I doubt that they will be the same after a few million years, no. But what kind of rock will it make? Won't it be unlike anything else?
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?
"microplastics everywhere". http://www.bbc.co.uk/news/science-environment-43363545
"Microplastic pollution in oceans is far worse than feared, say scientists", "pervasive and persistent nature" https://www.theguardian.com/environment/2018/mar/12/micropla...
"The deep sea is a major sink for microplastic debris" http://rsos.royalsocietypublishing.org/content/royopensci/1/...
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?