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.
Rust is iron oxide, which does not harm plants in moderate amounts, because it is not water soluble unless the soil ph is very low. In fact, oxidized iron is what gives most red subsoils their color.
Watering your plants with this water will not harm them at all. If you suppose there was nothing environmentally harmful in it before (which is NOT a good idea if you really don't know, because many things stored in steel barrels are not good for soil, and rust is very absorbent), then go ahead and use it. I would ask your friend if he/she wouldn't mind finding out what the barrel was used for.
This info is not totally true. Although the rust is not harmful, if you are planning to use it to water vegetables, it will leave an unwanted taste. Because of the metal and rust in the bin, your plants will grow and have a more salty flavor.
