(Tues. June 1, 2010, 8:00PM)
I know, I know. I started with two cans and suddenly there are three! Explanation following. First, the originals: The left container is just seawater (refreshed once) and ginger ale can. There is no noticeable change to the water or the can. The middle is seawater (also refreshed once), ginger ale can, and iron nails -- working on the evidence that seawater helps iron to erode aluminum. This can too seems unchanged.
Back on 5/28 I thought about this, and did some more research. It turns out that aluminum cans are coated inside and out at the factory -- there's no aluminum actually exposed to the elements. So to truly test why the cans at the beach are so rotten and eroded, I had to think how nature could expose the aluminum.
Didn't actually have to think hard. Presumably, a beach is sandpaper at its most pure. Add some scouring action from wind & waves, and it's a fair guess that a soda can will have its outer coating scraped away naturally (though again, exactly how long is unknown). So I helped the process along with a third can, using a bit of fine (180 grit) sandpaper to expose some (but not all) of the actual aluminum.
My prediction is that Can #1 (no iron) and Can #2 (iron & unsanded) will not change much even given weeks or months. Can #3 (iron & sanded) is what will make or break this test.
(Dating note: As of today, 6/1, the first two subjects are on Day 8 (having finished 7 days), and the third subject is on Day 5 (having finished 4 days). My notation for that is "Day 8-5," which is a system I'll keep until the experiment ends.)