In my last report (June 1: "Day 8-5"), I had added a third can, so I had three cans, all soaking in about 2 1/2 qts of seawater, left mostly undisturbed:
(1) A control can
(2) A can sitting with iron nails to test galvanic corrosion
(3) A sanded can in nails, to test if sand abrasion helps
Then about 6/3 a thought occurred. A can at the beach may accumulate more salt over time, as waves splash over it, and the water evaporates leaving salt behind. If I was trying to recreate conditions at the beach, perhaps I should steadily increase the salt levels. So I decided to remove the plastic coverings from the 3 containers and let the water evaporate naturally. This would increase the salt concentration more & more.
On 6/7 I added about another quart of seawater to each container to top them off.
And by 6/10, something neat started happening:
Around the cans' lips, little hard white deposits were starting to form. This was happening with all 3 cans, but most noticeably on can #3, the one I had pre-sanded.
This process is continuing, and seems to be accelerating. Just now I pulled out can #3 to get a better look:
I scratched off two of the deposits along the lip (leaving several others undisturbed) to see what was happening to the metal beneath. And sure enough, there seem to be some shallow divots in the aluminum itself. Also, the exposed aluminum on all 3 cans is starting to discolor & darken. Not corrode, but definitely oxidize. I don't know if this is salt action, or if there's just more oxygen in the water now that the covers are completely off.
Anyway, a pretty cool turn of events. The whole "needs iron nearby" thing seems more like a red herring. Meanwhile, the common-sense ideas of (1) sand abrading the cans to expose the metal and (2) heavier salt concentrations attacking the metal are both looking stronger.
I think I'm going to let this one ride now for another week without poking & prodding, and will post another update then. Oh, and if you were wondering, yes, this is fun! I highly recommend testing stuff.
* As a note, the corrosion isn't just salt. A quick (ill-advised?) taste test confirmed that it's a tasteless powder. It'll be fun seeing how fast the corrosion spreads now that it's taking hold. Especially if it helps answer why a can could last for centuries on the seabed but only months on a beach.
ReplyDelete