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 ai

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.