I am not sure why old and rusty things make such desirable photo subjects. Perhaps the interesting textures and color variations are the reason.
Some places for rusty things include:
|[7105 138th Avenue SE; Newcastle] This area was home to the coal mines that built the Seattle area. There is a rusty truck, various farm implements, some old buildings, and a few remaining mining pieces.
|Eastern Washington abounds in old, rusty, and interesting subjects. I prefer Hwy-2 when on a quest for the old and rusty. Just watch for subjects when driving and be sure to stop in the small towns along the way to look for photo opportunities.