We are using a used steel drum barrel as a rainwater container. Someone else in our house brought it from work, and we haven't bothered replacing with a plastic one yet.
Supposing that there is no harmful residue of the previous content in it: It is rusting a bit on the inside, so the water doesn't look that clean. Is the iron oxide actually harmful for vegetables? We are using it to irrigate plants such as peppers, salad, strawberries and some herbs.