Forcing a steel pipe to rust completely I understand rust.  Fe + H20 → OH- → Fe(OH)2 or 3 and/or FenOm
I understand rust prevention and steels that are resistant to rusting.  Trying to google this question gives thousands of answers on prevention.
But what I'm interested in is how to force steel to rust.  And not in an artsy way, so my steel stuff looks weathered and cool (these are the other thousands of search results for forcing iron/steel to rust).  I want to dispose of steel by turning it into iron oxide.  I don't actually really want to do this since it's a lot easier to just landfill or recycle, but I thought of it as an alternative and realized it's really not that easy.
The trouble seems to me that the passivation layer stops rust from completely going completely through the metal.  Can this be forced by running active corrosion circuit in reverse, or will the equation still slow to a crawl once a layer of rust builds up on the surface?
As we in the trade say &quot;rust never sleeps&quot;. Rust develops on the surface , rust is somewhat protective in some &quot;weathering&quot; steels such as Corten. Small additions of Cu ,Cr, and P , mostly provide limited slowing of rust . Otherwise there are many tables , based on conditions and location of exposure . The tables will give rust rates like 5 mpy ( mils per year ; mil is a thousandth of an inch , pardon my US units). The loss rates will be much higher on seacoasts or where rain often occurs. So with no other information ,you can guesstimate your steel will disappear in (THICKNESS divided by 0.005 ) years. Half that time for two sides exposed.