Wednesday, November 24, 2010

Soda Can Experiment - Postmortem

Longer-time readers here may remember an experiment I started back in May with soda cans. When I started my beach walks, I came across lots of badly mangled aluminum.
Like these
It occurred to me: If learning about the things that persist is important, so is learning how & why some things don't persist. After all, the Internet is flooded with sites claiming that an aluminum can will last for 80 to 200 years (or more). Which is not what I've been seeing.
Circa 1810? Anyone buying that?
So in late May I started an experiment to see what breaks an aluminum can down. I tried 3 cans: one in a bucket of seawater, one with iron nails added (to test for a galvanic reaction), and one I had scoured with sandpaper. At first, the scoured can corroded a little. But that soon stabilized. By mid-July, the experiment was pretty much DOA. (Feel free to catch up on all the details hereherehere, and here.) I've only just now finally emptied the buckets and officially killed the test.
Can #2 today, caked in salt, otherwise pristine
But I learned something. You see, aluminum is funny. It oxidizes easily. But that layer of oxidation actually protects the fresh aluminum underneath. It's like an impermeable film. For a can to corrode away, something needs to keep rubbing off that film. No rubbing of film, no corrosion. No corrosion, and boom, you've got a can that lasts for decades -- or centuries.

But take a can and place it at, say, the beach. Well, gee, what do we have at the beach that's abrasive and can rub off an outer protective film layer repeatedly? (This is a rhetorical question: there are no points for answering.)

So the future task is clear: toss a can into a bag of sand, shake it vigorously once a day, then let it spend the rest of its time in a bucket of seawater. And see what happens.

In the meantime, a last tidbit. In 2008, Americans recycled 53.2 billion cans. But they consumed 98.3 billion cans. Say only 1% of the 45.1 billion non-recycled cans got littered and found their way into the sea. That's 451 million cans added to the sea floor. Each year. And that's only from the U.S.

So maybe this year, a partier tossed a beer can into the ocean. From there, maybe it settled down to the bottom in a nice little nook. Where it lay, and lay. Then maybe in late 2209 a large storm will wash up that ancient soda can onto the beach. And maybe in the summer of 2210, your great-great-great-great-great-granddaughter will wander the beach and step down on a sharp, jagged bit of that can, as it finally rots away.
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  1. Harry,

    Just found your blog & am fascinated by the can experiments; I'm an offshore yacht captain & can tell you that common (& MARPOL compliant) practice is to throw aluminum cans overboard in deep water, after tearing them open. This saves space, weight, reduces possibility of laceration on empty cans, and prevents surface flotation while fully exposing the unpainted interior of the can to speed corrosion. Does your experience find this practice to be inappropriate?

  2. Hi!

    Wow, for one thing that answers a big mystery. It always surprised me how many pieces of can show up at the beach, and how they all seemed torn up. Didn't seem like a "typical" beachgoer thing to do.

    I'm probably not qualified to speak as to whether it's appropriate or not. The logic of it is understandable -- give the aluminum a chance to rot away in the corrosive seawater/seabed in the shortest time possible. But it does seem that what happens in the ocean isn't staying in the ocean. Even the stuff that sinks still manages to roll its way to shore, where it could be dangerous if it's still jagged & intact.

    It's the one thing that keeps amazing me. The ocean is so vast. One aluminum can (or whatever) tossed out in the Gulf of Maine, washing up on my beach seems like hitting the lottery -- and yet I hit the lottery most every week.

    Thx much for the note! Definite food for thought.