Ditch them and invest in better water storage containers. Brute cans are fine. Poly tanks are better. If you are buying used barrels they need to be food grade and have contained food product. Muriatic acid is a good way to get them clean, but, you shouldn't be mixing chemicals.
Pickle barrels are the best if you can find them since they are mostly vinegar already. That said, I love my ace roto mold tanks with large ~14" openings. I highly recommend going that route.