Interesting that the vast majority of 737s were flying so frequently that this was not found earlier.
For their entire 20-ish year service life, those planes are never left idle long enough for this part to rust! Amazing.
Goes to show how popular and successful the 737 line is.
Yeah I'm sure most commercial airliners are the same. Apart from maybe the Super-Heavies; I hear that there are dozens (hundreds?) of 747s and A380s sitting around doing nothing currently because they're too expensive to fly.
Those 747s and A380s are only sitting around not flying because of the pandemic. And it seems likely that many of them will never return to flight again at this point, except maybe after being modified for cargo hauling.
Prior to the pandemic, if you had a flight-worthy plane of any size, you were using it continuously. Once a plane was no longer being used continuously it was likely about to be permanently retired.
From my cursory glance at the doc, the part in question is inside the bleed-air cutoff system. This is a flapper valve (a bit like in your toilet) that takes air from the engine and uses it for other systems in the plane (air conditioning, cooling, starting other engines, etc).
When it's in use, the movement of the parts will prevent rust from building up in one spot. A little rust might accumulate, but it will be rubbed/wiped off when it's next used. The problem occurs when enough rust builds up that the flapper cannot move anymore.
In normal use these valves take air out of the turbine stage, so they see very very hot air with almost zero moisture in it in normal use. If you park them long enough I can imagine they get wet from rain or even just condensation.
When you say "cold air" I guess you mean that it's bled before the combustion stage. It's not cold at all after the compression.
And bleed air is indeed always taken in the compressor, never in the turbine. First there's no point taking it later than the compressor, you just want compressed air. And it's ultimately used to ventilate the cabin, you don't want fuel vapors or smell.
Stage 5 would be pretty early on. However generally when talking about the HPC you're talking about cold air. At this stage, cold air that's sprayed on the turbine housing when engine power is reduced to thermally shrink the metal to keep the turbine blades and casing within tolerance for efficiency's sake.
All is explained in the link I posted, would recommend it as good introductory reading on the subject.
Not specific to this part, but most mechanical items that move get oiled as part of their operation. When they don't move for a long time that oil evaporates, and now the moisture in the air can attack the surfaces. This is a common situation in general aviation if they're not flown regularly and can lead to early replacement of the engine.
