I'm at the mall and Yellowstone erupts - My, and everyone else's car is covered with ash, fire, and maybe even lava. Lets say it was a big shopping day too, like Black Friday or Christmas Eve, just to ensure the parking lot is packed -- That is a lot of steel in one place.
Now, everyone is dead, and our cars are covered in chemically reactive ash and chemically reactive fire. Also they're in a wasteland. Also, most everyone in the continental US is dead (or dying).
After a few decades, the US will be covered in plants, and in few centuries it will be covered in animals. Underneath all of that of course, perhaps a few dozens of feet, is me, the mall, my car, and my Christmas presents.
How long will it take for my and everyone else's car to look like a regular iron vein? Will it ever happen? What about the weird man made chemicals, like paint, motor oil, or antifreeze?
Also, Merry Christmas!