Okay, I feel like this is a bit of a dumb question, but whatever:
Does leaving metal to rust in place in the pasture/yard/garden/dump actually do anything harmful? It’s ugly and wasteful, but does it actually do anything bad to soils or people? Are there harmful additives or treatments in certain types of metal that make them toxic?
I'm reasonably confident that steel and iron are fairly safe in and of themselves...
The stuff I worry about is the paint, batteries, heavy metals, plastics, and oils/chemicals that may be connected to the steel.
The other interesting avenue for concern that comes to mind is contaminated recycled metal. Something built from recycled steel might have mercury from switches, for example..
Things like the below study are somewhat encouraging.. I am confident that the farm/sawmill that used to be here, never generated scrapyard levels of volume. I still get angry when I find a random oil filter in the bush, I have to assume the used oil went straight onto the ground as well... I really, really hope there are no hidden fuel tanks around.
I don't worry about anything ferrous, including stainless.
Copper will leach and it's an anti-microbial, so it may harm the soil bacteria, but I'm not sure how much. It's something that we need to varying amounts and some animals need it as a supplement, so it's going to get pissed out all over anyway, so I'm not too worried about it.
I've also seen zinc used as an anti-microbial, but we need some of that too, and some people slather it on their skin, so I'm not too worried.
Aluminum oxidises (rusts) very quickly, so quickly in fact that you need to use AC current when welding it to disturb the oxidation to make a good weld, so no worries there.
Lead is an issue, so keep it in a bucket out of the rain.
Titanium and tungsten aren't an issue as far as I'm aware.
Beyond that, I'm not sure what other metals we might have on hand. Paint on any of them is a wild card.
A lot of people cry when they cut onions. The trick is not to form an emotional bond. This tiny ad told me: