I can see your point on it being a common process by the metal manufacturer. For some reason I figured it was as Bill said.
In regard to hydrochloric acid vs phosphoric acid, I thought the use of any form of acid would be considered pickling and would resuld in the iron oxide coat mentioned( some stronger and some will eat the base material and some wont...). Its this inert layer that I figured would contribute to reducing rust over the years and alse give it that chassis black appearance.
My understanding is that most fo the "rust converter" products out there are actually acid based and convert the rust to iron oxide. If the iron oxide layer works so well, why would i remove it?
As an aside- my truck had a hoist put on it and you could see where the frame of the hoist which was bare steel ( even when butted up to the original frame) would rust away with little left and the frame was fine...
