TLDR: What portion of the cooling system is rust formed in (I.E. Engine block, radiator, all... etc)
I did a coolant flush in a 94 Ford Ranger 2.4L at 226K. It was a consistent brown color and the radiator had brown residue inside the pressure cap. I would not be surprised if the coolant had over 120K and 10 - 15 years on it. After four complete flush cycles it was still coming out brown. I did not have time, so I just filled it with coolant/water mixture and called it good for the next couple months (plan on replacing the radiator/water pump soon). This all got me wondering, is there a specific portion of the cooling system where rust is typically formed, or is it all the metal surfaces in the system.
Might be an obvious answer but I am new to cooling systems.
Lastly, the engine oil is not a cream color so I am not worried about the coolant discoloration being from a head gasket leak.