I've used similar buckets from my brothers store, some of them had pickels in them. Others I have used were from dunkin doughnuts, they had the frosting or something like that in them. I cleaned them really good and haven't had any problems.
I got some that were like 2 gallons from dunkin doughnuts they work great for transporting coral thats to big for a baggy, like a colony of sps etc etc. Very usefull to have. Also to do coral dips the 2 gallon bucket works great. I even think I have a 1 gallon somewhere too.
Edit: FYI, when my brother worked at dunkin doughnuts he told me that they had to throw away all of their buckets. They would get alot of buckets in a few times a week. I'de bet if you went to a dunkin doughnuts they might give you some or there is prob a millon out by their dumpster.
I asked him why they couldn't send them back to be refilled and he said they wouldn't do it, it was company policy to throw them out. I bet they could save a ton of money if they could figure a way to reuse those buckets.
All sizes too from 1 gallon 2 gallon 3 gallon and 5 gallon.