Aren't plastics hydrocarbons? Would the hydrogen be driven off (cracked) leaving something like coal.
I think that we might be more likely to find concentrations of metals left over from artifacts of civilization. If so, would these look different from naturally occuring metals?
Very few large things make it into the fossil record ( https://news.ycombinator.com/item?id=16837795 ), and a small fraction of those are discovered. It's possible, but I-beams are rare compared to plastic particles, which are basically carpeting the seabeds, rivers and beaches now.
We don't know what the effect of millions of years on this debris will be, but I am suggesting that due to the fact of the human species as a geological force, most of the rocks from this era will be a bit unusual in composition.
Your best bet for metals might be gold as it is not typically reactive with water or oxygen. And as a reminder, we do sometimes find solid blobs of gold... :-)
I also agree that the I-beams would rust away, as would rebar embedded in concrete. Would they remain as identifiable shapes or would the material migrate?
"remain as identifiable shapes" is basically fossilisation, which does happen. And it likely applies to I-beams as much as it does to coke cans, plastic bottles and cigarette buts. Except that the latter are going to be much more numerous.
I think that we might be more likely to find concentrations of metals left over from artifacts of civilization. If so, would these look different from naturally occuring metals?