they work just fine. Even if you cover them with aluminum foil, it’s just aluminum against aluminum; no two different metals, so no electrolytic corrosion.