Please let me explain what it is that you are seeing on the inside of the Z-Vent pipe.  What you see is a byproduct of the way that the Z-Vent pipe is cut as it comes off our tube mill machine at the factory.  You will see a brownish film (slag) that usually goes down the inside of the vent length for some distance.  This film can be wiped off with your fingers and appears to look like rusting or sooting.  You will also notice some minor pitting as you get closer to the end as this is the point of the highest heat concentration during the cutting process.  The seals may also pick up some of this “slag” giving them the appearance they are used. You will only see this on the pipe and not any of the fittings because the pipe is fed on a continuous coil of stainless steel where as the fittings use single sheets of stainless steel.