The 590 SPECIAL PURPOSE and 590A-1 have Parkerized receivers. You CANNOT Parkerize aluminum.
It's pretty obvious you don't own one, but you can google it and find 1000 posts saying the receiver is aluminum, or you can continue to believe it's steel. I touched a magnet to my 590A1 receiver and guess what, it doesn't stick. Who says the receiver is parkerized anyway? The barrel is parkerized.