Subject: How do 'rust converter' chemicals work?

I recently used Rust-Oleum (tm) "Rust Converter" on my tractor.  Out
of the bottle it's a gray liquid.  It's applied with a paint brush and 
dries clear, but causes a conversion of any red rust to a black
substance that looks like the black oxide found on the surface of 
hot-rolled steel.  Although the label claims a quick reaction, the 
liquid dries in a half hour or so and I found that it actually takes
a couple of days to completely convert the red rust.  Humidity in 
the air appears to help the reaction along once the coating has dried.

What is(are) the active ingredient(s) and what's the chemical reaction
going on?  Is this a permament change, or will the black substance
eventually revert back to red rust?  Why does iron oxidize naturally
into red rust instead of this black substance?  

P.S. I'm not talking about the various "rust stripper" types of 
products such as the one sold by Rust-Oleum or under the "Naval Jelly"
(tm) brand name.  I know one of the active ingredients in this type
of product is phosphoric acid, which appears to "dissolve" any red 
rust leaving shiny metal.  For comparison, what's the reaction going 
on here?

Re: How do 'rust converter' chemicals work?