Just a note about the paint... The black color where the plate was torn away is an oxygen deficient Fe corrosion product. It's between FeO and Fe3O4. It's stable in that crevass, because of the pressure and lack of O2. The normal red oxide is Fe2O3. I don't think there's any areas that weren't covered with paint. The stains are on the surface of the paint, except in a few small areas where the paint lifted. Fe corrodes by first producing Fe2+, which is water soluble. The Fe2+ remains in solution until it's oxidized by O2 from the atmosphere.
It's clear the drain dumped water on that gusset area and kept it wet. The corrosion occurred around and within the rivet crevasses. Fe2+ would wander out as a corrosion product and precipitate as the the water flowed down the struts, after being oxidized by atmospheric O2 to Fe3+. That's how the red rust color on the paint got there.
You are right, of course, about the anti-corrosive coating. I’ve never seen bare steel raised in an exposed design. It’s always covered with something.
In saying the U10 gusset “wasn’t painted”, my meaning was “wasn’t painted green, after assembly”, as opposed to “not painted, therefore bare steel”.
I appreciate you taking the time to add the oxidation resistance information, which I was just too otherwise directed to do.
Not sure if you agree or disagree, but I do NOT see rust on U10, before or after the collapse. I do NOT see dampness there either. It’s something else, because dampness would have gone away after the collapse, or spread considerably, This discoloration kept the same shape before, during and days after the collapse.
At the risk of being repetitive, the discoloration very closely follows the fracture lines too.