Awesome, awesome flavor. I might have to swipe this for my planar campaign; I think a character with this PrC and my own Ninefold Damnation discipline would be the perfect Dis emissary.

The only issue I have with it is all of the lower-level rusting abilities; you'd think a child of Dispater wouldn't want to harm iron, or would be healed by it, yet it doesn't get something like that until 4th level. Instead, I would add two things to Child of Iron--you heal 1d6 whenever you successfully use a rusting effect, and you are immune to rusting effects--and change Stance of the Iron Tyrant to basically be a permanent ironguard effect (making you immune to metal weaponry entirely) plus rusting.